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Title: Micromechanisms for plasticity and fracture of Si-Al-O-N ceramics
Author: Karunaratne, Baththanamudiyanse S. B.
ISNI:       0000 0001 3595 2309
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1980
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The broad aim of this research programme was to understand the structure, deformation and fracture processes associated with grain boundaries in nominally single phase ẞ Si-Al-O-N ceramics, hot- pressed with different sintering aids (MgO and Mn3O4). The major microstructural difference between the two types of ceramic is the occurrence of microscopic regions of triple-junction silicate glass in the Mn-containing ceramic. Lattice imaging shows that there is no resolvable phase within two-grain interfaces in both ceramics down to an approximate limit set by the prism-plane spacing of Si-Al-O-N crystals (6.6 Å). However, Auger electron spectroscopy indicates the presence of Mg, Mn, 0 and impurity Ca at grain boundaries. Creep tests have been performed in a newly designed and constructed apparatus which can easily be adapted to perform bend and uniaxial compressive creep. A double-torsion jig has been constructed to determine crack propagation data in vacuum. The dominant deformation mechanism in the Mn-containing ceramic is that of grain boundary sliding accompanied by cavitation at triple junctions nucleated within the silicate phase. The measured non­integral stress exponent (n » 1.5) and activation energy (Q “ 490 k Jmol-1) in the creep rate equation ἓ = const. ⥀n exp (-Q/RT) are typical of commercial Si3N4 ceramics. A similar cavity-inter linkage is the principal mechanism for subcritical crack growth, characterised by a low value for the stress intensity (K1) exponent (n) in the relation V(crack velocity) ■ const. K1n. Triple junction silicate, and hence cavitation, is absent in the Mg-containing ceramic, which exhibits a grain boundary diffusional (Coble) creep mechanism (n * 1). Subcritical crack growth is restricted to a narrow range of K1 and shows a higher 1--exponent (n ∼ 13) which favours a diffusive crack growth mechanism. The influence of heat-treatment on high-temperature creep and sub­critical crack growth has been analysed from microstructural evidence and determination of creep and crack growth parameters. Extraction of impurities (Mg, Mn, Ca etc.) from grain boundaries into a surface oxide film and consequent crystallisation of remaining glass components as ẞ- Si-Al-O-N, results in marked improvement in creep and resistance to subcritical crack growth. The most significant change is the elimination of triple-junction glass and hence suppression of cavitation during creep and the cavity-interlinkage mechanism for slow crack growth in the Mn-containing ceramics. A creep mechanism of grain boundary diffusion is characterised by stress exponent n “ 1 and high activation energy > 800 k Jmol-1. The slow crack growth behaviour favours a diffusive crack growth mechanism or that of a thermally- activated bond breaking.
Supervisor: Not available Sponsor: Commonwealth Scholarship Commission in the United Kingdom
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics