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Fully dense ZrB 2 –TiC composites were produced by reactive pressureless sintering using ZrB 2 and TiC as the starting reactants. The effect of TiC content on microstructure and mechanical properties of ZrB 2 –TiC composites was investigated. The densification and grain refinement were achieved due to the reaction and solid solution coupling effect. Besides, elongated (Ti, Zr)B 2 plate-like grains with a high aspect ratio were obtained for the first time by the reaction of ZrB 2 and TiC. The ZrB 2 -70 mol%TiC composite exhibited good comprehensive mechanical properties of high hardness of 29.0 GPa, flexural strength of 555 MPa, and fracture toughness of 5.4 MPa m1/2 due to grain-refinement and solid solution strengthening, and plate-like grains toughening effects. [ABSTRACT FROM AUTHOR]

Boron nitride-barium aluminosilicate (BN-BAS) composite ceramics were fabricated by the sol-gel method and hot-pressed sintering. The effects of the introduction method of BAS and sintering temperature on the phase assemblage, density, apparent porosity, mechanical properties and microstructures of the h-BN composite ceramics were investigated. The following two introduction methods were adopted: (A) BAS gel powder was prepared by the sol-gel method and then mixed with h-BN for hot pressing sintering, and (B) the BN-BAS composite powder was first prepared by the sol-gel method and then hot-pressed and sintered. The results indicate that the density, bending strength and fracture toughness of the latter are higher than those of the former when other conditions are the same. In the gel step of Group B, BAS has been coated on the surface of BN particles in dots, chemical bonding has existed, the distribution of BAS is more uniform, and it can better serve as a sintering aid. The BN-BAS composites prepared by both methods achieved the maximum mechanical properties at a sintering temperature of 1700 °C, and the density, apparent porosity, bending strength and fracture toughness of Group A were 2.40 g/cm3, 1.82%, 132.8 MPa and 2.3 MPa m1/2, while those of Group B were 2.42 g/cm3, 1.45%, 162.9 MPa and 2.7 MP m1/2. [ABSTRACT FROM AUTHOR]

MXenes have good application prospects due to the attractive mechanical and physicochemical properties. Here, for the first time, silicoboron carbonitride (Si–B–C–N) ceramics with the integration of MXene(Ti 3 C 2 T x) nanosheets were developed, and the influences of MXene additives (1.0–3.0 wt%) on mechanical properties and microstructures were investigated. The ceramics containing 3.0 wt% MXene(Ti 3 C 2 T x) show better mechanical properties including fracture toughness of 7.6 MPa‧m1/2, 36% higher than ceramics without any additives. The toughening mechanisms consist of pull-out, delamination, crack bridging, and crack deflection. This work demonstrates that MXenes are good reinforcements for enhancing the fracture toughness of structural ceramics, and also paves the way for the investigation of high-temperature properties of MXene-containing ceramics. [ABSTRACT FROM AUTHOR]

To improve the passive radiative heat dissipation property of aluminum alloy, a novel-structural SiC/ h BN nanocomposite ceramic coating with high emissivity value (>0.86) was fabricated by one-pot liquid-phase plasma assisted electrophoresis and sintering (LPES) method. The special formation conditions of nanocomposite coating were optimized as: applied voltage was not lower than 600 V, temperature of electrolyte was ranged from 70 to 90 °C and the concentrations of SiC and h BN nanoparticles were 8 g/L and 40 g/L, respectively. The unique parameters contributed to the plasma assisted SiC and hBN nanoparticles synergistic electrophoresis and then sintering together by SiO 2 based glassy phase. The infrared emissivity in the wavelength of 3–20 μm was enhanced by SiC/ h BN nanocomposite coating, especially the value was up to 0.86 in 3–8 μm wavelength. The nanocomposite coating enabled the temperature drop by 10.4 °C for 1 W LED compared with the bare aluminum alloy, which leds to the cooling efficiency of 19.8%, respectively. The SiC/ h BN ceramic nanocomposite coating fabricated by one-pot LPES process shows a novel strategy to design excellent radiative heat dissipation property coatings. [ABSTRACT FROM AUTHOR]

