Achieving Ultra-Low Friction in Ti-6Al-4V Alloy: Hydration Lubrication Mechanisms of HEC-Glycerol Composite

Achieving Ultra-Low Friction in Ti-6Al-4V Alloy: Hydration Lubrication Mechanisms of HEC-Glycerol Composite | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Achieving Ultra-Low Friction in Ti-6Al-4V Alloy: Hydration Lubrication Mechanisms of HEC-Glycerol Composite Dezun Sheng, Hongliang Yu, Xiao Zhang, Xin Zhou This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6963694/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 17 Sep, 2025 Read the published version in Tribology Letters → Version 1 posted 7 You are reading this latest preprint version Abstract Reducing alloy friction to achieve ultra-low friction is a valuable approach to save energy and reduce pollution from oil use, which is a major challenge for researchers. This study introduces a successful method to achieve ultra-low friction in Ti-6Al-4V using a hydrated lubricant composed of hydroxyethyl cellulose (HEC). And the effects of speed and concentration on lubricating were investigated. It was found that excessive sliding speeds may lead to lubricant detachment and consequent friction increase, indicating that the adsorption ability of HEC needs to be enhanced in future studies. In addition, when the concentration exceeds 5 wt.%, wear loss tends to stabilize across tests with different concentrations, while the friction force increases with rising concentrations. Based on these findings, microscopic studies were conducted to investigate the mechanism of friction reduction. Notably, distinct topographic features resembling 'valleys' and 'plateaus' were identified on the wear scars in a nanoscale scope. The movement of the surfaces induces the hydrated HEC lubricant to flow from the lower valleys to the higher plateaus, suggesting elastohydrodynamic lubrication mechanisms to form robust films. The valleys serve as lubricant reservoirs, while the plateau tops support the lubricant films to prevent contacts between Ti-6Al-4V and Si 3 N 4 . The schematic illustrations depict the microscopic mechanisms for achieving of ultra-low friction on Ti-6Al-4V alloy. Ultra-low friction microscopic mechanisms Hydration lubricant Ti-6Al-4V alloy Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 1 Introduction Friction and wear are two interrelated phenomena prevalent in engineering and biological systems, severely affecting the performance and lifetime of mechanical components and biomedical implants[1–3]. While conventional lubrication mechanisms have been extensively studied and applied to minimize friction and wear[4–7], recent studies have discovered a highly promising lubricating mechanism: hydration lubrication[8, 9], which is characterized by the formation of a hydrated layer at the interface between two sliding surfaces. Hydration lubrication has become a popular research area with important influences on tribology[10, 11]. This novel lubrication mechanism, derived from the physicochemical interactions between hydrated polymers and solid surfaces[12–14], offers a significant approach to achieving ultra-low friction characteristics, and even shows high potential to be applied to metal alloys such as Ti-6Al-4V[15–17]. Alloys typically operate under macroscale conditions. Remarkably, Li et al. discovered ultra-low friction through hydration lubrication in such conditions by using mixtures of acids and polymers. Notably, phosphoric acid demonstrated remarkable lubricating performance, with a friction coefficient as low as 0.004 [18–20]. Although acid solutions are unsuitable for alloy lubrication, these findings push forward the way for reducing alloy friction through hydration lubrication. Xu et al.[21] used microgels as lubricant additives in bovine calf serum to lubricate titanium alloy/ultra-high molecular weight polyethylene contacts, significantly reducing friction and demonstrating temperature-sensitive lubrication characteristics. Additionally, Cui et al. and Liu et al. discovered that bilayer coatings[22] and copolymer[23] on the Ti-6Al-4V surface enhance lubrication properties. Although these treatments did not achieve superlubricity, the hydrated polymers are believed to have the potential to significantly reduce friction on the Ti-6Al-4V alloy. Not only can hydrated polymers [24] reduce friction, but hydrated anions [25] also perform excellently in enhancing the antiwear ability of the Ti6Al4V alloy. Li et al. [26] explored the lubrication mechanism of halogen anions on positively charged surfaces at the atomic scale, discovering that the adsorption state of anions influences friction dissipation, and superlubricity can be achieved at low concentrations. The anions and polymers hydrated with water can improve the lubrication performance of Ti-6Al-4V alloys. Furthermore, they demonstrate significant potential for application in biotribology conditions. Yue et al. prepared chitosan-g-PMPC copolymers to achieve a very low coefficient of friction (µ < 0.01) on Ti-6Al-4V alloy in pure water. This remarkable reduction in friction is primarily attributed to the hydrated nature of the PMPC side chains, the interface adsorption of the copolymer, and the hydrodynamic effect[27]. Polymers serve multiple functions by forming various functional groups [28, 29]. In friction, these designed supramolecular structures demonstrate the ability to repair lubrication surfaces through hydration effects, significantly advancing the achievement of long-term lubrication mechanisms [30, 31]. Wang et al. prepared self-assembled polymer monolayers and visualized their dynamic repair process [32]. They discovered that, after friction-induced dissociation of the polymers, the polymer-to-surface interaction is restored through the reformation of host–guest complexes. This process renews the lubricity and maintains the reduction of wear. In contrast to boundary lubrication or other lubrication mechanisms, hydration lubrication relies on the hydration repulsions of the hydrated groups to resist contact and minimize friction[10]. Additionally, it uses the dynamic hydration-dehydration behavior to restore the lubricating function[25, 33]. The synergistic effect is a valuable approach to promoting the hydration behavior of lubricant[34]. Feng et al. synthesized a super-lubricated hydrogel with the synergy of phospholipid and hyaluronan and the coefficient of friction could reduce down to 0.004[35]. In addition, Liu et al. developed a novel layered soft hydrogel as cartilage prototype, which can exhibit a low friction coefficient (COF ≈ 0.006) under a wide range of contact stresses (0.2 to 2.4 MPa)[36]. It’s clear that the synergistic effect can enhance the effectiveness of hydration lubrication [37]. This study investigates the synergistic effect [38, 39] of hydroxyethyl cellulose (HEC) [40, 41] and glycerol in enhancing the tribological properties of Ti-6Al-4V alloys. By improving the hydration characteristics of the mixture, where HEC acts as a water-soluble polymer [42, 43] and glycerol functions as a coupling agent, the authors aim to develop a robust lubrication system to reduce friction in alloys. Considerable attention has been paid to friction tests on materials and mechanisms, and the testing methods have remained largely unchanged over the decades. In this work, the authors first reported the changed friction testing methods. Then, through experimental characterization and analysis, the authors investigated the fundamental mechanisms for the friction reduction of the hydrated HEC-glycerol, which aims to enhance the understanding of hydration lubrication and its role in improving the tribological properties of alloys. The achievement of ultra-low friction properties in Ti-6Al-4V alloys could expand the way to improved energy efficiency and enhanced reliability in automotive, and other applications. 2 Experiments and Methods 2.1 Preparation of Materials and Lubricants Hydroxyethyl cellulose (HEC) possesses numerous hydroxyl groups and is a water-soluble polymer extensively utilized across various industrial applications. The HEC utilized in this study was purchased from Aladdin Corporation (Catalog No. H104790, Aladdin Corp., CHN). It has a purity exceeding 99%, with the average viscosity of its 2.0 wt.% solution at 25°C falling within the range of 100–200 mPa·s. Notably, the 4.0 wt.% HEC solution was chosen based on prior research indicating its optimal lubricating properties[44]. The lubricant preparation process involved adding a quantified amount of HEC powder in deionized water, followed by continuous stirring for 30 minutes to achieve a uniform and transparent HEC solution. Glycerol, purchased from Aladdin (Catalog No. G116210, Aladdin Corp., CHN) with a purity exceeding 99.8%, was introduced into the 4.0 wt.% HEC base solutions in varying masses, producing seven distinct HEC-glycerol (HEC-G) mixed solutions with glycerol concentrations of 1.0, 5.0, 10.0, 15.0, 20.0, 25.0 and 30.0 wt.%. These mixed solutions underwent a 30-minute incubation period in an ultrasonic bath set at 25°C, during which their appearance remained transparent and homogenous. 2.2 Characterization of Additives and Lubricants Before tribological tests, the Fourier transform infrared spectra (FT-IR) of both Hydroxyethyl cellulose (HEC) and its glycerol-modified lubricant (HEC-G) were examined utilizing a Fourier transform infrared spectroscopy instrument (Nicolet 6700, ThermoFisher Corp., USA). The hydroxyl groups present in HEC possess strong attraction for water in bulk solutions, thereby contributing to its hydration effect. Glycerol, characterized by three hydroxyl groups per molecule, functions as an effective coupling agent, enhancing the synergistic effect within aqueous solutions. Consequently, investigating the chemical interactions occurring within HEC and HEC-G via FT-IR spectroscopy proves advantageous in comprehending the synergistic influence of glycerol on HEC solutions. Nonetheless, excessive hydration of the mixed solution may lead to the formation of numerous hydrogen bonds, resulting in high solution viscosity, which could adversely impact lubrication. Hence, to examine the macroscopic impact of glycerol on the fluidity and viscosity of HEC solutions, investigations were conducted utilizing a standard rheometer (Physica MCR301, Anton Paar Corp., USA) operating at 25°C. 2.3 Tribological Experiments and Analysis Friction experiments were conducted using a universal micro-tribometer (UMT-3, Bruker Corp., USA), operating in the ball-on-disk sliding configuration. The spherical balls utilized were fabricated from Si 3 N 4 with a radius of 2.0 mm and a surface roughness (Ra) of 25 nm. The disks employed were plates made of Ti-6Al-4V alloy, possessing dimensions of Φ25×3 mm. The Ti-6Al-4V samples underwent wet polishing using SiC paper. The surface roughness of the Ti-6Al-4V disks was quantified utilizing a 3D optical profilometer (NewView 5022, Zygo Corp., USA), yielding a measured Ra value of 1.6 ± 0.3 µm and a Root Mean Square (RMS) roughness of 278 ± 22 µm. The mechanical characteristics of both the Si 3 N 4 balls and Ti-6Al-4V disks are summarized in Table 1 , mirroring data found in previous publications[45, 46]. Prior to subsequent testing section, All samples were sequentially ultrasonically cleaned in acetone, ethanol, and deionized water (5 min each), followed by drying with nitrogen gas. Then, X-ray diffraction (XRD) patterns of the Ti-6Al-4V specimens were acquired, revealing features composed of α martensite phase, as depicted in Fig. 1 . Table 1 Mechanical properties of the Si 3 N 4 ball and Ti-6Al-4V disk Materials Density (g/cm 3 ) Hardness (kg/mm 2 ) Young’s modulus (GPa) Poisson’s ratio Si 3 N 4 3.3 1420 310 0.26 Si 3 N 4 * 3.2–3.5* 1390–2450* 290–315* 0.28* Ti-6Al-4V 4.5 320 118 0.34 Ti-6Al-4V* 4.2–4.5* 310–370* 110–220* 0.32–0.36* The data annotated with an asterisk (*) denote averaged parameters derived from preceding experimental trials. In the process of sliding friction, the rotational motion of the disc induces a sinusoidal variation in the applied normal load, due to the presence of a misalignment angle denoted as α, as illustrated in Fig. 2 . It is observed that larger misalignment angles correspond to increased measurement inaccuracies. To minimize the influence of the misalignment angle, this work developed a high-precision leveling apparatus aimed at reducing α to below 0.01°, which was published in our previous work[44]. Implementation of this leveling device effectively minimizes measurement errors, thereby enhancing the accuracy of obtained data. The efficacy of this approach was validated in Section 3.3 through examination of the measured results. 3 Results and Discussion 3.1 Spectra of HEC and HEC-G In the spectral analysis of HEC-G, the broader band observed between 3475 and 3133 cm − 1 arises from the presence of hydroxyl groups[47]. Peaks identified at 2917 cm − 1 correspond to the stretching band of C-H bonds. The faint peak observed at 2162 cm − 1 is attributed to C = C bonds within the sugar unit, while another weak peak at 1973 cm − 1 is associated with the bending band of C-H bonds. The peak appearing at 1563 cm − 1 is a consequence of the skeletal vibration of the sugar unit, whereas the peak at 1405 cm − 1 is linked to the bending vibration of C-H bonds. The intensity of the band observed at 1052 and 1021 cm − 1 is attributable to the bending of C-O-H bonds. The spectral profile of HEC-G exhibits similarities to that of HEC, particularly evident for peaks below 2917 cm − 1 . However, distinct differences in the relative intensities of hydroxyl group and C-O-H group peaks exist between HEC-G and HEC. Notably, for both HEC-G and HEC, the intensities of peaks at 1563 cm − 1 and 1405 cm − 1 are comparable, ranging from 75–80%, serving as a reference standard. Specifically, in the spectrum of HEC-G, transmittance intensities at 1052 cm − 1 and 1021 cm − 1 are notably lower compared to those at 1563 cm-1 and 1405 cm − 1 , while in the spectrum of HEC, the converse is observed, where transmittance intensities at 1052 cm − 1 and 1021 cm − 1 surpass those at 1563 cm − 1 and 1405 cm − 1 . Furthermore, the transmittance intensities of the broader band between 3475 and 3133 cm − 1 in both HEC-G and HEC exhibit a similar trend. This trend suggests a considerable increase in the number of hydroxyl groups in the mixed solution upon glycerol addition, resulting in change in the intensity of corresponding peaks. Additionally, Raman spectroscopy was conducted on both HEC-G and HEC, with results presented in Fig. 4 . The peaks denoted by circles (893.6, 931.6, 1122.5, 1180.5, 1413.6, 1472.0, 2893.6 and 2929.6 cm − 1 ) in Fig. 4 are the functional groups that appear in both HEC and HEC-G, indicating that the addition of glycerol tends not to change those original functional groups, but enhances them slightly. The five peaks at 931.6, 1122.5, 1472.0, 2893.6, and 2929.6 cm − 1 (marked with asterisks) are observed in the Raman spectra of all three polymers (glycerol, HEC, and HEC-G), with peaks 931.6 and 1122.5 cm − 1 representing the C-C group, and peaks 1472.0 and 2929.6 cm − 1 indicating the -CH2- group. The peak of 2893.6 cm − 1 represents the -OH, which is a weaker peak in the case of glycerol and a stronger peak in the case of HEC. The functional groups represented by the five peaks on HEC tend to be enhanced by the same functional groups in glycerol, which favors mutual attraction with water molecules. In addition, in the spectrum of glycerol, the peaks at 860.0 cm − 1 (represented by letter a) and 1066.5 cm − 1 (represented by letter b) are obviously strong[48]. In contrast, the peaks at the same positions are barely visible in the HEC spectrum, from which it can be inferred that the strong groups on glycerol are weakened by the formation of chemical bonds with the functional groups of HEC. As a result, the weak peaks of HEC-G at 874.4 cm − 1 (denoted by the letter A) and 1100.2 cm − 1 (denoted by the letter B) appear. The relatively strong peaks A and B compared to the same positions of HEC are attributed to the incorporation of glycerol. Glycerol molecules can interact strongly with the HEC functional groups and macroscopically enhance the viscosity and other properties of the HEC-G mixture. 