In-situ synthesis of Si3N4-SiC composites by reactive hot-pressing

Reactive hot-pressing was carried out with elemental powders to prepare Si3N4-SiC composite ceramics. Silicon and graphite powders with sintering additives (Al2O3 and Y2O3) were reacted at 1873 K in gaseous nitrogen (0.15 MPa) and hot-pressed at 2073 K for 7.2 ks with applied pressure of 70 MPa, and finally formed a Si3N4(beta-Si3N4)-SiC(beta-SiC) composite with a small amount of glassy and crystalline phases from additives. The resulting composites had 12, 21, and 34 vol% SiC. The Si3N4 matrix grains were rod-like in shape with sizes of about 0.3-5 mu m, and with aspect ratios of about 5-10. The SiC grains were granular with sizes of about 0.1-1 mu m, and homogeneously distributed and well bonded to the matrix. It was found that the composite ceramics had a relatively high bending strength both at ambient and elevated (1423 K) temperatures; about 1000 and 700 MPa, respectively. Fracture toughness of 5.3 MPa m(1/2) was obtained by indentation fracture tests. It was evident that reactive hot-pressing is a very effective process for in-situ synthesis of Si3N4-SiC composite ceramics.