Plasma sprayed hydroxyapatite coatings on titanium substrates Part 1: Mechanical properties and residual stress levels

Hydroxyapatite (HA) coatings have been sprayed on to substrates of Ti-6Al-4 V, using a range of input power levels and plasma gas mixtures. Coatings ha ve also been produced on substrates of mild steel and tungsten, in order to explore certain aspects of the mechanical behaviour of HA without the comp lication of yielding or creep in the substrate. Studies have been made of t he phase constitution, porosity, degree of crystallinity, OH- ion content, microstructure and surface roughness of the HA coatings. The Young's moduli in tension and in compression were evaluated by the cantilever beam bend t est using a tungsten/HA composite beam. The flexural Young's modulus was de termined using a free-standing deposit under the same test. Adhesion was ch aracterised using the single-edge notch-bend test; this is considered super ior to the tensile bond strength test in common use, measured interfacial f racture energies were of the order 1-10 J m(-2). Stress levels were investi gated using specimen curvature measurements in conjunction with a numerical process model. The quenching stress for HA was measured to be about 10-25 MPa and the residual stress level in HA coatings at room temperature are pr edicted to lie in the approximate range of 20-40 MPa (tensile). These resid ual stresses could be reduced in magnitude by maintaining the substrate at a low temperature (possibly below room temperature) during spraying and it may be worthwhile to explore this. Ideally, the HA coating should have low porosity, high cohesive strength, good adhesion to the substrate, a high de gree of crystallinity and high chemical purity and phase stability. In prac tice, such combinations are rather difficult to achieve by just varying the spraying parameters. (C) 1998 Published by Elsevier Science Ltd. All right s reserved.