Effects of Heat Treatment on Plasma-Sprayed Indian Clam Seashell-Derived Hydroxyapatite (HA) Coating Applied on Ti-Alloy

Initially, spray-dried Indian clam seashell-derived hydroxyapatite (HA) powder is sprayed by atmospheric plasma spray technique on titanium alloy (Ti-6Al-4 V ELI) substrate, and further the as-sprayed HA coating is heat-treated in a muffle furnace at different temperatures of 500, 600, 700, 800, and 900 °C for a 2 h of holding time under normal atmospheric conditions, followed by furnace cooling. Heat-treated and as-sprayed coatings are characterized to examine and study the effects of heating temperature on the physical, mechanical, and tribological properties of the coatings. A suitable heating condition is tried to find through this investigation. The phases and microstructure of the as-sprayed and heat-treated coatings are analyzed by equipments XRD (x-ray diffractometer) and FESEM (field emission scanning electron microscope), respectively. Average surface roughness, microhardness, and wear behavior are estimated through the stylus profilometer (contact-type), Vickers microhardness tester, and linear reciprocating tribometer, respectively. The transformation of the secondary phase such as CaO into the primary HA phase is noticed after heat treatment. Due to increased crystallinity and crystallite size, the volumetric expansion of limitedly spread HA splats is noticed at high heating temperatures. As a result, a relatively smoother coating surface is obtained. Also, at higher heating temperatures, the coating hardness is increased due to the reduction in coating porosity and improvement in the crystallinity. However, beyond 700 °C of heating temperature, the cracks have started to form and increase in number with an increase in heating temperature. The improved tribo-mechanical properties are obtained when the heating temperature of the HA coating is kept at 800 °C.