Reaction enthalpies as selection criteria for tribological coatings

A wide range of coatings are used for a variety of applications. Selection criteria for these coatings include coating-substrate adhesion, hardness, melting temperature, coefficient of thermal expansion mismatch, corrosion resistance, and cost. A factor often ignored is the possibility of reaction between the coating and the other surface which it contacts during use. This is especially important when frictional forces produce enough heat to locally melt asperities or even cause friction welding, as that heat will enhance the likelihood of chemical reactions at the interface. One aspect of this is to consider the reaction enthalpies between coatings and the surfaces they contact during operation. We explore the use of reaction enthalpies between coatings and opposing surfaces as a criteria for selecting coatings. Simple thermodynamic calculations are presented for several couples of coating - Opposing surfaces. Conclusions are drawn about the relevance based on experimental evidence.