Multiscale Friction in Lubricant-Surface Systems for High-Performance Transmissions Under Mild Wear

Abstract

The lubricant-surface system is complex in nature and can significantly affect the frictional performance of high-performance transmission systems. The complexity stems from the coupled mechanical and chemical phenomena that occur at the interfacial tooth conjunctions. A combined analytical and precision experimental approach is presented to analyse the salient parameters of the lubricant-surface system. A multiscale procedure comprising topographical measurement, pin-on-disc tribometry, atomic force microscopy in lateral force mode, X-ray photo-electron spectroscopy and continuum contact mechanics analysis under mixed non-Newtonian thermo-elastohydrodynamics is used to describe the formation of a tribo-film, as well as wear and frictional characteristics of the lubricant-surface system. The contribution of chemisorbed and physisorbed bonded tribo-film on the boundary coefficient of friction is ascertained at different physical scales. Therefore, the paper presents a novel multiscale analysis, promoting improved understanding of the complex interactions between mechanisms of friction, wear and surface chemistry.

Nano and microscale contact characteristics of tribofilms derived from fully formulated engine oil

“…It has been shown that under severe conditions, where physically-adsorbed layers may be removed completely or partially during the sliding process, the results obtained from dry LFM for the pressure coefficient of boundary shear strength may be more deemed to be more appropriate [82]. However, the changes in the frictional behaviour in micro-scale can also be linked to the changes in the surface topography resulting from the removal of the surface-adsorbed layers.…”

Multiscale boundary frictional performance of diamond like carbon coatings

“…A precision in-house manufactured uni-directional pin-on-disc tribometer ( Figure 1) replicates some aspects of high performance transmission gear meshing contacts through representative contact conditions [29]. The applied normal load on the disc sample through the pin is generated by a dual cantilever loading mechanism, resulting in a circular point contact.…”

“…A Ktype thermocouple feedback loop is used to maintain/regulate the surface temperature of the samples. A syringe driver supplies a fixed rate of lubricant feed onto the sample surface with the height of the inlet meniscus controlled by a roller wiper system [29]. Therefore, repeatable conditions are assured for all the reported experiments.…”

“…Typical operating conditions for high performance transmissions are replicated [30,31] using two methods to characterise the tribometer and transmission tribological contacts. Humphrey et al [29] showed that the dimensionless viscous and elastic parameters in the Greenwood chart [32] are: = 10 7 − 10 13 and = 10 5 − 10 11 for a high performance transmission and = 10 8 − 10 9 and = 10 9 − 10 10 for the pin-ondisc tribometric studies. This shows that both contact conditions reside within the piezo-viscous elastic (elastohydrodynamic) regime of lubrication.…”

“…This shows that both contact conditions reside within the piezo-viscous elastic (elastohydrodynamic) regime of lubrication. The Deborah number [33] was also calculated by Humphrey et al [29] to ascertain the prevailing tractive regime that the lubricant is subjected to in the transmission gearing, as well as the replicated conditions in the pin-on-disc tribometry. It was shown that the Deborah number (a measure of lubricant viscoelastic response) was greater than unity, indicating non-Newtonian traction [9].…”