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Thermal Degradation of Greases and the Effect on Lubrication Performance
Tribology for Energy Conservation / D. Dowson et al. (Editors) © 1998 Elsevier Science B.V. All rights reserved.
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Thermal Degradation of Greases and the Effect on Lubrication Performance S. Hurley, P.M. Cann and H.A Spikes Tribology Section, Department of Mechanical Engineering, Imperial College of Science, Technology & Medicine, London SW7 2BX, England.
The lubricating life of a grease in a rolling element bearing is reduced by operation at high temperatures and this can reaflt in premature failure of the bearing. A grease experiences severe conditions in an operating beating where the combination of high tempcraaLres and sustained mechanical working result in gross physical and chemical changes to the grease. These changes have a significant effect on the ability of the grease to replenish the contact and maintain a lubricating film, particularly under starved inlet conditions. Extended operation at high temperatures promotes evaporation of low molecular weight base oil components (1) and oxidation of one or both of the grease components (2)(3). The presence of small amounts of transition metals and their oxides can accelerate these processes and such material is commonly found as wear ( ~ r i s in bearings (4). It is difficult to disentangle these effects and this paper concentrates on the effeOs of thermal ageing on the lubricating ability of the grease. Simple thermal ageing tests have been carried out on two lithium hydroxystearate greases and the resulting changes in their chemistry characteriscd by infrared spectroscopy. The lubricating performance of the aged greases has been assessed by measuring film thiclmess and oil release in a rolling contact under starved conditions. Results from infrared analysis have shown that the oxidation process is accelerated at a temperature of 120° C forming carboxylic acids and related species. The film thickness results showed that the aged greases give a lower equilibmtm film thickness and this correlates with reduced oil release.
1. GREASE AGEING AND LUBRICATION PERFORMANCE Extended operation at high temperatures will limit the lubricating life of a grease in a rolling element bearing. At present we know little of the changes that occur in a grease as it is progressively worked and thermally stagssed, nor of the relationship between these changes and the failure of the grease to maintain a lubricating film. There are many papers (2) - (10) where the changes in grease chemistry during ageing have been studied, but little work where these changes have been explicitly related to the lubricating ability of the grease. In this paper the chemical changes that occur during thermal ageing of a grease have been characterised by IR ~ r o s c o p y . In addition the lubricating performance of the aged greases has been assessed by film thickness and oil release measurements.
It might be possible to use such an approach to develop a screening test to monitor grease condition in operating bearings. It is also not possible, at present, to predict 'lubricant life' from bulk grease properties. The grease should be considered as an integral component of the bearing and 'lubricant life' incorporated into any bearing life model. Such an approach is already being developed (l 1)(12), but we need to know more about the fundamental mechanisms of failure in such systems. A grease experiences both chemical and physical changes as it is worked at high ten~ratures in a bearing. These changes are not due solely to oxidation of the greases but are a combination of effects due to conditions experienced within the b e a ~ g . These include thermal and mechanical stress and the presence of debris and moisture.
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The p h ~ c a l effects include deterioration of thickener ~ , increased oil setm~tion and viscosity changes. For example the fibre network in a lithium hydroxystearate grease is broken down during working (13) and through thermal degradation. At low temperatures it is the physical changes which control grease life (12). Evaporation of the mineral base oil (14), especially the volatile, shorter-chain paraffins, will increase the viscosity and possibly affect contact replenishment. Perhaps the most influential factor on the service life of a grease is oxidation (12). For a given chemical structure and additive system, temperature has the greatest effect on the rate and mechanism. It is generally acknowledged that within a temperature range of 40-100°C, a rise of 10°C effectively doubles the rate of oxidation. Service life of a grease can be raised from I00 hours to 5000 hours if the working temperature is lowered from 100 to 50°C and can be sustained at ten~ratures 30-50°C above this if oxidation inhibitors are added (11)(15). Physical working also increases the amount of air present in the lubricant by as much as 3-5%. (I6) and this will promote oxidation. Debris which contains particular transition elements and their oxides will also catalyse oxidation. (9)(17)(18) The oxidation of paraffinic hydrocafoons is generally believed to occur via a free-radical chain reaction mechanism due to hydro-peroxide involvement. (17)-(22) It is believed to involve several series of chain b ~ n g and chain lengthening mechanisms. This chain reaction is both time and temperature dependent (35)-(39) and is observed to be accelerated by the presence of certain metals. (2X4)(17)(18)(33)(34) The initiation stage involves the homolytic cleavage of a hydrocarbon bond. This forms a highly reactive free radical species. This reacts with oxygen to form unstable species which decompose, creating peroxides and regenerating the radical. These peroxides in turn react to form products susceptible to oxidation. In this way, a complex autocatalytic process is established; the final products and concentrations of which are ,~miable. At the same time, thermal degradation of the grease components will occur and this is also considered a free-radical mechanism proceeding via a carbon-~u~n chain scission. Both of these reaction processes are likely to influence each other, possibly in competition, and produce a range of
