Yc. Tsui et al., Plasma sprayed hydroxyapatite coatings on titanium substrates Part 1: Mechanical properties and residual stress levels, BIOMATERIAL, 19(22), 1998, pp. 2015-2029
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.