Raman spectroscopy studies on the thermal stability of TiN, CrN, TiAlN coatings and nanolayered TiN/CrN, TiAlN/CrN multilayer coatings

About 1.5- mum-thick single-layer TiN, CrN, TiAIN coatings and nanolayered TiN/CrN, TiAIN/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 131 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 degreesC. Structural changes as a result of heating were characterized using Raman spectroscopy and XRD. Raman data showed that TiN, CrN, TiN/CrN, TiAIN, and TiAIN/CrN coatings started to oxidize at 500, 600, 750, 800, and 900 degreesC, 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 degreesC 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.