Surface analytical science and automotive lubrication

Abstract

This paper discusses the role surface analytical science has played in understanding the complex phenomena occurring during lubrication of modern internal combustion engines. The interactions between the multi-functional lubricant additives and the various metallic surfaces present in the tribological contacts are complex and lead to the formation of inhomogenous multi-layer protective surface films. Surface analysis by ultra-high vacuum (UHV) techniques such as x-ray photoelectron spectroscopy, Auger electron spectroscopy and secondary ion mass spectrometry has allowed model structures for these films to be derived through which the behaviour of the contact can be understood. Non-UHV probes have elucidated the role played by the soluble, partly reacted or degraded film precursors that occupy the interfacial region between the solid surface and the liquid lubricant, and have allowed the mechanical properties of the films to be investigated on a micro- and nano-scale. The use of surface analytical techniques to study interactions between lubricant species and combustion soot is also discussed.

Mechanisms of antiwear tribofilm growth revealed in situ by single-asperity sliding contacts

“…While ZDDP tribofilms can be formed between other non-ferrous material pairs, e.g., low-friction diamond-like carbon (DLC) films, they are often less durable than those formed when steel or iron is present for reasons not yet understood (9,10). It is desirable to reduce or replace ZDDP as it often increases frictional losses (3), and produces Zn-, Pand S-containing compounds in the exhaust, reducing the catalytic converterÕs efficiency and lifetime (1,3,11,12). Despite decades of research, no suitable substitute for ZDDP has yet been found (12), motivating research to understand the beneficial mechanisms underlying the growth and antiwear properties of ZDDP-derived tribofilms.…”

“…Direct experimental evidence for this model is lacking (3), and it does not explain tribofilm formation on non-ferrous surfaces (6,7). In contrast, Mosey et alÕs first principles atomistic simulations proposed that tribofilm formation results from contact pressure-induced cross-linking of zinc phosphate molecules (8) which are a thermal or catalytic decomposition product of ZDDP (11,15). Overall, there is no consensus on the growth mechanism, and no models conclusively explain either the tribofilm patchiness or why the film thickness is limited.…”

Microcharacterization of heavy‐duty diesel engine piston deposits

“…Their presence in the bulk of the deposit, but not near the deposit/substrate interface, suggests their formation during the normal operating temperature period of the engine tests and is in agreement with a growth mechanism based on thermal degradation and oxidation. 18 The fine structure of the deposit was found to be contained within a matrix consisting of a mix of calcium sulphates, zinc phosphates, organic material and possibly calcium carboxylates. These compounds are likely to have formed from thermal and oxidative degradation of the lubricant film to give a concentration of additive species in a gum-like material consisting of the less-volatile components of the original base oil.…”

“…The temperature of 150°C is sufficient to cause thermal decomposition of the ZDDP molecule, giving rise to both oil-soluble components, such as organic sulphides and organothiophosphates, and oil-insoluble components, such as zinc polyphosphates. 18 It is these oil-insoluble components that are most likely to have contributed to the initial stages of deposit formation. These initial layers were found to be ¾50 nm in total thickness.…”

Improvement of PEEM images from thick inhomogeneous antiwear films using a thin Pt coating

“…The most common, and the best antiwear additive used in engine oils today is zinc dialkyl-dithiophosphates (ZDDPs; figure 1). Extensive reviews of its formation, composition, and performance can be found in references [14][15][16][17][18]. The antiwear film is formed of an agglomeration of patches (termed antiwear pads) that are raised from the surface by up to 400 nm.…”