Zhanji Geng

In traditional cemented carbides, Co is mainly used as a binder. Recently, replacing Co with medium- to high-entropy alloys has shown significant improvements in hardness, fracture toughness, high-temperature oxidation resistance, and corrosion resistance, making it a research focus globally. Both the typical grain refiner Cr₃C₂ and medium- to high-entropy alloy binders affect the WC grain size in cemented carbides. This study investigates the synergistic grain refinement mechanism of Cr₃C₂ and medium- to high-entropy alloy binders on WC grains and their impact on the microstructure and mechanical properties of cemented carbides. The results show that increasing the Cr₃C₂ addition refined WC grains in the WC-CoNiFe alloy, increased coercivity, and enhanced hardness, with transverse rupture strength first increasing and then decreasing. The alloy achieved optimal performance at 0.6 wt.%Cr₃C₂, with a hardness of 91.25 HRA and transverse rupture strength of 3883.2 MPa.

To enhance the oxidation resistance of Mo-based TZM alloy (Mo-0.5Ti-0.1Zr-0.02C, wt%), a novel MoSi₂-ZrB₂ composite coating was applied on the TZM substrate by a two-step process comprising the in situ reaction of Mo, Zr, and B₄C to form a ZrB₂-MoB pre-layer followed by pack siliconizing. The as-packed coating was composed of a multi-layer structure, consisting of a MoB diffusion layer, an MoSi₂-ZrB₂ inner layer, and an outer layer of mixture of MoSi₂ and Al₂O₃. The composite coating could provide excellent oxidation-resistant protection for the TZM alloy at 1600 °C. The oxidation kinetic curve of the composite coating followed the parabolic rule, and the weight gain of the coated sample after 20 h of oxidation at 1600 °C was only 5.24 mg/cm². During oxidation, a dense and continuous SiO₂-baed oxide scale embedded with ZrO₂ and ZrSiO₄ particles showing high thermal stability and low oxygen permeability could be formed on the surface of the coating by oxidation of MoSi₂ and ZrB₂, which could hinder the inward diffusion of oxygen at high temperatures. Concurrently, the MoB inner diffusion layer played an important role in hindering the diffusion of Si inward with regard to the TZM alloy and could retard the degradation of MoSi₂, which could also improve the long life of the coating.

Cr₃C₂-modified NiCr–TiC composite coatings were prepared using the plasma spraying technique for different Cr₃C₂ contents on the microstructure and the properties of the Ni-based TiC cladding layer were investigated. The microstructures of the coatings were characterized using scanning electron microscopy, and the friction and wear performance of the coating was evaluated by the wear tests. The results revealed that the surfaces of the Cr₃C₂-modified NiCr–TiC composite coatings with varying Cr₃C₂ contents were dense and smooth. TiC was uniformly distributed throughout the entire coating, forming a gradient interface between the binder phase of the Ni-based alloy and the hard phase of TiC. At high temperatures, Cr₃C₂ decomposes, with some chromium diffusing and forming complex carbides around TiC, some chromium solubilizes with Fe, Ni, and other elements. An increase in chromium carbide content leads to an upward trend in hardness. The measured hardness of the coatings ranged from 600 to 850 HV3 and tended to increase with increasing Cr₃C₂ content. When the mass fraction of Cr₃C₂ reached 30%, the hardness increased to 850 HV3, and the cracks and defects were observed in the coating, resulting in a wear resistance decline.

To improve the oxidation resistance of Mo-based alloys, a novel (Mo,Hf)Si₂-Al₂O₃ composite coating was fabricated on a Mo-based alloy by the method of slurry sintering. The isothermal oxidation behavior of the coating was evaluated at 1400 °C. The microstructure evolution and phase composition of the coating before and after oxidation exposure were characterized. The anti-oxidant mechanism for the good performance of the composite coating during high-temperature oxidation was discussed. The coating had a double-layer structure consisting of a MoSi₂ inner layer and a (Mo,Hf)Si₂-Al₂O₃ outer composite layer. The composite coating could offer more than 40 h of oxidation-resistant protection at 1400 °C for the Mo-based alloy, and the final weight gain rate was only 6.03 mg/cm² after oxidation. A SiO₂-based oxide scale embedded with Al₂O₃, HfO₂, mullite, and HfSiO₄ was formed on the surface of the composite coating during oxidation. The composite oxide scale exhibited high thermal stability, low oxygen permeability, and enhanced thermal mismatch between oxide and coating layers, thus improving the oxidation resistance of the coating.