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IR Spectroscopic Analysis of Grease Lubricant Films in Rolling Contacts
Lubrication at the Frontier / D. Dowson et al. (Editors) 1999 Elsevier Science B.V.
589
IR Spectroscopic Analysis of Grease Lubricant Films in Rolling Contacts S. Hurley and P.M. Cann Tribology Section, Department of Mechanical Engineering Imperial College of Science, Technology & Medicine London SW7 2BX
Abstract
In this paper, the composition of grease lubricant films formed in rolling contacts has been studied using IR reflection spectroscopy. A simple ball-on-fiat test arrangement has been used. Greases are two-phase lubricants composed, primarily, of a thickener dispersed in a basestock. The lubricant film in the contact region however, does not necessarily reflect the bulk composition, but will depend critically on the mechanism of lubricant supply to the contact. If flow of bulk grease into the contact occurs then both thickener and base oil will be present. The grease structure however will be heavily degraded by the very high local shear stresses as it passes through the contact. If the contact is starved of bulk flow then film generation is due to local base oil flow from the grease reservoir. In this case, the separating film is primarily base oil. Thus the composition of the film depends on the lubricant supply and is likely to change with operating condition and time. In this work, the composition of grease films under both fully flooded and starved conditions has been studied using IR reflectance spectroscopy. This technique provides information on the concentration and distribution of the grease components both within the track and in the reservoir at the sides. Under fully flooded conditions both thickener and base oil are present in the inlet, although the thickener concentration varies depending on speed and proximity to the Hertzian zone. Under starved conditions there is an initial deposition of bulk grease in the track as fresh grease is overrolled at the start of the experiment. As the test continues however, the grease structure is broken down and base oil is expelled from the track. There is also evidence of shear degradation and oxidation of the thickener within the track. The condition of the grease close to the track also changes as the test proceeds and there are local thickener concentration changes in this region. These findings are discussed in the light of current theories of grease lubrication. 1. GREASE LUBRICANT FILMS IN ROLLING CONTACTS
Lubrication performance is essentially determined by the properties of the film formed in the contact region. The physical properties, chemical composition and thickness will determine the levels of friction and wear, and ultimately component life. In this paper the composition of lubricating films formed by greases in rolling contacts is examined. Although there have been many studies devoted to the analysis of films formed by liquid lubricant and additive systems, there is very little work in the literature where film formation by greases is considered.
Grease is a two-phase lubricant comprised primarily of a thickener physically and chemically dispersed in a basestock. However, the lubricant film in the contact region will not necessarily reflect the bulk composition but will depend critically on the mechanism of lubricant supply to the contact. Two supply regimes are identified in this work: fully flooded and starved. In the fully flooded condition the inlet is considered completely filled with lubricant. In the starved regime there is a limited amount of lubricant present and the inlet meniscus is close to, or at, the Hertzian circle. Under fully flooded conditions if flow of bulk grease into the inlet occurs then both thickener and base oil will be present. The grease structure
590
however will be heavily degraded by passage through the contact and it is likely that some deposition of thickener occurs in the rolled track. For the fully flooded condition film thickness will scale with the usual EHL criteria of inlet viscosity and rolling speed (1). If inlet flow is not maintained then the rules governing film formation are different. Under starved conditions base oil is thought to bleed out of the grease reservoir at the side of the track, replenishing the contact (2). This model implies that the film is composed of base oil alone. However, recent work has shown that the film composition is more complex and that both thickener and base oil are present (3). The thickener forms a thin (~5-10 nm) residual film on the metal surface which is augmented by a hydrodynamically-generated film (3). The operating requirements for bearing greases are low friction with a relatively long lubrication life. The ideal condition is therefore semi-starved as the friction losses and thermal effects are minimised. The lubricant film however must be sufficient to prevent surface damage and thus in this regime the thin film properties will be increasingly important in determining performance. Deposition of the thickener to form thin residual films which act as antiwear or boundary layers is one possible mechanism (4) and this has been observed with urea greases (5). An alternative mechanism is the formation of high viscosity layers (6) at the surface. Optimisation of the local rheology at the surface might have beneficial effects, particularly in the prevention of damage due to component vibration. The possibility of controlling grease formulation so as to optimise the surface film properties has not been explored as yet. Most of the research into grease has been concerned with their bulk rheologieal behaviour. There is very little published work concerning the properties of the grease lubricant films formed under different contact conditions. Therefore, the aims of the study were: • to develop techniques to characterise grease lubricating films • to apply these techniques to establish the chemical composition and physical properties of grease films / • to study film composition under fully flooded and starved conditions.
IR reflectance spectroscopy has been used to charactefise the grease films both in and out of contact. IR spectroscopy is ideally suited to the examination of greases as it can distinguish between the thickener and base oil phases and identify new organic species. IR transmission spectroscopy is routinely used to examine grease during manufacture and as samples from failed beatings. There is thus a large reference body of literature for comparison of results. This technique is also useful for providing information on structural changes such as alignment or destruction of soap thickener fibres (7). 2. EXPERIMENTAL PROGRAMME 2.1. Film thickness in rolling contacts
The first requirement was measure film thickness under both fully flooded and starved conditions. Once the lubrication behaviour had been established then IR analysis of the lubricant films could be carried out. In this study, grease film formation in a rolling point contact was measured for a ball on flat configuration using a high-resolution optical imerferometry technique (1)(8). The contact is formed by a loaded steel ball (diameter 19ram) rolling against a silica-coated glass disc. The thin film interferometry technique gives improved thickness resolution (1 nm) and minimum film thickness detection (1 rim) compared to the conventional methods. Two film thickness test procedures were used. All tests were carried out at 25°C: (i) Fully flooded - film thickness was measured with increasing speed, a small channelling device was used to push the overrolled grease back into the track. (ii) Starved - film thickness change with time was measured at constant speed. A single charge of grease was injected into the rolling track at the start of the test. 2.2 Characterisation of grease films in and out of contact
Film thickness measurements were carried out on the greases under fully flooded and starved conditions. In a corresponding series of tests the composition of the films formed under similar conditions was analysed by IR spectroscopy. IR
591
reflection-absorption techniques were used to analyse grease films both in and out of contact. Specular reflectance micro-spectroscopy was used to determine the composition and distribution of the grease components in the rolled track on a steel surface. A second method whereby IR reflection spectra were taken directly from the rolling contact was also developed to study the composition of the inlet and Hertzian region. These techniques are described in more detail below.
