Characterization of composite tribological coatings: composition, microstructure and mechanical properties

Abstract

The present study aims to characterize two kinds of plasma composite coatings (Cr 2O 3·CaF 2 and Cr 2O 3·BaF 2) with very good tribological properties. Chemical and quantitative image analyses show that solid lubricant content (CaF 2 and BaF 2) in the coatings is less than but proportional to the nominal solid lubricant content in mixed powders for spraying. The microstructure of the composite coatings is characteristic of a homogeneous distribution of solid lubricant powders within the Cr 2O 3 matrix: on a sliding surface, the coating's microstructure consists of the spheroidal solid lubricant particles dispersed in a continuous Cr 2O 3 matrix; on cross-section the distinct wavy multilayers (Cr 2O 3 and solid lubricant) have been compacted to build the composite coating. During sliding, the matrix has flowed and the lubricant particles have been removed partially, smashed to very fine fragments and incorporated with the matrix of which one thin soft film has been made. It is the thin soft film that offers the coatings' very good tribological properties. However, the solid lubricant results also in degradation of the mechanical properties. Consequently there is one optimal solid lubricant content (about 20 vol.%) at which the coating has the best tribological and mechanical properties.