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Effect of mechanical factors on tribological properties of palm oil methyl ester blended lubricant
Wear 239 Ž2000. 117–125 www.elsevier.comrlocaterwear
Effect of mechanical factors on tribological properties of palm oil methyl ester blended lubricant M.A. Maleque a,) , H.H. Masjuki a , A.S.M.A. Haseeb b a
Department of Mechanical and Materials Engineering, UniÕersity of Malaya, 50603 Kuala Lumpur, Malaysia b Department of Materials and Metallurgical Engineering, BUET, Dhaka 1000, Bangladesh Received 22 October 1999; received in revised form 4 January 2000; accepted 4 January 2000
Abstract The effects of mechanical factors viz. applied load and temperature on the tribological performance of 5% palm oil methyl ester ŽPOME. blended lubricant were studied using a steel–cast iron pair. Wear and frictional measurements were made using a stationary steel ball and a reciprocating cast iron plate in a modified universal wear and friction testing machine. The test conditions were contact pressure, 400 MPa; mean contact velocity, 0.34 mrs; reciprocating stroke, 80 mm; loads, 100–1100 N Žfixed temperature.; and temperature, 40–1408C Žfixed load.. Wear scar surfaces were investigated using scanning electron microscopy ŽSEM. to understand the wear mechanisms involved. Analysis of post bench test lubricating oils was performed using an ISL viscometer and TANrTBN analyzers to investigate the lubricating oil degradation properties. Results showed that at lower loads Žup to 500 N. and temperatures Žup to 1008C., the wear rates under 5% POME lubricant are lower, whereas at higher loads and temperatures, the wear rates are higher. The friction behavior of POME as an additive in commercial lubricant indicates the prevalence of the boundary lubrication regime. The viscosity test results showed that 5% POME can improve the viscosity index ŽVI. properties of mineral-based lubricant up to 500 N load. However, in this investigation, corrosive wear and pits on the damaged surface are the dominant wear mode at higher temperature. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Specific wear rate; Friction coefficient; Corrosive wear; Palm oil methyl ester; Viscosity
1. Introduction The tribological properties of mating components of engines and machines, where relative motion is involved, generally depend on factors such as load, temperature, speed, sliding time, base oil and additive formulation. As the operating conditions of the engines and machines become more severe, more trouble occurs on the contact surfaces due to damage caused by wear, seizure, fretting, pitting, etc. Wear is sometimes affected by corrosive environments under constant or varying contact load, resulting in failure of the components due to severe wear or seizure w1x. A lubricant performs a variety of functions in engines and machines, such as, it protects metal surfaces against corrosion, it acts as heat transfer agent, it flushes out contaminants and it absorbs shocks and it seals foam w2x. High performance engines and machines demand lubricat)
Corresponding author. Tel.: q60-3-7595263; fax: q60-3-7595317. E-mail address: [email protected] ŽM.A. Maleque..
ing oils, which contain tailored additive packages. Additives increase useful life and provide additional performance characteristics to the lubricant, such as improved flow, modified friction, and resistance to oxidation, extended pressure or temperature stability. The most common types of additive are detergents and dispersants, antiwear agents, anti-foams, emulsifiers, extreme pressure agents, pour point improvers, viscosity index ŽVI. improvers, corrosion inhibitors and friction modifiers w3x. Palm oil methyl ester ŽPOME. is a vegetable based oil. The vegetable oil is converted to alkyl ester by transesterification of the oil with an alcohol in the presence of a catalyst. The POME is mainly composed of triglycerides, glyceride, free fatty acids and non-glyceride substances. Triglycerides are esters of glycerol and fatty acids. Details of chemical compositions and properties of POME can be found elsewhere w4,5x. It is reported that a fatty acid of POME composition can provide effective boundary lubrication due to the presence of a polar structure. This polar structure would form an effective boundary layer. Hence,
0043-1648r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 0 0 . 0 0 3 1 9 - 7
M.A. Maleque et al.r Wear 339 (2000) 117–125
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it yields the potential of POME as an anti-wear additive for lubrication. There has been a growing awareness for the use of vegetable oils, which have an excellent biodegradability and also improved chemical stability by anti-wear additive formulation w6–8x. Different anti-wear additives are used for the reduction of wear and friction in tribological systems. Barwell and Kwon w9x have studied the behavior of such additives. They found that under severe conditions, thermal activation energy triggers the release of free hydrogen by exothermic decomposition of the hydrocarbons, which contributes to thermal instability at the frictional contact junctions, which in turn, results in film rupture and failure. Research has been done on a tricresylphosphate ŽTCP. additive to detect the tribochemical reactions with a metal surface w10,11x. It was found that additives react with the surface to form surface coatings, which are responsible for the anti-wear properties. Recently, many research work have been done and published on different lubricants w12–14x, anti-wear additives w9,15–18x and friction modifiers w19x. However, none of them have used POME as anti-wear agent or base lubricating oil. Two of the present authors performed four-ball wear tests, using different volume percentages of POME, viz., 0%, 3%, 5%, 7% and 10% with mineral oil-based lubricant w5x. The results indicated that 5% POME acts as an additive and improves the anti-wear characteristics. Therefore, in this paper, results, obtained with a 5% POME blended lubricant are presented and discussed. This paper will discuss the effects of mechanical factors, especially the role of applied load and temperature on the behavior of 5% POME as an additive in a mineral oil-based commercial lubricant on the wear and friction performance of steel–cast iron pair. Lubricant degradation parameters will also be presented. 2. Experimental details 2.1. Test apparatus The tribological behavior of a POME blended lubricant was examined in a modified universal wear and friction
Fig. 2. Block diagram of the experimental set-up.