2.2.1 Out-of-contact analysis In a separate series of tests rolled grease fdms were prepared on polished steel discs and then analysed by IR reflectance spectroscopy. An IR microscope (Spectratech) coupled to a FTIR spectrometer (Perkin Elmer 1740) was used to take single reflection spectra from small areas (- 150 l.tm diameter) of the grease film from in and around the rolled track. A simplified diagram of this technique is shown in Figure 1. Spectra were taken with 200 scans at a resolution of 8cm 1
spectrometer
I
rolled grease track
Figure 1 IR reflectance sampling from rolled track The grease films were prepared on the rolling contact rig but in this case the glass disc was replaced by a polished steel disc. The working tests were carried out at constant rolling speeds, typically 0.1ms 1 and 1.0ms "1. At the start of each test approximately 1 ml grease was injected as an even layer around the rolling track. The ball and disc motors were then set to the desired speed and a load of 20N applied. Previous film thickness tests (see Figure 4) have shown that under these conditions, the film thickness will decrease with time The tests were stopped at intervals, the disc removed for analysis and then replaced, with the grease film
undisturbed, allowing the test to be continued. The width of the track was approximately 280 lain. Spectra were taken at various positions: in the track, at the edge of the track and increasingly further away (see Figure 5 below).
2.2.2 In-contact analysis A second technique whereby IR spectra are taken directly from the operating EHL contact was also used (7). In this case an 1R transparent window was used instead of the glass disc. The FTIR microscope was used to take spectra from small areas in and around the contact. In earlier work the technique was limited to sliding contacts and a diamond window (3mm diameter) was used as the (stationary) counterface to the steel ball. In this study the technique has been applied for the first time to rolling contacts and a new test rig has been developed. One of the problems with this configuration is that a large IR transparent rotating disc is required and in this case a 10cm diameter CaF2 window was used. A diagram of the technique is shown in Figure 2. This approach means that it is now possible to monitor grease lubricant composition both in the starved and fully flooded regimes. In the fully flooded regime, the film composition in the contact and inlet regions has been studied. In the starved regime the film thickness in the contact is generally too low to be detected and there is little lubricant in the inlet. However it has been observed that once rolling has been halted lubricant flows out from the grease reservoir and forms a meniscus around the stationary Hertzian contact. It is likely that this reflow also occurs during rolling and thus provides a continual flow of lubricant into the track. By stopping the test rig and taking spectra from the reformed meniscus it is possible to analyse the composition of the lubricant replenishing the track under starved conditions. 2.3. Test Greases A set of four simple additive-flee model greases were used. These were lithium hydroxystearate: (CH3 (CH2)5 CHOH(CH2)10COOLi) and a tetraurea (general formula R1-NHCONH-R-NHCONH-R2NHCONH-R-NHCONH-R3 where R is an alkyl group), both containing 7 and 14 % w/w thickener. These greases had the same mineral base oil (viscosity @ 40°C 200cSt.)
592
,/sample areas sample area for results in Figure 9
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Figure 2 IR spectroscopy sampling from the contact region
3. RESULTS AND DISCUSSION 3.1 Film thickness results
Representative, fully flooded results are plotted as log film thickness against log rolling speed in Figure 3 for the 14% lithium hydroxystearate and tetraurea greases at 25°C. For both greases, the overall film thickness is higher than that of the base oil. However, two distinct speed regimes are observed: low speed region where the film thickness is unstable and tends to fluctuate wildly higher speed regime where the film steadily increases, with a log-log film thickness/speed exponent close to 0.7. The instability at low speed is due to the intermittent passage of thickener lumps through the contact distorting the EHL film. Such behaviour is probably one of the origins of grease 'noise' in beatings.
As the rolling speed increases this behaviour lessens and the film in the contact assumes the usual EHL shape. At higher speeds the film increase with increasing speed follows the usual EHL considerations, suggesting that the inlet rheology is essentially Newtonian. This implies that the grease structure has been fully degraded by the greater inlet shear stresses experienced at these speeds and that the grease has effectively reached a constant apparent viscosity. A typical starved test is shown in Figure 4 for the 14 % lithium hydroxystearate grease at 25°C and 0. lms "~. In this case film thickness is plotted against disc revolution (time). At this temperature the inlet is severely starved and film thickness decays steadily throughout the test giving a final thickness which is considerably less than the fully flooded case.
593
uneven residual film (typically 2-60 nm thick) is seen. On either side of the track, the bulk grease reservoir sits with a series of corrugations or 'fingers' reaching towards the rolled track; these are formed by the film cavitating in the exit region. The composition of the track (sample position 1) and at the edge (sample position 2) have been examined later in this paper.
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Figure 3 Fully flooded film thickness results for the 14% lithium hydroxystearate (open symbol) and tetraurea (closed symbol) greases at. 25°C. Base oil result shown as solid line.
160
Figure 5 Phase contrast microscopy photograph of a rolled grease track. The rolling direction is from fight to left. The rolled track corresponds to the Hertzian width of 280 ~tm.
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3.2. IR Spectroscopy Results
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3.2.1. Transmission spectra of fresh samples IR transmission spectra (limited wavelength range) are shown for two grease types (7% tetraurea and lithium hydroxystearate) in Figure 6 For the lithium hydroxystearate grease: the absorbances at 1580 and 1560cm "~ arise from the soap thickener. The characteristic double band corresponds to the asymmetric stretching of the carboxyl group (9). Peaks at 2955, 2925, 2850, 1463, 1377 and 720cm "1 are primarily due to the paraffinic base oil, arising from the CH2 and CH3 groups. However, it must be remembered that the hydrocarbon portion of the hydroxystearate thickener will also contribute to these peaks. The CH2 deformation for the pure soap occurs at 1453 cm "~. These assignments are summarised in Table 1 below.
