The high oxidation resistance of ceramics based on silicon nitride has contributed to their potential for use in high-temperature structural engineering applications. The oxidation behaviour and its effect on mechanical properties of a range of hot-pressed ceramic alloys of composition Si6-z Alz 0z N8-z (1 < z < 4) has been investigated at temperatures between 1200° and 1400°C. The mechanism of oxidation has been identified by determination of the kinetics of oxidation and the nature of the oxide scales using X- ray diffractometry, and scanning and transmission electron microscopy. The rate-controlling process in the oxidation of these ceramics is that of the inward diffusion of oxygen to the ceramic/oxide interface where the oxidation reaction takes place. In the early stages of oxidation the composition of the oxide scale and the rate of oxidation is modified by the diffusion of grain-boundary segregated additive and impurity ions such as Mg and Ca into the scale. The redistribution of these elements on oxidation creates a ceramic of more stabilised structure and of higher oxidation resistance. By heat-treatment of large blocks of ceramic the mechanism of this redistribution has been identified as that of diffusion into the oxide scale with negligible dissolution into the β' crystal phase of the ceramic. The out-diffusion of elements is accompanied by the removal of grain boundary phases by the local growth of the β' phase. Local enhancement of the rate of oxidation by the out-diffusion of the elements Mg and Ca towards the scale gives rise to a low creep rupture stress for the ceramic via the formation of intergranular cracks which grow at sub-critical stresses. This has been determined by tensile creep rupture tests which have been performed on two ceramics of composition Si5A10N7 at temperatures between 1275° and 1325°C.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:332768 |
| Date | January 1984 |
| Creators | Pickering, Colin Raymond |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/110574/ |
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