Improvement of wear and corrosion resistance of RPS Ti-TiN coatings by means of thermal oxidation

The reactive plasma spraying (RPS) of titanium powders in a nitrogen containing plasma gas produces thick coatings characterised by microdispersed titanium nitride phases in a titanium matrix. To increase the cohesion between the coating particles and decrease the coating porosity, an air oxidation is proposed as a post-coating treatment, and its influence on the microstructure, sliding wear resistance and corrosion resistance of Ti–TiN RPS coatings obtained on carbon steel substrates is studied. The oxidising treatment, carried out at 973 K for 2 h, promotes both the growth of titanium oxide on the coating pore walls, allowing a partial pore sealing, and interdiffusion processes, which increase the coating internal cohesion. After the post-coating treatment the average microhardness of the coating increases from about 650 to 800 HK0.2. Wear tests, performed in dry sliding conditions, show that for the sprayed and oxidised samples the wear volumes are lower, particularly at low sliding velocities (0.4, 0.8 m s^-1), in comparison with the as-sprayed samples. Moreover, corrosion resistance tests, carried out in 0.5 M NaCl and 0.1 M HCl deareated solutions, show that a reduction of the anodic current values is achieved for the sprayed and oxidised samples. This simple post-coating treatment is effective in improving the wear and corrosion protection of Ti–TiN RPS coatings on carbon steel substrates.