The lubricity of gases

The lubricity of gases | 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Download PDF Research Article The lubricity of gases Jie Zhang, Janet Wong, Hugh Spikes This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4445568/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 28 Aug, 2024 Read the published version in Tribology Letters → Version 1 posted 9 You are reading this latest preprint version Abstract A sealed reciprocating tribometer has been used to study the influence of different gaseous environments on the friction and wear properties of AISI52100 bearing steel at atmospheric pressure and 25°C. Helium, argon, hydrogen, carbon dioxide and nitrogen all give high friction and wear, suggestive of very little, if any tribofilm formation under the conditions studied. Dry air and oxygen also give high friction, slightly lower than the inert gases, but produce extremely high wear, much higher than the inert gases. This is characteristic of the phenomenon of “oxidational wear”. The two gases ammonia and carbon monoxide give relatively low friction and wear, and XPS analysis indicates that this is due to the formation of adsorbed ammonia/nitride and carbonate films respectively. For the hydrocarbon gases studied, two factors appear to control friction and wear, degree of unsaturation and molecular weight. For the saturated hydrocarbons, methane and ethane give high friction and wear but propane and butane give low friction after a period of rubbing that decreases with molecular weight. The unsaturated hydrocarbons all give an immediate reduction in friction with correspondingly low wear. Raman analysis shows that all the hydrocarbons that reduce friction and wear form a carbonaceous tribofilm of the rubbed surfaces. Gas lubrication ammonia carbon monoxide friction wear Raman XPS Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 1. Introduction Although most lubricated systems involve the use of a liquid or grease lubricant, there are some systems where the lubricating medium is a gas or vapour. This is the case, for example, in air bearings, gas processing, refrigeration, and when pumping and injecting gaseous fuels into a combustion chamber. In such systems, although coatings may be used to provide some degree of solid lubrication, the ability of the gas itself to provide protective lubricating films is potentially important. Study of the lubricating properties of relatively benign or inert gases such as air, carbon dioxide, nitrogen, and argon is quite straightforward. However, study of combustible or toxic gases such as methane, hydrogen and ammonia is more problematic and this may have constrained investigation into their lubricating properties in the past. To address this, the current paper employs a new, sealed, low volume tribometer to investigate the lubricating properties of a range of different gases of tribological interest. 2. Background When metallic surfaces are rubbed together in the absence of a liquid, grease or solid lubricant, interactions of the gaseous environment with the rubbing metal surfaces become of paramount importance in determining tribological performance. Such interactions can involve adsorption of gaseous molecules [ 1 – 4 ] and/or their chemical reaction with the surfaces, for example to form metal oxides from molecular oxygen [ 5 , 6 ] or carbonates from carbon dioxide [ 7 , 8 ]. Oxygen By far the most practically important effect of atmosphere stems from the presence or absence of molecular oxygen. This can be beneficial or deleterious. Thus, several researchers have found that the addition of low levels of oxygen can greatly reduce the high friction and wear observed when ferrous surfaces are rubbed in vacuum or in oxygen-free gases such as nitrogen, argon and hydrogen [ 4 , 2 , 9 , 10 ]. It is also widely accepted that the formation of oxide layers on steel provides general protection against adhesive wear [ 6 , 11 ]. On the other hand, higher wear levels of steel in air (21% O 2 ) as compared to inert gases have been reported in both sliding and fretting wear [ 12 – 14 ] and ascribed to the formation of easily removed or abrasive oxides. It appears that the impact of molecular oxygen on friction and wear is strongly dependent on steel hardness and on rubbing conditions such as load and the degree of sliding [ 15 ]. Other gases Several researchers have studied the properties of nominally oxygen-free gas environments on friction and wear. Most of this work has suggested that H 2, N 2 and inert gases (He, Ar) possess little intrinsic lubricity and impart high friction and wear to sliding steel/steel contacts [ 10 , 16 – 18 ]. Some authors have noted slightly lower friction of bearing steel in N 2 than in Ar and proposed that this might originate from the formation of iron or chromium nitride [ 10 , 17 ]. However, to date surface analysis has failed to detect any such nitrides formed on rubbing steel surfaces from a nitrogen atmosphere. One problem in comparing the friction and wear properties of very low lubricity gases is to be confident that small proportions of O 2 at the sub-100 ppm level, that can be present in some cylinder gases, are not influencing the observed behaviour. In corrosion studies it has been shown that oxide films start to form very rapidly on ferrous surfaces at room temperature at an oxygen partial pressure of 0.01 Pa, which, as a contaminant in inert gas, is equivalent to less than 1 ppm O 2 [ 19 , 5 ]. Mishina has also suggested that very low concentrations of O 2 , N 2 and H 2 in the Pa pressure range can significantly influence friction and wear by adsorbing on rubbing iron surfaces to promote the formation of non-adherent wear debris and thus wear [ 3 , 4 ]. Several researchers have explored the friction properties of metal surfaces rubbed in a CO 2 atmosphere, both because of the latter’s use in some nuclear reactors and due to its role as a refrigerant gas [ 7 , 8 , 16 – 18 , 20 ]. All have found CO 2 to give lower friction and wear than inert gases under some conditions, and surface analysis using XPS has indicated the formation of a ferrous carbonate tribofilm on and around the rubbed steel surfaces. There appears to have been relatively little work on the influence of gases other than the above on steel-steel friction and wear. Murray et al . studied the friction and wear properties of rubbing contacts in a range of gaseous fluoro- and chlorohydrocarbons and found that some of the latter, notably dichlorodifluoromethane, CCl 2 F 2 , gave low friction and wear [ 21 ]. Surface analysis showed the formation of a chloride tribofilm. Ramirez et al . showed that methane, CH 4 , formed a friction-reducing, graphene-like carbonaceous film on rubbing VN-Ni and VN-Cu coated AISI 52100 steel surfaces, but not on uncoated steel, suggesting a catalytically-driven tribochemical reaction [ 22 ]. However very recently the authors of the current paper used a sealed tribometer to show that propane gas formed friction-reducing carbon-based films on rubbing 52100 steel surfaces at room temperature [ 23 ]. Research has also been carried out on the ability of a range of carbon-based compounds including carbon monoxide, ethyne and butane, to form tribofilms on nickel and ceramic surfaces, both in the context of vapour phase lubrication and for fundamental study of tribochemical mechanisms [ 24 – 26 ]. While it is very straightforward to design and operate sealed tribometers to study the friction and wear properties of inert gases or relatively benign ones such as dry air and CO 2 , where health and safety concerns are minimal, this is not the case when studying combustible gases such as CH 4 and H 2 , or toxic ones such as NH 3 and CO. This may have constrained research on the lubricity of these latter gases. However, a low volume, sealed tribometer has recently become commercially available which makes it relatively straightforward to study the friction and reducing properties of a wide range of gases. This paper describes the application of this tribometer to study the ability of a range of nominally pure gases to lubricate rubbing steel-steel contacts. 3. TEST METHODS AND MATERIALS 3.1 HPR tribometer tests This study employs a fully sealed, reciprocating tribometer, the HPR, supplied by PCS Instruments, UK. This reciprocates a 6 mm diameter ball against the flat surface of a stationary disc and employs the same drive motor, test configuration and ball and disc test specimens as the conventional high frequency reciprocating rig (HFR). However, the reciprocating contact is fully enclosed in a stainless-steel cylinder. To achieve this, the shaft that drives the upper ball holder in reciprocating motion and is loaded externally by dead weights is sealed against the cylinder by two flexible bellows. The lower disc holder is flexibly-mounted and attached to an external force transducer to monitor friction via a thin metal membrane in the cylinder wall. The cylinder wall is also fitted with gas inlet and outlet ports, a break valve and a pressure-measuring gauge and pressure relief valve. The HPR can be used up to 130°C and 10 bar pressure but in this study most tests were carried out at 1 bar pressure and 25°C. The small size of this test rig enables its use in a fume cupboard, while its small internal volume of ca 20 ml facilitates the safe study of gases such as hydrogen, methane and ammonia. An additional benefit of this small internal volume is that it enables rapid switching of gases, which can be used to estimate the kinetics of formation and loss of tribofilms, as described later in this paper. In the current study, gases were supplied to the test chamber at a flow rate of 0.1 L/min using PC-controlled flow controllers, in both pure gas tests and gas switching tests. For the latter, two flow controllers were connected to two separate gas lecture bottles at one end and to two Swagelok valves at the other. By toggling the Swagelok valves, the gas supply could be switched between the two gases. The two flow controllers were both the MC series from Alicat Scientific, with accuracy at ± 0.6% of reading or ± 0.1% of full scale, whichever is greater. The overall set-up is shown in Fig. 1 while Fig. 2 shows a schematic of the gas supply system and rig. The test conditions and characteristics of the ball and disc test specimens are listed in Table 1 . Table 1 HPR test conditions Applied load 1.96 N (200 g), p max = 0.82 GPa Stroke length 1 mm Stroke frequency 50 Hz Max sliding speed (mid stroke) 0.157 m/s Test duration in single gas tests 75 minutes Test temperature 25°C ± 0.5°C Test pressure 1 bar Ball roughness, Rq 9.9 ± 1.2 nm Disc roughness, Rq 4.8 ± 0.8 nm Ball hardness 790 HV Disc hardness 800 HV Although all tests were carried out at 25°C it is important to consider possible effects of flash temperature rise within the contact during rubbing, especially when friction coefficients are high. Analysis indicated that the mean flash temperature rise in the contact at mid-stroke subject to the conditions in Table 1 and prior to any wear is DT fmean = 61µ where µ is the friction coefficient [ 27 , 28 ]. 3.2 Test gases The gases tested in this study are listed in Table 2 . These were supplied by BOC (Woking, UK) or CK gas (Newtown Unthank, UK). Hydrogen, ammonia and the hydrocarbons are of interest as possible combustion gases, while carbon dioxide, ammonia and propane are used in refrigeration. Table 2 Gases tested in this study Gas Supplier Purity Dry air N 2 , 21% O 2 BOC Oxygen O 2 CK Gas 99.999% Nitrogen N 2 BOC > 99.998% O 2 free Nitrogen N 2 CK Gas > 99.9999% Hydrogen H 2 BOC/CK Gas > 99.99% / >99.999% Argon Ar BOC > 99.998% Helium He BOC > 99.996% Ammonia NH 3 CK Gas 99.98% Carbon dioxide CO 2 CK Gas 99.995% Carbon monoxide CO CK Gas 99.97% Methane CH 4 CK Gas 99.9995% Ethane CH 3 CH 3 CK Gas 99.5% Propane CH3CH 2 CH 3 CK Gas 99.5% n-butane CH 3 CH 2 CH 2 CH 3 CK Gas 99.5% isobutane CH 3 CH(CH 3 )CH 3 CK Gas 99.5% Ethene CH 2 = CH 2 CK Gas 99.5% Propene CH 3 CH = CH 2 CK Gas 99.5% Ethyne CH ≡ CH BOC > 98.5% 3.3 Surface analysis methods Since the tests were run in dry conditions, post-test solvent cleaning prior to surface analysis was not necessary. For some tests with high wear rate, the disc wear track was partially covered by loose wear debris. In this case a gentle blow using N 2 gas was employed to remove this debris. All the disc specimens were immediately stored in 3 ml glass vials after disassembly from the test chamber. These vials were then promptly purged with N 2 gas to create an inert atmosphere, to avoid contamination and oxidation of the discs before any subsequent analysis. Confocal Raman spectroscopy was used to analyse disc surfaces from HPR tests. This employed an Alpha 300 RA (WITec, Germany) with a 532 nm laser source, a 600 lines/mm grating with 500 nm blaze and a Zeiss EC Epiplan 20× objective lens. Spectra were taken with an integration time of 0.5 s for 100 accumulations, giving a total acquisition time of 150 s. The laser power was ca 3 mW. Some HFR disc surfaces were analysed at end of test using XPS (Thermo Fisher K-Alpha spectrophotometer). Spectra were acquired from a 50 µm × 50 µm area. The XPS high resolution spectra were calibrated by using C1s as a reference at 284.8 eV. 4. RESULTS In this section the friction and wear results for each gas are presented and briefly described. Consideration of the origins of the behaviour observed in terms of wear mechanisms and tribofilm formation is deferred to the Discussion section. 