A crystal-chemical approach to lubrication by solid oxides

This paper introduces a new approach to the selection, classification, and mechanistic understanding of lubricious oxides that are used to combat friction and wear at elevated temperatures. Specifically, it describes a crystal-chemical model that enables one to predict the shear rheology or lubricity of an oxide or oxide mixture at elevated temperatures. This model can be used to formulate new alloy compositions or composite oxide structures that can provide low friction at high temperatures. In the case of composite oxides, the model allows one to estimate the solubility limits, chemical reactivity, compound forming tendencies, as well as the lowering of the melting point of one oxide when a second oxide is present. From a tribological standpoint, a prior knowledge of these details is important because they are strongly related to the extent of adhesive interactions, shear rheology, and hence to lubricity of oxides. In light of certain crystal-chemical considerations, general guidelines are provided for the selection of those oxides that can provide low friction at high temperatures. The major goal of this paper is to establish model relationships between relevant crystal-chemical and tribological properties of oxides that can be used as lubricants at high temperatures. Such a model may help eliminate guesswork in high-temperature lubrication and provide a new means to address the difficult lubrication problems experienced at high temperatures.