Structure and phase composition of hydroxyapatite plasma coating
Changes in the structure, phase composition of the hydroxyapatite (HA) plasma coating were established during deposition at an initial 20 °C and 550 °C temperature of the titanium substrate and a spraying distance of 95 and 150 mm. The structure of HA coatings was analyzed by SEM and optical microscopy. DSC analysis established the transition temperatures of HA coatings to the equilibrium state. When deposited on a substrate with an initial temperature of 20 °C at a deposition distance of 95 mm, a nanostructure with a crystallite size of 21 nm is fixed in the HA coating. With an increase in the deposition distance to 150 mm, the non-equilibrium phase composition increases, the crystallite size decreases to 12 nm, the HA content decreases from 72 to 61 %, TTCP from 10 to 5 %, and the α-TCP content increases from 17 to 30 %. A non-equilibrium nano-structural state passes into a more equilibrium state with the release of heat at temperatures of 615 - 727 °C in DSC studies. The high-temperature α-TCP phase is not fixed when the coating is deposited onto a substrate with an initial temperature of 550 °C at a spraying distance of 95 mm, the content of TTCP increases by 2 times, the size of the HA phase crystallites reaches 36 nm and their size in the sprayed powder is 75 nm. The HA coating has a dendritic microstructure and does not have a thermal effect during DSC heating at an initial substrate temperature of 550 °C.