INFLUENCE OF INTERFACIAL LAYER THICKNESS AND SUBSTRATE ROUGHNESS ON ADHESION OF TiN COATINGS DEPOSITED AT LOW TEMPERATURES BY IBAD

Ion beam assisted deposition (IBAD) was applied to produce TiN coatings on carburized steel substrates. Low deposition temperatures (~50°C) were applied to prevent distortion and softening of previously heat-treated substrates. Mechanical properties of all studied coatings are comparable to those obtained at usually used high temperatures. In order to improve adhesion between TiN coating and substrate, an interfacial layer was prepared by ion beam mixing of Ti atoms and steel substrate. The adhesion strength evaluation revealed significant improvement compared to the coatings produced without the ion beam mixed interfacial layer. Adhesion increased with increase in thickness of the interfacial layer. Substrate roughness was varied systematically in order to determine its influence on adhesion strength. The research was conducted for a rarely studied domain of low roughness (Average roughness Ra below 50 nm). The results of scratch tests revealed improvement of adhesion with increase in substrate roughness. This adhesion trend is different from the one reported by other authors who used rougher substrates. Two groups of opposing mechanisms acting during adhesion testing were identified. It appears that there exists an optimum roughness below which adhesion strength increases, and above which it decreases with the increase in substrate roughness. Accordingly, applying an expensive surface finish does not have to be a guarantee for achieving the appropriate adhesion of TiN coatings deposited at low temperatures.