testing apparatus, using a ball-on-plate configuration as shown in Fig. 1. The block diagram of the experimental set-up is given in Fig. 2. The balls were made of AISI 52100 bearing steel of 10 mm diameter Žhardness of about 920–940 Hv10 ., and the plate was made of gray cast iron. The chemical composition of the gray cast iron can be found elsewhere w20x. The as-received cast iron plate with dimensions of 115 = 20 = 30 mm was heat treated by austenitizing at 8308C for 1 h at a heating rate of 58Crmin followed by oil quenching. This yielded a hardness of 920 Hv10 . The plate was machined to a finish of 0.08 mm R a . The ball and plate assembly was submerged in a small bath which acted as the source of heat Žsee Fig. 1.. On-line measurements were made in the course of each test, including oil bath temperature, instantaneous friction force and the average wear from the sliding pair. All readings were logged by a microcomputer. 2.2. Test method The test conditions were as follows: load, 100–1100 N; temperature, 40–1408C; mean contact velocity, 0.34 mrs; frequency, 4.2 Hz; stroke length, 80 mm; and test duration, 1 h. Upon completion of each test, specimens were cleaned ultrasonically with ethyl alcohol and stored in a dessicator for subsequent scanning electron microscopy ŽSEM.. All results reported are the average of duplicate tests. 2.3. Lubricants Studies of wear and friction behavior were performed with a 5% POME blended lubricant. A commercial mineral oil-based lubricant ŽSAE20W-50. was pre-mixed with 5% Žby volume. POME to investigate the tribological behavior of POME. Mixing was effected using a stirrer. 2.4. Viscosity and total acid number (TAN) test
Fig. 1. Schematic illustration of ball-on-plate test machine.
The lubricant degradation characteristics were measured using an ISL viscometer and TANrTBN analyzers. The kinematic viscosity was measured at 408C as well as
M.A. Maleque et al.r Wear 339 (2000) 117–125
1008C, following the ASTM D2270 standard. For the TAN analysis, ASTM D664-81 test method was used with cŽKOH. s 0.2 molrl in isopropanol as the titrant.
3. Results and discussion 3.1. Effects of load and temperature on wear rate and friction coefficient Wear measurements were carried out using a linearly variable differential transformer ŽLVDT.. Specific wear rates ŽSWR., calculated from the LVDT measurements, are shown in Fig. 3 as a function of the sliding distance for
119
various applied loads and test temperatures. It is seen in Fig. 3a that the SWR increases with an increase in the applied load. Although the SWR increases fairly rapidly as the load is increased from 500 to 700 N, there is a prominent sudden jump in wear rate when the load is increased from 900 to 1100 N. At 1100 N, the SWR tends to increase drastically at a sliding distance of about 1000 m, leading to seizure. These results thus suggest that the 5% POME blended lubricant can be effective below 1100 N. The effect of temperature on the wear behavior under the 5% POME blended lubricant is shown in Fig. 3b. The SWR is seen to increase as the test temperature increases. The increase in the SWR is gradual at lower temperatures of up to about 1008C. Beyond this temperature, the rise in SWR with temperature is rather rapid.
Fig. 3. Specific wear rate vs. sliding distance Žm. for ball–plate configuration under 5% POME blended lubricant; speed, 0.34 mrs, Ža. varying load Žfixed temperature, 1208C., Žb. varying temperature Žfixed load, 100 N..
120
M.A. Maleque et al.r Wear 339 (2000) 117–125
Fig. 4a and b, respectively, shows the friction coefficient vs. sliding distance curves at different loads and temperatures. Fig. 4a reveals that the friction coefficient increases in a gradual manner as the load is increased up to 900 N. An increase in load from 900 to 1100 N results in a rapid rise in the friction coefficient. This corresponds fairly well with the trend in the variation of SWR with load as given in Fig. 3a. The friction coefficient is seen to increase
as the temperature increases from 408C to 1408C ŽFig. 4b.. In general, the rise in the friction coefficient is less rapid in the lower temperature range. It is to be noted that an analogous trend was also observed in the variation of SWR with temperature ŽFig. 3b.. In the recent past, a number of studies have been devoted to the effect of percentage of POME in the base lubricant on the wear and frictional behavior of different
Fig. 4. Friction coefficient vs. sliding distance Žm. for ball–plate configuration under 5% POME blended lubricant; speed, 0.34 mrs, Ža. varying loads Žfixed temperature, 1208C., Žb. varying temperatures Žfixed load, 100 N..
