Search

We investigated and thoroughly analyzed compressive response of ice-templated ceramics in quasistatic and dynamic regimes of strain rate. In the high pore volume regime, sintered scaffolds exhibited highly lamellar pore morphology and pore architecture transitioned to dendritic with the decreasing pore volume. Mechanical property measurements in the quasistatic regime of strain rate revealed that with increasing density, compressive response transitioned from a damageable, cellular-like failure to a brittle-like failure. We rationalized the measured results in terms of the propensity of the lamella walls to undergo buckling. Our conjecture is that in the high pore volume regime, lamella walls of the scaffolds are prone to buckling-induced elastic instability, which leads to a compressive response that manifests a gradual decrease of stress beyond peak stress. In contrast, we suggest that in the low pore volume regime thick lamella walls and extensive transverse bridging exhibit marked resistance to buckling-induced instability and scaffolds undergo a global failure, which is manifested in the measured quasistatic compressive response as a sharp drop of stress beyond peak stress. Dynamic compressive response of the scaffolds exhibited measurable differences relative to the quasistatic compressive response. Scaffolds exhibited a relatively gradual decrease of dynamic compressive stress beyond peak stress, and an overall improvement of compressive response and energy absorption capacity was measured under the dynamic loading conditions. We rationalized the measured differences in between the strain rate regimes in terms of the micro-inertia and other effects. This study is of critical significance for applications of ice-templated structures in dynamic environments.

Ice-templated ceramics have unique lamellar pore morphology that offers better compressive mechanical properties in comparison to the typical ceramic foams with isotropic pore morphology. However, for very high-level porosity (>65 vol%) strength difference diminishes. This investigation reveals that by inducing anisotropic grains within the matrix of a fine-grained ceramic, uniaxial compressive response of the ice-templated sintered scaffolds can be markedly enhanced without causing any considerable modification of the total porosity. To address this innovative materials design strategy, we synthesized a series of microstructures by systematically varying the anisotropic grains content in an aqueous suspension and the freezing kinetics to investigate the process-microstructure correlations and understand the structure-property relationships. Microstructural investigations revealed the unique arrangements of the platelets within and out of the lamella walls, where the upward moving ice fronts aligned the platelets' in-plane direction to the ice-growth direction. In the low freezing front velocity regime, platelets were observed to be mainly within the lamella walls, whereas platelets started to develop lamellar bridges with the increasing velocity. As a result, a transition of the pore morphology occurred with the increasing platelets content and the freezing front velocity. A novel method based on the rigorous microstructural analysis is developed to estimate the distribution of the platelets within and out of the walls and the variation of the platelets distribution with the composition and freezing front velocity. A drastic improvement of the compressive mechanical properties (stiffness, strength, energy absorption capacity) was measured due to the platelets' addition, which has been related to the platelets' distribution within and out of the walls and the pore morphology modifications. Results are rationalized based on the role of the platelets during the compressive deformation.

In boron carbide (B_(4)C), the loss of crystallinity (i.e. amorphization) during high-pressure loading is known to result in reduced hardness and inferior ballistic performance. In this investigation, a systematic procedure is developed to evaluate the size and shape of the amorphized zone as well as the spatial distribution of the amorphization intensity beneath a Vickers indentation. This is accomplished by successive metallographic polishing and material removal from the indented surface at submicron depth increments and then scanning of each surface using micro-Raman spectrometry to construct a three-dimensional map of the amorphized zone. The amorphized zone in B_(4)C extends to a depth of almost seven times the indentation depth. The self-similarity in stress fields at various depths is also confirmed from quantitative description of the Raman peaks for various loads. Damage evolution in the form of a Mescall zone and initiation of radial cracking from the amorphized zone were also observed. It is inferred that the amorphization process initiates as soon as the indenter tip makes contact with the B_(4)C surface and thereafter the indenter penetrates into the weakened amorphized material. The implications of this observation and the utility of the quantitative information on the size and shape of the measured amorphized zone for calibration of multiscale models on structural changes in B_(4)C are discussed.

An analytical framework for determination of scratch-induced residual stress within SiC grains of ZrB2–SiC composite is developed. Using a ‘‘secular equation” that relates strain to Raman-peak shift for zinc-blende structures and the concept of sliding blister field model for scratch-induced residual stress, explicit expressions are derived for residual stress calculation in terms of phonon deformation potentials and Raman peak shift. It is determined that, in the as-processed composite, thermal expansion coefficient mismatch between ZrB2 and SiC induces compressive residual stress of 1.731 GPa within the SiC grains and a tensile tangential stress of 1.126 GPa at the ZrB2– SiC interfaces. With increasing scratch loads, the residual stress within the SiC grains becomes tensile and increases in magnitude with scratch load. At a scratch load of 250 mN, the calculated residual stress in SiC was 2.6 GPa. Despite this high value, no fracture was observed in SiC grains, which has been rationalized based on fracture strength calculations from Griffith theory.

A ZrB2–5 wt.%SiC composite, consolidated using the plasma pressure compaction technique, was subjected to scratch loads in the range of 50–250 mN using a Berkovich nanoindenter. Microstructural analysis revealed that the scratch grooves consisted of numerous slip bands oriented at random angles to the scratch direction and microcracks oriented perpendicular to the scratch direction. The observed features were rationalized using an elastic stress field due to the combined Boussinesq- and Cerruti-field solutions. The analysis revealed that maximum shear stress occurs ahead of the scratch tool whereas the maximum principal tensile stress occurs in the wake of the scratch tool. Accordingly, it was argued that the slip bands occurred first in the region ahead of the scratch tool due to maximum shear stress while the microcracks developed later due to maximum tensile stress in the wake of the scratch tool. This result was further confirmed by the existence of slip bands ahead of the scratch tip at the exit end of the scratch grooves.