Some characteristics of hydroxylapatite powder particles after plasma spraying.

We have studied the particles of hydroxylapatite (HA) powder, the particles after plasma spraying, their distribution on substrate surface and their condition after transfer through the plasma torch. Mean particle size of HA powders was as follows: HA-A: 3.8 microm, HA-B 88.2 microm. The area of HA coating after plasma spraying, when the torch had a constant position against the substrate surface, shows two characteristic zones: the central part of coating formed mainly from deformed particles and the marginal part of coating with small non-deformed particles. These small non-deformed particles can be found in all zones of the coating and together with greater non-deformed particles and partially deformed particles will unfavourably affect the adhesive and cohesive strength of the coating and its porosity. The maximum diameter of the molten (spherical) particles in the conditions of Ar + H2 plasma, output P = 24 kW was: DA = 25 microm (HA-A) and DB = 65 microm (HA-B). The intervals of dimensions in which most of molten particles occurred were HA-A: 0-15 microm (98%), HA-B: 5-35 microm (84%). From comparison of HA-A and HA-B powders it can be concluded that the transport of HA-A powder was not continuous, the amount of molten HA-A particles was considerably greater (90%) than that of HA-B powder (63%). Phase decomposition and also solubility of HA-A powder (at in vitro tests) was greater. If we consider transport of particles, their melting, splitting and spraying efficiency, the suitable size of HA powder particles for the given spraying conditions is somewhere between the size of HA-A and HA-B particles--let us say--in the interval from 20 to 60 microm.