THE MECHANICAL CHARACTERIZATION OF PLASMA SPRAYED HYDROXYAPATITE (HA) COATINGS

Mechanical evaluations of plasma sprayed HA coatings have assumed vast importance in the orthopaedic applications where the demands of operational stresses of the coatings are stringently required. The determination of the mechanical properties such as Knoop hardness, elastic modulus, fracture toughness and bond strength are therefore essential and necessary for the assessment of the service behaviour and performance of the bioceramic coatings. The inherent properties of the coatings have been investigated and were found to have direct and impacting relationship with the feedstock characteristics, processing conditions as well as microstructural deformities. The presence of inter- and intralamellar thermal microcracks, voids and porosities with limited true contact between lamellae were found to degrade the mechanical characteristics of the coatings.

This paper aims to provide an insight to the mechanical properties of the HA coatings by plasma spray process, and the effect of microstructural defects on the resultant mechanical and structural integrity of the coatings. The elastic response behaviour and fracture toughness of both the as-sprayed and heat-treated HA coatings using Knoop and Vickers indentations at different loadings have been investigated. Results have shown that the mechanical properties (hardness, modulus, fracture toughness and bond strength) of the coatings have improved significantly despite increasing crack density after heat treatment. These properties were also found to deteriorate with increasing spray distance and particle size.