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Initial R-Curve Behaviour of SiAlON Ceramics

Many ceramics of interest for load-bearing applications exhibit the effect of an increasing crack growth
resistance, KR, during crack extension, Δa, that is closely related to its microstructure. For a rising R-curve
with KR=f(Δa), after exceeding the so-called crack-tip toughness, KI0, stable crack extension occurs initially
under increasing load until eventually a final critical fracture event occurs. Especially bridging effects
behind the propagating crack are responsible for this behaviour, which is known for silicon nitride-based
ceramics. In this paper, R-curves KIR(Δa), including crack tip toughness KI0, were determined for three different
neodymia-based α/β-SiAlON ceramics. Strongly rising crack-growth resistances were found in all
cases with plateau values of 4,8–6 MPa√m. The evaluation of crack opening measurements on Vickers
indentation cracks provided starting values of the R-curves of about 1,4 up to 1,6 MPa√m respectively,
depending slightly on the composition. As an example for the importance of the initial part of the
R-curves, the effect on strength predictions is discussed.

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