Observation of microcracks formed in HA-316L composites

Hydroxyapatite-stainless steel 316L fiber composites were prepared using as-supplied, 900oC calcined, and 15 vol% yttria stabilised zirconia added hydroxyapatite powders. The reinforcements were 20 vol% chopped 316L fibres of length 1 mm and diameter 55 mm. Both hot isostatic pressing and spark plasma sintering were used to densify the composites with the processing temperatures varying from 825 to 950 degrees Celsius, pressures from 20 to 140 MPa, and time from 10 to 180 min. The obtained composites were subjected to microstructural analysis using a scanning electron microscope. It was found that microcracking took place invariably in the hydroxyapatite matrices but near and around the 316L fibres. Such patterns of microcracks resulted from the thermal residual stresses developed during the cooling from the high temperatures and the intrinsic low mechanical strengths of the hydroxyapatite ceramics. Such microcracks may play important roles in the mechanical behaviour of the HA-316L fibre composites.