Complete densification of Si3N4-SiCceramic matrix composites (CMC's) by a pressureless sintering route

In this work, optimal conditions for the production of Si3N4 and Si3N4/SiC composites are advanced. Nearly 100% dense materials are obtained after 30min dwelling at 1750degreesC for composites with 5 and 110vol.% SiC. For the 30vol.% SiC composite and for the matrix, a 2 hours stage at 1750degreesC is needed before maximum density ( > 99%) is obtained. X-ray diffraction showed that only beta-Si3N4 and alpha-SiC crystalline phases are present. The mechanical properties of the composites were related to the microstructure and sintering cycle. SiC particles inhibit the Si3N4 matrix grain growth and contribute to the increase in hardness from 12.1GPa to 15.0GPa, relatively to the monolithic material. K-Ic, of the composites, with values in the range 6 and 8 MPa(.)m(1/2), is only slightly larger than that of the matrix, as expected for submicrometer dispersed particles of aspect ratio close to 1. The fabrication process allowed the production of monolithic Si3N4 matrix with a 4-point bending strength of 800MPa and a Weibull modulus of 11, in agreement with values reported for materials produced by pressure assisted techniques.