Low temperature preparation of SiO₂ reinforced hydroxyapatite coating on carbon/carbon composites

In the past decades, weak adhesive strength and thin thickness are headaches existed between carbon/carbon (C/C) composite and hydroxyapatite (HA) coating by one-step electrochemical deposition. To solve this problem, a monetite precursor coating was first synthesized on (NH4)₂S₂O₈ treated C/C (N-C/C) by hydrothermal electrochemical deposition, and then hydrothermally treated in a sodium silicate solution to transform into a SiO₂/HA composite coating. The morphology, microstructure, in-vitro bioactivity and biocompatibility of the SiO₂/HA coating were investigated. The distribution of spherical nano SiO₂ in the HA coating was homogeneous with uniform grain size of about 100 nm. The composite coating had a rod shaped morphology and a compact microstructure. For comparison, another HA coating was prepared on N-C/C under the same processing conditions except for sodium silicate addition in the hydrothermal system. Scratch and nano-indentation tests showed that this SiO₂/HA composite coating presented an adhesive strength of a critical load of 31.63 N, was nearly 1.5 times as high as one of the HA coating on N-C/C, and its modulus and hardness were 58.3 Gpa and 2.93 Gpa, far higher than those of the HA coating. In-vitro bioactivity and cell biocompatibility tests showed that the SiO₂/HA coating could induce apatite growth and had superior abilities of cell attachment and cell proliferation to naked C/C and the HA coating.