Preparation and Tribological Properties of (TiZrNbMoTa)B₂ High Entropy Ceramics

High-entropy diboride ceramics possess the advantages of refractory metal borides and high-entropy materials, with high hardness and good oxidation resistance, and are, therefore, expected to be applied in high-temperature wear-resistant fields. However, current research reports on high-entropy diboride ceramics mainly focus on synthesis, preparation, mechanical properties, and oxidation resistance, while tribological properties still need to be improved. In addition, high-entropy diboride ceramics still have problems, such as high synthesis temperature, poor density, element segregation, and poor fracture toughness. Therefore, it is necessary to improve the preparation process of high-entropy diboride ceramics and explore their tribological properties.

This study selected (TiZrNbMoTa)B₂ with high hardness as the basic component, and using five refractory transition metal element powders of Ta, Mo, Ti, Zr, Nb, and boron powder as raw materials, successfully prepared (TiZrNbMoTa)B₂ high-entropy ceramics through high-energy ball milling and SPS in-situ reaction sintering. The improvement process aimed to reduce the reaction energy and the introduction of impurities. The influence of the preparation process of (TiZrNbMoTa)B₂ high-entropy ceramics on its microstructure and mechanical properties was investigated. In order to further broaden the application of high-entropy diboride and provide technical and theoretical support for improving the tribological properties of high-entropy diboride, the tribological properties of (TiZrNbMoTa)B₂ high-entropy ceramics from 25 ℃ to 800 ℃ were emphatically explored. The changes in the composition and structure of (TiZrNbMoTa)B₂ high-entropy ceramics before and after friction were analyzed in detail using XPS, SEM, DSC-TG, and other characterization methods. The wear mechanism and wear resistance mechanism of (TiZrNbMoTa)B₂ high-entropy ceramics were analyzed.

The main results and conclusions were as follows: with the increase of milling speed, the distribution of elements in (TiZrNbMoTa)B₂ high entropy diborides tended to be uniformly distributed, and (TiZrNbMoTa)B₂ formed a single homogeneous solid solution phase of hexagonal metal diboride. Meanwhile, by virtue of the lattice distortion effect and solid solution strengthening effect inherent to high entropy materials, the mechanical properties of these ceramics were significantly enhanced with escalating milling speeds, as evidenced by an elevation in Vickers hardness from 23.42 to 28.18 GPa, an augmentation in fracture toughness from 2.13 MPa·m^1/2 to 2.47 MPa·m^1/2, and a reinforcement in bending strength from 83.33 MPa to 96.67 MPa. (TiZrNbMoTa)B₂ high entropy diborides had good high-temperature wear resistance, and the average wear rate at 600 °C was 8.98×10⁻⁶ mm³/(N·m), which was 36 times higher than that at room temperature. The good high-temperature wear resistance was contributed by the high-temperature self-lubrication effects of B₂O₃ and the formation of a dense enamel layer on the surface of the sample, which was composed of metal oxides and (TiZrNbMoTa)B₂ high entropy phase. The wear mechanism of (TiZrNbMoTa)B₂, high entropy diborides, was abrasive wear at low and medium temperatures. At high temperatures, the main wear mechanism was abrasive wear and oxidation wear. As a result of high-temperature oxidation, the tribological properties of (TiZrNbMoTa)B₂ high-entropy ceramics had seen a slight decline. Hence, bolstering the high-temperature oxidation resistance of (TiZrNbMoTa)B₂ high-entropy ceramics would be a pivotal strategy to improve their high-temperature wear resistance.