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A new high-entropy monoboride (Mo 0.2 Ta 0.2 Ni 0.2 Cr 0.2 W 0.2)B ceramic with a WB-type orthogonal structure was designed and synthesised by in-situ reactive hot pressing at 2000 °C and 30 MPa for 1.5 h under an argon atmosphere. The microstructure of the sintered samples was comprehensively characterised, and the formation of a high-entropy monoboride (Mo 0.2 Ta 0.2 Ni 0.2 Cr 0.2 W 0.2)B ceramic was confirmed. Owing to the high density of the dislocations and strengthening metal-boron bonds, the high-entropy (Mo 0.2 Ta 0.2 Ni 0.2 Cr 0.2 W 0.2)B ceramic exhibited a hardness of 48.51 ± 4.07 GPa, which enabled it to be classed as a new superhard material. In addition, the thermal conductivity (2.05 ± 0.10 W/(m·K) at 400 °C) and electric conductivity (132.30 S/cm) were determined. [ABSTRACT FROM AUTHOR]

Metal materials have been used as thin films in many optical applications because of its excellent high reflectivity in the near-infrared range. However, studies on thick metal coatings deposited by plasma spraying have rarely been conducted. The current study investigates the relationship between optical property and microstructure for different metal plasma-sprayed coatings (Al, Cu, and Ag) and discusses the corresponding mechanical properties. The phase structure and surface morphology of as-sprayed powders and corresponding coatings were examined by X-ray diffraction and scanning electron microscopy, respectively. The optical and mechanical properties were characterized by UV–visible-near infrared spectroscopy, Multi-Specimen machine testing and Vickers microhardness testing. Results indicate that the optical behavior of metal plasma-sprayed coatings is related to their phase structure, splat microstructure, and surface roughness. Optimization of powders in molten state and the surface microstructure is proposed as an effective method to improve the optical property of these coatings. The molten state and low porosity also help to improve the mechanical properties of the coating materials. [ABSTRACT FROM AUTHOR]

Ti 3 AlC 2 was used as the additive to fabricate a novel (Zr 0 · 2 Ta 0 · 2 Nb 0.2 Hf 0.2 Mo 0.2)B 2 -based (HEBTAC) ceramic using spark plasma sintering at 1900 °C. The as-prepared HEBTAC ceramic sample consists of (Ti, Zr, Hf, Ta, Nb, Mo)B 2 , multiscale (Zr, Hf, Ta, Nb, Mo)C, and Al 2 O 3 phases, which are produced by the decomposition of Ti 3 AlC 2 and subsequent exchange reaction & solid solution reactions during the sintering process. The relative density, hardness, fracture toughness, and flexural strength of the HEBTAC ceramic sample are 99.43 %, 27.92 ± 1.87 GPa, 5.84 ± 0.14 MPa m1/2, and 629 ± 8.7 MPa, respectively, which are even better than the same class of high-entropy diboride ceramics previously published. Solid-solution strengthening, dislocation strengthening and nano-HEC strengthening existed in the ceramic sample contribute to its excellent mechanical properties. The in situ formed Al 2 O 3 and (Zr, Hf, Ta, Nb, Mo)C effectively enhances the toughness of the HEBTAC ceramic sample through the synergistic effect of intrinsic and extrinsic toughening. This study presents a new approach for reactive sintering densification of high-entropy diborides with both high strength and high toughness. [ABSTRACT FROM AUTHOR]

The effects of high-energy ball milling (HEBM) on the sintering ability and microstructure of hafnium diboride (HfB 2) ultrahigh-temperature ceramics (UHTCs) were investigated. After HEBM, the average particle size range of the HfB 2 ceramics decreased to 10–20 nm with a slight alteration in the crystal structure. The microscopic grains of the sintered samples were homogeneous and fine in size. WC removed oxide impurities on the surface of the HfB 2 particles by reacting with the oxide impurities, which further improved the sintering ability of the HfB 2 ceramics. The densification mechanism of the HfB 2 ceramics was also investigated, and nearly fully dense HfB 2 (99.1%) was fabricated by reactive spark plasma sintering (R–SPS) at 2100 °C for 35 min under a pressure of 40 MPa. HEBM increased the surface energy of the fine particles, which provided a sufficient driving force to improve the sintering ability and significantly increase the densification of HfB 2. • HfB 2 ceramics with nearly full density (99.1% relative density) has been successfully prepared. • The high energy ball mill slightly causes lattice distortion and improves sintering property. • Study of the mechanism of densification and grain coarsening of HfB 2 during SPS. [ABSTRACT FROM AUTHOR]

The microstructure of thermal spray powders plays a significant role in the application performance of thermal protection coatings deposited by air plasma spraying. Based on the design of the spray powder structure, the YSZ@Cu core-shell structure powders have the potential to be a thermal conduction anisotropy material for thermal protection coating, owing to the special alternate-layered structure formed during plasma spraying. To study the mechanism of the thermal conduction anisotropy of YSZ/Cu composite coatings, the YSZ coatings, YSZ@Cu coatings and YSZ-Cu coatings were fabricated by YSZ powders, YSZ@Cu core-shell structure powders and YSZ-Cu mechanical mixture powders, respectively. There were YSZ vertical thermal insulation layer and Cu horizontal thermal condition layer in the YSZ@Cu coatings, which provides a structural basis for the realization of thermal conduction anisotropy. Compared with the YSZ-Cu coatings, the temperature difference between the back-surface center area and the edge area of the YSZ@Cu coatings is significantly reduced by approximately 37%, which is attributed to the synergistic effect of alternate-layered comprising the YSZ layer and the high thermal conductivity Cu layer. YSZ@Cu coatings can effectively prevent the rapid rise in local temperature by the analysis of finite-element simulation, which is beneficial for prolonging the service life of the thermal protection coating in a non-uniform temperature field. • To reveal thermal conduction anisotropy, YSZ@Cu core-shell structure powder was used to fabricate composite coatings by APS. • The YSZ@Cu coating has an alternate-layered structure including horizontal thermal conduction and vertical thermal isolation layer. • Based on experimental analysis, the novel YSZ@Cu coating make surface temperature difference reduce by approximately 37%. [ABSTRACT FROM AUTHOR]