Microstructural Degradation Accompanying Tensile Creep Deformation in Quasi-Ductile Silicon Nitride

Creep mechanisms in quasi-plastic silicon nitride by instrumented indentation

“…Our earlier studies found out that Yb and Al concentrations correlate with individual layers. [13][14][15][16] The concentration of Al increased in the zone from 30 to 100% of total thickness of the oxidation affected zone compared to the bulk, whereas Yb concentration was lower there with the exception of large precipitates of Yb 4 Al 2 O 9 . Sucking Yb and Al onto the surface resulted in a formation of the first, thin, porous subsurface layer composed from almost pure silicon nitride.…”

“…Fig. 2 illustrates this case in the terms of [13][14][15][16] engineering strain ε = l/l 0 and true tensile strain, ε t = ln (l/l 0 ), obtained at 1400 • C under the stress of 50 MPa. Both curves are composed of primary stage, short intermediate stage with a minimum strain rate and a prolonged accelerated stage.…”

“…Specimen geometry, creep machines and testing procedure were described earlier. [13][14][15][16][25][26][27] Microstructure Study. Creep introduced changes in the microstructure were studied by light and scanning electron microscopy (SEM) on polished cross sections, creep fracture surfaces and secondary fracture surfaces obtained by intentional fracture of the samples after testing at room temperature.…”

“…To distinguish such transitional behavior between creep and superplasticity, the terms "quasi-plasticity" or "quasi-ductile" behavior were suggested for this type of material. [13][14][15][16] However, though cavitation was considered to be the most important for producing large strains, the interaction between oxidation and creep was not fully understood.…”

“…Our earlier creep studies on this grade at the temperatures above 1300 • C revealed unusual creep behavior with the failure strains exceeding 20% during the prolonged accelerated stage. [13][14][15][16] They were accompanied by intensive cavitation in the core, strong oxidation enhanced by stress in surface layers and intensive cracking of these layers. 13 The obtained strains were up to five times higher than those in the standard silicon nitride grades, [17][18][19][20] however, at least half order of magnitude lower than the strains obtained during superplastic deformation.…”