Computer Simulation Study of the Effect of Surface Pre-Relaxation on the Adhesion of Apatite Thin Films to the (0001) Surface of α-Quartz

Abstract

Computer simulations of apatite thin films at a range of α-quartz surfaces have shown how the strength of adhesion between thin films of apatite material and ceramic silica surfaces is crucially dependent upon both the orientation of the film relative to the substrate and the nature of the silica surface − a finding which is important in a wide number of applications, from basic geological research into intergrowth of phosphate and silicate rock minerals to the search for more effective surgical implant materials. It is shown that although the unrelaxed quartz surface is more reactive toward the apatite film, the more regular thin film structures grown at the pre-relaxed quartz surfaces lead to more stable interfaces. Interfacial energies for a single apatite layer range from 0.64−1.22 Jm-2 at the unrelaxed quartz surface to 0.55−0.73 Jm-2 at the relaxed surface. Hence, the nature of the substrate surface before attachment of the film is more important in determining the structure and stability of the resulting interface than the initial reactivity of the substrate and/or the degree of bonding between the two materials across the interface. In addition, film growth at the unrelaxed quartz surface is energetically increasingly unfavorable, whereas growth at the pre-relaxed surface is calculated to continue beyond the first layer and we predict that the apatite thin film will form local domains of both (2 × 2) as well as clockwise and anticlockwise (2 × 2)R120° adsorbate layers.