Research On Plasma Surface Alloying And The Tribological Behavior Of Ti₂AlNb-based Alloys

Ti-Al intermetallic compounds have high specific strength, high specific modulus, excellent oxidation resistance, high temperature strength and so on. Recently, they have been regarded as the high temperature materials in aeronautics, astronautics, war industry and otherwise. However, the low hardness and poor wear resistance of Ti-Al intermetallic compounds restrict their practical application. The improvement of the tribological performance has become one of crucial project problems.The paper aims at solving the wear-resistant problem of elevated temperature motion components. The high hardness and wear-resistant alloying layers are prepared on Ti₂lNb O-phase alloys using an advanced technology-the double glow plasma surface alloying technique.The research firstly applied the"Empirical Electron Theory of Solid and Molecule"to calculate the molecule structure when the elements added to substrate. The results of analysis showed that the choice of Cr and W can be beneficial to improve the tribological properties of Ti₂lNb. Subsequently, the optimizing experiment of the plasma chromising, tungstening and the duplex treatment (carburization following plasma alloying) were performed using the double glow plasma surface alloying technique. Optical metallography (OM), scanning electron microscope (SEM), energy diffraction spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) were performed to study the phase formation and microstructure. Micro-hardness, nano-indentation and tribological properties were tested and analyzed for different alloying layers, respectively.The optimizing results indicated that the effective thickness of the plasma chromized layer was about 25μm. The surface chromium concentration reached 75%, and the content of Cr decrease in gradient with the increase of the alloying depth. Cr₂Nb, Al₈Cr₅ or Al₉Cr₄ were the major phases of the chromized layer; the thickness of the plasma tungstenzed layer was about 25μm. The surface tungsten concentration reached 55%, and the content of W decreased in gradient with the increase of the alloying depth. Ti_xW_1-x compound was the major phases of the tungstenzed layer; Cr-C duplex-treated layer mainly consisted of Cr₂₃C₆, Cr2Nb, and a few of carbon and Cr; W-C duplex-treated layer is primarily composed of W₂C or W₆C_2.54.The micro-hardness of plasma chromising and tungstening got to above 1100 HV, and the micro-hardness of duplex-treated layer reached 1650 HV. The four layers had higher hardness than 400 HV of the substrate. The results of nano-indenter showed that the elastic modulus of different layers was much higher than that of the substrate. The maximum elastic modulus of the tungstened layer reached about 700 GPa, that of other three layers distributed between 200 GPa to 500 GPa,while that of the substrate was only 100 GPa.Tribological test indicated that the double glow plasma chromising, tungstening and plasma duplex treatment all improved the wear resistance of Ti₂AlNbO-phase alloy. The friction coefficient of plasma chromising on Ti₂AlNb alloy decreased from 1.2 to 0.6 at the room temperature, while at the high temperature it decreased from 1.0 to 0.5. The friction coefficient at the room/high temperature reduced 0.2 below. At the room temperature, the wear rate of plasma chromising was about 10 times lower than that of the substrate, while at the high temperature, it reduced 60%. The wear rate of the duplex treatment lowered tenth to twenty-fifth of the substrate.At the room temperature, the friction coefficient of plasma tungstening decreased from 1.2 to 0.5, while at the high temperature it decreased from 0.5 to 0.15. However, the friction coefficient at the high temperature gradually ascending for long friction, while that of the duplex treatment can maintain about 0.15. At room temperature, the wear rate of plasma tungstening decreased tenth of that of substrate, while at the high temperature, it was only fifth of substrate. The wear rate of the duplex treatment was about 100 times lower than that of the substrate. At the high temperature, the lowest friction coefficient and wear rate was obtained using the carburization following plasma tungstening.The above experimental results confirmed that Cr and W as the alloying elements helped to improve of tribological properties of Ti₂AlNb alloy through the"Empirical Electron Theory of Solid and Molecule"forecasting.Finally, the vacancy mechanisms produced by the bombardment and sputtering under the double glow plasma alloying were studied. The diffusion model was built up for the alloying process based on the radiation enhanced diffusion theory.