Subcritical Crack Growth Resistance in Silicon Nitride Ceramics

Abstract

Long-term strength and lifetime of brittle ceramic materials at elevated temperatures are determined by their resistance to subcritical (slow) crack growth (SCG) and creep failure. Compared with creep deformation phenomena, however, elementary mechanisms of SCG process at high temperatures have been much less clarified. In the present work, HIPed high-purity Si₃N₄ without additives was used as a standard material, and influences of some dopants have been systematically investigated. Based on these experimental data, two SCG mechanisms have been proposed. 1) Grain boundary sliding/cavity formation mechanism. This mechanism is important especially when SiO₂-glass modifiers are segregated at the intergranular glassy phase. Typical examples can be seen in low Ca-doped and F-doped Si₃N₄ materials. SCG can be triggered by the grain boundary sliding which produces stress concentration at triple points and cavity formation as a result. Decrease in the “apparent” viscosity of the intergranular film can be clearly detected by internal friction measurements as a decrease in the relaxation peak-top temperature. 2) Solution effects. In dilute sialon materials (Si_6-zAl_zO_zN_8-z, z<0.25), SCG resistance was significantly reduced with little increase in steady state creep strain rate at 1400 ^°C. But, internal friction measurement showed that the “apparent” viscosity of the intergranular glass film was not so much decreased as in the case of low-Ca doped system. High density of dislocations were observed in transmission electron micrographs of crept materials and these dislocations are proposed to be directly responsible to the SCG process through piling up at grain boundaries. This mechanism may be important when the solute atom modifies the electronic structure of Si₃N₄ considerably and reduces the local covalency. Our electronic calculation suggests that Ga, Be and Li could show similar effects in β-Si₃N₄ matrix to Al.