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Figure 4 Starved film thickness decay for 14% lithium hydroxystearate grease at 25°C, 0.1 ms "1.
At the end of the test, the rolled grease track can be examined by a low power optical microscope and a photograph of this is shown in Figure 5. Several features should be noted as these are examined in the IR work. In the centre is the rolled track, where an
594
Polyurea thickeners give several characteristic absorption bands and those for the tetraurea are listed in Table 2. Urea thickeners have C=O, NH2 and NH bands similar to those of amides. The C-H bands are similar to those reported above for the lithium hydroxystearate grease. Transmission spectra can be compared directly to those from the reflection and in-contact studies.
Table 1 IR peak listing for lithium hydroxystearate thickener and base oil. Peak position cm
Structure
-1
2955
C-H asymmetric stretch (CH3)
2925
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Table 2 lit peak listing for tetraurea thickener. Peak position cm-1 3360-3260
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1631
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Figure 6 IR spectra for fresh greases: 7% lithium hydroxystearate (solid) and 7% tetraurea.
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595
3.2.2. In-contact analysis In this study IR spectra were taken from a rolling contact for fully flooded operation. The intemion was to determine the composition of the lubricant erecting the contact under differem speed conditions. Typical spectra are shown for a lithium hydroxystearate (14%) grease in Figure 7. Two speed levels were chosen corresponding to the 'noisy' (low speed) and 'EHL' (high speed) regimes noted in the film thickness work. The thickener content at low speeds is very high, in some cases it is higher than the average value recorded for the bulk grease. This suggests that in this region there is entrainmem of concentrated thickener bundles into the inlet. At higher speeds the concentration of thickener in the inlet region decreases to less than that of the bulk grease, although variations are also observed depending on the distance from the comact. Typically the thickener concentration decreases close to the contact both in the inlet and at the sides. An example of this for a tetraurea grease (7%) is shown in Figure 8. In this case, all the spectra have been normalised to the same value of the 1460 cm "1 band. The urea absorbance increases with distance from the Hertzian contact both in the inlet and at the side of the contact.
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Under heavily starved conditions there is very little lubricant in the inlet so this precludes IR analysis during rolling. The film in the comact was also generally too thin to be detected by this technique. However one observation from the starved studies is that once rolling has been halted lubricant flows from the grease reservoir to reform a meniscus around the contact. This process is due to capillary forces close to the contact and does not occur in the rolled track 'out-of-contact'. It is likely that this process operates during running to provide a local supply of lubricant to the contact. In-contact IR spectra have been taken from the meniscus to identify the composition of this lubricant and an example is shown in Figure 9. Spectra were taken at increasing distances from the stationary contact (this corresponds to increasing reflow time). The lubricant which reflows first is essentially base oil containing very little thickener. This free base oil has been released from the grease close to the track probably through the shear degradation of the thickener structure. With increasing time however, higher viscosity material containing some thickener returns to the track. This behaviour obviously provides a mechanism whereby flee oil and bulk grease can return to the track during periods when a beating is not in operation.
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Figure 7 IR spectra from inlet region for a fully flooded rolling contact at two speeds, ---0.011 ms1, - - - 0.08 ms 1. A spectrum of the bulk grease (14% lithium hydroxystearate) is also shown (solid colour ).
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Figure 9 IR spectra from lubricant reflowing into the track once rolling has been stopped. The distance from the contact reflects the length of time it takes for that material to reflow.
597
structure. In this case, thickener-rich particles were deposited in the track during the first few minutes of operation. These were clearly visible in the rolled track using an optical microscope. In Figure 10 spectra are shown of a deposited thickener particle and from the surrounding film. The increased concentration of urea thickener is seen by the intense absorption bands at 1636- 1515 and 1305-1234 cm "1 compared to the 'CH' bands at 1460 and 1377 cm1. The shift of the CH2 band to 1465 cm "1in the particle spectra is also indicative of almost pure urea. The thickener lumps gradually fragment with overrolling, and eventually break down to give a more uniform film. Cavitation patterns, seen within the track area, signify that the film is highly viscous rather than simply a solid layer. The 'high viscosity layer' mechanism for starved grease lubrication was suggested by Scarlett (6) and this result shows that it can be a viable mechanism.
3.2.3. IR reflection spectra from rolled grease films 'out-of-contact' The starved rolling tests show that the grease film decays very rapidly in the first few minutes of running. In a separate series of tests, the glass disc was replaced by a polished steel disc so that IR reflection-absorption spectra could be taken from the rolled track. The test was run in the usual way except the steel disc was removed periodically for analysis. IR spectra were taken from small areas (-150 lxm diameter) from within the rolled track and the 'fingers' at the side (sample positions 1 and 2 in Figure 5). Tests were run at two speed levels: 0.1 and 1.0 m/s. Initially, flesh grease is overrolled in the track. As the test proceeds, the film breaks down and base oil is released. At the lower speed the urea and high concentration lithium hydroxystearate greases were able to maintain a thick film in the track for very long periods. This was particularly seen with the high concentration urea greases which had a very lumpy
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Figure 10 IR reflectance spectra taken from a 14% tetraurea grease thickener particle deposited in the track ( - ) and the usual film within the track (- - -) 1.0 m/s after 20 seconds running.
