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Micromechanisms for plasticity and fracture of Si-A1-0-N ceramics
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Karunaratne, Baththanamudiyanse S. B. (1980) Micromechanisms for plasticity and fracture of Si-A1-0-N ceramics. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b1751733~S1
Abstract
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.
Item Type: | Thesis (PhD) | ||||
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Subjects: | Q Science > QC Physics | ||||
Library of Congress Subject Headings (LCSH): | Ceramic materials -- Plastic properties., Ceramic materials -- Fracture, Ceramic materials -- Surfaces., Grain boundaries, Superplasticity, Surfaces (Physics) | ||||
Official Date: | January 1980 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Department of Physics | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Lewis, M. H. | ||||
Sponsors: | Commonwealth Scholarship Commission in the United Kingdom | ||||
Extent: | [14], 133, [11] leaves : illustrations | ||||
Language: | eng |
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