organic s p i e s ; carboxylic acids and related ca~nyl co~ds being the main products. Both mechanisms are affected by the presence of small amounts of transition metal debris. From the reaction products it is difficult to distinguish between oxidation and thermal degradation processes. Grease life in a bearing operating at high temperatures is assessed in the ASTM D-3527 test which is run at 150°C until failure occurs. This approach is widely used to study grease failure in bearings and similar tests are reported in the research literature. In many cases, grease samples are removed from the ~ g and characterised by standard analytical techniques. There is evidence of viscosity increase (9) or decrease (4) both of which are thought to contnlmte to failure. Increased acidity (2)(4), colour changes (9) and IR ~ o s c o p i c analysis (2X9) have also been used to denote changes in grease chemistry. IR analysis of grease samples from bearing tests has shown qualitatively an increase in acidic species (corresponding to an increase in TAN) (4)(9), loss of anti-oxidant additives (9), and changes in lithium hydroxystearate concentration (2)(9)(23-30). Generally lubricant, and thus bearing failure, is linked to changes in bulk grease viscosity. Soflemng of the grease can result in leakage from the bearing (2). A significantly increased viscosity means that the grease fails to flow into the contact and thus the lubricating film is reduccd~ Increased frictional heating might thus result, which is coupled with r e ~ a ~ removal of heat (14). Bearing failure is signalled by a rapid increase in nmning temperature or torque level (12). It is only possible to draw very general conclusions from the bearing tests and to show qualitatively oxidation effects. It is not as yet possible to rink these changes directly to loss of lubricating performance. There are very many factors influencing degradation and it is necessary to be able to study each individually. This paper focuses on the effect of thermal ageing on the chemistry and lubrication performance of a grease. When considering the effect of grease degradation on lubric~on performance, it is necessary to ~ in mind the mechanisms of film formation in a bearing. Most bearings run under sta._rved or ~ l y starved inlet conditions. In this regime the bulk grease does not flow spontaneously
77
into the inlet and the lubricating film is maintained by local replenishment from the grease reservoir at the side of the tracE, It is changes in the ability of the grease ability to resupply the contact in this regime that are important in determining failure. The grease properties that control reflow around the contact will include oil bleeding or release (12)(13), base oil viscosity and polarity. All these things will influence reflow and hence lubricating performance, A brief review of the research literature has been presented, although few definite conclusions can be drawn. There has been no definite identification of the critical lubrication mechanism that ceases to function effectively and thus induces grease and bearing failure. The aims of the current programme are therefore to: (i) characterise the changes that occur in thermally stressed greases (ii) relate these changes to lubricating performance
2. RESEARCH PROGRAMME The research programme was developed around a series of simple ageing tests and uses infrared analysis to monitor grease chemistry; thus establishing the progress of oxidation. When a grease is run in a bearing it clears away from the rolling elements. There will thus be a distribution of grease within the bearing with most of it on the cage or in the housing. Close to the rolled track there will be a much thinner grease film. The ageing tests were designed to simulate both situations of bulk and thin film thermal degradation. Lubrication performance is assessed by film thickness decay and reflow tests under starved conditions and by examination of shear stability. Infrared transmission ~ r o s c o p y is used to characterise the chemical changes that occur in the grease during ageing. It provides information on the nature and concentration of oxidised species and it has been used to analyse samples both in simple laboratory tests and from bearing tests (23)-(28). It will therefore be possible to relate the results from this work to those from bearing tests. The ageing tests subject the grease to high temperatures for extended times, The greases will suffer both oxidation and thermal degradation reactions. It is
difficult to distinguish between the products of the two processes. The term thermal ageing is used to cover both decomposition processes. 3. Experimental 3.1 Ageing Tests The greases were aged as layers spread onto aluminium plates (layer thickness: 1.5mm). Samples were aged in a dry oven at 120°C operating for up to 1250 hours. Several samples were prepared at the start of the test and removed at intervals. Some of the samples contained metal and metal oxide panicles, similar to b e i n g wear debris (4)(33)(34).
3.2 Infrared Analysis All infrared analysis was carried out using a Perkin-Elmer 1740 Infrared Fourier Transform Spectrometer connected to a Venturis FX 5166 PC. All measurements were carried out at room temperature. The aged greases were sampled in transmission mode as thin films spread onto KBr discs. As the sample thickness was not constant, all spectra were normalised with r e ~ to the CH2 peak at 1460cm1. 3.3 Lubrication Performance Film thickness decay measurements were made on fresh and oxidised greases. These were earned out using a PCS Instruments EHL Ultra Thin Film Measurement System described in previous publications (31)(32). The apparatus consisted of a steel ball (diameter 19mm) loaded against a silicacoated glass disc. The disc and ball are driven by separate electric motors. This arrangement simulates contact conditions of a rolling element ball bearing. Film thickness in the centre of the contact was measured by the Thin Film Technique. Before use, the steel ball and disc were ultrasonic~ly cleaned in a toluene bath, washed with acetone and polished with a clean tissue. For these experiments, the disc was rotated at 0. l ms t and 20N load was applied to the ball. At the start of the test, I ml grease was injected as an even layer around the rolling track. The motor was then switched on and the load applied. Film thickness measurements were taken periodically for 60 minutes, or until the film had dropped to below 5 nm.
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Lubricant reflow properties of fresh and aged greases were measured in the same apparatus. In this case an optical microscope and a video camera were used to view the contact and a chromium-coated glass disc was employed. Approximately 0.5ml grease was injected into the rolling track using a syringe. An image of the contact was recorded using the video camera. The contact was starved by manual rotation of the glass disc about 25 ° forward and backward 15 times. The motion was then stopped and the lubricant allowed to flow back around the contact and into the track. The tests were carried out several times and video recorded. The image was analysed and the distance that the lubricant meniscus travelled with time was measured (Figure I).
Test Debris Iron trioxide - cx-Fe203: grain size 751am Iron tetra oxide- Fe~O4. grain size 751,tm Brass - 67% Cu, 33% Zn" grain size 53-63p,m Copper - Cu" grain size 38-45pm.
