Physical principles of radio-frequency magnetron sputter deposition of calcium-phosphate-based coating with tailored properties

Abstract The influence of radio-frequency (RF) magnetron sputter deposition conditions (RF power discharge density, working gas atmosphere, deposition time, and electrical substrate bias) on the properties (microstructure, texture, Ca/P ratio) of nanocomposite calcium-phosphate (CaP) coatings has been reported. A phenomenological model was developed to explain the formation of the RF magnetron sputter-deposited hydroxyapatite (HA) coating depending on the various deposition conditions. In the initial deposition stages, a nanocrystalline or quasi-amorphous CaP layer is formed. As both the film thickness and temperature gradient across the film thickness increased, the crystallisation of the HA phase occurred, which led to the formation of the fibre 〈002〉 texture in the films. The increase in the coating thickness also resulted in an increase in the grain size and a decrease in the residual microstress. The cross-section of the coating revealed a polycrystalline fibre structure with wedge-shaped columns. The addition of water vapour to an Ar atmosphere allowed to restore the hydroxyl groups in the composition of the HA coatings. The topography of the СaP coating surface that had irregular grains with different form and shape grown out of the coating plane was established, which extended the well-known structural zone models (Thornton, Monsieur, Movchan and Demchishin, Anders, etc.) and was formed both at a relatively low substrate temperature of 160–200 °С and RF power level when the substrate was bombarded with positive ions with an energy of ~100 eV, regardless of the working gas atmosphere used (argon or oxygen), resulting in a the growth of a coating with a preferably amorphous or nanocrystalline structure.