Preferred orientation and property anisotropy in silicon nitride

Farnjeng Lee, Purdue University

Abstract

Quantitative texture analysis, including calculation of the orientation distribution function, is used to demonstrate the degree of preferred orientation in $\beta$-$\rm Si\sb3N\sb4$ which has been produced by several forming processes, including: hot-pressing, hot-forging, and plane strain compression. Experimental results reveal that the texture is determined by the processing parameters including: temperature, sintering additives, and stress state. Grain rotation and preferred nucleation and/or grain growth contribute to texture development in $\beta$-$\rm Si\sb3N\sb4.$ Basal (00l) pole figures obtained from the orientation distribution function are consistent with microstructural observations and are reflected in indentation fracture toughness anisotropy. A grain rotation model based on fluid mechanics is applied to quantitatively predict the texture development and texture nonhomogenenity in hot-worked $\beta$-$\rm Si\sb3N\sb4.$ The results indicate this model can successfully predict the texture developed in $\beta$-$\rm Si\sb3N\sb4$ with significant differences observed only in highly strained materials. Further extension of this model is applied to investigate the texture development in materials with rod- or disc-shaped grains under various forming processes. A crack deflection model is used to evaluate the toughness anisotropy in textured $\beta$-$\rm Si\sb3N\sb4.$ The dependence of toughness anisotropy on the degree of texture, volume fraction, and aspect ratio of reinforcing grains is investigated in materials toughened by the crack deflection mechanism. The model predictions demonstrate that toughening is determined not only by the orientation of crack plane but also by the crack propagation direction in textured materials. The potential for application to other toughening mechanisms is also addressed. These model results are compared to experimental results on toughness anisotropy in textured silicon nitride.

Degree

Ph.D.

Advisors

Bowman, Purdue University.

Subject Area

Materials science

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