Optimisation of plasma sprayed hydroxyapatite coatings
Levingstone, Tanya J. (2008) Optimisation of plasma sprayed hydroxyapatite coatings. PhD thesis, Dublin City University.
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Hydroxyapatite, (HA), is a calcium phosphate bioceramic material which has an almost identical chemical composition to that of the mineral component of bone. Its biocompatibility and osteoconductivity have led to its use in a wide range of applications in both dentistry and orthopaedics. One such application is for the uncemented fixation of implants. The plasma spraying technique, a thermal spray process, is the most commonly used method for the production of HA coatings. This process is a complicated one, affected by a large number of parameters. Due to this complexity, the process – property – structure relationship is poorly understood.
The present work aims to clarify this relationship and use the knowledge gained to develop a novel bi-layer coating. Statistically designed experiments (DOE) were used to determine the effect of five process parameters (factors), Current, Gas Flow Rate, Powder Feed Rate, Spray Distance and Carrier Gas Flow Rate, on the coating properties. A screening design was first carried out to gain an initial understanding of the process. This was followed by a detailed Response Surface Methodology (RSM) experiment. Five properties (responses) were examined, crystallinity, purity, roughness, porosity and thickness.
Models describing the effects of the variables on these coating properties were then developed. The developed models were optimised using two separate optimisation criteria to develop a novel bi-layered coating, designed to provide improved in vivo performance over current HA coatings. The performance of this novel coating was evaluated using a cell culture experiment.
Statistically significant models were developed in this work for each of the measured responses. All factors were found to have a significant effect on the measured coating responses. Current, Gas Flow Rate, Spray Distance and the Current * Spray Distance interaction were found to be the parameters with greatest effect on the coating properties. Analysis of the bi-layered coating produced indicates that improved biological performance has been achieved.
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