ZDDP tribofilm formation under various rubbing conditions

Abstract

ZDDPs (zinc dialkyldithiophosphates) have been widely used as antiwear additives in engine oil for the last 70 years. Initially, since engine parts were manufactured from cast iron, ZDDPs were required to protect them by forming tribofilms on the surfaces of this material. However in order to save fossil fuel resources by increasing fuel economy, surface treatments such as coatings or texture to reduce friction coefficient and/or replacement of ferrous engine parts (e.g. engine block, piston, cylinder liner etc.) by aluminium alloys for weight reduction of vehicles have been recently employed. ZDDPs are therefore required to show good antiwear performance with aluminium alloys or surface coatings as well as with ferrous materials. However the mechanism of ZDDP tribofilm formation on non-ferrous surfaces is still not well understood. The purpose of this study is to clarify how ZDDP tribofilms form on non-ferrous surfaces. The main technique used in this study was the MTM-SLIM (mini traction machine - spacer layer interferometry) ball on disc tester. This machine can monitor ZDDP tribofilm formation during rubbing tests. Chemical and physical information of the rubbed area was also obtained using surface analyses. Firstly, as the fundamental study, the influence of slide roll ratio on ZDDP tribofilm formation on steel was investigated. In mixed sliding-rolling conditions, the tribofilms form on the rubbed areas while practically no tribofilm forms in pure-sliding conditions. However, in reciprocating conditions, tribofilm can be formed under pure sliding conditions. In addition, this formation process is quite different from that previously reported. In contacts using steel balls against aluminium silicon alloy discs, it was observed that the tribofilm on the counter steel surface was removed by etching so that silicon grains protruded from the alloy. Finally, the influence of the surface treatment on tribofilm formation was investigated. The film formation showed different results depending on the type of surface treatment. All of the above are discussed in this thesis.