1 Introduction
Premature failures and surface fatigue of bearings in wind turbine gearboxes significantly decrease their lifespan. Lubricant additives contribute to factors leading to micropitting, i.e., a prevalent surface fatigue in wind turbine gearboxes. ZDDP is an essential additive to mitigate excessive wear and scuffing, however, it has also been shown to accelerate micropitting [
1]. In our previous paper [
2], we showed that an amine-based OFM in combination with ZDDP can alleviate micropitting on steel surfaces under defined conditions. The effect of different OFMs on surface fatigue, however, cannot be generalised to be detrimental or beneficial owing to the broad OFM category encompassing molecules with different polar head chemistry. This can be evidenced from results showing an oleic acid OFM was not effective to inhibit micropitting in harsh tribo-contacts, while signs of improvement were observed in less severe contacts [
3]. Therefore, the chemical composition of OFMs has an impact on surface fatigue performance, and more systematic studies are required to reveal the impact of different chemistries of amine-based OFMs on micropitting performance of wind turbine gearbox lubricants. Moreover, the study of oleic acid [
3] was conducted in the absence of other lubricant additives, nevertheless chemical interactions between additives in the lubricant and their competition in terms of surface chemisorption [
4] are indications of the importance of studying OFMs in combination with other lubricant additives. Accordingly, this study elucidates the tribochemical influence of amine-based OFMs in combination with ZDDP anti-wear additive.
The chemistry of tribo-induced films from amine-based OFMs in combination with ZDDP is not fully understood nor is their impact on surface fatigue. This study investigates the influence of three different amine-based OFMs on friction, surface fatigue and composition of surface reactions films. The OFMs differ in the polar moiety. In this study, we show that OFM’s chemistry has an influence on the surface reaction film thickness and composition and hence alters tribological performance. Results in this study also highlight the influence of tribological parameters such as surface roughness and contact geometry on reaction film formation. Among three OFM chemistries, Ethoxylated (2) Tallowalkyl-Amine (ETA) showed the lowest friction, thinnest reaction film and larger mild wear and N-Tallow-1,3-DiaminoPropane (TDP) showed the highest film build-up kinetics and superior surface fatigue performance showing the impact of polar head chemistry of OFMs on tribological performance.
4 Discussion
OFMs on steel surfaces are, in general, believed to reduce friction through surface adsorption which might lead to chemical reactions with steel surfaces depending on the chemistry of the polar moiety. The friction performance of an OFM on steel surfaces in the absence of other additives strongly depends on its packing density, the bonding energy of the polar head to the surface, and the length of hydrocarbon tail. It has been shown that double bonds, especially
cis bonds in the hydrocarbon tail, deteriorate packing density resulting in less intra molecular van der Waals interactions between neighbouring tails and hence disrupt friction reduction performance of OFMs [
27]. The hydrocarbon chain in the current study was a tallow chain which has a high
cis to
trans bond ratio and a high degree of unsaturation (see Table
3) especially in C16 and C18 chains. Therefore, kinks in tallow result in a poor packing density. The low packing density of the OFMs used in this study provides free surfaces for the ZDDP film to form and grow.
The OFMs consisted of the same hydrocarbon tail and therefore different film formation kinetics, friction, and surface fatigue performances originated from disparate chemistries of polar heads. Adsorption of amine-based OFM molecules on iron oxide surfaces has been shown in previous studies [
14,
28] and can be suggested from Stribeck curves at the start of the test shown in Fig.
