Synthesis and mechanical and elevated temperature tribological properties of a novel high-entropy (TiVNbMoW)C <sub>4.375</sub> with carbon stoichiometry deviation
Jicheng Li, Yanchun Zhou, Yunfeng Su, Shuna Chen, Qiuan Sun, Hengzhong Fan, Junjie Song, Litian Hu, Yongsheng Zhang
Abstract
High-entropy carbides are a nascent group of ceramics that are promising for high-temperature applications due to the combination of good stability, high hardness (<i>H</i>), high strength, and superior creep resistance that they display. Due to high melting points and low lattice diffusion coefficients, however, the high-entropy carbides are usually difficult to consolidate to a nearly full density. To cope with this challenge, herein, binary carbides including TiC, V<sub>8</sub>C<sub>7</sub>, NbC, Mo<sub>2</sub>C, and WC with different carbon stoichiometry were used to prepare dense high-entropy (TiVNbMoW)C<sub>4.375</sub>, and the influence of carbon vacancy on formation ability and mechanical properties of carbon-deficient high-entropy (TiVNbMoW)C<sub>4.375</sub> were investigated. Intriguingly, although the starting binary carbides have different crystal structures and carbon stoichiometry, the as-prepared high-entropy material showed a rock-salt structure with a relatively high density (98.1%) and good mechanical properties with hardness of 19.4±0.4 GPa and fracture toughness (<i>K</i><sub>IC</sub>) of 4.02 MPa·m<sup>1/2</sup>. More importantly, the high-entropy (TiVNbMoW)C<sub>4.375</sub> exhibited low coefficient of friction (COF) at room temperature (RT) and 800 ℃. Wear rate (<i>W</i>) gradually increased with the temperature rising, which were attributed to the formation of low-hardness oxidation films at high temperatures to aggravate wear. At 800 ℃, lubricating films formed from sufficient oxidation products of V<sub>2</sub>O<sub>5</sub> and MoO<sub>3</sub> effectively improved tribological behavior of the high-entropy (TiVNbMoW)C<sub>4.375</sub>. Wear mechanisms were mainly abrasive wear resulting from grain pullout and brittle fracture as well as oxidation wear generated from high-temperature reactions. These results are useful as valuable guidance and reference to the synthesis of high-entropy ceramics (HECs) for sliding parts under high-temperature serving conditions.