An investigation of the wear mechanism leading to self-replenishing transfer films

Since the late 1980s, interest has risen in solid powder lubrication due to its proven ability to provide low friction and wear in interfaces unsuitable for traditional oils. This may be in the form of augmenting oil performance as an additive, or in the form of thin, solid transfer films since it was found that sliding materials sometimes inherently generate a film that can protect the contact interface during relative motion. In particular, in situ self- replenishing solid lubrication has shown the ability to maintain lubricious conditions through the continual deposition of thin powder transfer films to “dry” surface asperities from a sacrificial compact. An emerging class of self-lubricating compacts is being developed by compacting lamellar powder particles into different homogeneous and heterogeneous solid lubricant structures or “pellets.” An in situ self-replenishing solid lubricant system may be created by placing these pellets into tribosystems in such a way that their transferred film is continuously applied to dry asperities in load-bearing interfaces. Therefore, special emphasis must be given to understanding the method of transfer film deposition and depletion, if the lubrication process is to be predicted, controlled, and optimized. The purpose of this investigation is to examine the wear mechanisms which govern transfer film deposition and depletion in an in situ self-replenishing system, such that a more accurate modeling approach may be undertaken in the future. Surface analysis of contact interfaces was performed using an optical interferometer, while friction and wear relationships were gleaned from experiments on a pellet-on-disk with slider pad tribometer. Through an analysis of numerous qualitative and quantitative parameters that describe surface topography, it was found that abrasive wear is the predominant wear mechanism governing the transfer film process. Consequently, an alternate wear description of the in situ self-replenishing transfer film lubrication process is proposed.