Abstract
A comparison has been made of hot-pressed Si-Al-O-N ceramics, with different impurity sintering aids (MgO and Mn3O4), in relation to microstructure, high-temperature creep and fracture. The Mn-containing ceramic exhibits a mechanism for creep of grainboundary sliding accompanied by cavitation at triple junctions, nucleated within an impurity silicate residue. The measured non-integral stress exponent (n∼1.5) and activation energy Q in the creep equation \(\dot \varepsilon \) = const. σ nexp (-Q/kT) are typical of commercial silicon nitrides. A similar cavity-interlinkage is the principal mechanism for sub-critical crack growth, characterized by a low value for the stress-intensity exponent (n) in the relation V (crack velocity)=const. K n1 determined on double-torsion test specimens. Triple-junction silicate, and hence cavitation, is absent in the Mg-containing ceramic, which exhibits a Coble diffusional creep mechanism (stress exponent n=1). Sub-critical crack growth occurs only over a narrow range of stress intensity, near to K lC with n∼13 in the V-K n1 relation. A grain-boundary de-segregation caused mainly by extraction of impurities into an oxide film results in further improvement in creep and resistance to sub-critical crack growth.
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Karunaratne, B.S.B., Lewis, M.H. High-temperature fracture and diffusional deformation mechanisms in Si-Al-O-N ceramics. J Mater Sci 15, 449–462 (1980). https://doi.org/10.1007/BF00551698
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DOI: https://doi.org/10.1007/BF00551698