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 micro structure and the mechanical behavior of high-purity silicon nitride w as systematically investigated. Dense silicon nitride bodies doped res pectively with SiO2, Y2O3 and Yb2O3 were fabricated by hot isostatic p ressing (HIP). Two different compositions of the intergranular phase ( nominal oxide additions of 0.7 and 1.7 vol%) were examined for Y2O3 an d Yb2O3 in comparison with the same volume of pure SiO2. Only in the m aterial with the higher Y2O3 and Yb2O3 content was an improved level o f fracture toughness obtained. The mechanical properties at 1400 degre es C were evaluated with emphasis placed on time-dependent strength an d deformation behavior. The materials containing only SiO, or doped wi th the small amount of Y2O3 showed linear elastic K-1-controlled fract ure behavior at 1400 degrees C and the critical phenomenon for failure was subcritical crack growth (SCG) from preexisting defects. In the m aterials with additions of Yb2O3 or the larger amount of Y2O3, crack e xtension was governed by creep crack growth as a result of the exhibit ed strong creep effects. In the silicon nitride doped with 1.7 vol% Yb 2O3, however, a considerably improved creep behavior as a consequence of crystallization processes in the intergranular phase (Yb2Si2O7) cau sed by both thermal treatment and stress-initiated effects during the mechanical testing at 1400 degrees C was found.