FRACTURE-TOUGHNESS AND TIME-DEPENDENT STRENGTH BEHAVIOR OF LOW-DOPED SILICON NITRIDES FOR APPLICATIONS AT 1400-DEGREES-C

The influence of small additions of three selected oxides on the microstructure and the mechanical behavior of high-purity silicon nitride was systematically investigated. Dense silicon nitride bodies doped respectively with SiO2, Y2O3 and Yb2O3 were fabricated by hot isostatic pressing (HIP). Two different compositions of the intergranular phase (nominal oxide additions of 0.7 and 1.7 vol%) were examined for Y2O3 and Yb2O3 in comparison with the same volume of pure SiO2. Only in the material with the higher Y2O3 and Yb2O3 content was an improved level of fracture toughness obtained. The mechanical properties at 1400 degrees C were evaluated with emphasis placed on time-dependent strength and deformation behavior. The materials containing only SiO, or doped with the small amount of Y2O3 showed linear elastic K-1-controlled fracture behavior at 1400 degrees C and the critical phenomenon for failure was subcritical crack growth (SCG) from preexisting defects. In the materials with additions of Yb2O3 or the larger amount of Y2O3, crack extension was governed by creep crack growth as a result of the exhibited strong creep effects. In the silicon nitride doped with 1.7 vol% Yb2O3, however, a considerably improved creep behavior as a consequence of crystallization processes in the intergranular phase (Yb2Si2O7) caused by both thermal treatment and stress-initiated effects during the mechanical testing at 1400 degrees C was found.