Structural Changes in Hydroxyapatite Plasma Coatings Depending on the Plasma Power

The structure of hydroxyapatite (HA) plasma coatings obtained during changing of the power of a plasma torch (P) from 20 to 26 kW has been studied using X-ray diffraction analysis. All coatings contained the products of hydroxyapatite decomposition in the plasma flow, phases of tetracalcium phosphate (TTCP), and CaO. The increase in their contents with an increase in P and the plasma temperature (T_p) is due to the partial loss of phosphorus by the initial HA powder in plasma and due to the shift in the chemical composition of the coating to higher calcium contents. An increase in P and T_p is accompanied by an increase in the intensity of the HA base texture. In the range of 24 < P < 26 kW, the ratio of the intensity values of the HA X-ray reflections δ = I(200)/I(211) exceeds 3.0, while at P = 20 kW δ <1.5. The presence of the orientation relationships between the predominant crystallographic orientations of HA and TTCP crystals in coatings is associated with the similarity of their structures, the proximity of lattice periods, and the presence of identical fragments in their structures. The possibility of the formation of mixed-layered structures involving these compounds has been discussed. The following factors have been considered as those that most strongly affect the pattern of the obtained dependences: the substrate temperature, the particle size in the initial HA powder, and the composition of the plasma gases.