Fabrication fo Si₃N₄/SiC Composite Ceramics by Reactive Hot-Pressing from Elemental Powders

Abstract

Using elemental powders, reactive hot-pressing was carried out to prepare Si₃N₄/SiC composite ceramics. Silicon and graphite powders with additives (Al₂O₃ and Y₂O₃) were reacted at 1873K in gaseous nitrogen (0. 15MPa) and hot-pressed at 2073K for 7.2ks with applied pressure of 70MPa, and finally formed a Si₃N₄(β-Si₃N₄)-SiC(β-SiC) composite with a small amount of glassy and crystalline phases from additives. The resulting composites had 12-34vol%SiC, and the SiC grains were granular with sizes of about 0.1-1μm, and homogeneously distributed and well bonded to the matrix Si₃N₄. On the other hand, the Si₃N₄ matrix grains were rod-like in shape with sizes of 0.3-3μm. It was found that the composite ceramics had a relatively higher bending strength at both ambient and elevated (at 1423K) temperatures; about 1000 and 700 MPa, respectively. Fracture toughness of 5.3 MPam^1/2 was obtained by indentation fracture tests. It was evident that reactive hot-pressing was very effective process for in-situ synthesis of Si₃N₄/SiC composite ceramics.