Kinetics of layer growth and multiphase diffusion in ion- nitrided titanium
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Concentration profiles of nitrogen were measured in specimens ion nitrided at 900 ~ The profiles were in good agreement with the predicted ones at high nitrogen concentrations. However, at low nitrogen concentrations, a deviation was observed, assumed to be due to a concentration dependence of nitrogen diffusion in a titanium.
I.
INTRODUCTION
ION nitriding
is an extension of conventional nitriding processes, like gas and salt bath nitriding, using a nitriding plasma. The process utilizes a glow discharge operated at high voltage through which nitrogen ions are accelerated to impinge on the workpiece. This ion bombardment heats the workpiece, cleans the surface, and provides active nitrogen. [~,21 Although the process is mainly applied to iron-based alloys ~3,4,sl in an industrial scale, titanium and titanium alloys have also been treated to improve their wear properties. 16,71 Reaction of Ti with nitrogen is reported to be slower than that with oxygen is,91 and hydrogen, t~~ It is also established by many authors ts,~ ~31 that only thermodynamically stable phases form, although there is some controversy on the stability of Ti2N. Inward diffusion of nitrogen is the dominant mechanism delineated in these studies, but evidence of outward diffusion of titanium for longer diffusion times is also observed. Reaction kinetics obey parabolic rate law. During ion nitriding of titanium, a thin TiN (6) layer forms at the surface and offers excellent wear and friction properties, t6m This layer is usually found to be followed by TizN (e), a titanium, and transformed /3 (if treatment temperature is above the/3 transus). The growth EROL METIN, formerly Graduate Student, New Mexico Institute of Mining and Technology, is currently enlisted in military service in his native Turkey. OSMAN T. INAL is Professor and Chairman of the Material and Metallurgical Engineering Department, New Mexico Institute of Mining and Technology, Socorro, NM 87801. Manuscript submitted February 29, 1988. METALLURGICAL TRANSACTIONS A
of these layers, in many cases, is parabolic withtime, indicating a volume diffusion-controlled process, with possibly faster nitriding kinetics in hydrogen-containing plasma. 04,~51 The ion energies are seen to have minor influence on the nitrogen surface concentrations [~6,~7,~s] and the growth of the nitride layers. [~4j In summary, the above studies indicate that nitriding of Ti, depending on the temperature employed, forms 6, e, a, and/3 layers. The nitriding process, therefore, can be considered identical to the binary diffusion couples which form several intermetallic phases. It is usually possible to predict the layer growth or vice versa by using the solutions to multilayer diffusional growth problems. The purpose of this paper is to present data from the study of growth kinetics of these layers under ion-nitriding conditions and to measure diffusion coefficients. II.
EXPERIMENTAL
A. Procedures
The chemical composition of pure titanium specimens obtained from the RMI Company, Niles, OH, in the fo
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