Influence of powder preparation technique on microstructure, densification, mechanical and tribological properties of composites based on Al₂O₃ with addition of WC-Co hard metal

Influence of powder preparation technique on microstructure, densification, mechanical and tribological properties of composites based on Al₂O₃ with addition of WC-Co hard metal | 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 Influence of powder preparation technique on microstructure, densification, mechanical and tribological properties of composites based on Al₂O₃ with addition of WC-Co hard metal Heytor V.S.B. Azevêdo, Rafael A. Raimundo, Luís M.F. Morais, Cleber S. Lourenço, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5545783/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract In the present work, the influence of the powder preparation technique on the sintering behavior of 80 wt.% Al 2 O 3 -18 wt.% WC-2 wt.% Co composites was investigated. The powders were mixed with a mechanical stirrer for 10 min (MM) and by high-energy milling (HEM) for 50 h. They were then compacted at 200 MPa and sintered in a resistive dilatometric furnace for 1 h under argon atmosphere at a heating rate of 10°C/min for two sintering temperatures (1300°C and 1550°C). The powders prepared by MM and HEM were further characterized by XRD, SEM and dynamic light scattering (DLS), while the sintering kinetics was evaluated by dilatometry. The powders processed by HEM presented better dispersion and homogenization than those obtained by mechanical mixing (MM). The HEM approach enhances the sintering of immiscible systems that present low sinterability, as well as improves dispersion, phase refining and produces composite particles with greater interaction of the milled elements. In addition, HEM decreases the particle/crystallite size and increases the microstrain. The results showed that composites prepared by HEM and sintered at 1550 ºC presented better densification, Vickers microhardness and tribological behavior. Composites Mechanical Mixing High-Energy Milling Mechanical properties Tribological properties Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction Aluminum oxide (Al 2 O 3 ) is a chemical compound consisting of aluminum and oxygen. It is also widely known as alumina. Within the advanced ceramics, alumina is the most widely used because it offers good performance in terms of corrosion and wear resistance, and features low density, high hardness, high melting point, and good thermal conductivity, as well as offering a good cost-benefit ratio due to its low production cost [1–5]. Indeed, researchers have developed a series of new Al 2 O 3 -based materials combined with other phases to obtain new properties and lower cost in recent years, such as Al 2 O 3 -TiC-Ni [6], Al 2 O 3 -SiC-Ni [7], Al 2 O 3 -TiC-ZrO 2 [8], Al 2 O 3 -NbC [9], and Al 2 O 3 -WC [10]. Hence, it can combine mechanical and electrical properties, which favors the use of alumina in a wide range of applications [11–13]. Alumina intrinsically possesses ionic and covalent bonds with low fracture toughness [1]. Consequently, its plastic deformation is extremely limited [14]. The brittle nature of alumina has restricted its applications [15]. Therefore, the insertion of a ductile metal binder has been one of the most effective ways to improve its brittleness [16–20]. In turn, cobalt (Co) is the most used metallic binder in the world for the manufacture of sintered cermets due to its high wettability and solubility [21,22]. The addition of Co improves the sintering process and increases strength and toughness [23]. According to several researchers, tungsten carbide (WC) can be employed to reinforce Al 2 O 3 [10,24–26]. Certainly, the addition of WC has provided improvements in the microstructure, densification, and sintering of Al 2 O 3 -based composites, shaping these materials as attractive for using in abrasive machining and cutting tools [27,28]. Powder metallurgy is the technique employed to fabricate composites with immiscible components, such as Al 2 O 3 -Co [17], Al 2 O 3 -Fe [11], Al 2 O 3 -WC [29], Cu-WC [30], WC-Cu [31], and WC-Co [32]. According to Costa et al. [33], the component powders can be mixed (MM) to produce a conventional powder mixing or processed by high-energy milling (HEM) to produce a nanostructured composite powder. Conventional mechanical mixing (MM) is not able to produce a well-dispersed mixture and the components are randomly distributed, so there is a limited interaction between them. Thus, heterogeneous structures are formed that impair the sintering process [34]. On the other hand, HEM provides constant collisions between powders and grinding bodies resulting in deformation, cold welding, fracture, and cold re-welding of the powder particles. The deformation and fracturing processes favor the dispersion and homogenization of the components, determining the final powder microstructure [35–37]. Previous studies have investigated the influence of the powder preparation condition on the sintering behavior of immiscible systems. Costa et al. [38] studied three powder preparation conditions (mechanical mixing, rod milling and high-energy milling) on the sintering behavior of W-30 wt.% Cu composites in a resistive dilatometric furnace. Leal et al. [39] investigated the influence of the powder preparation technique (mechanical mixing and high-energy milling) on the microstructural properties and densification of 80 wt.% WC-10 wt.% Co-10 wt.% Al 2 O 3 composites sintered in a resistive dilatometric furnace. Results of these papers suggested that the composites prepared by high-energy milling achieved higher sinterability. Sintering alumina and its derivatives is a challenging task. The use of Al 2 O 3 as structural ceramics is limited by its low fracture toughness (3,0–3,3 MPa × m 1/2 ) [7]. However, several studies have sought to improve the properties of alumina to expand its industrial applications with insertion of other elements. Jing et al. [40] manufactured composites from Al 2 O 3 -TiC-4%Co by hot pressing (30 MPa) a 1650°C in vacuum for 30 min, achieved high toughness (7,8 ± 0,8 MPa × m 1/2 ) and strength (782 ± 60 MPa). Vleugels et al. [41] sintered via SPS composites based on Al 2 O 3 with variable content of niobium carbide (NbC), obtained high hardness 25.1 GPa and good fracture toughness of 5 MPa × m 1/2 , indicating potential application of this material as cutting tool inserts. Kasar et al. [42] used cold press sintering Al 2 O 3 -B 2 O 3 composites, varying the amount of B 2 O 3 (5–20 by weight%), evaluated that the presence of the aluminum borate phase in the composite increased hardness and wear resistance, these composites showed an increase in relative density percentage of 16–37% and hardness of 1.2–1.9. Parchovianský et al. [43] studied the mechanical and tribological properties of Al 2 O 3 -SiC composites with SiC contents ranging from 3 to 20 vol%, and observed that Vickers hardness increased linearly with the SiC content and that wear resistance of the composites was always higher with the increase of SiC volumetric fraction. Thus, the present study aims to investigate the influence of powder preparation method and sintering temperature (1300 and 1550°C) on the properties of composites 80 wt.% Al 2 O 3 -18 wt.% WC-2 wt.% Co. Powder samples (MM and HEM) and sintered samples were characterized for crystal structure, morphology, particle size, microstructure, relative density, and dilatometric shrinkage using several materials characterization techniques. Based on experiments and their results, composites were evaluated according to their mechanical and tribological properties in order to evaluate potential applications in industrial devices subject to wear, such as cylinder liners, abrasive machining, cutting tools, bearings, among others. 2. Materials and Methods Alumina (ALCOA, 99%), tungsten carbide (WBH, 99%) and cobalt (STARK, 99%) powders were used to prepare the composite 80 wt.% Al 2 O 3 -18 wt.% WC-2 wt.% Co. This composition was chosen based on hardmetals, which are materials widely used in abrasive machining, coatings, machining tools and mining [44,45]. In general, the cobalt metallic binder is used in less than 10 wt.%. Although it improves the sintering process and significantly increases the fracture toughness of the composite, it causes some problems such as toxicity, low melting point, high cost, and low corrosion resistance [46–48]. Figure 1 shows SEM images and particle size distribution curves of Al 2 O 3 , WC and Co powders. The Al 2 O 3 powder shows faceted morphology (Fig. 1 (a)) and average particle size of 4.27 µm (Fig. 1 (d)). Tungsten carbide powder exhibited small, agglomerated particles of irregular morphology (Fig. 1 (b)), with an average particle size of 2.95 µm (Fig. 1 (e)). In turn, cobalt powder exhibited spherical morphology (Fig. 1 (c)), with the presence of small particles adhered to the surface of larger particles. The average particle size of cobalt powder reached 20.09 µm (Fig. 1 (f)). Composite powders were prepared by mixing and by high-energy milling. In the former case, the powders were mixed in a glass container for 10 min in a mechanical stirrer. In the latter, the powders were milled for 50 h in a high-energy mill. The powder mixture consisted of 80 wt.% Al 2 O 3 -18 wt.% WC-2 wt.% Co, for all powder preparation methods. Milling was performed in a Planetary Pulverisette-7 mill using a crucible and carbide balls (5 mm diameter) in the presence of 20 mL of ethyl alcohol with a rotation speed of 400 RPM. The powder to balls mass ratio was 1:4.75 and the total powder mass was 20 g. After milling, the powder mixture was spread on an aluminum foil to allow the ethyl alcohol to evaporate. The crystal structure evolution of the phases present on the composite in powder form (MM and HEM) and sintered were evaluated by X-ray diffraction (XRD; Miniflex II from Rigaku) with Rietveld refinement, using the following parameters: angular variation from 10 to 120°, speed of 3°/minute and step of 0.02°. All diffractograms were fitted using TOPAS software (Total Pattern Analysis Solution, version 4.2, Bruker) [49]. The morphology of the powders and microstructure of the sintered samples were investigated by scanning electron microscopy (SEM, HITACHI, TM3000). Average particle sizes were measured by dynamic light scattering (DLS, CILAS 1090, liquid mode). Subsequently, the powder mixtures were then loaded into a cylindrical matrix with a diameter of 5 mm and compacted through a hydraulic press by cold uniaxial pressing at 200 MPa. The green samples were placed on an alumina support to be sintered in a resistive dilatometric furnace (DIL 402 PC, NETZSCH). The tests were performed with argon flow rate of 2.5 ml/s, heating rate of 10°C/min, constant sintering temperature for 1 h and cooling rate of 20°C/min. Circulating argon was used in the furnace to avoid contamination of the sample during heating and cooling processes. The samples were cooled inside the furnace until they reached room temperature. Sintering took place at two different temperatures: 1300°C and 1550°C. In addition, composites were sintered in a vacuum furnace model Vacuum Industries of CENTORR VI manufacturer, high vacuum with a pressure of 10 − 4 Torr, using the same sintering parameters of the resistive dilatometric furnace. The theoretical density of the sintered samples ( \(\:{\rho\:}_{t}\) = 4.66 g/cm 3 ) was determined by the mixing rule (Eq. 1 ), considering the theoretical densities of \(\:\rho\:{Al}_{2}{O}_{3}\) , \(\:\rho\:WC\) and \(\:\rho\:Co\) as 3.98 g/cm 3 , 15.63 g/cm 3 and 8.93 g/cm 3 , respectively. The weight percentages are designated by %Al 2 O 3 , %WC, and %Co. $$\:{\rho\:}_{t}=\frac{1}{(\:\frac{\%{Al}_{2}{O}_{3}}{\rho\:{Al}_{2}{O}_{3}}\:+\:\frac{\%WC}{\rho\:WC\:}+\:\frac{\%Co}{\rho\:Co\:})}$$ 1 The bulk density of the sintered samples was measured at room temperature and atmospheric pressure using the Archimedes principle. As established by ASTM B962-13 [50], a sintered sample was weighed in air ( \(\:Wa\) ), and then immersed in distilled water and weighed again ( \(\:Ww\) ) on a Shimadzu electronic balance (model: AUW220D) with an accuracy of 0.1 mg. The density of distilled water ( \(\:{\rho\:}_{w}\) ) at room temperature is 1 g/cm 3 . Therefore, the bulk density ( \(\:{\rho\:}_{a}\) ) can be calculated using Eq. 2 . Three repetitive tests per sample were performed and the average value was obtained. $$\:{\rho\:}_{a}=\frac{Wa}{(\:Wa-Ww)}*{\rho\:}_{w}$$ 2 In addition, the relative density ( \(\:{\rho\:}_{r}\) ), Eq. 3 , and apparent porosity (P), Eq. 4, of the sintered samples can be calculated from the relationship between the theoretical density ( \(\:{\rho\:}_{t}\) ), Eq. 1 , and the bulk density ( \(\:{\rho\:}_{a}\) ), Eq. 2 , as mentioned by references [51,52], follows: $$\:{\rho\:}_{r}\left(\%\right)=\frac{{\rho\:}_{a}}{{\rho\:}_{t}}*100$$ 3 \(\:{p}_{\left(\%\right)}=(1-\frac{{\rho\:}_{a}}{{\rho\:}_{t}}*100\) ) (4) The Vickers Microhardness was determined using a DIGIMESS digital micro hardness tester, model: HV-1000 (400.310), under a load of 4.9 N for 10 s at room temperature, considering the average of 5 measurements, according to standard ASTM E384-99 [53]. The sliding test was performed on a pin-on-disk tribometer (Magnum Engineers, model: TE-165LE, India) with sliding in non-lubricated conditions. The stationary pin using 80 wt.% Al 2 O 3 -18 wt.% WC-2 wt.% Co composite was loaded with an applied load of 10 N. The sliding tests were performed at room temperature. Test duration was 10 min (or 314 m distance) for all trials. The 1020 steel disc was maintained at a contact sliding speed of 0.5 m/s and disc rotation of 500 rpm. Before testing, the 1020 steel disc was polished with sandpaper up to 1200 mesh to reduce roughness. The composite wear volume ( \(\:V\) ), Eq. 5 , is calculated by the mass loss ratio \(\:\varDelta\:W\) (g), obtained by the differences of initial masses \(\:Wi\) and final masses \(\:Wf\) in the wear test, and the apparent density of composite \(\:\rho\:a\) (g/cm 3 ) determined by the Archimedes principle of composite. $$\:V=\frac{\varDelta\:W}{\rho\:a}=\frac{Wi-Wf}{\rho\:a}$$ 5 The evaluation of the composite in specific wear rate \(\:K\) (mm 3 /N·m) is normally calculated by the volume ratio of worn material \(\:V\) (mm 3 ) with the test load \(\:F\) (N) and the slip distance \(\:S\) (m). As described in Eq. 6 , developed by Archard [54]: $$\:K=\frac{V}{F.S}$$ 6 3. Results and Discussion 3.1 Effect of powder preparation on the crystal structure of Al 2 O 3 -WC-Co The refined X-ray diffraction patterns of the 80 wt.% Al 2 O 3 -18 wt.% WC-2 wt.% Co composite powders prepared by MM and HEM routes are shown in Fig. 2 (a). As observed, all peaks are characteristic of Al 2 O 3 (with lattice parameters a = b = 4.76 Å and c = 12.99 Å, ICSD n° 60419, space group R-3 c H (167)) [55] and WC (with lattice parameters a = b = 2.90 Å and c = 2.83 Å, ICSD n° 77738, space group P-6 m 2 (187)) [56] phases. Within the limit of the X-ray diffraction technique, the Co phase was not observed due to its low weight content (wt.%) on the composite. The crystallite sizes, lattice parameters, percentage of each phase (wt.%) and agreement indexes obtained through Rietveld analysis are assembled in Table 1 . The maximum values found for Rwp , Rexp and χ 2 were 31.82%, 26.95% and 1.18, respectively. The low χ 2 values indicate good agreement between experimental data and refined models. Figures 2 (b, c) show the magnifications of the main peaks of Al 2 O 3 (113) and WC (011) of the samples prepared by MM and HEM. As observed, three changes are observed in the X-ray patterns in both phases: peak shift to higher angles, intensity reduction, and broadening of the full width at half maximum. The shift of the peaks to higher angles indicates a reduction in the lattice parameters (as shown in lattice parameters "a" of both phases in Table 1 ). The reduction in intensity and the broadening of the full width at half maximum are consequences of the high microstructural refinement, reduction of the crystallite size, and increased crystalline micro-deformation. The obtained lattice parameters are similar to those of the initial Al 2 O 3 and WC powders. The crystallite sizes of Al 2 O 3 and WC phases reduced after HEM for 50 h and reached 27.47 nm and 30.55 nm, respectively, while the microstrains increased and reached values of 0.25% (Al 2 O 3 ) and 0.21% (WC). According to results observed in Fig. 2 (b) and Table 1 , the Al 2 O 3 phase is the most affected by HEM. Table 1. Crystallite size (D XRD ), lattice parameters (a), quantitative phase analysis, and Rietveld agreement indexes for samples obtained by MM and HEM. Data in square brackets correspond to the mass fraction of each phase. Sample Al 2 O 3 WC Rietveld agreement D XRD (nm) a (Å) c (Å) D XRD (nm) a (Å) c (Å) Rwp (%) Rexp (%) χ² MM 131 [91.29%] 4.7594 12.9933 60.3 [8.71%] 2.9060 2.8387 31.82 26.95 1.18 HEM 27.47 [88.38%] 4.7592 12.9988 30.55 [11.620%] 2.9058 2.8387 27.19 24.56 1.11 3.2 Effect of powder preparation method on microstructural and morphological properties Figure 3 shows the morphology and particle size curves of the powders (80 wt.% Al 2 O 3 -18 wt.% WC-2 wt.% Co) prepared by high-energy milling for 50 h and mixed for 10 min. Figure 3 (a) of the powders mixed for 10 min shows that original shape of the particles was maintained. There was no fragmentation of the brittle phases Al 2 O 3 (dark phase) and WC (bright phase) as well as welding of the brittle phases into the ductile matrix (Co). According to the particle distribution curve in Fig. 3 (b), the average particle diameter is 4.21 µm, an intermediate value between the particle diameter of the initial powders, indicating that it was a mixture, as shown in the SEM image. During HEM for 50 h, the powders were subjected to high-energy collisions, so that the ceramic particles (Al 2 O 3 , WC) were fragmented and incorporated into the ductile matrix phase (Co). The impregnation of Al 2 O 3 and WC on the Co metal surface produces composite Al 2 O 3 -WC-Co particles. The powders underwent successive cycles of plastic deformation, fracture, and cold welding, which caused dispersion and homogenization of the constituent phases of the composite powders, as observed by SEM (Fig. 3 (c)). The particle distribution curve of the powder milled for 50 h (Fig. 3 (d)) reveals that the average particle diameter was 2.02 µm, a 52% decrease compared to those mixed for 10 min, indicating that HEM significantly decreased the particle size of the powders. 3.3 Structural characterization of sintered composites XRD of the surface of composites sintered at 1300 and 1550°C are shown in Fig. 4 . As observed, three phases are identified: Al 2 O 3 , WC and W 2 C. When compared with the phases of the powders prepared by MM and HEM, there is phase transformation after sintering, i.e., the arising of the W 2 C phase. The Cobalt phase is not observed in Fig. 4 , and this may be related to the low cobalt content which is not included in the detection range of XRD. The phases identified in this work agree with previous reports. The crystallite sizes of the Al 2 O 3 phase were determined to be 123, 141, 92, and 115 nm for composites sintered with MM and HEM powders at 1300°C and 1550°C, respectively. As expected, an increase in the crystallite size of this phase occurs. However, the sintered specimens prepared by HEM presented smaller crystallites than those processed by MM. 3.4 Microstructure of sintered composites SEM images of sintered samples derived from powders mixed for 10 min and milled for 50 h at 1300°C and 1550°C are shown in Figs. 5 (a, b) and Figs. 5 (c, d), respectively. The samples sintered at 1300°C (Fig. 5 (a)) and 1550°C (Fig. 5 (b)) prepared by mechanical mixing exhibit less distributed grains in the microstructures of the samples, as well as less consolidated sintered phases. Meanwhile, the sintered samples prepared by high-energy milling for 50 h (Fig. 5 (c) and (d)) exhibit uniform grain distribution as well as better microstructural refinement. Besides, high-energy milling provided a decrease in the average particle sizes of the milled powders compared to the mixed powders, which caused an improvement in the sinterability of the milled powders. For the temperature of 1550°C in Figs. 5 (b, d), the sintered samples underwent a complete sintering process, characterized by the formation of more widely distributed and uniform binder regions in the microstructures, especially when milled for 50 h. In fact, the eutectic temperature of cobalt is reached, leading to the formation of a liquid phase, causing melting and fluidity through the structure, filling the remaining pores, resulting in higher densification of the composite. Higher grain growth is also observed compared to the samples sintered at 1300°C. These figures demonstrate that powder preparation and sintering temperature have a strong influence on the formation of a more uniform microstructure in the sintered samples. 3.5 Relative density and apparent porosity of sintered composites The relative density (Eq. 3 ) and apparent porosity (Eq. 4) of samples sintered in a resistive dilatometric furnace at 1300°C and 1550°C are shown in Table 2 . According to the obtained results, the relative densities and apparent porosities are strongly influenced by the powder preparation and sintering temperature. Indeed, high energy milling can cause (1) a decrease of interparticle distance, (2) a high phase interaction, (3) a uniform dispersion of Al 2 O 3 and WC powders in the ductile Co matrix, and (4) a decrease of average particle sizes, this last feature increases the surface area and improves the sinterability of the composite. Thus, the relative density was higher in the samples prepared by high-energy milling (1550°C; ρ mixed 67.44%, ρ milled 80.85%), probably due to their average diameter around 2.02 µm, 2.1 times smaller than that of the mixed particles. On the other hand, at 1550°C, comparing both powder preparation methods ( ρ mixed 67.44%, ρ milled 80.85%), the sintered samples achieved higher relative density and lower porosity than at 1300°C ( ρ mixed 59.25%, ρ milled 62.49%). Indeed, the relative density is also influenced by the sintering temperature, once increasing this temperature decreases porosity, increases densification, and results in denser composites [57,58]. These values are comparable to those of composites from literature with similar compositions. Fazili et al. [59] found that a WC-6 wt% Al 2 O 3 cermet sintered at 1350°C exhibited total densification of 67%. Leal et al. [39] concluded that WC-10 wt% Co-10 wt% Al 2 O 3 composites prepared by mechanical mixing and high energy milling achieved a total relative density of 68% after sintering at 1550°C. The densification of Al 2 O 3 -based immiscible systems is a challenging task, especially for powder compaction followed by sintering. Therefore, some studies have shown that sintering at high temperatures (1700°C − 1900°C) or the use of special sintering techniques such as hot vacuum pressing and spark plasma sintering (SPS) are necessary to obtain Al 2 O 3 composites -WC-Co totally dense [10,28,60]. Table 2 Relative density and apparent porosity of the 80 wt.