Fracture toughness and time-dependent strength behavior of low-doped silicon nitrides for applications at 1400 degree

The influence of small additions of three selection oxides on microstructure an mechanical behaviour of high-purity silicon nitride was systematically investigated. Dense silicon nitride bodies doped respectively by SiO2, Y203 and Yb203 were fabricated by hot isostatic pressing (HIP). Two different compositions of the intergranular phase (nominal oxide additions of 0,7 and 1,7 vol. percent) were respectively examined for Y203 and Yb203 in comparison with the same volume amount of pure SiO2. Only in the case of the material with the higher Y203 and Yb203 content an improved level in fracture toughness was obtained. The mechanical properties at 1400 degree were evaluated with emphasis placed on time dependent strength and deformation behaviour. The materials containing only SiO2 or doped with the small amount of Y203 showed linear elastic K-controlled fracture behaviour at 1400 degree and the critical phenomenon for failure was subcritical crack growth (SCG) from pre-existing defects. In the case of the materials with additions of Yb203 or the larger amount of Y203 the crack extension is governed by creep crack growth as the result of the exhibited strong creep effects. In the silicon nitride doped with 1,7 vol. percent Yb203, however, a considerable improved creep behaviour as a consequence of crystallisation processes in the intergranular phase (Yb2Si207) caused by both thermal treatment and stress initiated effects during the mechanical testing at 1400 degree was found.