Silicon cavitation creep model

The scheme also assumes that the redistribution of the secondary phase changes the size distribution of cavities within the material, which is supported by experimental observations [23,46], Final proof of the cavitation creep model is the fact that Eq. (2) fits the experimental data over a wide range of stresses for both SN 88 and NT 154. Thus, the model shovm in Figure 13.16 provides an understanding of the overall creep behavior of silicon nitride. Cavitation is the main creep mechanism in silicon nitride, and in other similarly bonded ceramics. 

In the absence of cavitation, creep in vitreous-bonded materials would occur by S-P, wherein material dissolves from one side of the grain and deposits on another [48, 49]. No definitive studies have been made to date that support the dislocation creep models in which the grains of silicon nitride deform by dislocation motion. Studies of deformed silicon nitride grains have provided no evidence of the types of dislocation pileup that should be present in order for this type of mechanism to be active [50]. 

Figure 13.16 Phenomenological model of the cavitation processes during tensile creep deformation in silicon nitride [15, 27].

Tensile creep in silicon nitride ceramics are best described by meso-mechanical models based on the dilatation of granular solids. These models provide a rationale for the exponential dependence of creep rate on applied stress, creep asymmetry, and the role of cavitation in the creep process. Meso-mechanical models are based on the assumption that grains of silicon nitride are rigid during deformation, so that displacements between adjacent grains can only occur along the grain boundaries. 

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