3.2 Fluidity of the mixed solutions The viscosities of HEC and HEC-G were measured using a rheometer to clarify their non-Newtonian fluid behavior characterized by shear-thinning properties, as shown in Fig. 5 (a). The dynamic viscosity of 4.0 wt.% HEC solution was 2150 mPa·s at a shear rate of 1 1/s, and decreased by 75% when the shear rate was increased to 10 3 1/s. In contrast, at a shear rate of 1 1/s, the dynamic viscosity of 1% HEC-G approximated 2180 mPa·s. It’s worthy note that the HEC-G demonstrated a gradual decline in viscosity with increasing shear rates, mirroring the shear-thinning characteristics observed in the 4.0 wt.% HEC solution. Since an increase in the shear rate of the lubricant can significantly reduce its viscosity, the mixed lubricant is likely to exhibit excellent lubrication efficacy at high velocities during friction. However, upon the addition of 5% glycerol to HEC, the dynamic viscosity notably surpassed that of the 4.0 wt.% HEC solution, as illustrated in the inset of Fig. 5 (a). Furthermore, at a glycerol concentration of 30%, the viscosity surged to 6859 mPa·s (marking an increase of 223.6%), indicating that the addition of a small quantity of glycerol marginally influences the dynamic viscosity of HEC. It is evident that the incorporation of glycerol negatively influenced the fluidity of HEC, with the viscosity of the HEC-G solution increasing proportionally with the glycerol content. This substantial increase in viscosity may impede the movement of the frictional pair adversely. Figure 5 (b) illustrates the variations in viscosity with increasing concentration at different shear rates. At a shear rate of 509 1/s, the viscosity of the mixed solution exhibited a modest increase from 489 mPa·s to 576 mPa·s, representing a 17.8% augmentation. This observation suggests a weak concentration effect on the lubricant's viscosity under conditions of rapid shear. In contrast, at a lower shear rate of 1 1/s, the viscosity of the mixed solution experienced a substantial augmentation from 2150 mPa·s to 6859 mPa·s, indicating a markedly robust concentration effect on the lubricant's viscosity[49, 50]. 3.3 Minimization of measurement errors The force transducer employed in this study was the DFM-1G model, characterized by a 10 N range and an low resolution of 0.1 mN (equivalent to 0.01 g). In order to raise the measurement precision, a comparative analysis of the coefficients of friction was conducted in both clockwise and counterclockwise sliding directions, according to the works established by Lee[51]. The comparative results are shown in Fig. 6 . Figure 6 presents the variations in normal load and friction force during the tests conducted at a speed of 410 rpm and a load of 1 N using a 4.0 wt.% HEC solution. These figures are modified reproductions obtained directly from UMT-3 equipment, where the force denoted as 1 gram (g) corresponds to 10 mN. Figure 6 (a) distinctly shows the results obtained in the clockwise direction. The friction force exhibited a sudden decrease from 18.95 g to 0.46 g upon the introduction of the lubricant at 200 seconds. Notably, the normal force (Fz) displayed considerable fluctuations during the wear-in phase within the initial 200 seconds. Subsequently, upon the application of the lubricant, it stabilized and exhibited flat variations, ranging between 99.92 g and 100.18 g (as indicated by the magnified partial view). Furthermore, the results obtained in the counterclockwise direction revealed a reduction in friction force from 18.60 g to 0.45 g following the addition of the lubricant. Simultaneously, the normal force exhibited slight fluctuations, ranging approximately between 99.82 g and 100.08 g. Comparative analysis between the tests conducted in opposite directions revealed that the fluctuation range of the normal force remained within 1.6 mN, while the measurement error of the friction force was minimal, standing at 2.5%. These findings underscore the conformity of the measurement outcomes with the requisite standards for scientific investigations. 3.4 Difference in friction coefficients under two friction strategies In this study, we employed two distinct methodologies to execute the friction tests. Method 1 involved the initial application of approximately 0.2 mL of a 10% HEC-G lubricant onto the friction track, followed by the start of the friction test. The variation of the friction coefficient under this method is depicted in Fig. 7 (a). Initially, a notable reduction in the friction coefficient was observed, followed by a brief period where the friction coefficient as low as 0.12, as indicated by the inset. Subsequently, the friction coefficient exhibited an increasing trend and fluctuated within the range of 0.012 to 0.025, with an average friction coefficient of approximately 0.017. It is evident from these results that the lubricating effect of HEC-G, as assessed through this method, failed to sufficiently reduce the frictional force. Consequently, an alternative approach was devised for conducting the friction tests. Method 2 was devised as follows: Initially, a load of 1 N was applied to the ball, and friction was allowed to proceed for approximately 250 seconds without the application of any lubricant; Subsequently, the mixed HEC-G was introduced into the friction track. The variation of the friction coefficient under this method, as depicted in Fig. 7 (b), exhibited notable distinctions from that observed in Method 1. Throughout the wearing-in stage, the friction coefficient demonstrated a gradual increase from 0.029 to 0.045. Then, it stabilized within the range of 0.038 to 0.048, indicating the attainment of a relatively stable condition for the wear surfaces of the ball and disk. Upon the introduction of the lubricant at the end of the wearing-in stage, the friction coefficient exhibited a sudden decrease to approximately 0.004. Notably, during this phase, the average friction coefficient remained stable until the end of the test, which extended for a duration exceeding one hour. Given the remarkable lubricating performance observed in the results obtained from Method 2, all subsequent tests were conducted in reference to this method. The inset in Fig. 7 (b) is an optical picture and displays the worn surface of the disk produced during the wearing-in stage, which will be examined in detail in the following sections. It is noteworthy to highlight the beneficial role of the wearing-in stage in achieving low friction forces, which need to be investigated in future research endeavors. 3.5 Lubrication results enhanced by HEC-G Velocity has a significant effect on the formation of the lubricant film, which is a key factor in the lubrication process. Initially, we evaluated the lubrication efficacy of HEC-G at different velocities to determine the optimal test velocity for subsequent experiments. A lubricant consisting of 10% HEC-G was chose, with a load of 5 N. The rotational speed ranged from 80 rpm to 850 rpm, incremented by 110 rpm steps. Since the radius of the contact trace measures 4.1 millimeters, the corresponding linear velocity at the friction center varied between 34.3 mm/s and 364.7 m/s. Notably, the duration of the wearing-in stage varied for each test, deliberately configured to ensure uniform sliding distances across all experiments during wearing-in stage. Specifically, the sliding distance was standardized to 33.3 meters, as shown in Table 2 . Table 2 Duration of wearing-in stage for each test with the same sliding distance Speed (rpm) 80 190 300 410 520 630 740 850 Velocity (mm/s) 34.3 81.5 128.6 175.8 222.9 270.7 317.6 364.7 Duration (s) 964 406 257 188 148 122 104 91 Sliding distance (m) 33.3 33.3 33.3 33.3 33.3 33.3 33.3 33.3 Figure 8 illustrates a comprehensive analysis of the friction coefficient over the course of a one-hour test duration. At a rotational speed of 80 rpm, The friction coefficient showed significant fluctuations (amplitude > 30% of mean value) during the initial wearing-in phase. Subsequently, upon the introduction of lubricant, the friction coefficient immediately decreases to a lower value and continued to fluctuate with a lesser extent. Notably, the average friction coefficient decreased by approximately 61%, from 0.059 to 0.023. At a speed of 190 rpm, the variation in friction coefficient became smoother for a brief period following the addition of lubricant, subsequently fluctuating within the range of 0.008 to 0.018. Moreover, at rotational speeds of 410 rpm and 740 rpm, the variation in friction coefficient exhibited notably smoother trends and minimal fluctuations. As the rotational speed increased, the duration of the wearing-in stage diminished, accompanied by a reduction in the magnitude of friction coefficient fluctuations. Additionally, the average friction coefficient plummeted to an impressively low value of 0.004, indicative of a highly effective lubrication stage. It is reasonable to infer that under these conditions, the lubricating film successfully separates the contact surfaces, thereby ensuring optimal lubrication of the friction pairs. Furthermore, at a speed of 740 rpm, the average friction coefficient remained stable and low at 0.006. However, with further increases in speed, the lubricating performance of HEC-G become progressively worse. At a speed of 850 rpm, the friction coefficient fluctuated throughout the entire test duration, even with the addition of lubricant. It was observed that, under high-speed situations, the lubricant was ejected from the friction disk, likely due to the substantial centrifugal forces generated at such high rotational speeds. It can be assumed that the increase in friction coefficient at higher speeds is not directly attributable to the diminished lubrication performance of HEC-G, but rather to the inadequate supply of lubricant. Unfortunately, under conditions of elevated rotational speeds, the actual lubrication effectiveness could not be accurately assessed. To clarify the negative effect of inadequate supply of lubricant, several drops of HEC-G were introduced into the friction track during a dry test with speed of 850 rpm, as illustrated in Fig. 9 . During standard frictional testing, a single drop of HEC-G was added at approximately 1000 seconds. It was observed that the friction coefficient decreased from 0.018 to 0.009, a reduction sustained for approximately 10 seconds, followed by an increase in friction coefficient to about 0.019. Subsequently, at around 1058 seconds, 10 drops of HEC-G were added. This resulted in a further reduction of the friction coefficient to 0.009, which lasted for approximately 30 seconds. Evidently, the duration of the friction coefficient at 0.009 increased with the greater supply of lubricant, confirming the notion that insufficient lubricant is responsible for the raised friction coefficient observed at 850 rpm. Subsequently, the friction coefficient stabilized at approximately 0.015–0.025, akin to typical frictional conditions. It is crucial to note that the addition of HEC-G during friction momentarily reduces the friction coefficient and underutilizes the lubricating ability of HEC-G. According to Dowson-Hamrock theory, increased velocity facilitates the formation of a lubricating film. However, if the adhesive force of the lubricant on the friction surfaces surface cannot withstand the centrifugal force, the lubricant was centrifugally ejected from the wear track, thereby leading to an increase in the friction coefficient. The lubrication mechanism investigated in this study is presumed to be elastohydrodynamic lubrication, which was generated by the hydrated HEC-G lubricant. Accordingly, the lubricating films were analyzed utilizing the Dowson-Hamrock theory, enabling the calculation of film thickness. The calculated film thickness values are presented, alongside the corresponding average friction coefficient data, in Fig. 10 . It is evident that film thickness increases with rising speed, with a corresponding decrease in friction coefficient observed at lower speeds. Notably, at a speed of 410 rpm, the friction coefficient reaches its lowest value of 0.004. However, as speeds surpass 410 rpm, although the calculated film thickness continues to increase, the friction coefficient begins to rise due to lubricant depletion. In such instances, friction coefficients at speeds exceeding 410 rpm were extrapolated and predicted based on the observed trend, depicted by the unfilled circle symbol in Fig. 10 . The recorded friction coefficient values notably exceeded the predicted values, indicating a pronounced detrimental effect of high speeds on HEC-G lubrication efficacy. For rotating friction systems, it is observed that friction coefficient augmentation with speed occurs beyond a certain threshold, indicating the Dowson-Hamrock theory inadequate for predicting the relationship between film thickness and friction coefficient. This emphasizes the need for alternative perspectives when analyzing lubrication models at high velocity conditions. Furthermore, these findings emphasize the critical role of lubricant adsorption onto friction surfaces in achieving effective lubrication. Enhancing the adsorption capability of lubricants to resist detachment is crucial for achieving optimal lubrication performance. The profiles of worn tracks were determined. Five characteristic tracks under different speeds were chose and depicted in Fig. 11 (a). The yellow dashed circles indicate the worn tracks covered by lubricants, while the cleaned tracks are depicted by the insets. Notably, the measured diameter of the tracks is 4.1 mm, though it may appear different between pictures due to camera placement. It is evident that the tested lubricant at 80 rpm was darker than the others, attributable to a higher concentration of worn particles immersed in the lubricant. Similarly, the tested lubricant at 850 rpm was equally dark, despite appearing less in quantity, due to substantial centrifugal forces generated