Xiaohu Chen

Surface modification of pure titanium by plasma tantalumising

Abstract In order to improve corrosion resistance of pure titanium in strong acid solution, the tantalum modified layer was formed on the pure titanium surface by double glow plasma surface tantalumising. The modification layer was comprised of deposition layer and diffusion layer, which metallurgically adhered to the substrate. Tantalum element decreased with the case depth. The corrosion resistance of the tantalum modified layer in HCl solution was investigated by weight loss method and electrochemical measurements, and the corrosion morphology (SEM) was studied by SEM and X-ray diffraction. The annual corrosion rate of the modified layer was only 12 and 21% of pure titanium in 10 and 20 wt-% HCl solution respectively, and the corrosion rate was still small in 30% HCl solution. In concentrated acid for 120 h, it was found that there was significant pitting corrosion on the surface of pure titanium, while no pitting corrosion could be perceived in the modified layer. The main reason for the good corrosion resistance was a compact, stable tantalum oxidation film formed at the surface of the modified layer.

Oxidation behaviour of plasma surface alloying on Ti6Al4V alloy

ABSTRACT Ti–CrNi alloy coatings with 20, 40, 60 and 80 at-%Ni were prepared on Ti6Al4V alloy surface by double glow plasma surface alloying technique. The cyclic oxidation behaviour of the coatings was investigated at 750, 850 and 950°C. The results show that each coating has a complete structure of settled layer, mutual diffusion layer and sputtering affected zone. When the Ni content was 40 at-%, the Ti–CrNi alloy coating showed the best cyclic oxidation resistance. The oxide scale with ‘Cr2O3 film+NiO film’ structure showed the best resistance to peeling.

Tribological behaviour of double-glow plasma zirconium-yttrium alloying on γ-TiAl

ABSTRACT A Zr–Y alloyed layer was prepared onto the surface of γ-TiAl using double-glow plasma surface alloying technique (DG technique). The results showed that the Zr–Y alloyed layer was comprised of deposited sub-layer and diffused sub-layer, with a total layer thickness of about 20 μm. The nano-indentation test result showed that the nano-hardness of Zr–Y alloyed layer was 2.35 times that of γ-TiAl. The scratch test was carried to evaluate the adhesion strength of Zr–Y alloyed layer. The tribological behaviours were investigated both at ambient temperature and 500°C. Under dry sliding condition, the friction coefficient of the Zr–Y alloyed layer was lower than that of the γ-TiAl. The specific wear rate of γ-TiAl in the presence of Zr–Y alloyed layer was approximately one fifth that in the absence of the alloyed layer. The excellent wear resistance of Zr–Y alloyed layer could be attributed to the high hardness or load-carrying ability.

Tribological Behavior of Aluminum Slurry Coating on 300M Steel

Al slurry coatings, an alternative of Cd coating to protect against corrosion in the aerospace industry, are currently being explored to satisfy the stringent technology and safety requirements for aeronautical applications. The results of salt fog corrosion exposure, fluid corrosion exposure (immersion), humidity resistance, electrical conductivity, galvanic corrosion, embrittlement, and fatigue indicate that Al slurry coatings can be used as an alternative of Cd coatings. However, the tribological property of Al slurry coatings has still not been investigated. Two types of aluminum slurry coatings on 300M steel were produced using the innovative technological process and characterized by scanning electron microscope, nanoindentation tests, and adhesion tests. The H/E and H3/E2 ratios of the two-layer Al slurry-coated sample were almost four times higher than the Cd-Ti plating-coated sample. The dry wear test results show that the friction coefficient of the two-layer Al slurry coating independently from the load was lower than the Cd-Ti plating. The width of the wear track of the two-layer Al slurry coating was significantly narrower, only 62% of the Cd-Ti plating wear scar. Compared to the Cd-Ti plating, an excellent wear resistance of the two-layer Al slurry coating can be attributed to its high resistance to plastic deformation and good load-bearing capacity. The results indicate that the two-layer Al slurry coating is an excellent alternative to Cd coatings in the aerospace industry.

Oxidation Behavior of TiAl-Based Alloy Modified by Double-Glow Plasma Surface Alloying with Cr–Mo

Abstract A Cr–Mo alloyed layer was prepared on a TiAl-based alloy using plasma surface alloying technique. The isothermal oxidation kinetics of the untreated and treated samples was examined at 850 °C. The microstructure and phase composition of the alloyed layer were analyzed by scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray powder diffraction (XRD). The morphology and constituent of the oxide scales were also analyzed. The results indicated that the oxidation resistance of TiAl was improved significantly after the alloying treatment. The oxide scale eventually became a mixture of Al2O3, Cr2O3 and TiO2. The oxide scale was dense and integrated throughout the oxidation process. The improvement was mainly owing to the enhancing of scale adhesion and the preferential oxidation of aluminum brought by the alloying effect for TiAl-based alloy.