Plasma-sprayed hydroxyapatite (HA) coatings with flame-spheroidized feedstock: microstructure and mechanical properties

Flame-spheroidized feedstock, with excellent known heat transfer and consis tent melting capabilities, were used to produce hydroxyapatite (HA) coating s via plasma spraying. The characteristics and inherent mechanical properti es of the coatings have been investigated and were found to have direct and impacting relationship with the feedstock characteristics, processing para meters as well as microstructural deformities. Processing parameters such a s particle sizes (SHA: 20-45, 45-75 and 75-125 mu m) and spray distances (1 0, 12 and 14 cm) have been systematically varied in the present study. It w as found that the increase of particle sizes and spray distances weakened t he mechanical properties (microhardness, modulus, fracture toughness and bo nd strength) and structural stability of the coatings. The presence of inte r- and intralamellar thermal microcracks, voids and porosities with limited true contact between lamellae were also found to degrade the mechanical ch aracteristics of the coatings, especially in coatings produced from large-s ized HA particles. An effort was made to correlate the effects of microstru ctural defects with the resultant mechanical properties and structural inte grity of the plasma-sprayed hydroxyapatite (HA) coatings. The effects of di fferent heat treatment temperatures (600, 800 and 900 degrees C) on the mec hanical properties of the coatings were also studied. It was found that a h eat treatment temperature of 800 degrees C does enhance the microhardness a nd elastic modulus of the coatings significantly (P < 0.05) whereas a furth er increment in heat treatment temperature to 900 degrees C did not show an y discernable improvements (P > 0.1). The elastic response behaviour and fr acture 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 of the coatings have im proved significantly despite increasing crack density after heat treatment in air. Coatings produced from the spheroidized feedstock of 20-45 mu m (SH A 20-45 mu m) sprayed at a stand-off distance of 10 cm were found to posses s the most favourable mechanical properties. (C) 2000 Elsevier Science Ltd. All rights reserved.