2a. The Stribeck curves exhibited significantly lower friction coefficients in lubricants with an OFM compared to friction in BO + ZDDP. Iron oxide on steel surfaces catalyses ZDDP film formation [
29] and hence the surface blocked by OFM molecules is not available for ZDDP-film formation, delaying ZDDP-tribofilm formation. Therefore, the competition between the OFM molecules and ZDDP molecules to form a film governs the kinetics of overall tribofilm formation. In addition, the metal surface may be exposed to ZDDP (or ZDDP’s decomposition products) through the removal of OFMs from the surface in the presence of shear stresses and wear at the tribological contact, leading to ZDDP-tribofilm formation. Eriksson [
30] studied adsorption behaviour of an OleylAmine (OA) and a Ethoxylated OleylAmine (EOA) using a Quartz Crystal Microbalance (QCM) and showed a greater adsorption and higher surface coverage of EOA over OA, implying superior adsorption of ETA compared to TA and TDP. Furthermore, two hydroxyl groups in ETA prompt relatively strong nucleophilic properties and consequently ETA coordinates to the surface’s positive sites (e.g. iron cations). Therefore, the slower kinetics of film formation from ZDDP + ETA can be attributed to the greater propensity of ETA to steel surfaces. The larger polar head of ETA in comparison to TA and TDP which occupies a larger surface area might also be a contributing factor.
Amine-based OFMs can also chemically interact with ZDDP molecules in the bulk lubricant which influences film formation kinetics and tribological performance. N can act as a ligand through its lone pair of electrons in hybridized
sp3 orbitals. Shiomi et al. [
11] showed that (di)amines as ligands can form metal complexes with the Zn atom in ZDDP. The metal complexes generated are larger molecules which result in larger steric hindrance and inferior adsorption on the surface, and also lead to a reduction in the concentration of free ZDDP molecules to from ZDDP-film. Therefore, metal complex formation in the bulk lubricant may induce a further delay in tribofilm formation. Conversely, the hydroxyethyl group ((CH
2)
2OH) in ETA exerts steric hindrance around N which might decay complex formation between Zn and N resulting in more free ETA to adsorb on the surface and hence less surface for ZDDP to form a film.
TA showed a stronger delaying effect compared to TDP. Siegel et al. [
31] showed that diamine has a greater dipole moment (µ
b) compared to amine while µ
b in diamine is not twice the µ
b in amine. This means that TA has more adsorption potential compared to TDP, if TDP is not protonated and is adsorbed through terminal N. Protonation of TDP and TA disturbs electron density pattern around the polar head. This might hinder the accessibility of electron pair in N resulting in altered surface adsorption. In addition, TA has a smaller molecular structure, indicating that TA adsorbs more easily on the steel surface due to the less steric hindrance and hence can induce greater delaying effect when compared to TDP. Therefore, among the OFMs examined in this study, the greater delaying effect on ZDDP-film formation results from superior extent of the OFM adsorption onto the surface which increases along the series of OFMs TDP < TA < ETA. The friction performance of an OFM in an oil with no other additive depends on its surface adsorption characteristics including chemistry of the polar head and packing density [
32]. Accordingly, in MTM SS and MPR tests, where thin films in the order of 5–20 nm (Fig.
7) were formed on surfaces, the friction performance of OFMs most probably depends on the extent of OFM adsorption onto the surface. Therefore, an enhanced friction reduction was observed with the OFM showing greater adsorption characteristics; when considering that the hydrocarbon tail was the same for all the OFMs.
The impact of OFMs on tribofilm thickness and formation rate can be addressed through shear-assisted ZDDP-tribofilm formation [
33,
34] as a potential alternative mechanism. Since the change in the lubricant viscosity as a result of OFM addition was insignificant and so too the change in Hertzian contact area, the friction coefficient is proportional to the contact shear stresses. Adsorption of OFMs onto steel surfaces reduced shear stresses within the contact. This indicates that for ZDDP to form a film, higher energy inputs are required to overcome its activation energy barrier. Therefore, a thinner tribofilm, formed at a slower rate, is expected for the OFM that reduced the friction coefficient to a greater extent, as demonstrated in MTM SS test results. In our previous paper [
2], we showed that addition of TDP to ZDDP blends generated smoother tribofilms (suggested in Fig.
3) leading to enhanced lubricant entrainment into the contact and hence reduced frictional forces [
2,
35]. Pad features in ZDDP-tribofilms are believed to from at asperity conjunctions which promote the film-growth (since they are subjected to higher shear stresses) [
33]. Therefore, smoother tribofilms from OFM + ZDDP blends hindered the growth of tribofilms and hence thinner tribofilms were formed (Figs.
1,
7).
