Influence of exposure orientation on the microstructure and mechanical properties of microwave-sintered SS316L-5 wt.% hydroxyapatite composites

Abstract

This study investigates the effect of exposure orientation on the microstructure and mechanical properties of SS316L-5 wt.% HA composites fabricated via microwave sintering. Nano-sized hydroxyapatite (HA) particles were blended with SS316L powder through planetary ball milling and sintered at 1250 °C in a hybrid-mode microwave furnace using horizontal and vertical sample orientations. The vertically oriented samples exhibited superior densification (∼83%), reduced porosity (∼17%), and enhanced micro hardness (288 HV₀.₀₅), attributed to uniform microwave absorption and controlled thermal gradients. X-ray diffraction using Cu Kα radiation and scanning electron microscopy with energy-dispersive X-ray spectroscopy analyses confirmed the partial decomposition of HA [Ca₁₀(PO₄)₆(OH)₂] into calcium phosphate phases such as oxyapatite-type (Ca₁₀P₆O₂₆), tricalcium phosphate (Ca₃(PO₄)₂) and tetracalcium phosphate (Ca₄P₂O₉), alongside a uniform elemental distribution of Calcium (Ca) and Phosphorus (P). These findings demonstrate that exposure orientation significantly affects phase transformation, microstructure, and mechanical behaviour, offering a controlled design strategy for optimizing implant-grade microstructural performance through exposure orientation engineering.