MgO-Al 2 O 3 -SiO 2 (MAS) glass ceramic was used as liquid sintering aid to densify h -BN ceramics, and effects of sintering temperature on the phase composition of MAS, texture behavior of h -BN grains and properties of the composite ceramics were studied. h -BN grains will react with MAS by forming N-M (Al3+ and Si4+) coordinate-covalent bonds and inhibit the crystallization of glass phase. The h -BN grains will rotate parallel to the pressure direction due to the joint action of the liquid sintering aid and the pressure. The sintering temperature will significantly affect the properties of the composite ceramics by effecting the texture behavior of h -BN grains. And higher sintering temperature of 1900 °C will result in the decline of mechanical properties and increase of thermal conductivity due to the decrease of texture degree caused by the grain growth of h -BN. [ABSTRACT FROM AUTHOR]

Abstract TiB 2 –40 wt% TiC composites were prepared by pressureless sintering. Prior to the conventional heating at 2000 °C for 1 h for final densification, addition of a low temperature hold at 1500 °C for 4 h was found effective in densification of the composite, refining the microstructure and promoting the anisotropic growth of TiB 2 grains. The TiB 2 -TiC composite prepared by two-step sintering had better mechanical properties with flexural strength of 776 MPa and fracture toughness of 7.43 MPa m1/2, in comparison with the values of 419 MPa and 4.81 MPa m1/2 for composite by conventional one-step sintering. [ABSTRACT FROM AUTHOR]

To meet the ultrahigh temperature requirements of a thermal protection system, an ultrahigh temperature phase of ZrB 2 was introduced into a SiBCN matrix that was fabricated using a reactive spark plasma sintering method. The thermal ablation behavior of SiBCN-Zr composites was investigated using an oxyacetylene flame test. The test results indicated that the ablation behavior of the modified ceramic composites was significantly improved over that of a monolithic SiBCN ceramic. The linear and mass ablation rates of the SiBCN-Zr material were found to be 0.004 mm/s and 4.75×10 −4 g/s, which was indicative of excellent ablation resistance. Analysis of the material after thermal ablation testing showed that ablation products mainly consisted of the ZrSiO 4 , SiO 2 and ZrO 2 phases. A reaction occurred between the SiO 2 and ZrO 2 phases in the central region of the ceramic forming ZrSiO 4 that protected the material from further thermal damage. A loose and porous oxidation layer was found from the matrix based on analysis of a cross-section image. [ABSTRACT FROM AUTHOR]

Porous boron nitride/silicon oxynitride (BN/Si 2 N 2 O) composites were fabricated by pressureless sintering at 1650 °C with Li 2 O as sintering aid. The influence of Li 2 O and hexagonal boron nitride (h-BN) contents on phase, microstructure, mechanical, dielectric and thermal properties of the resulting porous BN/Si 2 N 2 O composites was investigated. Increasing Li 2 O content facilitated densification and decomposition of Si 2 N 2 O into Si 3 N 4 . The apparent porosity of the composites increases with the h-BN content increases and Si 2 N 2 O grain growth was restrained by the dispersed h-BN particles. The dielectric properties and thermal conductivities (TC) were affected mainly by porosity. Porous BN/Si 2 N 2 O ceramic composites with 4 mol% Li 2 O and 25 mol% BN exhibit both low dielectric constant (3.83) and dielectric loss tangent (0.008) with good mechanical and thermal performance, suggesting possible use as high-temperature structural/functional materials. [ABSTRACT FROM AUTHOR]

Since its invention, alumina ceramics have been extensively investigated for potential various applications. However, their intrinsic brittle nature is still an insurmountable obstacle when they are applied as structural components. This paper provides a simple routs to prepare ductile alumina based composites with the addition of chopped carbon fiber (C sf /Al 2 O 3 -BN). Effects of fiber length and sintering temperature on the microstructure, phase composition, mechanical properties together with fracture behavior were systematically investigated. The results showed that composites with mixed fiber lengths of 12 mm and 1 mm exhibited homogeneous microstructure and striking enhancement in mechanical performances compared with composites with other fiber length. With the increase in sintering temperature from 1500 °C to 1650 °C, interfacial bonding strength increased and interface state converted from mechanical interlocking at 1500 °C into chemical bonding at 1650 °C. Chemical reaction in the composites degraded carbon fiber properties, which resulted in the decrease in mechanical performance of the composites. [ABSTRACT FROM AUTHOR]