HVOF has the potential to produce Hydroxyapatite HA (Bio-ceramic) coatings based on its experience with other sprayed ceramic materials. This technique should offer mechanical and biological results comparable to other thermal spraying processes such as plasma spray currently FDA approved for HA deposition. Deposition of HA via HVOF is a new venture especially using the Sulzer Metco Diamond Jet (DJ) process, hence the aim of this paper. In this research, a Design of Experiment (DOE) model as developed to optimize the HVOF process for the deposition of HA. Five parameters (factors) were researched over two levels namely: oxygen flow rate, propylene flow rate, air flow rate, spray distance and powder flow rate. Coating crystallinity and purity were measured as the responses to the factors used. The research showed that: propylene, air flow rate, spray distance and powder feed rate had the largest effect on the responses and the study aimed to find the desired optimised settings. This research found crystallinity and purity values of 93.8% and 99.8% respectively for a set of HVOF parameters which were improved findings compared to the crystallinity and purity of 87.6 % and 99.4 % respectively found using the FDA approved Plasma thermal spray process. Hence a new technique for HA deposition now exists using the DJ HVOF facility. Future research aims to evaluate the biological response to these coatings through in vitro tests.

The application of FGMs is quite difficult, but thermal spray processes like Plasma spray have demonstrated their unique potential in producing graded deposits, where researchers have used twin powder feed systems to mix different proportions of powders. FGMs vary in composition and/or microstructure from one boundary (substrate) to another (top service surface), and innovative characteristics result from the gradient from metals to ceramics or non-metallic to metals. The present study investigates an innovative modification of a HVOF (High Velocity Oxy- Fuel) thermal spray process to produce functionally graded thick coatings. In order to deposit thick coatings, certain problems have to be overcome. Graded coatings enable gradual variation of the coating composition and/or microstructure, which offers the possibility of reducing residual stress build-up with in coatings. In order to spray such a coating, modification to a commercial powder feed hopper was required to enable it to deposit two powders simultaneously which allows deposition of different layers of coating with changing chemical compositions, without interruption to the spraying process. Various concepts for this modification were identified and one design was selected, having been validated through use of a process model, developed using ANSYS Flotran Finite Element Analysis. In the current research the mixing of different proportions of powders were controlled by a computer using LabVIEW software and hardware, which allowed the control and repeatability of the microstructure when producing functionally graded coatings.