Process Parameters Optimization And Residual Stress Analysis For Plasma Sprayed HA Composite Coating

Due to its favorable biocompatibility and mechanical properties, hydroxyapatite (HA)-coated Ti–6A1–4V alloy has been accepted as one of the most promising implant materials for orthopaedic and dental applications. But it doesn't have so wide application because of its weak bonding strength between Ti–6A1–4V alloy and HA resulted from difference of physical and chemical properties between them. This dissertation aims to study the bonding strength of plasma sprayed HA composite coating on titanium alloy substrate, to get the optimized parameters through uniform design experiment, and to analyze the main parameters that may affect the bonding strength. The bonding strength, micro-hardness and porosity of coatings with different thickness are investigated, and the residual stress is analyzed with the commercial software ANSYS.A uniform experiment was used to investigate the influence of spraying distance, spraying electrical current, primary gas flow rate and secondary gas flow rate on the plasma spraying HA top layer. The experimental results were analyzed basing on regression analysis, and a regression equation is obtained by using DPS. X₁ spraying distance, X₂ spraying electrical current, X₃ primary gas flow rate, X₄ secondary gas flow rate.According to their correlation coefficient, the effect of these parameters on the bonding strength of HA composite coating is as follows: X₃X₄ > X₁X₂> X₄²> X₁> X₁X₃. HA composite coating is sprayed adopting optimized process parameters, the bonding strength is obtained to be 40.5MPa, micro-hardness of 4.5GPa, porosity of 7%. Coating properties has been enhanced obviously after optimization.According to the analysis of the HA composite coating's thickness between bonding strength, micro-hardness and porosity, the result is: bonding strength and miro-hardness decreased as coating's thickness increased, while porosity increased as coating's thickness increased.A mathematical model is developed to simulate the residual stress of HA composite coating. HA top layer and Zr02 bonding layer interface radial stress increases with radial distance and the large compressive stress on central region changes to the tensile stress on the outer region. Shear stress is small near to zero on the central region. However, Shear stress rapidly increases in the external part, the maximum shear stress appears at the edge of the interface. The axial stress is small tensile stress near the central region. As the radius increases, the tensile stress changes to compressive stress rapidly.Changing the coating thickness can change the interface stress level, but can not change the distribution pattern of the stress. 50%HA+50%Zr02 middle layer can reduce the maximum radial stress in the coating.