Microwave hydrothermal (MH) combining supersonic atmospheric plasma sprayed (SAPS) calcium phosphorus (Ca-P) bio-coatings on carbon/carbon (C/C) composite has been widely used due to their osteoconductivity and osteoproductivity. However, the erratic shear strength between coatings prepared only by SAPS (outer coating) and C/C substrates has attached more attention over the implant failure. Adding a coating prepared by MH (inner coating) before SAPS can possess superior shear strength to conventional outer coating. The inner coating with fine Ca-P particles was prepared through a unique MH method under different concentrations (10, 500 and 1000 mmol/L). The influence of concentration on microstructure, phase composition, roughness and shear strength are investigated in this paper. In particularly, the roughness of inner coatings on C/C substrates was found to related to the morphologies and particle size. Results showed that inner coatings have higher roughness which was beneficial for the promotion of shear strength between the obtained Ca-P big-coating and the C/C substrates. Subsequently, the specimens were immersed in a simulated body fluid (SBF) to investigate the bioactivity. (C) 2017 Elsevier B.V. All rights reserved.