598
The lower concentration lithium hydroxystearates tended to decay very quickly giving much thinner residual films. There was also evidence of thickener structure breakdown giving physically or chemically transformed layers. This occurred both within the track and in the grease at the side. Figure 11 shows a series of spectra that have been taken at different times from the track (position 1 in Figure 5). Results showed broadening of the asymmetric carboxyl band at 1580cm1 and gradual disappearance of the corresponding one at 1560cm1 as rolling proceeded. The absorbance of the carboxylate band also increased relative to the CH2 band suggesting that as the grease breaks down in the track, a thickener rich layer is left and base oil is expelled. This free base oil is thus available for reflow, as seen in Figure 9 above, and the formation of a hydrodynamic film during rolling. The CH2 absorption at 1463 cm~ also appears to resolve into two peaks at 1460 and 1465cm"x. One possible explanation for this is that the soap or base oil has been chemically transformed within the track. In this case mechanical degradation of the soap fibres, followed by chemical reaction of the reactive molecular fragments is a more likely option. The 'scissoring' frequency of CH2 groups is very sensitive to the molecular environment. When a CH2 group is adjacent to oxygen atoms this frequency decreases and there is also an increase in intensity (9). Thus one interpretation of the decrease in the deformation frequency observed in the spectra of the rolled track is that oxidation has occurred and acidic products are beginning to form. Alternatively, the second peak may be another CH2 deformation at a decreased frequency from a new chemical species with a shorter hydrocarbon chain. The CH3 absorption at 1378cm"~ decreased in height and gradually increased in frequency, becoming a shoulder of a new peak arising at 1411cm~. This latter peak is due to the CH2 deformation frequency of short chain carboxylic acids such as ethanoic acid (9). This again would suggest that the soap thickener is being physically degraded by the high shear stresses in the contact and that new shorter chain or oxidised species are being formed. The condition of the grease at the side of the track will have an influence on replenishment and IR spectra have been taken from the grease 'fingers' (position 2 in Figure 5) to study thickener
concentration and breakdown (see Figure 12 below). Increasing the speed from 0.1 to 1.0ms"1 increased the rate of track depletion. Initially base oil is expelled from the track but at a later stage thickener is also lost. Thus at the edges of the track the (relative) soap absorbance increased slightly with extended running time. This was also seen in the characteristic shift of the CH2 deformation peak towards the lower frequency of 1453 cml; which is characteristic of the soap. These spectra also exhibited further evidence of chemical degradation. All spectra taken from the edge of the track showed an increase in the absorbance band at 1410cm1 which is associated with short chain acidic species (9) and is taken as an indication of oxidation. There is also further evidence of this: in some of the spectra taken in the high speed tests, a series of overlapping absorption bands appeared between 1630 and 1800cm 1. These are due to C=O stretching of acidic species, particularly ketones (~1700crn"1) aldehydes (-~1710cm"l) and carboxylic acids (>1720cmI). All these effects were only observed in spectra taken in, or close to, the track. In IR spectra taken well away from the edge of the track, the soap ratio decreased and the degradation bands were not present. The lit results from the out-of-contact analysis show that deposition of grease occurs in the track. forming a thickener-rich film. The stability and adherence of such a film could have significant implications for the continued operation of beatings under conditions where a hydrodynamic film is not formed. For instance: when severely starved, during start-up or at low speeds. Thickener films might also help to prevent surface damage through damping of transient effects, for example the low load vibrations that result in false brinelling. IR spectra taken from the films after extended running under starved conditions have shown that degradation of the grease film occurs. Evidence of structural breakdown, concentration changes and oxidation to form acidic species has been found. In the starved regime the conditions within the contact are very severe; in this case films below 20 nm have been measured. It is likely therefore, that the combination of high local shear stresses, thermal effects and the proximity of the steel surface will contribute to the breakdown of the thickener to form oxidised, possibly acidic species. This has implications for oxidative wear of the metal surface.
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Figure 11 IR spectra from rolled grease track, position 1 in p h o t o g r a p h in Figure 5, 14% lithium hydroxystearate, 25°C, 0.4 m/s.
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4. CONCLUSIONS
ACKNOWLEDGMENTS
The findings from this study are summarised below:
The authors would like to thank the EPSRC and the NLGI Fundamental Research Committee for financial support of this study.
Development of IR analysis methods: - The in-contact IR technique has been extended to rolling contacts and has been. applied to the study of grease film composition both in the contact and inlet regions. Grease EHL film thickness: - Fully flooded: two distinct speed regimes, low speed 'noisy' regime where thickener lumps survive passage through the inlet and enter the contact. At higher speeds, film thickness increases according to usual EHL rules. - Starved: at ambient temperatures film thickness decays rapidly to a lower value than the fully flooded level. Analysis of grease lubricant film composition: - Fully flooded: at low speeds, much higher thickener concentrations are obtained in the inlet region confirming the presence of local thickener 'bundles' although this is an intermittent occurrence. At higher speeds, the inlet thickener concentration drops close to the contact and this has important implications for the development of inlet rheological and film thickness prediction models. - Starved: a thick grease film is deposited in the initial stages which breaks down releasing base oil and leaving a thickener rich layer. Concentration changes are seen in the 'finger' region at the side of track. - There are indications of a breakdown in the thickener chemical structure forming oxidised, acidic hydrocarbon species. - Lubricant that reflows around the stationary contact is flee base oil released from grease close to the track. This mechanism could provide continual replenishment during running. The work has shown that the Scarlett (6) model of a high viscosity deposited layer is valid although the life is limited unless it is renewed. Such films might offer very different surface protection properties to the conventional fluid EHL film and it should be possible to optimise grease composition to exploit these properties for particular applications.
REFERENCES
1. "Starved Grease Lubrication of Rolling Contacts," P.M. Cann, accepted for publication STLE Trans. 2. "Minimum Oil Requirements of Ball Bearings," E.R., Booser and D.F Wilcock, Lub Eng., 9, 140 (1953). 3. "Understanding Grease Lubrication," P.M. Cann, Proceedings 22na Leeds-Lyon Symp. on Trib., (1996). 4. "Friction of Grease and Grease Components during Boundary Lubrication," D.J. Godfrey, Lub. ASLE Trans., 7, 24, (1964). 5. "Recent Developments in Diurea Greases," T. Endo, NLGI Spokesman, 5_7.7,532, (1993). 6. "Use of Grease in Rolling Bearings," N.A.Scarlett, Proc. IMecE. Part 3A, 182, 585, (1967). 7. "In Lubro Studies of Lubricants in EHL Contacts Using FTIR Absorption Spectroscopy," P.M. Cann and H.A. Spikes, Trib. Trans. 34, 248, (1991). 8. "The Measurement and Study of Very Thin Lubricant Films in Concentrated Contacts," G.J Johnston, R.W. Wayte and H.A. Spikes, Trib. Trans., 34,187, (1991). 9. "The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules," D. Lin-Vien, N.B. Colthup, W.G. Fately and J.G Grasselli, Academic Press, (1991).
