Design of experiment analysis of high velocity oxy-fuel coating of hydroxyapatite
Hasan, Shahriar (2009) Design of experiment analysis of high velocity oxy-fuel coating of hydroxyapatite. Master of Engineering thesis, Dublin City University.
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Due to the flexibility and cost effectiveness the high velocity oxy-fuel (HVOF) thermal spray process has been widely adopted by many industries. Though there are different types of flame spraying processes, the HVOF thermal spray process utilises only powder as the coating material rather than wire or rod. Carbon-hydrogen group of gases like propane, propylene, acetylene or even just pure hydrogen can be used as fuel gas in the HVOF process. In the Sulzer Metco Diamond Jet HVOF thermal spraying process powder material is melted by the use of combustion of oxygen and propylene gas and accelerated through a series of nozzles and deposited onto a work-piece. When the gun is in operation, air is used as the main cooling agent. The molten or semi molten particles are propelled out of the gun nozzle at supersonic velocities towards the substrate or forming die. By changing the propylene and oxygen ratio influences both the temperature and the velocity of the particles. HVOF has the potential to produce Hydroxyapatite HA (Bio-ceramic) coatings based on its experience with other sprayed materials, this technique should offer mechanical and biological results compared to other thermal spraying process such as plasma spray. Deposition of HA via HVOF is a new venture especially using the Diamond Jet, and the aim is for HVOF to provide superior coating results compared to Plasma spray, the process currently used in orthopaedic implants.
In this research Design of Experiment (DOE) models were developed to optimise the HVOF process for the deposition of HA through an optimised screening set of experiments. A preliminary process investigation was carried out before beginning the Design of Experiment study. In order to select suitable ranges for the production of HA coatings the parameters were first investigated. Currently five parameters (factors) were researched over two levels (maximum and minimum values for each factor) for Oxygen flow rate, Propylene flow rate, Air flow rate, Spray distance and Powder flow rate. Coating crystallinity and coating 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 current studies are now aiming to find the desired optimised settings to achieve high Crystallinity and Purity greater than 95%. This research found crystallinity and purity values of 93.81 % and 99.84 % respectively for a set of HVOF parameters which was even better than that achieved by the FDA approved plasma spray. Hence a new technique for HA deposition exist. The coating will allow bone within the femur to attach (graft) onto the implant using the HA material. HA is synthetic bone and turns to natural bone once in contact with living cells such as blood and human bone. HA does not cause any side effects in the human body.
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