The influence of ductile interlayers on the mechanical performance of tungsten nitride coatings

Multilayers between ceramic materials and metallic materials can combine th e high hardness and wear resistance of the ceramic layers with the toughnes s and mechanical strength of the metallic layers. This work is aimed at the production and characterisation of tungsten nitride/titanium (or nickel) m ultilayers. The coatings, deposited by magnetron sputtering, are characteri sed with respect to their structure, morphology and hardness. The adhesion to the substrate is evaluated by a scratch-test technique, as it constitute s a fundamental prerequisite. The choice of titanium and nickel as interlayering materials was made in or der to study the role of materials with elevated plasticity, but with diffe rent reactivities. Titanium tends to react chemically with other materials, namely, ferrous oxides and nitrides. Nickel has low chemical reactivity bu t has the same crystallographic structure as the W2N phase. All of the mult ilayer coatings produced have a total thickness of 4 mu m. The hardness of the multilayers, deposited with different ceramic/metal thickness ratios, d ecreases with the thickness of the ductile metallic layers. When the metal thickness is too high it causes the spalling of the coatings. The optimal m edium critical load (65 N) is obtained with a ceramic/metal ratio equal to four. Even though the type of bond is different, the adhesion of the multil ayers is not influenced by the substitution of titanium with nickel. The de position process yields well-adhered and sufficiently hard multilayer coati ngs when compared with the ceramic single layers. (C) 1999 Elsevier Science S.A. All rights reserved.