In vitro bio-tribological behaviour of textured nitride coating on selective laser melted Ti-6Al-4V alloy

This work reports a simple and convenient surface modification method to enhance the bio-tribological properties of additive manufactured (AM) biomedical titanium alloys. The Ti-6Al-4V samples with textured surface of different texture densities were designed and prepared in a single-step by selective laser melting (SLM), followed by heat treatment in N-2. A uniform dense nitride coating consisted of similar to 2.4 mu m TiN layer and similar to 3.6 mu m diffusion layer was created. The nitride coating had high bonding strength to the substrate, and remarkably improved the hardness of the substrate (from similar to 1.7 GPa to 8.6 GPa) and the resistance to plastic deformation, which caused an enhancement in bio-tribological performance. In comparison with the sample without nitride coating, the average frictional coefficient and wear rate of the coated samples decreased from 0.34 and 18.27 x 10(-6) mm(3)/N.m to 0.29 and 0.33 x 10(-6) mm(3)/N.m, respectively. With the texture density increasing from 40% to 75%, the frictional coefficient decreased firstly and then rose to a small extent, and the results of contact angles of the samples followed the same trend. The sample with texture density of 65% obtained the lowest frictional coefficient, which was 0.19. The samples without coating exhibited a typical abrasive wear and adhesive wear; however, the wear behaviour of the nitride coated samples was brittle peeling. The surface texture decreased the propagation of cracks on the nitride coating. The wear productions of the nitride coated samples in simulated body fluids mainly contained TiO2, Ti2O3, SiO2, CaCO3 and calcium phosphate.