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Sintering method greatly affects the sintering behavior, microstructure and properties of the ceramic composites. In the present work, TiB 2 – h -BN composites prepared by reactive spark plasma sintering (RSPS) and conventional spark plasma sintering (CSPS) are compared. Due to the in-situ synthesized TiB 2 and h -BN particles during sintering process, the reactive sintering not only inhibits the orientation of TiB 2 grains but also prevented the formation of the Card-house structure. TiB 2 – h -BN composites obtained by RSPS (R series) exhibit higher sintering activity and finer microstructure, which results in better mechanical performance than those prepared by CSPS (C series). As the SPS temperature is 1600 °C, the relative density of the composite in R series is as high as 97.3%, much higher than that (89.3%) in C series. The composite obtained by RSPS at 1600 °C also exhibits excellent mechanical properties with a bending strength of 205.3 MPa and a Vickers hardness of 2.50 GPa, respectively, which exceeds all composites prepared by CSPS at 1600–2000 °C. When the SPS temperature is 1800 °C or above, for both series, the increase of SPS temperature greatly enhances the Vickers hardness but has litter effect on the strength and electrical resistivity, which is attributed to the comprehensive effect of the density increase and microstructure coarsening. The present work indicates that in-situ reactive sintering has obvious advantages in preparing TiB 2 – h -BN composites with excellent mechanical properties at low temperature. [ABSTRACT FROM AUTHOR]

To reduce the negative effects of the long-time and B 2 O 3 phase on the traditional sintering process for B 4 C-based composite ceramics, nearly fully dense B 4 C–VB 2 composite ceramics were prepared by reactive spark plasma sintering (SPS) technology at 2000 °C with B and V 8 C 7 powders as raw materials in this paper. The effects of the degassing time during SPS on the microstructure and the mechanical properties of the final products were investigated in detail. The results revealed that the proper degassing time was beneficial for the vent of B 2 O 3 during the sintering process, which refined the grain size, promoted densification and improved the mechanical properties of the composite ceramic. However, the redundant degassing time increased the holding time at high temperature, resulting in abnormal grain growth and mechanical performance deterioration. In the present work, the optimal degassing time was 6 min, and the final product prepared under the above conditions exhibited excellent comprehensive performance with a relative density of 99.2%, Vickers hardness of 31.2 GPa, bending strength of 654 MPa and fracture toughness of 5.7 MPa m1/2. In addition, the strengthening and toughening mechanisms of the products were mainly attributed to the residual thermal stresses and bridging structure caused by the fine B 4 C and VB 2 grains distributed uniformly. [ABSTRACT FROM AUTHOR]

Previous research have reported that B 4 C–TiB 2 composites could be prepared by the reactive sintering of TiC–B powder mixtures. However, due to spontaneous oxidation of raw powders, using TiC–B powder mixtures with a B/TiC molar ratio of 6: 1 introduced an intermediate phase of C during the sintering process, which deteriorated the hardness of the composites. In this report, the effects of B excess on the phase composition, microstructure, and mechanical properties of B 4 C–TiB 2 composites fabricated by reactive hot pressing TiC–B powder mixtures were investigated. XRD and Raman spectra confirmed that lattice expansion occurred in B-rich boron carbide and B x C–TiB 2 (x > 4) composites were obtained. The increasing B content improved the hardness and fracture toughness but decreased the flexural strength of B x C–TiB 2 (x > 4) composites. When the molar ratio of B/TiC increased from 6.6:1 to 7.8:1, the Vickers hardness and the fracture toughness of the composites were enhanced from 26.7 GPa and 4.53 MPa m1/2 to 30.4 GPa and 5.78 MPa m1/2, respectively. The improved hardness was attributed to the microstructural improvement, while the toughening mechanism was crack deflection, crack bridging and crack branching. [ABSTRACT FROM AUTHOR]