A ZrB 2 -SiC composite was prepared from a mixture of zirconium, silicon, and B 4 C via reactive hot pressing. The three-point bending strength was 506 ؎ 43 MPa, and the fracture toughness was 4.0 MPa⅐m 1/2 . The microstructure of the composite was observed via scanning electron microscopy; the in-situ-formed ZrB 2 and SiC were found in agglomerates with a size that was in the particle-size ranges of the zirconium and silicon starting powders, respectively. A model of the microstructure formation mechanism of the composite was proposed, to explain the features of the phase distributions. It is considered that, in the reactive hot-pressing process, the B and C atoms in B 4 C will diffuse into the Zr and Si sites and form ZrB 2 and SiC in situ, respectively. Because the diffusion of Zr and Si atoms is slow, the microstructure (phase distributions) of the obtained composite shows the features of the zirconium and silicon starting powders.

Crack-tip bridging by particles is considered to be one of the primary strengthening mechanisms of ceramic nanocomposites. Small, brittle particulate inclusions have been shown to cause crack-tip bridging at short distances behind the crack tip. This mechanism leads to modest toughness but a very steep R-curve, and it is the latter that produces the very high fracture strength of the ceramic nanocomposite. Localized high residual stress around the particles (particularly in the case of silicon carbidealumina material) causes the strengthening mechanism to operate effectively, even at a small volume fraction of 5%. The present study predicts the magnitude of the toughness increase and the extent of R-curve behavior for the nanocomposite.

a b s t r a c tSilicon nitride (Si 3 N 4 ) with high thermal conductivity has emerged as one of the most promising substrate materials for the next-generation power devices. This paper gives an overview on recent developments in preparing high-thermal-conductivity Si 3 N 4 by a sintering of reaction-bonded silicon nitride (SRBSN) method. Due to the reduction of lattice oxygen content, the SRBSN ceramics could attain substantially higher thermal conductivities than the Si 3 N 4 ceramics prepared by the conventional gas-pressure sintering of silicon nitride (SSN) method. Thermal conductivity could further be improved through increasing the ␤/␣ phase ratio during nitridation and enhancing grain growth during post-sintering. Studies on fracture resistance behaviors of the SRBSN ceramics revealed that they possessed high fracture toughness and exhibited obvious R-curve behaviors. Using the SRBSN method, a Si 3 N 4 with a record-high thermal conductivity of 177 Wm −1 K −1 and a fracture toughness of 11.2 MPa m 1/2 was developed. Studies on the influences of two typical metallic impurity elements, Fe and Al, on thermal conductivities of the SRBSN ceramics revealed that the tolerable content limits for the two impurities were different. While 1 wt% of impurity Fe hardly degraded thermal conductivity, only 0.01 wt% of Al caused large decrease in thermal conductivity.

Porous silicon carbide (SiC) ceramics were fabricated by an oxidation‐bonding process in which the powder compacts are heated in air so that SiC particles are bonded to each other by oxidation‐derived SiO2 glass. Because of the crystallization of amorphous SiO2 glass into cristobalite during sintering, the fracture strength of oxidation‐bonded SiC ceramics can be retained to a relatively high level at elevated temperatures. It has been shown that the mechanical strength is strongly affected by particle size. When 0.6 μm SiC powders were used, a high strength of 185 MPa was achieved at a porosity of ∼31%. Moreover, oxidation‐bonded SiC ceramics were observed to exhibit an excellent oxidation resistance.