Effect of van der Waals interactions on the adhesion strength at the interface of the hydroxyapatite-titanium...

Hydroxyapatite (HAP) is frequently used as biocompatible coating on Ti-based implants. In this context, the HAP-Ti adhesion is of crucial importance. Here, we report ab initio calculations to investigate the influence of Si incorporation into the amorphous calcium-phosphate (a-HAP) structure on the interfacial bonding mechanism between the a-HAP coating and an amorphous titanium dioxide (a-TiO₂) substrate, contrasting two different density functionals: PBE-GGA, and DFT-D3, which are capable of describing the influence of the van der Waals (vdW) interactions. In particular, we discuss the effect of dispersion on the work of adhesion (W _ad), equilibrium geometries, and charge density difference (CDD). We find that replacement of P by Si in a-HAP (a-Si-HAP) with the creation of OH vacancies as charge compensation results in a significant increase in the bond strength between the coating and substrate in the case of using the PBE-GGA functional. However, including the vdW interactions shows that these forces considerably contribute to the W _ad. We show that the difference (W _ad - W _ad(vdW)) is on average more than 1.1 J m^-2 and 0.5 J m^-2 for a-HAP/a-TiO₂ and a-Si-HAP/a-TiO₂, respectively. These results reveal that including vdW interactions is essential for accurately describing the chemical bonding at the a-HAP/a-TiO₂ interface.