Combined lubricant–surface system perspective: Multi-scale numerical–experimental investigation

Abstract

Frictional losses are one of the main causes of reduced energy efficiency in all machines and mechanisms. In particular, there is mounting pressure upon manufacturers of all forms of vehicle to comply with increasingly stringent legislation and directives with regard to harmful emissions. Therefore, reduction of friction has become an imperative issue. The traditional approach of dealing with surface material and lubricant formulation in isolation has been replaced by a lubricant-surface system approach. This paper presents multi-scale experimentation from nano/meso-scale lateral force microscopy of ultra-thin surface adsorbed films through to micro-scale precision sliding tribometry to investigate lubricant-surface friction optimisation within the mixed regime of lubrication, using lubricants with different organic and inorganic friction modifying species. These affect the parameters of the system, commonly used as input to models for mixed and boundary regimes of lubrication. Therefore, the precise measurement of these parameters at different physical scales is important. The study also makes use of detailed numerical predictions at micro-scale through combined solution of the average Reynolds equation as well as interaction of wetted asperities in mixed and boundary regimes of lubrication. Good agreement is found between the predictions and measurements at micro-scale tribometric interactions. Furthermore, the same trends are observed in testing across the physical scales.

Multiscale boundary frictional performance of diamond like carbon coatings

“…In conjunction with lubricant additive pack interactions within the system, an optimised lubricant additive package would allow for durable low friction, low wear operation. This leads to the lubricant-surface system approach in modern tribological perspective [14,15], an approach also undertaken in the current paper.…”

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

“…AFM has been used extensively for tribological investigations as it allows for highly sensitive measurements of surface forces at a scale similar to those of a single asperity [50][51][52][53][54][55][56]. The pressure coefficient of boundary shear strength of a surface, relates the boundary shear stress at the AFM tipsample conjunction to the applied load.…”

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

“…Post tribometry the samples are analysed for nanoscale boundary friction with LFM, following the detailed approach described by Leighton et al [17]. This is to determine the interfacial pressure coefficient of boundary shear strength between asperity contacts, ζ and the nanoscale boundary friction properties.…”

“…Calibration is performed for each probe using a silicon-carbide calibration specimen with a known coefficient of friction of 0.19. For wet (lubricated) LFM, a tip holder is used to ensure that formed menisci are kept far away from the tip-sample conjunction, thus not affecting the measurements [17].…”

“…The advent of nanoscale mechanical testing using Atomic Force Microscopy (AFM) has provided an impetus in the fundamental understanding of tribo-film formation and mechanical behaviour [15][16][17][18]. Pidduck and Smith [18] showed that Lateral Force Mode (LFM) with AFM can be used to investigate tribo-films formed through tribometric activation.…”