4. RESULTS
4.1 Ageing of Grease Samples During the ageing treatment, the appearance of the grease samples changed. The colour of the pure samples gradually deepened from a gold to a dull red/brown with a 'dry' appearance. Fe203 and Fe304 caused a deeper eolour change, but the greases retained their wet, shiny appearance. Brass produced a much deeper and redder colour change with no evidence of drying. Copper effected a similar colour change to brass, but greatly dried the samples.
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All the spectra are shown normalised to the CH2 absorbance at 1460 cm "~. The spectra obtained of the fresh grease were those of a typical lithium hydroxystearate soap thickener in paraffmic mineral oil. The major absorbances are listed below in Table
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Figure 1 contact
Lubricant reflow around a stationary
3.4 Test Materials Test samples were chosen from a series of simple lithium hydroxystearate greases with: 9% and 15% w/w thickener. These greases had the same base oil (viscosity @ 40°C 30eSt) and contained no anti-oxidants or other additives. In some tests, metal powder was included to simulate the effect of wear debris. Each was added to the grease at a concentration of 0.1% by mass by stirring manually. Care was taken to ensure that all the samples experienced a similar degree of mixing and were not excessively worked.
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The oxidised sample produces a similar peak pattern, but with different absorbance ratios. A broad absorbance exists between ! 740-1685crn "l due to the presence of a range of oxidation products (aldehydes, ketones, carboxylic acids). In addition,
79
small peaks a ~ r ageing.
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cm l. The 15% and 9% thicker film samples, analysed by transmittance gave lower ratios. The carboxylate soap absorl~nce at 1580cm ~, was observed to increase with time for all samples. This is not a real effect, but the result of base oil evaporation. Furthermore, there is a reduction in the rate of soap increase with time. This suggests that oxidation of soap may occur after an initial 'induction period.' The presence of debris, especially brass, significantly affects the infrared spectra of the aged grease (Figure 4 ).
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Figure 2 Infrared transmittance spectrum of fresh (shaded) and oxidised (clear) 9% grease.
Figure 3 shows how the transmittance spectra change with increasing oxidation time. A significant increase in the broad band absorbance at 1700 - 1740 cm l is observed.
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as ageing proceeds. The CTI3/CH2 ratio increases significamly; indicating chain-breaking reactions. The most obvious change exhibiteA is the massive reduction in the carboxylate absorbanc¢ with time and which, at 66.25 hours, ceases to exist. A corresponding increase in the acid a b s o ~ c ¢ gives a linear relationship between the acid appearance and carboxylate disappearance. This suggests that the brass not only accelerates the oxidation process, but modifies the dominant reaction mechanism. This results in a total loss of carboxylate a b s o ~ and presumably thickener structure. The appearance of the brass-contaminated sample was also different: it a ~ e d to be very soil again suggesting an effective viscosity loss rather than the increase associated with conventional ageing, The results for the other debris materials were not so extreme.
80
and Lubrication Performance Film thickness decay tests showed that with the fresh greases the 9% maintained a higher film thickness than the 15% at the end of the test (Figure 5). The 15% grease initially gives a thicker film but this breaks down to give a lower equilibrium film thickness at the end of the test. The lower thickener content facilitates oil release and hence contact replenishment. The equilibrium film thickness reflects therefore the ability of the grease to supply the contact with lubricant. 4.2 Oil Release
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Figure 6 Film decay with time for flesh and aged (1250 hours) 15% greases.
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After oxidation for 1250 hours, both greases gave significantly lower equilibrium film thickness under starved conditions than their fresh counterparts. This is shown in Figures 6 and 7 for ..................the 15% ~ d 9 % ~ ~ s ~ s ~ v e l y : .....................................................................
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81 The films also decayed at a faster rate. Decay is this region is due to breakdown of the deposited layer which releases base oil. The faster breakdown indicates that the stability of the thickener structure is reduced by ageing, rendering it less resistant to the shear stress within the contact region. This lower equilibrium film thickness is due to poorer replenishment of the contact as this is determined by the amount of oil available for reflow (40) and the viscosity or surface spreading properties of the oil. The faster film decay suggests that sufficient oil release has occurred both from inside the contact and in the grease reservoir. The alternative explanation is therefore that the oil viscosity has increased sufficiently to inhibit reflow or that the base oil spreading properties have been adversely affected by ageing. It is also possible to quantify the reflow by measuring the time for the oil meniscus to reform around the contact. Reflow test results are plotted below in Figure 8. Reflow distance (meniscus radius/Hertzian radius) is plotted against elapsed time after rolling has stopped. Results are shown for 9% and 15% greases both fresh (solid line), and after 1250 hours ageing (dotted line). They show that the fresh 9% grease rapidly reforms a memscus around the contact and that ageing significantly reduces this ability. Similar effects are seen with the 15% grease. Reflow has almost stopped for the 15% aged grease and the lubricant flow barely manages to reform around the contact even after 160 seconds. One interesting observation from these experiments was the way in which reflow changed during the test. The test procedure was as follows: the disc was rotated backwards and forwards 15 times, movement was then mopped and reflow measured. This was repeated 5 times with each grease sample and the results below were taken on the 4 th stoppage. It was noted that reflow for the fresh grease was fairly repeatable for each stoppage. The aged greases, however, gave very different reflow rates and these increased dramatically with successive stoppages. The difference between fresh and aged grease reflow was therefore more marked in the early stages of the test.
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Figure 8 Reflow distance (meniscus r a d i ~ r t z i a n radius) against time for flesh and aged (9% & 15%) grease.