at such high rotational speeds. The flung lubricant is still visible beside the wear track, as indicated by green arrows. A similar phenomenon occurred at a speed of 740 rpm, where scattered lubricant is easier to find. At 410 rpm, the worn track was easy to observe since the lubricant remained transparent. It should be noted that the lubricants are rounded within the blue dotted circle, which represents the trajectory generated during the minimization of the misalignment angle α. Additionally, the cross-sectional profiles of the worn tracks were measured and are depicted in Fig. 11 (b). The height drop (ΔH) was calculated manually. It is evident that the ΔH at 80 rpm, which measured 11.2 ± 0.1, was the largest, while the lowest ΔH of 5.4 ± 0.1 was observed at 410 rpm. This significantly indicates that the wear occurring at 80 rpm was the most severe, causing the track to erode deeply, resembling a crater. It is believed that the wear particles immersed in the lubricant were responsible for the extensive wear observed at 80 rpm, a condition under which the lubricant failed to form effective lubricating films. In contrast, the worn track at 410 rpm appeared relatively shallow. As the speed increased beyond 410 rpm, the ΔH became larger, indicating progressively worse wear. However, the trends in surface roughness (Ra) of the worn tracks did not align with those of ΔH, as Ra increased with rising speed. Notably, the Ra at 190 rpm was the lowest, almost equal to that at 80 rpm. This discrepancy can be attributed to two factors. First, the calculation methods for Ra and ΔH are different. Second, at 80 rpm, the wear particles tend to be smaller due to the poor lubricating efficacy, causing the asperities to be worn down and resulting in a lower average height of asperities for calculating Ra. In contrast, at 410 rpm, the asperities are more likely to be separated by the lubricating films, resulting in fewer worn asperities and pits, and thus a higher average height for calculating Ra. At 850 rpm, the lubricant was thrown off, leading to increased contact and wear of the asperities and wear particles. However, it is unlikely that the Ra at 850 rpm would decrease to the same extent as at 80 rpm due to the continuous wear, as the wear mechanisms differ between low and high-speed situations. The worn zone on the Si 3 N 4 balls were also detected and depicted by the blue dashed circles in Fig. 12 (a). The five blue circles have the same diameter, indicating that the wear on the balls is similar in size. This is likely due to the high hardness of the balls, with most of the wear occurring during the initial wear-in period. When the lubricant was introduced, the wear zone on the balls showed minimal further wear, whereas the Ti-6Al-4V plates exhibited significant wear instead. The section profiles across the yellow dashed line, which is perpendicular to the movement direction (indicated by the green arrow) of the balls, are shown in Fig. 12 (b). The insets provide partial enlargements for clarity. From these section profiles, the differences in the details of the worn zones are clearly evident. At 80 rpm, the worn zone exhibits burring and an uneven surface, which is similar to the condition observed at 190 rpm. In contrast, the worn surface at 410 rpm is relatively smooth, whereas at the high speed of 850 rpm, the surface once again becomes burring. The surface roughness of the worn zone was measured, and both Ra and Rz values are provided. The Ra value is 2.73 ± 0.03 for 410 rpm, the lowest among the tested speeds, while it is higher for 80 and 850 rpm. Although the Ra values for 190 and 740 rpm are not significantly greater than that of 410 rpm, the Rz values for 190 and 740 rpm are substantially higher. Rz represents the average value of the heights of the five highest-profile peaks and the depths of the five deepest valleys within the worn zone. Given the trend of Rz, the worn zone at 410 rpm is the smoothest. It is believed that at speeds of 80 and 850 rpm, the lubricants cannot adequately separate the contact surfaces within the worn zone, resulting in higher Ra and Rz values due to increased surface wear. In conclusion, while high speeds can promote the formation of lubricating films, they can also lead to the detachment of the lubricant from the surface. Therefore, considering the interactions between the lubricant and the friction surface, the optimal speed for achieving the best lubricating performance may vary depending on the specific lubricating conditions. 3.6 Impact of concentration on the lubrication provided by HEC-G Velocity has a significant effect on the lubricating film and, consequently, the lubrication efficacy of HEC-G. This section examines how the concentration of glycerol impacts the lubrication efficacy of HEC at a speed of 410 rpm. Using 4.0 wt.% HEC as the base lubricant, various concentrations of glycerol were added. The wear loss and friction coefficient were recorded and are presented in Fig. 13 . The friction coefficient of 4.0 wt.% HEC was about 0.024. With the addition of glycerol, the friction coefficient began to decrease sharply, reaching its lowest point of approximately 0.004 at a glycerol concentration of 10 wt.%, as also shown in Fig. 7 . The friction coefficient at 5 wt.% glycerol was also low, around 0.008, indicating very good lubrication efficiency. However, when glycerol concentrations exceeded 10 wt.%, the friction coefficient increased rapidly, though it remained close to 0.01. This suggests that excessive glycerol can negatively affect the lubricating performance of HEC. Additionally, the wear loss of Ti-6Al-4V disks and Si 3 N 4 balls was evaluated after the tests. For Ti-6Al-4V, wear loss tended to decrease with increasing glycerol concentration until it exceeded 10 wt.%, at which point the wear slowly increased again. This trend mirrors the changes in the friction coefficient, suggesting a correlation between enhanced lubrication conditions and reduced wear between the friction pairs. Specifically, as the glycerol concentration increased from 0 to 10 wt.%, the wear loss of Ti-6Al-4V decreased from 110 mg to 45 mg, a reduction of 59.1%. However, when the concentration increased from 10 to 30 wt.%, the wear loss of Ti-6Al-4V increased slightly. In contrast, the wear trend for Si 3 N 4 balls remained relatively stable, with little fluctuation as the glycerol concentration increased. Notably, the wear amounts of Si 3 N 4 balls ranged from 0.03 to 0.04 mg, whereas the wear amounts of Ti-6Al-4V disks ranged from 45 to 110 mg. This significant difference in wear amounts is likely due to the considerable hardness difference between Ti-6Al-4V and Si 3 N 4 , with Si 3 N 4 being significantly harder, as shown in Table 1 . The Ti-6Al-4V disk exhibited more noticeable wear when in contact with the Si 3 N 4 ball. However, a novel perspective emerges here focusing solely on trends within the red dashed rectangle (Fig. 13 ), with glycerol concentrations from 5 wt.% to 30 wt.%. Within this range, the wear loss of Ti-6Al-4V disks demonstrates consistent stability with minimal fluctuations, mirroring the behavior observed in Si 3 N 4 balls. In this context, the impact of glycerol concentration on wear loss of both disks and balls appears negligible, whereas the addition of glycerol has a significant effect on the friction, which is reflected in the friction coefficient. Essentially, HEC-G lubricants (5–30%) effectively facilitate separation between contacting surfaces. It is suggested that thicker lubricants may contribute to increased friction, primarily attributable to the viscous resistance of the lubricant, which suggests that hydration lubrication can be excellent in the right concentrations. Given the weak correlation observed between wear loss (representing direct contact) and concentration, this highlights the dominance of viscous resistance as a primary frictional source in this situation. The contact zones of the friction tracks on the disks were observed using SEM, while the section profiles were detected using ZYGO. Figure 14 presents the results for HEC and HEC-G (1, 10, 20, 30 wt.%). The red lines denote the cross-section profiles selected to be perpendicular to the direction of motion. To maintain consistency in length across all five tracks, the red lines do not traverse the entire dimensions of the images. The significant observation is that the profiles of HEC-G at concentrations of 10, 20, and 30 wt.% exhibit less smoothness compared to those of HEC and HEC-G at 1 wt.%, with Ra roughness values of 1.57 µm and 1.99 µm, respectively. Additionally, distinct plateaus are evident on the friction tracks of HEC-G with higher concentrations, with Ra values ranging from 3.78 µm to 5.84 µm, and the plateaus did not exist under low concentrations instead they became some flatten scars indicated by blue ellipse in Fig. 14 (a) and (b). Through comparison, it is suggested that these plateaus on the surface are not completely worn down during friction when using thicker HEC-G lubricants. This observation implies that thicker HEC-G lubricants can establish robust lubricating films, whereas HEC and low-concentration HEC-G may not adequately resist the wear of plateaus. Furthermore, it is noteworthy that the formation of plateaus can be attributed to both friction and frictional adhesion, with their upper surfaces appearing nearly flat, characterized by Ra values ranging from 0.01 to 0.02 µm. These unique topographies are particularly notable within the entire section, as illustrated in Fig. 14 (c). Importantly, these plateaus are distributed throughout the entire tracks, suggesting a significant influence on lubricating efficiency. Conventionally, a flat surface is advantageous for lubricating efficiency, while a rough surface typically leads to increased friction. However, the identification of these distinct plateaus in nanoscale friction strongly indicates that deliberate modification in surface topography can significantly improve the lubricating efficiency of hydration lubrication at a relatively larger scale. The modification in surface topography means the plateaus with flat top cut. Then, the microscopic details of “plateau” were explored and discussed in Fig. 15 . Figures 15 (a) and 15(b) present SEM images of the topographies surrounding the plateau, clearly depicting valleys and plateaus analogous to topographic features. Notably, these images are captured from the nanoscale details of the friction track. On Earth, valleys are more efficient at containing water than plateaus, leading to the formation of lakes and playing an important role in directing water flow. Similarly, at the nanoscale, these “valleys” serve in a dual function: they contain lubricant and direct its flow. Specifically, the “valleys” direct lubricant across the plateau, enabling the hydrated HEC-G to form a continuous lubricant film. These lubricant "lakes" ensure a steady supply of lubricant and contain the working lubricant, whether sourced from other valleys or from the plateau. When discussing the formation of lubricating films, molecular interactions play a significant role alongside the effect of lubricant flow. Figures 15 (c-f) illustrate the distribution of elements on both the “valley” and “plateau” surfaces. The primary elements identified are Ti, Al, and V, originating from the Ti-6Al-4V disk materials. Insets in Fig. 15 (d) provide detailed views of the distribution of Ti, Al, and V along the red lines across the SEM image, perpendicular to the direction of motion. It is evident that, perpendicular to the direction of motion, the primary elements exhibit a relatively uniform distribution. In stark contrast, the distribution of Si, O, and N elements is notably non-uniform. Figure 15 (e) shows that Si, O, and N elements predominantly cover the plateau areas, while their presence is less noticeable in the valleys. This distribution trend corresponds to both the plateau and valley regions, suggesting that the valleys in the friction track are likely formed by the plowing action of wear particles from the Ti-6Al-4V material during the wearing-in period. Consequently, the Si and N from the balls do not contact the valley areas, whereas the plateau surfaces are worn down by the Si 3 N 4 balls throughout the entire friction process. The Si, O, and N elements distributed on the plateau can interact with lubricant molecular groups, such as -OH and -H. Previous research indicates that silicon and oxygen elements can strongly attract -OH and -H groups to form hydrogen bonds[52], which are classified as strong non-covalent interactions, a behavior also exhibited by titanium. Consequently, these elements can interact with the hydroxyl groups of HEC or glycerol, facilitating the robust formation of lubricating films. These interactions likely result in the formation of a nanoscale adsorbed polymer film covering the surface. As the Ti-6Al-4V wears against the Si 3 N 4 balls during friction, the surface details of the wear scars on the Si 3 N 4 balls were examined, with the results presented in Fig. 16 . The plateaus observed on the Ti-6Al-4V surface were formed through abrasion with the Si 3 N 4 balls. The interaction between the Ti, Al, and V elements with the Si 3 N 4 was examined using XPS spectra, as shown in Fig. 16 , which depicted energy peaks from wear scars on the Si 3 N 4 balls. Clear and distinct peaks corresponding to Si 2s, Si 2p, and N 1s were observed, unequivocally attributed to the Si 3 N 4 base material. However, peaks corresponding to Ti 2p, Al 2p, and V 2p (as delineated by the dashed box) were notably absent, indicating that it’s difficult for Ti-6Al-4V material to embed into the hard surface of Si 3 N 4 , despite undergoing significant wear during the wearing-in period. Remarkably, the spectra exhibited consistent peak positions for all elements, suggesting the absence of any chemical reactions between them. Consequently, it is inferred that the primary interactions during friction mainly involve solid contact and non-covalent bonds, leading to a significant reduction in friction. Solid contact occurs during the wearing-in period, modifying the beneficial topographies. Concurrently, non-covalent bonds form within the lubricant molecules, enhancing the fluidity characteristics necessary for the formation of robust lubricating films. Given this, when the lubricant is introduced into the friction track, the friction coefficient decreases abruptly without an incubation period. It is certain that, during this brief interval, little to no incubation process between HEC-G and Si 3 N 4 takes place. Therefore, the formation of the lubricating film is entirely attributed to a physical mechanism, partly analogous to elastohydrodynamic lubrication, facilitated by both solid contact and non-covalent bonding. Figure 17 provides a microscopic, three-dimensional view of the worn surface, accompanied by illustrations detailing the lubricant flow over the “plateau” and “valley” regions. The model vividly presents the elevated plateaus and the recessed valleys. On the top of plateau, the detected elements of silicon, oxygen, and nitrogen attract naturally hydrated HEC-G polymer chains, which tend to remain dispersed in all directions due to hydration repulsion. During friction, the dispersed chains were driven to flow in the same direction, thereby enhancing hydration repulsion. The lakes, or lubricant reservoirs, scattered across the friction tracks serve as lubricant suppliers. As the Si 3 N 4 balls move, they drive the lubricant to flow from the lakes to the plateau tops, similar to the elastohydrodynamic lubrication mechanism. This process generates robust lubricating films, with well-aligned polymer chains enhancing hydration repulsion, effectively preventing solid contact between the plateaus and the Si 3 N 4 balls. Additionally, minor plateaus, referred to as mesas, play a crucial role during friction, though their specific contributions require further investigation. 