Suspension Plasma Sprayed Titanium Oxide and Hydroxyapatite Coatings

The paper aims at development of TiO₂ and Ca₅(PO₄)₃OH (hydroxyapatite, HA) coatings onto stainless steel, titanium and aluminum substrate using suspension plasma spraying. The total thickness of the coatings was in the range 10 to 150 µm. The suspension on the base of distilled water, ethanol and their mixtures was formulated with the use of fine commercial TiO₂ pigment crystallized as rutile and HA milled from commercial spray-dried powder or synthesized from calcium nitrate and ammonium phosphate in an optimized reaction. The HA powder was crystallized as mainly hydroxyapatite. Two liquid feeders were applied. A mechanical feeder enabled to deliver to the atomizer two suspensions in a controlled fashion. A pneumatic one was used to feed the to the continuous stream injector. The injection of suspension to the plasma jet was studied carefully with the use of atomizer injector or a continuous stream one. The injectors was placed inside or outside the anode-nozzle of the plasma torch and the stream of liquid was directed under angle right or slightly backwards with regard to the torch axis. The sprayed deposits were submitted to the phase analysis by the use of X-ray diffraction. The content of anatase and rutile was calculated in the titanium oxide deposits as well as the content of the decomposition phases in the hydroxyapatite ones. The micro-Raman spectroscopy was used to visualize the area of appearance of some phases and X-ray photoelectron spectroscopy was applied to characterize the chemical composition of coatings and the oxidation level of some elements. Scratch test enabled to characterize the adhesion of the deposits, their microhardness and friction coefficient. The electric properties including electron emission, impedance spectroscopy, dielectric properties of some coatings were equally tested. Several properties of the coatings were expressed by the deposition variables by the use of statistical regression equations.