Changing the N content in the Ti₃AlC_2−yN_y MAX phase solid solutions allows for the fine-tuning of their properties. However, systematic studies on the synthesis and properties of Ti₃AlC_2−yN_y solid solution bulks have not been reported thus far. Here, previously reported Ti₃AlC_2−yN_y solid solution bulks (y = 0.3, 0.5, 0.8, and 1.0) were synthesized via hot pressing of their powder counterparts under optimized conditions. The prepared Ti₃AlC_2−yN_y bulks are dense and have a fine microstructure with grain sizes of 6–8 μm. The influence of the N content on the mechanical properties, electrical conductivities, and coefficients of thermal expansion (CTEs) of the prepared Ti₃AlC_2−yN_y bulk materials was clarified. The flexural strength and Vickers hardness values increased with increasing N content, suggesting that solid solution strengthening effectively improved the mechanical properties of Ti₃AlC_2−yN_y. Ti₃AlCN (y = 1) had the highest Vickers hardness and flexural strength among the studied samples, reaching 5.54 GPa and 550 MPa, respectively. However, the electrical conductivity and CTEs of the Ti₃AlC_2−yN_y solid solutions decreased with increasing N content, from 8.93×10⁻⁶ to 7.69×10⁻⁶ K⁻¹ and from 1.33×10⁶ to 0.95×10⁶ S/m, respectively. This work demonstrated the tunable properties of Ti₃AlC_2−yN_y solid solutions with varying N contents and widened the MAX phase family for fundamental studies and applications.

Two-dimensional (2D) MoB metal borides (MoB MBene) have attracted much attention due to their fascinating properties and functional applications. So far, work on the synthesis of 2D MoB nanosheets by acid or alkaline etching of MoAlB has not been very successful. It has been proposed that the 2D MoB MBene may be fabricated by chemical etching of a Mo₂AlB₂ precursor, but further investigations were not performed possibly due to the difficult preparation of the metastable Mo₂AlB₂ compound at high temperatures by solid-state reactions. Here, we report on the successful synthesis of the Mo₂AlB₂ compound and 2D MoB nanosheets by the deintercalation of Al from MoAlB through a ZnCl₂ molten salt etching approach at relatively low temperatures. The influence of etching temperature, etching time, and starting mixtures on the formation of desirable phases have been investigated. A pure Mo₂AlB₂ compound was synthesized at temperatures below 600 ℃, while the 2D MoB MBene nanosheets were obtained at 700 ℃ through the molten salt etching of MoAlB. In addition, the present work further confirms that the MoB MBene can be prepared by etching the as-synthesized Mo₂AlB₂ precursor in LiF–HCl solution. Our work demonstrates that the molten salt etching is an effective method to prepare 2D MoB MBene.

Ti₂AlC, a MAX phase ceramic, has an attractive self-healing ability to restore performance via the oxidation-induced crack healing mechanism upon healing at high temperatures in air (high oxygen partial pressures). However, such healing ability to repair damages in vacuum or low oxygen partial pressure conditions remains unknown. Here, we report on the self-healing behavior of Ti₂AlC at a low oxygen partial pressure of about 1 Pa. The experimental results showed that the strength recovery depends on both healing temperature and time. After healing at 1400 ℃ for 1–4 h, the healed samples exhibited the recovered strengths even exceeding the original strength of 375 MPa. The maximum recovered strength of ~422 MPa was achieved in the healed Ti₂AlC sample after healing at 1400 ℃ for 4 h, about 13% higher than the original strength. Damages were healed by the formed TiC_x from the decomposition of Ti₂AlC. The decomposition-induced crack healing as a new mechanism in the low oxygen partial pressure condition was disclosed for the MAX ceramics. The present study illustrates that key components made of Ti₂AlC can prolong their service life and keep their reliability during use at high temperatures in low oxygen partial pressures.

Novel MoAlB composites reinforced with 5-15 vol% SiC have been firstly prepared and characterized in the present study. The SiC reinforcement is stable with MoAlB at a sintering temperature of 1200 ℃ in Ar. The 5 vol% SiC/MoAlB composite exhibited improved mechanical properties and enhanced oxidation resistance. A flexural strength of 380 MPa and a Vickers hardness of 12.7 GPa were achieved and increased by 24% and 51%, respectively, as compared with those for MoAlB, indicating the enhanced strengthening effect of SiC. Cyclic oxidation tests at 1200 and 1300 ℃ for 10 h in air showed that the 5 vol% SiC/MoAlB composite has better oxidation resistance than MoAlB due to the formation of a dense and continuous scale composed of Al₂O₃ and SiO₂, which prevents the oxygen inward diffusion and the evaporation of oxides. We expect that the general strategy of second phase reinforcing for materials will help to widen the applications of MoAlB composites.

MAX phases (Ti₃SiC₂, Ti₃AlC₂, V₂AlC, Ti₄AlN₃, etc.) are layered ternary carbides/nitrides, which are generally processed and researched as structure ceramics. Selectively removing A layer from MAX phases, MXenes (Ti₃C₂, V₂C, Mo₂C, etc.) with two-dimensional (2D) structure can be prepared. The MXenes are electrically conductive and hydrophilic, which are promising as functional materials in many areas. This article reviews the milestones and the latest progress in the research of MAX phases and MXenes, from the perspective of ceramic science. Especially, this article focuses on the conversion from MAX phases to MXenes. First, we summarize the microstructure, preparation, properties, and applications of MAX phases. Among the various properties, the crack healing properties of MAX phase are highlighted. Thereafter, the critical issues on MXene research, including the preparation process, microstructure, MXene composites, and application of MXenes, are reviewed. Among the various applications, this review focuses on two selected applications: energy storage and electromagnetic interference shielding. Moreover, new research directions and future trends on MAX phases and MXenes are also discussed.