M.A. Maleque et al.r Wear 339 (2000) 117–125
ferrous alloy pairs w4,5x. It has been found that the wear and friction coefficient of the systems studied attain minimum values at a POME content of 5%. This has been
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attributed to the presence of aliphatic fatty acid of the general formula C n H 2 nq1COOH, such as stearic acid in POME w4x. Each molecule of such acid consists of a long
Fig. 5. SEM micrographs of worn surfaces of ball specimens under 5% POME blended lubricant at different applied loads and fixed temperature Ž1208C.: Ža. 100 N, Žb. 300 N, Žc. 500 N, Žd. 700 N, Že. 900 N and Žf. 1100 N.
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M.A. Maleque et al.r Wear 339 (2000) 117–125
covalently bonded hydrocarbon chain and these long chains align themselves normal to the surface, acting as an effective barrier to metal-to-metal contact. In general, esters are
considered to show better wear and scuffing protection behavior than hydrocarbon-based lubricants. Konishi et al. w16x explained that esters have a high affinity towards a
Fig. 6. SEM micrographs of worn surfaces of ball specimens under 5% POME blended lubricant at different temperatures and fixed load Ž100 N.: Ža. 408C, Žb. 608C, Žc. 808C, Žd. 1008C, Že. 1208C and Žf. 1408C.
M.A. Maleque et al.r Wear 339 (2000) 117–125
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3.2. Worn surface characteristics
Fig. 7. Viscosity test results of 5% POME blended lubricant after 1 h running in wear machine. Ža. Varying loads, Žb. varying temperatures.
metal surface, owing to their polar functional groups and, thus, form a protective layer on the surface. In the present study, it has been observed that the protection offered by POME decreases with applied normal load, and that at 1100 N, the wear rate and the friction coefficient increase drastically. It has further been observed that POME is effective at temperatures up to about 80–1008C. Above this temperature, its effectiveness decreases rather rapidly. This is ascribed to the fact that the layer of lubricant, present in between the rubbing surfaces, becomes thinner, resulting in the breakdown of the film. Desorption and degradation of POME at higher temperature can also contribute to the loss of its effectiveness in keeping the wear rate and friction coefficient low. Both higher loads and temperatures cause more metal-to-metal contact through the destruction of the protective film with a consequent increase in wear and friction. At loads up to 900 N and at a temperature up to 1208C, the friction coefficient ranged typically from 0.09 to 0.25, which indicated boundary lubrication. Again, this can be attributed to the fact that the POME contains fatty acid, which provides effective boundary lubrication resulting from the formation of a surface film from polar molecules.
The appearance of the surface damage in the wear scar on the ball specimens is shown in Figs. 5 and 6. Surface examination of the worn samples was done by SEM. Figs. 5 and 6 show that mild abrasion, pitting corrosion, and severe delamination of the specimen surface increased significantly when run at higher load and higher temperature. Individual specimen displayed the above wear mechanisms in different forms and intensities. On the worn surface of the ball specimens, tested at different temperatures ŽFig. 6., pits, erosion, and corrosion were observed. The major wear mechanisms were abrasive wear, pitting, polishing andror smoothing, rough erosion and corrosion. Corrosive wear occurs in situations where the environment surrounding a sliding surface interacts chemically with it. In this case, the POME additive reacted with the metal surface at higher temperature and the reaction products were worn off from the surface ŽFig. 6e and f., leading to higher wear and friction. This can be attributed to the fact that at higher temperature, the TAN value is increased, indicating higher susceptibility of the contact surface to corrosion andror oxidation. It can be concluded that, at higher temperature, the corrosive wear and pit formation are the dominant wear modes in this investigation. The chemical reactivity of additives plays an important role in wear protection properties. Interaction between additive and metal surface consists, in principle, of two competing effects. By reacting with a metal surface, antiwear additives reduce adhesive wear, but also produce chemical wear. Furthermore, not all surface reaction films are effective at reducing friction and wear w21x.
Fig. 8. Showing VI parameter: Ža. VI as a function of load and Žb. VI as a function of temperature.
M.A. Maleque et al.r Wear 339 (2000) 117–125
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Table 1 Effect of temperature on TAN value of used lubricant Žafter simulation test.. The TAN value of unused lubricant is 1.34 mg KOHrg Test sample Žused lubricant.
Temperature Ž8C.
TAN Žmg KOHrg.
TAN increase
1 2 3 4 5 6
40 60 80 100 120 140
1.50 1.59 3.01 3.16 4.10 4.69
0.16 0.25 1.67 1.81 2.76 3.35
3.3. Lubricating oil degradation Viscosity is an important property of a lubricant, as it affects the film thickness and, thus, the wear rate of sliding surfaces. It is used for the identification of individual grades of oil and for monitoring the changes occurring in oil while in service. Viscosity increase usually indicates that a used oil has deteriorated by oxidation or by contamination, while a decrease usually indicates dilution by a lower viscosity oil or by fuel. Fig. 7 shows the viscosity test results of 5% POME blended lubricant after 1 h running at various loads and temperatures. From Fig 7a, it can be seen that in general, the viscosity increased with increasing loads. This is due to the oxidation process during the test resulting into sludge or insoluble product formation, having an increased length of molecular chain and hence, resulting in increased viscosity of the used lubricating oil w22x. From the viscosity test, using an ISL viscometer at 408C and 1008C, the VI was determined. The VI compares the rate of change of viscosity of the sample with the rate of change of two types of lubricant having the highest and lowest viscosity indices at the same time. An oil with a low VI changes greatly in viscosity with change in temperature; an oil with high VI changes relatively small in viscosity for the same temperature range w23x. The VI of the used oil after 1 h simulation test is plotted in Fig. 8. A higher VI was found at lower loads Žup to 500 N.. This is because of the lower rate of change of viscosity with temperature in the presence of the POME at lower loads. Therefore, it can be concluded that 5% POME in commercial lubricant acts as a viscosity improver up to 500 N load. Fig. 8b shows that the VI decreases with increasing temperature up to 808C. Above this temperature, the VI slightly increases and becomes almost constant. This is attributed to the oxidation process during the wear and friction test at higher temperature. The total acid number ŽTAN. is a measure for the total amount of both weak and strong organic acids present in the lubricant and is expressed in mg KOHrg, i.e., the number of milligrams of potassium hydroxide required to neutralize one gram of lubricating oil. The TAN test result is shown in Table 1. A higher change in TAN value with
the wear simulation was obtained at a higher temperature. The higher change Žin terms of increase. in TAN is possibly caused by the depletion of some additives w24x. Another explanation is that at higher temperature, the fatty acid molecules or other organic acids are decomposed during operation and hence, increase the TAN value.
4. Conclusions Ž1. At lower loads and temperatures, the wear rates under 5% POME lubricant are lower, whereas at higher loads and temperatures, the wear rates are higher. Ž2. The friction behavior of POME as additive in mineral-based lubricant indicates the prevalence of the boundary lubrication regime. However, at higher temperature, the friction coefficient is higher while at lower temperature, the friction coefficient is lower. Ž3. The viscosity slightly increases with increasing load but decreases with an increase in temperature. A 5% POME in mineral-based oil increases the VI up to 500 N load and decreases up to 808C temperature. Ž4. At a higher temperature, an increased TAN value is obtained. Ž5. The typical wear modes at different loads are mild abrasive, pitting, and severe delamination. Ž6. Corrosive wear and pit formation are the dominant wear modes at higher temperature.
Acknowledgements The authors would like to acknowledge the financial support of The Ministry of Science, Technology and The Environment, Malaysia, under vote IRPA No. 03-02-030329.
References w1x H. Goto, Influence of mechanical and chemical factors on transition between severe and mild wear in saline solution, J. Tribol. 119 Ž1997. 619–625. w2x B.A. Khoorramian, G.R. Iyer, S. Kodali, P. Natarajan, R. Tupil, Review of antiwear additives for crankcase oils, Wear 169 Ž1993. 87–95. w3x L.A. Toms, Machinery Oil Analysis — Methods, Automation and Benefits, A Guide for Maintenance Managers and Supervisors, in: 1995, pp. 50–54, Pensacola, FL. w4x H.H. Masjuki, M.A. Maleque, The effect of palm oil diesel fuel contaminated lubricant on sliding wear of cast irons against mild steel, Wear 198 Ž1996. 293–299. w5x H.H. Masjuki, M.A. Maleque, Investigation of the anti-wear characteristics of palm oil methyl ester using a four-ball tribometer test, Wear 206 Ž1997. 179–186. w6x A. Arnsek, J. Vizintin, Scuffing load capacity of rapeseed-based oils, Lubr. Eng. 55 Ž1999. 11–18.
M.A. Maleque et al.r Wear 339 (2000) 117–125 w7x U.S. Choi, B.G. Ahn, O.K. Kwon, Tribological behavior of some antiwear additives in vegetable oils, Tribol. Int. 30 Ž1997. 677–683. w8x S. Gunsel, F.E. Lockwood, The influence of Copper-containing additives on oil oxidation and corrosion, Tribol. Trans. 38 Ž1995. 485–496. w9x F.T. Barwell, O.K. Kwon, Interaction of chemical, thermal and mechanical factors in the lubrication of machine elements, in: Proc. of Plenary Session of 3rd Int. Tribology Congress, 1 September, 1981, pp. 40–50. w10x J.M. Perez, C.S. Ku, P. Pei, B.E. Hegemann, S.M. Hsu, Characterization of tricresylphosphate lubricating films by micro-Fourier transform infrared spectroscopy, Tribol. Trans. 33 Ž1990. 131. w11x A. Gauthier, H. Montes, J.M. Georges, Boundary lubrication with tricresylphosphate ŽTCP. important of corrosive, ASLE Trans. 25 Ž1982. 445. w12x J.J. Liu, Y. Chen, Y.Q. Cheng, The generation of wear debris of different morphology in the running-in process of iron and steels, Wear 154 Ž1992. 259–267. w13x S. Asadauskas, J.M. Perez, J.L. Duda, Lubrication properties of castor oil-potential base stock for biodegradable lubricants, Lubr. Eng. 53 Ž1997. 35–40. w14x R.L. Goyan, R.E. Melley, P.A. Wissner, W.C. Ong, Biodegradable lubricants, Lubr. Eng. 54 Ž1998. 10–17. w15x J. Zhang, W. Liu, Q. Xue, The friction and wear behaviors of some O-containing organic compounds as additives in liquid paraffin, Wear 219 Ž1998. 124–127.
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w16x T. Konishi, E.E. Klaus, J.L. Duda, Wear characteristics of aluminum–silicon alloy under lubricated sliding conditions, Tribol. Trans. 39 Ž1996. 811–818. w17x Y. Wan, Q. Xue, Friction and wear characteristics of P-containing antiwear and extreme pressure additives in the sliding of steel against aluminum alloy, Wear 188 Ž1995. 27. w18x Y. Wan, Q. Xue, Effects of antiwear and extreme pressure additives on the wear of aluminum alloy in the sliding of steel against aluminum alloy, Tribol. Int. 28 Ž1995. 553. w19x F.A. Davis, T.S. Eyre, The effect of a friction modifier on piston ring and cylinder bore friction and wear, Tribol. Int. 23 Ž1990. 163–171. w20x H.H. Hassan, M.A. Maleque, S. Kumar, Lubricated wear and frictional behaviour study of cast iron–cast iron pair, in: Proc. Int. Conf. On Structure, Processing and Properties of Materials, SPPM ’97, 15–17 November, 1997, pp. D57–D66. w21x Y. Wan, L. Cao, Q. Xuet, Friction and wear characteristics of ZDDP in the sliding of steel against aluminium alloy, Tribol. Int. 30 Ž1997. 767–772. w22x H.H. Zuidema, The Performance of Lubricating Oil, American Chemical Society, Reinhold Publishing, 115. w23x BS 4231, Viscosity Classification for Industrial Liquid Lubricants, British Standards Institution, London, 1975. w24x W.S. Moon, Y. Kimura, Wear-preventing properties of used gasoline engine oils, Wear 139 Ž1990. 351–365.
Effect of mechanical factors on tribological properties of palm oil methyl ester blended lubricant M.A. Maleque a,) , H.H. Masjuki a , A.S.M.A. Haseeb b a
Department of Mechanical and Materials Engineering, UniÕersity of Malaya, 50603 Kuala Lumpur, Malaysia b Department of Materials and Metallurgical Engineering, BUET, Dhaka 1000, Bangladesh Received 22 October 1999; received in revised form 4 January 2000; accepted 4 January 2000
Abstract The effects of mechanical factors viz. applied load and temperature on the tribological performance of 5% palm oil methyl ester ŽPOME. blended lubricant were studied using a steel–cast iron pair. Wear and frictional measurements were made using a stationary steel ball and a reciprocating cast iron plate in a modified universal wear and friction testing machine. The test conditions were contact pressure, 400 MPa; mean contact velocity, 0.34 mrs; reciprocating stroke, 80 mm; loads, 100–1100 N Žfixed temperature.; and temperature, 40–1408C Žfixed load.. Wear scar surfaces were investigated using scanning electron microscopy ŽSEM. to understand the wear mechanisms involved. Analysis of post bench test lubricating oils was performed using an ISL viscometer and TANrTBN analyzers to investigate the lubricating oil degradation properties. Results showed that at lower loads Žup to 500 N. and temperatures Žup to 1008C., the wear rates under 5% POME lubricant are lower, whereas at higher loads and temperatures, the wear rates are higher. The friction behavior of POME as an additive in commercial lubricant indicates the prevalence of the boundary lubrication regime. The viscosity test results showed that 5% POME can improve the viscosity index ŽVI. properties of mineral-based lubricant up to 500 N load. However, in this investigation, corrosive wear and pits on the damaged surface are the dominant wear mode at higher temperature. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Specific wear rate; Friction coefficient; Corrosive wear; Palm oil methyl ester; Viscosity
1. Introduction The tribological properties of mating components of engines and machines, where relative motion is involved, generally depend on factors such as load, temperature, speed, sliding time, base oil and additive formulation. As the operating conditions of the engines and machines become more severe, more trouble occurs on the contact surfaces due to damage caused by wear, seizure, fretting, pitting, etc. Wear is sometimes affected by corrosive environments under constant or varying contact load, resulting in failure of the components due to severe wear or seizure w1x. A lubricant performs a variety of functions in engines and machines, such as, it protects metal surfaces against corrosion, it acts as heat transfer agent, it flushes out contaminants and it absorbs shocks and it seals foam w2x. High performance engines and machines demand lubricat)
Corresponding author. Tel.: q60-3-7595263; fax: q60-3-7595317. E-mail address: [email protected] ŽM.A. Maleque..
ing oils, which contain tailored additive packages. Additives increase useful life and provide additional performance characteristics to the lubricant, such as improved flow, modified friction, and resistance to oxidation, extended pressure or temperature stability. The most common types of additive are detergents and dispersants, antiwear agents, anti-foams, emulsifiers, extreme pressure agents, pour point improvers, viscosity index ŽVI. improvers, corrosion inhibitors and friction modifiers w3x. Palm oil methyl ester ŽPOME. is a vegetable based oil. The vegetable oil is converted to alkyl ester by transesterification of the oil with an alcohol in the presence of a catalyst. The POME is mainly composed of triglycerides, glyceride, free fatty acids and non-glyceride substances. Triglycerides are esters of glycerol and fatty acids. Details of chemical compositions and properties of POME can be found elsewhere w4,5x. It is reported that a fatty acid of POME composition can provide effective boundary lubrication due to the presence of a polar structure. This polar structure would form an effective boundary layer. Hence,
0043-1648r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 0 0 . 0 0 3 1 9 - 7
M.A. Maleque et al.r Wear 339 (2000) 117–125
118
it yields the potential of POME as an anti-wear additive for lubrication. There has been a growing awareness for the use of vegetable oils, which have an excellent biodegradability and also improved chemical stability by anti-wear additive formulation w6–8x. Different anti-wear additives are used for the reduction of wear and friction in tribological systems. Barwell and Kwon w9x have studied the behavior of such additives. They found that under severe conditions, thermal activation energy triggers the release of free hydrogen by exothermic decomposition of the hydrocarbons, which contributes to thermal instability at the frictional contact junctions, which in turn, results in film rupture and failure. Research has been done on a tricresylphosphate ŽTCP. additive to detect the tribochemical reactions with a metal surface w10,11x. It was found that additives react with the surface to form surface coatings, which are responsible for the anti-wear properties. Recently, many research work have been done and published on different lubricants w12–14x, anti-wear additives w9,15–18x and friction modifiers w19x. However, none of them have used POME as anti-wear agent or base lubricating oil. Two of the present authors performed four-ball wear tests, using different volume percentages of POME, viz., 0%, 3%, 5%, 7% and 10% with mineral oil-based lubricant w5x. The results indicated that 5% POME acts as an additive and improves the anti-wear characteristics. Therefore, in this paper, results, obtained with a 5% POME blended lubricant are presented and discussed. This paper will discuss the effects of mechanical factors, especially the role of applied load and temperature on the behavior of 5% POME as an additive in a mineral oil-based commercial lubricant on the wear and friction performance of steel–cast iron pair. Lubricant degradation parameters will also be presented. 2. Experimental details 2.1. Test apparatus The tribological behavior of a POME blended lubricant was examined in a modified universal wear and friction
Fig. 2. Block diagram of the experimental set-up.
testing apparatus, using a ball-on-plate configuration as shown in Fig. 1. The block diagram of the experimental set-up is given in Fig. 2. The balls were made of AISI 52100 bearing steel of 10 mm diameter Žhardness of about 920–940 Hv10 ., and the plate was made of gray cast iron. The chemical composition of the gray cast iron can be found elsewhere w20x. The as-received cast iron plate with dimensions of 115 = 20 = 30 mm was heat treated by austenitizing at 8308C for 1 h at a heating rate of 58Crmin followed by oil quenching. This yielded a hardness of 920 Hv10 . The plate was machined to a finish of 0.08 mm R a . The ball and plate assembly was submerged in a small bath which acted as the source of heat Žsee Fig. 1.. On-line measurements were made in the course of each test, including oil bath temperature, instantaneous friction force and the average wear from the sliding pair. All readings were logged by a microcomputer. 2.2. Test method The test conditions were as follows: load, 100–1100 N; temperature, 40–1408C; mean contact velocity, 0.34 mrs; frequency, 4.2 Hz; stroke length, 80 mm; and test duration, 1 h. Upon completion of each test, specimens were cleaned ultrasonically with ethyl alcohol and stored in a dessicator for subsequent scanning electron microscopy ŽSEM.. All results reported are the average of duplicate tests. 2.3. Lubricants Studies of wear and friction behavior were performed with a 5% POME blended lubricant. A commercial mineral oil-based lubricant ŽSAE20W-50. was pre-mixed with 5% Žby volume. POME to investigate the tribological behavior of POME. Mixing was effected using a stirrer. 2.4. Viscosity and total acid number (TAN) test
Fig. 1. Schematic illustration of ball-on-plate test machine.
The lubricant degradation characteristics were measured using an ISL viscometer and TANrTBN analyzers. The kinematic viscosity was measured at 408C as well as
M.A. Maleque et al.r Wear 339 (2000) 117–125
1008C, following the ASTM D2270 standard. For the TAN analysis, ASTM D664-81 test method was used with cŽKOH. s 0.2 molrl in isopropanol as the titrant.