4 Conclusion This study explored a novel friction method aimed at enhancing the anti-friction properties of Ti-6Al-4V to achieve ultra-low friction under hydrated lubrication. Two methods were evaluated, with Method 2 showing a significant reduction in friction. When the mixed lubricant of HEC and glycerol was introduced into the friction tracks, the friction sharply decreased to the ultra-low region and remained at 0.004 for approximately 50 minutes. Notably, during this ultra-low friction, the wear loss of the friction pairs remained consistent across different glycerol concentrations. This consistency indicates that solid contact was largely prevented under Method 2, owing to the robust formation of lubricating films. Considering the critical role of friction methods in reducing friction, the microscopic mechanisms were examined in detail. SEM and XPS analyses revealed unique topographies featuring plateaus and valleys on the wear scars, along with the corresponding distribution of elements. The valleys containing lubricants acted as effective reservoirs, supplying the necessary lubricant for lubrication. Lubricant flowed from the valleys to the plateaus, forming robust films, a process similar to elastohydrodynamic lubrication but occurring at the nanoscale. In addition to these valleys, the flat tops of the plateaus facilitated steady lubricant flow, aided by the adsorption between the base elements and the chemical groups of the lubricant. Declarations Declaration of Artificial Intelligence All the manuscript was originally written by the authors. The authors only used generative artificial intelligence to improve readability and language. Then, each improved sentence was carefully reviewed and manually re-edited by the authors. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Institutional Affiliations All affiliations are listed on the title page of the manuscript. Funding All funding sources for this study are listed in the “Acknowledgments” section of the manuscript. Availability of data and material The datasets used or analyzed during the current study are available from the corresponding author on reasonable request. Authors' contributions Conceptualization : Dezun Sheng, Xiao Zhang; Methodology : All the authors; Formal analysis and investigation : Dezun Sheng, Xiao Zhang, Hongliang Yu; Writing - original draft preparation : Dezun Sheng, Xiao Zhang; Writing - review and editing : Dezun Sheng Funding acquisition : Xin Zhou; Supervision : Dezun Sheng Acknowledgement This work is financially supported by the Natural Science Foundation of Shandong Province (Grant No.: ZR2019BEE073), the Youth Innovation team Project of Higher Education Institutions in Shandong Province (Grant No.: 2022KJ272), the Natural Science Foundation of Shandong Province (Grant No.: ZR2023ME067), and Basic Research Project of Yantai Science and Technology Innovation Development Plan (Grant No.: 2023JCYJ054). References K. Holmberg, A. Erdemir: Influence of tribology on global energy consumption, costs and emissions. Friction. 5(3), 263-284 (2017). https://doi.org/10.1007/s40544-017-0183-5 W. Wang, B. Shen, Y. Li, Q. Ni, L. 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Cite Share Download PDF Status: Published Journal Publication published 17 Sep, 2025 Read the published version in Tribology Letters → Version 1 posted Editorial decision: Revision requested 21 Jul, 2025 Reviews received at journal 18 Jul, 2025 Reviewers agreed at journal 27 Jun, 2025 Reviewers invited by journal 26 Jun, 2025 Editor assigned by journal 25 Jun, 2025 Submission checks completed at journal 25 Jun, 2025 First submitted to journal 24 Jun, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6963694","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":477357735,"identity":"c0d60bd3-be38-4592-bf6e-80bd8524c3ee","order_by":0,"name":"Dezun Sheng","email":"","orcid":"","institution":"Yantai University","correspondingAuthor":false,"prefix":"","firstName":"Dezun","middleName":"","lastName":"Sheng","suffix":""},{"id":477357737,"identity":"85ccca17-08c5-4013-ae38-228f07d8c38e","order_by":1,"name":"Hongliang Yu","email":"","orcid":"","institution":"Yantai University","correspondingAuthor":false,"prefix":"","firstName":"Hongliang","middleName":"","lastName":"Yu","suffix":""},{"id":477357739,"identity":"98840581-2f6a-4383-8600-476797725ccf","order_by":2,"name":"Xiao Zhang","email":"","orcid":"","institution":"Yantai University","correspondingAuthor":false,"prefix":"","firstName":"Xiao","middleName":"","lastName":"Zhang","suffix":""},{"id":477357740,"identity":"933c5844-5391-4f1f-ae4a-8e119047cabc","order_by":3,"name":"Xin Zhou","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAy0lEQVRIiWNgGAWjYDACCRBhwCAH4bGRoMUYopp4LQwMiQ1Ea5Gf3XxMmqfgTvqG+z0GDB/KDjPwz27Ar4VxzrE0yRkGz3I3HOMxYJxx7jCDxJ0D+LUwS+SYSXwwOAzWwszbdpjBQCIBvxY2kJYEg8PpBiAtf4nRwgO1JQGshZEYLRISacmWMwwOG848llZwsOdcOo/EDQJa5GckH7zN8+ewPN/hwxsf/CizluOfQUALHCgcYGA4AHIpkepB1jUQr3YUjIJRMApGGAAA96o+tVCc7NIAAAAASUVORK5CYII=","orcid":"","institution":"Yantai University","correspondingAuthor":true,"prefix":"","firstName":"Xin","middleName":"","lastName":"Zhou","suffix":""}],"badges":[],"createdAt":"2025-06-24 08:53:42","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6963694/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6963694/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11249-025-02068-y","type":"published","date":"2025-09-17T15:57:43+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":85687244,"identity":"34054cc4-5538-46fb-a3b0-6b4f563d256a","added_by":"auto","created_at":"2025-06-30 16:09:52","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":3577878,"visible":true,"origin":"","legend":"\u003cp\u003eXRD spectra of Ti-6Al-4V and the unworn surface\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/f40a85848dbb19b7ce4b4f6e.jpeg"},{"id":85687241,"identity":"5ae5cd9f-5a01-4169-9106-6f7fd54460bf","added_by":"auto","created_at":"2025-06-30 16:09:52","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":179604,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of rotating friction pairs and misalignment angle α\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/a1533ebea716109cd64c4271.jpeg"},{"id":85688527,"identity":"082399a4-fae8-41c7-b4fc-9db24c6da25b","added_by":"auto","created_at":"2025-06-30 16:25:52","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1438269,"visible":true,"origin":"","legend":"\u003cp\u003eFT-IR spectra of 4 wt.% HEC and 10 wt.% HEC-G\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/7bb26a6e96b63e4c52717e60.jpeg"},{"id":85687689,"identity":"f953d111-ef5a-468d-8d71-3335454d32be","added_by":"auto","created_at":"2025-06-30 16:17:52","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1265908,"visible":true,"origin":"","legend":"\u003cp\u003eRaman spectra of 4 wt.% HEC and 10 wt.% HEC-G\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/c87d847074c30cfff76b4675.jpeg"},{"id":85688528,"identity":"6e82561e-f6c5-424a-a355-8be581813f29","added_by":"auto","created_at":"2025-06-30 16:25:52","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":245651,"visible":true,"origin":"","legend":"\u003cp\u003eShear-thinning performance of HEC-G (a) and the effect of concentration (b)\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/c52ec51333f75cbab48f8616.jpeg"},{"id":85687695,"identity":"46f7bba4-3a6e-42b2-b262-e4ff95c42f4d","added_by":"auto","created_at":"2025-06-30 16:17:52","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":571250,"visible":true,"origin":"","legend":"\u003cp\u003eComparisons of friction force (Fx) and normal load (Fz) in two reverse directions\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/d1e07f7b32c2020a40938605.jpeg"},{"id":85687253,"identity":"98bba994-f66d-4bc5-a190-872bcf4ee617","added_by":"auto","created_at":"2025-06-30 16:09:52","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":2485822,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of the results of the two test methods: (a) Method 1, (b) Method 2\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/d6b8eb81b4784572eb4b8338.jpeg"},{"id":85687696,"identity":"7956210d-26f0-46b8-a222-fff0f72525f6","added_by":"auto","created_at":"2025-06-30 16:17:52","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":2752887,"visible":true,"origin":"","legend":"\u003cp\u003eVariations of friction coefficient with time under different speeds\u003c/p\u003e","description":"","filename":"floatimage8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/79488af5281fc32274408ad1.jpeg"},{"id":85687256,"identity":"2cb1a40f-1ea8-43d4-a758-8398b942c357","added_by":"auto","created_at":"2025-06-30 16:09:52","extension":"jpeg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":1594250,"visible":true,"origin":"","legend":"\u003cp\u003eVariations of friction coefficient when lubricant is added during the friction\u003c/p\u003e","description":"","filename":"floatimage9.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/7673f6ae62a0764dd8396079.jpeg"},{"id":85688531,"identity":"40c22672-27cc-41f0-9199-90ef9fd3c4c9","added_by":"auto","created_at":"2025-06-30 16:25:52","extension":"jpeg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":1234496,"visible":true,"origin":"","legend":"\u003cp\u003eCalculated film thickness and the variation of measured friction coefficient\u003c/p\u003e","description":"","filename":"floatimage10.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/40f3efaedd98d61aa56ef541.jpeg"},{"id":85687700,"identity":"1ab70f27-9fab-47b3-b6ce-e00e9babb790","added_by":"auto","created_at":"2025-06-30 16:17:52","extension":"jpeg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":1768506,"visible":true,"origin":"","legend":"\u003cp\u003eOptical micrographs of wear tracks on Ti-6Al-4V with corresponding cross-sectional profiles\u003c/p\u003e","description":"","filename":"floatimage11.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/f53caafa9765bde33a24cde6.jpeg"},{"id":85687267,"identity":"7b709bc6-0cc7-41cd-b26e-9684fe2b0c1e","added_by":"auto","created_at":"2025-06-30 16:09:52","extension":"jpeg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":3862231,"visible":true,"origin":"","legend":"\u003cp\u003eWorn zone and the profile on the friction balls\u003c/p\u003e","description":"","filename":"floatimage12.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/941e6cdab077c5e062e0df54.jpeg"},{"id":85687255,"identity":"d59377ed-afa1-4b39-8e62-aaef51216d40","added_by":"auto","created_at":"2025-06-30 16:09:52","extension":"jpeg","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":2113132,"visible":true,"origin":"","legend":"\u003cp\u003eImpact of concentration on the wear loss and friction coefficient\u003c/p\u003e","description":"","filename":"floatimage13.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/598ef504cf28936229c43a82.jpeg"},{"id":85687263,"identity":"6b4e70f4-6495-43b4-8565-c37d6f9e1995","added_by":"auto","created_at":"2025-06-30 16:09:52","extension":"jpeg","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":1506897,"visible":true,"origin":"","legend":"\u003cp\u003eSEM and section profiles of HEC and HEC-G: (a) HEC, (b) HEC-G 1 wt.%, (c) HEC-G 10 wt.%, (d) HEC-G 20 wt.%, (e) HEC-G 30 wt.\u003c/p\u003e","description":"","filename":"floatimage14.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/1bf2c434ffb29f87ab204352.jpeg"},{"id":85687274,"identity":"cfe1d349-6ab0-481e-a062-a01ec82844fc","added_by":"auto","created_at":"2025-06-30 16:09:53","extension":"jpeg","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":4714238,"visible":true,"origin":"","legend":"\u003cp\u003eDetailed view of friction track and the distribution of elements\u003c/p\u003e","description":"","filename":"floatimage15.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/94f1741487662e71cbd420f8.jpeg"},{"id":85687276,"identity":"dfb7e597-f682-4c49-b46d-06be640d993e","added_by":"auto","created_at":"2025-06-30 16:09:53","extension":"jpeg","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":2466467,"visible":true,"origin":"","legend":"\u003cp\u003eXPS spectra of wear scars on the Si3N4 balls\u003c/p\u003e","description":"","filename":"floatimage16.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/61d2276d2265b7ed42e4c7d6.jpeg"},{"id":85687701,"identity":"c28e55ec-61e5-4e50-968c-5a325d04296c","added_by":"auto","created_at":"2025-06-30 16:17:52","extension":"jpeg","order_by":17,"title":"Figure 17","display":"","copyAsset":false,"role":"figure","size":1961010,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic view of worn surface and the microscopic mechanism of friction reduction\u003c/p\u003e","description":"","filename":"floatimage17.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/8d8840a3683e7e6eef75892b.jpeg"},{"id":91889857,"identity":"e6fe79fd-1758-499f-b900-10f678d492d4","added_by":"auto","created_at":"2025-09-22 16:02:49","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":33573676,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6963694/v1/286fd4fc-2eb3-425f-91f3-84d6704e0d87.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eAchieving Ultra-Low Friction in Ti-6Al-4V Alloy: Hydration Lubrication Mechanisms of HEC-Glycerol Composite\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eFriction and wear are two interrelated phenomena prevalent in engineering and biological systems, severely affecting the performance and lifetime of mechanical components and biomedical implants[1\u0026ndash;3]. While conventional lubrication mechanisms have been extensively studied and applied to minimize friction and wear[4\u0026ndash;7], recent studies have discovered a highly promising lubricating mechanism: hydration lubrication[8, 9], which is characterized by the formation of a hydrated layer at the interface between two sliding surfaces. Hydration lubrication has become a popular research area with important influences on tribology[10, 11]. This novel lubrication mechanism, derived from the physicochemical interactions between hydrated polymers and solid surfaces[12\u0026ndash;14], offers a significant approach to achieving ultra-low friction characteristics, and even shows high potential to be applied to metal alloys such as Ti-6Al-4V[15\u0026ndash;17].