Dissolution and adhesion behaviour of radio-frequency magnetron-sputtered Ca–P coatings

Radio-frequency magnetron sputtering deposition was used to produce calcium phosphate sputter coatings with three different thicknesses (0.1, 1 and 4 μm) on titanium discs. Half of the as-sputtered coatings were subjected to an additional heat treatment for 2 h at 500°C. X-ray diffraction demonstrated that annealing at 500°C changed the amorphous 1 and 4 μm sputtered coatings into an amorphous–crystalline structure, while the amorphous 0.1 μm changed in a crystalline apatite structure. Further, scanning electron microscopy (SEM) inspection demonstrated that annealing of the 1 and 4 μm coatings resulted in the appearance of some cracks. The dissolution behaviour of these Ca–P coatings was determined in a simulated body fluid. It was found that after incubation for 4 weeks the dissolution was determined by the crystallinity of the deposited coating. SEM and Fourier transform infrared evaluation showed that all the heat-treated sputter coating appeared to be stable under the test conditions and a Ca–P precipitate was always deposited on the coating surface. On the other hand, the amorphous 0.1 and 1 μm coatings dissolved completely within 4 weeks, while the amorphous 4 μm coating showed only signs of surface dissolution. Scratch testing demonstrated that there is a linear correlation between the critical load, Lc, and the coating thickness. A heat treatment for the CaP-4 coating resulted in an additional decrease in the critical load. On the basis of these findings, we conclude that already a 0.1 μm heat-treated Ca-P sputter coating is of sufficient thickness to show in-vitro adequate bioactive and adhesive properties.