5.1 Low lubricity gases He, Ar, H2, N2, CO2
He, Ar and H2 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 H2 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. N2 appears to give slightly lower friction than the inert gases, in the range 0.8 to 1.0. Previous researchers have suggested that N2 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 N2. 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 O2 or H2O in the cylinder gases, two types of cylinder N2 were used, highly pure N2 N6.0 and also “N2 oxygen free” from BOC. However, both gave similar friction behaviour to that shown in Fig. 3.
Figure 4 shows that CO2 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 CO2 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 CO2 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 CO2 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 CO2 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 O2 ≤ 7 ppm, H2O ≤ 5ppm.
5.2 Dry air and O2
Dry air and O2 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 O2 gas gave slightly higher wear and slightly lower friction than dry air (21% O2).
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 O2 is present than in oxygen-free N2, 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 (Fe2O3) HPR ball and disc in helium. This is shown in Supplementary Information and indicates that dry Fe2O3/Fe2O3 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, NH3 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 H2 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 N2. However, for NH3 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 NH3 and CO are shown in Supplementary Information. A strong nitrogen signal was only observed from the sample rubbed in NH3 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 NH3 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 CO2 [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.