Development of a tensile creep apparatus for advanced ceramics and tensile creep investigations of silicon nitride materials

The critical components of the developed tensile creep concept for temperatures up to 1400deg C are described, i.e. geometry of the test pieces, gripping and aligning system and the high temperature extensometer. Misalignment of less than 5% can be achieved. The creep behaviour of two sintered Si3N4-materials is studied. One material contains Y2O3 and MgO as sintering additives and the second Yb2O3. Whereas the first material shows crystalline as well as amorphous grain boundary phases, the Yb-containing material is to a large extension crystalline. Both materials exhibit high nonlinear stress dependencies of the minimal creep rates, which can be assigned to different creep mechanisms. For the material containing an amorphous grain boundary phase the change of the composition of the glassy phase leads to extended transient creep ranges. The dependency of the minimal creep rate on the annealing time suggests a time independent analysis of the primary creep behaviour to determine the stress and temperature dependency of the creep rates. The static fatigue behaviour of both materials can be described as a superposition of creep rupture and fracture by slow crack growth. Tensile and bending creep results are compared indicating clearly the stress relaxation effect which occurs in bending. (orig./MM)