589
IR Spectroscopic Analysis of Grease Lubricant Films in Rolling Contacts S. Hurley and P.M. Cann Tribology Section, Department of Mechanical Engineering Imperial College of Science, Technology & Medicine London SW7 2BX
Abstract
In this paper, the composition of grease lubricant films formed in rolling contacts has been studied using IR reflection spectroscopy. A simple ball-on-fiat test arrangement has been used. Greases are two-phase lubricants composed, primarily, of a thickener dispersed in a basestock. The lubricant film in the contact region however, does not necessarily reflect the bulk composition, but will depend critically on the mechanism of lubricant supply to the contact. If flow of bulk grease into the contact occurs then both thickener and base oil will be present. The grease structure however will be heavily degraded by the very high local shear stresses as it passes through the contact. If the contact is starved of bulk flow then film generation is due to local base oil flow from the grease reservoir. In this case, the separating film is primarily base oil. Thus the composition of the film depends on the lubricant supply and is likely to change with operating condition and time. In this work, the composition of grease films under both fully flooded and starved conditions has been studied using IR reflectance spectroscopy. This technique provides information on the concentration and distribution of the grease components both within the track and in the reservoir at the sides. Under fully flooded conditions both thickener and base oil are present in the inlet, although the thickener concentration varies depending on speed and proximity to the Hertzian zone. Under starved conditions there is an initial deposition of bulk grease in the track as fresh grease is overrolled at the start of the experiment. As the test continues however, the grease structure is broken down and base oil is expelled from the track. There is also evidence of shear degradation and oxidation of the thickener within the track. The condition of the grease close to the track also changes as the test proceeds and there are local thickener concentration changes in this region. These findings are discussed in the light of current theories of grease lubrication. 1. GREASE LUBRICANT FILMS IN ROLLING CONTACTS
Lubrication performance is essentially determined by the properties of the film formed in the contact region. The physical properties, chemical composition and thickness will determine the levels of friction and wear, and ultimately component life. In this paper the composition of lubricating films formed by greases in rolling contacts is examined. Although there have been many studies devoted to the analysis of films formed by liquid lubricant and additive systems, there is very little work in the literature where film formation by greases is considered.
Grease is a two-phase lubricant comprised primarily of a thickener physically and chemically dispersed in a basestock. However, the lubricant film in the contact region will not necessarily reflect the bulk composition but will depend critically on the mechanism of lubricant supply to the contact. Two supply regimes are identified in this work: fully flooded and starved. In the fully flooded condition the inlet is considered completely filled with lubricant. In the starved regime there is a limited amount of lubricant present and the inlet meniscus is close to, or at, the Hertzian circle. Under fully flooded conditions if flow of bulk grease into the inlet occurs then both thickener and base oil will be present. The grease structure
590
however will be heavily degraded by passage through the contact and it is likely that some deposition of thickener occurs in the rolled track. For the fully flooded condition film thickness will scale with the usual EHL criteria of inlet viscosity and rolling speed (1). If inlet flow is not maintained then the rules governing film formation are different. Under starved conditions base oil is thought to bleed out of the grease reservoir at the side of the track, replenishing the contact (2). This model implies that the film is composed of base oil alone. However, recent work has shown that the film composition is more complex and that both thickener and base oil are present (3). The thickener forms a thin (~5-10 nm) residual film on the metal surface which is augmented by a hydrodynamically-generated film (3). The operating requirements for bearing greases are low friction with a relatively long lubrication life. The ideal condition is therefore semi-starved as the friction losses and thermal effects are minimised. The lubricant film however must be sufficient to prevent surface damage and thus in this regime the thin film properties will be increasingly important in determining performance. Deposition of the thickener to form thin residual films which act as antiwear or boundary layers is one possible mechanism (4) and this has been observed with urea greases (5). An alternative mechanism is the formation of high viscosity layers (6) at the surface. Optimisation of the local rheology at the surface might have beneficial effects, particularly in the prevention of damage due to component vibration. The possibility of controlling grease formulation so as to optimise the surface film properties has not been explored as yet. Most of the research into grease has been concerned with their bulk rheologieal behaviour. There is very little published work concerning the properties of the grease lubricant films formed under different contact conditions. Therefore, the aims of the study were: • to develop techniques to characterise grease lubricating films • to apply these techniques to establish the chemical composition and physical properties of grease films / • to study film composition under fully flooded and starved conditions.
IR reflectance spectroscopy has been used to charactefise the grease films both in and out of contact. IR spectroscopy is ideally suited to the examination of greases as it can distinguish between the thickener and base oil phases and identify new organic species. IR transmission spectroscopy is routinely used to examine grease during manufacture and as samples from failed beatings. There is thus a large reference body of literature for comparison of results. This technique is also useful for providing information on structural changes such as alignment or destruction of soap thickener fibres (7). 2. EXPERIMENTAL PROGRAMME 2.1. Film thickness in rolling contacts
The first requirement was measure film thickness under both fully flooded and starved conditions. Once the lubrication behaviour had been established then IR analysis of the lubricant films could be carried out. In this study, grease film formation in a rolling point contact was measured for a ball on flat configuration using a high-resolution optical imerferometry technique (1)(8). The contact is formed by a loaded steel ball (diameter 19ram) rolling against a silica-coated glass disc. The thin film interferometry technique gives improved thickness resolution (1 nm) and minimum film thickness detection (1 rim) compared to the conventional methods. Two film thickness test procedures were used. All tests were carried out at 25°C: (i) Fully flooded - film thickness was measured with increasing speed, a small channelling device was used to push the overrolled grease back into the track. (ii) Starved - film thickness change with time was measured at constant speed. A single charge of grease was injected into the rolling track at the start of the test. 2.2 Characterisation of grease films in and out of contact
Film thickness measurements were carried out on the greases under fully flooded and starved conditions. In a corresponding series of tests the composition of the films formed under similar conditions was analysed by IR spectroscopy. IR
591
reflection-absorption techniques were used to analyse grease films both in and out of contact. Specular reflectance micro-spectroscopy was used to determine the composition and distribution of the grease components in the rolled track on a steel surface. A second method whereby IR reflection spectra were taken directly from the rolling contact was also developed to study the composition of the inlet and Hertzian region. These techniques are described in more detail below.
