The thickness, friction and wear of lubricant films

Transcription

1 The thickness, friction and wear of lubricant films Hugh Spikes SAE Powertrain & Fluid Systems Conference & Exhibition, San Antonio, TX October 25 th 2005

2 Why care about the thickness of lubricant films? Historically we simply measure the friction and wear properties of lubricants by rubbing bits of metal together But what underlies and controls friction and wear is the protective film formed by the lubricant We can understand a lot more about friction and wear by studying this protective film And a good way to start is to measure its thickness

3 Layout of talk The Stribeck curve Measuring film thickness & friction Boundary films - base oil blends Antiwear films ZDDP New way to measure wear

5 Stribeck (or Hersey or GümbelG mbel) curve Journal Bearings Friction coefficient Thurston found that friction versus speed has a minimum (1879) Stribeck measured variation of friction coefficient with speed (1902) Gümbel & Hersey plotted variation of friction coefficient with (uη/w) (both in 1914) Uη/W

6 Origin of Origin of Stribeck Stribeck curve curve log(uηw) Friction coefficient BL mixed hydrodynamic log(film thickness) h α (Uη/W) n μ α (Uη/W) 1-n n s n s W U k u U A W U k W u = = 1 η μ η η μ W A h u W A W F A F s η μ τ μ τ = = = =

7 Non-conforming contacts Film thickness and friction are decoupled. Film thickness determined by oil viscosity in the inlet Friction determined by highly viscous, non-newtonian oil in the high pressure contact h = k (μu) 0.7 α 0.5 W -0.1 μ EHL = 0.02 to 0.12

8 Stribeck curve in non-conforming contact (e.g. gears, cams) log(film thickness) h α (Uη) 0.7 Friction coefficient EHD friction/traction BL mixed Elastohydrodynamic (EHD) log(uη)

9 How might lubricants influence friction in concentrated contacts? By controlling the EHL friction of the base fluid Friction coefficient BL EHL log(uη) By controlling the thickness of EHL film (via viscosity and α-value) Friction coefficient BL EHL log(uη)

10 How might lubricants influence friction in concentrated contacts? By forming a thin, solid-like film with low (or high) shear strength BL EHL By forming a viscous film on surfaces BL EHL By changing the roughness of the surfaces BL EHL

12 Measuring film thickness, 1885 h max = 73.5 μm ± 2.5 μm Goodman, Proc Inst. Civ. Eng., lxxxix, , part III, pp 1-19

13 Measuring film thickness ultrathin film interferometry Spectrometer Video camera Microscope Coated glass disc Steel ball ( N φ) λ=2 nοιlhoil + 2nspacerhspacer

14 Ultrathin film interferometry Film Thickness (nm) C 80 C 120 C Entrainment Speed, U (m/s) SN150 Group I mineral oil h = k( Uη) 0.7 α 0.5

15 Ultrathin film interferometry Film thickness (nm) % stearic acid, wet hexadecane Entrainment speed (m/s) Boundary films shown by enhanced film thickness at low speeds

16 Measuring friction, 1884 Tower B, Proc. I. Mech. E., 1884, pp

17 Measuring friction - MTM Test lubricant Steel disc Steel ball Mixed rolling-sliding contact

18 MTM friction curve 0.12 Friction coefficient C 80 C 120 C Entrainment speed, U (m/s) SN150 Group I mineral oil

19 MTM Stribeck curves 0.12 boundary friction Friction coefficient C 80 C 120 C Film thickness (nm) SN150 Group I mineral oil EHL friction

21 Layout of talk The Stribeck curve Measuring film thickness & friction Boundary films - base oil blends Antiwear films ZDDP Measuring wear

22 Viscous boundary films Modern, low polarity hydrocarbon base fluids generally require addition of a significant proportion of higher polarity base fluid (e.g. ester) to help additive solubility and deposit formation control. The polar base fluid molecules should be preferentially attracted to polar, metal surfaces Can we make use of this?

23 Surface fractionation Film thickness (nm) % Ester blend PAO (low η 13 cp) Ester (high η, 72 cp ) Entrainment speed (m/s) Guangteng et al., Wear (1996)

24 Why? The more polar molecules concentrate close to the solid surfaces because of solid/liquid attractive forces If the more polar fluid is more viscous, a viscous surface layer is formed

25 Surface fractionation 100 Film thickness (nm) 10 10% Ester blend PAO (high η 314 cp) Ester (low η, 9 cp ) Entrainment speed (m/s)

26 Effect of surface fractionation % Ester blend EHL friction coefficient PAO (low vis) Ester (high vis) Entrainment speed (m/s) Guangteng et al., Trib. Trans. (1997)

27 Effect of surface fractionation 0.16 SN100 Friction coefficient ESTH+SN100 ESTH 100 C Entrainment speed (m/s)

29 ZDDP antiwear films mins 30mins Film thickness (nm) mins 60mins % wt. P, 80 C Rubbing time (hrs) 90mins 120mins ZDDPs form quite thick (100 nm) solid-like films on rubbing surfaces Fujita et al., Trib. Trans. (2005 )

