The Influence of Aluminium–Silicon Alloy on ZDDP Tribofilm Formation on the Counter-Surface

Abstract

In order to reduce vehicle weight and thus improve fuel economy, aluminium (Al) alloys have been increasingly adopted as contact surfaces such as piston skirts and cylinder liners in current engines. In general, hypereutectic Al-Si alloys are used, in which hard silicon grains are embedded in a softer Al-Si single phase matrix. It is reported that after rubbing, the matrix is removed to leave silicon grains protruding from the surface. However, the response of the counter-surface by these silicon grains is rarely investigated. In this study, mini traction machinespace layer imaging (MTM-SLIM) has been used to monitor tribofilm formation in situ and investigate the evolution of both surfaces in the contact of a steel ball on an Al-Si disc lubricated by ZDDP solution. In low-load conditions, the top layer of aluminium on the Al-Si disc is removed physically to leave silicon grains protruding from the surface, while ZDDP tribofilm pads are formed mainly on the grains. On the counter-surface (steel ball), ZDDP tribofilms are formed and build up with no wear scars. In high-load conditions, deep gaps are observed to form initially round the silicon grains on the disc. During rubbing, these become shallower, while the silicon grains start to protrude. On the steel ball, ZDDP tribofilm is generated initially over the whole rubbing track, but then the tribofilm in the middle of the track is almost completely removed by rubbing against the protruding silicon grains. Wear of the underlying steel surface then ensues.

What stress components drive mechanochemistry? A study of ZDDP tribofilm formation

“…4 Under mixed and boundary lubrication conditions, ZDDPs decompose and form a protective surface-bound layer, known as a tribolm, at locations where the surfaces experience sliding contact. The tribolm prevents severe wear of the underlying substrate, particularly for ferrous-based materials, including steel, but also for other materials, such as silicon, 5 metals or metal alloys, [6][7][8][9] ceramics, [10][11][12] and diamond-like carbon (DLC) coatings. 13,14 Many classes of additives form tribolms, however, due to the widespread use of ZDDPs and their crucial importance for wear protection, most studies have focused on ZDDP-derived tribolms.…”

“…According to molecular dynamics simulations by Mosey et al of a simplified phosphorous-based tribofilm precursor molecule, cross-linking of a polyphosphate tribofilm increases strongly with compressive stress once the stress exceeds a threshold value of 5 GPa. 30–33 This may explain why ZDDP-derived tribofilms do not form well on softer Al-based alloys, since their low hardness compared to steel (750–920 HV vs. 170–190 HV 8 ) will prevent contact stresses from reaching the threshold value. Consistent with this idea, Gosvami et al observed, using the AFM technique, that initially no ZDDP-derived tribofilm forms, and substantial plastic deformation of the Al occurs.…”

ZDDP Tribofilm Formation on Non-Ferrous Surfaces

“…In addition, most studies on the behaviour of ZDDP with non-ferrous surfaces have been conducted using a non-ferrous on ferrous tribopair. These studies have reported presence of tribofilms on various nonferrous surfaces including Al-Si alloy, Si 3 N 4 , SiC, ZrO 2 and DLC coatings [10][11][12][13][14][15] and researchers have generally suggested that these tribofilms have been transferred from the ferrous counterface during rubbing. This possible transfer of tribofilm or of ferrous or ferric ions from ferrous surfaces complicates the understanding of reaction mechanism of tribofilm formation on non-ferrous surfaces.…”

Nanotribology of Ionic Liquids as Lubricant Additives for Alumina Surfaces

“…Common hydrocarbon-based oils adsorb weakly to surfaces, so oil-miscible surfactants and polymers are added which adsorb to the surface and remain in place even under high loads. , The effectiveness of surfactant and polymer additives depends on the affinity it has for the surface via Coulombic, van der Waals, H-bonding, and solvophobic interactions. Unfortunately, many common additives which are effective on steel surfaces are not effective on lightweight surfaces such as aluminum and aluminum alloys. , Thus, there is a push toward developing additives to match these surfaces.…”

“…Unfortunately, many common additives which are effective on steel surfaces are not effective on lightweight surfaces such as aluminum and aluminum alloys. 3,4 Thus, there is a push toward developing additives to match these surfaces.…”