| Titanium alloys are one of the most widely applied structural materials in aerospace, chemical, petrochemical and marine industries because of their unique property combinations of low density, high specific strength, exceptional corrosion resistance and high temperature mechanical properties. However, titanium alloys are restricted from industrial applications because of their poor tribological properties and poor high-temperature oxidation resistance, etc. Fabricating a novel wear-resistant and oxidation-resistant coating on TC4 using appropriate advanced surface engineering methods is one of the most economic and flexible means to enhance the tribological and oxidation properties for titanium alloys.Transition metal silicides alloys such as the Mo-Si, Cr-Si, Nb-Si binary and their multi-components alloys represent the largest family of intermetallic compounds having many attractive attributes for elevated temperature mechanical and environmental properties and hence, are widely recognized as a new generation of high- and/or ultrahigh-temperature candidate structural materials for the aerospace and energy-processing industries because of the combinations of high melting point, low density, high elastic modulus and excellent high-temperature oxidation and creep resistance. Unfortunately, because of metal silicides suffer from serious room temperature brittleness which has hindered their use as structural materials.A series of nanocrystalline (MoxCr1-x)5Si3 films with the average grain size of 8nm have been prepared on Ti-6A1-4V alloy substrates by glow discharge. Phase compositions of the as-deposited films were characterized with X-ray diffractometry. The cross-sectional morphology and chemical composition of the as-deposited films were studied by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), respectively. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were observed by JEOL JEM-2010. The films are devoid of pores and micro-cracks, and appears compact and quite well adhesive to the Ti–6Al–4V alloy substrate. The results revealed that with the increasing amount of Cr addition the hardness and elastic modulus of nanocrystalline (MoxCr1-x)5Si3 (x=1.00, 0.78,0.75,0.64,0.57) film slightly decreased, whereas the ratio between H and E of five kinds (MoxCr1-x)5Si3 films increase , the plastic deformability significantly increased. During cyclic oxidation test at 800°C in air, the unalloyed Mo5Si3 film shows a pest reaction immediately after a few minutes of oxidation in air. On the contrary, spallation resistance to thermal cycling for nanocrystalline Cr-alloyed Mo5Si3 films are significantly improved with increasing Cr-addition, and particularly for (Mo0.57Cr0.43)5Si3 film, no sign of spallation or microcracking occurred after exposure at 800°C in ambient air for 240 h, because of the formation of a fine-grain protective scale. During wear test at room-temperature and 600°C, respectively. The results indicate that the friction coefficient increases with the increase of load between 3.3 N and 4.8 N. Meantime, the steady-state friction coefficient and wear rate of nanocrystalline (MoxCr1-x)5Si3 films decreases linearly with the increase of content of Cr in the films. The wear track of (MoxCr1-x)5Si3 films become smooth and flat with the increase Cr-addition. The wear resistance of nanocrystalline (MoxCr1-x)5Si3 films evidently improve with the increase of content of Cr in the films. |
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