THE FABRICATION AND FAILURE OF LAMINAR CERAMIC COMPOSITES

Rather than using fibres or other dense inclusions as a means of introducing weak interfaces to deflect cracks, a much simpler route has been developed where composite elements, in this case sheets, are made from silicon carbide powder. After coating, these sheets are stacked, compacted together and sintered without any pressure. It is shown that the graphite layers will deflect cracks preventing catastrophic failure whilst raising the apparent fracture toughness from 3.6 MPa, square-root m to 17.7 MPa, square-root m and the work required to break the sample from 28 J m-2 to 6152 J m-2. Further crack growth through the sample occurs when the unbroken part reaches its failure stress as determined from an unnotched beam. This allows the apparent fracture toughness to be simply related to the failure stress and this has been confirmed by experiments where the strength of the beam has been controlled by adding artificial flaws. Crack growth is not much affected by the thickness of the interface except at low thicknesses where small gaps in the interface are more likely to occur, allowing the delamination crack to kink out of the interface and cross the next lamina. The thermal stability of the laminate up to 1500-degrees-C in air is limited only by the oxidation resistance of the graphite. The potential for the process is also discussed.