Tough and adhesive nanostructured calcium phosphate thin films deposited by the pulsed plasma deposition method

Abstract

Calcium phosphate (CaP) coatings are commonly employed to improve the bioactivity of bone and dental metal implants, due to the chemical–physical similarity with the mineral phase of bone, envisaged to enhance the integration with the surrounding bone tissue. However, growing concerns about the use of commercial thick CaP coatings, mainly related to their weak mechanical properties, poor interface strength and micro-structural inhomogeneity, are leading the investigation of new and alternative CaP deposition techniques. In this study the feasibility to deposit tough and well adherent CaP thin films by a novel Pulsed Plasma Deposition (PPD) method on Ti alloy was investigated. Microstructural, chemical and morphological properties of the coatings as well as the nano-mechanical properties and their adhesion to the Ti-alloy substrate were extensively characterized. In vitro biocompatibility was also preliminary assessed evaluating the adhesion and proliferation of primary mouse osteoblasts. As-deposited CaP films were amorphous and exhibited dense and uniform surface composed of sub-micrometric aggregated globular particles. Noteworthy, mechanical properties of as-deposited films were comparable to the ones of commercial plasma-sprayed coatings despite the significant difference of thickness (a few hundred nanometers vs. tens of micrometers). After the thermal annealing of the as deposited films at 600 °C for 1 h in air, a transformation from amorphous calcium phosphate to crystalline hydroxyapatite (HA) phase occurred. The mechanical properties as well as the adhesion to substrate of the annealed films strongly improved respect to those of the as deposited films, displaying interesting high hardness, elastic strain to failure and plastic deformation resistance values. Finally, biological in vitro tests indicated good biocompatibility of both as-deposited and annealed films, with this latter showing better cells adhesion and proliferation compared to the former.