Influences of oxygen impurity contained in nitrogen on the reaction of titanium with nitrogen
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In this work, Ti pellets were selected as a model system to investigate the influences of oxygen impurity in nitrogen gas on the reaction of a metal with the nitrogen. Analyzing changes in the in situ oxygen partial pressures when titanium specimens were annealed in the oxygen-containing nitrogen shows that the dissolution of oxygen in Ti and TiNx is exothermic, and the solubility decreases as the temperature increases. X-ray diffraction results show that nitridation of Ti occurred before oxidation, even in an oxygencontaining nitrogen atmosphere. Kinetics apparently predominates over thermodynamics at low temperatures in such a system. I. INTRODUCTION
Commercially available nitrogen usually possesses a small amount of oxygen, depending on the purity level of the nitrogen. Such nitrogen may sufficiently react with many metals or nonoxide ceramics at elevated temperatures. Thus far, there is still a lack of systematic research focusing on the study of reactions of materials with the oxygen-containing nitrogen. Titanium nitride was used as a model nitride system for the study, as reported in our previous work.1 By analyzing changes in both in situ oxygen partial pressures and crystalline phases when titanium nitride specimens were annealed in a gas-tight furnace under such a nitrogen atmosphere, we found that the dissolution of oxygen in TiN was exothermic, and the solubility decreased as the temperature increased. The oxidation of TiN consists of the dissolution of oxygen, predominating at low temperatures, and the oxide scale formation, governing at high temperatures.1 This research deals with a more basic but complex system—titanium because nitridation and oxidation would occur concurrently during annealing in the nitrogen that contains oxygen impurities even in parts per million levels. Either oxidation or nitridation of Ti has been intensively studied in the literature.2–5 Nevertheless, the topic dealing with the concurrent reactions has rarely been investigated. Saillard et al.6 investigated the role of oxygen contamination on titanium nitridation. Although the subject sounds similar, the objective is considerably different. They focused on the influence of oxygen contamination in the substrate under a NH3 plasma environment a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0288
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 7, Jul 2009 Downloaded: 18 Mar 2015
and found that increasing the bulk oxygen contamination before nitridation would enhance the nitridation kinetics. Moreover, most of the research studied the nitridation of Ti by exposing it to NH32,7 or a prepurified N24 atmosphere to prevent the oxygen contamination from the reactive gas. A similar topic has been studied by one of the authors in a rather different system—Cr.8 Because more nitride phases are expected in Ti, the results in the Ti system may be also more complex. Because of the phase versatility and its wide applicability, Ti has been selected as a model system for th
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