The Relation between Wear and Irreversible Entropy Generation in the Dry Sliding of Metals

This paper examines the relationship between wear and the generation of entropy within the mechanically affected zone, MAZ, of a sliding material. It is postulated that wear is related to irreversible entropy generation within the MAZ. This accumulation takes a place due to the degradation in the ability of the rubbing material to maintain a noncongested flow of the generated entropy. This, in turn, is affected by the nature of the change in the thermal transport properties of the material with respect to the thermal and the mechanical loads acting on the interface. A model, that treats the MAZ as a heat engine in the Carnot sense is presented. The model assumes that the MAZ is a heat engine that transports heat from a high temperature reservoir, represented by the asperity contact layer, to a low temperature reservoir, represented by the sub-contact layer. Consequently an entropy generation source that represents the irreversibilities within the MAZ is defined and a study of the entropy generation is attempted. Wear data, published elsewhere, of two materials, Oxygen Free High conductivity Copper, and Commercially Pure Titanium are analyzed using the developed model. It is found that wear for both materials is correlated to entropy generation, and to the entropy flow within the MAZ. Interestingly, moreover, in view of the contrasting wear trends of the test materials, the relationship of the mass wear rate and the specific wear rate of each material with respect to entropy generation are totally opposite to each other. A common feature between the behavior of the two materials, was found in wear behavior with respect to a so called Ratio of Residual Entropy. This ratio determines wether the system is over or under supplied with entropy. It is found that when the capacity to transport entropy exceeds the entropy supply the mass wear rate increases, and when the entropy transport capacity of the system is exceeded and entropy generation takes place the mass wear rate decreases. This is attributed to the re-establishment of equilibrium within the system.