Pingze Zhang

Investigation of W–Mo alloyed layer synthesised by double glow plasma surface metallurgy

Abstract In the present study, the W–Mo alloyed layer was deposited on surface of Ti–6Al–4V. Microhardness and nanoindentation testing was used to evaluate mechanical properties of the alloyed layer. The tribological behaviour of W–Mo alloyed layer was estimated using a pin on disc tribometer. The friction coefficient and the wear mass were measured. Results showed that the tungsten and molybdenum content in the alloyed layer decreased gradually from surface to substrate, while the microhardness decreased gradually as the depth increased. The W–Mo alloyed layer had a smaller penetration depth and larger elastic modulus than the substrate. The friction coefficient of the W–Mo alloyed layer was lower than the substrate. The wear mass of W–Mo alloyed layer was one‐third lower than that of the substrate. It can be attributed to the higher hardness of the W–Mo alloyed layer surface which limited plastic deformation of the friction surface and decreased wear mass.

High-temperature tribological behaviors of Ti2AlNb-based alloys by plasma surface duplex treatment

Abstract Plasma tungstening followed by carburization (W-C duplex treatment) was performed on the Ti2AlNb-based (O phase) alloy by using the double glow plasma process to enhance its wear resistance. The microstructure and high-temperature tribological behaviors of the un-treated and W-C duplex-treated samples were investigated. The results show that the duplex-treated layer is mainly composed of W2C or W6C2.54 phases and the contents of W and C elements in the alloyed layer change gradually along the depth by surface plasma duplex treatment. The diffusion depth of W is about 12 μm, while the carbon atoms most exist in the depth more than 12 μm. High temperature tribometer tests indicate that the friction coefficient of the W-C duplex-treated layer is approximately 1/6 that of substrate. The wear rate of the duplex-treated layer is about 28% that of the untreated one. So, plasma surface W-C duplex treatment can obviously improve the high-temperature tribological resistance of Ti2AlNb-based alloy. The tribological mechanism of the duplex-treated layer is discussed by dividing the friction process of the duplex-treated layer into three fluctuate stages. The first stage is the formation of oxide film between W-C duplex-treated layer and counterface. The second stage is the detachment of oxide film, acting as “the third body”. The last stage is the period that the friction and wear occur between the compact particle layer and counterface.

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 behaviors of Fe–Al–Cr–Nb alloyed layer deposited on 45 steel via double glow plasma surface metallurgy technique

Abstract Double glow plasma surface metallurgy technique was used to fabricate a Fe–Al–Cr–Nb alloyed layer onto the surface of the 45 steel. The microstructures and composition of the Fe–Al–Cr–Nb alloyed layer were analyzed by scanning electronic microscopy, X-ray diffraction and energy dispersive spectroscopy. The results indicate that the 20 μm alloyed layer is homogeneous and compact. The alloyed elements exhibit a gradient distribution along the cross section. Microhardness and nanoindentation tests imply that the surface hardness of the alloyed layer reaches HV 580, which is almost 2.8 times that of the substrate. Compared with the substrate, the alloyed layer has a much smaller displacement and a larger elastic modulus. According to the friction and wear tests at room temperature, the Fe–Al–Cr–Nb alloyed layer has lower friction coefficient and less wear mass, implying that the Fe–Al–Cr–Nb alloyed layer can effectively improve the surface hardness and wear resistance of the 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.

Improving corrosion resistance of Q235 steel by Ni-Cr alloyed layer

Ni-Cr alloyed layer was formed on surface of Q235 steel by double glow plasma surface metallurgy to improve the corrosion resistance of substrate. The composition and microstructure of alloyed layer was analyzed by SEM and XRD. Potentiodynamic polarization and electrochemical impedance spectroscopy was applied to evaluate the corrosion resistance of the alloyed layer. The results showed working pressure had a great effect on structure of Ni-Cr alloyed layer, and the dense and smooth alloyed layer was prepared at 50 Pa working pressure. Compared with substrate, Ni-Cr alloyed layer exhibited higher corrosion potential, lower corrosion current density and larger charge transfer resistance, which indicated that Ni-Cr alloyed layer significantly modified the corrosion resistance of Q235 steel.

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 BEHAVIOR OF Al–Cr COATING OBTAINED BY DGPSM AND IIP COMPOSITE TECHNOLOGY

An Al–Cr composite alloyed layer composed of an Al enriched layer, a Cr enriched layer and a transition layer from the surface to the bulk along the cross-section was deposited on a 45# steel substrate by composite technology, where Cr was deposited using double glow plasma surface metallurgy (DGPSM), and Al was then implanted by ion implantation (IIP) to achieve higher micro-hardness and excellent abrasive resistance. The composite alloyed layer is approximately 5μm, and as metallurgical adherence to the substrate. The phases are Al8Cr5, Fe2AlCr, Cr23C6, Cr (Al) and Fe (Cr, Al) solid solution. The wear resistance tests were performed under various rotational speed (i.e. 280, 560 and 840r/min) with silicon nitride balls as the counterface material at ambient temperature. The Al–Cr composite alloyed layer exhibits excellent wear resistance when the speed is 280r/min with a friction coefficient as low as 0.3, which is attributed to Al8Cr5 in the Al implanted layer that withstands abrasive wear. Better wear resistance (friction coefficient: 0.254) at 560r/min is resulted from the formation of a high micro-hardness zone, and an oxidation layer with lubrication capacity. In addition, the composite alloyed layer suffers severe oxidative wear and adhesive wear at 840r/min due to the increment of the frictional heating. When compared to the 45# steel substrate, the enhanced wear resistance of the Al–Cr composite alloyed layer demonstrates the viable method developed in this work.

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.

Effect of plasma surface tungstenising on the friction and wear of Ti2AlNb-based alloys

ABSTRACT To obtain a strong bond between W coatings and the substrate, a novel graded tungstenised layer on Ti-Al-Nb alloys was produced using a double glow plasma surface alloying technology and a special graded tribological coating was designed. The microstructural results showed that the tungstenised layer was distributed in a graded manner and was mainly comprised of W- or Ti-rich TixW1−x phases. Varying the friction conditions indicated that an increase in the load and sliding speed led to an increase of the friction coefficient and wear rate of the tungstenised layer at room temperature. These changes were mainly caused by the graded distribution of the W composition and the change in surface contact status. The results indicated that the friction and wear properties of Ti-Al-Nb alloys were greatly improved by the surface tungstenising.