Effect of film thickness and Ti interlayer on the structure and properties of nanocrystalline TiN thin films on AISI D2 steel

Nanocrystalline TiN thin films were deposited on AISI D2 steel substrates using unbalanced magnetron (UBM) sputtering method. The objective of this study was to investigate the effect of TiN film thickness and Ti interlayer thickness on the N/Ti ratio, structure, mechanical properties and corrosion resistance of the films. The results showed that (111) was the dominant preferred orientation in the TiN films, which became less distinct for the specimen with Ti interlayer. Roughness and grain size of the TiN films were only slightly dependent on the film thickness. The packing factor was almost constant with film thickness and even the thinnest TiN coating, 140 nm, reached a quite high packing factor of 0.8. The nanohardness of the films, ranging from 21–26 GPa, did not show significant dependence on film thickness. All specimens contained compressive residual stresses, and decreased with increasing TiN film thickness. In the specimens with Ti interlayer, there was a critical interlayer thickness, between 130 and 180 nm, at which the residual stress in the TiN films could be substantially relieved. The results of potentiodynamic scan in both 5% NaCl and 0.5 M H2SO4 + 0.05 M KSCN solutions indicated that packing factor was more effective than film thickness on the corrosion resistance of the coatings. Furthermore, increasing film thickness or adding a Ti interlayer could effectively protect the substrate from the corrosive medium, if the packing factor was sufficiently high. The increase of interlayer thickness may also increase the corrosion resistance of the film.