Thermal treatment in air of direct current (DC) magnetron sputtered TiN coatings

TiN coatings were deposited onto mirror polished stainless steel substrates by reactive DC magnetron sputtering using a pure Ti target and Ar+N2 atmosphere. The deposited TiN coatings were thermally treated in ambient air at temperatures ranging from 500 to 700°C for times between 1 and 16 h. The as-deposited and thermally treated coatings were characterized using glow discharge optical emission spectroscopy, x-ray diffraction and scanning electron microscopy. Titanium oxide layers were identified at the surface of thermally treated TiN coatings, which grow according to oxygen diffusion controlled parabolic time law. Phase composition of the oxide layers is found to depend strongly on temperature and exposure time. At low temperatures and shorter exposure times the oxide layers were found to be a mixture of anatase and rutile polymorphs of TiO2, while at high temperatures and longer exposure times the oxide layers consisted only of the rutile polymorph of TiO2. The results show that the microstructure of the oxide layers is porous and non-uniform across the oxide layer thickness. The porous microstructure is explained by the accumulation of nitrogen by short-range diffusion and transition into a gaseous state. Key words: TiN, coating, magnetron sputtering, rutile, thermal treatment, X-ray diffraction (XRD), glow discharge optical emission spectroscopy (GD-OES).