The Influence of Base Oil Properties on the Friction Behaviour of Lithium Greases in Rolling/Sliding Concentrated Contacts

The list of custom greases and their technical data are presented in Table 1. A grease kettle for small batch production, consisting of a reactor connected to a heating and cooling system, was used to fabricate the custom greases. The same general recipe, including the actual quantity of ingredients, and manufacturing protocol, were used for all the greases. The aim was to keep constant manufacturing conditions, so as to maintain consistent results. Since the focus of this work was the influence of base oil properties, all the greases were fabricated using the same amount of 12 w/w% lithium 12-hydroxystearate thickener. This was produced through the saponification reaction between 12-hydroxystearic acid (HSA) and a dispersion of micron-sized anhydrous lithium hydroxide (LiOH) particles in mineral oil [23], conducted in the specific base oil chosen for the given grease batch. A common process [24] for simple lithium greases consisting of four consecutive stages (saponification, soap dissolution, re-crystallization and homogenisation) was employed. A reference thermal cycle [23] was followed during the saponification, soap dissolution and re-crystallization stages, which were carried out in the reactor. The last stage, i.e. homogenisation, was conducted in a roller mill and consisted in working the grease between the rollers in successive steps by progressively reducing the rollers gaps, thus increasingly refining the thickener microstructure [24].

Better understanding of the effect of base oil viscosity on friction can only be achieved if greases made of the same base oil type are compared [25]. PAO was chosen as the most suitable oil type for this investigation, due to its definite and consistent composition [26], with consequent improvements in repeatability of the results and validity of the described comparisons. In addition, the use of PAO allowed for a direct comparison between these custom greases and the commercial LiPAO grease, studied in the previous work by the authors [19], which is formulated with the same type of thickener and base oil. Five PAO-based greases were fabricated with PAOs of different viscosities. The designation ‘LPx’ is used throughout this paper to denote these lithium-PAO greases, with ‘x’ between 1 and 5. The viscosities of the base oils were chosen to be distributed in the following way:where ν is the kinematic viscosity and the subscript identifies each of the five LP greases.

In addition, with the aim of assessing the potential influence of friction modifiers, three extra greases were prepared by blending oleic acid, a common friction modifier in oils, with LP1, LPE and LM1 greases using 5% w/w oleic acid concentration. For ease of reference, the resulting greases are labelled as LP1a, LPEa and LM1a. Finally, the additised base oils of LP1, LPE and LM1 were also prepared with the same concentration of oleic acid and tested for comparison.

The base oil viscosities in Table 1 are the values measured using Anton Paar Stabinger SVM 3001 viscometer. The viscosity was also measured at 60 and 80 °C, as the friction tests were also carried out at these temperatures. The measured viscosities follow Walther’s equation (ASTM D341-722) [27], and hence this formula was used to predict the base oil viscosity in the film thickness measurements, which were carried out at ambient temperature.

Laboratory ball-on-disc tribometers were used to measure friction and film thickness. Friction was measured on the MTM rig and film thickness on the EHD rig, both supplied by PCS Instruments UK. These rigs and the experimental procedures adopted in the current paper are the same as those described in the previous work by the authors [19]. Therefore, only the basic information is given here, and the reader is referred to the previous publication for full details. Fully flooded conditions were ensured in all tests by the use of a ‘grease scoop’, which was mounted ahead of the contact so that it continually pushes the test grease back into the rolling track. Friction was measured over a series of temperatures and entrainment speeds at a fixed slide-to-roll ratio of 0.1. Film thickness was measured to support the investigation into the friction behaviour of the greases, which is the main focus of this study. Every friction and film thickness test for each grease and test conditions was carried out at least twice, with excellent repeatability. All friction and film thickness results were plotted as an average of at least two tests. The test conditions are summarised in Table 2.

