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Dense, fine grained B 4 C–SiC composites were successfully fabricated with the hot-pressing pyrolyzed mixtures of polycarbosilane-coated B 4 C powder and the Si additive at 1950 °C for 1 h under 30 MPa. The addition of Si not only improved the densification process due to the formation of liquid phase Si, but also greatly enhanced the mechanical properties of B 4 C ceramics. However, the added Si showed no significant effect on fracture toughness. The improvement in mechanical properties was attributed to the decrease of porosity and the elimination of free carbon derived from polycarbosilane and B 4 C powder. The introduction of Si improved the microstructure homogeneity and further refined the layer structure of in-situ SiC grains. The flexural strength, Vickers hardness, and fracture toughness of the sample prepared with 11.4 wt% Si, respectively, reached 389 MPa, 33.2 GPa, and 5.64 MPa m 1/2 . [ABSTRACT FROM AUTHOR]

Abstract: B4C-SiC composites with fine grains were fabricated with hot-pressing pyrolyzed mixtures of polycarbosilane-coated B4C powder without or with the addition of Si at 1950°C for 1h under the pressure of 30MPa. SiC derived from PCS promoted the densification of B4C effectively and enhanced the fracture toughness of the composites. The sinterability and mechanical properties of the composites could be further improved by the addition of Si due to the formation of liquid Si and the elimination of free carbon during sintering. The relative density, Vickers hardness and fracture toughness of the composites prepared with PCS and 8wt% Si reached 99.1%, 33.5GPa, and 5.57MPam1/2, respectively. A number of layered structures and dislocations were observed in the B4C-SiC composites. The complicated microstructure and crack bridging by homogeneously dispersed SiC grains as well as crack deflection by SiC nanoparticles may be responsible for the improvement in toughness. [Copyright &y& Elsevier]

In order to prepare dense B4C–SiC composites under relatively low temperature without any additive, stacking disordered B4C–SiC ultrafine composite powders were fabricated by mechanical alloying firstly. Subsequently, hot pressing sintering was applied to produce a dense B4C–SiC composite under relatively low temperature (1800–1950°C) without any additive. For the samples sintered at 1950°C for 30min, the obtained relative density, Vickers hardness, flexural strength and fracture toughness were 96%, 24GPa, 430MPa, and 4.6MPa.m1/2, respectively. The microstructural characterization showed that the main fracture mode was transgranular. The transgranular facture is caused by the powerful interfacial bonding between B4C and SiC. The density of the samples sintered with composite powders was 13% higher than the density of the samples sintered with mixed powders under the same conditions, indicating that the composite powders had an improved sintering activity due to their disordered structure. Meanwhile, the role of disorder–order transformation-driven sintering of ceramics was demonstrated. [ABSTRACT FROM AUTHOR]

High-performance TiB 2 –SiC composites were fabricated with a new reaction system of B 4 C–TiC–Si by reactive hot pressing. The reaction process was controlled by the diffusion of boron atoms and hindered by the accumulated product layer of TiB 2 and C. The melting Si can offer another diffusion path for boron atoms, thereby promoting the reaction process. The Vickers hardness, flexural strength, and fracture toughness of the fabricated TiB 2 –SiC composites respectively reached 28.6 ± 0.6 GPa, 654 ± 49 MPa, and 6.8 ± 0.5 MPa m 1/2 . Both the intergranular fracture resulting from mismatch in thermal expansion coefficient between TiB 2 and SiC and the transgranular fracture resulting from the network structure of TiB 2 can enhance the fracture toughness of the TiB 2 –SiC composite. [ABSTRACT FROM AUTHOR]