The addition of OFMs reduced the boundary friction in the first Stribeck curve at the start of the tests, suggesting OFM adsorption onto the surface. Also, the OFMs reduced the friction-enhancing behaviour of the ZDDP-film. An earlier shift of mixed/boundary regime to fluid film lubrication in Stribeck curves was achieved with OFMs. The less effective friction reduction by OFMs in MPR tests in comparison to MTM SS tests may be attributed to higher distortion in the OFM film structure [
36] due to the higher combined-roughness and/or the surface fatigue features on the surfaces of MPR rollers. Simulation work suggested that nanoscale roughness can result in a disordered monolayer [
36] which affected friction performance. Ewen et al. [
37] suggested that friction reduction by OFMs was achieved through easy slip planes formed between solid-like OFM monolayers and lubricant films at high surface coverages. They postulated liquid-like monolayers at low surface coverages with inferior friction reduction properties. A simulation work by Doig et al. [
28] suggested that, at a lower surface coverage, amines lay more flat towards the surface and greater lubricant penetration into the monolayer developed a layered structure. Therefore, micropits and the higher surface roughness on the surface of MPR rollers compared to MTM SS specimens disrupted OFM adsorption resulting in a higher friction coefficient. In contrast, the surfaces on MTM SR samples were fully covered with relatively thick tribofilms. The detected N and P signals in XPS results (Fig.
4 and Table
4) suggest chemical interactions of amine with the ZDDP film. This showed that OFMs used in this study not only adsorbed onto the steel surface but also interacted with phosphates in ZDDP-film. Miklozic et al. [
12] examined a variety of OFMs with different polar heads in combination with ZDDP and suggested that an effective OFM should also deliver friction reduction on the surface of anti-wear tribofilms.
Tribo-contacts in BO + ZDDP-lubricated systems induce zinc (poly)phosphates on steel surfaces and Zn to P ratio in the zinc (poly)phosphates has been used in evaluating the phosphate chain length [
10,
29]. The Zn 3s–P 2p
3/2 ΔBE values for tribofilms in this study suggested a similar phosphate chain length but greater Zn/P and S/P ratios and α’ values for tribofilms from OFMs (Table
4) suggested changes in Zn state in the tribofilm probably implying enhanced ZnS formation [
23,
24]. These imply that Zn
2+ cations were partly exchanged with protonated organic compounds. According to Brønsted–Lowry theory, amines can bind to a proton (H
+) to form a weak R-NH
3+ acid. The protonation of amines were confirmed in our XPS data. Following protonation, R-NH
3+ might react with phosphate ions (hard base) according to Hard and Soft Acids and Bases (HSAB) concept and from amine/ammonium phosphates. Therefore, the OFMs used in this study adsorbed onto the ZDDP-tribofilm and chemically interacted with the phosphate chains which influenced wear and friction performance.
As shown in Figs.
3 and
5, OFMs improved the wear performance to a certain extent compared to BO + ZDDP under test conditions defined in this study. Our experiments using MPR with TDP blended in BO (not shown here) and post-test XPS analysis evidenced wear protection characteristics of TDP and N interaction with the surface. Our results are in agreement with Eriksson [
30] observations, showing anti-wear properties of OA and EOA with and without ZDDP. Eriksson [
30] reported that OFM enhances anti-wear performance when added to ZDDP up to concentration ratios of amine/ZDDP ≤ 1 under sliding conditions. The wear protection characteristics of TDP indicate a stable adsorption on the steel surface with a contribution of chemical interactions with the surface. The chemical interaction of an amine with Fe surfaces has been reported by Wood et al. [
14], who studied the adsorption of hexadecylamine from a model oil (hexadecane) onto an iron oxide surface. They found a dense tilted hexadecylamine monolayer with a thickness of 16–20 Å formed through lone pair electron donation from N in amine to iron cations. They also suggested interdigitation of oil molecules with the monolayer, which probably impacts friction and wear performances through providing the contact with a retained oil layer. The retained oil between the contacting bodies enhances oil entrainment into the contact resulting in less asperity–asperity contacts. At lower surface coverages the interdigition of the lubricant with the monolayer has been suggested to become pronounced [
28]. Therefore, with kinks in the OFMs used in this study, a greater lubricant penetration into the monolayer might reduce the chance of lubricant starvation and hence surfaces were separated more effectively. Ewen et al. [
36] modelled OFM monolayers and showed that adsorbed OFM molecules on the surface asperities can prevent asperity–asperity contacts under pressures of the order of GPa. Therefore, OFM adsorption can alleviate solid–solid contacts under certain conditions. Asperity–asperity contacts between surfaces induce micropitting and hence monolayers formed from OFMs in this study alleviated micropitting nucleation on the surface (Fig.