4.1 Individual gases All gases were tested at least twice and friction traces and wear results for all individual tests are included in Supplementary Information. In the friction graphs below, individual, representative friction traces are shown for clarity. Figure 3 compares friction coefficient traces in dry air, He, Ar, N 2 and H 2 at 1 bar pressure and 25°C while Fig. 4 shows traces for dry air, O 2 , CO, CO 2 and NH 3 . Dry air is included in both graphs for comparative purposes. All gases except for CO and NH 3 show high friction coefficient in the range 0.8 to 1.2. He, Ar, N 2 , He and H 2 give friction coefficient values of ca 1 to 1.2 while dry air and O 2 give slightly lower friction than these gases. N 2 also gives marginally lower friction than the inert gases, especially towards the end of test. Carbon monoxide and ammonia give much lower friction than the other gases shown. The trace for CO is very noisy and this was also seen in two repeat tests (see Supplementary Information). Figures 5 and 6 show friction coefficient traces from tests with saturated and unsaturated hydrocarbon gases respectively. Note that the friction scale in Fig. 6 is different from that in Figs. 3 to 5 . For the saturated hydrocarbons, methane and ethane show high friction, similar to N 2 , H 2 and the inert gases, suggesting that they do not form effective tribofilms under these conditions. There is however some indication of a slight reduction of friction for ethane in the later stage of the test shown and this was also seen in a repeat test. Propane shows a pronounced friction reduction to a quite low value after about 30 minutes rubbing and this was also seen in a repeat test. n-butane and isobutane both show very rapid friction reduction to values of ca 0.2. These friction drops occur between 1 and 6 minutes after the start of a test. The unsaturated hydrocarbons, ethene and propene show low friction coefficients of ca 0.2 throughout the test, with no initial high friction at the start as seen with the butanes. Ethyne gives slightly a higher friction coefficient of ca 0.27 but again this low friction is established at the start of the test, indicative of very rapid tribofilm formation. Figure 7 summarises friction and wear results from all tests, showing the average values from all repeats for each gas. There is generally good correlation between friction and wear reduction, except for dry air, oxygen and propane. Dry air and oxygen show high friction, slightly lower than the inert gases, but extremely high wear, much greater than for the inert gases. This is further discussed in later in the paper. Propane shows intermediate friction but wear that is only slightly lower than the inert gases. This reflects the fact that friction reduction, presumably originating from tribofilm formation, occurs part-way through the tests. This reduces the mean friction, but it is likely that most of the measured wear occurred prior to formation of a protective tribofilm. 4.2 Switching gases Some tests were carried out in which two supplied gases were switched periodically to investigate the kinetics and durability of tribofilms using the set-up shown schematically in Fig. 2 . To illustrate the rate at which gases were exchanged, Fig. 8 shows how O 2 level varied when switching between ethene and dry air, with O 2 level being monitored using a fast response oxygen sensor in the outlet flow. Gas exchange was rapid, matching the gas supply rate. Figure 9 shows two repeat tests in which the atmosphere was switched between propene and dry air. Propene gives low friction, as seen previously in Fig. 6 , but friction then rises as soon as dry air is introduced, suggesting very rapid loss of tribofilm. Then, when propene replaces dry air, friction reduces once more, indicating rapid tribofilm reformation. It is not clear from the tests shown in Fig. 9 whether the loss of tribofilm and consequent rise in friction in air occurred because the film was weak and thus easily removed by rubbing once no propene was available to regenerate it, or whether the oxygen in air had a specifically detrimental chemical effect on the propene tribofilm. To assess this, tests were carried out switching between propene and nitrogen as shown in Fig. 10 . The results were quite variable over time, so four repeats were carried out. It is evident that friction rises when the propene atmosphere is replaced by nitrogen, implying that the tribofilm formed by propene is initially quite weak and being continually formed and removed. It is also evident that friction rises more slowly when propene is replaced by nitrogen than when it is replaced by air, indicating that there is also a direct chemical contribution to tribofilm removal by oxygen, as suggested from in-contact Raman studies [ 23 ]. Most striking is that in three of the four repeats carried out, the tribofilm appears to become stronger over rubbing time in propene since friction takes longer to increase and eventually only partially increases when the atmosphere is switched to nitrogen. Tests were also carried out switching between NH 3 and N 2 and between CO and N 2 to assess the strength of the films formed by these gases. Figure 11 shows results for switching between NH 3 and N 2 . During the first switch to N 2 the friction rises sharply, suggesting that the film is rapidly removed. However, during subsequent switches to N 2 the film is lost more slowly, implying that it is becoming more resistant to removal, as was seen with propene. Figure 12 shows friction traces from tests in which CO and N 2 gases were switched. Friction rises rapidly after switching to N 2, indicating loss of CO-originating tribofilm. However, during each subsequent switch to N 2 the friction stabilises at a lower value, possibly suggesting that less and less low friction tribofilm is being removed by rubbing in N 2 . However, the friction in a CO atmosphere appears to increase slightly during successive switches. Other switching results are included in Supplementary Information. 5. DISCUSSION 5.1 Low lubricity gases He, Ar, H 2 , N 2 , CO 2 He, Ar and H 2 all showed high friction coefficients in the range 0.9 to 1.2, indicative of little or no lubricity. This is in accord with previous studies [ 1 , 10 , 16 , 17 ]. Some previous work has suggested that H 2 might give higher friction than the inert gases because it might reduce any iron oxides present [ 10 ], but this cannot be concluded from the current study. N 2 appears to give slightly lower friction than the inert gases, in the range 0.8 to 1.0. Previous researchers have suggested that N 2 might react with steel during rubbing to form a nitride and that this could be responsible for some friction reduction [ 10 , 17 ]. XPS was therefore carried out on HFR discs from tests with N 2 . These did show very slight traces of nitrogen compounds at ca 400 eV within the rubbed track, but similar traces were also seen on fresh discs and so are not believed to indicate a tribofilm. To assess possible effects of traces of O 2 or H 2 O in the cylinder gases, two types of cylinder N 2 were used, highly pure N 2 N6.0 and also “N 2 oxygen free” from BOC. However, both gave similar friction behaviour to that shown in Fig. 3 . Figure 4 shows that CO 2 gives high friction coefficient of ca 1.0, comparable to that of the inert gases. This is contrary to some previous work. Nunez et al . found that CO 2 reduced friction of grey cast iron but that this only occurred close to the gas-liquid transition, i.e. at high pressure and/or low temperature [ 8 ]. At atmospheric pressure they found no friction reduction, in agreement with the current work. Wu et al . obtained low friction of 52100 steel in CO 2 and related this to the formation of carbonate and bicarbonate [ 7 ]. However, their study was at considerably lower speed (lower frequency and longer stroke length) than the current work. Velkahvr et al . used a reciprocating rig at conditions quite similar to the current study and found that CO 2 gave variable friction, ranging between 0.8 and 0.2, with rapid transition between the two values suggesting loss and reformation of a tribofilm. They also detected a carbonate or bicarbonate tribofilm using XPS [ 17 , 21 ]. It appears that CO 2 may have very marginal lubricity that depends critically on the conditions studied. All five gases show high wear, in the range 500 to 600 µm in wear scar diameter, with helium giving somewhat lower wear than the others. This might originate from very small levels of impurity in the gas since the specification for the He used was only O 2 ≤ 7 ppm, H 2 O ≤ 5ppm. 5.2 Dry air and O 2 Dry air and O 2 gave high friction coefficients in the 0.8 to 0.9 range, slightly lower than the five gases above, but they both produced extremely high wear, much larger than the wear seen with all other gases. Indeed, this wear was so large that the final wear scar diameters exceeded the reciprocating stroke length, so towards the end of tests the contacts may have been operating in fretting rather than sliding wear mode [ 29 ]. Pure O 2 gas gave slightly higher wear and slightly lower friction than dry air (21% O 2 ). This very high wear probably represents classical oxidational wear, where, in dry conditions, iron oxides are formed during rubbing and then removed by mechanical action – effectively corrosive-abrasive wear. Such wear in air was first identified in the 1950s [ 12 ] but the concept of oxidational wear was formalised and modelled in a series of papers by Quinn from the 1960s [ 30 – 33 ]. Quinn’s oxidational wear model is based on the assumption that wear occurs at high temperatures generated by rubbing, but in the current work the calculated mean flash temperature at mid-stroke is relatively low ( ca 60°C at a friction coefficient of 1.0), implying an initial total surface temperature of less than 100°C. Of course, Quinn’s oxidational model preceded knowledge of, and thus takes no account of, the possibility of mechanochemical effects. Although the wear is much higher when O 2 is present than in oxygen-free N 2 , the friction coefficient is similar at ca 0.8. This might be because, although wear is dominated by oxide loss, friction is dominated by adhesion between metallic asperities. Alternatively, this friction coefficient may reflect the actual value of friction coefficient of iron oxide on iron oxide. To explore this a test was carried out using haematite (Fe 2 O 3 ) HPR ball and disc in helium. This is shown in Supplementary Information and indicates that dry Fe 2 O 3 /Fe 2 O 3 gives a friction coefficient approaching 1.0. Thus the friction measured when steel surfaces are rubbed in dry air or oxygen may simply reflect the value of iron(III) oxide against iron(III) oxide, although it may equally well originate from iron/iron asperity adhesion; or quite possibly a combination of both. A key difference between an oxygen-free and an oxygen-containing atmosphere may simply be that in the former, many metallic wear particles remain adhered to the rubbing surfaces, supressing material loss, while in the latter, as ferrous oxide they do not adhere strongly and thus are easily dislodged from the rubbing surfaces, so enabling high rate of material loss. 5.3. Lubricious non-hydrocarbon gases In Fig. 4 it can be seen that the two gases, NH 3 and CO, give much lower wear than the other non-hydrocarbon gases studied, indicating that these form lubricious tribofilms on the rubbing surfaces. Little previous research has been carried out on the friction and wear of these gases although Blanchet et al . have suggested that blends of CO with H 2 may generate carbonaceous tribofilms of rubbing ceramic surfaces at high temperature [ 25 ]. From the gas switching results in Figs. 11 and 12 it is evident that the initially-formed tribofilms are not very strong in that they are removed by rubbing in N 2 . However, for NH 3 the tribofilm appears to become more slowly removed after several switching cycles, while for CO the film seems to become fully resistant to rubbing over time. To explore the origins of this friction reduction, XPS and Raman surface analysis were carried out on the rubbed tracks of HPR discs at the end of tests in the two gases. XPS survey spectra from a fresh steel disc surface and from rubbed tracks tests in NH 3 and CO are shown in Supplementary Information. A strong nitrogen signal was only observed from the sample rubbed in NH 3 and its high resolution N1s and C1s XPS spectra are shown in Fig. 13 . Corresponding spectra from a fresh, unrubbed, disc surface are included in Supplementary Information. The N1s spectrum shows evidence of iron nitride (397.94 eV and 398.67 eV) as well as adsorbed ammonia (400 eV) [ 34 – 36 ]. Interestingly, the C1s spectrum contains peaks from C-N and C = N. This supports the interpretation that the tribofilm formed in NH 3 contains nitrides. The peak from C-C bonds most likely originates from adventitious carbon. It should be noted that a Raman spectrum of this worn surface did not contain an obvious nitride-related peak, probably because the tribofilm is very thin. This may explain the difficulty in identifying this film in previous studies. The XPS spectrum obtained from the disc tracks rubbed in CO (Fig. 14 ) contains a -C-C- peak from adventitious carbon. The peak at 286 cm − 1 may corresponds to adsorbed CO or adventitious carbon [ 37 ]. Ther are also peaks from polar carbon and carbonate moieties. This is consistent with the literature, where metal carbonate has been detected in tribofilms formed on steel surfaces rubbed in CO 2 [ 7 , 20 ]. 