Determining microhardness and elastic modulus of plasma-sprayed Cr 3C 2–NiCr coatings using Knoop indentation testing

Abstract

In this paper, the microhardness and elastic moduli of plasma-sprayed Cr 3C 2–NiCr coatings were measured using the Knoop indentation technique, and the effects of coating process, measurement directions and indenter loads were investigated. The measured data sets were then statistically analyzed employing the Weibull distribution to assess their variability within the coatings. It was found that the measured Knoop hardness and elastic moduli of the coatings were related to microstructure and displayed highly anisotropic behavior of the coatings. In particular, the Knoop hardness, when measured with the major diagonal of the indenter parallel to the interlamellar boundaries of the coatings, was obviously lower than that done where the major diagonal was vertical to the boundaries for an identical indenter load. The measured elastic modulus data exhibited much more scatter than those of the corresponding Knoop hardness. Because it depends on the applied indenter load and it contains systematical error, the Knoop indentation test is not practicable to measure the elastic modulus of thermal sprayed coatings when the major diagonal of the indenter is parallel to the interlamellar boundaries. The results are mainly explained in terms of interlamellar boundary sliding caused by the indenter loads.