Effects of silicon doping on strengthening adhesion at the interface of the hydroxyapatite–titanium biocomposite: A first-principles study

In this paper we employ first-principles calculations to investigate the effect of substitutional Si doping in the amorphous calcium-phosphate (a-HAP) structure on the work of adhesion, integral charge transfer, charge density difference and theoretical tensile strengths between an a-HAP coating and amorphous titanium dioxide (a-TiO₂) substrate systemically. Our calculations demonstrate that substitution of a P atom by a Si atom in a-HAP (a-Si-HAP) with the creation of OH-vacancies as charge compensation results in a significant increase of the bonding strength of the coating to the substrate. The work of adhesion of the optimized Si-doped interfaces reaches a value of up to −2.52 J m⁻², which is significantly higher than for the stoichiometric a-HAP/a-TiO₂. Charge density difference analysis indicates that the dominant interactions at the interface have significant covalent character, and in particular two Ti–O and three Ca–O bonds are formed for a-Si-HAP/a-TiO₂ and one Ti–O and three Ca–O bonds for a-HAP/a-TiO₂. From the stress-strain curve, the Young's modulus of a-Si-HAP/a-TiO₂ is calculated to be about 25% higher than that of the a-HAP/a-TiO₂, and the yielding stress is about 2 times greater than that of the undoped model. Our calculations therefore demonstrate that the presence of Si in the a-HAP structure strongly alters not only the bioactivity and resorption rates, but also the mechanical properties of the a-HAP/a-TiO₂ interface. The results presented here provide an important theoretical insight into the nature of the chemical bonding at the a-HAP/a-TiO₂ interface, and are particularly significant for the practical medical applications of HAP-based biomaterials.