ZHOU Yinghan ¹, YAN Jianhui ^{1, 2}

(1. Materials Science and Engineering, Hunan University of Science and Technology, Xiangtan 411201, Hunan, China;

2. Hunan Provincial Key Defense Laboratory of High Temperature Wear Resisting Materials and Preparation

Technology, Hunan University of Science and Technology, Xiangtan 411201, Hunan, China)

Extended abstract:

[Background and purposes] High-entropy carbide ceramics exhibit higher hardness, superior high-temperature stability and wear resistance, as compared with the traditional single-component carbides, demonstrating promising application potential in aerospace thermal protection systems and ultra-high-speed cutting tools. However, like most ceramic materials, high-entropy carbides still face inherent brittleness issues, while achieving full densification typically requires sintering temperatures of above 2000 ℃, which limits their application under various working conditions. Therefore, developing preparation routes with reduced sintering temperature, while simultaneously improving material toughness, is of significant importance. In recent years, the rise of high-entropy alloys has prompted researchers to explore their feasibility as binder phases in cemented carbides or cermets. It has been shown that CoCrFeNiAl and CoCrFeNiMn high-entropy alloy systems possess competitive comprehensive properties, as compared with their traditional counterparts. Nevertheless, the CoCrFeNiMo high-entropy alloy system, which combines good plasticity with strengthening effects as a toughening binder, has been rarely studied. With this background, to achieve high-entropy carbide ceramics with both high hardness and high toughness, (Ti_0.2V_0.2Nb_0.2Ta_0.2W_0.2)C/CoCrFeNiMo_x composite ceramics were prepared. The effects of the content of CoCrFeNiMo on microstructure, hardness and fracture toughness of the composite ceramics, were systematically evaluated, while the mechanism of the enhancement of overall mechanical properties, particularly fracture toughness was examined.

[Methods] The initial powders were prepared via the carbothermal reduction method, using carbon black, TiO₂, Ta₂O₅, Nb₂O₅, V₂O₅ and WO₃ as raw materials, with a stoichiometric ratio corresponding to equiatomic amounts of each metal. The powder was subjected to carbothermal reduction at 1400 ℃ for 1 h, at a heating rate of 10 ℃·min⁻¹ and pressure of 40 MPa. The carburized powder was subsequently mixed with CoCrFeNiMo high-entropy alloy powder. The mixed composite powder was loaded into a graphite mold and sintered using a ZT-40-21Y vacuum hot-pressing sintering furnace. Phase composition of the samples was characterized by using a German-made Bruker D8 Advance X-ray diffractometer (XRD), over scanning range of 10° to 90°, at a speed of 5 (°)·min⁻¹. Microstructure was observed by using a Tescan Mira4 field-emission scanning electron microscope (SEM), while the composition analysis was carried out in conjunction with an Oxford Xplore30. Vickers hardness and fracture toughness were measured via the indentation method using a Vickers hardness tester (SHYCHVT-30Z).

[Results] The samples after hot-pressing sintering exhibited the main peak of (Ti, W, Ta, Nb, V)C. With increasing content of HEA, pronounced FCC diffraction peaks appear at 43.6°, 51.5° and 76.0°, which belong to the CoCrFeNiMo high-entropy alloy phase. Additionally, secondary phase TiO₂ can be observed in all samples. As the HEA content increases, the fracture toughness shows an initial increase, followed by a decrease, as the sintering temperature rises. The sample with 5 wt.% HEA exhibited the lowest toughness [(6.65±0.31) MPa·m^1/2]. The sample with 10 wt.% HEA had the maximum toughness of (7.49±0.28) MPa·m^1/2. The high-entropy carbide ceramics without toughening measures, the fracture toughness is increased by about 1.97 times. Despite only partial residual FeCoNi present in the sample, the toughness is significantly enhanced, due to the beneficial contribution of FeCoNi's ductility. However, the sample with 15 wt.% HEA shows a fracture toughness of 7.03 MPa·m^1/2. This is because the substantial content of HEA promotes the carbothermal reduction during sintering, leading to a reduction in content of the Ti(O_0.4, C_0.5) solid solution and ultimately decreasing the fracture toughness. From the images of crack propagation, crack bridging and deflection can be observed in the samples. Crack deflection prolongs the crack propagation path, thereby increasing resistance to crack advancement. Additionally, crack bridging and branching alter the crack propagation path, which greatly dissipates the energy of crack propagation and serves as an important toughening mechanism. When a crack encounters Ti(O_0.4, C_0.5) particles and the binder phase, it has two possible growth paths. One is to bypass the Ti(O_0.4, C_0.5) particles and binder phase, while the other is to pass through them. Both growth paths result in a toughening effect by extending the propagation path and increasing energy dissipation.

[Conclusions] (Ti_0.2V_0.2Nb_0.2Ta_0.2W_0.2)C/CoCrFeNiMo_x composite ceramics were prepared via carbothermal reduction for initial powder synthesis and subsequent hot-pressing sintering. Dense (Ti_0.2V_0.2Nb_0.2Ta_0.2W_0.2)C/CoCrFeNiMo_x composite ceramics were successfully obtained at 1400 ℃. During the sintering process, the addition of CoCrFeNiMo promoted the progress of carbothermal reduction, while the secondary phases gradually diminished with increasing content of HEA. Due to the extrusion of some FeCoNi phases, only a small fraction of FeCoNi remained in the sintered product. Hardness and toughness of the composite strongly depended on the content of the Ti(O_0.4, C_0.5) solid solution phase and the residual FeCoNi after densification. As the HEA content increased, the hardness slightly decreased from (21.75±0.67) GPa to (19.24±0.54) GPa, while the toughness was improved from (6.65±0.31) MPa·m^1/2 to (7.49±0.28) MPa·m^1/2, due primarily to the toughening mechanisms, such as crack deflection and crack bridging. By incorporating CoCrFeNiMo as a sintering aid, materials with high hardness and high toughness were produced, enhancing their potential for industrial applications.

Key words: high-entropy carbide ceramics; mechanical properties; second phase particles; toughening mechanism