Dependence of the mechanical and structural properties of (Ti,Al)N films on the nitrogen content
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Dependence of the mechanical and structural properties of (Ti,Al)N films on the nitrogen content C. Jime´nez, C. Sa´nchez-Ferna´ndez, C. Morant, and J.M. Martı´nez-Duart Dpto. Fı´sica Aplicada C-12, Universidad Auto´noma de Madrid, Cantoblanco 28049, Madrid, Spain
M. Ferna´ndez Inst. Ciencia de Materiales, CSIC, Cantoblanco 28049, Madrid, Spain
J. Sa´nchez-Olı´as Dpto. Corrosio´n y Proteccio´n, Centro Nacional de Investigacio´n Metalu´rgicas, CSIC, Avda. Gregorio del Amo, 8.28040, Madrid, Spain (Received 26 February 1998; accepted 18 March 1999)
(Ti,Al)N films with increasing nitrogen content were grown by reactive cosputtering and characterized by auger electron spectroscopy, grazing x-ray diffraction, polarization curves, electrochemical impedance spectroscopy, nanoindentation, and atomic force microscopy. For Ti/Al ≈ 1 the Ti1−x Alx N phase is always present, but lower nitrogen contents lead to an additional phase, probably ␣–Ti(Al), which causes a decrease in hardness and Young’s modulus. The increase of nitrogen content results in a decrease of surface roughness or a more compacted surface coating, according to the impedance results.
I. INTRODUCTION
The most important criteria of suitability of a material for tribological applications are high hardness, stiffness, resistance against oxidation, and low coefficient of friction.1 The ternary system obtained by the deposition of (Ti, Al)N films is very promising because of the formation of the metastable phase, Ti1−x Alx N, which presents optimal mechanical properties.2 The main feature to be highlighted relates to the improvement in corrosion and oxidation resistance that the material presents compared with TiN.2,3 The Ti1−x Alx N metastable phase is a substitutional solution of the face-centered-cubic (fcc) ␦–TiN phase in which Ti atoms of the TiN lattice are replaced by Al atoms.4 It has already been established that the Al content can be varied from x ⳱ 0 to x ⳱ 0.7; for higher x values, the final product is a mixture of TiN and AlN.5 In most cases, the deposition methods used to obtain the metastable phase are based on physical vapor deposition (PVD) techniques,6 although chemical vapor deposition (CVD) processes also have been studied to decrease the processing temperature.7 Reactive sputtering is considered one of the best techniques to grow films with phases that are not thermodynamically favorable. In the case of Ti1−x Alx N, the main difficulty comes from the differences in crystal structure and chemical bonds. Many attempts have been made to establish the optimal Al:Ti ratio to improve the properties of the Tix Aly N.8 In this work, we have studied the changes in the properties of the Tix Aly N films relative to the nitrogen content. We have found that the presence of phases other than the 2830
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J. Mater. Res., Vol. 14, No. 7, Jul 1999 Downloaded: 17 Mar 2015
metastable one, even at similar compositions, influences the film’s hardness, roughness, adherence and other proper
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