Intrinsic fracture energy of polycrystalline silicon nitride

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.