Effect of silicate doping on the structure and mechanical properties of thin nanostructured RF magnetron sputter-deposited hydroxyapatite films

Silicon-doped hydroxyapatite-based (Si-HA) coatings were deposited via radio frequency (RF) magnetron sputtering on the surface of titanium that was treated with a pulsed electron beam. This study aimed to evaluate the effect of Si doping on the structure and mechanical properties of thin HA films. The content of the silicon was 1.2 and 4.6 at.% for the coatings prepared using the Si-HA precursor powders with a chemical formula Ca10(PO4)6 − x(SiO4)x(OH)2 − x where, x = 0.5 and 1.72. Pure HA (Ca10(PO4)6(OH)2) coatings were deposited for comparison. The as-deposited films were analysed with respect to their composition, state of chemical binding and microstructure using XPS, FTIR, XRD, and SEM. We hypothesized that the addition of Si would affect the mechanical features of the coatings due to microstructure changes. The effect of the introduction of Si on the nanohardness and the Young's modulus as well as the adhesion strength and scratch resistance of the HA coating was investigated using nanohardness testing and a scratch test, respectively. Examination of the coating microstructure using SEM and AFM revealed that Si doping influenced the surface morphology and led to a smaller grain size. The tendency to form an amorphous structure also increased with an increase in the Si content. A monotonous decrease in both the nanohardness and the elastic modulus was observed with an increase in the Si content. A maximum nanohardness of ~ 7 GPa was obtained for the Si-free HA coating, whereas the hardness decreased to ~ 4.3 GPa for the films with a Si content of 1.2 at.%. The addition of 4.6 at.% Si to the HA coating resulted in a reduction in the elastic modulus, whereas the nanohardness was very similar to that of the uncoated substrate. The adhesion behaviour of the coatings demonstrated different responses. In the case of pure HA coatings, failure occurred due to the low cohesion of the coating, whereas the crystalline Si-HA coatings with a Si content of 1.2 at.% deformed plastically without crack formation and without detaching from the titanium substrate, which resulted in a greater coating stability.