% Al 2 O 3 -18 wt.% WC-2 wt.% Co composite. Data obtained from a batch of three samples. Preparation technique Sintering temperature (ºC) Apparent Porosity (%) Relative Density (%) Mechanical mixing 1300 39.10 ± 0.73 59.25 ± 1.51 Mechanical mixing 1550 31.86 ± 0.65 62.49 ± 1.53 High-energy milled 1300 31.73 ± 0.81 67.44 ± 0.17 High-energy milled 1550 19.15 ± 1.11 80.85 ± 1.11 The results of relative density of the sintered composites by vacuum furnace at 1300°C and 1550°C are presented in Table 3 . Composites sintered by vacuum furnaces achieved better relative density results than those obtained by conventional furnaces can be observed. Sintering in a vacuum furnace has several advantages such as uniformity of heating and cooling with beneficial effects on dimensional variation, uniform microstructure and absence of oxidation, lower risk of crack development, ease of monitoring and electronic recording of process information [61]. In addition, sintering in vacuum furnaces is often used in the state of liquid phase sintering. This process is accompanied to obtain maximum densification of the sintered parts. For the sintering of cemented alloy powders, high temperatures above the melting point of metal binder are normally required [62,63]. Table 3 Relative density and apparent porosity of 80 wt.% Al 2 O 3 -18 wt.% WC-2 wt.% Co composites sintered in a vacuum furnace. Preparation technique Sintering temperature (ºC) Apparent Porosity (%) Relative Density (%) Mechanical mixing 1300 40.75 ± 1.51 60.90 ± 0.73 Mechanical mixing 1550 37.51 ± 1.53 68.14 ± 0.65 High-energy milled 1300 32.56 ± 0.17 68.27 ± 0.81 High-energy milled 1550 8.63 ± 0.59 91.37 ± 0.59 3.6 Dilatometric shrinkage Figure 6 shows the dilatometric curves for each composite sample (80 wt.% Al 2 O 3 -18 wt.% WC-2 wt.% Co) sintered from room temperature (25°C) to sintering temperatures of 1300 and 1550°C, with a constant sintering plateau of 1 h. The sintering of the milled (HEM) and mechanically mixed (MM) samples up to the temperature of 1300°C is related to the solid-state diffusion processes such as grain boundary diffusion, surface diffusion and volumetric diffusion [64]. With the increase of sintering temperature to 1550°C the eutectic point of cobalt (1495°C) is reached; thus the solid structure continuously disappears during the onset of liquid phase formation promoted by cobalt. The structure contracts rapidly due to the capillary force caused by the still open pores and the meniscus at the edges of the sample, as long as there is sufficient liquid formation. This stage only ends with the densification of the sample or when the capillary force responsible for closing the pores reaches insufficient levels to promote the closure of the remaining pores [65,66]. This dilatometric shrinkage behavior was also observed by Acchar et al. [67] in pure alumina and niobium carbide reinforced alumina composites, where the dilatometric curves showed two distinct regions. The first refers to the temperature range before the onset of material shrinkage. One can cite that in this region the alumina has not yet begun to densify, and the material shows only rearrangement, coalescence, and early formation of the particle contact point. The second region can be associated with the shrinkage of the material, starting at approximately 1100°C. Therefore, with increasing the sintering temperature to 1550°C the mixed and milled samples exhibited greater shrinkage. The samples processed by HEM for 50 h experienced the highest dilatometric shrinkage, indicating an average variation from the starting point of approximately − 0.082, thus confirming the improved densification of this sintered sample. However, the samples mixed for 10 min and sintered at 1550°C experienced approximate dilatometric shrinkage of -0.035. The obtained results of relative density of the mixed and milled samples confirm an increasing tendency with increasing sintering temperature for alumina-based composites [68–70]. 3.7 Mechanical properties Figure 7 shows Vickers Microhardness of the samples sintered by vacuum furnaces. As can be seen in the graph, the Vickers Microhardness suffers positive influence with high grinding. According to Raimundo [71], the improvement of performance in mechanical properties of microhardness promoted by grinding is caused by factors that follow: decreased distance between particles, high interaction of phases, uniform dispersion of ceramic powders in the metal matrix of Co, and decrease of the average particle sizes, this last characteristic tends to increase the surface area, considerably improving the sintering of Al 2 O 3 -WC-Co composites. The composites prepared by HEM showed Vickers Microhardness values of 11.60 ± 0.85 GPa (1300°C) e 20.61 ± 0.85 GPa (1550°C) while composites prepared by MM obtained values of 5.35 ± 0.85 GPa and 8.35 ± 0.85 GPa for temperatures of 1300°C and 1550°C, respectively. According to the literature, the microhardness values reported for Al 2 O 3 can vary between 20.6 e 29.4 GPa, using powerful sintering techniques [72]. The results of Microhardness Vickers of powder ground for 50 h and consolidated to 1550°C are superior compared to various literature works. Zawrah and Taha [73] achieved Microdrureza Vickers values of 9–13 GPa of Al 2 O 3 -ZrO 2 -Ni composites sintered in a tubular furnace at 1500°C. Zhu et al. [74] achieved 17.13–17.86 GPa of Vickers Microhardness in WC-Al 2 O 3 -Ni composites sintered in a vacuum furnace (ZT-40-20YB) at 1540°C. Mariana et al. [75] sintered via SPS composites WC-Al 2 O 3 to 1550°C and obtained Vickers Microhardness between 16.9–19 GPa. 3.8 Tribological properties Notably, the size of particles prepared by different powder preparation techniques (MM and HEM) plays an important role in the wear behavior of sintered composites, as shown in Fig. 8 . In the initial softening period (from 0 to 80 s), for both methods of powder preparation, the contact surface suffers plastic deformation due to small roughness that increases the surface roughness of the initial materials, causing higher contact pressures which leads to debris removal from tribological pair, resulting rapid increase of the friction coefficient curve with significant fluctuations. This behavior is also observed in our previous studies in Al 2 O 3 -WC-Co and SiC-WC-Co composites, obtained by Spark plasma Sintering [28,76]. After the softening period, combination of friction and temperature favors adhesive phenomena between the surfaces of the tribological pair. This step is characterized by the formation of an adhesion layer in 1020 steel caused by continuous sliding of the pin (composite) over previously worn sections, leading to a sliding process where surfaces of the same material (tribofilm) are in touch with each other. The atmosphere where the wear test happens also influences the adhesion layer, because molecules of water or oxygen present in the ambient air creates a film on the surface of tribological pairs [77,78]. For composites prepared by HEM, nanoparticles evenly distributed by the action of the powder preparation technique play an essential role in the friction process, since it converts sliding friction into rolling friction to prevent adhesive wear and preventing crack propagation, as noted by Zhengjun et al. [79] in Al 2 O 3 -Fe-Al composites. Thus, the curves of friction coefficient reveal smooth fluctuation of the mean values 0.52 ± 0.03 and 0.49 ± 0.01 for composites prepared by HEM at 1300°C and 1550°C, respectively. On the other hand, composites with larger particle size prepared by MM have a decrease in wear resistance, leading to a drastic fluctuation of the friction coefficient curve with a mean of 0.58 ± 0.06 and 0.54 ± 0.04 for sintered composites at 1300°C and 1550°C, respectively. This can be explained by the low interaction between the particles, leading to the detachment of ceramic particles (Al 2 O 3 and WC) and the formation of numerous wear debris. Composites with higher microhardness and relative densities obtained lower coefficients of friction, this is beneficial for tribological applications, since they can present high hardness even at high temperatures, good thermal stability and excellent wear resistance, allowing better performances during use [80]. 4. Conclusions This work compared two techniques for powder preparation of the 80 wt.% Al 2 O 3 -18 wt.% WC-2 wt.% Co composite: mechanical mixing for 10 min and high-energy milling for 50 h, associated with two sintering temperatures, 1300 and 1550 ° C. It was observed that mechanical mixing preserves the intrinsic characteristics of the starting powders; the particles do not fracture, however, they remained clustered. On the other hand, after 50 h of high-energy milling, the particles of the ceramic phases (Al 2 O 3 , WC) were refined, deformed, and welded into the Co metal matrix. HEM also promoted dispersion and homogenization of the powders. Besides, the average particle sizes of the milled powders decreased in comparison to the mixed powders, which caused an improvement in its sinterability. The best values of relative density and dilatometric shrinkage were obtained for the samples milled for 50 h and further sintered at 1550°C (91.37% and − 0.082, respectively), confirming that the liquid phase formation promoted by cobalt caused greater pore filling, resulting in greater densification of the composite. In addition, the powder preparation method also strongly influenced the microstructure, densification, mechanical and tribological properties of the composites. The results of mechanical and tribological tests showed that composites processed by HEM were promising materials for devices subject to wear. Declarations Conflicts of Interest: The authors declare that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its out-come. Funding: General financial support received from CAPES. No interference with study design and data analysis. Author Contribution Credit Author StatementHeytor V.S.B. Azevêdo: Conceptualization, Validation, Methodology, Software, Investigation, Writing – original draft, Writing – review & editing. Rafael A. Raimundo: Conceptualization, Validation, Software, Methodology, Investigation, Writing – review & editing. Luís M.F. Morais: Methodology, Investigation, Writing – review & editing. Cleber S. Lourenço: Methodology, Writing – review & editing. Nailton T. Câmara: Methodology, Investigation, Writing – review & editing. Daniel A. Macedo: Methodology, Investigation, Writing – review & editing. Danielle G.L. Cavalcante: Methodology, Investigation, Writing – review & editing. Uílame U. Gomes: Conceptualization, Validation, Methodology, Software, Investigation, Supervision, Writing – original draft, Writing – review & editing. Acknowledgments: This research was supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brazil (CAPES) – Finance Code 001. 