2.2.1 Out-of-contact analysis In a separate series of tests rolled grease fdms were prepared on polished steel discs and then analysed by IR reflectance spectroscopy. An IR microscope (Spectratech) coupled to a FTIR spectrometer (Perkin Elmer 1740) was used to take single reflection spectra from small areas (- 150 l.tm diameter) of the grease film from in and around the rolled track. A simplified diagram of this technique is shown in Figure 1. Spectra were taken with 200 scans at a resolution of 8cm 1
spectrometer
I
rolled grease track
Figure 1 IR reflectance sampling from rolled track The grease films were prepared on the rolling contact rig but in this case the glass disc was replaced by a polished steel disc. The working tests were carried out at constant rolling speeds, typically 0.1ms 1 and 1.0ms "1. At the start of each test approximately 1 ml grease was injected as an even layer around the rolling track. The ball and disc motors were then set to the desired speed and a load of 20N applied. Previous film thickness tests (see Figure 4) have shown that under these conditions, the film thickness will decrease with time The tests were stopped at intervals, the disc removed for analysis and then replaced, with the grease film
undisturbed, allowing the test to be continued. The width of the track was approximately 280 lain. Spectra were taken at various positions: in the track, at the edge of the track and increasingly further away (see Figure 5 below).
2.2.2 In-contact analysis A second technique whereby IR spectra are taken directly from the operating EHL contact was also used (7). In this case an 1R transparent window was used instead of the glass disc. The FTIR microscope was used to take spectra from small areas in and around the contact. In earlier work the technique was limited to sliding contacts and a diamond window (3mm diameter) was used as the (stationary) counterface to the steel ball. In this study the technique has been applied for the first time to rolling contacts and a new test rig has been developed. One of the problems with this configuration is that a large IR transparent rotating disc is required and in this case a 10cm diameter CaF2 window was used. A diagram of the technique is shown in Figure 2. This approach means that it is now possible to monitor grease lubricant composition both in the starved and fully flooded regimes. In the fully flooded regime, the film composition in the contact and inlet regions has been studied. In the starved regime the film thickness in the contact is generally too low to be detected and there is little lubricant in the inlet. However it has been observed that once rolling has been halted lubricant flows out from the grease reservoir and forms a meniscus around the stationary Hertzian contact. It is likely that this reflow also occurs during rolling and thus provides a continual flow of lubricant into the track. By stopping the test rig and taking spectra from the reformed meniscus it is possible to analyse the composition of the lubricant replenishing the track under starved conditions. 2.3. Test Greases A set of four simple additive-flee model greases were used. These were lithium hydroxystearate: (CH3 (CH2)5 CHOH(CH2)10COOLi) and a tetraurea (general formula R1-NHCONH-R-NHCONH-R2NHCONH-R-NHCONH-R3 where R is an alkyl group), both containing 7 and 14 % w/w thickener. These greases had the same mineral base oil (viscosity @ 40°C 200cSt.)
592
,/sample areas sample area for results in Figure 9
optical view
FTIR ' ] microscope
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,,
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, ,
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,
,
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it
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at film
Figure 2 IR spectroscopy sampling from the contact region
3. RESULTS AND DISCUSSION 3.1 Film thickness results
Representative, fully flooded results are plotted as log film thickness against log rolling speed in Figure 3 for the 14% lithium hydroxystearate and tetraurea greases at 25°C. For both greases, the overall film thickness is higher than that of the base oil. However, two distinct speed regimes are observed: low speed region where the film thickness is unstable and tends to fluctuate wildly higher speed regime where the film steadily increases, with a log-log film thickness/speed exponent close to 0.7. The instability at low speed is due to the intermittent passage of thickener lumps through the contact distorting the EHL film. Such behaviour is probably one of the origins of grease 'noise' in beatings.
As the rolling speed increases this behaviour lessens and the film in the contact assumes the usual EHL shape. At higher speeds the film increase with increasing speed follows the usual EHL considerations, suggesting that the inlet rheology is essentially Newtonian. This implies that the grease structure has been fully degraded by the greater inlet shear stresses experienced at these speeds and that the grease has effectively reached a constant apparent viscosity. A typical starved test is shown in Figure 4 for the 14 % lithium hydroxystearate grease at 25°C and 0. lms "~. In this case film thickness is plotted against disc revolution (time). At this temperature the inlet is severely starved and film thickness decays steadily throughout the test giving a final thickness which is considerably less than the fully flooded case.
593
uneven residual film (typically 2-60 nm thick) is seen. On either side of the track, the bulk grease reservoir sits with a series of corrugations or 'fingers' reaching towards the rolled track; these are formed by the film cavitating in the exit region. The composition of the track (sample position 1) and at the edge (sample position 2) have been examined later in this paper.
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.
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Figure 3 Fully flooded film thickness results for the 14% lithium hydroxystearate (open symbol) and tetraurea (closed symbol) greases at. 25°C. Base oil result shown as solid line.
160
Figure 5 Phase contrast microscopy photograph of a rolled grease track. The rolling direction is from fight to left. The rolled track corresponds to the Hertzian width of 280 ~tm.
. . . . . . . . . . . . . . . .
140 ,D D.......................................................... 120 - j ......................................................... o
3.2. IR Spectroscopy Results
~.~ 100 ..............................................................
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3.2.1. Transmission spectra of fresh samples IR transmission spectra (limited wavelength range) are shown for two grease types (7% tetraurea and lithium hydroxystearate) in Figure 6 For the lithium hydroxystearate grease: the absorbances at 1580 and 1560cm "~ arise from the soap thickener. The characteristic double band corresponds to the asymmetric stretching of the carboxyl group (9). Peaks at 2955, 2925, 2850, 1463, 1377 and 720cm "1 are primarily due to the paraffinic base oil, arising from the CH2 and CH3 groups. However, it must be remembered that the hydrocarbon portion of the hydroxystearate thickener will also contribute to these peaks. The CH2 deformation for the pure soap occurs at 1453 cm "~. These assignments are summarised in Table 1 below.
