Two MgO-doped Si3N4 specimens (Norton, NC-132) were tested in ultra-long flexure stress rupture experiments with an external load of 266 MPa, i.e., 14.941 and 17.376 hours. Following mechanical testing, the materials were characterized by transmission electron microscopy, in order to correlate the mechanical response with the microstructural changes observed upon long-term exposure to 1100 C. Although the starting material revealed the presence of a secondary glass phase both at multi-grain junctions and along grain boundaries, the tested specimens contained no remainder of any residual glass. Former secondary triple pockets filled with amorphous phase showed epitaxial precipitation of Si3N4 after long-term testing. In addition, continuous amorphous grain-boundary films, observed prior to annealing, evolved into glass-free interfaces. Concurrently to the elimination of the secondary glass phase, a continuous network of cracked boundaries was observed after long-term flexure testing. It is therefore concluded that these materials reveal are affected by creep deformation via microcrack nucleation and growth based on the depletion of the amorphous siliceous grain-boundary phase.
Citation: Journal of the American Ceramic Society
Pub Type: Journals
creep, glass boundary phase, silicon nitride, stress rupture, transmission electron microscopy