5. DISCUSSION AND CONCLUSIONS This paper has attempted to characterise thermal ageing of a grease using IR spectral analysis and to relate this to changes in lubricating ability. The results have shown that, as expected, the following chemical changes occur in a thermally aged grease (i) increased acidic species through oxidation of the base oil and, to a limited extent, thickener (ii) chain scission and evaporation of low molecular weight ~ i e s (iii) the presence of metal debris accelerates production of acidic slx~es and can change the dominant reaction pathway The presence of brass changes the grease degradation pathway in the most dramatic fashion. In this case both the oil and the thickener are degraded and complete loss of the c a ~ x y l a t e peak was observed after only 66 hours heating, The soap appears to be attacked preferentially in the early stages. Carboxylate loss suggests that the thickener
82
structure has broken down completely. The grease sample at this stage a ~ d to have a very loose consistency. If this occurred in a beating then failure might be due to bulk grease churning within the bearing; which would lead to excessive heat generation and thermal runaway. In tests where such debris is not present then the conventional degradation path is followed, Evaporation and oxidation of base oil species occurs resulting in increased viscosity, both of the bulk grease and the base oil. The increased viscosity of the bulk grease will diminish its heat removal capabilities. The increased viscosity of the base oil will impair its ability to reflow around the contact and replenish the track. The film thickness will thus drop and the system will accelerate towards failure. The conclusions are summarised as follows: (i) Ageing tests: oxidation initially of the base oil and then thickener. Presence of debris accelerates the process and there are significant changes in the oxidation route depending on debris. (ii)Aged greases give lower equilibrium film thickness in starved tests, suggesting that oil release and reflow has deteriorated with ageing. (iii) Lubricant released from aged greases has reduced ability to reflow around the contact.
ACKNOWLEDGMENTS The authors would like to thank the EPSRC for financial support of this project.
REFERENCES (1) E.R. Booser and A.E. Baker, "Evaporation - A Factor in Ball Bearing Grease Life." NLGI Spokesman Vol. 40 (1976) 60. (2) S. Hosoya and M. Hayano. "Deterioration of Lithium Soap Greases and Functional Life in Ball Bearings." NLGI Spokesman Vol. 53 (1989) 246. (3) C. Araki, H. Kanzaki and T. Taguchi, "A Study on the Thermal Degradation of Lubricating Greases." NLGI Spokesman Vol. 59 (1995) 15.
(4) W.W. Bailey and S. Pratt. " ~ m i c Oxidation Stability of Lubricating Greases." NLGI
Spokesman (1982) 15. (5) E. Brandolese, R. Santorelli, G. Pisaniello and G. Ponti. "An Accelerated Test Method for Oxidation Stability Usable to Predict the Behaviour of Lubricating Greases Under Severe Dynamic Conditions. NLGI 6Yd Annual Meeting, Arizona, October (I 996). (6)S. Gunsel, E.E. KLaus and J.L. Bailey. "Evaluation of Some Poly-Alpha-Olefins in a Pressured Penn State Microoxidation Test." Lubrication Engineering Vol. 43, (1987) 629. (7) Designation IP 280/80 (1982) "Oxidation Stability of Inhibited Mineral Turbine Oils." Oxidation Stability (1982) (8) J.tL Barnes and J.C. Bell. "Laboratory Screening of Engine Lubricants for High Temperature Performance." Lubrication Engineering Vol. 45, (1989) 549. (9) D.J. CarrY, R. Bauer and P.D. Fleischauer, "Chemical Analysis of Hydrocarbon Grease from Spin-lxaring Tests." ASLE Trans. Vol. 26, (1982) 475. (10) H.H. Abou El Naga and M.A. A!xtel Ghany. "Chemical Structure Bases for Oxidation Stability of Neutral Base Oils." ASLE Trans. Vol. 30, (1987) 261. (11) W.J. Bartz "Long-Life and Life-Time Lubrication - Possibilities and Limitations." Lubrication Engineering Vol. 49, (1993) 5!8. (12) H. Ito, K. Hideki and M. Naka. Proc. Int. Trib. Conf. Yokohama. Vol. H (1995) 93 I. (13) P.M. Cann, B.P. Williamson, R.C. Coy and H.A. Spikes. "The Behaviour of Greases in Elastohydrodynamic Contacts. Applied Physics Vol. 25, A124-A132. (1992) (14) A~ Lansdown and R.B. Gupta. "The Influence of Evaporation on Grease Life." NLGI Spokesman July (1985) 149. (15) A.N. Smith.. "Turbine Lubricant Oxidation: Testing, Experience and Prediction." "Aspects of Lubricant Oxidation." ASTM STP 916, W.H. Stadtmiller and A.N. Smith, Eds., American Society for Testing and Materials, Philadelphia (1986) 1. (16) M. Jungk and D. Hesse, "Silicone Oil Base Fluids as a Tool to Tailor High Performance Lubricating Greases", presented at the 9 th ELGI
83
Conference (1997). To be published in Eurogrease. (17) R.A. Newley, H . A . Spikes and P.B. Macpherson. "Oxidative Wear in Lubricated Contact." Journal of Lubrication Technology Vol. 102 (1980) 539. (18) S.M. Hsu, C~S. Ku and P.T. Pei. "Oxidative Degradation Mechanisms of Lubricants." "Aspects of Lubricant Oxidation." ASTM STP 916, W.H. Stadtmiller and A~N. Smith, Eds., American Society for Testing and Materials, Philadelphia (1986) 27. (19)D.W. Murray, C.T. Clarke, G.A. MacAlpine and P.G. Wright. "The Effect of Base Stock Comtx~sition on Lubricant Performance." SAE SP 526, "Base Oils for Automotive Lubricants." SAE Warrendale, PA (1982) (20) S.Y. Shen and E.E. Klaus. "A Kinetic Study of Oil Oxidation in Concentrated Contacts." ASLE Trans. Vol. 27, (1984) 45. (21) J.K. Kochi., ed. "Free Radicals." Vol. 1, Wiley, New York (1973) (22) M.R Bach. "Pyrolysis of Hydrocarbons," NBS SP 3 5 7 , "The Mechanisms of Pyrolysis, Oxidation and Bunting of Organic Molecules." (1972) 13. (23)P.M~ Cann and H . A . Spikes. "Foune "r Transform Infrared Study of the Behaviour of Grease in Lubricated Contacts." Lubrication Engineering Vol. 48, (1991) (24) J.P. Coates and L.C. Setti. "Infrared Spectroscopy as a Tool for Monitoring Oil Degradation." Aspects of Lubricant Degradation, ASTM STP 916, W.H. Stadtmiller and A.N. Smith, Eds., American Society for Testing and Materials, Philadelphia (1986) 57. (25) A. Izcue and S . A . Kra~. "Infrared Spectroscopy in the Development and Manufacture of Lubricating Greases." NLGI Spokesman Special Issue, August (1988) 165. (26) J.H. Marino, J.C. Root and K.L. Thomas. "Infrared Spectrophotometric Analysis of Additives Used in Lubricating Grease." NLGI Spokesman, July (1972) 120.
