Effect of sprayed techniques on the surface microstructure and in vitro behavior of nano-HAp coatings

Supersonic atmospheric plasma spray (SAPS) technique is a classical method which is employed to coat the carbon/carbon (C/C) composites by nano-hydroxyapatite (HAp) powders to decrease the biologically inert, hydrophobic drawbacks of substrate surfaces. In recent years, profiting from the promoting of energy conservation and environmental protection, more emphasis was placed on industrial manufacturing to simplify the experimental steps. This paper aims to study the preparation of nano-HAp coatings by suspension plasma spray (SPS) instead of the original SAPS technique. A denser, more uniform and less defective coating is successfully fabricated on C/C substrate using SPS technique. More important, fewer surface flaws in SPS coating could be observed by scanning electron microscopy (SEM) which shows that the large drawbacks of the coating have disappeared during spraying process. Meanwhile, except for HAp, phase compositions of the SPS coating appear with slight calcium oxide (CaO, 0.8%) and tricalcium phosphate (TCP, 30.7%), and then all CaO as well as TCP phases transform into dicalcium phosphate anhydrous (DCPA, 60.6%) after microwave-hydrothermal (MH) treatment. Thermal analysis (TG/DSC) reveals that SPS coating (97.51%) has a higher thermal stability than that of the SAPS coating (82.37%). Also, in comparison, the SPS coating after MH treatment (SPS-MH coating) exhibits better thermal properties (92.76%). In addition, compared to the SAPS and SPS coatings, due to the more flaw reduction and phase transformation, SPS-MH coating shows a better biological properties according to the surface microstructure in simulation body fluid (SBF) solution and cell spreading area on coating. The highest corrosion resistance with the current density of 3.9798 × 10^-7 A/cm² and a potential of 0.0419 V is achieved for SPS-MH coating.