A range of composites have been prepared by dispersing various sources of SiC whiskers and platelets into a Si3N4 based matrix intended for high temperature applications. The main objective was to improve the fracture toughness of the Si3N4 monolith without any detrimental effect on other properties such as the excellent high temperature deformation behaviour. The matrix composition was tailored so that the intergranular phase formed desirable crystallisation products within the Si3N4 - Si2N20- Y2Si207 phase system. A slip casting technique was used to prepare the composites, in order to preferentially align the dispersoids, and conventional powder pressing methods were used to achieve a random distribution of dispersoids. Composites containing at least 30 weight % dispersoid content were fully densified using ABB Cerama's glass encapsulated HIP process. The resulting composites showed steadily increasing fracture toughness values with increasing SiC content, although the maximum increment was only approximately 30%. Platelet containing composites had slightly higher values than composites containing a similar volume fraction of whiskers, although there was a significantly lower fracture strength in the former. A microstructural survey of the materials indicated that the mechanical properties were influenced by the dispersoid /matrix interfacial characteristics. Fracture surfaces of the platelet containing composites showed there was significant debonding at the interface, whereas the whiskers appeared strongly bonded to the matrix. Whisker reinforced Si3N4 composites showed excellent creep resistance at 1400°C, which appeared slightly better than the monolith. Both materials had a similar stress exponent value close to 1, which suggests the deformation mechanisms in both these materials are similar and follow a grain boundary diffusional flow mechanism. There appeared to be a threshold stress level of 250 MPa at 1400°C for both materials where failure did not occur after prolonged deformation. This result and the absence of cavity formation wiihin the intergranular region suggests there is a sub- critical residual glass volume at this stress level for the nucleation of cavities and hence for slow crack growth. Both monolith and composites show good oxidation resistance at these elevated temperatures.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:332511 |
| Date | January 1992 |
| Creators | Ketchion, Stephen Martin |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/110523/ |
Page generated in 0.0016 seconds