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.
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.
.
.
.
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Figure 4 Starved film thickness decay for 14% lithium hydroxystearate grease at 25°C, 0.1 ms "1.
At the end of the test, the rolled grease track can be examined by a low power optical microscope and a photograph of this is shown in Figure 5. Several features should be noted as these are examined in the IR work. In the centre is the rolled track, where an
594
Polyurea thickeners give several characteristic absorption bands and those for the tetraurea are listed in Table 2. Urea thickeners have C=O, NH2 and NH bands similar to those of amides. The C-H bands are similar to those reported above for the lithium hydroxystearate grease. Transmission spectra can be compared directly to those from the reflection and in-contact studies.
Table 1 IR peak listing for lithium hydroxystearate thickener and base oil. Peak position cm
Structure
-1
2955
C-H asymmetric stretch (CH3)
2925
C-H asymmetric stretch (CH2)
2850
C-H symmetric stretch (CH2)
1580
COO asymmetric stretch
1560
COO asymmetric stretch
1463
CH deformation (CH2)
1402
COO symmetric stretch
1377
CH deformation (CH3)
Table 2 lit peak listing for tetraurea thickener. Peak position cm-1 3360-3260
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1631
Amide I band (C=O)
1565
.~maide II (C-N and N-H)
1233 &1302 1310-1175
Structure
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Figure 6 IR spectra for fresh greases: 7% lithium hydroxystearate (solid) and 7% tetraurea.
em "1 1300
595
3.2.2. In-contact analysis In this study IR spectra were taken from a rolling contact for fully flooded operation. The intemion was to determine the composition of the lubricant erecting the contact under differem speed conditions. Typical spectra are shown for a lithium hydroxystearate (14%) grease in Figure 7. Two speed levels were chosen corresponding to the 'noisy' (low speed) and 'EHL' (high speed) regimes noted in the film thickness work. The thickener content at low speeds is very high, in some cases it is higher than the average value recorded for the bulk grease. This suggests that in this region there is entrainmem of concentrated thickener bundles into the inlet. At higher speeds the concentration of thickener in the inlet region decreases to less than that of the bulk grease, although variations are also observed depending on the distance from the comact. Typically the thickener concentration decreases close to the contact both in the inlet and at the sides. An example of this for a tetraurea grease (7%) is shown in Figure 8. In this case, all the spectra have been normalised to the same value of the 1460 cm "1 band. The urea absorbance increases with distance from the Hertzian contact both in the inlet and at the side of the contact.
t
.
Under heavily starved conditions there is very little lubricant in the inlet so this precludes IR analysis during rolling. The film in the comact was also generally too thin to be detected by this technique. However one observation from the starved studies is that once rolling has been halted lubricant flows from the grease reservoir to reform a meniscus around the contact. This process is due to capillary forces close to the contact and does not occur in the rolled track 'out-of-contact'. It is likely that this process operates during running to provide a local supply of lubricant to the contact. In-contact IR spectra have been taken from the meniscus to identify the composition of this lubricant and an example is shown in Figure 9. Spectra were taken at increasing distances from the stationary contact (this corresponds to increasing reflow time). The lubricant which reflows first is essentially base oil containing very little thickener. This free base oil has been released from the grease close to the track probably through the shear degradation of the thickener structure. With increasing time however, higher viscosity material containing some thickener returns to the track. This behaviour obviously provides a mechanism whereby flee oil and bulk grease can return to the track during periods when a beating is not in operation.
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1600
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1500
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Figure 7 IR spectra from inlet region for a fully flooded rolling contact at two speeds, ---0.011 ms1, - - - 0.08 ms 1. A spectrum of the bulk grease (14% lithium hydroxystearate) is also shown (solid colour ).
1400
596
Normalised CH: peak
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Figure 8 IR spectra taken from the inlet region for tetraurea grease (7%) at 0.1 ms "1. For positions 1 (-~2801xrn) and 2 (--560~tm) from the centre of the contact, above. The bulk transmission spectrum is also shown ( .... ).
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much"further from contact
1581
1377
no thickener present essentially base oil
thickener band at 1581 cm "1 appears
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cm
Figure 9 IR spectra from lubricant reflowing into the track once rolling has been stopped. The distance from the contact reflects the length of time it takes for that material to reflow.
597
structure. In this case, thickener-rich particles were deposited in the track during the first few minutes of operation. These were clearly visible in the rolled track using an optical microscope. In Figure 10 spectra are shown of a deposited thickener particle and from the surrounding film. The increased concentration of urea thickener is seen by the intense absorption bands at 1636- 1515 and 1305-1234 cm "1 compared to the 'CH' bands at 1460 and 1377 cm1. The shift of the CH2 band to 1465 cm "1in the particle spectra is also indicative of almost pure urea. The thickener lumps gradually fragment with overrolling, and eventually break down to give a more uniform film. Cavitation patterns, seen within the track area, signify that the film is highly viscous rather than simply a solid layer. The 'high viscosity layer' mechanism for starved grease lubrication was suggested by Scarlett (6) and this result shows that it can be a viable mechanism.
3.2.3. IR reflection spectra from rolled grease films 'out-of-contact' The starved rolling tests show that the grease film decays very rapidly in the first few minutes of running. In a separate series of tests, the glass disc was replaced by a polished steel disc so that IR reflection-absorption spectra could be taken from the rolled track. The test was run in the usual way except the steel disc was removed periodically for analysis. IR spectra were taken from small areas (-150 lxm diameter) from within the rolled track and the 'fingers' at the side (sample positions 1 and 2 in Figure 5). Tests were run at two speed levels: 0.1 and 1.0 m/s. Initially, flesh grease is overrolled in the track. As the test proceeds, the film breaks down and base oil is released. At the lower speed the urea and high concentration lithium hydroxystearate greases were able to maintain a thick film in the track for very long periods. This was particularly seen with the high concentration urea greases which had a very lumpy
1636
1564
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1234
A 'thickener bundle' 1465
1305 1459 1410 1377
"1
1700
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1600
I
1500
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1200
1110 "
-
-
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Figure 10 IR reflectance spectra taken from a 14% tetraurea grease thickener particle deposited in the track ( - ) and the usual film within the track (- - -) 1.0 m/s after 20 seconds running.