The base oils were also tested for comparison under the same conditions. The base oil film thickness measurements were used to estimate the pressure–viscosity coefficient α of each oil, by back-calculation using the Hamrock–Dowson formula [28]. The resulting α values are reported in Table 3. All PAO oils exhibit relatively similar values of α, while the corresponding values for the mineral oils of LM1 and LM2 are much higher. The values are comparable with those reported in the literature for these types of oils [29]. Although the pressure–viscosity coefficients are reported to depend on the contact operating conditions [30], in this study they were assumed to be constant within the range of conditions employed, and were later used to estimate film thickness in the friction tests.

The friction curves obtained with the custom greases are shown in Fig. 2 at (a) 40 °C, (b) 60 °C and (c) 80 °C. The majority of the friction curves have the shape of a bell, i.e. the friction coefficient increases with speed up to a maximum value (the curve ‘bump’), after which it starts decreasing. This is particularly evident at 80 °C in Fig. 2c. LPE grease displays a more accentuated bump, due to the relatively large difference between the values of minimum (at low and high speed) and maximum friction (at the bump speed). All the LP greases produce very low friction at all speeds. LM1 and LM2 give overall higher friction coefficients, especially at high speeds.

The comparison in friction behaviour between LP1 and LPE greases and their base oils, representative of the whole range of behaviour observed for this type of analysis, is shown in Figs. 3 and 4 respectively. At low speeds the greases provide much lower friction than their base oils. With increasing speed, the greases and base oils friction curves get closer. The transition of the greases into their base oil-like behaviour appears to occur at the speeds corresponding to the bump of the grease friction curve: at speeds higher than this the grease and base oil friction curves are generally the same. This transition speed increases with temperature and is lower with LPE than with LP1.

To examine the morphology of the lubricant film deposited in the wake of the contact, the friction tests carried out with these greases at 40 °C were stopped in sequence at speeds of 20 mm s⁻¹, 50 mm s⁻¹, 200 mm s⁻¹ and 1000 mm s⁻¹. Next, pictures of the running tracks on the discs were taken with an optical microscope at the location where the grease had just been overrolled by the ball. These pictures are also shown in Figs. 3 and 4 just below the corresponding friction curve. With LP1 in Fig. 3a, a uniform thick layer of lubricant is left in the rolling track at 20 mm s⁻¹. This gets gradually removed as the speed is increased, until the maximum speed of 1000 mm s⁻¹ when only a dispersion of oil droplets is left in the track, possibly separated by thickener particles.

The equivalent pictures for LPE grease are shown in Fig. 4. As in LP1, the morphology of the film deposit changes as the speed is increased, from a grainy structure to a gradually fluid-like appearance, until only oil drops are found in the track at the maximum speed of 1000 mm s⁻¹. However, the deposited film at low speeds looks patchier and thinner than in LP1.

The influence of oleic acid in the custom greases was investigated by comparing the friction results obtained with additised and non-additised base oils and greases.

Figure 5 plots the curves obtained at 80 °C, the test temperature at which the effect of oleic acid was the most apparent. Friction is evidently reduced in boundary and partly mixed lubrication conditions when oleic acid is used in the base oils of LP1, LPE and LM1. In contrast, the friction behaviour of additised and non-additised greases appears to be practically the same, i.e. no beneficial effect of oleic acid is apparent under these conditions.

The effect of base oil viscosity on friction can be examined by comparing the behaviour of the lithium–PAO (LP) greases, or that of the lithium–mineral oil (LM) greases, since each of these groups contains greases made with the same type of base oil but different viscosities. Figure 2, where the friction results are plotted against the entrainment speed, indicates some basic trends in this respect, in that for example the transition point (i.e. at the curve bump) between the thickener-dominated, low-speed and oil-dominated, high-speed regions on the LP greases moves to higher speed with decreasing base oil viscosity. However, no obvious correlation between the base oil viscosity and the absolute value of friction is apparent in these figures. Since base oil viscosity affects film thickness as well as friction, a better way to investigate the potential mechanisms at play is through relating the obtained friction results to some parameter related to contact film thickness. Figure 7 plots the friction measurements for the LP and LM greases against the nominal specific film thickness Λ, i.e. the specific film thickness calculated using the base oil properties only, not accounting for any influence of the thickener. Although the film thickness calculated in this way is clearly not the actual one in the contact within the thickener-dominated region (see Fig. 6), it is used here instead of the actual grease film thickness, which cannot be predicted using standard EHL film thickness equations, as it is relatively easy to calculate and hence useful for illustrating basic correlations. The nominal Λ ratio used here is therefore calculated as the ratio of the film thickness estimated using the Hamrock–Dowson equation [28], with the base oil viscosity as measured at the temperature of each test, the oil pressure–viscosity values stated in Table 3, and the composite Rq surface roughness of the ball and disc specimens measured as 18.5 nm. Although Λ is a dimensionless parameter, the reader should be aware that the friction versus Λ relationship shown in Fig. 7 is not intended for use in general predictions of absolute friction in lithium–PAO greases under other operating conditions resulting in the same Λ ratios, not least because all current results are generated under one contact pressure and one surface roughness, and changing either of these is likely to have effects that are not accounted for by the nominal Λ value alone.