\u003c/p\u003e \u003cp\u003eAlloys typically operate under macroscale conditions. Remarkably, Li et al. discovered ultra-low friction through hydration lubrication in such conditions by using mixtures of acids and polymers. Notably, phosphoric acid demonstrated remarkable lubricating performance, with a friction coefficient as low as 0.004 [18\u0026ndash;20]. Although acid solutions are unsuitable for alloy lubrication, these findings push forward the way for reducing alloy friction through hydration lubrication. Xu et al.[21] used microgels as lubricant additives in bovine calf serum to lubricate titanium alloy/ultra-high molecular weight polyethylene contacts, significantly reducing friction and demonstrating temperature-sensitive lubrication characteristics. Additionally, Cui et al. and Liu et al. discovered that bilayer coatings[22] and copolymer[23] on the Ti-6Al-4V surface enhance lubrication properties. Although these treatments did not achieve superlubricity, the hydrated polymers are believed to have the potential to significantly reduce friction on the Ti-6Al-4V alloy. Not only can hydrated polymers [24] reduce friction, but hydrated anions [25] also perform excellently in enhancing the antiwear ability of the Ti6Al4V alloy. Li et al. [26] explored the lubrication mechanism of halogen anions on positively charged surfaces at the atomic scale, discovering that the adsorption state of anions influences friction dissipation, and superlubricity can be achieved at low concentrations. The anions and polymers hydrated with water can improve the lubrication performance of Ti-6Al-4V alloys. Furthermore, they demonstrate significant potential for application in biotribology conditions. Yue et al. prepared chitosan-g-PMPC copolymers to achieve a very low coefficient of friction (\u0026micro;\u0026thinsp;\u0026lt;\u0026thinsp;0.01) on Ti-6Al-4V alloy in pure water. This remarkable reduction in friction is primarily attributed to the hydrated nature of the PMPC side chains, the interface adsorption of the copolymer, and the hydrodynamic effect[27]. Polymers serve multiple functions by forming various functional groups [28, 29]. In friction, these designed supramolecular structures demonstrate the ability to repair lubrication surfaces through hydration effects, significantly advancing the achievement of long-term lubrication mechanisms [30, 31]. Wang et al. prepared self-assembled polymer monolayers and visualized their dynamic repair process [32]. They discovered that, after friction-induced dissociation of the polymers, the polymer-to-surface interaction is restored through the reformation of host\u0026ndash;guest complexes. This process renews the lubricity and maintains the reduction of wear. In contrast to boundary lubrication or other lubrication mechanisms, hydration lubrication relies on the hydration repulsions of the hydrated groups to resist contact and minimize friction[10]. Additionally, it uses the dynamic hydration-dehydration behavior to restore the lubricating function[25, 33]. The synergistic effect is a valuable approach to promoting the hydration behavior of lubricant[34]. Feng et al. synthesized a super-lubricated hydrogel with the synergy of phospholipid and hyaluronan and the coefficient of friction could reduce down to 0.004[35]. In addition, Liu et al. developed a novel layered soft hydrogel as cartilage prototype, which can exhibit a low friction coefficient (COF\u0026thinsp;\u0026asymp;\u0026thinsp;0.006) under a wide range of contact stresses (0.2 to 2.4 MPa)[36]. It\u0026rsquo;s clear that the synergistic effect can enhance the effectiveness of hydration lubrication [37].\u003c/p\u003e \u003cp\u003eThis study investigates the synergistic effect [38, 39] of hydroxyethyl cellulose (HEC) [40, 41] and glycerol in enhancing the tribological properties of Ti-6Al-4V alloys. By improving the hydration characteristics of the mixture, where HEC acts as a water-soluble polymer [42, 43] and glycerol functions as a coupling agent, the authors aim to develop a robust lubrication system to reduce friction in alloys. Considerable attention has been paid to friction tests on materials and mechanisms, and the testing methods have remained largely unchanged over the decades. In this work, the authors first reported the changed friction testing methods. Then, through experimental characterization and analysis, the authors investigated the fundamental mechanisms for the friction reduction of the hydrated HEC-glycerol, which aims to enhance the understanding of hydration lubrication and its role in improving the tribological properties of alloys. The achievement of ultra-low friction properties in Ti-6Al-4V alloys could expand the way to improved energy efficiency and enhanced reliability in automotive, and other applications.\u003c/p\u003e"},{"header":"2 Experiments and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Preparation of Materials and Lubricants\u003c/h2\u003e \u003cp\u003eHydroxyethyl cellulose (HEC) possesses numerous hydroxyl groups and is a water-soluble polymer extensively utilized across various industrial applications. The HEC utilized in this study was purchased from Aladdin Corporation (Catalog No. H104790, Aladdin Corp., CHN). It has a purity exceeding 99%, with the average viscosity of its 2.0 wt.% solution at 25\u0026deg;C falling within the range of 100\u0026ndash;200 mPa\u0026middot;s. Notably, the 4.0 wt.% HEC solution was chosen based on prior research indicating its optimal lubricating properties[44]. The lubricant preparation process involved adding a quantified amount of HEC powder in deionized water, followed by continuous stirring for 30 minutes to achieve a uniform and transparent HEC solution. Glycerol, purchased from Aladdin (Catalog No. G116210, Aladdin Corp., CHN) with a purity exceeding 99.8%, was introduced into the 4.0 wt.% HEC base solutions in varying masses, producing seven distinct HEC-glycerol (HEC-G) mixed solutions with glycerol concentrations of 1.0, 5.0, 10.0, 15.0, 20.0, 25.0 and 30.0 wt.%. These mixed solutions underwent a 30-minute incubation period in an ultrasonic bath set at 25\u0026deg;C, during which their appearance remained transparent and homogenous.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Characterization of Additives and Lubricants\u003c/h2\u003e \u003cp\u003eBefore tribological tests, the Fourier transform infrared spectra (FT-IR) of both Hydroxyethyl cellulose (HEC) and its glycerol-modified lubricant (HEC-G) were examined utilizing a Fourier transform infrared spectroscopy instrument (Nicolet 6700, ThermoFisher Corp., USA). The hydroxyl groups present in HEC possess strong attraction for water in bulk solutions, thereby contributing to its hydration effect. Glycerol, characterized by three hydroxyl groups per molecule, functions as an effective coupling agent, enhancing the synergistic effect within aqueous solutions. Consequently, investigating the chemical interactions occurring within HEC and HEC-G via FT-IR spectroscopy proves advantageous in comprehending the synergistic influence of glycerol on HEC solutions. Nonetheless, excessive hydration of the mixed solution may lead to the formation of numerous hydrogen bonds, resulting in high solution viscosity, which could adversely impact lubrication. Hence, to examine the macroscopic impact of glycerol on the fluidity and viscosity of HEC solutions, investigations were conducted utilizing a standard rheometer (Physica MCR301, Anton Paar Corp., USA) operating at 25\u0026deg;C.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Tribological Experiments and Analysis\u003c/h2\u003e \u003cp\u003eFriction experiments were conducted using a universal micro-tribometer (UMT-3, Bruker Corp., USA), operating in the ball-on-disk sliding configuration. The spherical balls utilized were fabricated from Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e with a radius of 2.0 mm and a surface roughness (Ra) of 25 nm. The disks employed were plates made of Ti-6Al-4V alloy, possessing dimensions of Φ25\u0026times;3 mm. The Ti-6Al-4V samples underwent wet polishing using SiC paper. The surface roughness of the Ti-6Al-4V disks was quantified utilizing a 3D optical profilometer (NewView 5022, Zygo Corp., USA), yielding a measured Ra value of 1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3 \u0026micro;m and a Root Mean Square (RMS) roughness of 278\u0026thinsp;\u0026plusmn;\u0026thinsp;22 \u0026micro;m. The mechanical characteristics of both the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e balls and Ti-6Al-4V disks are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, mirroring data found in previous publications[45, 46]. Prior to subsequent testing section, All samples were sequentially ultrasonically cleaned in acetone, ethanol, and deionized water (5 min each), followed by drying with nitrogen gas. Then, X-ray diffraction (XRD) patterns of the Ti-6Al-4V specimens were acquired, revealing features composed of α martensite phase, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMechanical properties of the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e ball and Ti-6Al-4V disk\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMaterials\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDensity (g/cm\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eHardness (kg/mm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eYoung\u0026rsquo;s modulus (GPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ePoisson\u0026rsquo;s ratio\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSi\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1420\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e310\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.26\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSi\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3.2\u0026ndash;3.5*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1390\u0026ndash;2450*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e290\u0026ndash;315*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.28*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTi-6Al-4V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e4.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e320\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e118\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.34\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTi-6Al-4V*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e4.2\u0026ndash;4.5*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e310\u0026ndash;370*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e110\u0026ndash;220*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.32\u0026ndash;0.36*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe data annotated with an asterisk (*) denote averaged parameters derived from preceding experimental trials.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn the process of sliding friction, the rotational motion of the disc induces a sinusoidal variation in the applied normal load, due to the presence of a misalignment angle denoted as α, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. It is observed that larger misalignment angles correspond to increased measurement inaccuracies. To minimize the influence of the misalignment angle, this work developed a high-precision leveling apparatus aimed at reducing α to below 0.01\u0026deg;, which was published in our previous work[44]. Implementation of this leveling device effectively minimizes measurement errors, thereby enhancing the accuracy of obtained data. The efficacy of this approach was validated in Section \u003cspan refid=\"Sec9\" class=\"InternalRef\"\u003e3.3\u003c/span\u003e through examination of the measured results.\u003c/p\u003e \u003c/div\u003e"},{"header":"3 Results and Discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Spectra of HEC and HEC-G\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn the spectral analysis of HEC-G, the broader band observed between 3475 and 3133 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e arises from the presence of hydroxyl groups[47]. Peaks identified at 2917 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e correspond to the stretching band of C-H bonds. The faint peak observed at 2162 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is attributed to C\u0026thinsp;=\u0026thinsp;C bonds within the sugar unit, while another weak peak at 1973 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is associated with the bending band of C-H bonds. The peak appearing at 1563 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is a consequence of the skeletal vibration of the sugar unit, whereas the peak at 1405 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is linked to the bending vibration of C-H bonds. The intensity of the band observed at 1052 and 1021 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is attributable to the bending of C-O-H bonds.\u003c/p\u003e \u003cp\u003eThe spectral profile of HEC-G exhibits similarities to that of HEC, particularly evident for peaks below 2917 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. However, distinct differences in the relative intensities of hydroxyl group and C-O-H group peaks exist between HEC-G and HEC. Notably, for both HEC-G and HEC, the intensities of peaks at 1563 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1405 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are comparable, ranging from 75\u0026ndash;80%, serving as a reference standard. Specifically, in the spectrum of HEC-G, transmittance intensities at 1052 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1021 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are notably lower compared to those at 1563 cm-1 and 1405 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, while in the spectrum of HEC, the converse is observed, where transmittance intensities at 1052 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1021 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e surpass those at 1563 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1405 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eFurthermore, the transmittance intensities of the broader band between 3475 and 3133 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in both HEC-G and HEC exhibit a similar trend. This trend suggests a considerable increase in the number of hydroxyl groups in the mixed solution upon glycerol addition, resulting in change in the intensity of corresponding peaks. Additionally, Raman spectroscopy was conducted on both HEC-G and HEC, with results presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe peaks denoted by circles (893.6, 931.6, 1122.5, 1180.5, 1413.6, 1472.0, 2893.6 and 2929.6 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e are the functional groups that appear in both HEC and HEC-G, indicating that the addition of glycerol tends not to change those original functional groups, but enhances them slightly. The five peaks at 931.6, 1122.5, 1472.0, 2893.6, and 2929.6 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (marked with asterisks) are observed in the Raman spectra of all three polymers (glycerol, HEC, and HEC-G), with peaks 931.6 and 1122.5 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e representing the C-C group, and peaks 1472.0 and 2929.6 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e indicating the -CH2- group. The peak of 2893.6 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e represents the -OH, which is a weaker peak in the case of glycerol and a stronger peak in the case of HEC. The functional groups represented by the five peaks on HEC tend to be enhanced by the same functional groups in glycerol, which favors mutual attraction with water molecules. In addition, in the spectrum of glycerol, the peaks at 860.0 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (represented by letter a) and 1066.5 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (represented by letter b) are obviously strong[48]. In contrast, the peaks at the same positions are barely visible in the HEC spectrum, from which it can be inferred that the strong groups on glycerol are weakened by the formation of chemical bonds with the functional groups of HEC. As a result, the weak peaks of HEC-G at 874.4 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (denoted by the letter A) and 1100.2 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (denoted by the letter B) appear. The relatively strong peaks A and B compared to the same positions of HEC are attributed to the incorporation of glycerol. Glycerol molecules can interact strongly with the HEC functional groups and macroscopically enhance the viscosity and other properties of the HEC-G mixture.