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B4C–SiC intergranular/intragranular nanocomposites with high hardness and high toughness were fabricated through mechanochemical processing with B4C, Si, and graphite powders and subsequent hot pressing without any sintering aid. The milled powders are composed of stacking-disordered SiC and nanocrystalline B4C. Most nano/micron-sized SiC particles are homogeneously dispersed in B4C matrix, and some nano-sized SiC and B4C particles are embedded into B4C grains to form an intergranular/intragranular structure. The disordered characteristic of the milled powders is the essential factor for the formation of the intragranular structure, sudden densification within the narrow temperature range (1700–1900°C), and the preparation of dense samples under a relatively low temperature (1900°C). The relative density, Vickers hardness, and fracture toughness of the samples sintered at 1950°C are 98.6%, 34.3GPa, and 6.0MPam1/2, respectively. The intergranular/intragranular structure plays an important role in improving fracture toughness and hardness of the composites. [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]

Abstract: B4C–SiC nanocomposite powders were synthesized successfully through mechanochemical processing with B4C, Si and graphite powders. Structural evolution of the powder mixtures was characterized by XRD and IR spectra. Microstructure investigations of the end products were performed in HRTEM. Phase transition of the end products associated with heat treatment was monitored by DSC and XRD. The results indicate that the end products are composed of nanocrystalline B4C and amorphous SiC. The synthesis of the amorphous SiC is only controlled by the gradual diffusion reaction mechanism. During ball milling, the C atoms are firstly dissolved into the interstices of Si lattice to form an interstitial solid solution Si (C). After continuous milling, the C atoms located into the interstices of Si lattice gradually occupy the positions of Si to form amorphous SiC. However, no C or Si atom is dissolved into the interstices of B4C lattice. The large quantity of B4C dilutes the reactants of graphite and Si and hinders the self-propagating reaction to form crystal SiC. The end products (nanocrystalline B4C and amorphous SiC) are gradually transformed into crystalline B4C and SiC during heat treatment. [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]