(70ds) Hydroxylapatite Particles for Plasma Sprayed Medical Coatings

We describe the synthesis of hydroxylapatite (HA) powders of medical grade for coating orthopaedic implants by the plasma spraying technique. HA was made by precipitation from aqueous solution followed by filtering and oven drying. The product was then calcined at temperatures from 900 to 1200°C. Centimetre sized chunks were ground by ball milling in a ceramic jar or in a metal grinder. The yields in four size categories were compared as a function of calcination temperature. The categories were (in µm): greater than 300, 45 to 300, less than 45, and lost as dust. Results showed that for both fragmentation methods, the higher temperatures led to greater amounts of larger particles (42%), smaller amounts of small particles (24%) and a nearly constant quantity of desired product (30%) and lost fines (4%). The smaller particles could be reintroduced into the following HA syntheses up to 25% without affecting subsequent particle strength. The particles intended for plasma spraying were sieved and characterized with or without the addition of 1% of colloidal silica to help powder flow. The DigitalMicrograph software was used to evaluate particle aspect ratios, which were 0.33 for metal grinding and 0.40 for ball milling. Accordingly, the ball milled product had a better flow rate (2.4 s versus 3.1s). Steady flow is crucial for even coating thickness by plasma spraying. The flow times and specific surface areas decreased as the particles were sintered at higher temperatures, and their apparent densities increased. The more sintered particles were harder to grind but easier to sieve. Ball milled products had fewer impurities, notably no metal grinder wear particles. On the other hand, ball milling was more time consuming and required multiple operations to avoid production of fines. For larger scale productions, it was found to be convenient to grind chunks heated to 900°C and separate particles from 45 µm to 2mm diameters. The powder is then sintered at 1200°C, water washed to remove surface adsorbed submicron sized particles, and finally sieved to the desired final range. The surfaces of the particles were observed with a SEM, and showed grain sizes less than 2 µm with pores covering 6% of the surface and of 0.6 µm length. Wavelength dispersive X ray analysis did not reveal homogeneous coating of the HA particles with silica, but showed occasional molten silica rich particles. Infrared and X ray diffraction analysis indicated silica could produce some non-HA phases. The best plasma sprayed coatings had tensile strengths of 12MPa and crystallinities near 69% for powder of mean diameter 151 µm and D(v, 0.1) 102; D(v, 0.9) 216. The smaller HA particle size distributions led to more amorphous coatings. HA was the only crystalline phase clearly identified in the coatings.