The effect of base oil viscosity on grease friction is now more apparent. Measured grease friction for LP greases is correlated with the PAO base oil viscosity so that friction values are similar at a given value of nominal Λ and form a common ‘master’ friction versus Λ, inverse-Stribeck-like curve (Fig. 7a–c). For all five LP greases and all three test temperatures, the location of the bump in the friction curves corresponds to the same friction value and to a relatively constant Λ value (1–2) for all LP greases. This clearly suggests that the point of transition between the two operating regions is determined by the base oil film thickness at the prevailing contact conditions. This behaviour seems to be in line with that reported by Kanazawa et al. [22], who proposed that for a given thickener, the mechanism of grease lubrication is such that it is the characteristic ‘transition film thickness’, and not the ‘transition speed’, that determines the demarcation point between the thickener-dominated and the oil-dominated regions in the tribological behaviour of greases. Indeed, this is further confirmed by Fig. 6, where it is shown that the measured film thickness at the transition speed, above which the curve becomes linear, is comparable for all LP greases.

Distinct regions may be identified on the friction versus nominal Λ ratio curve for the LP greases. At Λ ratios higher than the transition point demarking the oil- and the thickener-dominated regions, the friction coefficient behaves in a manner that is relatively well established for lubricating oils. In full-film EHL conditions, at Λ values of about 5 and over, the friction coefficient is in the range of 0.01–0.02, which, at the applied Hertz contact pressure of 0.96 GPa, corresponds to a maximum shear stress range of 10–20 MPa for all PAO-based greases and temperatures tested. The friction versus log Λ plot produces a straight line within this region, as may be expected at these shear stresses. At the highest speeds (the highest Λ values in Fig. 7), some thermal effects may occur within the contact, however small, reducing the effective contact friction further. As Λ decreases from 5 down to Λ ≈ 1–2, which marks the transition point between the thickener and oil-dominated regions, friction increases as the contact enters mixed regime. This again is in line with the behaviour expected of the base oil alone as is evident in Figs. 3 and 4, where the frictional behaviour of greases and their base oil is compared. As Λ decreases further, from 1–2 at the transition point down to about Λ = 0.2, friction decreases linearly with decreasing log Λ for all lithium–PAO greases tested, but with the gradient that is larger than that observed in the full-film region. The mechanisms responsible for friction within this region are likely to be rather complex. On the one hand, Fig. 6, all be it at a lower temperature of 22 °C, shows that within this region, the film thickness increases from about 80 nm to approximately 150 nm with decreasing speed, so that the effective shear rate decreases considerably and some drop in friction may be expected from this. However, it is not possible to say whether this is the dominant friction mechanisms, not least because some contribution to friction from roughness effects may still be present, even if a full lift occurs, in a similar manner reported for oils [31, 32], and this contribution will also decrease as speed decreases. Moreover, in this region the lubricating film is likely to be made of a concentrated mixture of thickener and base oil, which will have its own characteristic high-pressure rheology. However, a thorough understanding of the effect of high-pressure rheology on friction, which is currently a subject of fierce debate even for simple base oils [33], is outside of the scope of this paper. In any case, grease friction is clearly lower than that of the base oils throughout this region, suggesting that the film enhancement through thickener action is clearly having a positive effect by reducing asperity contact. Finally, it should be noted that at even lower nominal Λ values, the nature of the lubricant film and the resulting contact friction response may change. Some evidence of this is present in Fig. 7c, showing friction results at the highest temperatures, and hence the lowest nominal Λ, where friction coefficient appears to reach a plateau at a very low value of less than 0.01, with no further reduction with decreasing Λ. The reasons for this are not entirely clear, but it may suggest that the contact is lubricated by some low-friction solid-like deposited film leading to a low, constant friction.

