Preparation And Properties Of Plasma Zr-alloyed And Zr-N Composite Layers On TC4Alloy Surface

TC4titanium alloy as an important engineering material has been widely used in the aerospace, automotive, chemical and biomedical industries due to its low density, excellent strength to weight ratio, high corrosion resistance and biocompatibility, etc. But the deficient tribological property of TC4and its poor corrosion resistance in reducing medium such as sulfuric acid limit the application in engine valves, turbochargers and corrosion environments.Using double glow plasma alloying technique, TC4substrate was alloyed to form a Zr-alloyed layer in TC4surface, and then treated by plasma nitriding to make a Zr-N composite layer. Both plasma-treated TC4exhibit an improved wear and corrosion resistances. Effects of alloying temperature on microstructure, phase structure and hardness of Zr and Zr-N treated TC4were evaluated, and the formation mechanisms for Zr and Zr-N alloyed layers were studied. The tribological property and electrochemical behavior of TC4after Zr alloying and Zr-N duplex treatments as well as the mechanisms of wear and corrosion resistances were investigated in this paper. The experimental results show as follows.(1) A compact Zr-alloyed layer can be fabricated on TC4surface by using the double glow plasma surface alloying technique. The thickness of Zr-alloyed layer increases from40μm to80μm when the alloying temperature increases from800℃to900℃. The alloyed layer, bonding strongly to TC4substrate with the diffusion interface, mainly consists of a (Ti、Zr) and (3(Ti, Zr). With alloying temperature increasing, a phase increases, but (3phase decreases, and the surface roughness also increases after Zr alloying. The micro-hardness of the Zr-alloyed layer is higher compared with untreated TC4and shows a gradient distribution along the alloyed depth similar to the composition distribution.(2) A Zr-N composite layer is formed in alloyed TC4surface after nitriding at850℃, which containing4μm thick N-diffusion layer and70μm thick Zr-diffusion layer and is mainly composed of ZrN, a small amount of TiN0.3and A1N. The micro-hardness of the Zr-N alloyed layer increases significantly and decreases gradually along the alloyed depth.(3) Under the loads of2N and5N, the average values of friction coefficient after Zr alloying are about0.2and0.15, respectively, which are lower than those of untreated TC4(0.45and0.32). And the wear rates are48%and56%of untreated TC4, respectively, showing an improved combination of antifriction and wear resistance. When the load reaches10N, the alloyed layer is worn out and the coefficient friction of Zr-alloyed TC4is about0.55, but the wear rate is55%of untreated TC4, which means it plays some part in wear resistance. Solid solution strengthening of Zr in TC4after plasma Zr alloying is the main reason of the improved tribological property. In addition, the gradient distribution of hardness in the alloyed layer with a strongly supporting to load is also helpful to increasing wear resistance. (4) Both friction coefficient and wear rate after Zr-N duplex treatment decrease compared with untreated TC4, showing an excellent antifriction and wear resistance. The friction coefficients of Zr-N treated TG4at different loads of2N、5N、10N are0.10,0.13and0.25, respectively, and lower than those of TC4substrate (0.45,0.3,0.3). The wear rate is7%,1%and9%of the untreated substrate. The dispersion strengthening of nitrides plays a very important role in improving the tribological property, besides solid solution strengthening of Zr and supporting of gradient hardness.(5) The corrosion resistance in10%H2SO4solution for Zr-alloyed TC4formed at different temperatures is improved obviously and the sample alloyed at900℃shows the best performance. The corrosion potentials of Zr-alloyed TC4at800℃,850℃and900℃are about3.16×10-2μA.cm-2,3.10×10-2μA.cm-2and3.02×10-2μA.cm-2, respectively, lowing by an order of magnitude than that of untreated TC4(3.50×10-1μA.cm-2). Passive current density of Zr-alloyed samples obtained at three temperatures,6.27μA.cm-2,6.33μA.cm-2and3.27μA.cm-2, decrease compared with untreated substrate (8.90μA.cm-2). The impedance values for all Zr-alloyed samples are higher than that of the untreated TC4. The excellent corrosion resistance for Zr-alloyed TC4is attributed to the formation of a dense inert ZrO2film when Zr exposed to air. The best property after Zr alloying at900℃is due to the higher concentration of Zr and more a phase in the alloyed surface treated at this temperature.(6) The corrosion resistance is further improved after Zr-N duplex treated compared with only Zr alloying. By contrast to the Zr-alloyed TC4, the corrosion potential of the Zr-N treated TC4(266mV) shifts markedly to positive direction and the passive current density (0.63μA.cm-2) decreases significantly. The impedance values of the Zr-N treated TC4are also greater than that of zirconium alloyed TC4.