Plasma-sprayed hydroxyapatite (HA) coatings with flame-spheroidized feedstock: microstructure and mechanical properties
S.W.K. Kweh, K.A. Khor, P. Cheang
Biomaterials
Abstract
Flame-spheroidized feedstock, with excellent known heat transfer and consistent melting capabilities, were used to produce hydroxyapatite (HA) coatings via plasma spraying. The characteristics and inherent mechanical properties of the coatings have been investigated and were found to have direct and impacting relationship with the feedstock characteristics, processing parameters as well as microstructural deformities. Processing parameters such as particle sizes (SHA: 20}45, 45}75 and 75}125 m) and spray distances (10, 12 and 14 cm) have been systematically varied in the present study. It was found that the increase of particle sizes and spray distances weakened the mechanical properties (microhardness, modulus, fracture toughness and bond strength) and structural stability of the coatings. The presence of inter- and intralamellar thermal microcracks, voids and porosities with limited true contact between lamellae were also found to degrade the mechanical characteristics of the coatings, especially in coatings produced from large-sized HA particles. An eort was made to correlate the eects of microstructural defects with the resultant mechanical properties and structural integrity of the plasma-sprayed hydroxyapatite (HA) coatings. The eects of dierent heat treatment temperatures (600, 800 and 9003C) on the mechanical properties of the coatings were also studied. It was found that a heat treatment temperature of 8003C does enhance the microhardness and elastic modulus of the coatings signi"cantly (P(0.05) whereas a further increment in heat treatment temperature to 9003C did not show any discernable improvements (P'0.1). The elastic response behaviour and fracture toughness of both the as-sprayed and heat-treated HA coatings using Knoop and Vickers indentations at dierent loadings have been investigated. Results have shown that the mechanical properties of the coatings have improved signi"cantly despite increasing crack density after heat treatment in air. Coatings produced from the spheroidized feedstock of 20}45 m (SHA 20}45 m) sprayed at a stand-o distance of 10 cm were found to possess the most favourable mechanical properties.
keywords:Hydroxyapatite; Microstructure; Elastic modulus; Fracture toughness; Bond strength; Plasma spray; Knoop hardness
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