"Nanomechanical Measures of Robustness for Highly Sensitive Brittle Materials" 

Morgan Chamberlain, MSE PhD Candidate 

Advisor: Professor David Bahr

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ABSTRACT

The nanomechanical response of materials to an induced stress is critical to understanding a holistic pathway to failure; starting with the onset of plasticity, to propagation of plastic deformation, to eventual fracture. This road to failure is less easily traveled when the material is highly sensitive either mechanically or chemically, making the measurement itself non-trivial. Nanomechanical measures of robustness are investigated in this dissertation using nanoindentation for two such materials that bookend the breadth of materials science: delicate amorphous silica aerogels, and the energetic molecular crystal pentaerythritol tetranitrate (PETN).

The onset of dislocation-mediated plasticity is first investigated for PETN, an elastically and plastically anisotropic material. The influence of tip radius, geometry, and acuity is assessed using three indenter tips (Berkovich, conical, Knoop) on the yield behavior, mechanical measurements, and propensity for fracture of the (110)⟨001⟩ slip system. A critical crystal orientation is identified at which the azimuthal angle of the Knoop is 45° from the ⟨001⟩ direction on the (110) plane of the crystal. At this angle, a 69% higher perceived hardness was measured that was not previously captured in foundational micro-hardness Knoop studies.

A precise measure of toughness has not been previously feasible for PETN due to its complex slip systems, which has inhibited the use of most indentation toughness models. This current study verified that fracture thresholds can be found for the 45° azimuthal angle. Consequentially, the fracture behavior along this major slip system of PETN was isolated and repeated, which enabled the implementation of Morris and Cook’s three-field indentation wedging model. As a result, a measure of toughness for PETN is presented for the first time to be 0.106 MPa m1/2. This result provides an additional metric by which mock materials can be compared to for the safe testing of PETN surrogates.

For an amorphous porous material, such as silica aerogel, deformation is not controlled by atomistic or molecular defects like dislocations but rather through complex particle flow, shear, and fracture of silica clusters. Due to this fragile network, silica aerogels face a bottleneck of durability despite their ultra-low thermal conductivity having significant potential for heat management applications in computer packaging. A safe method of indenting aerogels is developed through this work; additionally, the impact on mechanical robustness of two processing methods to decrease porosity in the aerogels is quantified. A reduction of porosity accomplished by modifying the silica content as well as increasing the temperature during gel solidification is shown to increase the hardness and elastic modulus by up to 500%, while minimally affecting the thermal conductivity. These results provide a foundational step in the continued efforts towards overcoming durability concerns in expanding aerogel applications.