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Title: The microstructural stability of Si-Al-O-N ceramics in severe thermal environments
Author: Heath, Gary Robert
ISNI:       0000 0001 3551 355X
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1985
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The work described in this thesis is broadly a study of the stability of the microstructure of a range of pressureless sintered Y-Si-Al-O-N ceramics in various high temperature environments. More specifically» the stability is assessed with respect to resistance to; oxidation, fracture, annealing and laser irradiation. The techniques used for microstructural assessment are; Transmission and Scanning Electron Microscopy, Energy Dispersive X-ray Analysis together with Electron and X-ray Diffraction. Microstructural modifications are facilitated by changing the content of the phase 21R in the initial composition. The as-sintered material has a glass matrix and 21R is demonstrated to alter the glass Nitrogen level. Nitrogen is concluded to be a critical factor influencing the observed mechanical properties. The matrix can be crystallised to Yttriun Disilicate (Y2S) or Yttrium Aluminium Garnet (YAG) and their relative merits are assessed. Oxidation behaviour for all the Y-Si-Al-O-Ns is presented, with emphasis on sub-oxide modifications. Material with a Y2S matrix exhibits excellent oxidation resistance, comparable to Hot-Pressed (H.P.) material, whilst YAG-containing material catastrophically oxidises above a threshold temperature of 1300oC. The degradation mechanism is a reaction between YAG and SÍO2 (the oxide) producing a ternary eutectic liquid. This degradation was overcome by a novel modification of the surface matrix to the Silicon Oxynitride phase. The fracture toughness (Kjc) together with slow crack growth rates and mechanisms are studied at high temperatures using Notched Beam and Double Torsion specimens respectively. A critical dependence on the presence of any residual glass was demonstrated and an exceptionally high Kjc level recorded. Laser-cut surfaces are studied and identify the material loss mechanisms. This work is the first such study and identifies H.P. material as highly suitable for laser machining.
Supervisor: Not available Sponsor: Joseph Lucas (Industries)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics