Surface hardening of nickel alloys by means of plasma nitriding
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INTRODUCTION
NICKEL and its alloys are widely used as industrial materials, but an industrially available surface hardening process to enhance wear resistance for these materials has not yet been established. In general, there are two types of surface hardening processes, one is a diffusion process such as carburizing, nitriding, or boronizing for iron and steels, and the other is a coating process such as chemical vapor deposition (CVD), physical vapor deposition (PVD), or plating. Among these diffusion processes, boronizing is the only method applicable to surface hardening of Ni and Ni alloys.[1] However, boronizing treatment has not been established as an industrial surface hardening process as have carburizing and nitriding. On the other hand, hard, ceramics coating such as TiN by CVD and PVD is applicable to any metal and alloy, but it is much more costly in comparison with the diffusion process. On the contrary, the plasma nitriding process, assisted by a glow discharge, is one of the nitriding processes widely used in an industrial surface hardening process due to its beneficial features such as energy and labor savings, good reproducibility of the property of nitrided layer, and no requirement of antipollution equipment.[2] Therefore, the plasma nitriding process is applied not only to iron and steels but also to nonferrous metals such as titanium, zirconium, etc.[3] However, the nitriding treatment has not been applied to Ni, because metals such as Ni, Co, Cu, Au, and Ag, which do not have the solubility of nitrogen and no stable nitride, cannot be hardened by nitriding treatment. It is well known that the typical nitride forming elements of IVa, Va, and VIa families in the periodic table are easily TAKASHI MAKISHI, Associate Professor, is with the Department of Mechanical Systems Engineering, University of Ryukyus, Okinawa 903-0213, Japan. Contact e-mail: [email protected] KAZUHIRO NAKATA, Professor, is with the Joining and Welding Research Institute, Osaka University, Osaka 567-0047, Japan. Manuscript submitted September 24, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
hardened by nitriding treatment. This suggests when some nitriding forming elements are alloyed to pure nickel, surface hardening of Ni alloy by nitriding may become possible. At the nitriding process, the nitrogen diffuses into Ni alloy from its surface and reacts with alloying elements to make their nitrides, and this will form the hardened layer on the surface of Ni alloy. Therefore, in this article, the probability of surface hardening of Ni alloy by means of the plasma nitriding process has been investigated. The individual effect of nitride forming elements on surface hardening of Ni alloys has been investigated by using tentative Ni binary alloys, and a beneficial alloying element and its effective content for surface hardening of Ni alloy have been examined. Moreover, the surface hardening mechanism of nitrided Ni alloys has been discussed mainly using transmission electron microscopy (TEM), X-ray diffraction (X
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