(27) T.M. Verdura. "Infrared Spectra of Lubricating Grease Base Oils and Thickeners - Part I." NLGI Spokesman, Oct (1971) 235. (28) T.M. Verdura. "Infrared Spectra of Lubricating Grease Base Oils and Thickel~rs - Part II." NLGI Spokesman, November (197 I) 268. (29) D.L Wooton and D.W. Hughes. "At~lication of R~fl~'tanc~ Infrared Spectroscopy to the Lubrication Industry." Lubrication Engineering Vol. 43, (1987) 736. (30) P.M. Calm and H.A. Spikes. "In Lubro Studies of Lubricants in EHD Contacts Using FTIR Absorption Spectroscopy." Tribology Trans. Vol. 34, (1991) 248. (31) P.M. Cann and H.A. Spikes. "Film Thickness Measurements of Lubricating Greases Under Normally Starved Conditions." NLGI Spokesman Vol. 56 (1992) 21. (32)G.J Johnston, 1LW.Wayte and H.A. Spikes, "'The Measurement and Study of Very Thin Lubricant Films in Concentrated Contacts", Trib. Trans., Vol. 34, (I 991), ! 87. (33) S.M. Hsu. Lubrication Engineering Vol. 37, (1981) 722. (34) R.D. Dwyer-Joyce, "The Effect of Lubricant Contamination on Rolling Bearing Performance", Phl) thesis London University, (1994). (35) J. Vcprek-Siska. OxicL Commun. Vol.8, (1985) 301. (36) A.K. Vijh. Wear Vol. 104 (1985) 151. (37)E.E. Klaus, J.L. Duda and J.C. Wang. Tribology Trans. Vol. 35, (316) 1992. (38) M. Lazar, J. Rychly, V. Klimo, P. Pelican and L. Valko. "Free Radicals in Chemistry and Biology." CRC Press, (1989) (39) E.T. Denisov and I.V. Khudyakor. Chem. Rev. (1987) 1313. (40) G. Guantengo P.M. Cann and H.A. Spikes, "A Study of Parched Lubrication", Wear, Vol 153, (1992) 91.
75
Thermal Degradation of Greases and the Effect on Lubrication Performance S. Hurley, P.M. Cann and H.A Spikes Tribology Section, Department of Mechanical Engineering, Imperial College of Science, Technology & Medicine, London SW7 2BX, England.
The lubricating life of a grease in a rolling element bearing is reduced by operation at high temperatures and this can reaflt in premature failure of the bearing. A grease experiences severe conditions in an operating beating where the combination of high tempcraaLres and sustained mechanical working result in gross physical and chemical changes to the grease. These changes have a significant effect on the ability of the grease to replenish the contact and maintain a lubricating film, particularly under starved inlet conditions. Extended operation at high temperatures promotes evaporation of low molecular weight base oil components (1) and oxidation of one or both of the grease components (2)(3). The presence of small amounts of transition metals and their oxides can accelerate these processes and such material is commonly found as wear ( ~ r i s in bearings (4). It is difficult to disentangle these effects and this paper concentrates on the effeOs of thermal ageing on the lubricating ability of the grease. Simple thermal ageing tests have been carried out on two lithium hydroxystearate greases and the resulting changes in their chemistry characteriscd by infrared spectroscopy. The lubricating performance of the aged greases has been assessed by measuring film thiclmess and oil release in a rolling contact under starved conditions. Results from infrared analysis have shown that the oxidation process is accelerated at a temperature of 120° C forming carboxylic acids and related species. The film thickness results showed that the aged greases give a lower equilibmtm film thickness and this correlates with reduced oil release.
1. GREASE AGEING AND LUBRICATION PERFORMANCE Extended operation at high temperatures will limit the lubricating life of a grease in a rolling element bearing. At present we know little of the changes that occur in a grease as it is progressively worked and thermally stagssed, nor of the relationship between these changes and the failure of the grease to maintain a lubricating film. There are many papers (2) - (10) where the changes in grease chemistry during ageing have been studied, but little work where these changes have been explicitly related to the lubricating ability of the grease. In this paper the chemical changes that occur during thermal ageing of a grease have been characterised by IR ~ r o s c o p y . In addition the lubricating performance of the aged greases has been assessed by film thickness and oil release measurements.
