The in¯uence of ductile interlayers on the mechanical performance of tungsten nitride coatings M.T. Vieira * , A.S. Ramos ICEMS ± Faculdade de Cie Ãncias e Tecnologia da Universidade de Coimbra, 3030 Coimbra, Portugal Abstract Multilayers between ceramic materials and metallic materials can combine the high hardness and wear resistance of the ceramic layers with the toughness and mechanical strength of the metallic layers. This work is aimed at the production and characterisation of tungsten nitride/titanium (or nickel) multilayers. The coatings, deposited by magnetron sputtering, are characterised with respect to their structure, morphology and hardness. The adhesion to the substrate is evaluated by a scratch-test technique, as it constitutes a fundamental pre- requisite. The choice of titanium and nickel as interlayering materials was made in order to study the role of materials with elevated plasticity, but with different reactivities. Titanium tends to react chemically with other materials, namely, ferrous oxides and nitrides. Nickel has low chemical reactivity but has the same crystallographic structure as the W 2 N phase. All of the multilayer coatings produced have a total thickness of 4 mm. The hardness of the multilayers, deposited with different ceramic/metal thickness ratios, decreases with the thickness of the ductile metallic layers. When the metal thickness is too high it causes the spalling of the coatings. The optimal medium 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 multilayers is not in¯uenced by the substitution of titanium with nickel. The deposition process yields well-adhered and suf®ciently hard multilayer coatings when compared with the ceramic single layers. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Multilayers; Ductile interlayers; Ultra-microhardness; Adhesion 1. Introduction If the different layers are selected adequately, multilayer coatings can provide a way to enhance the performance of the system substrate/coating. Multilayer coatings have a wide range of applications being used as X-ray mirrors, monochromators, corrosion and erosion protections and diffusion barriers [1±5]. More recently, multilayer coatings have been used for mechanical application, namely to improve the wear resistance of coated components [3,6,7]. It seems that the wear resistance results from a speci®c favourable combination of hardness and toughness, which is dif®cult to obtain with a single coating [8]. The low toughness of hard ceramic coatings could be improved by bonding them with a ductile layer. Based on this principle the toughness of alumina coatings was increased by the incorporation of thin ductile nickel layers [8]. The introduc- tion of the metal ®lms aims at: (i) enhancement of the plastic deformation of the coating; (ii) stress relaxation; (iii) reduc- tion of porosity; and (iv) crack de¯ection. However, particularly when dealing with multilayer coat- ings, the adhesion is very important because it is necessary to promote not only the bonding to the substrate but also that between all of the layers involved. The objective of the present work is to study the effect of ductile metal interlayers on the adhesion and mechanical performance of hard ceramic coatings. The ceramic material adopted was tungsten nitride, type W 2 N, which has an fcc structure, and the ductile metals were titanium (hcp) or nickel (fcc). This choice was made in order to enhance the in¯uence of the chemical reactivity and structure on the adhesion and mechanical properties of the coated samples. Titanium has a high chemical reactivity in opposition to nickel, which has the same structure as W 2 N. In the case of multilayer coatings, parameters such as the number of layers, the period thickness and the relationship between the thickness of the two deposited materials can Journal of Materials Processing Technology 92±93 (1999) 156±161 *Corresponding author. Tel: +351(0)39-7000745; fax: +351(0)39- 7000701 E-mail address: teresa.vieira@mail.dem.uc.pt (M.T. Vieira) 0924-0136/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII:S0924-0136(99)00235-6