High temperature gas nitridation and wear resistance of TiAl based alloys
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High temperature gas nitridation and wear resistance of TiAl based alloys Bin Zhao, Jian Sun, Jiansheng Wu and Fei Wang Key Laboratory of the Ministry of Education for High Temperature Materials and Tests School of Materials Science and Engineering, Shanghai Jiao Tong Univ., Shanghai 200030, P. R. China ABSTRACT Gas nitridation of TiAl based alloys in an ammonia atmosphere was carried out in the present work. The nitride layers were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The evaluation of the surface hardness and wear resistance was performed to compare with those of the non-nitrided alloys. It is concluded that the nitride layers are composed of Ti2AlN as the inward-growing layer and TiN as the outward-growing layer. The nitridation temperature and time were two major factors influencing the thickness of the nitride layers of the alloys. The high temperature nitridation raised the surface hardness and the wear resistance of the TiAl based alloys markedly. The tribological behaviors of the nitrided alloys were also discussed. INTRODUCTION It is well known that the intermetallic compound TiAl has attractive comprehensive properties, especially high-temperature properties, such as high modulus, elevated temperature strength, good oxidation resistance and low density, which are particularly useful for applications in aircraft and automobile structures [1,2]. Therefore, in recent years more and more attention has been paid to the development of the engineering intermetallic alloys TiAl [3,4]. However, the TiAl alloy has two major drawbacks: very limited ductility at room temperature and poor surface properties at high temperatures. The former has been gradually enhanced by modification of the microstructure using ternary element addition [5] and thermo-mechanical processing, or powder metallurgy [6,7]. The latter can be improved to some extent by surface treatments. As we known, tribological problems are always involved in the structural materials, especially in case of a turbine engine or a turbocharger rotor. Some tribological events, such as adhesive wear, erosive wear, oxidation wear and fretting wear, could result in loss or damage of the parts. Therefore, the surface improvement of the intermetallic compounds by various surface modification methods [8-10] is being performed. Due to its unique physical, mechanical and metallurgical properties, a thin titanium nitride (TiN) is effective in reducing tool wear so that the lifetime and cutting speed are considerably increased [11,12]. Because TiN is more thermodynamically stable than AlN, TiN layers can be formed on TiAl by direct gas nitridation in different atmospheres [13,14], implantation with N [10] and by ion nitridation [15]. Coatings of TiN applied by chemical vapor deposition (CVD) or physical vapor deposition (PVD) are often applied, but among various methods the direct gas nitridation is a less costly process. In the present work, high-temperature gas nitridation behaviors of TiAl based alloys in ammonia are investig
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