In contrast to the LP greases, the mineral oil-based greases LM1 and LM2 produce values of friction that are very different even when compared at the same nominal Λ (Fig. 7d–f). However, unlike the PAO base oils, the mineral base oils of LM1 and LM2 greases are likely to contain very different chemical compounds, as mineral oils are generally complex mixtures of different components [34]. The investigation into the effect of base oil viscosity on friction for these two greases is therefore somewhat futile, as the two mineral oils of LM1 and LM2 cannot be directly compared.

The effect of base oil type on friction can be investigated by comparing the behaviour of greases made with base oils of similar viscosity but different type, namely LP1 with LM1 and LPE, and LP3 with LM2. The friction results obtained with these two groups of greases are plotted against the entrainment speed in Fig. 8. It is immediately apparent that the mineral oil-based LM1 and LM2 greases give higher friction than the PAO-based LP1 and LP3, respectively, at all speeds and temperatures tested. Such behaviour is expected at high speeds, where grease friction is determined by its base oil properties as discussed above. Indeed, EHL friction with oil is known to be strongly dependant on the molecular structure of the oil, including molecular shape and flexibility, since these characteristics determine the ‘fluid’ friction arising from shearing the film molecular layers at high pressure [35‐37]. Low shear strength, and thus low friction, is favoured by oils containing linear, aligned and flexible molecules. These are all common features of the alkyl chains that compose the PAO oils, which consequently are expected to produce lower friction than mineral oils [37].

However, the reasons for the higher friction observed with the mineral oil-based greases within the low-speed, thickener-dominated region are somewhat less obvious, given that all greases were formulated with the same lithium thickener. In addition, the minimum film thicknesses given by all these greases are very similar (Fig. 6), so that the observed differences in friction cannot be attributed to different degrees of asperity interaction. This result clearly suggests that the oil type does have an influence on friction even in the thickener-dominated region. Within this region, the friction coefficient seems to increase approximately linearly with log of speed, again suggesting that the contact is not lubricated by a solid layer of thickener only, but most likely some concentrated mixture of thickener and base oil. Consequently, within this region, the relative friction coefficients of greases also follow the trend that may be expected from their corresponding base oil properties, being lower for PAO-based greases than for the equivalent mineral oil-based greases.