It might be possible to use such an approach to develop a screening test to monitor grease condition in operating bearings. It is also not possible, at present, to predict 'lubricant life' from bulk grease properties. The grease should be considered as an integral component of the bearing and 'lubricant life' incorporated into any bearing life model. Such an approach is already being developed (l 1)(12), but we need to know more about the fundamental mechanisms of failure in such systems. A grease experiences both chemical and physical changes as it is worked at high ten~ratures in a bearing. These changes are not due solely to oxidation of the greases but are a combination of effects due to conditions experienced within the b e a ~ g . These include thermal and mechanical stress and the presence of debris and moisture.
76
The p h ~ c a l effects include deterioration of thickener ~ , increased oil setm~tion and viscosity changes. For example the fibre network in a lithium hydroxystearate grease is broken down during working (13) and through thermal degradation. At low temperatures it is the physical changes which control grease life (12). Evaporation of the mineral base oil (14), especially the volatile, shorter-chain paraffins, will increase the viscosity and possibly affect contact replenishment. Perhaps the most influential factor on the service life of a grease is oxidation (12). For a given chemical structure and additive system, temperature has the greatest effect on the rate and mechanism. It is generally acknowledged that within a temperature range of 40-100°C, a rise of 10°C effectively doubles the rate of oxidation. Service life of a grease can be raised from I00 hours to 5000 hours if the working temperature is lowered from 100 to 50°C and can be sustained at ten~ratures 30-50°C above this if oxidation inhibitors are added (11)(15). Physical working also increases the amount of air present in the lubricant by as much as 3-5%. (I6) and this will promote oxidation. Debris which contains particular transition elements and their oxides will also catalyse oxidation. (9)(17)(18) The oxidation of paraffinic hydrocafoons is generally believed to occur via a free-radical chain reaction mechanism due to hydro-peroxide involvement. (17)-(22) It is believed to involve several series of chain b ~ n g and chain lengthening mechanisms. This chain reaction is both time and temperature dependent (35)-(39) and is observed to be accelerated by the presence of certain metals. (2X4)(17)(18)(33)(34) The initiation stage involves the homolytic cleavage of a hydrocarbon bond. This forms a highly reactive free radical species. This reacts with oxygen to form unstable species which decompose, creating peroxides and regenerating the radical. These peroxides in turn react to form products susceptible to oxidation. In this way, a complex autocatalytic process is established; the final products and concentrations of which are ,~miable. At the same time, thermal degradation of the grease components will occur and this is also considered a free-radical mechanism proceeding via a carbon-~u~n chain scission. Both of these reaction processes are likely to influence each other, possibly in competition, and produce a range of
organic s p i e s ; carboxylic acids and related ca~nyl co~ds being the main products. Both mechanisms are affected by the presence of small amounts of transition metal debris. From the reaction products it is difficult to distinguish between oxidation and thermal degradation processes. Grease life in a bearing operating at high temperatures is assessed in the ASTM D-3527 test which is run at 150°C until failure occurs. This approach is widely used to study grease failure in bearings and similar tests are reported in the research literature. In many cases, grease samples are removed from the ~ g and characterised by standard analytical techniques. There is evidence of viscosity increase (9) or decrease (4) both of which are thought to contnlmte to failure. Increased acidity (2)(4), colour changes (9) and IR ~ o s c o p i c analysis (2X9) have also been used to denote changes in grease chemistry. IR analysis of grease samples from bearing tests has shown qualitatively an increase in acidic species (corresponding to an increase in TAN) (4)(9), loss of anti-oxidant additives (9), and changes in lithium hydroxystearate concentration (2)(9)(23-30). Generally lubricant, and thus bearing failure, is linked to changes in bulk grease viscosity. Soflemng of the grease can result in leakage from the bearing (2). A significantly increased viscosity means that the grease fails to flow into the contact and thus the lubricating film is reduccd~ Increased frictional heating might thus result, which is coupled with r e ~ a ~ removal of heat (14). Bearing failure is signalled by a rapid increase in nmning temperature or torque level (12). It is only possible to draw very general conclusions from the bearing tests and to show qualitatively oxidation effects. It is not as yet possible to rink these changes directly to loss of lubricating performance. There are very many factors influencing degradation and it is necessary to be able to study each individually. This paper focuses on the effect of thermal ageing on the chemistry and lubrication performance of a grease. When considering the effect of grease degradation on lubric~on performance, it is necessary to ~ in mind the mechanisms of film formation in a bearing. Most bearings run under sta._rved or ~ l y starved inlet conditions. In this regime the bulk grease does not flow spontaneously
77
into the inlet and the lubricating film is maintained by local replenishment from the grease reservoir at the side of the tracE, It is changes in the ability of the grease ability to resupply the contact in this regime that are important in determining failure. The grease properties that control reflow around the contact will include oil bleeding or release (12)(13), base oil viscosity and polarity. All these things will influence reflow and hence lubricating performance, A brief review of the research literature has been presented, although few definite conclusions can be drawn. There has been no definite identification of the critical lubrication mechanism that ceases to function effectively and thus induces grease and bearing failure. The aims of the current programme are therefore to: (i) characterise the changes that occur in thermally stressed greases (ii) relate these changes to lubricating performance
2. RESEARCH PROGRAMME The research programme was developed around a series of simple ageing tests and uses infrared analysis to monitor grease chemistry; thus establishing the progress of oxidation. When a grease is run in a bearing it clears away from the rolling elements. There will thus be a distribution of grease within the bearing with most of it on the cage or in the housing. Close to the rolled track there will be a much thinner grease film. The ageing tests were designed to simulate both situations of bulk and thin film thermal degradation. Lubrication performance is assessed by film thickness decay and reflow tests under starved conditions and by examination of shear stability. Infrared transmission ~ r o s c o p y is used to characterise the chemical changes that occur in the grease during ageing. It provides information on the nature and concentration of oxidised species and it has been used to analyse samples both in simple laboratory tests and from bearing tests (23)-(28). It will therefore be possible to relate the results from this work to those from bearing tests. The ageing tests subject the grease to high temperatures for extended times, The greases will suffer both oxidation and thermal degradation reactions. It is
difficult to distinguish between the products of the two processes. The term thermal ageing is used to cover both decomposition processes. 3. Experimental 3.1 Ageing Tests The greases were aged as layers spread onto aluminium plates (layer thickness: 1.5mm). Samples were aged in a dry oven at 120°C operating for up to 1250 hours. Several samples were prepared at the start of the test and removed at intervals. Some of the samples contained metal and metal oxide panicles, similar to b e i n g wear debris (4)(33)(34).