3. Results and discussion 3.1. Effects of load and temperature on wear rate and friction coefficient Wear measurements were carried out using a linearly variable differential transformer ŽLVDT.. Specific wear rates ŽSWR., calculated from the LVDT measurements, are shown in Fig. 3 as a function of the sliding distance for
119
various applied loads and test temperatures. It is seen in Fig. 3a that the SWR increases with an increase in the applied load. Although the SWR increases fairly rapidly as the load is increased from 500 to 700 N, there is a prominent sudden jump in wear rate when the load is increased from 900 to 1100 N. At 1100 N, the SWR tends to increase drastically at a sliding distance of about 1000 m, leading to seizure. These results thus suggest that the 5% POME blended lubricant can be effective below 1100 N. The effect of temperature on the wear behavior under the 5% POME blended lubricant is shown in Fig. 3b. The SWR is seen to increase as the test temperature increases. The increase in the SWR is gradual at lower temperatures of up to about 1008C. Beyond this temperature, the rise in SWR with temperature is rather rapid.
Fig. 3. Specific wear rate vs. sliding distance Žm. for ball–plate configuration under 5% POME blended lubricant; speed, 0.34 mrs, Ža. varying load Žfixed temperature, 1208C., Žb. varying temperature Žfixed load, 100 N..
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M.A. Maleque et al.r Wear 339 (2000) 117–125
Fig. 4a and b, respectively, shows the friction coefficient vs. sliding distance curves at different loads and temperatures. Fig. 4a reveals that the friction coefficient increases in a gradual manner as the load is increased up to 900 N. An increase in load from 900 to 1100 N results in a rapid rise in the friction coefficient. This corresponds fairly well with the trend in the variation of SWR with load as given in Fig. 3a. The friction coefficient is seen to increase
as the temperature increases from 408C to 1408C ŽFig. 4b.. In general, the rise in the friction coefficient is less rapid in the lower temperature range. It is to be noted that an analogous trend was also observed in the variation of SWR with temperature ŽFig. 3b.. In the recent past, a number of studies have been devoted to the effect of percentage of POME in the base lubricant on the wear and frictional behavior of different
Fig. 4. Friction coefficient vs. sliding distance Žm. for ball–plate configuration under 5% POME blended lubricant; speed, 0.34 mrs, Ža. varying loads Žfixed temperature, 1208C., Žb. varying temperatures Žfixed load, 100 N..
M.A. Maleque et al.r Wear 339 (2000) 117–125
ferrous alloy pairs w4,5x. It has been found that the wear and friction coefficient of the systems studied attain minimum values at a POME content of 5%. This has been
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attributed to the presence of aliphatic fatty acid of the general formula C n H 2 nq1COOH, such as stearic acid in POME w4x. Each molecule of such acid consists of a long
Fig. 5. SEM micrographs of worn surfaces of ball specimens under 5% POME blended lubricant at different applied loads and fixed temperature Ž1208C.: Ža. 100 N, Žb. 300 N, Žc. 500 N, Žd. 700 N, Že. 900 N and Žf. 1100 N.
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covalently bonded hydrocarbon chain and these long chains align themselves normal to the surface, acting as an effective barrier to metal-to-metal contact. In general, esters are
considered to show better wear and scuffing protection behavior than hydrocarbon-based lubricants. Konishi et al. w16x explained that esters have a high affinity towards a
Fig. 6. SEM micrographs of worn surfaces of ball specimens under 5% POME blended lubricant at different temperatures and fixed load Ž100 N.: Ža. 408C, Žb. 608C, Žc. 808C, Žd. 1008C, Že. 1208C and Žf. 1408C.
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3.2. Worn surface characteristics
Fig. 7. Viscosity test results of 5% POME blended lubricant after 1 h running in wear machine. Ža. Varying loads, Žb. varying temperatures.
metal surface, owing to their polar functional groups and, thus, form a protective layer on the surface. In the present study, it has been observed that the protection offered by POME decreases with applied normal load, and that at 1100 N, the wear rate and the friction coefficient increase drastically. It has further been observed that POME is effective at temperatures up to about 80–1008C. Above this temperature, its effectiveness decreases rather rapidly. This is ascribed to the fact that the layer of lubricant, present in between the rubbing surfaces, becomes thinner, resulting in the breakdown of the film. Desorption and degradation of POME at higher temperature can also contribute to the loss of its effectiveness in keeping the wear rate and friction coefficient low. Both higher loads and temperatures cause more metal-to-metal contact through the destruction of the protective film with a consequent increase in wear and friction. At loads up to 900 N and at a temperature up to 1208C, the friction coefficient ranged typically from 0.09 to 0.25, which indicated boundary lubrication. Again, this can be attributed to the fact that the POME contains fatty acid, which provides effective boundary lubrication resulting from the formation of a surface film from polar molecules.
