Influences of oxygen impurity contained in nitrogen gas on the reactions of chromium with nitrogen
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Fong-Shung Huang Department of Medical Technology, Foo-Ying University, Ta-Liao Hsiang, Kaohsiung Hsien 831, Taiwan, Republic of China
Fu-Hsing Lu Department of Materials Engineering, National Chung Hsing University, Taichung 402, Taiwan, Republic of China (Received 19 February 2005; accepted 6 July 2005)
In this research, the influences of the oxygen impurity contained in the commercially available nitrogen gas on the reactions of chromium pellets with nitrogen were investigated in the temperature range 600–1350 °C. A small amount of oxygen competed with the majority N2 to react with chromium in the annealing process. Analyzing the in situ oxygen partial pressure changes during annealing proved that the dissolution of oxygen in Cr and/or resultant CrxN (CrN or Cr2N) was exothermic and the solubility decreased with increasing temperature. It was found that the oxygen partial pressure decreased drastically to about 10−22 atm when specimens were annealed at 600 °C compared to a mere 10−5 atm for a blank test, while its value increased with temperature. The oxidation involved simultaneous dissolution of oxygen in specimens and formation of oxide scale. Moreover, comparing the aforementioned results with those obtained from additional annealing experiments preformed in argon gas showed that the formation of Cr2O3 might stem mainly from oxidation of the resultant nitrides instead of the metallic chromium.
I. INTRODUCTION
In material processing, the formation of metal nitrides often involves the reaction of metals with nitrogen gas at moderate or elevated temperatures. However, commercially available pure nitrogen gas (4N–6N) usually contains a small amount of oxygen impurity, which may play an important role during heat treatment or chemical reactions. Since the oxygen incorporation may greatly influence the properties of materials, it is essential to explore the role of oxygen impurity in annealing under oxygen-contained gas environment. Recently, one of the authors and his research group, Lu and Lo,1 studied the influence of the oxygen impurity in nitrogen on the annealing of titanium nitride. From the results, it was concluded that the dissolution of oxygen into titanium nitrides predominates at low temperatures while the formation of oxide scale predominates at high temperatures. A significant oxygen pressure decrease was observed in a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0341 J. Mater. Res., Vol. 20, No. 10, Oct 2005
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the gas-tight furnace during the dissolution of oxygen into titanium nitrides. Analyzing the in situ oxygen partial pressure changes gave that the dissolution of oxygen in TiN is exothermic and the solubility decreases with increasing temperature. As the annealing temperature reached above 700 °C, titanium dioxide appeared. TiN was used as a model system for this study because no phase transformation or chemical reaction would occur other than oxidation involving dissolution and oxide
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