The high-entropy design of MAX phases is expected to confer superior properties, but its study was hindered by the complex synthesis method and limited purity of samples. In this work, two noteworthy types of high-entropy MAX phase structural ceramics, high-entropy (TiVNbTaM)₂AlC (M = Zr, Hf), were designed and prepared by the in-situ synthesis using spark plasma sintering (SPS). The microstructure and lattice parameters of sintered samples were determined. Compared with the single-component MAX phases, the highly pure high-entropy (TiVNbTaZr)₂AlC sample had good physical and mechanical properties, including electrical conductivity of 0.96×10⁶ Ω⁻¹·m⁻¹, thermal expansion coefficient of 3.65×10⁻⁶ K⁻¹, thermal conductivity of 8.98 W·m⁻¹·K⁻¹, Vickers hardness of 9.80 GPa, flexural strength of 507 MPa, fracture toughness of 5.62 MPa·m^1/2, and compressive strength of 1364 MPa, which exhibited the remarkable hardening-strengthening effect.

Textured Nb₄AlC₃ ceramics were rapidly and efficiently prepared by hot forging through spark plasma sintering (SPS). The longitudinal compression ratio of textured Nb₄AlC₃ ceramics was −78.3%, and the lateral expansion ratio was 32.1%. The grains grew preferentially along the direction perpendicular to the c-axis, forming the texture microstructure. The Lotgering orientation factor f_(00l) was calculated to be 0.63. The thermal conductivity of textured Nb₄AlC₃ ceramics along the c-axis direction (11.23 W·m⁻¹·K⁻¹) (25 ℃) was lower than that of untextured ceramics (13.75 W·m⁻¹·K⁻¹) (25 ℃). The electrical conductivity perpendicular to the c-axis direction reached 4.37×10⁶ S·m⁻¹ at room temperature. The ordered layered grains increased the resistance of crack propagation, resulting in a higher fracture toughness parallel to the c-axis direction (9.41 MPa·m^1/2), which was higher than that of untextured ceramics (6.88 MPa·m^1/2). The Vickers hardness tested at 10 N on the texture top surface (7.18 GPa) was higher than that on the texture side surface (6.45 GPa).

The ternary or quaternary layered compounds called MAB phases are frequently mentioned recently together with the well-known MAX phases. However, MAB phases are generally referred to layered transition metal borides, while MAX phases are layered transition metal carbides and nitrides with different types of crystal structure although they share the common nano-laminated structure characteristics. In order to prove that MAB phases can share the same type of crystal structure with MAX phases and extend the composition window of MAX phases from carbides and nitrides to borides, two new MAB phase compounds Zr₂SeB and Hf₂SeB with the Cr₂AlC-type MAX phase (211 phase) crystal structure were discovered by a combination of first-principles calculations and experimental verification in this work. First-principles calculations predicted the stability and lattice parameters of the two new MAB phase compounds Zr₂SeB and Hf₂SeB. Then they were successfully synthesized by using a thermal explosion method in a spark plasma sintering (SPS) furnace. The crystal structures of Zr₂SeB and Hf₂SeB were determined by a combination of the X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The lattice parameters of Zr₂SeB and Hf₂SeB are a = 3.64398 Å, c = 12.63223 Å and a = 3.52280 Å, c = 12.47804 Å, respectively. And the atomic positions are M at 4f (1/3, 2/3, 0.60288 [Zr] or 0.59889 [Hf]), Se at 2c (1/3, 2/3, 1/4), and B at 2a (0, 0, 0). And the atomic stacking sequences follow those of the Cr₂AlC-type MAX phases. This work opens up the composition window for the MAB phases and MAX phases and will trigger the interests of material scientists and physicists to explore new compounds and properties in this new family of materials.

In this paper, Zr₂SB ceramic with purity of 82.95 wt% (containing 8.96 wt% ZrB₂ and 8.09 wt% zirconium) and high relative density (99.03%) was successfully synthesized from ZrH₂, sublimated sulfur, and boron powders by spark plasma sintering (SPS) at 1300 ℃. The reaction process, microstructure, and physical and mechanical properties of Zr₂SB ceramic were systematically studied. The results show that the optimum molar ratio to synthesize Zr₂SB is n(ZrH₂):n(S):n(B) = 1.4:1.6:0.7. The average grain size of Zr₂SB is 12.46 μm in length and 5.12 μm in width, and the mean grain sizes of ZrB₂ and zirconium impurities are about 300 nm. In terms of physical properties, the measured thermal expansion coefficient (TEC) is 7.64×10⁻⁶ K⁻¹ from room temperature to 1200 ℃, and the thermal capacity and thermal conductivity at room temperature are 0.39 J·g⁻¹·K⁻¹ and 12.01 W∙m⁻¹∙K⁻¹, respectively. The room temperature electrical conductivity of Zr₂SB ceramic is measured to be 1.74×10⁶ Ω⁻¹∙m⁻¹. In terms of mechanical properties, Vickers hardness is 9.86±0.63 GPa under 200 N load, and the measured flexural strength, fracture toughness, and compressive strength are 269±12.7 MPa, 3.94±0.63 MPa·m^1/2, and 2166.74±291.34 MPa, respectively.

