Dongbo Wei

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.

Effects of elements W and C on microstructure and wear property of γ-TiAl surface alloying layer

To solve the poor tribological property of the γ-TiAl alloy, the duplex treatment of plasma carburisation on plasma alloying γ-TiAl with tungsten was conducted by double glow plasma surface alloying technique. The results indicated the alloying layer consisted of TiC and W2C phase, metallurgically adhered to the substrate. In terms of the H/E and H3/E2 ratios, the duplex treated specimen was nearly three times larger than the untreated specimen. The wear property of the specimens was tested on a pin-on-disk tribometer by rubbing against Si3N4 ceramic ball. The wear rate of the duplex-treated specimens is 1·056×10−4 mm3 N−1 m−1, which is only 25·9% of that of the untreated specimens. The relationship between alloying layer mechanical property and wear resistance indicates that the carbide phase formed within the alloying layer contributes to the improvement of the wear performance.

High temperature oxidation behaviour of Al2O3/Al composite coating on γ-TiAl

Abstract An Al2O3/Al composite coating was introduced on the substrate γ-TiAl alloy by magnetron sputtering. The isothermal oxidation behaviour of the coated γ-TiAl alloy was investigated at 1000°C. The results suggested that the Al2O3/Al composite coating improved the oxidation resistance of γ-TiAl alloy at 1000°C air exposure. No spallation or crack was observed in the oxide scale of coated specimen, which was composed of α-Al2O3 and TiAl3 after the oxidation test. The outward Al diffusion from interlayer provided sufficient Al source for the formation of Al2O3 layer in the coating surface during the high temperature oxidation test. As an active intermetallic element with Ti, Al suppressed the outward of Ti effectively due to the formation of Ti–Al intermetallics. Owing to inward and outward diffusion, the Al interlayer was consumed after 100 h oxidation test and a Ti–Al interdiffusion zone was formed, which was beneficial for the adhesion between coating and substrate.

Tribological Properties of Double-Glow Plasma Surface Niobizing on Low-Carbon Steel

The niobized layer was formed on Q235 low-carbon steel by double-glow plasma surface niobizing to improve its wear resistance. The microstructure, phase composition, and microhardness were determined. The friction and wear properties of the niobized samples and the untreated alloys were tested on a ball-on-disk tribometer by rubbing against GCr15 and silicon nitride (Si3N4) balls at room temperature and 400°C, respectively. The results indicated that the alloyed layer that contained a sediment layer and diffusion layer is about 35 μm in thickness, metallurgically adhered to the base metal. Niobium content was gradually decreased along the depth direction from the surface, which was similar to the change in the microhardness. The alloying layer mainly consisted of Nb, Fe2Nb, and FeNb phases. Under unlubricated sliding conditions, the friction coefficients and the specific wear rates were lower than those of the untreated carbon steel at room and high temperatures. The wear mechanism of the niobized specimen at room temperature is dominated by slightly abrasive wear, whereas the predominant wear mechanism is abrasive wear and fatigue delamination at high temperature.

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.

Mechanical and tribological properties of Cr–Nb double-glow plasma coatings deposited on Ti–Al alloy

Abstract Double-glow plasma (DGP) coatings are recommended for metallic components to mitigate the damage induced by complex working conditions in previous studies. In this paper, Nb-rich (Cr–Nb4) and Cr-rich (Cr4–Nb) -alloyed layers were formed onto the Ti–Al substrate via a DGP process to enhance its wear resistance. Scratch and Nano-indentation tests were used to evaluate the mechanical properties of the coatings. The tribological behaviour of the coatings were investigated using a pin-on-disc tribometer by rubbing against the GCr15 ball. Results from surface analysis techniques showed that the coatings mainly comprised Cr, Nb and Cr2–Nb phases, and were well bonded to the substrate. The hardness of the Cr–Nb4 coating was 11.61GPa and the Cr4–Nb coating was 9.66 GPa which all higher than that of the uncoated Ti–Al which was 5.65 GPa. However, the critical load of the Cr4–Nb coating ~21.64 was higher than that of the Cr–Nb4 coating ~17.6. And the specific wear rate of Cr–Nb4 coating, Cr4–Nb coating and uncoated Ti–Al were 3.54 × 10−4, 0.01 × 10−4 and 1.53 × 10−4mm3 N−1 m−1, respectively. The low-wear mechanism of the coatings is discussed in detail in this paper.

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.

Fretting wear behaviour of double-glow plasma Cr–Nb coating on γ-TiAl alloy

Double-glow plasma coatings are recommended for metallic components to mitigate the damage induced by complex loading conditions. In this paper, Cr–Nb alloyed layer was formed onto the TiAl substrate via a double-glow plasma process to enhance its anti-fretting wear performance. Nano-indentation and scratch tests were used to evaluate the mechanical properties of the coating. The fretting wear behaviour of the coating was investigated using a pin-on-plate fretting rig by rubbing against the Si3N4 ball. The 7 µm thick Cr–Nb coating was well bonded to the substrate with the 2 µm thick diffusion layer. The hardness of the coating was 9.5 GPa, which was 1.6 times greater than that of the uncoated TiAl substrate. Scratch tests showed that the critical load of Cr–Nb coating was 17.6 N. The fretting wear mechanism of the coatings was discussed in detail in this paper.

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.