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 fric tion and wear at elevated temperatures. Specifically, it describes a crysta l-chemical model that enables one to predict the shear rheology or lubricit y 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 ox ides, the model allows one to estimate the solubility limits, chemical reac tivity, compound forming tendencies, as well as the lowering of the melting point of one oxide when a second oxide is present. From a tribological sta ndpoint, a prior knowledge of these details is important because they are s trongly related to the extent of adhesive interactions, shear rheology, and hence to lubricity of oxides. In light of certain crystal-chemical conside rations, 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 hi gh temperatures. Such a model may help eliminate guesswork in high-temperat ure lubrication and provide a new means to address the difficult lubricatio n problems experienced at high temperatures.