5.4 Saturated Hydrocarbons Figures 5 and 7 showed the influence of five saturated gaseous hydrocarbons on friction and wear and indicates that their lubricity increases markedly with molecular weight. Methane and ethane give friction and wear values similar to the inert gases, suggesting negligible tribofilm formation, except for a possible slight reduction in friction for ethane towards end of test. Propane apparently forms a tribofilm slowly, such that friction drops to a relatively low value part-way through each test, and this is reflected in an intermediate wear result. The two butanes show very low friction after 1–5 minutes rubbing and low wear, suggesting quite rapid tribofilm formation. There is no apparent difference in performance between the linear and the branched butane. Previous work using both gaseous and liquid saturated hydrocarbons has shown that these can form lubricious carbonaceous films readily during rubbing in oxygen-free atmospheres [ 23 , 38 , 39 ]. It is generally accepted that this film formation is driven by the severe conditions present in rubbing contacts that break C-C and/or C-H covalent bonds to form free radical species that then rearrange, lose hydrogen atoms, and combine to form graphitic material on the rubbing surfaces. There is some debate about the relative importance of the impact of mechanical stresses and surface catalysis on the reaction, and it is probable that catalysis is most important with surfaces that contain catalytically-active atoms such as Cu, Ni and Pt, but that mechanochemical effects are more prevalent with bearing steels. Previous work has shown that propane forms a low friction carbonaceous film on rubbing AISI 52100 surfaces [ 23 ] and that butane reacts on rubbing ceramic surfaces to form a high molecular weight material [ 26 ]. The formation of such films by methane or ethane on steel surfaces has not been reported but has been shown that methane will form such films on highly catalytic nickel-containing surfaces [ 22 ]. The fact that methane does not appear to form effective tribofilms on steel surfaces but does so on catalytic ones may imply that its reaction is controlled by a catalytic rather than mechanical response. Two effects might explain the fact that carbonaceous tribofilm formation by saturated hydrocarbons increases with molecular weight. One is simply that there should be stronger adsorption of the gaseous molecules on the surfaces as the molecular weight increases. The second is that the mechanical forces experienced by interior C-C bonds will increase with the size of the groups attached to each C atom as forces are transmitted along the chain. Also, of course, methane, with just one carbon atom, cannot experience C-C bond cleavage to form a hydrocarbyl free radical. To confirm the formation of a carbonaceous tribofilm, an HPR disc from a test with n-butane was analysed using surface Raman as shown in Fig. 15 . Characteristic carbon D- and G- bands can be seen in spectra from some parts of the worn surface, while only iron oxide peaks are observed in other places. The morphology of the tribofilm shown by optical micrographs confirmed that the tribofilm is heterogeneous. It should be noted that the iron oxide may have been formed after tests, when samples were exposed briefly to air for Raman analysis. 5.5 Unsaturated Hydrocarbons Figures 6 and 7 showed that ethene, propene and ethyne give very low friction and wear, implying that they form protective tribofilms very readily. It is well known that unsaturated hydrocarbons form carbonaceous films more easily that saturated ones [ 36 , 40 ]. The precise mechanism of formation is not yet clear – it is evidently promoted by rubbing, but the extent to which simple polymerisation as opposed to stress-driven radical reactions occurs in not known. Raman spectra from various regions of disc surfaces rubbed in ethene and ethyne are presented in Fig. 16 . Compared to results from butane, the surface coverage of carbonaceous materials increases as we go from this to ethene, then to ethyne. This supports the interpretation that the ease of carbonaceous film formation increases with the degree of unsaturation of the gas. Although the formation of carbonaceous film increases with unsaturation, this does not result in a progressive reduction in friction and wear; wear is similar for all three unsaturated hydrocarbons studied while friction is actually slightly higher for ethyne than for ethene. 6. CONCLUSIONS This study has used a sealed, reciprocating, ball of flat tribometer, the HPR, to measure the friction and wear properties a steel/steel sliding contact in a range of gaseous environments of tribological interest. It is found that the inert gases helium and argon and also nitrogen and hydrogen give high friction and wear indicating that they form little or no protective tribofilm. Dry air and pure oxygen give friction similar to nitrogen but produce extremely high wear. This is believed to originate from oxidational wear when rapidly-forming iron oxide is abraded from the rubbing surfaces. Ammonia and carbon monoxide both show relatively low friction and wear, indicating that they form tribofilms on the rubbing surfaces. Gas-switching studies suggest that these tribofilms are initially weak but that they become stronger and have lower friction after repeated switching. XPS analysis on the rubbed surfaces shows that ammonia both adsorbs on the surfaces and forms nitrides, while carbon monoxide forms a carbonate. The saturated hydrocarbon gases show decreasing friction and wear reduction with increase of molecular weight. Thus methane and ethane show no friction reduction, propane forms a film only after about 30 minutes rubbing and n-butane and isobutane form films after between 1 and 5 minutes rubbing. Raman analysis of a disc track rubbed in n-butane shows the presence of D- and G- carbon bands, characteristic of graphitic carbon. The unsaturated gases, ethene, propene and ethyne all show an immediate reduction in friction and give low wear. Raman analysis of the rubbed surfaces shows the formation of a carbonaceous film, with this increasing with the level of unsaturation of the hydrocarbon. Declarations ACKNOWLEDGEMENTS The authors wish to thank PCS Instruments for supply of an HPR test rig, the Shell UTC for Mobility and Lubricants for support for Jie Zhang, and Shouyi Yin for the help with XPS. For the purpose of open access, the authors have applied for a Creative Commons Attribution (CC BY) license to any Author Accepted Manuscript version arising. CRediT authorship contribution statement J.Z.: Investigation, Analysis; J.S.S.W.: analysis, writing – review and editing; H.A.S.: conceptualisation, supervision, project administration, writing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. DATA AVAILABILITY Data that supports the findings of this study are available from the corresponding author upon reasonable request. Supporting Information The Supporting Information is available free of charge at XXX. It contains friction test and XPS results. References Bowden, F.P., Hughes, T.P. The friction of clean metals and the influence of adsorbed gases. The temperature coefficient of friction. Proc. Roy. Soc. Lond. A172, 263-279, (1939). Soda, N., Sasada, T., Mechanism of lubrication by surrounding gas molecules in adhesive wear. Trans ASME J. Lub. Techn. 100, 492-497, (1978) Mishina, H. Atmospheric characteristics in friction and wear of metals. Wear 152, 99-110 (1992). Mishina, H. Chemisorption of diatomic gas molecules and atmospheric characteristics in adhesive wear and friction of metals. Wear, 180(1-2), 1-7, (1995). Wilson, J. E., F. H. Stott, Wood, G.C. The development of wear-protective oxides and their influence on sliding friction. Proc. Roy. Soc. Lond. A369, 557-574, (1980). Zhang, Q. Y., Chen, K. M., Wang, L., Cui, X.H., Wang, S.Q. Characteristics of oxidative wear and oxidative mild wear. Tribol. Intern., 61, 214-223, (2013). Wu, X., Cong, P., Nanao, H., Minami, I., Mori, S. Tribological behaviors of 52100 steel in carbon dioxide atmosphere. Tribol. Lett., 17, 925-930, (2004). Nunez, E.E, Polychronopoulou, K., Polycarpou, A.A. Lubricity effect of carbon dioxide used as an environmentally friendly refrigerant in air-conditioning and refrigeration compressors. Wear 270, 46-56, (2010). Rosenberg, S.J., Jordan, L. Influence of oxide films on the wear of steels. J. of Res. of the National Bureau of Standards, 13, RP708, pp. 267-279, (1934). Buckley, D.H, Johnson, R.L. Effect of inert, reducing and oxidising atmospheres on friction and wear of metals to 1000°F. NASA Techn. Note D-1103, Oct 1961 Batchelor, A.W., Stachowiak, G.W., Cameron, A. The relationship between oxide films and the wear of steels. Wear, 113, 203-223, (1986). Fink M. Wear oxidation – a new component of wear. Trans. Amer. Soc for Steel Treating 18, 1026-1034, (1930) Smith, A.F. Influence of environment on the unlubricated wear of 316 stainless steel at room temperature. Tribol. Intern. 19(1), 3-10, (1986). Feng, I.M., Uhlig, H.H. Fretting corrosion of mild steel in air and in nitrogen. Trans ASME J. Appl. Mech . 21, 395-400, (1954) So, H. The mechanism of oxidational wear. Wear, 184, 161-167, (1995). Cornelius, D.E., Roberts, W.H., Friction and wear of metals in gases up to 600 C. ASLE Trans. 4, 20-32 (1961) Velkavrh, I., Ausserer, F., Klien, S., Brenner, J., Forêt, P., Diem, A. The effect of gaseous atmospheres on friction and wear of steel–steel contacts. Tribol. Intern., 79, 99-110, (2104). Zhang, Y., Jourani, A. Combined effect of microstructure and gaseous environments on oxidative and adhesive wear of dual-phase steel. J. of Mats Eng. & Perf. 30, 9333-9351, (2021) Kruger, J., Yolken, H.T. Room temperature oxidation of iron at low pressures. Corrosion 20, 29t-33t, (1964). Velkavrh, I., Ausserer, F., Klien, S., Voyer, J., Ristow, A., Brenner, J., Forêt, P., Diem, A., The influence of temperature on friction and wear of unlubricated steel/steel contacts in different gaseous atmospheres. Tribol. Intern. 98, 155-171, (2016). Murray, S.F., Johnson R.L., Swikert , M.A., Boundary lubrication of steel with fluorine- and chlorine-substituted methane and ethane gases. NACA Techn Note 3401, Lewis Flight Propulsion Laboratory, Cleveland, Ohio, (1955). Ramirez, G., Eryilmaz, O.L., Fatti, G., Righi, M.C., Wen, J., Erdemir, E. Tribochemical conversion of methane to graphene and other carbon nanostructures: implications for friction and wear. ACS Applied Nano Materials 3, 8060-8067, (2020). Zhang, J., Bolle, B., Wong, J.S.S., Spikes H.A. Influence of atmosphere on carbonaceous film formation in rubbing, metallic contacts. Tribol. Lett. 72:4, (2024). Lauer, J.L., Dwyer, S.R. (1991). Tribochemical lubrication of ceramics by carbonaceous vapors. Tribol. Trans. 34, 521-528, (1991). Blanchet, T.A., Lauer, J.L., Liew, Y.F., Rhee, S.J., Sawyer, W.G. Solid lubrication by decomposition of carbon monoxide and other gases. Surface & Coatings Techn. 68 446-452, (1994). Nakayama, K., Hashimoto, H. Triboemission, tribochemical reaction, and friction and wear in ceramics under various n-butane gas pressures. Tribol. Intern. 29, 385-393, (1996). Greenwood, J.A.: An interpolation formula for flash temperatures. Wear 150, 153-158, (1991) Reddyhoff, T., Schmidt, A., Spikes, H.A., Thermal conductivity and flash temperature. Tribol. Lett. 67:22, (2019) Fouvry, S., Kapsa, P., Vincent, L.: Quantification of fretting damage. Wear 200, 186–205 (1996) Quinn, T.F.J. Role of oxidation in the mild wear of steel. Brit. J. of Appl. Phys. 13, 33-37, (1962) Quinn, T.F.J. Oxidational wear. Wear, 18, 413-419, (1971) Quinn, T.F.J. Review of oxidational wear: Part I: The origins of oxidational wear. Tribol. Intern. 16, 257-271, (1983) Quinn, T.F.J. Review of oxidational wear part II: recent developments and future trends in oxidational wear research. Tribol. Intern. 16 305-315, (1983) Vinod, K. R., Saravanan, P., Sakar, M., Balakumar, S. Insights into the nitridation of zero-valent iron nanoparticles for the facile synthesis of iron nitride nanoparticles. RSC Advances, 6 (51), 45850-45857. (2016). Peng, H., Mo, Z., Liao, S., Liang, H., Yang, L., Luo, F., Song, H. Zhong, Y., Zhang, B. High performance Fe-and N-doped carbon catalyst with graphene structure for oxygen reduction. Scientific reports, 3(1), 1765, (2013). Wojciechowski, P., Lewandowski, M. Iron nitride thin films: Growth, structure, and properties. Crystal Growth & Design, 22(7), 4618-4639, (2022). Lukashuk, L., Yigit, N., Rameshan, R., Kolar, E., Teschner, D., Hävecker, M., Knop-Gericke, A., Schlögl, R., Föttinger, K., Rupprechter, G. Operando insights into CO oxidation on cobalt oxide catalysts by NAP-XPS, FTIR, and XRD. ACS catalysis 8.9. 