Synthesis and growth mechanism of calcium phosphate layer on SiC-coated carbon/carbon composites with surface pre-oxidation

Substrates surface containing with polar components has a great influence on the synthesis of sol-gel derived calcium phosphate coatings. This paper reports the synthesis of calcium phosphates coatings on native and pre-oxidated C/C-SiC (SiC-coated carbon/carbon) substrates, and compares their differences in composition, structure and in vitro bioactivity. C/C-SiC composites were prepared by CVD (chemical vapor deposition) at 1100 °C for 25 h and modified with aqua regia (HCl/HNO₃) for 2 h. Then the coatings were manufactured by sol-gel combined with dip-coating technique. It is shown that the pre-oxidation promotes the formation of dispersed beta-tricalcium phosphate (β-TCP) particles, and the conversion of calcium phosphates coatings from hydroxyapatite (HA) to BCP (HA/β-TCP). The formed β-TCP is composed of hexagonal and orthorhombic systems, and HA is mainly hexagonal crystal. Additionally, the BCP coating on C/C-SiC exhibits better bioactivity compared to pure HA coating during immersion in simulated body fluid (SBF), which is result from the negatively functional groups OH and COOH formed on pre-oxidated surface. These groups disturb the stability of sol via electrostatic interaction and induce the nucleation of β-TCP. Hence, the BCP coating with preferential dissolution of β-TCP phases reveals better apatite formation capability.