THE INFLUENCE OF TITANIUM INTERLAYERS ON THE ADHESION OF PVD-TIN COATINGS ON OXIDIZED STAINLESS-STEEL SUBSTRATES

It has been shown that the use of thin titanium interlayers improves t he coating-substrate adhesion of physical vapour deposition (PVD) tita nium nitride thin films on a stainless steel substrate. This improveme nt arises from a combination of chemical gettering and mechanical comp liance effects. The improved adhesion of plasma-assisted chemical vapo ur deposition TiN coatings with increasing interlayer thickness has be en shown to be largely attributable to the compliance effect (S. J. Bu ll, P. R. Chalker, C. F. Ayres and D. S. Rickersby, Mater. Sci. Eng. A , 139 (1991) 71). The development of practical methods to improve adhe sion is hampered by the difficulties involved in quantitative measurem ents of the effect. To avoid the influence of the intrinsic and extrin sic parameters involved in scratch test and microhardness measurements , efforts have been made to apply fracture mechanical testing methods to the determination of the adhesion strength of the film on the subst rate (S. Berg, S. W. Kim, V. Grajewski and E. Fromm, Mater. Sci. Eng. A, 139 (1991) 345). In our study the influence of Ti interlayers on th e adhesion of PVD TiN coatings on oxidized stainless steel substrates was investigated using a pull-off test for adhesion measurements and s canning tunnelling microscopy and secondary ion mass spectrometry for analysis of the fractured surfaces. It was shown that the thickness of the Ti layer must be chosen according to the thickness of the oxide l ayer. An excess of Ti leads to lower adhesion values due to failure in the Ti layer, while a shortage of Ti leads to unreacted oxide and min imum adhesion due to brittle fracture in the oxide layer, which was sh own to be amorphous.