Mechanical property anisotropy in textured silicon nitride
Abstract
Several prior investigations have documented the deformation-induced rotation of $\beta$-$\rm Si\sb3N\sb4$ grains resulting in texture and property anisotropy. Highly oriented microstructures have been produced by hot deformation of full density silicon nitride in the plane strain condition due to the rearrangement of the elongated $\beta$-grains. Predictive modeling of textures was conducted for a range of deformation conditions, and compared with experimentally determined crystallographic orientation distributions. The quantification of crystallographic orientation was performed by measuring X-ray pole figures. Predictive modeling compares favorably with experimental results for a limited degree of deformation. At high deformation levels, grain clustering, which effectively reduces the aspect ratio in modeling is observed, and accounts for deviation of experimental results from modeling. Fracture toughness anisotropy is observed for these materials and is primarily due to crack deflection along oriented grain boundaries. Modeling is used to predict crack path in anisotropic materials by equating an orientation dependent fracture toughness with crack deflection mechanisms. Results from this modeling approach are in reasonable agreement with experimental observations of crack deflection in Vickers indentation and three-point bending, however, this approach only considers mode I of a mixed mode loading. Hertzian contact tests were conducted on fine and coarse grain materials. A definite trend from cone cracking behavior to a shear microcracking mechanism was observed with increasing grain size. However, from observation, cone cracking initiates before the microcracking at all grain sizes, but is impeded in cases where the grain size is finite relative to the contact diameter. Direct measurement of damage associated with indentations was obtained through surface profilometry, allowing for analysis of contact behavior. Hertzian contacts in textured materials result in non-circular cone cracking behavior. This is attributed to the observed fracture toughness anisotropy. Implications regarding enhanced contact properties were demonstrated through sliding ball wear tests which showed dramatic effects of orientation on crack initiation.
Degree
Ph.D.
Advisors
Bowman, Purdue University.
Subject Area
Materials science|Metallurgy|Mechanical engineering
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