Improvement of Oxidation Behavior in Cr(N,O) By Increasing the Oxygen Content

Abstract

Chromium oxynitride(Cr(N,O)) thin films have been developed by controlling the oxygen partial pressure during the deposition 1). The Cr(N,O) thin films show superior properties not only the hardness but also the oxidation behavior than those of CrN, which are suitable for machining tool applications. However, the reason of the oxidation improvement was not known because of the presence of grain boundaries. Recently, we have succeeded in preparation of epitaxially grown Cr(N,O) on MgO and other single crystal substrates to reduce the density of grain boundaries2). In this work, oxidation behavior of the epitaxially grown Cr(N,O) thin films was investigated from crosssectional transmission electron microscope (TEM) observations. Thin films of Cr(N,O) were epitaxially grown by a pulsed laser deposition method. In the deposition chamber, oxygen gas with a partial pressure of 2x10-5Pa and N radicals from an RF radical source were introduced. A plate of Cr was rotated in the deposition chamber was irradiated by a Nd:YAG pulsed laser with a wavelength of 355 nm. Thin films were deposited on MgO (100) single crystal substrates kept at 973 K in the deposition chamber. After the deposition, oxidation tests were carried out in a furnace at 673-1173 K in air for one hour. After the oxidation tests, powder X-ray diffraction (XRD), crosssectional TEM and electron energy loss spectroscopy (EELS) observations/analyses were conducted for precise phase, microstructure and composition analysis. From the XRD and TEM analyses, the as-deposited thin films were of epitaxially grown Cr(N,O) on the substrates. After the oxidation tests, from XRD patterns, peaks for Cr2O3were seen from samples heated above 873 K. On the other hand, peaks for Cr(N,O) remained up to 1073 K. From TEM and EELS analyses of the sample heated at 1073K, on the existing Cr(N,O) epitaxially grown thin film, a polycrystalline oxide layer was covering. It was found that the oxide layer works as an oxygen diffusion barrier from the surface. References 1) T. Suzuki, J. Inoue, H. Saito, M. Hirai, H. Suematsu, W. Jiang, and K. Yatsui, Thin Solid Films 515, 2161 (2006). 2) K. Suzuki, T. Endo, A. Sato, T. Suzuki, T. Nakayama, H. Suematsu, and K. Niihara, Mater. Trans. 54, 1957 (2013).