The ability of relatively unreactive organic molecules such as hydrocarbons to form tribofilms on rubbing metal surfaces has been noted and explored several times over the last seventy years in the context of various research fields. It was first reported in electrical switchgear when, in 1958, Hermance and Egan noted the formation of insulating organic deposits on non-arcing palladium and other platinum-group electrical switches [1]. They ascribed these to the reaction of adventitious organic vapour contaminants on rubbing surfaces and termed the product “frictional polymer” [1]. Considerable further research followed in this area [2–5] and although ability to analyse the product was limited it was determined that the mechanism of formation was most probably a free radical process promoted by surface catalysis and rubbing [4]. The phenomenon is still being researched to the present day [6, 7].
In the 1960s there was considerable interest in the boundary lubricating ability of hydrocarbon base oils and it was found that these formed carbon-containing films on rubbing steel ball surfaces in 4-ball testers in some conditions [8, 9]. The role of oxygen was explored and it was noted that carbon-based films were formed during rubbing in argon, resulting in superior tribological performance at low loads. However, an oxygen atmosphere gave better performance at very high loads due to oil oxidation products [8].
From ca 2000 there was concern that the release of active hydrogen from organic lubricants during rubbing might embrittle steel to promote rolling contact fatigue and, especially in recent years, might help generate white etching cracks. There was also interest in lubricants for use in space. Both these areas of interest led to research on the behaviour of lubricants in vacuum, where hydrogen and other molecular fragments generated during rubbing of steel surfaces in liquid lubricants and greases could be monitored using a mass spectrometer [10–14]. John et al. detected small hydrocarbon fragments formed from a high MWt hydrocarbon base oil and found that more and smaller fragments were detected when rubbing took place than without rubbing [10]. Tests were also carried out in nitrogen and produced similar hydrocarbon fragments and a black surface tribofilm, which Raman analysis showed to contain graphitic carbon. Kohara et al. tested a range of base fluid types and found that hydrogen was evolved during high friction asperity contact events. By comparing the behaviour of hydrocarbon and fluorocarbon base oils they demonstrated that the hydrogen originated from the lubricant [11]. Lu et al. found that hydrogen evolution only occurred above a critical applied load and then was both load and sliding speed dependent. They proposed a mechanism based on hydrocarbon adsorption on nascent metal followed by surface-catalysed unzipping of the molecular chain [13].
Over the last decade, research on the formation of carbonaceous films has accelerated considerably. Now a key focus appears to be whether tribofilms formed in this way can be of practical value in terms of reducing friction and wear, although other work has simply reported the formation of such films.
Erdemir and co-workers have explored the possibility of using catalytically-active, metal-containing coatings to promote the formation of carbonaceous tribofilms [15]. They found that steel surfaces coated with a composite of copper with molybdenum or vanadium nitride gave lower friction and surface damage than uncoated steel when lubricated with both PAO base oil and a formulated engine oil. A carbonaceous tribofilm was detected on the coated surfaces but not the uncoated ones. The work has subsequently been extended to other types of organic lubricant including methane and ethanol [16–18], and to other catalytic metals [19]. Molecular modelling was carried out to help elucidate the mechanism of film formation. This was performed at 1000K to mimic asperity contact temperatures and from it the authors suggested a combination of C-C bond and C-H bond scission leading to dehydrogenation followed by aromatisation. Normally C-H bonds are stronger than C-C ones, but it was suggested that catalysis served to promote a classical dehydrogenation reaction [20].
Wang and co-workers have studied strained ring molecules based on cyclopropane that might undergo C-C scission more easily than other hydrocarbons [21–24]. They found that the addition of 2.5% wt. of cyclopropyl-carboxylic acid to PAO produced a very large reduction in wear and a more modest reduction in friction. Raman surface analysis indicated the formation of a carbonaceous film on the rubbed surfaces. The researchers proposed that the acid group promotes adsorption on the metal surfaces, where pressure and flash temperature transform the additive to a carbon-based solid film.
Carbonaceous tribofilms have also been detected and credited with reducing wear in tests with an ester biofuel [25], and synthetic [26] and natural [27] ester lubricants, as well as on retrieved metal-metal hip joints, where they were presumed to be formed from denatured proteins [28].
