Raman spectroscopy studies on the thermal stability of TiN, crN, TiAlN coatings and nanolayered TiN/CrN, TiAlN/CrN multi
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Raman spectroscopy studies on the thermal stability of TiN, CrN, TiAlN coatings and nanolayered TiN/CrN, TiAlN/CrN multilayer coatings Harish C. Barshiliaa) and K.S. Rajam Surface Engineering Division, National Aerospace Laboratories, Bangalore 560 017, India (Received 10 May 2004; accepted 1 July 2004)
About 1.5-m-thick single-layer TiN, CrN, TiAlN coatings and nanolayered TiN/CrN, TiAlN/CrN multilayer coatings were deposited on silicon (111) substrates using a reactive direct current magnetron sputtering process. Structural characterization of the coatings was done using x-ray diffraction (XRD) and micro-Raman spectroscopy. All the coatings exhibited NaCl B1 structure in the XRD data. Raman spectroscopy data of as-deposited coatings exhibited two broad bands centered at 230–250 and 540–630 cm−1. These bands have been assigned to acoustical and optical phonon modes, respectively. Thermal stability of the coatings was studied by heating the coatings in air in a resistive furnace for 30 min in the temperature range 400–900 °C. Structural changes as a result of heating were characterized using Raman spectroscopy and XRD. Raman data showed that TiN, CrN, TiN/CrN, TiAlN, and TiAlN/CrN coatings started to oxidize at 500, 600, 750, 800, and 900 °C, respectively. To isolate the oxidation-induced spectral changes as a result of heating of the coatings in air, samples were also annealed in vacuum at 800 °C under similar conditions. The Raman data of vacuum-annealed coatings showed no phase transformation, and intensity of the optical phonon mode increased and shifted to lower frequencies. The origin of these spectral changes is discussed in terms of defect structure of the coatings. Our results indicate that the thermal stability of nanolayered multilayer coatings is superior to the single-layer coatings.
I. INTRODUCTION
A variety of transition metal nitride hard coatings such as TiN and CrN were developed in the 1980s for various dry and high-speed operations. These coatings exhibit high hardness, wear resistance, and corrosion resistance. However, for many technological applications, the performance of these coatings at higher temperature is severely affected because of oxide formation on the surface of the coating, which modifies their wear and friction properties. Large difference in the molar volumes of oxides and nitride coating leads to considerable amount of stress in the coating, which eventually affects adhesion of the coating, and in extreme cases the coating may not adhere at all to the substrate. It has been reported that substitution of Ti by Al in TiN coatings improves the thermal stability considerably.1 In recent years, nanolayered multilayer coatings (also known as artificial superlattices) have been the subject of much discussion.2–6
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0444 3196
J. Mater. Res., Vol. 19, No. 11, Nov 2004
These coatings exhibit superior properties as compared to single-layer coatings. For example, it has been shown that T
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