5). In industrial applications, fully-formulated gear lubricants, which comprise the same amine-based OFMs used in this study, have been shown to prevent pitting and spalling of gear teeth [
38].
The XPS results (Table
4) evidenced a reduction in the contribution of C–O and O–C=O bonds when either TDP or TA was present in the lubricant implying anti-oxidant activity of the amine-based additives [
39]. It has been shown that alkylated diphenylamine molecules reduce wear and friction coefficient of sliding steel–steel surfaces [
39]. The friction coefficient and wear reduction was attributed to the free radical scavenging behaviour and/or suppression of the catalytic activity of metallic surfaces. The tribofilms formed through interactions between the lone pair electron of N and/or π electrons in the aromatic ring and iron surfaces were proposed to mask the catalytic activity of the worn steel surfaces [
39]. Therefore, anti-oxidant behaviour and organic films generated from TDP/TA and through its interaction with ZDDP can contribute to factors causing wear and friction coefficient reductions observed in this study.
Figure
5, however, showed a substantial rise in wear as a result of ETA addition to BO + ZDDP. This contradicts the previous results showing the impact of EOA on wear reduction in combination with ZDDP under sliding conditions [
30]. This can be attributed to the high affinity of ETA to the surface which almost hindered ZDDP film formation on the MPR roller surface. The higher surface adsorption of an ethoxylated amine in comparison to the non-ethoxylated amine has been reported in the literature [
30]. Our XPS results showed that P concentration and Zn/P atomic concentration ratio in tribofilms reduced from around 11 at% and 1.1 in TA + ZDDP to 4.5 at% and 0.3 in ETA + ZDDP respectively. In addition, a contribution of metallic iron and a significant oxide peak were observed in the tribofilm from ETA + ZDDP-(Fig.
8) indicating extensive wear of the surface. The XPS results confirmed that ETA mitigated phosphate formation in micropitting tests resulting in excessive wear. Costa et al. [
40] showed that addition of ethanol to a lubricant delayed ZDDP-tribofilm formation and eventually led to a thinner film on the surface. In addition, hydrated ethanol removed a preformed ZDDP-tribofilm which was attributed to localised chemical-etching action of hydrated ethanol [
40]. The functional group in ethanol is hydroxyl group and ETA with two hydroxyl groups is expected to induce similar effects. Therefore, ETA blocked the surface hindering ZDDP-tribofilm formation and digested formed phosphates on the surface leading to substantial wear of the roller surfaces. The surface adsorption of ethoxylated amine molecules has been suggested to form a close to a monolayer structure [
30]. However, under shear forces, a proportion of ETA molecules can form surface reverse-micelles as a result of relatively higher polarity of ETA in the non-polar base oil (PAO) used in this study [
41]. The micelles are bulkier and can block larger areas on the surface bringing about deteriorated wear performance.
In spite of reduced micropitting through addition of the OFMs, abrasive marks were more pronounced on surfaces from lubricants with the OFMs suggesting intensified abrasive wear (Fig.
5). Abrasive wear is induced by hard metallic oxides (i.e. iron oxide) in tribological steel–steel contacts. The observation of enhanced abrasive wear can be addressed considering HSAB reactions. Long chain zinc phosphates are hard bases and according to HSAB react with iron cations (Fe
3+) to eliminate iron oxide wear particles and hence mitigate abrasive wear [
29]. OFMs tampered with ZDDP film formation and induced thinner phosphate films (Fig.
1), and therefore, they favour abrasive wear which can cause catastrophic damages in some applications.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.