Based on the above, it is evident that the formation of carbonaceous tribofilms can be strongly promoted by surface catalysis. This need not be from deliberately modified surface coatings but may also result from the presence of catalytic additives. Two studies have used Raman analysis to study tribofilm formation by the friction modifier additive molybdenum dialkydithiocarbamate (MoDTC) and noted that a carbon-based film was formed quite readily alongside MoS2 when MoDTC was present, but was not formed from MoDTC-free base oil [29, 30]. The propensity of carbonaceous tribofilm formation on Cu and Pt-group metals was noted above and it is also possible that the carbonaceous film observed on retrieved metal hip implants [29], and in wear tests on such implants [31], is promoted by the presence of catalytic metals in the CoCrMo alloy employed. Recently Khan et al. showed that the extent of carbonaceous film formation and also the level of wear in a reciprocating ball on flat contact lubricated by PAO varied between steels and suggested that this variation originates from the presence of catalytically-active metals and oxides in some alloys [32].
A key aspect of research on carbonaceous tribofilms since the 1990s [33] has been the use of surface Raman spectroscopy to detect a pair of adsorption bands at wavenumbers ca 1350 and 1570 cm− 1. These bands, known respectively as the D and G bands, are indicative of the presence of amorphous or graphitic carbon [34] and their existence on cleaned, rubbed surfaces serves as a relatively straightforward way to establish the presence of a carbonaceous tribofilm. Indeed, detection of these bands was used in all the post-2000 references cited above except for refs 11 to 13. Recently it has been suggested that these bands should not be taken as proving unequivocally that amorphous or graphitic carbon is produced during rubbing, since it is possible that other types of carbon-rich material may be converted into a graphitic structure during exposure to the intense laser beam used in Raman analysis [35]. This does not, of course, nullify the value of the method for demonstrating the presence of a carbonaceous tribofilm, simply the interpretation of this film as having a graphitic content.
Most of the experimental research outlined above, except for the work in vacuum, was carried out in laboratory air. In early work Vinogradov et al. compared wear and seizure of hydrocarbons in argon and oxygen in four ball tests and found that a hard coating which they ascribed to carbide was formed on the ball surface in argon [8]. Argibay and co-workers carried out controlled atmosphere tests in which a sapphire ball was rubbed against flat coated in a thin film of Pt/Au alloy [6, 7, 36]. Organic lubricants were either adventitious trace hydrocarbons or, in some tests, a stream of isopropanol/water delivered via the vapor phase. Tests in air gave high friction, but in nitrogen friction dropped to a very low value during rubbing and a carbonaceous film that the authors suggested to be amorphous carbon DLC was identified using Raman. FIB/TEM showed this to be 50 to 100 nm thick.
Recently the authors of the current paper have carried out a systematic study of the influence of oxygen level on the friction and wear of a reciprocating steel ball on steel disc contact lubricated by isooctane and hexadecane [37]. Typical results are shown in Fig. 1. They indicate that both friction and wear are much lower in nitrogen (and argon) than in dry air.
Raman surface analysis showed that at zero and very low oxygen levels a carbonaceous film was always formed rapidly on the rubbing surfaces and was present inside and around the rubbed track. In tests in air and above about 5% O2, only iron oxides were detected on the rubbed surfaces.
The drop in friction as oxygen level is reduced shown in Fig. 1 appears at first sight to be progressive but, as can be seen in Fig. 2, when friction is monitored during a test at intermediate oxygen levels it is evident that friction switches intermittently between a high and low value to give the average value seen in Fig. 1.
Very recently, Li et al have studied the influence of carrier gas on the lubrication of stainless steel ball on flat contacts by hydrocarbons delivered using vapour phase lubrication [38]. They found similar behaviour to the above, i.e. saturated hydrocarbons produced carbonaceous tribofilms and gave low friction and wear in both 100%N2 and a blend of 10%H2 in N2 carrier gas, but not in O2. When an O2 stream was employed, only an unsaturated hydrocarbon was able form such a tribofilm.
These studies, together with the work of Argibay [6] and also observations of tests in a vacuum chamber in air and N2 [10] suggest that carbon tribofilm formation can be greatly facilitated, even in the absence of overtly catalytic surfaces, by replacing the oxygen in the atmospheric environment by nitrogen. The current paper explores this further to consider the kinetics of carbonaceous film formation, the composition of the tribofilm, its mechanism of formation and its potential utility.