The crystallinity of hydroxyapatite (HA) coatings used in femoral implant applications is a crucial factor. A coating containing a large percentage of amorphous phases will dissolve quickly. This leaves the coating mechanical weak and thus reduces its functional life. The crystallinity of the final coating largely depends on the parameters selected during the spraying process. In this study the design of experiment technique was used to investigate the parameters that have the greatest effect on the crystallinity of the coating. The effect of furnace heat treatment in air at 600°C, 700°C and 800°C on the crystallinity of the coating was also investigated. The coatings were analysed using scanning electron microscopy (SEM) and X-ray diffraction (XRD).

Due to the recent advances in thermal spraying technology, considerable research emphasis has been placed on the development of models capable of predicting deposition mechanisms at various stages during the process. In order to gain a deeper knowledge of the mechanisms involved in thermal spraying, it is necessary to isolate the factors affecting these constitutive properties (for example residual stress generation) and in doing so quantify the effect of the individual factors. Finite Element Analysis (FEA) is used in the present research to predict the residual stress generated in a WC-Co deposit produced via the HVOF process. The model is compared to an analytical technique and validated experimentally, the result of which provides a thermomechanical modelling procedure with an accuracy greater than 80% of that found experimentally. Combining FEA techniques with analytical and experimental results will enhance the understanding of residual stress in thermal spray techniques.

This paper presents a study of the residual stress and microstructural properties of thick, spray-formed components, produced using the High Velocity Oxy-Fuel (HVOF) thermal spraying process. The forming material used is Tungsten carbide cobalt (WC-Co), a material which is more usually processed using expensive press and sinter technology. The aim of this study is to examine the effect of production parameters on the formation of thick components. In order to fabricate thick specimens, certain problems have to be overcome. More specifically these problems include the minimizing residual stresses, which cause shape distortion in the components and maining the integrity of the coating on a microstructural scale. The dependence of residual stress, and sprayed material characteristics on spraying distance, and powder feed rate conditions is presented. Results show that cylindrical WC-Co components up to a thickness of 9mm can successfully be produced, by careful control of these parameters. This represents a significant improvement on maximum thickness values previously reported for WC-Co [1,2].