3.2 Infrared Analysis All infrared analysis was carried out using a Perkin-Elmer 1740 Infrared Fourier Transform Spectrometer connected to a Venturis FX 5166 PC. All measurements were carried out at room temperature. The aged greases were sampled in transmission mode as thin films spread onto KBr discs. As the sample thickness was not constant, all spectra were normalised with r e ~ to the CH2 peak at 1460cm1. 3.3 Lubrication Performance Film thickness decay measurements were made on fresh and oxidised greases. These were earned out using a PCS Instruments EHL Ultra Thin Film Measurement System described in previous publications (31)(32). The apparatus consisted of a steel ball (diameter 19mm) loaded against a silicacoated glass disc. The disc and ball are driven by separate electric motors. This arrangement simulates contact conditions of a rolling element ball bearing. Film thickness in the centre of the contact was measured by the Thin Film Technique. Before use, the steel ball and disc were ultrasonic~ly cleaned in a toluene bath, washed with acetone and polished with a clean tissue. For these experiments, the disc was rotated at 0. l ms t and 20N load was applied to the ball. At the start of the test, I ml grease was injected as an even layer around the rolling track. The motor was then switched on and the load applied. Film thickness measurements were taken periodically for 60 minutes, or until the film had dropped to below 5 nm.
78
Lubricant reflow properties of fresh and aged greases were measured in the same apparatus. In this case an optical microscope and a video camera were used to view the contact and a chromium-coated glass disc was employed. Approximately 0.5ml grease was injected into the rolling track using a syringe. An image of the contact was recorded using the video camera. The contact was starved by manual rotation of the glass disc about 25 ° forward and backward 15 times. The motion was then stopped and the lubricant allowed to flow back around the contact and into the track. The tests were carried out several times and video recorded. The image was analysed and the distance that the lubricant meniscus travelled with time was measured (Figure I).
Test Debris Iron trioxide - cx-Fe203: grain size 751am Iron tetra oxide- Fe~O4. grain size 751,tm Brass - 67% Cu, 33% Zn" grain size 53-63p,m Copper - Cu" grain size 38-45pm.
4. RESULTS
4.1 Ageing of Grease Samples During the ageing treatment, the appearance of the grease samples changed. The colour of the pure samples gradually deepened from a gold to a dull red/brown with a 'dry' appearance. Fe203 and Fe304 caused a deeper eolour change, but the greases retained their wet, shiny appearance. Brass produced a much deeper and redder colour change with no evidence of drying. Copper effected a similar colour change to brass, but greatly dried the samples.
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All the spectra are shown normalised to the CH2 absorbance at 1460 cm "~. The spectra obtained of the fresh grease were those of a typical lithium hydroxystearate soap thickener in paraffmic mineral oil. The major absorbances are listed below in Table
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. '~'"
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The absorbances at 1580 and 1560cm "~ are due to the soap thickener. Peaks at 1460, 1377 and 720cm "~are primarily due to the base oil. The acidic products absorb in the range 1740-1685cm "~. Table I IR Peak listing of grease samples .
Figure 1 contact
Lubricant reflow around a stationary
3.4 Test Materials Test samples were chosen from a series of simple lithium hydroxystearate greases with: 9% and 15% w/w thickener. These greases had the same base oil (viscosity @ 40°C 30eSt) and contained no anti-oxidants or other additives. In some tests, metal powder was included to simulate the effect of wear debris. Each was added to the grease at a concentration of 0.1% by mass by stirring manually. Care was taken to ensure that all the samples experienced a similar degree of mixing and were not excessively worked.
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The oxidised sample produces a similar peak pattern, but with different absorbance ratios. A broad absorbance exists between ! 740-1685crn "l due to the presence of a range of oxidation products (aldehydes, ketones, carboxylic acids). In addition,
79
small peaks a ~ r ageing.
at 1410 and 1302cm~ after
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cm l. The 15% and 9% thicker film samples, analysed by transmittance gave lower ratios. The carboxylate soap absorl~nce at 1580cm ~, was observed to increase with time for all samples. This is not a real effect, but the result of base oil evaporation. Furthermore, there is a reduction in the rate of soap increase with time. This suggests that oxidation of soap may occur after an initial 'induction period.' The presence of debris, especially brass, significantly affects the infrared spectra of the aged grease (Figure 4 ).
!
Figure 2 Infrared transmittance spectrum of fresh (shaded) and oxidised (clear) 9% grease.
Figure 3 shows how the transmittance spectra change with increasing oxidation time. A significant increase in the broad band absorbance at 1700 - 1740 cm l is observed.