Like the mineral oil-based LM1 grease, the ester-containing LPE grease also gives higher friction than the PAO-based LP1 at low speeds. In this case, however, the result can largely be attributed to the significantly lower film thickness obtained with LPE within this region, as evident from Fig. 6, and hence increased contribution of asperity contacts to the overall friction. The film thickness results show that effectively, the extent of the thickener-dominated region with LPE grease is smaller than that with LP1. These differences in film thickness and friction between LP1 and LPE greases are clearly evident, despite the fact that the compositions of LPE and LP1 are very similar, and only differ by the presence of 20% of ester oil in the formulation of LPE. A similar trend was previously observed by the authors with commercial greases of the same basic lithium thickener—ester oil composition [19]. Various characteristics of ester-based greases have been proposed in literature as negatively affecting their lubrication behaviour [38, 39], with increased propensity for contact starvation suggested as perhaps the most significant factor [40]. However, in the current study the onset of starvation can be excluded, as the tests were performed in fully flooded conditions by directing the supply of grease to the contact by means of a grease scoop. Instead, it is thought that it is the characteristic structure of the ester-containing lithium greases that is responsible for the observed decreased film thickness, and in turn the increased friction. With this in mind, the structure of the lithium thickener of the test greases was observed in an SEM. The SEM samples were prepared by first spreading a thin layer of each grease on a cylinder mount and then repeatedly flushing the grease layers with toluene to dissolve and separate the base oil until only the residual thickener was left on the mounts. An agar sputter coater was used to cover the thickener samples with gold before the SEM analysis to improve conductivity. Figure 9 shows SEM images of selected lithium thickener samples, including those of the custom LP1 and LPE greases tested here, as well as the commercial PAO- and ester-based greases tested in the previous study [19], denoted as LiPAO and LiEs respectively. Although the shown thickener structure is not necessarily the one existing in the contact, these images can be used to make basic relative comparison in terms of grease thickener structure. It is clearly evident that the lithium fibres of the PAO-based greases (LP1 and LiPAO) are much thicker and longer than those of the ester-based LPE and LiEs greases. All the remaining non-ester greases tested in this work display very similar fibre structure to that of LP1, as is evident in the examples of LP3 and LM2 in Fig. 9. Literature [40‐42] suggests that shorter lithium thickener fibres can produce thinner films at low speeds, and consequently higher friction due to increased asperity contacts [43]. Kanazawa et al. [22] suggest that the ‘transition film thickness’ for a given grease, and hence the extent of the region where thickener film enhancement is evident, is determined by the ratio of thickener fibre size to the film thickness obtained by the base oil alone, i.e. the thickener-dominated zone ends when the film thickness attainable with the base oil alone exceeds a characteristic dimension of the thickener. Indeed, in the present study, a much lower transition film thickness value was found with LPE, while approximately the same value was recorded for the LP and LM greases. Therefore, the results suggest that the thinner lithium fibres obtained with ester greases are responsible for the diminished film enhancement at low speeds and the associated increase in friction due to asperity contact. This assertion is also supported by the differences observed in the comparison between the deposited grease film in LP1 (Fig. 3) and LPE (Fig. 4) during the friction tests.

Based on the current results, the potential mechanisms through which base oil type may influence lithium grease friction can be summarised as follows:

The results of the present study indicate that, as far as base oil properties are concerned, lithium greases formulated with a relatively low viscosity PAO base oil produce lower friction than greases made with mineral or ester oils. It is interesting to compare these results to those obtained with an equivalent fully formulated commercial lithium grease, based on a PAO base oil of similar viscosity, which was found to produce the lowest friction out of a large set of commercial greases tested in the previous study by the authors [19]. To this end, the friction curves of the custom LP1 grease and the commercially available lithium–PAO grease, denoted as LiPAO and marketed as a low-friction grease [19], are plotted together in Fig. 10 for comparison. The curves are remarkably similar, which verifies that the model greases tested in this paper are representative of their commercial counterparts, and confirms the suitability of lithium thickener and PAO oil as a potential low-fiction formulation, as initially suggested by the previous study with commercial greases. In addition, the fact that the two greases produce very similar friction despite the fact the commercial LiPAO grease is very likely to contain additives in its formulation, also shows that its low-friction performance is primarily determined by the basic formulation (base oil and thickener) rather than the additive package, at least under the conditions considered here. This observation is further supported by the result obtained with the LP1 grease additised with oleic acid, a friction modifier, which showed no significant effect on the frictional performance of LP1 (Fig. 5). In contrast, oleic acid did show a reduction in friction with base oils within the low-speed, mixed lubrication region where metal-to-metal contact may be expected to occur in case of oil lubrication. The most plausible reason for the apparent ineffectiveness of oleic acid in this grease formulation is that at these low speeds, where oleic acid would normally be expected to act, the thick film formed by the lithium grease already provides low friction due to much higher real Λ values than attainable with corresponding base oils, rendering the oleic acid somewhat redundant. Obviously, these results are not exhaustive and do not exclude the influence of oleic acid, or other friction modifiers, on friction under different contact conditions. However, they do suggest that under certain conditions, additive-free grease formulations may be just as acceptable as their fully formulated, and more expensive, equivalents.