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Azevêdo","email":"data:image/png;base64,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","orcid":"","institution":"Petróleo Brasileiro S. A, PETROBRAS","correspondingAuthor":true,"prefix":"","firstName":"Heytor","middleName":"V.S.B.","lastName":"Azevêdo","suffix":""},{"id":409104481,"identity":"f2c18c4d-fa8c-45fb-8309-8a0f09d6b420","order_by":1,"name":"Rafael A. Raimundo","email":"","orcid":"","institution":"UFPB","correspondingAuthor":false,"prefix":"","firstName":"Rafael","middleName":"A.","lastName":"Raimundo","suffix":""},{"id":409104482,"identity":"9663d640-e94f-4d90-bcb0-d91799203f3f","order_by":2,"name":"Luís M.F. Morais","email":"","orcid":"","institution":"UFRN","correspondingAuthor":false,"prefix":"","firstName":"Luís","middleName":"M.F.","lastName":"Morais","suffix":""},{"id":409104483,"identity":"0c8a2f63-d529-4e73-9a32-3a32e639ac50","order_by":3,"name":"Cleber S. Lourenço","email":"","orcid":"","institution":"UFRN","correspondingAuthor":false,"prefix":"","firstName":"Cleber","middleName":"S.","lastName":"Lourenço","suffix":""},{"id":409104486,"identity":"8d4e7ec7-0322-4931-85ed-571b9cb539ab","order_by":4,"name":"Nailton T. Câmara","email":"","orcid":"","institution":"UFRN","correspondingAuthor":false,"prefix":"","firstName":"Nailton","middleName":"T.","lastName":"Câmara","suffix":""},{"id":409104488,"identity":"a8494926-b147-4943-8169-123983f1a68d","order_by":5,"name":"Daniel A. Macedo","email":"","orcid":"","institution":"UFPB","correspondingAuthor":false,"prefix":"","firstName":"Daniel","middleName":"A.","lastName":"Macedo","suffix":""},{"id":409104489,"identity":"65aa6f03-d6af-4695-8b32-22bcb1bf64b7","order_by":6,"name":"Danielle G.L. Cavalcante","email":"","orcid":"","institution":"UFPB","correspondingAuthor":false,"prefix":"","firstName":"Danielle","middleName":"G.L.","lastName":"Cavalcante","suffix":""},{"id":409104491,"identity":"cd1fcb6a-eb32-4d5e-a356-077c6c42b933","order_by":7,"name":"Uílame U. Gomes","email":"","orcid":"","institution":"UFRN","correspondingAuthor":false,"prefix":"","firstName":"Uílame","middleName":"U.","lastName":"Gomes","suffix":""}],"badges":[],"createdAt":"2024-11-29 03:08:23","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5545783/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5545783/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":75296926,"identity":"487fadd5-f4ec-452e-b29d-ba446c124bc7","added_by":"auto","created_at":"2025-02-03 07:00:18","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":270220,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images and particle size distribution curves of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (a, d), WC (b, e) and Co (c, f) powders.\u003c/p\u003e","description":"","filename":"Figure1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5545783/v1/504e7c876a948c641bccc444.jpeg"},{"id":75296907,"identity":"7dc25831-abc6-42db-9c2d-9316db7be260","added_by":"auto","created_at":"2025-02-03 07:00:17","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":210199,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Refined XRD patterns and details of broadening and angular shift of diffraction peaks of the (b) Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (113) and (c) WC (011) phases.\u003c/p\u003e","description":"","filename":"Figure2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5545783/v1/661c79a951a0ee45eb01ed61.jpeg"},{"id":75296911,"identity":"84eab9c9-9c9f-45f2-809d-55867e04dce5","added_by":"auto","created_at":"2025-02-03 07:00:17","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":295488,"visible":true,"origin":"","legend":"\u003cp\u003ePowder mixed for 10 min: (a) SEM image and (b) particle size distribution curve. Powder milled for 50\u003c/p\u003e\n\u003cp\u003eh: (c) SEM image and (d) particle size distribution curve.\u003c/p\u003e","description":"","filename":"Figure3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5545783/v1/1210002897c00071dafff2ef.jpeg"},{"id":75296924,"identity":"46cc40c7-4af1-495f-911a-cfc14a79c34e","added_by":"auto","created_at":"2025-02-03 07:00:18","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":130005,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XRD patterns of composites sintered at 1300 °C and 1550 °C.\u003c/p\u003e","description":"","filename":"Figure4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5545783/v1/2a56f1493ce62053afdc50d4.jpeg"},{"id":75296945,"identity":"c1365d39-9817-435e-89f4-593c6812d26f","added_by":"auto","created_at":"2025-02-03 07:00:19","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":521716,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of composites 80 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-18 wt.% WC-2 wt.% Co prepared with powders mixed for 10 min and sintered at 1300 °C (a) and 1550 °C (b) and powders milled for 50 h and sintered at 1300 °C (c) and 1550 °C (d).\u003c/p\u003e","description":"","filename":"Figure5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5545783/v1/c14d62e038b89e9a037be161.jpeg"},{"id":75297154,"identity":"57a575f9-2ca8-4a75-b980-81c1e944e7d6","added_by":"auto","created_at":"2025-02-03 07:08:17","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":123035,"visible":true,"origin":"","legend":"\u003cp\u003eDilatometric curves for two 80 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-18 wt.% WC-2 wt.% Co composites.\u003c/p\u003e","description":"","filename":"Figure6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5545783/v1/b00922ca0da6f45a69072a25.jpeg"},{"id":75296908,"identity":"d0e340f2-27b2-4ca9-a910-84f52f518a40","added_by":"auto","created_at":"2025-02-03 07:00:17","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":5389703,"visible":true,"origin":"","legend":"\u003cp\u003eVickers microhardness of 80 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-18 wt.% WC-2 wt.% Co composites.\u003c/p\u003e","description":"","filename":"Figure7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5545783/v1/0ea1eec3bae2f7efd57be893.jpg"},{"id":75296933,"identity":"79c99545-b2fb-4034-b9bc-777e8f7a63d9","added_by":"auto","created_at":"2025-02-03 07:00:18","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":8147399,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of friction coefficient curves in composites as a function of sliding time under a contact load of 10 N against 1020 steel discs.\u003c/p\u003e","description":"","filename":"Figure8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5545783/v1/b5674bdd4ab13711e91793d6.jpg"},{"id":75299749,"identity":"465b0c7b-9695-45f6-913e-b0e1ff3751ec","added_by":"auto","created_at":"2025-02-03 07:24:28","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":16020904,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5545783/v1/108a7762-56cd-48ce-a143-73a742e7a359.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Influence of powder preparation technique on microstructure, densification, mechanical and tribological properties of composites based on Al₂O₃ with addition of WC-Co hard metal","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAluminum oxide (Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) is a chemical compound consisting of aluminum and oxygen. It is also widely known as alumina. Within the advanced ceramics, alumina is the most widely used because it offers good performance in terms of corrosion and wear resistance, and features low density, high hardness, high melting point, and good thermal conductivity, as well as offering a good cost-benefit ratio due to its low production cost [1\u0026ndash;5]. Indeed, researchers have developed a series of new Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-based materials combined with other phases to obtain new properties and lower cost in recent years, such as Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-TiC-Ni [6], Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-SiC-Ni [7], Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-TiC-ZrO\u003csub\u003e2\u003c/sub\u003e [8], Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-NbC [9], and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-WC [10]. Hence, it can combine mechanical and electrical properties, which favors the use of alumina in a wide range of applications [11\u0026ndash;13].\u003c/p\u003e \u003cp\u003eAlumina intrinsically possesses ionic and covalent bonds with low fracture toughness [1]. Consequently, its plastic deformation is extremely limited [14]. The brittle nature of alumina has restricted its applications [15]. Therefore, the insertion of a ductile metal binder has been one of the most effective ways to improve its brittleness [16\u0026ndash;20]. In turn, cobalt (Co) is the most used metallic binder in the world for the manufacture of sintered cermets due to its high wettability and solubility [21,22]. The addition of Co improves the sintering process and increases strength and toughness [23]. According to several researchers, tungsten carbide (WC) can be employed to reinforce Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e [10,24\u0026ndash;26]. Certainly, the addition of WC has provided improvements in the microstructure, densification, and sintering of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-based composites, shaping these materials as attractive for using in abrasive machining and cutting tools [27,28].\u003c/p\u003e \u003cp\u003ePowder metallurgy is the technique employed to fabricate composites with immiscible components, such as Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-Co [17], Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-Fe [11], Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-WC [29], Cu-WC [30], WC-Cu [31], and WC-Co [32]. According to Costa et al. [33], the component powders can be mixed (MM) to produce a conventional powder mixing or processed by high-energy milling (HEM) to produce a nanostructured composite powder. Conventional mechanical mixing (MM) is not able to produce a well-dispersed mixture and the components are randomly distributed, so there is a limited interaction between them. Thus, heterogeneous structures are formed that impair the sintering process [34]. On the other hand, HEM provides constant collisions between powders and grinding bodies resulting in deformation, cold welding, fracture, and cold re-welding of the powder particles. The deformation and fracturing processes favor the dispersion and homogenization of the components, determining the final powder microstructure [35\u0026ndash;37].\u003c/p\u003e \u003cp\u003ePrevious studies have investigated the influence of the powder preparation condition on the sintering behavior of immiscible systems. Costa et al. [38] studied three powder preparation conditions (mechanical mixing, rod milling and high-energy milling) on the sintering behavior of W-30 wt.% Cu composites in a resistive dilatometric furnace. Leal et al. [39] investigated the influence of the powder preparation technique (mechanical mixing and high-energy milling) on the microstructural properties and densification of 80 wt.% WC-10 wt.% Co-10 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e composites sintered in a resistive dilatometric furnace. Results of these papers suggested that the composites prepared by high-energy milling achieved higher sinterability.