30 Increased friction due to ZDDP film Friction coefficient C start of test 10 minutes minutes 3 hours Entrainment speed (m/s) These films result in increased friction. Not in the boundary but in the mixed lubrication regime

31 How? Forms a high friction surface film? or Inhibits formation of an EHD film? Friction coefficient Entrainment speed (m/s)

32 Does the ZDDP film inhibit formation of an EHL film? (a) roll/slide steel disc on steel ball to form ZDDP reaction film (50% SRR, 0.1 m/s) (b) replace steel disc by spacer-layer one and measure film thickness /speed behaviour in pure rolling using optical interferometry Taylor et al., Trib. Trans. (2003)

33 Does the ZDDP film inhibit formation of an EHL film? 300 Film thickness (nm) Solid ZDDP film EHD theory Entrainment speed m/s

34 Why? Possibilities Formation of low viscosity liquid layer next to ZDDP film Formation of low α-value layer next to ZDDP film Starvation due to loose ZDDP film material blocking the inlet Slip of lubricant at the ZDDP film surface Increase in surface roughness due to the ZDDP film μm

35 Conclusions so far We can gain insight into lubrication mechanisms by coupling measurement of both film thickness and friction taken over a range of entrainment speeds Enables us to identify the operating regime of lubrication Enables us to explore impact of lubricant rheology on film formation Enables us to see how additives influence friction Next stage is to combine this with wear measurements

37 Measuring wear 1803 Hatchett, C. Phil. Trans. R. Soc. Lond. Part 1, , (1803)

38 Limitations of existing wear tests most current mild wear tests are based on pure sliding, nonconforming contact with high sliding speeds Wear is measured from scar dimensions at end of test Because of high sliding speed, test often operates in mixed-lubrication. Thus sensitive to viscosity Contact area and pressure change markedly during test as wear occurs on the stationary surface wsd (μm) test time (mins) pressure (GPa)

39 Mixed sliding-rolling One obvious solution is to rotate both surfaces, to give mixed sliding-rolling This distributes wear around contact so there is much less change in local geometry and thus contact pressure But it requires a different method of measuring wear (no large, clearly defined, localised wear scar)

40 Sliding speed vs entrainment speed However mixed sliding-rolling also provides a very important opportunity Wear depends on sliding distance, so to obtain lots of wear in a reasonable time you need a high sliding speed, u s But a high sliding speed normally means a high entrainment speed and thus a high EHL film thickness, and so less wear Sliding speed u s = u 1 u 2 Wear rate u s Entrainment speed U = u 1 u EHD film thickness U 0.7

41 Contra-rotation rotation In pure sliding, where u 2 = 0 u s = u = 1 u2 u1 u u U = + = u 1 But is mixed sliding-rolling we can have any u s and U combination we want If we move the surfaces in opposite direction (contra-rotation) we can have high u s with low U

42 For example - speed 1 = 0.5 m/s, speed 2 = 0 m/s Pure sliding u s = 0.5 m/s U = 0.25 m/s speed 1 = 0.35 m/s, speed 2 = m/s Counter-rotation u s = 0.5 m/s U = 0.05 m/s

43 To measure wear ICP-AES (Atomic emission spectroscopy with inductively coupled plasma) Monitors wear (remove samples of oil periodically) ppb resolution Must be no metal from other wear positions (or package) Needs ICP equipment Not on line

44 Test protocol Set up MTM with test fluid and specimens Reach test temperature without load applied Withdraw two 0.5 ml samples of test oil for ICP Rub surfaces in contra-rotating sliding-rolling with entrainment speed = 0.05 m/s (boundary lubrication) and sliding speed = 0.25 m/s. Load = 30 N. Temperature normally 80ºC. Periodically halt rubbing, bubble in N 2 gas and withdraw two samples of oil for ICP (and take Stribeck curve and interference image if needed) End test after 2-4 hours and take two more oil samples Analyse oil samples and carry out profilometry and imaging on wear tracks

45 Wear results base oil Fe conc (ppm) Base oil, 60 C Base oil, 80 C Base oil, 80 C (repeat) Base oil, 100 C λ λ Time (hrs.) Wear rate increases with temperature (lower viscosity, thinner film) Good repeatability

46 Wear results ZDDP solutions Fe conc (ppm) base oil 0.005% P 0.01% P 0.02% P 0.03% P 0.05% P Time (hrs.) Very low ZDDP concentrations give higher wear than base oil alone For high ZDDP concs, almost no wear (unless other additives present)

47 Wear results ZDDP solutions k 1 (m 3 /J)* k 1 (m 3 /J)* [P] wt% [P] wt.% Q = k 1 u s Wt

48 Conclusions We now have experimental techniques to measure the film thickness and friction properties of lubricants over the range of entrainment speed on a routine basis. These enables us to relate EHL and boundary lubricating properties to lubricant composition and optimise these properties We are currently developing tools for measuring wear rate under similar, controlled conditions

49 Acknowledgements Lots of hard-working PhD students and research assistants, especially: Tina Fan Hiroshi Fujita Gao Guangteng Lisa Taylor