Mechanism of apatite formation on wollastonite coatings in simulated body fluids

The formation mechanism of apatite on the surface of wollastonite coating was examined. Plasma-sprayed wollastonite coatings were soaked in a lactic acid solution (pH=2.4) to result in the dissolution of calcium from the coating to form silanol ( Si–OH) on the surface. Some calcium-drained samples were soaked in a trimethanol aminomethane solution (pH=10) for 24 h to create a negatively charged surface with the functional group ( Si–O−). These samples before and after treatment in a trimethanol aminomethane solution were immersed in simulated body fluids (SBF) to investigate the precipitation of apatite on the coating surface. The results indicate that the increase of calcium in the SBF solution is not the critical factor affecting the precipitation of apatite on the surface of the wollastonite coating and the apatite can only form on a negatively charged surface with the functional group ( Si–O−). The mechanism of apatite formation on the wollastonite coating is proposed. After the wollastonite coatings are immersed into the SBF, calcium ions initially exchange with H+ leading to the formation of silanol ( Si–OH) on the surface of the layer and increase in the pH value at the coating–SBF interface. Consequently, a negatively charged surface with the functional group ( Si–O−) forms on the surface. Due to the negatively charged surface, Ca2+ ions in the SBF solution are attracted to the interface between the coating and solution, thereby increasing the ionic activity of the apatite at the interface to the extent that apatite precipitates on the coating surface.