\u003c/p\u003e \u003cp\u003eSintering alumina and its derivatives is a challenging task. The use of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e as structural ceramics is limited by its low fracture toughness (3,0\u0026ndash;3,3 MPa \u0026times; m\u003csup\u003e1/2\u003c/sup\u003e) [7]. However, several studies have sought to improve the properties of alumina to expand its industrial applications with insertion of other elements. Jing et al. [40] manufactured composites from Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-TiC-4%Co by hot pressing (30 MPa) a 1650\u0026deg;C in vacuum for 30 min, achieved high toughness (7,8\u0026thinsp;\u0026plusmn;\u0026thinsp;0,8 MPa \u0026times; m\u003csup\u003e1/2\u003c/sup\u003e) and strength (782\u0026thinsp;\u0026plusmn;\u0026thinsp;60 MPa). Vleugels et al. [41] sintered via SPS composites based on Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e with variable content of niobium carbide (NbC), obtained high hardness 25.1 GPa and good fracture toughness of 5 MPa \u0026times; m\u003csup\u003e1/2\u003c/sup\u003e, indicating potential application of this material as cutting tool inserts. Kasar et al. [42] used cold press sintering Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e composites, varying the amount of B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (5\u0026ndash;20 by weight%), evaluated that the presence of the aluminum borate phase in the composite increased hardness and wear resistance, these composites showed an increase in relative density percentage of 16\u0026ndash;37% and hardness of 1.2\u0026ndash;1.9. Parchoviansk\u0026yacute; et al. [43] studied the mechanical and tribological properties of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-SiC composites with SiC contents ranging from 3 to 20 vol%, and observed that Vickers hardness increased linearly with the SiC content and that wear resistance of the composites was always higher with the increase of SiC volumetric fraction.\u003c/p\u003e \u003cp\u003eThus, the present study aims to investigate the influence of powder preparation method and sintering temperature (1300 and 1550\u0026deg;C) on the properties of composites 80 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-18 wt.% WC-2 wt.% Co. Powder samples (MM and HEM) and sintered samples were characterized for crystal structure, morphology, particle size, microstructure, relative density, and dilatometric shrinkage using several materials characterization techniques. Based on experiments and their results, composites were evaluated according to their mechanical and tribological properties in order to evaluate potential applications in industrial devices subject to wear, such as cylinder liners, abrasive machining, cutting tools, bearings, among others.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cp\u003eAlumina (ALCOA, 99%), tungsten carbide (WBH, 99%) and cobalt (STARK, 99%) powders were used to prepare the composite 80 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-18 wt.% WC-2 wt.% Co. This composition was chosen based on hardmetals, which are materials widely used in abrasive machining, coatings, machining tools and mining [44,45]. In general, the cobalt metallic binder is used in less than 10 wt.%. Although it improves the sintering process and significantly increases the fracture toughness of the composite, it causes some problems such as toxicity, low melting point, high cost, and low corrosion resistance [46\u0026ndash;48]. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows SEM images and particle size distribution curves of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, WC and Co powders. The Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powder shows faceted morphology (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (a)) and average particle size of 4.27 \u0026micro;m (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (d)). Tungsten carbide powder exhibited small, agglomerated particles of irregular morphology (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (b)), with an average particle size of 2.95 \u0026micro;m (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (e)). In turn, cobalt powder exhibited spherical morphology (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (c)), with the presence of small particles adhered to the surface of larger particles. The average particle size of cobalt powder reached 20.09 \u0026micro;m (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (f)).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eComposite powders were prepared by mixing and by high-energy milling. In the former case, the powders were mixed in a glass container for 10 min in a mechanical stirrer. In the latter, the powders were milled for 50 h in a high-energy mill. The powder mixture consisted of 80 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-18 wt.% WC-2 wt.% Co, for all powder preparation methods. Milling was performed in a Planetary Pulverisette-7 mill using a crucible and carbide balls (5 mm diameter) in the presence of 20 mL of ethyl alcohol with a rotation speed of 400 RPM. The powder to balls mass ratio was 1:4.75 and the total powder mass was 20 g. After milling, the powder mixture was spread on an aluminum foil to allow the ethyl alcohol to evaporate. The crystal structure evolution of the phases present on the composite in powder form (MM and HEM) and sintered were evaluated by X-ray diffraction (XRD; Miniflex II from Rigaku) with Rietveld refinement, using the following parameters: angular variation from 10 to 120\u0026deg;, speed of 3\u0026deg;/minute and step of 0.02\u0026deg;. All diffractograms were fitted using TOPAS software (Total Pattern Analysis Solution, version 4.2, Bruker) [49]. The morphology of the powders and microstructure of the sintered samples were investigated by scanning electron microscopy (SEM, HITACHI, TM3000). Average particle sizes were measured by dynamic light scattering (DLS, CILAS 1090, liquid mode).\u003c/p\u003e \u003cp\u003eSubsequently, the powder mixtures were then loaded into a cylindrical matrix with a diameter of 5 mm and compacted through a hydraulic press by cold uniaxial pressing at 200 MPa. The green samples were placed on an alumina support to be sintered in a resistive dilatometric furnace (DIL 402 PC, NETZSCH). The tests were performed with argon flow rate of 2.5 ml/s, heating rate of 10\u0026deg;C/min, constant sintering temperature for 1 h and cooling rate of 20\u0026deg;C/min. Circulating argon was used in the furnace to avoid contamination of the sample during heating and cooling processes. The samples were cooled inside the furnace until they reached room temperature. Sintering took place at two different temperatures: 1300\u0026deg;C and 1550\u0026deg;C. In addition, composites were sintered in a vacuum furnace model Vacuum Industries of CENTORR VI manufacturer, high vacuum with a pressure of 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e Torr, using the same sintering parameters of the resistive dilatometric furnace.\u003c/p\u003e \u003cp\u003eThe theoretical density of the sintered samples (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\rho\\:}_{t}\\)\u003c/span\u003e\u003c/span\u003e = 4.66 g/cm\u003csup\u003e3\u003c/sup\u003e) was determined by the mixing rule (Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), considering the theoretical densities of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\rho\\:{Al}_{2}{O}_{3}\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\rho\\:WC\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\rho\\:Co\\)\u003c/span\u003e\u003c/span\u003e as 3.98 g/cm\u003csup\u003e3\u003c/sup\u003e, 15.63 g/cm\u003csup\u003e3\u003c/sup\u003e and 8.93 g/cm\u003csup\u003e3\u003c/sup\u003e, respectively. The weight percentages are designated by %Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, %WC, and %Co.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{\\rho\\:}_{t}=\\frac{1}{(\\:\\frac{\\%{Al}_{2}{O}_{3}}{\\rho\\:{Al}_{2}{O}_{3}}\\:+\\:\\frac{\\%WC}{\\rho\\:WC\\:}+\\:\\frac{\\%Co}{\\rho\\:Co\\:})}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe bulk density of the sintered samples was measured at room temperature and atmospheric pressure using the Archimedes principle. As established by ASTM B962-13 [50], a sintered sample was weighed in air (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:Wa\\)\u003c/span\u003e\u003c/span\u003e), and then immersed in distilled water and weighed again (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:Ww\\)\u003c/span\u003e\u003c/span\u003e) on a Shimadzu electronic balance (model: AUW220D) with an accuracy of 0.1 mg. The density of distilled water (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\rho\\:}_{w}\\)\u003c/span\u003e\u003c/span\u003e) at room temperature is 1 g/cm\u003csup\u003e3\u003c/sup\u003e. Therefore, the bulk density (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\rho\\:}_{a}\\)\u003c/span\u003e\u003c/span\u003e) can be calculated using Eq.\u0026nbsp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Three repetitive tests per sample were performed and the average value was obtained.\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:{\\rho\\:}_{a}=\\frac{Wa}{(\\:Wa-Ww)}*{\\rho\\:}_{w}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eIn addition, the relative density (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\rho\\:}_{r}\\)\u003c/span\u003e\u003c/span\u003e), Eq.\u0026nbsp;\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, and apparent porosity (P), Eq.\u0026nbsp;4, of the sintered samples can be calculated from the relationship between the theoretical density (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\rho\\:}_{t}\\)\u003c/span\u003e\u003c/span\u003e), Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, and the bulk density (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\rho\\:}_{a}\\)\u003c/span\u003e\u003c/span\u003e), Eq.\u0026nbsp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, as mentioned by references [51,52], follows:\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:{\\rho\\:}_{r}\\left(\\%\\right)=\\frac{{\\rho\\:}_{a}}{{\\rho\\:}_{t}}*100$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:{p}_{\\left(\\%\\right)}=(1-\\frac{{\\rho\\:}_{a}}{{\\rho\\:}_{t}}*100\\)\u003c/span\u003e \u003c/span\u003e) (4)\u003c/p\u003e \u003cp\u003eThe Vickers Microhardness was determined using a DIGIMESS digital micro hardness tester, model: HV-1000 (400.310), under a load of 4.9 N for 10 s at room temperature, considering the average of 5 measurements, according to standard ASTM E384-99 [53].\u003c/p\u003e \u003cp\u003eThe sliding test was performed on a pin-on-disk tribometer (Magnum Engineers, model: TE-165LE, India) with sliding in non-lubricated conditions. The stationary pin using 80 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-18 wt.% WC-2 wt.% Co composite was loaded with an applied load of 10 N. The sliding tests were performed at room temperature. Test duration was 10 min (or 314 m distance) for all trials. The 1020 steel disc was maintained at a contact sliding speed of 0.5 m/s and disc rotation of 500 rpm. Before testing, the 1020 steel disc was polished with sandpaper up to 1200 mesh to reduce roughness.\u003c/p\u003e \u003cp\u003eThe composite wear volume (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:V\\)\u003c/span\u003e\u003c/span\u003e), Eq.\u0026nbsp;\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e5\u003c/span\u003e, is calculated by the mass loss ratio \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:W\\)\u003c/span\u003e\u003c/span\u003e (g), obtained by the differences of initial masses \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:Wi\\)\u003c/span\u003e\u003c/span\u003e and final masses \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:Wf\\)\u003c/span\u003e\u003c/span\u003e in the wear test, and the apparent density of composite \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\rho\\:a\\)\u003c/span\u003e\u003c/span\u003e (g/cm\u003csup\u003e3\u003c/sup\u003e) determined by the Archimedes principle of composite.