8630-8641, (2018) Zhang, J., Campen, S., Wong, J., Spikes, H. Oxidational wear in lubricated contacts–Or is it? Tribol. Intern., 165, 107287, (2022). Li, Y.S., Jang, S., Bhuiyan, F.H., Martini, A., Kim, S.H. Molecular structure and environment dependence of shear-driven chemical reactions: tribopolymerization of methylcyclopentane, cyclohexane and cyclohexene on stainless steel. Tribol. Letters, 71, 49, (2023). He, X., Kim, S.H. Mechanochemistry of physisorbed molecules at tribological interfaces: molecular structure dependence of tribochemical polymerization. Langmuir 33, 2717-2724, (2017) Additional Declarations No competing interests reported. Supplementary Files v8supplementaryinformationgases.docx Cite Share Download PDF Status: Published Journal Publication published 28 Aug, 2024 Read the published version in Tribology Letters → Version 1 posted Editorial decision: Revision requested 24 Jul, 2024 Reviews received at journal 12 Jul, 2024 Reviews received at journal 24 Jun, 2024 Reviewers agreed at journal 07 Jun, 2024 Reviewers agreed at journal 25 May, 2024 Reviewers invited by journal 23 May, 2024 Submission checks completed at journal 20 May, 2024 Editor assigned by journal 20 May, 2024 First submitted to journal 19 May, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4445568","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":308306945,"identity":"f4de453b-1323-439b-8ec1-377ec7f79c30","order_by":0,"name":"Jie Zhang","email":"","orcid":"","institution":"Imperial College London","correspondingAuthor":false,"prefix":"","firstName":"Jie","middleName":"","lastName":"Zhang","suffix":""},{"id":308306946,"identity":"c410c136-35e0-494b-a2a5-eb62b4fa4dc4","order_by":1,"name":"Janet Wong","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA40lEQVRIiWNgGAWjYBACxgYwZcMDphIYLJBF8WpJg2mRIKwFCg7DGERoYW7gMXxc8Ou8jO7s5mMfHtRIMPC3H2CTnIHXYTzGxjP7bvOY3TmWPCPhmASDxJkENskNeLXwbpPm7QFquZFjzJDABnTYDQY2yQeEtZwDasn/zJDwT4JBnigtPD8OgGxhZkhsk2AwAGnB67Bm/s/GvA3JIL8YMyT2SfAYnklstsTnfcP2tsTHPH/s7M1uNz9m/PHNRk7u+OGDN3vwaWkGWdXGAIsRBh6CESkPJv8gtIyCUTAKRsEowAAAlW5ICPsIrmMAAAAASUVORK5CYII=","orcid":"","institution":"Imperial College London","correspondingAuthor":true,"prefix":"","firstName":"Janet","middleName":"","lastName":"Wong","suffix":""},{"id":308306947,"identity":"0b5b3c24-3290-4735-b2ff-99aabf3b11de","order_by":2,"name":"Hugh Spikes","email":"","orcid":"","institution":"Imperial College London","correspondingAuthor":false,"prefix":"","firstName":"Hugh","middleName":"","lastName":"Spikes","suffix":""}],"badges":[],"createdAt":"2024-05-19 20:23:12","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4445568/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4445568/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11249-024-01911-y","type":"published","date":"2024-08-28T15:58:08+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":57478342,"identity":"0bbc61ee-7636-4dd2-acc7-7c04487dc718","added_by":"auto","created_at":"2024-05-31 08:35:53","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":13709029,"visible":true,"origin":"","legend":"\u003cp\u003ePhotograph of HPR\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/741e4b170122a3eadc61de36.png"},{"id":57478333,"identity":"d81f859b-eff5-401d-8adc-f18a55201144","added_by":"auto","created_at":"2024-05-31 08:35:52","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":51216,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of HPR gas supply set up.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/fc1b7e805e01942e6f7f52a3.png"},{"id":57479815,"identity":"39022e89-c674-41e7-86b0-90d110c9cf42","added_by":"auto","created_at":"2024-05-31 08:51:52","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":61379,"visible":true,"origin":"","legend":"\u003cp\u003eFriction coefficient traces for dry air, He, Ar, N\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/1b3d9e41905402b66c7168fb.png"},{"id":57478335,"identity":"74a45fd9-6c1e-4d24-915a-dcf9b77e32a6","added_by":"auto","created_at":"2024-05-31 08:35:52","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":61040,"visible":true,"origin":"","legend":"\u003cp\u003eFriction coefficient traces for dry air, O\u003csub\u003e2\u003c/sub\u003e, CO, CO\u003csub\u003e2\u003c/sub\u003e and NH\u003csub\u003e3 \u003c/sub\u003eat 25°C\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/c0692078a57417cfe7bd250c.png"},{"id":57478970,"identity":"2064fd8b-98ae-42da-8983-60aa25a51aa9","added_by":"auto","created_at":"2024-05-31 08:43:52","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":35181,"visible":true,"origin":"","legend":"\u003cp\u003eFriction coefficient traces for five saturated hydrocarbon gases\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/7d98de1160f3edb9be3d75bc.png"},{"id":57478338,"identity":"a448df10-5031-4d2a-8050-6a900c4000d9","added_by":"auto","created_at":"2024-05-31 08:35:52","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":41285,"visible":true,"origin":"","legend":"\u003cp\u003eFriction coefficient traces for three unsaturated hydrocarbon gases\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/fa8ef09877023e83338f4857.png"},{"id":57478341,"identity":"4f2c93ed-dda6-4934-a86f-b9823a0a7b1c","added_by":"auto","created_at":"2024-05-31 08:35:52","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":37646,"visible":true,"origin":"","legend":"\u003cp\u003eSummary of average wear scar diameter (WSD) on the balls and friction coefficient, (μ), results for all gases tested\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/a07eb21b03e076c4e3215ba7.png"},{"id":57478973,"identity":"af3fb3d2-fd47-4da4-94a8-ff86c9030852","added_by":"auto","created_at":"2024-05-31 08:43:53","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":15382,"visible":true,"origin":"","legend":"\u003cp\u003eRate of variation of O\u003csub\u003e2\u003c/sub\u003e content during switch test between ethene and dry air\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/8a1e611e2067a964ccf3d3bb.png"},{"id":57478350,"identity":"a8950620-be53-4551-b6c4-f785c2dcde0e","added_by":"auto","created_at":"2024-05-31 08:35:53","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":58166,"visible":true,"origin":"","legend":"\u003cp\u003eFriction coefficient variation during switching between propene and dry air\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/3360c79e2ea5be2ef63956aa.png"},{"id":57478339,"identity":"a76211ed-9f78-482b-b85c-19d59de51cb0","added_by":"auto","created_at":"2024-05-31 08:35:52","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":73160,"visible":true,"origin":"","legend":"\u003cp\u003eFriction coefficient variation during switching between propene and nitrogen gas\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/a37a4b7ae220c164a01f2008.png"},{"id":57479816,"identity":"5a32acf8-9770-43c5-b883-2f4d19f906c2","added_by":"auto","created_at":"2024-05-31 08:51:53","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":36182,"visible":true,"origin":"","legend":"\u003cp\u003eFriction coefficient variation during switching between ammonia and nitrogen gas\u003c/p\u003e","description":"","filename":"floatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/33dc00c200896e213d1eb11d.png"},{"id":57478972,"identity":"b0737d40-040b-40a8-ab49-095947f08248","added_by":"auto","created_at":"2024-05-31 08:43:52","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":61236,"visible":true,"origin":"","legend":"\u003cp\u003eFriction coefficient variation during switching between carbon monoxide and nitrogen gas\u003c/p\u003e","description":"","filename":"floatimage12.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/2771ba7acba40e6f4b483287.png"},{"id":57478347,"identity":"14e78abf-c3a0-40be-8f8c-524169316bb7","added_by":"auto","created_at":"2024-05-31 08:35:53","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":109798,"visible":true,"origin":"","legend":"\u003cp\u003e(a) N1s and (b) C1s spectra from disc track rubbed in NH\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"floatimage13.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/e524019604e62ab389fe37cc.png"},{"id":57478348,"identity":"6fe81da9-ce87-4c4d-ba42-7919e071d612","added_by":"auto","created_at":"2024-05-31 08:35:53","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":113241,"visible":true,"origin":"","legend":"\u003cp\u003e(a) C1s and (b) O1s spectra from track rubbed in CO\u003c/p\u003e","description":"","filename":"floatimage14.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/a019518a2300288f3cc5d27d.png"},{"id":57478343,"identity":"897f9b75-177d-44bf-a3f6-4cda6eae2d8a","added_by":"auto","created_at":"2024-05-31 08:35:53","extension":"png","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":10636177,"visible":true,"origin":"","legend":"\u003cp\u003eRaman spectra from HPR disc track rubbed in n-butane\u003c/p\u003e","description":"","filename":"floatimage15.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/fb42f3e9af0d214fbbcb1774.png"},{"id":57478349,"identity":"ab031f60-5aae-46bb-bc64-4e8cd42ee948","added_by":"auto","created_at":"2024-05-31 08:35:53","extension":"png","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":511910,"visible":true,"origin":"","legend":"\u003cp\u003eRaman spectra of disc surfaces rubbed in (a) ethene and (b) ethyne\u003c/p\u003e","description":"","filename":"floatimage16.png","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/928e711d0236ad5e1769ecc3.png"},{"id":63821163,"identity":"e5643a62-fcc9-4df5-8048-3e7477866b84","added_by":"auto","created_at":"2024-09-02 16:12:41","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":29198299,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/7b7559d7-f9c6-421d-9e3d-cb960fd3786b.pdf"},{"id":57478340,"identity":"1696521e-6a20-45f4-85dd-6fe8184184ec","added_by":"auto","created_at":"2024-05-31 08:35:52","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1165407,"visible":true,"origin":"","legend":"","description":"","filename":"v8supplementaryinformationgases.docx","url":"https://assets-eu.researchsquare.com/files/rs-4445568/v1/2456583df77a3c786d4d4365.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"The lubricity of gases","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAlthough most lubricated systems involve the use of a liquid or grease lubricant, there are some systems where the lubricating medium is a gas or vapour. \u0026nbsp; This is the case, for example, in air bearings, gas processing, refrigeration, and when pumping and injecting gaseous fuels into a combustion chamber. In such systems, although coatings may be used to provide some degree of solid lubrication, the ability of the gas itself to provide protective lubricating films is potentially important. \u0026nbsp;Study of the lubricating properties of relatively benign or inert gases such as air, carbon dioxide, nitrogen, and argon is quite straightforward. However, study of combustible or toxic gases such as methane, hydrogen and ammonia is more problematic and this may have constrained investigation into their lubricating properties in the past. \u0026nbsp;To address this, the current paper employs a new, sealed, low volume tribometer to investigate the lubricating properties of a range of different gases of tribological interest. \u0026nbsp;\u003c/p\u003e\n"},{"header":"2. Background","content":"\u003cp\u003eWhen metallic surfaces are rubbed together in the absence of a liquid, grease or solid lubricant, interactions of the gaseous environment with the rubbing metal surfaces become of paramount importance in determining tribological performance. Such interactions can involve adsorption of gaseous molecules [\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] and/or their chemical reaction with the surfaces, for example to form metal oxides from molecular oxygen [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] or carbonates from carbon dioxide [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eOxygen\u003c/span\u003e \u003c/p\u003e \u003cp\u003eBy far the most practically important effect of atmosphere stems from the presence or absence of molecular oxygen. This can be beneficial or deleterious. Thus, several researchers have found that the addition of low levels of oxygen can greatly reduce the high friction and wear observed when ferrous surfaces are rubbed in vacuum or in oxygen-free gases such as nitrogen, argon and hydrogen [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. It is also widely accepted that the formation of oxide layers on steel provides general protection against adhesive wear [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. On the other hand, higher wear levels of steel in air (21% O\u003csub\u003e2\u003c/sub\u003e) as compared to inert gases have been reported in both sliding and fretting wear [\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] and ascribed to the formation of easily removed or abrasive oxides. It appears that the impact of molecular oxygen on friction and wear is strongly dependent on steel hardness and on rubbing conditions such as load and the degree of sliding [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eOther gases\u003c/span\u003e \u003c/p\u003e \u003cp\u003eSeveral researchers have studied the properties of nominally oxygen-free gas environments on friction and wear. Most of this work has suggested that H\u003csub\u003e2,\u003c/sub\u003e N\u003csub\u003e2\u003c/sub\u003e and inert gases (He, Ar) possess little intrinsic lubricity and impart high friction and wear to sliding steel/steel contacts [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Some authors have noted slightly lower friction of bearing steel in N\u003csub\u003e2\u003c/sub\u003e than in Ar and proposed that this might originate from the formation of iron or chromium nitride [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. However, to date surface analysis has failed to detect any such nitrides formed on rubbing steel surfaces from a nitrogen atmosphere. One problem in comparing the friction and wear properties of very low lubricity gases is to be confident that small proportions of O\u003csub\u003e2\u003c/sub\u003e at the sub-100 ppm level, that can be present in some cylinder gases, are not influencing the observed behaviour. In corrosion studies it has been shown that oxide films start to form very rapidly on ferrous surfaces at room temperature at an oxygen partial pressure of 0.01 Pa, which, as a contaminant in inert gas, is equivalent to less than 1 ppm O\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Mishina has also suggested that very low concentrations of O\u003csub\u003e2\u003c/sub\u003e, N\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e in the Pa pressure range can significantly influence friction and wear by adsorbing on rubbing iron surfaces to promote the formation of non-adherent wear debris and thus wear [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSeveral researchers have explored the friction properties of metal surfaces rubbed in a CO\u003csub\u003e2\u003c/sub\u003e atmosphere, both because of the latter\u0026rsquo;s use in some nuclear reactors and due to its role as a refrigerant gas [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. All have found CO\u003csub\u003e2\u003c/sub\u003e to give lower friction and wear than inert gases under some conditions, and surface analysis using XPS has indicated the formation of a ferrous carbonate tribofilm on and around the rubbed steel surfaces.