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Figure 3 Infrared transmittance spectrum of 9% grease with increasing oxidation time (0, 112, 400 hours). It was possible to monitor the progress of oxidation by plotting the change in ratio of peak heights with time. The acid absorbance was normalised against the base oil absorbance at 1460
as ageing proceeds. The CTI3/CH2 ratio increases significamly; indicating chain-breaking reactions. The most obvious change exhibiteA is the massive reduction in the carboxylate absorbanc¢ with time and which, at 66.25 hours, ceases to exist. A corresponding increase in the acid a b s o ~ c ¢ gives a linear relationship between the acid appearance and carboxylate disappearance. This suggests that the brass not only accelerates the oxidation process, but modifies the dominant reaction mechanism. This results in a total loss of carboxylate a b s o ~ and presumably thickener structure. The appearance of the brass-contaminated sample was also different: it a ~ e d to be very soil again suggesting an effective viscosity loss rather than the increase associated with conventional ageing, The results for the other debris materials were not so extreme.
80
and Lubrication Performance Film thickness decay tests showed that with the fresh greases the 9% maintained a higher film thickness than the 15% at the end of the test (Figure 5). The 15% grease initially gives a thicker film but this breaks down to give a lower equilibrium film thickness at the end of the test. The lower thickener content facilitates oil release and hence contact replenishment. The equilibrium film thickness reflects therefore the ability of the grease to supply the contact with lubricant. 4.2 Oil Release
1000
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Figure 6 Film decay with time for flesh and aged (1250 hours) 15% greases.
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After oxidation for 1250 hours, both greases gave significantly lower equilibrium film thickness under starved conditions than their fresh counterparts. This is shown in Figures 6 and 7 for ..................the 15% ~ d 9 % ~ ~ s ~ s ~ v e l y : .....................................................................
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81 The films also decayed at a faster rate. Decay is this region is due to breakdown of the deposited layer which releases base oil. The faster breakdown indicates that the stability of the thickener structure is reduced by ageing, rendering it less resistant to the shear stress within the contact region. This lower equilibrium film thickness is due to poorer replenishment of the contact as this is determined by the amount of oil available for reflow (40) and the viscosity or surface spreading properties of the oil. The faster film decay suggests that sufficient oil release has occurred both from inside the contact and in the grease reservoir. The alternative explanation is therefore that the oil viscosity has increased sufficiently to inhibit reflow or that the base oil spreading properties have been adversely affected by ageing. It is also possible to quantify the reflow by measuring the time for the oil meniscus to reform around the contact. Reflow test results are plotted below in Figure 8. Reflow distance (meniscus radius/Hertzian radius) is plotted against elapsed time after rolling has stopped. Results are shown for 9% and 15% greases both fresh (solid line), and after 1250 hours ageing (dotted line). They show that the fresh 9% grease rapidly reforms a memscus around the contact and that ageing significantly reduces this ability. Similar effects are seen with the 15% grease. Reflow has almost stopped for the 15% aged grease and the lubricant flow barely manages to reform around the contact even after 160 seconds. One interesting observation from these experiments was the way in which reflow changed during the test. The test procedure was as follows: the disc was rotated backwards and forwards 15 times, movement was then mopped and reflow measured. This was repeated 5 times with each grease sample and the results below were taken on the 4 th stoppage. It was noted that reflow for the fresh grease was fairly repeatable for each stoppage. The aged greases, however, gave very different reflow rates and these increased dramatically with successive stoppages. The difference between fresh and aged grease reflow was therefore more marked in the early stages of the test.
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-,! ............ L :i ....... ,~~........ 15% fresh
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t ..........
15% aged !
"
i
~
-i . . . . . . . . . . . . . . . . . . . . . . .
;
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,
20
40
I
......
60
~ 1 ~"' "
~
°
"
HenzJan radius " ..
i ,
i .'.
80 I0() 120 140 160
Reflow time (sees)
Figure 8 Reflow distance (meniscus r a d i ~ r t z i a n radius) against time for flesh and aged (9% & 15%) grease.
5. DISCUSSION AND CONCLUSIONS This paper has attempted to characterise thermal ageing of a grease using IR spectral analysis and to relate this to changes in lubricating ability. The results have shown that, as expected, the following chemical changes occur in a thermally aged grease (i) increased acidic species through oxidation of the base oil and, to a limited extent, thickener (ii) chain scission and evaporation of low molecular weight ~ i e s (iii) the presence of metal debris accelerates production of acidic slx~es and can change the dominant reaction pathway The presence of brass changes the grease degradation pathway in the most dramatic fashion. In this case both the oil and the thickener are degraded and complete loss of the c a ~ x y l a t e peak was observed after only 66 hours heating, The soap appears to be attacked preferentially in the early stages. Carboxylate loss suggests that the thickener
82
structure has broken down completely. The grease sample at this stage a ~ d to have a very loose consistency. If this occurred in a beating then failure might be due to bulk grease churning within the bearing; which would lead to excessive heat generation and thermal runaway. In tests where such debris is not present then the conventional degradation path is followed, Evaporation and oxidation of base oil species occurs resulting in increased viscosity, both of the bulk grease and the base oil. The increased viscosity of the bulk grease will diminish its heat removal capabilities. The increased viscosity of the base oil will impair its ability to reflow around the contact and replenish the track. The film thickness will thus drop and the system will accelerate towards failure. The conclusions are summarised as follows: (i) Ageing tests: oxidation initially of the base oil and then thickener. Presence of debris accelerates the process and there are significant changes in the oxidation route depending on debris. (ii)Aged greases give lower equilibrium film thickness in starved tests, suggesting that oil release and reflow has deteriorated with ageing. (iii) Lubricant released from aged greases has reduced ability to reflow around the contact.
ACKNOWLEDGMENTS The authors would like to thank the EPSRC for financial support of this project.
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83
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