Joining of reaction-bonded silicon carbide using a preceramic polymer

Ceramic joints between reaction-bonded silicon carbide (RBSiC) were produced using a preceramic polymer (GE SR350 silicone resin) as joining material; samples were heat treated in an argon flux at temperatures ranging from 800-1200 degrees C without applying any pressure. The strength of the joints was determined by four-point bending, shear and indentation tests. Microstructural and microchemical analyses were performed by optical microscopy, SEM, TEM and AEM. The room-temperature strength of the joints increased with the joining temperature. Maximum values as high as 220 MPa in bending and 39 MPa in shear tests were reached for samples joined at 1200 degrees C. No detectable residual stresses were observed both in the joining material and the joined parts, and the fracture mechanism was nearly always cohesive. The joint thickness was shown to depend on the processing temperature, and ranged from about 2-7 mu m. The joining material was a silicon oxycarbide amorphous ceramic, with no oxygen diffusion occurring between this and the RBSiC joined parts. The rack of compositional gradients, precipitates or reaction layers indicate that the SiOC ceramic acted as an inorganic adhesive, and that the joining mechanism involved the direct formation of chemical bonds between the RBSiC parts and the joining material. (C) 1998 Chapman & Hall.