\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:V=\\frac{\\varDelta\\:W}{\\rho\\:a}=\\frac{Wi-Wf}{\\rho\\:a}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe evaluation of the composite in specific wear rate \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:K\\)\u003c/span\u003e\u003c/span\u003e (mm\u003csup\u003e3\u003c/sup\u003e/N\u0026middot;m) is normally calculated by the volume ratio of worn material \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:V\\)\u003c/span\u003e\u003c/span\u003e (mm\u003csup\u003e3\u003c/sup\u003e) with the test load \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:F\\)\u003c/span\u003e\u003c/span\u003e (N) and the slip distance \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:S\\)\u003c/span\u003e\u003c/span\u003e (m). As described in Eq.\u0026nbsp;\u003cspan refid=\"Equ5\" class=\"InternalRef\"\u003e6\u003c/span\u003e, developed by Archard [54]:\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$$\\:K=\\frac{V}{F.S}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Effect of powder preparation on the crystal structure of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-WC-Co\u003c/h2\u003e \u003cp\u003eThe refined X-ray diffraction patterns of the 80 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-18 wt.% WC-2 wt.% Co composite powders prepared by MM and HEM routes are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (a). As observed, all peaks are characteristic of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (with lattice parameters a\u0026thinsp;=\u0026thinsp;b\u0026thinsp;=\u0026thinsp;4.76 \u0026Aring; and c\u0026thinsp;=\u0026thinsp;12.99 \u0026Aring;, ICSD n\u0026deg; 60419, space group R-3 c H (167)) [55] and WC (with lattice parameters a\u0026thinsp;=\u0026thinsp;b\u0026thinsp;=\u0026thinsp;2.90 \u0026Aring; and c\u0026thinsp;=\u0026thinsp;2.83 \u0026Aring;, ICSD n\u0026deg; 77738, space group P-6 m 2 (187)) [56] phases. Within the limit of the X-ray diffraction technique, the Co phase was not observed due to its low weight content (wt.%) on the composite.\u003c/p\u003e \u003cp\u003eThe crystallite sizes, lattice parameters, percentage of each phase (wt.%) and agreement indexes obtained through Rietveld analysis are assembled in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The maximum values found for \u003cem\u003eRwp\u003c/em\u003e, \u003cem\u003eRexp\u003c/em\u003e and \u003cem\u003eχ\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e were 31.82%, 26.95% and 1.18, respectively. The low \u003cem\u003eχ\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e values indicate good agreement between experimental data and refined models. Figures\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (b, c) show the magnifications of the main peaks of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (113) and WC (011) of the samples prepared by MM and HEM. As observed, three changes are observed in the X-ray patterns in both phases: peak shift to higher angles, intensity reduction, and broadening of the full width at half maximum. The shift of the peaks to higher angles indicates a reduction in the lattice parameters (as shown in lattice parameters \"a\" of both phases in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The reduction in intensity and the broadening of the full width at half maximum are consequences of the high microstructural refinement, reduction of the crystallite size, and increased crystalline micro-deformation. The obtained lattice parameters are similar to those of the initial Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and WC powders. The crystallite sizes of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and WC phases reduced after HEM for 50 h and reached 27.47 nm and 30.55 nm, respectively, while the microstrains increased and reached values of 0.25% (Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) and 0.21% (WC). According to results observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (b) and Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase is the most affected by HEM.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eTable 1.\u003c/strong\u003e Crystallite size (D\u003csub\u003eXRD\u003c/sub\u003e), lattice parameters (a), quantitative phase analysis, and Rietveld agreement indexes for samples obtained by MM and HEM. Data in square brackets correspond to the mass fraction of each phase.\u003c/p\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"747\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd rowspan=\"2\" style=\"width: 6.2035%;\"\u003e\n \u003cp\u003eSample\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" valign=\"top\" style=\"width: 11.2726%;\"\u003e\n \u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" valign=\"top\" style=\"width: 19.0535%;\"\u003e\n \u003cp\u003eWC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" style=\"width: 10.989%;\"\u003e\n \u003cp\u003eRietveld agreement\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 8.9507%;\"\u003e\n \u003cp\u003eD\u003csub\u003eXRD\u003c/sub\u003e (nm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 5.4945%;\"\u003e\n \u003cp\u003ea (\u0026Aring;)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 5.5831%;\"\u003e\n \u003cp\u003ec (\u0026Aring;)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"\" valign=\"top\" style=\"width: 10.4573%;\"\u003e\n \u003cp\u003eD\u003csub\u003eXRD\u003c/sub\u003e (nm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 4.8742%;\"\u003e\n \u003cp\u003ea (\u0026Aring;)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 4.8742%;\"\u003e\n \u003cp\u003ec (\u0026Aring;)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"\" valign=\"top\" style=\"width: 4.6969%;\"\u003e\n \u003cp\u003eRwp (%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 4.6969%;\"\u003e\n \u003cp\u003eRexp (%)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 3.279%;\"\u003e\n \u003cp\u003e\u0026chi;\u0026sup2;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 6.2035%;\"\u003e\n \u003cp\u003eMM\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 10.9004%;\"\u003e\n \u003cp\u003e131 [91.29%]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 5.4945%;\"\u003e\n \u003cp\u003e4.7594\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 5.5831%;\"\u003e\n \u003cp\u003e12.9933\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"\" valign=\"top\" style=\"width: 10.4573%;\"\u003e\n \u003cp\u003e60.3 [8.71%]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 4.8742%;\"\u003e\n \u003cp\u003e2.9060\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 4.8742%;\"\u003e\n \u003cp\u003e2.8387\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"\" valign=\"top\" style=\"width: 4.6969%;\"\u003e\n \u003cp\u003e31.82\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 4.6969%;\"\u003e\n \u003cp\u003e26.95\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 3.279%;\"\u003e\n \u003cp\u003e1.18\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 6.2035%;\"\u003e\n \u003cp\u003eHEM\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 10.9004%;\"\u003e\n \u003cp\u003e27.47 [88.38%]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 5.4945%;\"\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp;4.7592\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 5.5831%;\"\u003e\n \u003cp\u003e12.9988\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"\" valign=\"top\" style=\"width: 10.4573%;\"\u003e\n \u003cp\u003e30.55 [11.620%]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 4.8742%;\"\u003e\n \u003cp\u003e2.9058\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 4.8742%;\"\u003e\n \u003cp\u003e2.8387\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"\" valign=\"top\" style=\"width: 4.6969%;\"\u003e\n \u003cp\u003e27.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 4.6969%;\"\u003e\n \u003cp\u003e24.56\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 3.279%;\"\u003e\n \u003cp\u003e1.11\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Effect of powder preparation method on microstructural and morphological properties\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the morphology and particle size curves of the powders (80 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-18 wt.% WC-2 wt.% Co) prepared by high-energy milling for 50 h and mixed for 10 min. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (a) of the powders mixed for 10 min shows that original shape of the particles was maintained. There was no fragmentation of the brittle phases Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (dark phase) and WC (bright phase) as well as welding of the brittle phases into the ductile matrix (Co). According to the particle distribution curve in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (b), the average particle diameter is 4.21 \u0026micro;m, an intermediate value between the particle diameter of the initial powders, indicating that it was a mixture, as shown in the SEM image. During HEM for 50 h, the powders were subjected to high-energy collisions, so that the ceramic particles (Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, WC) were fragmented and incorporated into the ductile matrix phase (Co). The impregnation of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and WC on the Co metal surface produces composite Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-WC-Co particles. The powders underwent successive cycles of plastic deformation, fracture, and cold welding, which caused dispersion and homogenization of the constituent phases of the composite powders, as observed by SEM (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (c)). The particle distribution curve of the powder milled for 50 h (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (d)) reveals that the average particle diameter was 2.02 \u0026micro;m, a 52% decrease compared to those mixed for 10 min, indicating that HEM significantly decreased the particle size of the powders.\u003c/p\u003e\u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Structural characterization of sintered composites\u003c/h2\u003e \u003cp\u003eXRD of the surface of composites sintered at 1300 and 1550\u0026deg;C are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. As observed, three phases are identified: Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, WC and W\u003csub\u003e2\u003c/sub\u003eC. When compared with the phases of the powders prepared by MM and HEM, there is phase transformation after sintering, i.e., the arising of the W\u003csub\u003e2\u003c/sub\u003eC phase. The Cobalt phase is not observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, and this may be related to the low cobalt content which is not included in the detection range of XRD. The phases identified in this work agree with previous reports. The crystallite sizes of the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase were determined to be 123, 141, 92, and 115 nm for composites sintered with MM and HEM powders at 1300\u0026deg;C and 1550\u0026deg;C, respectively. As expected, an increase in the crystallite size of this phase occurs. However, the sintered specimens prepared by HEM presented smaller crystallites than those processed by MM.