Plasma sprayed hydroxyapatite coatings on titanium substrates Part 1: Mechanical properties and residual stress levels
Y.C. Tsui, C. Doyle, T.W. Clyne
Biomaterials
Abstract
Hydroxyapatite (HA) coatings have been sprayed on to substrates of Ti 6Al 4V, using a range of input power levels and plasma gas mixtures. Coatings have also been produced on substrates of mild steel and tungsten, in order to explore certain aspects of the mechanical behaviour of HA without the complication of yielding or creep in the substrate. Studies have been made of the phase constitution, porosity, degree of crystallinity, OH ion content, microstructure and surface roughness of the HA coa...Moretings. The Young s moduli in tension and in compression were evaluated by the cantilever beam bend test using a tungsten/HA composite beam. The exural Young s modulus was determined using a free-standing deposit under the same test. Adhesion was characterised using the single-edge notch-bend test; this is considered superior to the tensile bond strength test in common use. Measured interfacial fracture energies were of the order 1 10 J m . Stress levels were investigated 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 predicted to lie in the approximate range of 20 40 MPa (tensile). These residual 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 degree of crystallinity and high chemical purity and phase stability. In practice, such combinations are rather dicult to achieve by just varying the spraying parameters.
keywords:Plasma spraying; Hydroxyapatite; Adhesion; Young's modulus; Residual stresses
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