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Fluidity of the mixed solutions\u003c/h2\u003e \u003cp\u003eThe viscosities of HEC and HEC-G were measured using a rheometer to clarify their non-Newtonian fluid behavior characterized by shear-thinning properties, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a). The dynamic viscosity of 4.0 wt.% HEC solution was 2150 mPa\u0026middot;s at a shear rate of 1 1/s, and decreased by 75% when the shear rate was increased to 10\u003csup\u003e3\u003c/sup\u003e 1/s. In contrast, at a shear rate of 1 1/s, the dynamic viscosity of 1% HEC-G approximated 2180 mPa\u0026middot;s. It\u0026rsquo;s worthy note that the HEC-G demonstrated a gradual decline in viscosity with increasing shear rates, mirroring the shear-thinning characteristics observed in the 4.0 wt.% HEC solution. Since an increase in the shear rate of the lubricant can significantly reduce its viscosity, the mixed lubricant is likely to exhibit excellent lubrication efficacy at high velocities during friction. However, upon the addition of 5% glycerol to HEC, the dynamic viscosity notably surpassed that of the 4.0 wt.% HEC solution, as illustrated in the inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a). Furthermore, at a glycerol concentration of 30%, the viscosity surged to 6859 mPa\u0026middot;s (marking an increase of 223.6%), indicating that the addition of a small quantity of glycerol marginally influences the dynamic viscosity of HEC. It is evident that the incorporation of glycerol negatively influenced the fluidity of HEC, with the viscosity of the HEC-G solution increasing proportionally with the glycerol content. This substantial increase in viscosity may impede the movement of the frictional pair adversely. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b) illustrates the variations in viscosity with increasing concentration at different shear rates. At a shear rate of 509 1/s, the viscosity of the mixed solution exhibited a modest increase from 489 mPa\u0026middot;s to 576 mPa\u0026middot;s, representing a 17.8% augmentation. This observation suggests a weak concentration effect on the lubricant's viscosity under conditions of rapid shear. In contrast, at a lower shear rate of 1 1/s, the viscosity of the mixed solution experienced a substantial augmentation from 2150 mPa\u0026middot;s to 6859 mPa\u0026middot;s, indicating a markedly robust concentration effect on the lubricant's viscosity[49, 50].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Minimization of measurement errors\u003c/h2\u003e \u003cp\u003eThe force transducer employed in this study was the DFM-1G model, characterized by a 10 N range and an low resolution of 0.1 mN (equivalent to 0.01 g). In order to raise the measurement precision, a comparative analysis of the coefficients of friction was conducted in both clockwise and counterclockwise sliding directions, according to the works established by Lee[51]. The comparative results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e presents the variations in normal load and friction force during the tests conducted at a speed of 410 rpm and a load of 1 N using a 4.0 wt.% HEC solution. These figures are modified reproductions obtained directly from UMT-3 equipment, where the force denoted as 1 gram (g) corresponds to 10 mN. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a) distinctly shows the results obtained in the clockwise direction. The friction force exhibited a sudden decrease from 18.95 g to 0.46 g upon the introduction of the lubricant at 200 seconds. Notably, the normal force (Fz) displayed considerable fluctuations during the wear-in phase within the initial 200 seconds. Subsequently, upon the application of the lubricant, it stabilized and exhibited flat variations, ranging between 99.92 g and 100.18 g (as indicated by the magnified partial view). Furthermore, the results obtained in the counterclockwise direction revealed a reduction in friction force from 18.60 g to 0.45 g following the addition of the lubricant. Simultaneously, the normal force exhibited slight fluctuations, ranging approximately between 99.82 g and 100.08 g. Comparative analysis between the tests conducted in opposite directions revealed that the fluctuation range of the normal force remained within 1.6 mN, while the measurement error of the friction force was minimal, standing at 2.5%. These findings underscore the conformity of the measurement outcomes with the requisite standards for scientific investigations.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Difference in friction coefficients under two friction strategies\u003c/h2\u003e \u003cp\u003eIn this study, we employed two distinct methodologies to execute the friction tests. Method 1 involved the initial application of approximately 0.2 mL of a 10% HEC-G lubricant onto the friction track, followed by the start of the friction test. The variation of the friction coefficient under this method is depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a). Initially, a notable reduction in the friction coefficient was observed, followed by a brief period where the friction coefficient as low as 0.12, as indicated by the inset. Subsequently, the friction coefficient exhibited an increasing trend and fluctuated within the range of 0.012 to 0.025, with an average friction coefficient of approximately 0.017. It is evident from these results that the lubricating effect of HEC-G, as assessed through this method, failed to sufficiently reduce the frictional force. Consequently, an alternative approach was devised for conducting the friction tests.\u003c/p\u003e \u003cp\u003eMethod 2 was devised as follows: Initially, a load of 1 N was applied to the ball, and friction was allowed to proceed for approximately 250 seconds without the application of any lubricant; Subsequently, the mixed HEC-G was introduced into the friction track. The variation of the friction coefficient under this method, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(b), exhibited notable distinctions from that observed in Method 1. Throughout the wearing-in stage, the friction coefficient demonstrated a gradual increase from 0.029 to 0.045. Then, it stabilized within the range of 0.038 to 0.048, indicating the attainment of a relatively stable condition for the wear surfaces of the ball and disk. Upon the introduction of the lubricant at the end of the wearing-in stage, the friction coefficient exhibited a sudden decrease to approximately 0.004. Notably, during this phase, the average friction coefficient remained stable until the end of the test, which extended for a duration exceeding one hour. Given the remarkable lubricating performance observed in the results obtained from Method 2, all subsequent tests were conducted in reference to this method. The inset in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(b) is an optical picture and displays the worn surface of the disk produced during the wearing-in stage, which will be examined in detail in the following sections. It is noteworthy to highlight the beneficial role of the wearing-in stage in achieving low friction forces, which need to be investigated in future research endeavors.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Lubrication results enhanced by HEC-G\u003c/h2\u003e \u003cp\u003eVelocity has a significant effect on the formation of the lubricant film, which is a key factor in the lubrication process. Initially, we evaluated the lubrication efficacy of HEC-G at different velocities to determine the optimal test velocity for subsequent experiments. A lubricant consisting of 10% HEC-G was chose, with a load of 5 N. The rotational speed ranged from 80 rpm to 850 rpm, incremented by 110 rpm steps. Since the radius of the contact trace measures 4.1 millimeters, the corresponding linear velocity at the friction center varied between 34.3 mm/s and 364.7 m/s. Notably, the duration of the wearing-in stage varied for each test, deliberately configured to ensure uniform sliding distances across all experiments during wearing-in stage. Specifically, the sliding distance was standardized to 33.3 meters, as shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDuration of wearing-in stage for each test with the same sliding distance\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpeed (rpm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e80\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e190\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e300\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e410\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e520\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e630\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003e740\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003e850\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVelocity (mm/s)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e34.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e81.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e128.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e175.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e222.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e270.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e317.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e364.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDuration (s)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e964\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e406\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e257\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e188\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e148\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e122\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e104\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e91\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSliding distance (m)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e33.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e33.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e33.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e33.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e33.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e33.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e33.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e33.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e illustrates a comprehensive analysis of the friction coefficient over the course of a one-hour test duration. At a rotational speed of 80 rpm, The friction coefficient showed significant fluctuations (amplitude\u0026thinsp;\u0026gt;\u0026thinsp;30% of mean value) during the initial wearing-in phase. Subsequently, upon the introduction of lubricant, the friction coefficient immediately decreases to a lower value and continued to fluctuate with a lesser extent. Notably, the average friction coefficient decreased by approximately 61%, from 0.059 to 0.023. At a speed of 190 rpm, the variation in friction coefficient became smoother for a brief period following the addition of lubricant, subsequently fluctuating within the range of 0.008 to 0.018. Moreover, at rotational speeds of 410 rpm and 740 rpm, the variation in friction coefficient exhibited notably smoother trends and minimal fluctuations. As the rotational speed increased, the duration of the wearing-in stage diminished, accompanied by a reduction in the magnitude of friction coefficient fluctuations. Additionally, the average friction coefficient plummeted to an impressively low value of 0.004, indicative of a highly effective lubrication stage. It is reasonable to infer that under these conditions, the lubricating film successfully separates the contact surfaces, thereby ensuring optimal lubrication of the friction pairs. Furthermore, at a speed of 740 rpm, the average friction coefficient remained stable and low at 0.006. However, with further increases in speed, the lubricating performance of HEC-G become progressively worse. At a speed of 850 rpm, the friction coefficient fluctuated throughout the entire test duration, even with the addition of lubricant. It was observed that, under high-speed situations, the lubricant was ejected from the friction disk, likely due to the substantial centrifugal forces generated at such high rotational speeds. It can be assumed that the increase in friction coefficient at higher speeds is not directly attributable to the diminished lubrication performance of HEC-G, but rather to the inadequate supply of lubricant. Unfortunately, under conditions of elevated rotational speeds, the actual lubrication effectiveness could not be accurately assessed.