The creep properties of silicon nitride ceramics have usually been discussed by reference to the dependence of the secondary creep rate (εs) on stress (σ) and temperature (T) using a power law relationship of the form:-hskip 0.5cm εs = Aσn(-Q_c/RT) where 'A' and 'n' are constants and 'Q_c' is the activation energy for creep. Studies show, however, that the creep properties of ceramics can not be adequately characterised by measurement of only the secondary creep rate. The evidence indicates that traditional approaches to creep should be abandoned in favour of a new method of analysing creep data termed the <θ> projection concept. This provides equations which describe the creep curve shape and its dependence on test conditions. In this investigation the creep data for five silicon nitride ceramics [obtained in other studies] has been extended by carrying out creep tests on two of the materials at a range of conditions of stress and temperature. The creep data for all five materials has been analysed using both traditional approaches and the θ projection concept. Microstructural studies of the materials have also been carried out. The creep properties of the materials have been related to their microstructural features. It is shown that creep properties are strongly dependent on the character of the intergranular amorphous phase. The investigation shows that the limitations of traditional approaches to creep of silicon nitride ceramics can be overcome by using the θ projection concept to analyse creep behaviour. The θ analysis has been used to identify the dominant influences on creep behaviour patterns. The evidence indicates that the creep behaviour patterns of these materials can be strongly affected by oxidation influenced changes in the character of the intergranular phase. The time and temperature dependence of these changes can influence the stress and temperature dependence of creep curve shape.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:636155 |
| Date | January 1993 |
| Creators | Briggs, D. J. E. |
| Publisher | Swansea University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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