Influence of spraying conditions on thermal and velocity properties of plasma sprayed hydroxyapatite

Hydroxyapatite (HA), classified as a bioactive material, was proven to be well suitable for orthopaedic and dental applications due to its ability to integrate into bone structures and support bone ingrowth. Therefore it is used as a coating material for common metallic implant materials which carry most of the load. Covered by a layer of HA, the implants are much better accepted by surrounding tissues. To date, the only reliable method of applying HA coatings to metallic implants is plasma spraying. The aim of this research was to study the conditions to which the HA powder is exposed during spraying, which, in return, highly influence the resulting quality of the final coating. The two main characteristics describing and quantifying the plasma spraying process — temperature and velocity of the HA particles — were investigated. To study the changes of both characteristics under different spraying conditions, Taguchi design of experiment together with analysis of variance (ANOVA) method were utilized. Results showed the three main parameters influencing the powder particles' in-flight properties are gun power, main gas flow and spraying distance, while other parameters have a little (feed rate) or no effect (auxiliary gas).