\u003c/p\u003e \u003cp\u003eTo clarify the negative effect of inadequate supply of lubricant, several drops of HEC-G were introduced into the friction track during a dry test with speed of 850 rpm, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. During standard frictional testing, a single drop of HEC-G was added at approximately 1000 seconds. It was observed that the friction coefficient decreased from 0.018 to 0.009, a reduction sustained for approximately 10 seconds, followed by an increase in friction coefficient to about 0.019. Subsequently, at around 1058 seconds, 10 drops of HEC-G were added. This resulted in a further reduction of the friction coefficient to 0.009, which lasted for approximately 30 seconds. Evidently, the duration of the friction coefficient at 0.009 increased with the greater supply of lubricant, confirming the notion that insufficient lubricant is responsible for the raised friction coefficient observed at 850 rpm. Subsequently, the friction coefficient stabilized at approximately 0.015\u0026ndash;0.025, akin to typical frictional conditions. It is crucial to note that the addition of HEC-G during friction momentarily reduces the friction coefficient and underutilizes the lubricating ability of HEC-G. According to Dowson-Hamrock theory, increased velocity facilitates the formation of a lubricating film. However, if the adhesive force of the lubricant on the friction surfaces surface cannot withstand the centrifugal force, the lubricant was centrifugally ejected from the wear track, thereby leading to an increase in the friction coefficient.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe lubrication mechanism investigated in this study is presumed to be elastohydrodynamic lubrication, which was generated by the hydrated HEC-G lubricant. Accordingly, the lubricating films were analyzed utilizing the Dowson-Hamrock theory, enabling the calculation of film thickness. The calculated film thickness values are presented, alongside the corresponding average friction coefficient data, in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. It is evident that film thickness increases with rising speed, with a corresponding decrease in friction coefficient observed at lower speeds. Notably, at a speed of 410 rpm, the friction coefficient reaches its lowest value of 0.004. However, as speeds surpass 410 rpm, although the calculated film thickness continues to increase, the friction coefficient begins to rise due to lubricant depletion. In such instances, friction coefficients at speeds exceeding 410 rpm were extrapolated and predicted based on the observed trend, depicted by the unfilled circle symbol in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. The recorded friction coefficient values notably exceeded the predicted values, indicating a pronounced detrimental effect of high speeds on HEC-G lubrication efficacy. For rotating friction systems, it is observed that friction coefficient augmentation with speed occurs beyond a certain threshold, indicating the Dowson-Hamrock theory inadequate for predicting the relationship between film thickness and friction coefficient. This emphasizes the need for alternative perspectives when analyzing lubrication models at high velocity conditions. Furthermore, these findings emphasize the critical role of lubricant adsorption onto friction surfaces in achieving effective lubrication. Enhancing the adsorption capability of lubricants to resist detachment is crucial for achieving optimal lubrication performance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe profiles of worn tracks were determined. Five characteristic tracks under different speeds were chose and depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(a). The yellow dashed circles indicate the worn tracks covered by lubricants, while the cleaned tracks are depicted by the insets. Notably, the measured diameter of the tracks is 4.1 mm, though it may appear different between pictures due to camera placement. It is evident that the tested lubricant at 80 rpm was darker than the others, attributable to a higher concentration of worn particles immersed in the lubricant. Similarly, the tested lubricant at 850 rpm was equally dark, despite appearing less in quantity, due to substantial centrifugal forces generated at such high rotational speeds. The flung lubricant is still visible beside the wear track, as indicated by green arrows. A similar phenomenon occurred at a speed of 740 rpm, where scattered lubricant is easier to find. At 410 rpm, the worn track was easy to observe since the lubricant remained transparent. It should be noted that the lubricants are rounded within the blue dotted circle, which represents the trajectory generated during the minimization of the misalignment angle α. Additionally, the cross-sectional profiles of the worn tracks were measured and are depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(b). The height drop (ΔH) was calculated manually. It is evident that the ΔH at 80 rpm, which measured 11.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1, was the largest, while the lowest ΔH of 5.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 was observed at 410 rpm. This significantly indicates that the wear occurring at 80 rpm was the most severe, causing the track to erode deeply, resembling a crater. It is believed that the wear particles immersed in the lubricant were responsible for the extensive wear observed at 80 rpm, a condition under which the lubricant failed to form effective lubricating films. In contrast, the worn track at 410 rpm appeared relatively shallow. As the speed increased beyond 410 rpm, the ΔH became larger, indicating progressively worse wear. However, the trends in surface roughness (Ra) of the worn tracks did not align with those of ΔH, as Ra increased with rising speed. Notably, the Ra at 190 rpm was the lowest, almost equal to that at 80 rpm. This discrepancy can be attributed to two factors. First, the calculation methods for Ra and ΔH are different. Second, at 80 rpm, the wear particles tend to be smaller due to the poor lubricating efficacy, causing the asperities to be worn down and resulting in a lower average height of asperities for calculating Ra. In contrast, at 410 rpm, the asperities are more likely to be separated by the lubricating films, resulting in fewer worn asperities and pits, and thus a higher average height for calculating Ra. At 850 rpm, the lubricant was thrown off, leading to increased contact and wear of the asperities and wear particles. However, it is unlikely that the Ra at 850 rpm would decrease to the same extent as at 80 rpm due to the continuous wear, as the wear mechanisms differ between low and high-speed situations.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe worn zone on the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e balls were also detected and depicted by the blue dashed circles in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e(a). The five blue circles have the same diameter, indicating that the wear on the balls is similar in size. This is likely due to the high hardness of the balls, with most of the wear occurring during the initial wear-in period. When the lubricant was introduced, the wear zone on the balls showed minimal further wear, whereas the Ti-6Al-4V plates exhibited significant wear instead. The section profiles across the yellow dashed line, which is perpendicular to the movement direction (indicated by the green arrow) of the balls, are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e(b). The insets provide partial enlargements for clarity. From these section profiles, the differences in the details of the worn zones are clearly evident. At 80 rpm, the worn zone exhibits burring and an uneven surface, which is similar to the condition observed at 190 rpm. In contrast, the worn surface at 410 rpm is relatively smooth, whereas at the high speed of 850 rpm, the surface once again becomes burring. The surface roughness of the worn zone was measured, and both Ra and Rz values are provided. The Ra value is 2.73\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 for 410 rpm, the lowest among the tested speeds, while it is higher for 80 and 850 rpm. Although the Ra values for 190 and 740 rpm are not significantly greater than that of 410 rpm, the Rz values for 190 and 740 rpm are substantially higher. Rz represents the average value of the heights of the five highest-profile peaks and the depths of the five deepest valleys within the worn zone. Given the trend of Rz, the worn zone at 410 rpm is the smoothest. It is believed that at speeds of 80 and 850 rpm, the lubricants cannot adequately separate the contact surfaces within the worn zone, resulting in higher Ra and Rz values due to increased surface wear. In conclusion, while high speeds can promote the formation of lubricating films, they can also lead to the detachment of the lubricant from the surface. Therefore, considering the interactions between the lubricant and the friction surface, the optimal speed for achieving the best lubricating performance may vary depending on the specific lubricating conditions.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.6 Impact of concentration on the lubrication provided by HEC-G\u003c/h2\u003e \u003cp\u003eVelocity has a significant effect on the lubricating film and, consequently, the lubrication efficacy of HEC-G. This section examines how the concentration of glycerol impacts the lubrication efficacy of HEC at a speed of 410 rpm. Using 4.0 wt.% HEC as the base lubricant, various concentrations of glycerol were added. The wear loss and friction coefficient were recorded and are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e. The friction coefficient of 4.0 wt.% HEC was about 0.024. With the addition of glycerol, the friction coefficient began to decrease sharply, reaching its lowest point of approximately 0.004 at a glycerol concentration of 10 wt.%, as also shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. The friction coefficient at 5 wt.% glycerol was also low, around 0.008, indicating very good lubrication efficiency. However, when glycerol concentrations exceeded 10 wt.%, the friction coefficient increased rapidly, though it remained close to 0.01. This suggests that excessive glycerol can negatively affect the lubricating performance of HEC. Additionally, the wear loss of Ti-6Al-4V disks and Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e balls was evaluated after the tests. For Ti-6Al-4V, wear loss tended to decrease with increasing glycerol concentration until it exceeded 10 wt.%, at which point the wear slowly increased again. This trend mirrors the changes in the friction coefficient, suggesting a correlation between enhanced lubrication conditions and reduced wear between the friction pairs. Specifically, as the glycerol concentration increased from 0 to 10 wt.%, the wear loss of Ti-6Al-4V decreased from 110 mg to 45 mg, a reduction of 59.1%. However, when the concentration increased from 10 to 30 wt.%, the wear loss of Ti-6Al-4V increased slightly. In contrast, the wear trend for Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e balls remained relatively stable, with little fluctuation as the glycerol concentration increased. Notably, the wear amounts of Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e balls ranged from 0.03 to 0.04 mg, whereas the wear amounts of Ti-6Al-4V disks ranged from 45 to 110 mg. This significant difference in wear amounts is likely due to the considerable hardness difference between Ti-6Al-4V and Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e, with Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e being significantly harder, as shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The Ti-6Al-4V disk exhibited more noticeable wear when in contact with the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e ball. However, a novel perspective emerges here focusing solely on trends within the red dashed rectangle (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e), with glycerol concentrations from 5 wt.% to 30 wt.%. Within this range, the wear loss of Ti-6Al-4V disks demonstrates consistent stability with minimal fluctuations, mirroring the behavior observed in Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e balls. In this context, the impact of glycerol concentration on wear loss of both disks and balls appears negligible, whereas the addition of glycerol has a significant effect on the friction, which is reflected in the friction coefficient. Essentially, HEC-G lubricants (5\u0026ndash;30%) effectively facilitate separation between contacting surfaces. It is suggested that thicker lubricants may contribute to increased friction, primarily attributable to the viscous resistance of the lubricant, which suggests that hydration lubrication can be excellent in the right concentrations. Given the weak correlation observed between wear loss (representing direct contact) and concentration, this highlights the dominance of viscous resistance as a primary frictional source in this situation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe contact zones of the friction tracks on the disks were observed using SEM, while the section profiles were detected using ZYGO. Figure\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e presents the results for HEC and HEC-G (1, 10, 20, 30 wt.%). The red lines denote the cross-section profiles selected to be perpendicular to the direction of motion. To maintain consistency in length across all five tracks, the red lines do not traverse the entire dimensions of the images. The significant observation is that the profiles of HEC-G at concentrations of 10, 20, and 30 wt.% exhibit less smoothness compared to those of HEC and HEC-G at 1 wt.