Particle size effect of starting SiC on processing, microstructures and mechanical properties of liquid phase-sintered SiC

Dense SiC (97.3-99.2% relative density) of 1.1-3.5 mu m average grain size was prepared by the combination of colloidal processing of bimodal SiC particles with sintering additives (Al2O3 plus Y2O3, 2-4 vol%) and subsequent hot-pressing at 1900-1950 degrees C. The fracture toughness of SiC was sensitive to the grain boundary thickness which was controlled by grain size and amount of oxide additives. A maximum fracture toughness (6.2 MPa m(1/2)) was measured at 20 nm of grain boundary thickness. The mixing of 30 nm SiC (25 vol%) with 800 nm SiC (75 vol%) was effective to reduce the flaw size of fracture origin, in addition to a high fracture toughness, leading to the increase of flexural strength. However, the processing of a mixture of 30 nm SiC (25 vol%)-330 nm SiC (75 vol%) provided too small grains (1.1 mu m average grain size), resultant thin grain boundaries (12 nm), decreased fracture toughness, and relatively large defect of fracture origin, resulting in the decreased strength. (C) 2010 Elsevier Ltd. All rights reserved.