Pathway to tailor the phase composition, microstructure and mechanical properties of pulsed laser deposited cobalt-substituted calcium phosphate coatings on titanium

Dense calcium phosphate-based ceramics were fabricated to be used as targets for pulsed laser deposition (PLD). Nanostructured cobalt-substituted hydroxyapatite (Co:HAP) was used as a starting powder. To vary phase composition and microstructure of targets, two sintering approaches were applied, conventional (CS) and two-step sintering (TSS). The obtained results show that in both cases biphasic calcium phosphate (BCP) ceramics (targets) were prepared, with slightly different HAP-to-β–TCP amount ratio and a significantly different microstructure. While the CS method yielded fully dense ceramics with an average grain size of 1.3 μm, the ceramics prepared by TSS had a density of 98.5%, with a predominant grain size below 100 nm. (Ca + Co)P coatings were prepared by PLD of (Ca + Co)P targets. The temperature of the Ti substrate was adjusted to be 25 and 500 °C. The results show that the phase composition of (Ca + Co)P coatings depended on the phase composition of targets as well as on the temperature of the Ti substrate. The coating prepared at 25 °C using CS target consisted of three calcium phosphate phases, HAP, β–TCP and α–TCP; when the TSS target was used, the coating was biphasic, containing HAP and β–TCP. When the substrate was heated to 500 °C, regardless of whether the CS or the TSS target was used, the deposited coatings were composed of HAP and α–TCP. Due to different phase compositions, the (Ca + Co)P coatings deposited at 25 °C showed an improved hardness compared to those deposited at 500 °C. The obtained results confirmed that the phase composition, morphology and mechanical properties of 0.3 μm thick (Ca + Co)P coatings on a Ti substrate can be tailored by employing (Ca + Co)P targets with different microstructures, and also by varying the temperature of the Ti substrate during deposition experiments.