%, with Ra roughness values of 1.57 \u0026micro;m and 1.99 \u0026micro;m, respectively. Additionally, distinct plateaus are evident on the friction tracks of HEC-G with higher concentrations, with Ra values ranging from 3.78 \u0026micro;m to 5.84 \u0026micro;m, and the plateaus did not exist under low concentrations instead they became some flatten scars indicated by blue ellipse in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e (a) and (b). Through comparison, it is suggested that these plateaus on the surface are not completely worn down during friction when using thicker HEC-G lubricants. This observation implies that thicker HEC-G lubricants can establish robust lubricating films, whereas HEC and low-concentration HEC-G may not adequately resist the wear of plateaus. Furthermore, it is noteworthy that the formation of plateaus can be attributed to both friction and frictional adhesion, with their upper surfaces appearing nearly flat, characterized by Ra values ranging from 0.01 to 0.02 \u0026micro;m. These unique topographies are particularly notable within the entire section, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e(c). Importantly, these plateaus are distributed throughout the entire tracks, suggesting a significant influence on lubricating efficiency. Conventionally, a flat surface is advantageous for lubricating efficiency, while a rough surface typically leads to increased friction. However, the identification of these distinct plateaus in nanoscale friction strongly indicates that deliberate modification in surface topography can significantly improve the lubricating efficiency of hydration lubrication at a relatively larger scale. The modification in surface topography means the plateaus with flat top cut. Then, the microscopic details of \u0026ldquo;plateau\u0026rdquo; were explored and discussed in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigures \u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e(a) and 15(b) present SEM images of the topographies surrounding the plateau, clearly depicting valleys and plateaus analogous to topographic features. Notably, these images are captured from the nanoscale details of the friction track. On Earth, valleys are more efficient at containing water than plateaus, leading to the formation of lakes and playing an important role in directing water flow. Similarly, at the nanoscale, these \u0026ldquo;valleys\u0026rdquo; serve in a dual function: they contain lubricant and direct its flow. Specifically, the \u0026ldquo;valleys\u0026rdquo; direct lubricant across the plateau, enabling the hydrated HEC-G to form a continuous lubricant film. These lubricant \"lakes\" ensure a steady supply of lubricant and contain the working lubricant, whether sourced from other valleys or from the plateau. When discussing the formation of lubricating films, molecular interactions play a significant role alongside the effect of lubricant flow. Figures\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e (c-f) illustrate the distribution of elements on both the \u0026ldquo;valley\u0026rdquo; and \u0026ldquo;plateau\u0026rdquo; surfaces. The primary elements identified are Ti, Al, and V, originating from the Ti-6Al-4V disk materials. Insets in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e (d) provide detailed views of the distribution of Ti, Al, and V along the red lines across the SEM image, perpendicular to the direction of motion. It is evident that, perpendicular to the direction of motion, the primary elements exhibit a relatively uniform distribution. In stark contrast, the distribution of Si, O, and N elements is notably non-uniform. Figure\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e(e) shows that Si, O, and N elements predominantly cover the plateau areas, while their presence is less noticeable in the valleys. This distribution trend corresponds to both the plateau and valley regions, suggesting that the valleys in the friction track are likely formed by the plowing action of wear particles from the Ti-6Al-4V material during the wearing-in period. Consequently, the Si and N from the balls do not contact the valley areas, whereas the plateau surfaces are worn down by the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e balls throughout the entire friction process. The Si, O, and N elements distributed on the plateau can interact with lubricant molecular groups, such as -OH and -H. Previous research indicates that silicon and oxygen elements can strongly attract -OH and -H groups to form hydrogen bonds[52], which are classified as strong non-covalent interactions, a behavior also exhibited by titanium. Consequently, these elements can interact with the hydroxyl groups of HEC or glycerol, facilitating the robust formation of lubricating films. These interactions likely result in the formation of a nanoscale adsorbed polymer film covering the surface. As the Ti-6Al-4V wears against the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e balls during friction, the surface details of the wear scars on the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e balls were examined, with the results presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe plateaus observed on the Ti-6Al-4V surface were formed through abrasion with the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e balls. The interaction between the Ti, Al, and V elements with the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e was examined using XPS spectra, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e, which depicted energy peaks from wear scars on the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e balls. Clear and distinct peaks corresponding to Si 2s, Si 2p, and N 1s were observed, unequivocally attributed to the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e base material. However, peaks corresponding to Ti 2p, Al 2p, and V 2p (as delineated by the dashed box) were notably absent, indicating that it\u0026rsquo;s difficult for Ti-6Al-4V material to embed into the hard surface of Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e, despite undergoing significant wear during the wearing-in period. Remarkably, the spectra exhibited consistent peak positions for all elements, suggesting the absence of any chemical reactions between them. Consequently, it is inferred that the primary interactions during friction mainly involve solid contact and non-covalent bonds, leading to a significant reduction in friction. Solid contact occurs during the wearing-in period, modifying the beneficial topographies. Concurrently, non-covalent bonds form within the lubricant molecules, enhancing the fluidity characteristics necessary for the formation of robust lubricating films. Given this, when the lubricant is introduced into the friction track, the friction coefficient decreases abruptly without an incubation period. It is certain that, during this brief interval, little to no incubation process between HEC-G and Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e takes place. Therefore, the formation of the lubricating film is entirely attributed to a physical mechanism, partly analogous to elastohydrodynamic lubrication, facilitated by both solid contact and non-covalent bonding. Figure\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e17\u003c/span\u003e provides a microscopic, three-dimensional view of the worn surface, accompanied by illustrations detailing the lubricant flow over the \u0026ldquo;plateau\u0026rdquo; and \u0026ldquo;valley\u0026rdquo; regions. The model vividly presents the elevated plateaus and the recessed valleys. On the top of plateau, the detected elements of silicon, oxygen, and nitrogen attract naturally hydrated HEC-G polymer chains, which tend to remain dispersed in all directions due to hydration repulsion. During friction, the dispersed chains were driven to flow in the same direction, thereby enhancing hydration repulsion. The lakes, or lubricant reservoirs, scattered across the friction tracks serve as lubricant suppliers. As the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e balls move, they drive the lubricant to flow from the lakes to the plateau tops, similar to the elastohydrodynamic lubrication mechanism. This process generates robust lubricating films, with well-aligned polymer chains enhancing hydration repulsion, effectively preventing solid contact between the plateaus and the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e balls. Additionally, minor plateaus, referred to as mesas, play a crucial role during friction, though their specific contributions require further investigation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eThis study explored a novel friction method aimed at enhancing the anti-friction properties of Ti-6Al-4V to achieve ultra-low friction under hydrated lubrication. Two methods were evaluated, with Method 2 showing a significant reduction in friction. When the mixed lubricant of HEC and glycerol was introduced into the friction tracks, the friction sharply decreased to the ultra-low region and remained at 0.004 for approximately 50 minutes. Notably, during this ultra-low friction, the wear loss of the friction pairs remained consistent across different glycerol concentrations. This consistency indicates that solid contact was largely prevented under Method 2, owing to the robust formation of lubricating films.\u003c/p\u003e \u003cp\u003eConsidering the critical role of friction methods in reducing friction, the microscopic mechanisms were examined in detail. SEM and XPS analyses revealed unique topographies featuring plateaus and valleys on the wear scars, along with the corresponding distribution of elements. The valleys containing lubricants acted as effective reservoirs, supplying the necessary lubricant for lubrication. Lubricant flowed from the valleys to the plateaus, forming robust films, a process similar to elastohydrodynamic lubrication but occurring at the nanoscale. In addition to these valleys, the flat tops of the plateaus facilitated steady lubricant flow, aided by the adsorption between the base elements and the chemical groups of the lubricant.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eDeclaration of Artificial Intelligence\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll the manuscript was originally written by the authors. The authors only used generative artificial intelligence to improve readability and language. Then, each improved sentence was carefully reviewed and manually re-edited by the authors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of Competing Interest\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eInstitutional Affiliations\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll affiliations are listed on the title page of the manuscript.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll funding sources for this study are listed in the “Acknowledgments” section of the manuscript.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and material\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets used or analyzed during the current study are available from the corresponding author on reasonable request.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors' contributions\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConceptualization\u003c/strong\u003e: Dezun Sheng, Xiao Zhang;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMethodology\u003c/strong\u003e: All the authors;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFormal\u003c/strong\u003e \u003cstrong\u003eanalysis\u003c/strong\u003e \u003cstrong\u003eand\u003c/strong\u003e \u003cstrong\u003einvestigation\u003c/strong\u003e: Dezun Sheng, Xiao Zhang, Hongliang Yu;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eWriting\u003c/strong\u003e-\u003cstrong\u003eoriginal\u003c/strong\u003e \u003cstrong\u003edraft\u003c/strong\u003e \u003cstrong\u003epreparation\u003c/strong\u003e: Dezun Sheng, Xiao Zhang;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eWriting\u003c/strong\u003e-\u003cstrong\u003ereview\u003c/strong\u003e \u003cstrong\u003eand\u003c/strong\u003e \u003cstrong\u003eediting\u003c/strong\u003e: Dezun Sheng\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e \u003cstrong\u003eacquisition\u003c/strong\u003e: Xin Zhou;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupervision\u003c/strong\u003e: Dezun Sheng\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgement\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work is financially supported by the Natural Science Foundation of Shandong Province (Grant No.: ZR2019BEE073), the Youth Innovation team Project of Higher Education Institutions in Shandong Province (Grant No.: 2022KJ272), the Natural Science Foundation of Shandong Province (Grant No.: ZR2023ME067), and Basic Research Project of Yantai Science and Technology Innovation Development Plan (Grant No.: 2023JCYJ054).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eK. 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Chemical Reviews. 95(5), 1589-1673 (2002). http://dx.doi.org/10.1021/cr00037a600\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"tribology-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"tril","sideBox":"Learn more about [Tribology Letters](https://www.springer.com/journal/11249)","snPcode":"11249","submissionUrl":"https://submission.nature.com/new-submission/11249/3","title":"Tribology Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Ultra-low friction, microscopic mechanisms, Hydration lubricant, Ti-6Al-4V alloy","lastPublishedDoi":"10.21203/rs.3.rs-6963694/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6963694/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eReducing alloy friction to achieve ultra-low friction is a valuable approach to save energy and reduce pollution from oil use, which is a major challenge for researchers. This study introduces a successful method to achieve ultra-low friction in Ti-6Al-4V using a hydrated lubricant composed of hydroxyethyl cellulose (HEC). And the effects of speed and concentration on lubricating were investigated. It was found that excessive sliding speeds may lead to lubricant detachment and consequent friction increase, indicating that the adsorption ability of HEC needs to be enhanced in future studies. In addition, when the concentration exceeds 5 wt.%, wear loss tends to stabilize across tests with different concentrations, while the friction force increases with rising concentrations. Based on these findings, microscopic studies were conducted to investigate the mechanism of friction reduction. Notably, distinct topographic features resembling 'valleys' and 'plateaus' were identified on the wear scars in a nanoscale scope. The movement of the surfaces induces the hydrated HEC lubricant to flow from the lower valleys to the higher plateaus, suggesting elastohydrodynamic lubrication mechanisms to form robust films. The valleys serve as lubricant reservoirs, while the plateau tops support the lubricant films to prevent contacts between Ti-6Al-4V and Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e. The schematic illustrations depict the microscopic mechanisms for achieving of ultra-low friction on Ti-6Al-4V alloy.\u003c/p\u003e","manuscriptTitle":"Achieving Ultra-Low Friction in Ti-6Al-4V Alloy: Hydration Lubrication Mechanisms of HEC-Glycerol Composite","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-30 16:09:47","doi":"10.21203/rs.3.rs-6963694/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-07-21T09:53:33+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-07-18T14:30:05+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"287470307645156546597737417406838629651","date":"2025-06-27T11:20:25+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-06-26T12:37:53+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-06-25T04:52:28+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-06-25T04:52:27+00:00","index":"","fulltext":""},{"type":"submitted","content":"Tribology Letters","date":"2025-06-24T08:46:42+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"tribology-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"tril","sideBox":"Learn more about [Tribology Letters](https://www.springer.com/journal/11249)","snPcode":"11249","submissionUrl":"https://submission.nature.com/new-submission/11249/3","title":"Tribology Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"967204e9-2f7b-40f5-9fa5-aa450890c3ca","owner":[],"postedDate":"June 30th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-09-22T16:00:23+00:00","versionOfRecord":{"articleIdentity":"rs-6963694","link":"https://doi.org/10.1007/s11249-025-02068-y","journal":{"identity":"tribology-letters","isVorOnly":false,"title":"Tribology Letters"},"publishedOn":"2025-09-17 15:57:43","publishedOnDateReadable":"September 17th, 2025"},"versionCreatedAt":"2025-06-30 16:09:47","video":"","vorDoi":"10.1007/s11249-025-02068-y","vorDoiUrl":"https://doi.org/10.1007/s11249-025-02068-y","workflowStages":[]},"version":"v1","identity":"rs-6963694","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6963694","identity":"rs-6963694","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}