Hydroxyapatite-coated Ti-6Al-4V part 1: the effect of coating thickness on mechanical fatigue behaviour

The present investigation examines the effect of coating thickness on the fatigue behaviour of hydroxyapatite (HA)-coated Ti-6Al-4V. Uniaxial fatigue tests were conducted on grit blasted Ti-6Al-4V coupons with HA coatings deposited by atmospheric plasma-spray at thicknesses of 0, 25, 50, 75, 100, and 150 microm, as well as on grit blasted specimens that had received a stress relieving heat treatment. Coupons with 150 microm HA coatings were shown to have significantly decreased fatigue resistances, with lives similar to those of the stress relieved specimens, while coatings of thickness 25-100 microm were found to have no affect on fatigue resistance. Residual stresses generated during deposition, cracks propagating towards the substrate from within the coating, and stress relief due to heat inputs from the spraying process were all considered with respect to their potential effects on fatigue behaviour. Stress relief in the substrate due to enhanced heat transfer mechanisms was identified as the most likely source of the observed reductions in substrate fatigue life in the 150 microm coupons.