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 machine–space 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.
Highlights
Despite considerable improvements in recent years, there is still great concern to further reduce the fuel consumption of internal combustion engines in passenger vehicles
The zinc dialkyldithiophosphate (ZDDP) tribofilms formed on steel balls after rubbing against Al–Si alloy discs and steel discs were compared. (Some results were obtained in previous research [21].) Figures 3 and 4 show interference images on the MTM steel balls at 5 and 20 N, respectively, while Fig. 5 shows mean film thicknesses at the middle of the worn area on the balls which were calculated from these interference images
This indicates that ZDDP tribofilms are formed and build up progressively on the rubbed tracks on the steel balls during rubbing tribofilm formation speeds are different
Summary
Despite considerable improvements in recent years, there is still great concern to further reduce the fuel consumption of internal combustion engines in passenger vehicles. The compression ratio and the specific power of engines have been progressively increased [1, 2] resulting in higher cylinder liner temperatures near the combustion chamber, especially at top dead centre This produces conditions that are more likely to produce wear and scuffing of piston ring–cylinder liner contacts [3]. Non-ferrous materials have been introduced in both engines and transmissions to reduce vehicle weight and improve fuel economy In some cases, these materials are used as one of the rubbing components in tribological contacts, for example in piston rings and in cylinder liners, and in the latter case, hypereutectic aluminium–silicon alloys (Al–Si alloys), which provide reasonable antiwear performance, have been increasingly adopted [4,5,6]. Engine oil formulations are required to show good antiwear and antiseizure performance under these more severe conditions with Al–Si alloys as well as with ferrous materials
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