Hydroxyapatite (HA) coatings have been widely used to provide a biocompatible surface on the dental and prosthetic implants. To improve the mechanical strength of the coating, we choose HA-glass functionally graded materials as coating materials. The functionally graded structure has been proved to be able to mitigate the residual stresses in materials near the interface of the coating and the substrate. The residual stresses are mainly caused by the mismatch in thermal expansions of the coating material and the substrate material. However, it is also a crucial requirement to evaluate the effect of the spatial distribution of constituent phases on the thermal residual stress distributions in the functionally graded coatings. With this aim, we measure the thermal residual states in the coatings by means of X-ray diffraction technique and simulate them using a computational model which applies the finite element method at the microscale. The experimental and the computational results show that the graded compound HA-glass interlayer structure can mitigate internal stresses and control the density and kinetics of misfit emanating from interfaces effectively.
Hydroxyapatite ceramic (Ca10(PO4)6(OH)2; HA) has been used successfully as a substitute material for defective bone issue because of its good osteointegration and biocompatibility. (Holmes, 1986; Klein, 1994) However, the HA material still cannot be used for implants under heavy loads directly due to its low tensile strength and low resistance to fatigue failure. (Brown, 1994) Using HA as a coating on metallic substrates is an innovation, which can compensate for the insufficient mechanical strength and maintain the biocompatibility of HA. (Geesink, 1987) Various coating techniques, such as plasma spraying, dip coating, hot isostatic pressing (HIP) and ion-beam sputtering, have been used successfully in coating HA on the metallic substrate. (Berndt, 1990; Li, 1996; Lacefield, 1988; Ong, 1994)
