Xuehui Zhang

Ultrafine cobalt powders are indispensable for high-end applications (cemented carbides, magnetic alloys, and new energy batteries) because of their unique properties; however, conventional methods have high costs, energy consumption, coarse sizes, agglomeration, and complex processes. This study used a one-step supercritical methanol hydrothermal method to synthesize ultrafine Co powder, investigating key parameters (reducing/chelating agents, concentrations, pH) and characterizing via SEM, XRD, FT-IR, XPS, TG/DTG. Under optimal conditions (glycerol 2 mol/L, PVP 0.2 mol/L, pH=10.82, precursor 0.05 mol/L), fcc/hcp Co powders with min average size 522±117 nm were obtained. PVP inhibited agglomeration via C=O-Co coordination, the powders had excellent antioxidation resistance (~300°C onset, stable up to 600°C).This study proposes an efficient and energy-saving method.

This study systematically explores and expands upon the research questions, revealing the scientific principles and engineering value of chromium–aluminum (Cr-Al) co-diffusion coatings in enhancing high-temperature friction performance. This study addresses the critical need for wear resistance in GH5188 cobalt-based alloy stator bushings operating in high-temperature environments. The high-temperature wear resistance mechanism of aluminized coatings modified with Cr elements on the GH5188 alloy, based on thermal diffusion technology, was investigated. The experimental results indicate that the high-temperature wear resistance of the samples was directly related to the type and content of oxides in the wear scars and debris. After friction at 700 °C, the aluminized coating on the GH5188 alloy showed the lowest oxide content in the wear scars, primarily composed of CoAl₂O₄. The oxides in the wear scars of the GH5188 alloy and Al-Cr co-aluminized coatings were mainly CoCr₂O₄ and Cr₂O₃, with the Al-Cr co-aluminized coating showing the highest amount of wear debris. The Cr-rich oxide debris not only has high thermodynamic stability but also exhibits relatively low high-temperature growth stress, making it difficult to spall. Additionally, the higher diffusion coefficient of Cr³⁺ accelerates the reoxidation of wear debris pits, resulting in excellent high-temperature wear resistance. The wear rate of the Al-Cr co-aluminized coating was reduced by 30% compared with the GH5188 substrate and by 69% compared with the aluminized coating. In summary, the key findings are not only applicable to cobalt-based alloys but can also be extended to a broader range of material systems and engineering applications. This provides new perspectives and methodologies for the design of high-temperature coatings, the development of materials for extreme conditions, and interdisciplinary applications.

In order to improve the poor comprehensive performance of aluminum bronze surface, a duplex surface modification technology consisting of laser cladding and followed by plasma nitriding was adopted to prepare the infiltration composite modified layer. The effects of different surface treatment techniques on the microstructure and properties of the modified layer were investigated. The results show that the laser cladding surface is mainly composed of Cu-rich intermetallic compounds, and the AlN layer appears on the surface after plasma nitriding treatment, which significantly improves the hardness and tribological properties. The surface modification treatment changes the surface morphology. The microhardness of the alloy surface after composite modification is much higher than that of untreated and single treatment. It is about six times that of the Cu-Al matrix. At the same time, the composite modified layer has a lower friction coefficient and wear volume, showing excellent tribological properties. These research results can provide a new way for the surface design and development of metal materials.

Modified surface layers with refined grain size and high microhardness characteristics were prepared on Cu-Ti alloy using ultrasonic shot peening (USP) by controlling the shot diameter. The phase structure and organization morphology of the modified layer were analyzed using x-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscope (SEM). The microhardness, wear resistance and corrosion resistance were systematically characterized. The results showed that the comprehensive surface properties of the sample was optimized at a shot diameter of 3.0 mm. The average grain size was refined to 12.4 nm under the high-frequency impact of steel balls. Owing to the grain refinement and work hardening, the surface microhardness, wear resistance and corrosion resistance of the sample was significantly improved. Among these, the surface microhardness of the sample was enhanced from 257.68 HV_0.1 to 313.51 HV_0.1, the volume wear rate was decreased from 0.3305 um² N⁻¹ to 0.1707 um² N⁻¹, and the self-corrosion current density was also decreased from 1.658 × e⁻⁵ mA μm⁻² to 4.053 × e⁻⁶ mA μm⁻². It is proved that USP is one of the effective methods for preparing Cu-Ti alloys with excellent surface properties.

Cu–Al alloy with a gradient nanostructured surface layer is prepared by ultrasonic shot peening (USP). The effect of different shot peening times on the microstructure and properties of the modified layer is systematically investigated. The results show that the surface phase structure is not changed by USP technology, but a gradient nanocrystalline layer is formed on the surface, and the surface grains are obviously refined. The nanocrystalline mechanism is characterized by dislocation movement and mechanical twinning. USP can greatly improve the mechanical properties, tribological properties, and corrosion properties of the alloy. When the shot peening time is 15 min, the modification effect is the best. The hardness is as high as 254.1 HV_0.2, which is 2.4 times that of the matrix, and the average coefficient of friction is only 0.231. The volume wear amount and volume wear rate are 4.434 × 10⁷ μm³ and 0.1848 μm² N⁻¹, respectively. The strengthening mechanism of nanosized samples is grain refinement and work hardening, and the wear mechanism is abrasive wear and slight oxidation wear. The improvement of the corrosion performance of nanosized samples is mainly due to the formation of the dense passivation film on the surface.

In this work, Ni-W-Ti₂AlC composite coatings with anti-wear and anti-corrosion abilities were prepared using direct current electrodeposition. The study primarily focuses on the influence of different current densities on the structure and properties of Ni-W-Ti₂AlC composite coatings. The microstructural, mechanical properties, and electrochemical performances were studied by using SEM, XRD, EDS, and Tafel polarization techniques. The results indicated that Ti₂AlC is successfully co-deposited into the Ni-W coating, and the average grain size of the Ni-W-Ti₂AlC composite coating is 15.2 nm. The significant improvement in hardness, wear resistance, and corrosion resistance of Ni-W-Ti₂AlC composite coating is attributed to the effects of solid solution strengthening and the uniform distribution of the microstructure. The suitable current density has a great influence on the properties. The microhardness of Ni-W-Ti₂AlC composite coatings was increased by 99.59% at a current density of 5 A dm⁻² compared to 3 A dm⁻². At the same time, the friction coefficient decreased by 13.89%.