Intrinsic fracture energy of polycrystalline silicon nitride

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Abstract
The fracture surface energy of a dense pure polycrystalline sample of silicon nitride (Si3N4) has been measured at room temperature by the work-of-fracture method on chevron-notched bend bars. Dense specimens were fabricated by hot isostatically pressing highly pure Si3N4 starting powder at 1950°C under 180 MPa via glass encapsulation. Annealing experiments at 1750°C in a nitrogen atmosphere were also conducted after sintering in order to release internal residual stresses possibly arising from cooling down under high pressure. A value of 10–7 ± 0–8 J m−2was measured in both as-sintered and annealed specimens. According to fractographic observation and calculation of the theoretical compliance of the notched specimen, the present value is regarded as the intrinsic fracture energy of Si3N4 and is to be compared with the markedly higher values commonly measured in alloyed materials.