598
The lower concentration lithium hydroxystearates tended to decay very quickly giving much thinner residual films. There was also evidence of thickener structure breakdown giving physically or chemically transformed layers. This occurred both within the track and in the grease at the side. Figure 11 shows a series of spectra that have been taken at different times from the track (position 1 in Figure 5). Results showed broadening of the asymmetric carboxyl band at 1580cm1 and gradual disappearance of the corresponding one at 1560cm1 as rolling proceeded. The absorbance of the carboxylate band also increased relative to the CH2 band suggesting that as the grease breaks down in the track, a thickener rich layer is left and base oil is expelled. This free base oil is thus available for reflow, as seen in Figure 9 above, and the formation of a hydrodynamic film during rolling. The CH2 absorption at 1463 cm~ also appears to resolve into two peaks at 1460 and 1465cm"x. One possible explanation for this is that the soap or base oil has been chemically transformed within the track. In this case mechanical degradation of the soap fibres, followed by chemical reaction of the reactive molecular fragments is a more likely option. The 'scissoring' frequency of CH2 groups is very sensitive to the molecular environment. When a CH2 group is adjacent to oxygen atoms this frequency decreases and there is also an increase in intensity (9). Thus one interpretation of the decrease in the deformation frequency observed in the spectra of the rolled track is that oxidation has occurred and acidic products are beginning to form. Alternatively, the second peak may be another CH2 deformation at a decreased frequency from a new chemical species with a shorter hydrocarbon chain. The CH3 absorption at 1378cm"~ decreased in height and gradually increased in frequency, becoming a shoulder of a new peak arising at 1411cm~. This latter peak is due to the CH2 deformation frequency of short chain carboxylic acids such as ethanoic acid (9). This again would suggest that the soap thickener is being physically degraded by the high shear stresses in the contact and that new shorter chain or oxidised species are being formed. The condition of the grease at the side of the track will have an influence on replenishment and IR spectra have been taken from the grease 'fingers' (position 2 in Figure 5) to study thickener
concentration and breakdown (see Figure 12 below). Increasing the speed from 0.1 to 1.0ms"1 increased the rate of track depletion. Initially base oil is expelled from the track but at a later stage thickener is also lost. Thus at the edges of the track the (relative) soap absorbance increased slightly with extended running time. This was also seen in the characteristic shift of the CH2 deformation peak towards the lower frequency of 1453 cml; which is characteristic of the soap. These spectra also exhibited further evidence of chemical degradation. All spectra taken from the edge of the track showed an increase in the absorbance band at 1410cm1 which is associated with short chain acidic species (9) and is taken as an indication of oxidation. There is also further evidence of this: in some of the spectra taken in the high speed tests, a series of overlapping absorption bands appeared between 1630 and 1800cm 1. These are due to C=O stretching of acidic species, particularly ketones (~1700crn"1) aldehydes (-~1710cm"l) and carboxylic acids (>1720cmI). All these effects were only observed in spectra taken in, or close to, the track. In IR spectra taken well away from the edge of the track, the soap ratio decreased and the degradation bands were not present. The lit results from the out-of-contact analysis show that deposition of grease occurs in the track. forming a thickener-rich film. The stability and adherence of such a film could have significant implications for the continued operation of beatings under conditions where a hydrodynamic film is not formed. For instance: when severely starved, during start-up or at low speeds. Thickener films might also help to prevent surface damage through damping of transient effects, for example the low load vibrations that result in false brinelling. IR spectra taken from the films after extended running under starved conditions have shown that degradation of the grease film occurs. Evidence of structural breakdown, concentration changes and oxidation to form acidic species has been found. In the starved regime the conditions within the contact are very severe; in this case films below 20 nm have been measured. It is likely therefore, that the combination of high local shear stresses, thermal effects and the proximity of the steel surface will contribute to the breakdown of the thickener to form oxidised, possibly acidic species. This has implications for oxidative wear of the metal surface.
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Figure 11 IR spectra from rolled grease track, position 1 in p h o t o g r a p h in Figure 5, 14% lithium hydroxystearate, 25°C, 0.4 m/s.
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.........
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4. CONCLUSIONS
ACKNOWLEDGMENTS
The findings from this study are summarised below:
The authors would like to thank the EPSRC and the NLGI Fundamental Research Committee for financial support of this study.
Development of IR analysis methods: - The in-contact IR technique has been extended to rolling contacts and has been. applied to the study of grease film composition both in the contact and inlet regions. Grease EHL film thickness: - Fully flooded: two distinct speed regimes, low speed 'noisy' regime where thickener lumps survive passage through the inlet and enter the contact. At higher speeds, film thickness increases according to usual EHL rules. - Starved: at ambient temperatures film thickness decays rapidly to a lower value than the fully flooded level. Analysis of grease lubricant film composition: - Fully flooded: at low speeds, much higher thickener concentrations are obtained in the inlet region confirming the presence of local thickener 'bundles' although this is an intermittent occurrence. At higher speeds, the inlet thickener concentration drops close to the contact and this has important implications for the development of inlet rheological and film thickness prediction models. - Starved: a thick grease film is deposited in the initial stages which breaks down releasing base oil and leaving a thickener rich layer. Concentration changes are seen in the 'finger' region at the side of track. - There are indications of a breakdown in the thickener chemical structure forming oxidised, acidic hydrocarbon species. - Lubricant that reflows around the stationary contact is flee base oil released from grease close to the track. This mechanism could provide continual replenishment during running. The work has shown that the Scarlett (6) model of a high viscosity deposited layer is valid although the life is limited unless it is renewed. Such films might offer very different surface protection properties to the conventional fluid EHL film and it should be possible to optimise grease composition to exploit these properties for particular applications.
REFERENCES
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