\u003c/p\u003e \u003cp\u003eThere appears to have been relatively little work on the influence of gases other than the above on steel-steel friction and wear. Murray \u003cem\u003eet al\u003c/em\u003e. studied the friction and wear properties of rubbing contacts in a range of gaseous fluoro- and chlorohydrocarbons and found that some of the latter, notably dichlorodifluoromethane, CCl\u003csub\u003e2\u003c/sub\u003eF\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e, gave low friction and wear [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Surface analysis showed the formation of a chloride tribofilm. Ramirez \u003cem\u003eet al\u003c/em\u003e. showed that methane, CH\u003csub\u003e4\u003c/sub\u003e, formed a friction-reducing, graphene-like carbonaceous film on rubbing VN-Ni and VN-Cu coated AISI 52100 steel surfaces, but not on uncoated steel, suggesting a catalytically-driven tribochemical reaction [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. However very recently the authors of the current paper used a sealed tribometer to show that propane gas formed friction-reducing carbon-based films on rubbing 52100 steel surfaces at room temperature [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Research has also been carried out on the ability of a range of carbon-based compounds including carbon monoxide, ethyne and butane, to form tribofilms on nickel and ceramic surfaces, both in the context of vapour phase lubrication and for fundamental study of tribochemical mechanisms [\u003cspan additionalcitationids=\"CR25\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWhile it is very straightforward to design and operate sealed tribometers to study the friction and wear properties of inert gases or relatively benign ones such as dry air and CO\u003csub\u003e2\u003c/sub\u003e, where health and safety concerns are minimal, this is not the case when studying combustible gases such as CH\u003csub\u003e4\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003e, or toxic ones such as NH\u003csub\u003e3\u003c/sub\u003e and CO. This may have constrained research on the lubricity of these latter gases. However, a low volume, sealed tribometer has recently become commercially available which makes it relatively straightforward to study the friction and reducing properties of a wide range of gases. This paper describes the application of this tribometer to study the ability of a range of nominally pure gases to lubricate rubbing steel-steel contacts.\u003c/p\u003e"},{"header":"3. TEST METHODS AND MATERIALS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e3.1 \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eHPR tribometer tests\u003c/span\u003e\u003c/h2\u003e \u003cp\u003eThis study employs a fully sealed, reciprocating tribometer, the HPR, supplied by PCS Instruments, UK. This reciprocates a 6 mm diameter ball against the flat surface of a stationary disc and employs the same drive motor, test configuration and ball and disc test specimens as the conventional high frequency reciprocating rig (HFR). However, the reciprocating contact is fully enclosed in a stainless-steel cylinder. To achieve this, the shaft that drives the upper ball holder in reciprocating motion and is loaded externally by dead weights is sealed against the cylinder by two flexible bellows. The lower disc holder is flexibly-mounted and attached to an external force transducer to monitor friction \u003cem\u003evia\u003c/em\u003e a thin metal membrane in the cylinder wall. The cylinder wall is also fitted with gas inlet and outlet ports, a break valve and a pressure-measuring gauge and pressure relief valve. The HPR can be used up to 130\u0026deg;C and 10 bar pressure but in this study most tests were carried out at 1 bar pressure and 25\u0026deg;C.\u003c/p\u003e \u003cp\u003eThe small size of this test rig enables its use in a fume cupboard, while its small internal volume of \u003cem\u003eca\u003c/em\u003e 20 ml facilitates the safe study of gases such as hydrogen, methane and ammonia. An additional benefit of this small internal volume is that it enables rapid switching of gases, which can be used to estimate the kinetics of formation and loss of tribofilms, as described later in this paper.\u003c/p\u003e \u003cp\u003eIn the current study, gases were supplied to the test chamber at a flow rate of 0.1 L/min using PC-controlled flow controllers, in both pure gas tests and gas switching tests. For the latter, two flow controllers were connected to two separate gas lecture bottles at one end and to two Swagelok valves at the other. By toggling the Swagelok valves, the gas supply could be switched between the two gases. The two flow controllers were both the MC series from Alicat Scientific, with accuracy at \u0026plusmn;\u0026thinsp;0.6% of reading or \u0026plusmn;\u0026thinsp;0.1% of full scale, whichever is greater.\u003c/p\u003e \u003cp\u003eThe overall set-up is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e while Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows a schematic of the gas supply system and rig.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe test conditions and characteristics of the ball and disc test specimens are listed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003e HPR test conditions\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\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 \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eApplied load\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.96 N (200 g), \u003cem\u003ep\u003c/em\u003e\u003csub\u003e\u003cem\u003emax\u003c/em\u003e\u003c/sub\u003e = 0.82 GPa\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStroke length\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1 mm\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStroke frequency\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e50 Hz\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMax sliding speed (mid stroke)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.157 m/s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTest duration in single gas tests\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e75 minutes\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTest temperature\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e25\u0026deg;C\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u0026deg;C\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTest pressure\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1 bar\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBall roughness, Rq\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e9.9\u0026thinsp;\u0026plusmn;\u0026thinsp;1.2 nm\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDisc roughness, Rq\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8 nm\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBall hardness\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e790 HV\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDisc hardness\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e800 HV\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\u003eAlthough all tests were carried out at 25\u0026deg;C it is important to consider possible effects of flash temperature rise within the contact during rubbing, especially when friction coefficients are high. Analysis indicated that the mean flash temperature rise in the contact at mid-stroke subject to the conditions in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and prior to any wear is \u003cem\u003eDT\u003c/em\u003e\u003csub\u003e\u003cem\u003efmean\u003c/em\u003e\u003c/sub\u003e = 61\u0026micro; where \u003cem\u003e\u0026micro;\u003c/em\u003e is the friction coefficient [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e3.2 \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eTest gases\u003c/span\u003e\u003c/h2\u003e \u003cp\u003eThe gases tested in this study are listed in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. These were supplied by BOC (Woking, UK) or CK gas (Newtown Unthank, UK). Hydrogen, ammonia and the hydrocarbons are of interest as possible combustion gases, while carbon dioxide, ammonia and propane are used in refrigeration.\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\u003e Gases tested in this study\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\u003eGas\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSupplier\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePurity\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDry air\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eN\u003csub\u003e2\u003c/sub\u003e, 21% O\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eBOC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOxygen\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCK Gas\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e99.999%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNitrogen\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eN\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eBOC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026gt;\u0026thinsp;99.998% O\u003csub\u003e2\u003c/sub\u003e free\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNitrogen\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eN\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCK Gas\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026gt;\u0026thinsp;99.9999%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHydrogen\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eH\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eBOC/CK Gas\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026gt;\u0026thinsp;99.99% / \u0026gt;99.999%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eArgon\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eBOC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026gt;\u0026thinsp;99.998%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHelium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eHe\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eBOC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026gt;\u0026thinsp;99.996%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAmmonia\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNH\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCK Gas\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e99.98%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCarbon dioxide\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCK Gas\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e99.995%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCarbon monoxide\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCK Gas\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e99.97%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMethane\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCH\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCK Gas\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e99.9995%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEthane\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCH\u003csub\u003e3\u003c/sub\u003eCH\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCK Gas\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e99.5%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePropane\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCH3CH\u003csub\u003e2\u003c/sub\u003eCH\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCK Gas\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e99.5%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003en-butane\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCH\u003csub\u003e3\u003c/sub\u003eCH\u003csub\u003e2\u003c/sub\u003eCH\u003csub\u003e2\u003c/sub\u003eCH\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCK Gas\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e99.5%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eisobutane\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCH\u003csub\u003e3\u003c/sub\u003eCH(CH\u003csub\u003e3\u003c/sub\u003e)CH\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCK Gas\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e99.5%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEthene\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCH\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;CH\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCK Gas\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e99.5%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePropene\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCH\u003csub\u003e3\u003c/sub\u003eCH\u0026thinsp;=\u0026thinsp;CH\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCK Gas\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e99.5%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEthyne\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCH\u0026thinsp;\u0026equiv;\u0026thinsp;CH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eBOC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026gt;\u0026thinsp;98.5%\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=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Surface analysis methods\u003c/h2\u003e \u003cp\u003eSince the tests were run in dry conditions, post-test solvent cleaning prior to surface analysis was not necessary. For some tests with high wear rate, the disc wear track was partially covered by loose wear debris. In this case a gentle blow using N\u003csub\u003e2\u003c/sub\u003e gas was employed to remove this debris. All the disc specimens were immediately stored in 3 ml glass vials after disassembly from the test chamber. These vials were then promptly purged with N\u003csub\u003e2\u003c/sub\u003e gas to create an inert atmosphere, to avoid contamination and oxidation of the discs before any subsequent analysis.