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Microstructure of sintered composites\u003c/h2\u003e \u003cp\u003eSEM images of sintered samples derived from powders mixed for 10 min and milled for 50 h at 1300\u0026deg;C and 1550\u0026deg;C are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (a, b) and Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (c, d), respectively. The samples sintered at 1300\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (a)) and 1550\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (b)) prepared by mechanical mixing exhibit less distributed grains in the microstructures of the samples, as well as less consolidated sintered phases. Meanwhile, the sintered samples prepared by high-energy milling for 50 h (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (c) and (d)) exhibit uniform grain distribution as well as better microstructural refinement. Besides, high-energy milling provided a decrease in the average particle sizes of the milled powders compared to the mixed powders, which caused an improvement in the sinterability of the milled powders. For the temperature of 1550\u0026deg;C in Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (b, d), the sintered samples underwent a complete sintering process, characterized by the formation of more widely distributed and uniform binder regions in the microstructures, especially when milled for 50 h. In fact, the eutectic temperature of cobalt is reached, leading to the formation of a liquid phase, causing melting and fluidity through the structure, filling the remaining pores, resulting in higher densification of the composite. Higher grain growth is also observed compared to the samples sintered at 1300\u0026deg;C. These figures demonstrate that powder preparation and sintering temperature have a strong influence on the formation of a more uniform microstructure in the sintered samples.\u003c/p\u003e \n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Relative density and apparent porosity of sintered composites\u003c/h2\u003e \u003cp\u003eThe relative density (Eq.\u0026nbsp;\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) and apparent porosity (Eq.\u0026nbsp;4) of samples sintered in a resistive dilatometric furnace at 1300\u0026deg;C and 1550\u0026deg;C are shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. According to the obtained results, the relative densities and apparent porosities are strongly influenced by the powder preparation and sintering temperature. Indeed, high energy milling can cause (1) a decrease of interparticle distance, (2) a high phase interaction, (3) a uniform dispersion of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and WC powders in the ductile Co matrix, and (4) a decrease of average particle sizes, this last feature increases the surface area and improves the sinterability of the composite.\u003c/p\u003e \u003cp\u003eThus, the relative density was higher in the samples prepared by high-energy milling (1550\u0026deg;C; \u003cem\u003eρ\u003c/em\u003e mixed 67.44%, \u003cem\u003eρ\u003c/em\u003e milled 80.85%), probably due to their average diameter around 2.02 \u0026micro;m, 2.1 times smaller than that of the mixed particles. On the other hand, at 1550\u0026deg;C, comparing both powder preparation methods (\u003cem\u003eρ\u003c/em\u003e mixed 67.44%, \u003cem\u003eρ\u003c/em\u003e milled 80.85%), the sintered samples achieved higher relative density and lower porosity than at 1300\u0026deg;C (\u003cem\u003eρ\u003c/em\u003e mixed 59.25%, \u003cem\u003eρ\u003c/em\u003e milled 62.49%). Indeed, the relative density is also influenced by the sintering temperature, once increasing this temperature decreases porosity, increases densification, and results in denser composites [57,58]. These values are comparable to those of composites from literature with similar compositions. Fazili et al. [59] found that a WC-6 wt% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e cermet sintered at 1350\u0026deg;C exhibited total densification of 67%. Leal et al. [39] concluded that WC-10 wt% Co-10 wt% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e composites prepared by mechanical mixing and high energy milling achieved a total relative density of 68% after sintering at 1550\u0026deg;C. The densification of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-based immiscible systems is a challenging task, especially for powder compaction followed by sintering. Therefore, some studies have shown that sintering at high temperatures (1700\u0026deg;C \u0026minus;\u0026thinsp;1900\u0026deg;C) or the use of special sintering techniques such as hot vacuum pressing and spark plasma sintering (SPS) are necessary to obtain Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e composites -WC-Co totally dense [10,28,60].\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\u003eRelative density and apparent porosity of the 80 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-18 wt.% WC-2 wt.% Co composite. Data obtained from a batch of three samples.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\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 \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePreparation technique\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSintering temperature\u003c/p\u003e \u003cp\u003e(\u0026ordm;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eApparent Porosity\u003c/p\u003e \u003cp\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRelative Density\u003c/p\u003e \u003cp\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMechanical mixing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e39.10\u0026thinsp;\u0026plusmn;\u0026thinsp;0.73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e59.25\u0026thinsp;\u0026plusmn;\u0026thinsp;1.51\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMechanical mixing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1550\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e31.86\u0026thinsp;\u0026plusmn;\u0026thinsp;0.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e62.49\u0026thinsp;\u0026plusmn;\u0026thinsp;1.53\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHigh-energy milled\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e31.73\u0026thinsp;\u0026plusmn;\u0026thinsp;0.81\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e67.44\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHigh-energy milled\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1550\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e19.15\u0026thinsp;\u0026plusmn;\u0026thinsp;1.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e80.85\u0026thinsp;\u0026plusmn;\u0026thinsp;1.11\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 results of relative density of the sintered composites by vacuum furnace at 1300\u0026deg;C and 1550\u0026deg;C are presented in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Composites sintered by vacuum furnaces achieved better relative density results than those obtained by conventional furnaces can be observed. Sintering in a vacuum furnace has several advantages such as uniformity of heating and cooling with beneficial effects on dimensional variation, uniform microstructure and absence of oxidation, lower risk of crack development, ease of monitoring and electronic recording of process information [61]. In addition, sintering in vacuum furnaces is often used in the state of liquid phase sintering. This process is accompanied to obtain maximum densification of the sintered parts. For the sintering of cemented alloy powders, high temperatures above the melting point of metal binder are normally required [62,63].\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eRelative density and apparent porosity of 80 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-18 wt.% WC-2 wt.% Co composites sintered in a vacuum furnace.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\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 \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePreparation technique\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSintering temperature\u003c/p\u003e \u003cp\u003e(\u0026ordm;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eApparent Porosity\u003c/p\u003e \u003cp\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRelative Density\u003c/p\u003e \u003cp\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMechanical mixing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e40.75\u0026thinsp;\u0026plusmn;\u0026thinsp;1.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e60.90\u0026thinsp;\u0026plusmn;\u0026thinsp;0.73\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMechanical mixing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1550\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e37.51\u0026thinsp;\u0026plusmn;\u0026thinsp;1.53\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e68.14\u0026thinsp;\u0026plusmn;\u0026thinsp;0.65\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHigh-energy milled\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e32.56\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e68.27\u0026thinsp;\u0026plusmn;\u0026thinsp;0.81\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHigh-energy milled\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1550\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8.63\u0026thinsp;\u0026plusmn;\u0026thinsp;0.59\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e91.37\u0026thinsp;\u0026plusmn;\u0026thinsp;0.59\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.6 Dilatometric shrinkage\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the dilatometric curves for each composite sample (80 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-18 wt.% WC-2 wt.% Co) sintered from room temperature (25\u0026deg;C) to sintering temperatures of 1300 and 1550\u0026deg;C, with a constant sintering plateau of 1 h. The sintering of the milled (HEM) and mechanically mixed (MM) samples up to the temperature of 1300\u0026deg;C is related to the solid-state diffusion processes such as grain boundary diffusion, surface diffusion and volumetric diffusion [64]. With the increase of sintering temperature to 1550\u0026deg;C the eutectic point of cobalt (1495\u0026deg;C) is reached; thus the solid structure continuously disappears during the onset of liquid phase formation promoted by cobalt. The structure contracts rapidly due to the capillary force caused by the still open pores and the meniscus at the edges of the sample, as long as there is sufficient liquid formation. This stage only ends with the densification of the sample or when the capillary force responsible for closing the pores reaches insufficient levels to promote the closure of the remaining pores [65,66]. This dilatometric shrinkage behavior was also observed by Acchar et al. [67] in pure alumina and niobium carbide reinforced alumina composites, where the dilatometric curves showed two distinct regions. The first refers to the temperature range before the onset of material shrinkage. One can cite that in this region the alumina has not yet begun to densify, and the material shows only rearrangement, coalescence, and early formation of the particle contact point. The second region can be associated with the shrinkage of the material, starting at approximately 1100\u0026deg;C.\u003c/p\u003e \u003cp\u003eTherefore, with increasing the sintering temperature to 1550\u0026deg;C the mixed and milled samples exhibited greater shrinkage. The samples processed by HEM for 50 h experienced the highest dilatometric shrinkage, indicating an average variation from the starting point of approximately \u0026minus;\u0026thinsp;0.082, thus confirming the improved densification of this sintered sample. However, the samples mixed for 10 min and sintered at 1550\u0026deg;C experienced approximate dilatometric shrinkage of -0.035. The obtained results of relative density of the mixed and milled samples confirm an increasing tendency with increasing sintering temperature for alumina-based composites [68\u0026ndash;70].