Investigation into the Fabrication of Hydroxyapatite Coatings from Calcium and Phosphorous Rich Precursors using Plasma Spraying

Abstract

Hydroxyapatite (HA) is one of many calcium phosphate phases and is of particular interest due to its biocompatible nature and its similar properties to natural bone. It is extensively used in the biomedical industry, as well as in catalysis, fertilisers, and pharmaceuticals. In many applications HA coatings are used and are often made in a two-step process, where HA is firstly synthesised, and then applied as a coating. One way to make the process more efficient, is to combine the HA synthesis and coating processes into a single step. Plasma spray is a commonly used coating method for deposition of high melting point ceramic materials, including HA. The main aim of this research was to fabricate HA coatings from composite precursor powders using plasma spray and to investigate the reaction mechanisms which occurred during the process. Various composite precursor powders (calcium carbonate + brushite, calcium oxide + brushite, calcium carbonate + calcium pyrophosphate and calcium oxide + calcium pyrophosphate) were plasma sprayed under a range of parameters. Phase analysis was carried out using X-ray diffraction (XRD), and the morphology of the coatings was observed using scanning electron microscopy (SEM). All precursor powders successfully formed coatings composed of well-molten splats consisting of CaO and a mixture of calcium phosphate phases, including low concentrations of HA. The plasma spray parameters which influenced the particle velocity were found to have the largest effect on the concentration of HA with higher velocities generating greater HA formation on average. In flight reaction mechanisms were hypothesised to account for the observed results, based on the pseudo binary CaO-P₂O₅ phase diagrams with and without water. The majority of as-sprayed coating compositions were accounted for according to a mechanism that followed tetracalcium phosphate (TTCP) formation over tricalcium phosphate (TCP) formation in the coatings. Heat treatment of the coatings informed from simultaneous thermogravimetric analysis (STA) showed that the as-sprayed calcium phosphate phases transformed into HA between 750-1150°C. This phase transformation mechanism was distinctly different to the chemical reaction mechanism which occurred during heat treatment of the feedstock powders to form HA. Overall, this thesis confirmed that HA coatings could be produced from precursor feedstock compounds using plasma spraying.