\u003c/p\u003e \u003cp\u003eConfocal Raman spectroscopy was used to analyse disc surfaces from HPR tests. This employed an Alpha 300 RA (WITec, Germany) with a 532 nm laser source, a 600 lines/mm grating with 500 nm blaze and a Zeiss EC Epiplan 20\u0026times; objective lens. Spectra were taken with an integration time of 0.5 s for 100 accumulations, giving a total acquisition time of 150 s. The laser power was \u003cem\u003eca\u003c/em\u003e 3 mW.\u003c/p\u003e \u003cp\u003eSome HFR disc surfaces were analysed at end of test using XPS (Thermo Fisher K-Alpha spectrophotometer). Spectra were acquired from a 50 \u0026micro;m \u0026times; 50 \u0026micro;m area. The XPS high resolution spectra were calibrated by using C1s as a reference at 284.8 eV.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. RESULTS","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eIn this section the friction and wear results for each gas are presented and briefly described. Consideration of the origins of the behaviour observed in terms of wear mechanisms and tribofilm formation is deferred to the \u003cspan refid=\"Sec9\" class=\"InternalRef\"\u003eDiscussion\u003c/span\u003e section.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e4.1 \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eIndividual gases\u003c/span\u003e\u003c/h2\u003e \u003cp\u003eAll gases were tested at least twice and friction traces and wear results for all individual tests are included in Supplementary Information. In the friction graphs below, individual, representative friction traces are shown for clarity.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e compares friction coefficient traces in dry air, He, Ar, N\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003e at 1 bar pressure and 25\u0026deg;C while Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows traces for dry air, O\u003csub\u003e2\u003c/sub\u003e, CO, CO\u003csub\u003e2\u003c/sub\u003e and NH\u003csub\u003e3\u003c/sub\u003e. Dry air is included in both graphs for comparative purposes. All gases except for CO and NH\u003csub\u003e3\u003c/sub\u003e show high friction coefficient in the range 0.8 to 1.2. He, Ar, N\u003csub\u003e2\u003c/sub\u003e, He and H\u003csub\u003e2\u003c/sub\u003e give friction coefficient values of \u003cem\u003eca\u003c/em\u003e 1 to 1.2 while dry air and O\u003csub\u003e2\u003c/sub\u003e give slightly lower friction than these gases. N\u003csub\u003e2\u003c/sub\u003e also gives marginally lower friction than the inert gases, especially towards the end of test. Carbon monoxide and ammonia give much lower friction than the other gases shown. The trace for CO is very noisy and this was also seen in two repeat tests (see Supplementary Information).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigures \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e show friction coefficient traces from tests with saturated and unsaturated hydrocarbon gases respectively. Note that the friction scale in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e is different from that in Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e to \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. For the saturated hydrocarbons, methane and ethane show high friction, similar to N\u003csub\u003e2\u003c/sub\u003e, H\u003csub\u003e2\u003c/sub\u003e and the inert gases, suggesting that they do not form effective tribofilms under these conditions. There is however some indication of a slight reduction of friction for ethane in the later stage of the test shown and this was also seen in a repeat test. Propane shows a pronounced friction reduction to a quite low value after about 30 minutes rubbing and this was also seen in a repeat test. n-butane and isobutane both show very rapid friction reduction to values of ca 0.2. These friction drops occur between 1 and 6 minutes after the start of a test.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe unsaturated hydrocarbons, ethene and propene show low friction coefficients of \u003cem\u003eca\u003c/em\u003e 0.2 throughout the test, with no initial high friction at the start as seen with the butanes. Ethyne gives slightly a higher friction coefficient of \u003cem\u003eca\u003c/em\u003e 0.27 but again this low friction is established at the start of the test, indicative of very rapid tribofilm formation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e summarises friction and wear results from all tests, showing the average values from all repeats for each gas. There is generally good correlation between friction and wear reduction, except for dry air, oxygen and propane. Dry air and oxygen show high friction, slightly lower than the inert gases, but extremely high wear, much greater than for the inert gases. This is further discussed in later in the paper. Propane shows intermediate friction but wear that is only slightly lower than the inert gases. This reflects the fact that friction reduction, presumably originating from tribofilm formation, occurs part-way through the tests. This reduces the mean friction, but it is likely that most of the measured wear occurred prior to formation of a protective tribofilm.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e4.2 \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eSwitching gases\u003c/span\u003e\u003c/h2\u003e \u003cp\u003eSome tests were carried out in which two supplied gases were switched periodically to investigate the kinetics and durability of tribofilms using the set-up shown schematically in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. To illustrate the rate at which gases were exchanged, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows how O\u003csub\u003e2\u003c/sub\u003e level varied when switching between ethene and dry air, with O\u003csub\u003e2\u003c/sub\u003e level being monitored using a fast response oxygen sensor in the outlet flow. Gas exchange was rapid, matching the gas supply rate.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e shows two repeat tests in which the atmosphere was switched between propene and dry air. Propene gives low friction, as seen previously in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, but friction then rises as soon as dry air is introduced, suggesting very rapid loss of tribofilm. Then, when propene replaces dry air, friction reduces once more, indicating rapid tribofilm reformation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIt is not clear from the tests shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e whether the loss of tribofilm and consequent rise in friction in air occurred because the film was weak and thus easily removed by rubbing once no propene was available to regenerate it, or whether the oxygen in air had a specifically detrimental chemical effect on the propene tribofilm. To assess this, tests were carried out switching between propene and nitrogen as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. The results were quite variable over time, so four repeats were carried out. It is evident that friction rises when the propene atmosphere is replaced by nitrogen, implying that the tribofilm formed by propene is initially quite weak and being continually formed and removed. It is also evident that friction rises more slowly when propene is replaced by nitrogen than when it is replaced by air, indicating that there is also a direct chemical contribution to tribofilm removal by oxygen, as suggested from in-contact Raman studies [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Most striking is that in three of the four repeats carried out, the tribofilm appears to become stronger over rubbing time in propene since friction takes longer to increase and eventually only partially increases when the atmosphere is switched to nitrogen.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTests were also carried out switching between NH\u003csub\u003e3\u003c/sub\u003e and N\u003csub\u003e2\u003c/sub\u003e and between CO and N\u003csub\u003e2\u003c/sub\u003e to assess the strength of the films formed by these gases. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e shows results for switching between NH\u003csub\u003e3\u003c/sub\u003e and N\u003csub\u003e2\u003c/sub\u003e. During the first switch to N\u003csub\u003e2\u003c/sub\u003e the friction rises sharply, suggesting that the film is rapidly removed. However, during subsequent switches to N\u003csub\u003e2\u003c/sub\u003e the film is lost more slowly, implying that it is becoming more resistant to removal, as was seen with propene.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e shows friction traces from tests in which CO and N\u003csub\u003e2\u003c/sub\u003e gases were switched. Friction rises rapidly after switching to N\u003csub\u003e2,\u003c/sub\u003e indicating loss of CO-originating tribofilm. However, during each subsequent switch to N\u003csub\u003e2\u003c/sub\u003e the friction stabilises at a lower value, possibly suggesting that less and less low friction tribofilm is being removed by rubbing in N\u003csub\u003e2\u003c/sub\u003e. However, the friction in a CO atmosphere appears to increase slightly during successive switches.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOther switching results are included in Supplementary Information.\u003c/p\u003e \u003c/div\u003e"},{"header":"5. DISCUSSION","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e5.1 \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eLow lubricity gases He, Ar, H\u003c/span\u003e\u003csub\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e2\u003c/span\u003e\u003c/sub\u003e, \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eN\u003c/span\u003e\u003csub\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e2\u003c/span\u003e\u003c/sub\u003e, \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eCO\u003c/span\u003e\u003csub\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e2\u003c/span\u003e\u003c/sub\u003e\u003c/h2\u003e \u003cp\u003eHe, Ar and H\u003csub\u003e2\u003c/sub\u003e all showed high friction coefficients in the range 0.9 to 1.2, indicative of little or no lubricity. This is in accord with previous studies [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Some previous work has suggested that H\u003csub\u003e2\u003c/sub\u003e might give higher friction than the inert gases because it might reduce any iron oxides present [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], but this cannot be concluded from the current study. N\u003csub\u003e2\u003c/sub\u003e appears to give slightly lower friction than the inert gases, in the range 0.8 to 1.0. Previous researchers have suggested that N\u003csub\u003e2\u003c/sub\u003e might react with steel during rubbing to form a nitride and that this could be responsible for some friction reduction [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. XPS was therefore carried out on HFR discs from tests with N\u003csub\u003e2\u003c/sub\u003e. These did show very slight traces of nitrogen compounds at \u003cem\u003eca\u003c/em\u003e 400 eV within the rubbed track, but similar traces were also seen on fresh discs and so are not believed to indicate a tribofilm. To assess possible effects of traces of O\u003csub\u003e2\u003c/sub\u003e or H\u003csub\u003e2\u003c/sub\u003eO in the cylinder gases, two types of cylinder N\u003csub\u003e2\u003c/sub\u003e were used, highly pure N\u003csub\u003e2\u003c/sub\u003e N6.0 and also \u0026ldquo;N\u003csub\u003e2\u003c/sub\u003e oxygen free\u0026rdquo; from BOC. However, both gave similar friction behaviour to that shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows that CO\u003csub\u003e2\u003c/sub\u003e gives high friction coefficient of \u003cem\u003eca\u003c/em\u003e 1.0, comparable to that of the inert gases. This is contrary to some previous work. Nunez \u003cem\u003eet al\u003c/em\u003e. found that CO\u003csub\u003e2\u003c/sub\u003e reduced friction of grey cast iron but that this only occurred close to the gas-liquid transition, i.e. at high pressure and/or low temperature [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. At atmospheric pressure they found no friction reduction, in agreement with the current work. Wu \u003cem\u003eet al\u003c/em\u003e. obtained low friction of 52100 steel in CO\u003csub\u003e2\u003c/sub\u003e and related this to the formation of carbonate and bicarbonate [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. However, their study was at considerably lower speed (lower frequency and longer stroke length) than the current work. Velkahvr \u003cem\u003eet al\u003c/em\u003e. used a reciprocating rig at conditions quite similar to the current study and found that CO\u003csub\u003e2\u003c/sub\u003e gave variable friction, ranging between 0.8 and 0.2, with rapid transition between the two values suggesting loss and reformation of a tribofilm. They also detected a carbonate or bicarbonate tribofilm using XPS [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. It appears that CO\u003csub\u003e2\u003c/sub\u003e may have very marginal lubricity that depends critically on the conditions studied.