The synthesis, microstructure, and properties of high purity dense bulk Mo₂TiAlC₂ ceramics were studied. High purity Mo₂TiAlC₂ powder was synthesized at 1873 K starting from Mo, Ti, Al, and graphite powders with a molar ratio of 2:1:1.25:2. The synthesis mechanism of Mo₂TiAlC₂ was explored by analyzing the compositions of samples sintered at different temperatures. It was found that the Mo₂TiAlC₂ phase was formed from the reaction among Mo₃Al₂C, Mo₂C, TiC, and C. Dense Mo₂TiAlC₂ bulk sample was prepared by spark plasma sintering (SPS) at 1673 K under a pressure of 40 MPa. The relative density of the dense sample was 98.3%. The mean grain size was 3.5 μm in length and 1.5 μm in width. The typical layered structure could be clearly observed. The electrical conductivity of Mo₂TiAlC₂ ceramic measured at the temperature range of 2-300 K decreased from 0.95 × 10⁶ to 0.77 × 10⁶ Ω^-1·m^-1. Thermal conductivity measured at the temperature range of 300-1273 K decreased from 8.0 to 6.4 W·(m·K)^-1. The thermal expansion coefficient (TEC) of Mo₂TiAlC₂ measured at the temperature of 350-1100 K was calculated as 9.0 × 10^-6 K^-1. Additionally, the layered structure and fine grain size benefited for excellent mechanical properties of low intrinsic Vickers hardness of 5.2 GPa, high flexural strength of 407.9 MPa, high fracture toughness of 6.5 MPa·m^1/2, and high compressive strength of 1079 MPa. Even at the indentation load of 300 N, the residual flexural strength could hold 84% of the value of undamaged one, indicating remarkable damage tolerance. Furthermore, it was confirmed that Mo₂TiAlC₂ ceramic had a good oxidation resistance below 1200 K in the air.

Guided by the theoretical prediction, a new MAX phase V₂SnC was synthesized experimentally for the first time by reaction of V, Sn, and C mixtures at 1000 ℃. The chemical composition and crystal structure of this new compound were identified by the cross-check combination of first-principles calculations, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and high resolution scanning transmission electron microscopy (HR-STEM). The stacking sequence of V₂C and Sn layers results in a crystal structure of space group P6₃/mmc. The a- and c-lattice parameters, which were determined by the Rietveld analysis of powder XRD pattern, are 0.2981(0) nm and 1.3470(6) nm, respectively. The atomic positions are V at 4f (1/3, 2/3, 0.0776(5)), Sn at 2d (2/3, 1/3, 1/4), and C at 2a (0, 0, 0). A new set of XRD data of V₂SnC was also obtained. Theoretical calculations suggest that this new compound is stable with negative formation energy and formation enthalpy, satisfied Born-Huang criteria of mechanical stability, and positive phonon branches over the Brillouin zone. It also has low shear deformation resistance c₄₄ (second-order elastic constant, c_ij) and shear modulus (G), positive Cauchy pressure, and low Pugh’s ratio (G/B = 0.500 < 0.571), which is regarded as a quasi-ductile MAX phase. The mechanism underpinning the quasi-ductility is associated with the presence of a metallic bond.

Tribological property of c-axis textured shell-like Ti₃AlC₂ ceramic was investigated using reciprocating sliding balls (SUS304) under loads of 1, 5, and 9 N. It was found that the textured top surface (TTS), corresponding to the (000l) plane, shows the lowest mean coefficient of friction in comparison with those measured on the textured side surface (TSS), where the sliding directions are parallel (TSS-1) and perpendicular (TSS-2) to c axis, under the same load. Among all the tested orientations, the TSS-2 exhibited the lowest wear rate of 1.51×10^-3 mm³/(N·m) under the load of 9 N. The worn mechanisms on the TTS and TSS-1 were delamination, grain fracture, and grain spalling-off. On the TSS-2, plowing effect against balls was the dominating mechanism. This work suggests the criteria to maximize the wear resistance in the load range of 1-9 N.