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.7 Mechanical properties\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows Vickers Microhardness of the samples sintered by vacuum furnaces. As can be seen in the graph, the Vickers Microhardness suffers positive influence with high grinding. According to Raimundo [71], the improvement of performance in mechanical properties of microhardness promoted by grinding is caused by factors that follow: decreased distance between particles, high interaction of phases, uniform dispersion of ceramic powders in the metal matrix of Co, and decrease of the average particle sizes, this last characteristic tends to increase the surface area, considerably improving the sintering of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-WC-Co composites. The composites prepared by HEM showed Vickers Microhardness values of 11.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.85 GPa (1300\u0026deg;C) e 20.61\u0026thinsp;\u0026plusmn;\u0026thinsp;0.85 GPa (1550\u0026deg;C) while composites prepared by MM obtained values of 5.35\u0026thinsp;\u0026plusmn;\u0026thinsp;0.85 GPa and 8.35\u0026thinsp;\u0026plusmn;\u0026thinsp;0.85 GPa for temperatures of 1300\u0026deg;C and 1550\u0026deg;C, respectively. According to the literature, the microhardness values reported for Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e can vary between 20.6 e 29.4 GPa, using powerful sintering techniques [72]. The results of Microhardness Vickers of powder ground for 50 h and consolidated to 1550\u0026deg;C are superior compared to various literature works. Zawrah and Taha [73] achieved Microdrureza Vickers values of 9\u0026ndash;13 GPa of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e-Ni composites sintered in a tubular furnace at 1500\u0026deg;C. Zhu et al. [74] achieved 17.13\u0026ndash;17.86 GPa of Vickers Microhardness in WC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-Ni composites sintered in a vacuum furnace (ZT-40-20YB) at 1540\u0026deg;C. Mariana et al. [75] sintered via SPS composites WC-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e to 1550\u0026deg;C and obtained Vickers Microhardness between 16.9\u0026ndash;19 GPa.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.8 Tribological properties\u003c/h2\u003e \u003cp\u003eNotably, the size of particles prepared by different powder preparation techniques (MM and HEM) plays an important role in the wear behavior of sintered composites, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. In the initial softening period (from 0 to 80 s), for both methods of powder preparation, the contact surface suffers plastic deformation due to small roughness that increases the surface roughness of the initial materials, causing higher contact pressures which leads to debris removal from tribological pair, resulting rapid increase of the friction coefficient curve with significant fluctuations. This behavior is also observed in our previous studies in Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-WC-Co and SiC-WC-Co composites, obtained by Spark plasma Sintering [28,76]. After the softening period, combination of friction and temperature favors adhesive phenomena between the surfaces of the tribological pair. This step is characterized by the formation of an adhesion layer in 1020 steel caused by continuous sliding of the pin (composite) over previously worn sections, leading to a sliding process where surfaces of the same material (tribofilm) are in touch with each other. The atmosphere where the wear test happens also influences the adhesion layer, because molecules of water or oxygen present in the ambient air creates a film on the surface of tribological pairs [77,78].\u003c/p\u003e \u003cp\u003eFor composites prepared by HEM, nanoparticles evenly distributed by the action of the powder preparation technique play an essential role in the friction process, since it converts sliding friction into rolling friction to prevent adhesive wear and preventing crack propagation, as noted by Zhengjun et al. [79] in Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-Fe-Al composites. Thus, the curves of friction coefficient reveal smooth fluctuation of the mean values 0.52\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 and 0.49\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 for composites prepared by HEM at 1300\u0026deg;C and 1550\u0026deg;C, respectively. On the other hand, composites with larger particle size prepared by MM have a decrease in wear resistance, leading to a drastic fluctuation of the friction coefficient curve with a mean of 0.58\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06 and 0.54\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04 for sintered composites at 1300\u0026deg;C and 1550\u0026deg;C, respectively. This can be explained by the low interaction between the particles, leading to the detachment of ceramic particles (Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and WC) and the formation of numerous wear debris. Composites with higher microhardness and relative densities obtained lower coefficients of friction, this is beneficial for tribological applications, since they can present high hardness even at high temperatures, good thermal stability and excellent wear resistance, allowing better performances during use [80].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThis work compared two techniques for powder preparation of the 80 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-18 wt.% WC-2 wt.% Co composite: mechanical mixing for 10 min and high-energy milling for 50 h, associated with two sintering temperatures, 1300 and 1550 \u0026deg; C. It was observed that mechanical mixing preserves the intrinsic characteristics of the starting powders; the particles do not fracture, however, they remained clustered. On the other hand, after 50 h of high-energy milling, the particles of the ceramic phases (Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, WC) were refined, deformed, and welded into the Co metal matrix. HEM also promoted dispersion and homogenization of the powders. Besides, the average particle sizes of the milled powders decreased in comparison to the mixed powders, which caused an improvement in its sinterability. The best values of relative density and dilatometric shrinkage were obtained for the samples milled for 50 h and further sintered at 1550\u0026deg;C (91.37% and \u0026minus;\u0026thinsp;0.082, respectively), confirming that the liquid phase formation promoted by cobalt caused greater pore filling, resulting in greater densification of the composite. In addition, the powder preparation method also strongly influenced the microstructure, densification, mechanical and tribological properties of the composites. The results of mechanical and tribological tests showed that composites processed by HEM were promising materials for devices subject to wear.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eConflicts of Interest:\u003c/h2\u003e \u003cp\u003eThe authors declare that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its out-come.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding:\u003c/h2\u003e \u003cp\u003eGeneral financial support received from CAPES. No interference with study design and data analysis.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eCredit Author StatementHeytor V.S.B. Azev\u0026ecirc;do: Conceptualization, Validation, Methodology, Software, Investigation, Writing \u0026ndash; original draft, Writing \u0026ndash; review \u0026amp; editing. Rafael A. Raimundo: Conceptualization, Validation, Software, Methodology, Investigation, Writing \u0026ndash; review \u0026amp; editing. Lu\u0026iacute;s M.F. Morais: Methodology, Investigation, Writing \u0026ndash; review \u0026amp; editing. Cleber S. Louren\u0026ccedil;o: Methodology, Writing \u0026ndash; review \u0026amp; editing. Nailton T. C\u0026acirc;mara: Methodology, Investigation, Writing \u0026ndash; review \u0026amp; editing. Daniel A. Macedo: Methodology, Investigation, Writing \u0026ndash; review \u0026amp; editing. Danielle G.L. Cavalcante: Methodology, Investigation, Writing \u0026ndash; review \u0026amp; editing. U\u0026iacute;lame U. Gomes: Conceptualization, Validation, Methodology, Software, Investigation, Supervision, Writing \u0026ndash; original draft, Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\u003ch2\u003eAcknowledgments:\u003c/h2\u003e \u003cp\u003eThis research was supported by the Coordena\u0026ccedil;\u0026atilde;o de Aperfei\u0026ccedil;oamento de Pessoal de N\u0026iacute;vel Superior \u0026ndash; Brazil (CAPES) \u0026ndash; Finance Code 001.\u003c/p\u003e\u003ch2\u003eData availability:\u003c/h2\u003e \u003cp\u003eThe raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of a continuing study.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eS. Said, S. Mikhail, M. Riad, Recent processes for the production of alumina nano-particles, Mater. Sci. Energy Technol. 3 (2020) 344\u0026ndash;363. https://doi.org/10.1016/j.mset.2020.02.001.\u003c/li\u003e\n\u003cli\u003eR. 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II, Elsevier, 2017: pp. 165\u0026ndash;203. https://doi.org/10.1016/B978-0-12-803581-8.09795-2.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Composites, Mechanical Mixing, High-Energy Milling, Mechanical properties, Tribological properties","lastPublishedDoi":"10.21203/rs.3.rs-5545783/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5545783/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn the present work, the influence of the powder preparation technique on the sintering behavior of 80 wt.% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-18 wt.% WC-2 wt.% Co composites was investigated. The powders were mixed with a mechanical stirrer for 10 min (MM) and by high-energy milling (HEM) for 50 h. They were then compacted at 200 MPa and sintered in a resistive dilatometric furnace for 1 h under argon atmosphere at a heating rate of 10\u0026deg;C/min for two sintering temperatures (1300\u0026deg;C and 1550\u0026deg;C). The powders prepared by MM and HEM were further characterized by XRD, SEM and dynamic light scattering (DLS), while the sintering kinetics was evaluated by dilatometry. The powders processed by HEM presented better dispersion and homogenization than those obtained by mechanical mixing (MM). The HEM approach enhances the sintering of immiscible systems that present low sinterability, as well as improves dispersion, phase refining and produces composite particles with greater interaction of the milled elements. In addition, HEM decreases the particle/crystallite size and increases the microstrain. The results showed that composites prepared by HEM and sintered at 1550 \u0026ordm;C presented better densification, Vickers microhardness and tribological behavior.\u003c/p\u003e","manuscriptTitle":"Influence of powder preparation technique on microstructure, densification, mechanical and tribological properties of composites based on Al₂O₃ with addition of WC-Co hard metal","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-03 07:00:11","doi":"10.21203/rs.3.rs-5545783/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"e85d8ded-662d-4921-a146-f85d69416b75","owner":[],"postedDate":"February 3rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-02-25T12:08:30+00:00","versionOfRecord":[],"versionCreatedAt":"2025-02-03 07:00:11","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5545783","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5545783","identity":"rs-5545783","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}