\u003c/p\u003e \u003cp\u003eAll five gases show high wear, in the range 500 to 600 \u0026micro;m in wear scar diameter, with helium giving somewhat lower wear than the others. This might originate from very small levels of impurity in the gas since the specification for the He used was only O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;\u0026le;\u0026thinsp;7 ppm, H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;\u0026le;\u0026thinsp;5ppm.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e5.2 \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eDry air and O\u003c/span\u003e\u003csub\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e2\u003c/span\u003e\u003c/sub\u003e\u003c/h2\u003e \u003cp\u003eDry air and O\u003csub\u003e2\u003c/sub\u003e gave high friction coefficients in the 0.8 to 0.9 range, slightly lower than the five gases above, but they both produced extremely high wear, much larger than the wear seen with all other gases. Indeed, this wear was so large that the final wear scar diameters exceeded the reciprocating stroke length, so towards the end of tests the contacts may have been operating in fretting rather than sliding wear mode [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Pure O\u003csub\u003e2\u003c/sub\u003e gas gave slightly higher wear and slightly lower friction than dry air (21% O\u003csub\u003e2\u003c/sub\u003e).\u003c/p\u003e \u003cp\u003eThis very high wear probably represents classical oxidational wear, where, in dry conditions, iron oxides are formed during rubbing and then removed by mechanical action \u0026ndash; effectively corrosive-abrasive wear. Such wear in air was first identified in the 1950s [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] but the concept of oxidational wear was formalised and modelled in a series of papers by Quinn from the 1960s [\u003cspan additionalcitationids=\"CR31 CR32\" citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Quinn\u0026rsquo;s oxidational wear model is based on the assumption that wear occurs at high temperatures generated by rubbing, but in the current work the calculated mean flash temperature at mid-stroke is relatively low (\u003cem\u003eca\u003c/em\u003e 60\u0026deg;C at a friction coefficient of 1.0), implying an initial total surface temperature of less than 100\u0026deg;C. Of course, Quinn\u0026rsquo;s oxidational model preceded knowledge of, and thus takes no account of, the possibility of mechanochemical effects.\u003c/p\u003e \u003cp\u003eAlthough the wear is much higher when O\u003csub\u003e2\u003c/sub\u003e is present than in oxygen-free N\u003csub\u003e2\u003c/sub\u003e, the friction coefficient is similar at \u003cem\u003eca\u003c/em\u003e 0.8. This might be because, although wear is dominated by oxide loss, friction is dominated by adhesion between metallic asperities. Alternatively, this friction coefficient may reflect the actual value of friction coefficient of iron oxide on iron oxide. To explore this a test was carried out using haematite (Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) HPR ball and disc in helium. This is shown in Supplementary Information and indicates that dry Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e gives a friction coefficient approaching 1.0. Thus the friction measured when steel surfaces are rubbed in dry air or oxygen may simply reflect the value of iron(III) oxide against iron(III) oxide, although it may equally well originate from iron/iron asperity adhesion; or quite possibly a combination of both.\u003c/p\u003e \u003cp\u003eA key difference between an oxygen-free and an oxygen-containing atmosphere may simply be that in the former, many metallic wear particles remain adhered to the rubbing surfaces, supressing material loss, while in the latter, as ferrous oxide they do not adhere strongly and thus are easily dislodged from the rubbing surfaces, so enabling high rate of material loss.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e5.3. \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eLubricious non-hydrocarbon gases\u003c/span\u003e\u003c/h2\u003e \u003cp\u003eIn Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e it can be seen that the two gases, NH\u003csub\u003e3\u003c/sub\u003e and CO, give much lower wear than the other non-hydrocarbon gases studied, indicating that these form lubricious tribofilms on the rubbing surfaces. Little previous research has been carried out on the friction and wear of these gases although Blanchet \u003cem\u003eet al\u003c/em\u003e. have suggested that blends of CO with H\u003csub\u003e2\u003c/sub\u003e may generate carbonaceous tribofilms of rubbing ceramic surfaces at high temperature [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFrom the gas switching results in Figs.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e and \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e it is evident that the initially-formed tribofilms are not very strong in that they are removed by rubbing in N\u003csub\u003e2\u003c/sub\u003e. However, for NH\u003csub\u003e3\u003c/sub\u003e the tribofilm appears to become more slowly removed after several switching cycles, while for CO the film seems to become fully resistant to rubbing over time.\u003c/p\u003e \u003cp\u003eTo explore the origins of this friction reduction, XPS and Raman surface analysis were carried out on the rubbed tracks of HPR discs at the end of tests in the two gases.\u003c/p\u003e \u003cp\u003eXPS survey spectra from a fresh steel disc surface and from rubbed tracks tests in NH\u003csub\u003e3\u003c/sub\u003e and CO are shown in Supplementary Information. A strong nitrogen signal was only observed from the sample rubbed in NH\u003csub\u003e3\u003c/sub\u003e and its high resolution N1s and C1s XPS spectra are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e. Corresponding spectra from a fresh, unrubbed, disc surface are included in Supplementary Information. The N1s spectrum shows evidence of iron nitride (397.94 eV and 398.67 eV) as well as adsorbed ammonia (400 eV) [\u003cspan additionalcitationids=\"CR35\" citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. Interestingly, the C1s spectrum contains peaks from C-N and C\u0026thinsp;=\u0026thinsp;N. This supports the interpretation that the tribofilm formed in NH\u003csub\u003e3\u003c/sub\u003e contains nitrides. The peak from C-C bonds most likely originates from adventitious carbon. It should be noted that a Raman spectrum of this worn surface did not contain an obvious nitride-related peak, probably because the tribofilm is very thin. This may explain the difficulty in identifying this film in previous studies.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe XPS spectrum obtained from the disc tracks rubbed in CO (Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e) contains a -C-C- peak from adventitious carbon. The peak at 286 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e may corresponds to adsorbed CO or adventitious carbon [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. Ther are also peaks from polar carbon and carbonate moieties. This is consistent with the literature, where metal carbonate has been detected in tribofilms formed on steel surfaces rubbed in CO\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e5.4 \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eSaturated Hydrocarbons\u003c/span\u003e\u003c/h2\u003e \u003cp\u003eFigures \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e and \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e showed the influence of five saturated gaseous hydrocarbons on friction and wear and indicates that their lubricity increases markedly with molecular weight. Methane and ethane give friction and wear values similar to the inert gases, suggesting negligible tribofilm formation, except for a possible slight reduction in friction for ethane towards end of test. Propane apparently forms a tribofilm slowly, such that friction drops to a relatively low value part-way through each test, and this is reflected in an intermediate wear result. The two butanes show very low friction after 1\u0026ndash;5 minutes rubbing and low wear, suggesting quite rapid tribofilm formation. There is no apparent difference in performance between the linear and the branched butane.\u003c/p\u003e \u003cp\u003ePrevious work using both gaseous and liquid saturated hydrocarbons has shown that these can form lubricious carbonaceous films readily during rubbing in oxygen-free atmospheres [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. It is generally accepted that this film formation is driven by the severe conditions present in rubbing contacts that break C-C and/or C-H covalent bonds to form free radical species that then rearrange, lose hydrogen atoms, and combine to form graphitic material on the rubbing surfaces. There is some debate about the relative importance of the impact of mechanical stresses and surface catalysis on the reaction, and it is probable that catalysis is most important with surfaces that contain catalytically-active atoms such as Cu, Ni and Pt, but that mechanochemical effects are more prevalent with bearing steels. Previous work has shown that propane forms a low friction carbonaceous film on rubbing AISI 52100 surfaces [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e] and that butane reacts on rubbing ceramic surfaces to form a high molecular weight material [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. The formation of such films by methane or ethane on steel surfaces has not been reported but has been shown that methane will form such films on highly catalytic nickel-containing surfaces [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The fact that methane does not appear to form effective tribofilms on steel surfaces but does so on catalytic ones may imply that its reaction is controlled by a catalytic rather than mechanical response. Two effects might explain the fact that carbonaceous tribofilm formation by saturated hydrocarbons increases with molecular weight. One is simply that there should be stronger adsorption of the gaseous molecules on the surfaces as the molecular weight increases. The second is that the mechanical forces experienced by interior C-C bonds will increase with the size of the groups attached to each C atom as forces are transmitted along the chain. Also, of course, methane, with just one carbon atom, cannot experience C-C bond cleavage to form a hydrocarbyl free radical.\u003c/p\u003e \u003cp\u003eTo confirm the formation of a carbonaceous tribofilm, an HPR disc from a test with n-butane was analysed using surface Raman as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e. Characteristic carbon D- and G- bands can be seen in spectra from some parts of the worn surface, while only iron oxide peaks are observed in other places. The morphology of the tribofilm shown by optical micrographs confirmed that the tribofilm is heterogeneous. It should be noted that the iron oxide may have been formed after tests, when samples were exposed briefly to air for Raman analysis.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e5.5 \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eUnsaturated Hydrocarbons\u003c/span\u003e\u003c/h2\u003e \u003cp\u003eFigures \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e and \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e showed that ethene, propene and ethyne give very low friction and wear, implying that they form protective tribofilms very readily. It is well known that unsaturated hydrocarbons form carbonaceous films more easily that saturated ones [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. The precise mechanism of formation is not yet clear \u0026ndash; it is evidently promoted by rubbing, but the extent to which simple polymerisation as opposed to stress-driven radical reactions occurs in not known.\u003c/p\u003e \u003cp\u003eRaman spectra from various regions of disc surfaces rubbed in ethene and ethyne are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e. Compared to results from butane, the surface coverage of carbonaceous materials increases as we go from this to ethene, then to ethyne. This supports the interpretation that the ease of carbonaceous film formation increases with the degree of unsaturation of the gas.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAlthough the formation of carbonaceous film increases with unsaturation, this does not result in a progressive reduction in friction and wear; wear is similar for all three unsaturated hydrocarbons studied while friction is actually slightly higher for ethyne than for ethene.\u003c/p\u003e \u003c/div\u003e"},{"header":"6. CONCLUSIONS","content":"\u003cp\u003eThis study has used a sealed, reciprocating, ball of flat tribometer, the HPR, to measure the friction and wear properties a steel/steel sliding contact in a range of gaseous environments of tribological interest. It is found that the inert gases helium and argon and also nitrogen and hydrogen give high friction and wear indicating that they form little or no protective tribofilm. Dry air and pure oxygen give friction similar to nitrogen but produce extremely high wear. This is believed to originate from oxidational wear when rapidly-forming iron oxide is abraded from the rubbing surfaces.\u003c/p\u003e \u003cp\u003eAmmonia and carbon monoxide both show relatively low friction and wear, indicating that they form tribofilms on the rubbing surfaces. Gas-switching studies suggest that these tribofilms are initially weak but that they become stronger and have lower friction after repeated switching. XPS analysis on the rubbed surfaces shows that ammonia both adsorbs on the surfaces and forms nitrides, while carbon monoxide forms a carbonate.