MECHANICAL TESTING OF ELECTROPHORETICALLY DEPOSITED HYDROXYAPATITE

In the present study, hydroxyapatite (HAp) was electrophoretically deposited on Ti, Ti6A14V and 316L stainless steel substrates. The effects of sintering temperatures ranging from 875°C to 1000°C on the bonding strength between the HAp coating and metal substrates were investigated. In general, sintering causes decomposition of the HAp coating even at a temperature of 875°C. In the present study, dual HAp coatings were prepared. The use of dual HAp coatings enabled decomposition to be confined to the “undercoat”, while the surface coating remained decomposition free. The bonding strength between the HAp coating and the metal substrates was assessed by shear strength testing, according to ASTM F 1044-87. It was found that the coating on stainless steel sintered at 925°C gave the best result among the three substrates, 25 MPa of adhesive strength. The high adhesive strength of stainless steel was mainly due to the high thermal expansion coefficient of this metal. This resulted in residual compressive stresses in the coating, whereas the low thermal expansion coefficients of Ti and Ti6A14V produced residual tensile stresses. The tensile stress in the coating over Ti and Ti6A14V substrates caused the coatings to buckle and crack, thereby greatly decreasing their bonding strengths.