The appearance of the surface damage in the wear scar on the ball specimens is shown in Figs. 5 and 6. Surface examination of the worn samples was done by SEM. Figs. 5 and 6 show that mild abrasion, pitting corrosion, and severe delamination of the specimen surface increased significantly when run at higher load and higher temperature. Individual specimen displayed the above wear mechanisms in different forms and intensities. On the worn surface of the ball specimens, tested at different temperatures ŽFig. 6., pits, erosion, and corrosion were observed. The major wear mechanisms were abrasive wear, pitting, polishing andror smoothing, rough erosion and corrosion. Corrosive wear occurs in situations where the environment surrounding a sliding surface interacts chemically with it. In this case, the POME additive reacted with the metal surface at higher temperature and the reaction products were worn off from the surface ŽFig. 6e and f., leading to higher wear and friction. This can be attributed to the fact that at higher temperature, the TAN value is increased, indicating higher susceptibility of the contact surface to corrosion andror oxidation. It can be concluded that, at higher temperature, the corrosive wear and pit formation are the dominant wear modes in this investigation. The chemical reactivity of additives plays an important role in wear protection properties. Interaction between additive and metal surface consists, in principle, of two competing effects. By reacting with a metal surface, antiwear additives reduce adhesive wear, but also produce chemical wear. Furthermore, not all surface reaction films are effective at reducing friction and wear w21x.
Fig. 8. Showing VI parameter: Ža. VI as a function of load and Žb. VI as a function of temperature.
M.A. Maleque et al.r Wear 339 (2000) 117–125
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Table 1 Effect of temperature on TAN value of used lubricant Žafter simulation test.. The TAN value of unused lubricant is 1.34 mg KOHrg Test sample Žused lubricant.
Temperature Ž8C.
TAN Žmg KOHrg.
TAN increase
1 2 3 4 5 6
40 60 80 100 120 140
1.50 1.59 3.01 3.16 4.10 4.69
0.16 0.25 1.67 1.81 2.76 3.35
3.3. Lubricating oil degradation Viscosity is an important property of a lubricant, as it affects the film thickness and, thus, the wear rate of sliding surfaces. It is used for the identification of individual grades of oil and for monitoring the changes occurring in oil while in service. Viscosity increase usually indicates that a used oil has deteriorated by oxidation or by contamination, while a decrease usually indicates dilution by a lower viscosity oil or by fuel. Fig. 7 shows the viscosity test results of 5% POME blended lubricant after 1 h running at various loads and temperatures. From Fig 7a, it can be seen that in general, the viscosity increased with increasing loads. This is due to the oxidation process during the test resulting into sludge or insoluble product formation, having an increased length of molecular chain and hence, resulting in increased viscosity of the used lubricating oil w22x. From the viscosity test, using an ISL viscometer at 408C and 1008C, the VI was determined. The VI compares the rate of change of viscosity of the sample with the rate of change of two types of lubricant having the highest and lowest viscosity indices at the same time. An oil with a low VI changes greatly in viscosity with change in temperature; an oil with high VI changes relatively small in viscosity for the same temperature range w23x. The VI of the used oil after 1 h simulation test is plotted in Fig. 8. A higher VI was found at lower loads Žup to 500 N.. This is because of the lower rate of change of viscosity with temperature in the presence of the POME at lower loads. Therefore, it can be concluded that 5% POME in commercial lubricant acts as a viscosity improver up to 500 N load. Fig. 8b shows that the VI decreases with increasing temperature up to 808C. Above this temperature, the VI slightly increases and becomes almost constant. This is attributed to the oxidation process during the wear and friction test at higher temperature. The total acid number ŽTAN. is a measure for the total amount of both weak and strong organic acids present in the lubricant and is expressed in mg KOHrg, i.e., the number of milligrams of potassium hydroxide required to neutralize one gram of lubricating oil. The TAN test result is shown in Table 1. A higher change in TAN value with
the wear simulation was obtained at a higher temperature. The higher change Žin terms of increase. in TAN is possibly caused by the depletion of some additives w24x. Another explanation is that at higher temperature, the fatty acid molecules or other organic acids are decomposed during operation and hence, increase the TAN value.
4. Conclusions Ž1. At lower loads and temperatures, the wear rates under 5% POME lubricant are lower, whereas at higher loads and temperatures, the wear rates are higher. Ž2. The friction behavior of POME as additive in mineral-based lubricant indicates the prevalence of the boundary lubrication regime. However, at higher temperature, the friction coefficient is higher while at lower temperature, the friction coefficient is lower. Ž3. The viscosity slightly increases with increasing load but decreases with an increase in temperature. A 5% POME in mineral-based oil increases the VI up to 500 N load and decreases up to 808C temperature. Ž4. At a higher temperature, an increased TAN value is obtained. Ž5. The typical wear modes at different loads are mild abrasive, pitting, and severe delamination. Ž6. Corrosive wear and pit formation are the dominant wear modes at higher temperature.
Acknowledgements The authors would like to acknowledge the financial support of The Ministry of Science, Technology and The Environment, Malaysia, under vote IRPA No. 03-02-030329.
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