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It is a substantial challenge to obtain single-phase ceramics with self-lubricating properties. Most conventional lubricated ceramics benefit from the addition of solid lubricants, which largely deteriorate theirs mechanical properties. Here, we report a single-phase (HfMoNbTaTi)C system high-entropy ceramic with self-lubricity inspired by the novel high-entropy design concept and regulation of high-temperature lubrication components (with different Mo contents). The formation mechanism of the single-phase structure is ascribed to the increasing of carbon vacancies with the increasing of Mo content. Meanwhile, the tribological performances are greatly enhanced at high temperature. Especially for (Hf 0.149 Mo 0.404 Nb 0.149 Ta 0.149 Ti 0.149)C 0.745 ceramic (with 20 wt% Mo) against Al 2 O 3 ball at 900 °C, the average friction coefficient is as low as 0.6, and its wear rate is reduced an order of magnitude. The results show that the self-lubrication mechanisms are tribo-chemical reactions to generate lubricating tribo-films. • Single-phase (Hf-Mo-Nb-Ta-Ti)C system ceramic with self-lubricity was prepared. • The formation mechanism of the single-phase structure was investigated. • Regulation of Mo content has a significant effect on tribological properties. • Tribo-films with lubricity have a vital impact on friction reduction and anti-wear. [ABSTRACT FROM AUTHOR]

In this work, the Cr-Ti-Mo ternary o -MAX ceramics based on the Cr 2 TiAlC 2 phase are synthesized, and their tribological performance at elevated temperatures up to 800 ºC in the air is evaluated. With Si 3 N 4 as a tribocouple, an unexpected ultra-low wear rate reaching 6.1 × 10−8 mm3 N−1 m−1 is observed in Cr 1.9 Ti 0.9 Mo 0.2 AlC 2 at 800 ºC, accompanied by a stable coefficient of friction (COF) around 0.33 in a wide temperature range. X-ray photoelectron spectroscopy (XPS) depth profiling confirms a tribofilm with gradient composition, which simultaneously offers fluid lubricating at elevated temperatures and self-healing after cooling. Particularly, confirmed by the theoretical simulations, the doping of Mo improves the interlayer binding as well as alters the oxidation behaviors of Cr 2 TiAlC 2. With an optimal interlayer binding strength and oxidation rate, the Cr 1.9 Ti 0.9 Mo 0.2 AlC 2 can generate a tribofilm possessing ideal composition, which simultaneously promotes lubrication and anti-wear performance at elevated temperatures. [ABSTRACT FROM AUTHOR]

Ceramic design based on reducing friction and wear-related failures in moving mechanical systems has gained tremendous attention due to increased demands for durability, reliability and energy conservation. However, only few materials can meet these requirements at high temperatures. Here, we designed and prepared a Sn-containing Si 3 N 4 -based composite, which displayed excellent tribological properties at high temperatures. The results showed that the friction coefficient and wear rate of the composites were reduced to 0.27 and 4.88 × 10−6 mm3 N−1 m−1 in air at 800 °C. The wear mechanism of the sliding pairs at different temperatures was revealed via detailed analyses of the worn surfaces. In addition, the tribo-driven graphitization was detected on the wear surfaces and in the wear debris, and the carbon phase was identified by SEM, TEM, and Raman spectrum. [ABSTRACT FROM AUTHOR]