Wear and sulphur chemistry of the tribolayer

In the piston-ring cylinder-liner tribosystem, the friction and wear behaviour is thought to be closely connected to the formation of a tribolayer. Lubricant additives, or their decomposition products, together with the native oxide layer and the metal substrate, form a protective layer. For steel surfaces, Martin et al describe the formation of short and/or long-chain polyphosphates as part of the tribolayers [1]. An important factor influencing the tribolayer formation and composition, is the lubricant used during the tribological testing. Differences in lubricant additivation [2] and condition (i.e. aged or fresh) [3][4] directly affect the tribolayer and subsequently also friction and wear behaviour. A previous investigation of the authors [4] describes the connection between the steady-state wear rates and tribolayer thicknesses for model tests with fresh and aged lubricants. The present paper will go into more detail concerning the composition of the tribolayer. A special focus is laid on the sulphur chemistry in the wear zone, which was also found to be correlated to the steady-state wear rate.