Study On The Residual Stress Of Plasma-Sprayed Hydroxyapatite Coatings

The residual stress has great impact on the adhesion of the plasma sprayedceramic coatings. This study aims at analyzing the influence of process variations interms of the fundamental mechanism of the residual stress formation. A self-madecurvature tester was developed firstly to measure the residual stress of thermallysprayed coatings. The effect of the principal variations, such as substrate temperature,spraying current, gas flow rate, and spraying distance, etc., on the elastic modulus andthe residual stress of plasma sprayed ZrO₂ coatings on Ti-6Al-4V substrate wasstudied, respectively. Furthermore, the influence of process variations on the residualstress of plasma sprayed hydroxyapatite(HA) coatings were studied, and the keyfactors determining the adhesion of plasma sprayed HA coatings were preliminarilystudied.The main results are as follows:The substrate temperature has a great influence on the elastic modulus and theresidual stress of the plasma sprayed ZrO₂ coatings. A rise in substrate temperaturewould increase the elastic modulus of coatings, and the quenching stress decreases aswell, due to the reduced temperature difference between the splat and the substrate. Inthe case of non-preheating and inferior cooling condition, the elastic modulus of ZrO₂coating was 61GPa and the mean quenching stress was 122MPa. When the substratepreheating temperature is 160oC, the elastic modulus was increased to 129GPa whilethe mean residual stress was decreased to 65MPa. The variation of the thermal stresswas the same with that of the elastic modulus of the coating if temperature of thecoating formation was unchanged. The inter-laminar stress is negligible when thefluctuation of substrate temperature is not significant.The increase of the spray current leads to the increase of the input power from36kW to 39kW, and the porosity of the ZrO₂ coating was markedly decreased, from8.9% to 4.8%. The elastic modulus was increased from 49GPa to 120GPa and themean residual stress was decreased from 42MPa to 83MPa. The further increment ofthe spray power from 39kW to 45.5kW by increasing the hydrogen flow rate and theargon flow rate would increase the porosity of the coating to 6.1%. The elasticmodulus was accordingly decreased to 112GPa and the mean residual stress wasdecreased to 64MPa.The elastic modulus and the residual stress of ZrO₂ coating increased with theincreasing of stand-off distance, and then decreased. In this study, the elastic modulus of ZrO₂ coating increased, from 86GPa to120GPa and residual stress increased from64MPa to 86MP when the distance increased form 80mm to 90mm. The elasticmodulus decreased to 27GPa and residual stress decreased to 35MPa when thestand-off distance further increased to 105mm.In the case of plasma sprayed HA coatings, it was found that within the range of90mm to 100mm of stand-off distance and 600A to 700A of spray current, both theelastic modulus and residual stress increased with the increase of the stand-offdistance and the spray current. Consequently, the adhesion of the plasma sprayed HAcoatings decreased in the presence of the higher residual stress. The quenching stressplayed more important role than thermal stress in the plasma sprayed HA coatings, inwhich the latter measured by the self-made tester is roughly as same as the expectedvalue.The study shows that the adhesive strength and crystallinity of HA coatings isaccord with the ISO13779-2. The optimized processing parameters are: Argon flowrate: 42L/min, hydrogen flow rate: 7L/min, current: 600A; stand-off distance: 90mm,in which the plasma sprayed HA coatings present the highest bonding strength andlower residual stresses.