\u003c/p\u003e \u003cp\u003eThe saturated hydrocarbon gases show decreasing friction and wear reduction with increase of molecular weight. Thus methane and ethane show no friction reduction, propane forms a film only after about 30 minutes rubbing and n-butane and isobutane form films after between 1 and 5 minutes rubbing. Raman analysis of a disc track rubbed in n-butane shows the presence of D- and G- carbon bands, characteristic of graphitic carbon.\u003c/p\u003e \u003cp\u003eThe unsaturated gases, ethene, propene and ethyne all show an immediate reduction in friction and give low wear. Raman analysis of the rubbed surfaces shows the formation of a carbonaceous film, with this increasing with the level of unsaturation of the hydrocarbon.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eACKNOWLEDGEMENTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors wish to thank PCS Instruments for supply of an HPR test rig, the Shell UTC for Mobility and Lubricants for support for Jie Zhang, and Shouyi Yin for the help with XPS.\u003c/p\u003e\n\u003cp\u003eFor the purpose of open access, the authors have applied for a Creative Commons Attribution (CC BY) license to any Author Accepted Manuscript version arising.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCRediT authorship contribution statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eJ.Z.: Investigation, Analysis; J.S.S.W.: analysis, writing \u0026ndash; review and editing; H.A.S.: conceptualisation, supervision, project administration, writing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of competing interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDATA AVAILABILITY\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData that supports the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eSupporting Information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Supporting Information is available free of charge at XXX. It contains friction test and XPS results.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBowden, F.P., Hughes, T.P. The friction of clean metals and the influence of adsorbed gases. The temperature coefficient of friction. Proc. Roy. Soc. Lond. A172, 263-279, (1939).\u003c/li\u003e\n\u003cli\u003eSoda, N., Sasada, T., Mechanism of lubrication by surrounding gas molecules in adhesive wear. Trans ASME J. Lub. Techn. 100, 492-497, (1978)\u003c/li\u003e\n\u003cli\u003eMishina, H. Atmospheric characteristics in friction and wear of metals. Wear 152, 99-110 (1992).\u003c/li\u003e\n\u003cli\u003eMishina, H. Chemisorption of diatomic gas molecules and atmospheric characteristics in adhesive wear and friction of metals. Wear, 180(1-2), 1-7, (1995).\u003c/li\u003e\n\u003cli\u003eWilson, J. E., F. H. Stott, Wood, G.C. The development of wear-protective oxides and their influence on sliding friction. Proc. Roy. Soc. Lond. A369, 557-574, (1980).\u003c/li\u003e\n\u003cli\u003eZhang, Q. Y., Chen, K. M., Wang, L., Cui, X.H., Wang, S.Q. Characteristics of oxidative wear and oxidative mild wear. Tribol. Intern., 61, 214-223, (2013).\u003c/li\u003e\n\u003cli\u003eWu, X., Cong, P., Nanao, H., Minami, I., Mori, S. Tribological behaviors of 52100 steel in carbon dioxide atmosphere. Tribol. Lett., 17, 925-930, (2004).\u003c/li\u003e\n\u003cli\u003eNunez, E.E, Polychronopoulou, K., Polycarpou, A.A. Lubricity effect of carbon dioxide used as an environmentally friendly refrigerant in air-conditioning and refrigeration compressors. Wear 270, 46-56, (2010).\u003c/li\u003e\n\u003cli\u003eRosenberg, S.J., Jordan, L. Influence of oxide films on the wear of steels. J. of Res. of the National Bureau of Standards, 13, RP708, pp. 267-279, (1934).\u003c/li\u003e\n\u003cli\u003eBuckley, D.H, Johnson, R.L. Effect of inert, reducing and oxidising atmospheres on friction and wear of metals to 1000\u0026deg;F. NASA Techn. Note D-1103, Oct 1961\u003c/li\u003e\n\u003cli\u003eBatchelor, A.W., Stachowiak, G.W., Cameron, A. The relationship between oxide films and the wear of steels. Wear, 113, 203-223, (1986).\u003c/li\u003e\n\u003cli\u003eFink M. Wear oxidation \u0026ndash; a new component of wear. Trans. Amer. Soc for Steel Treating 18, 1026-1034, (1930)\u003c/li\u003e\n\u003cli\u003eSmith, A.F. Influence of environment on the unlubricated wear of 316 stainless steel at room temperature. Tribol. Intern. 19(1), 3-10, (1986).\u003c/li\u003e\n\u003cli\u003eFeng, I.M., Uhlig, H.H. Fretting corrosion of mild steel in air and in nitrogen. Trans ASME \u003cem\u003eJ. Appl. Mech\u003c/em\u003e\u003cem\u003e.\u003c/em\u003e 21, 395-400, (1954)\u003c/li\u003e\n\u003cli\u003eSo, H. The mechanism of oxidational wear. Wear, 184, 161-167, (1995). \u003c/li\u003e\n\u003cli\u003eCornelius, D.E., Roberts, W.H., Friction and wear of metals in gases up to 600 C. ASLE Trans. 4, 20-32 (1961)\u003c/li\u003e\n\u003cli\u003eVelkavrh, I., Ausserer, F., Klien, S., Brenner, J., For\u0026ecirc;t, P., Diem, A. The effect of gaseous atmospheres on friction and wear of steel\u0026ndash;steel contacts. Tribol. Intern., 79, 99-110, (2104).\u003c/li\u003e\n\u003cli\u003eZhang, Y., Jourani, A. Combined effect of microstructure and gaseous environments on oxidative and adhesive wear of dual-phase steel. J. of Mats Eng. \u0026amp; Perf. 30, 9333-9351, (2021)\u003c/li\u003e\n\u003cli\u003eKruger, J., Yolken, H.T. Room temperature oxidation of iron at low pressures. Corrosion 20, 29t-33t, (1964).\u003c/li\u003e\n\u003cli\u003eVelkavrh, I., Ausserer, F., Klien, S., Voyer, J., Ristow, A., Brenner, J., For\u0026ecirc;t, P., Diem, A., The influence of temperature on friction and wear of unlubricated steel/steel contacts in different gaseous atmospheres. Tribol. Intern. 98, 155-171, (2016).\u003c/li\u003e\n\u003cli\u003eMurray, S.F., Johnson R.L., Swikert , M.A., Boundary lubrication of steel with fluorine- and chlorine-substituted methane and ethane gases. NACA Techn Note 3401, Lewis Flight Propulsion Laboratory, Cleveland, Ohio, (1955).\u003c/li\u003e\n\u003cli\u003eRamirez, G., Eryilmaz, O.L., Fatti, G., Righi, M.C., Wen, J., Erdemir, E. Tribochemical conversion of methane to graphene and other carbon nanostructures: implications for friction and wear. ACS Applied Nano Materials 3, 8060-8067, (2020).\u003c/li\u003e\n\u003cli\u003eZhang, J., Bolle, B., Wong, J.S.S., Spikes H.A. Influence of atmosphere on carbonaceous film formation in rubbing, metallic contacts. Tribol. Lett. 72:4, (2024).\u003c/li\u003e\n\u003cli\u003eLauer, J.L., Dwyer, S.R. (1991). Tribochemical lubrication of ceramics by carbonaceous vapors. Tribol. Trans. 34, 521-528, (1991).\u003c/li\u003e\n\u003cli\u003eBlanchet, T.A., Lauer, J.L., Liew, Y.F., Rhee, S.J., Sawyer, W.G. Solid lubrication by decomposition of carbon monoxide and other gases. Surface \u0026amp; Coatings Techn. 68 446-452, (1994).\u003c/li\u003e\n\u003cli\u003eNakayama, K., Hashimoto, H. Triboemission, tribochemical reaction, and friction and wear in ceramics under various n-butane gas pressures. Tribol. Intern. 29, 385-393, (1996).\u003c/li\u003e\n\u003cli\u003eGreenwood, J.A.: An interpolation formula for flash temperatures. Wear 150, 153-158, (1991)\u003c/li\u003e\n\u003cli\u003eReddyhoff, T., Schmidt, A., Spikes, H.A., Thermal conductivity and flash temperature. Tribol. Lett. 67:22, (2019)\u003c/li\u003e\n\u003cli\u003eFouvry, S., Kapsa, P., Vincent, L.: Quantification of fretting damage. Wear 200, 186\u0026ndash;205 (1996)\u003c/li\u003e\n\u003cli\u003eQuinn, T.F.J. Role of oxidation in the mild wear of steel. Brit. J. of Appl. Phys. 13, 33-37, (1962)\u003c/li\u003e\n\u003cli\u003eQuinn, T.F.J. Oxidational wear. Wear, 18, 413-419, (1971)\u003c/li\u003e\n\u003cli\u003eQuinn, T.F.J. Review of oxidational wear: Part I: The origins of oxidational wear. Tribol. Intern. 16, 257-271, (1983)\u003c/li\u003e\n\u003cli\u003eQuinn, T.F.J. Review of oxidational wear part II: recent developments and future trends in oxidational wear research. Tribol. Intern. 16 305-315, (1983)\u003c/li\u003e\n\u003cli\u003eVinod, K. R., Saravanan, P., Sakar, M., Balakumar, S. Insights into the nitridation of zero-valent iron nanoparticles for the facile synthesis of iron nitride nanoparticles. RSC Advances, \u003cem\u003e6\u003c/em\u003e(51), 45850-45857. (2016).\u003c/li\u003e\n\u003cli\u003ePeng, H., Mo, Z., Liao, S., Liang, H., Yang, L., Luo, F., Song, H. Zhong, Y., Zhang, B. High performance Fe-and N-doped carbon catalyst with graphene structure for oxygen reduction. Scientific reports, 3(1), 1765, (2013).\u003c/li\u003e\n\u003cli\u003eWojciechowski, P., Lewandowski, M. Iron nitride thin films: Growth, structure, and properties. Crystal Growth \u0026amp; Design, 22(7), 4618-4639, (2022).\u003c/li\u003e\n\u003cli\u003eLukashuk, L., Yigit, N., Rameshan, R., Kolar, E., Teschner, D., H\u0026auml;vecker, M., Knop-Gericke, A., Schl\u0026ouml;gl, R., F\u0026ouml;ttinger, K., Rupprechter, G. Operando insights into CO oxidation on cobalt oxide catalysts by NAP-XPS, FTIR, and XRD. ACS catalysis 8.9. 8630-8641, (2018)\u003c/li\u003e\n\u003cli\u003eZhang, J., Campen, S., Wong, J., Spikes, H. Oxidational wear in lubricated contacts\u0026ndash;Or is it? Tribol. Intern., 165, 107287, (2022).\u003c/li\u003e\n\u003cli\u003eLi, Y.S., Jang, S., Bhuiyan, F.H., Martini, A., Kim, S.H. Molecular structure and environment dependence of shear-driven chemical reactions: tribopolymerization of methylcyclopentane, cyclohexane and cyclohexene on stainless steel. Tribol. Letters, 71, 49, (2023).\u003c/li\u003e\n\u003cli\u003eHe, X., Kim, S.H. Mechanochemistry of physisorbed molecules at tribological interfaces: molecular structure dependence of tribochemical polymerization. Langmuir 33, 2717-2724, (2017)\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"tribology-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"tril","sideBox":"Learn more about [Tribology Letters](https://www.springer.com/journal/11249)","snPcode":"11249","submissionUrl":"https://submission.nature.com/new-submission/11249/3","title":"Tribology Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Gas lubrication, ammonia, carbon monoxide, friction, wear, Raman, XPS","lastPublishedDoi":"10.21203/rs.3.rs-4445568/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4445568/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eA sealed reciprocating tribometer has been used to study the influence of different gaseous environments on the friction and wear properties of AISI52100 bearing steel at atmospheric pressure and 25°C. Helium, argon, hydrogen, carbon dioxide and nitrogen all give high friction and wear, suggestive of very little, if any tribofilm formation under the conditions studied. Dry air and oxygen also give high friction, slightly lower than the inert gases, but produce extremely high wear, much higher than the inert gases. This is characteristic of the phenomenon of “oxidational wear”. The two gases ammonia and carbon monoxide give relatively low friction and wear, and XPS analysis indicates that this is due to the formation of adsorbed ammonia/nitride and carbonate films respectively.\u003c/p\u003e\n\u003cp\u003eFor the hydrocarbon gases studied, two factors appear to control friction and wear, degree of unsaturation and molecular weight. For the saturated hydrocarbons, methane and ethane give high friction and wear but propane and butane give low friction after a period of rubbing that decreases with molecular weight. The unsaturated hydrocarbons all give an immediate reduction in friction with correspondingly low wear. Raman analysis shows that all the hydrocarbons that reduce friction and wear form a carbonaceous tribofilm of the rubbed surfaces.\u003c/p\u003e","manuscriptTitle":"The lubricity of gases","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-05-31 08:35:48","doi":"10.21203/rs.3.rs-4445568/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-07-24T10:11:06+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-07-12T17:34:00+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-06-24T10:04:02+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"161630692349754005139432512230369576637","date":"2024-06-07T22:32:50+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"27311980854467238256188432306173830687","date":"2024-05-25T06:55:50+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-05-23T18:44:53+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-05-20T11:37:43+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-05-20T11:37:43+00:00","index":"","fulltext":""},{"type":"submitted","content":"Tribology Letters","date":"2024-05-19T20:19:13+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"tribology-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"tril","sideBox":"Learn more about [Tribology Letters](https://www.springer.com/journal/11249)","snPcode":"11249","submissionUrl":"https://submission.nature.com/new-submission/11249/3","title":"Tribology Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"13ed42b0-200d-486b-b19b-adb86c2a4443","owner":[],"postedDate":"May 31st, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-09-02T16:05:11+00:00","versionOfRecord":{"articleIdentity":"rs-4445568","link":"https://doi.org/10.1007/s11249-024-01911-y","journal":{"identity":"tribology-letters","isVorOnly":false,"title":"Tribology Letters"},"publishedOn":"2024-08-28 15:58:08","publishedOnDateReadable":"August 28th, 2024"},"versionCreatedAt":"2024-05-31 08:35:48","video":"","vorDoi":"10.1007/s11249-024-01911-y","vorDoiUrl":"https://doi.org/10.1007/s11249-024-01911-y","workflowStages":[]},"version":"v1","identity":"rs-4445568","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4445568","identity":"rs-4445568","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}