Elastic modulus of plasma-sprayed coatings determined by indentation and bend tests

Abstract

The elastic moduli of plasma-sprayed thick coatings including two metallic and three cermet materials as well as three ceramic materials were assessed using Knoop indentation and three-point bend tests. The anisotropic elastic behavior and reduced elastic modulus determined by Knoop indentation technique are closely related to their microstructures in the plasma-sprayed coatings. Most elastic modulus values in the in-plane direction are higher than those in the perpendicular direction since pores and weak splat boundaries contribute to the reduction of elastic modulus values in the perpendicular direction. Three-point bend tests clearly reveal the non-linear stress-strain behavior of the plasma-sprayed coatings resulting from microcracks and lamellar structures. The elastic modulus values measured by three-point bend tests are significantly low and around 6–21 GPa irrespective of coating compositions investigated. As compared to the Knoop indentation test which considers only the elastic component in evaluating elastic modulus values, the three-point bend test accounts for inelastic deformation due to splat boundary sliding and propagation of cracks and pores to evaluate the elastic modulus values. Therefore, it is considered that the elastic modulus values determined by bend tests for plasma-sprayed coatings having a porosity of around 10% are always significantly low compared to those measured by Knoop indentation test irrespective of their chemical compositions.