Characterization of the Thermal Shock Resistance of Paper-Derived Ceramics and Comparison with current Modeling Results

In the continuous development of novel refractory ceramics, the introduction of additive manufacturing processes, such as Laminated Object Manufacturing (LOM), offers a valuable approach in the production of multilayered and complex shaped parts. Such ceramic multilayers can range in size from millimeters to centimeters or even larger, and they may feature gradients in the material type or the porosity. Papermaking as a streamlined and effective manufacturing method can be utilized to produce multilayered ceramic components from preceramic papers. Typically, preceramic papers contain ceramic fillers, either particulate fillers and/or ceramic fibers, besides organic paper components, such as pulp fibers and binder starches, and can be categorized as green bodies. By help of a heat treatment at temperatures up to 550 °C in an oxygen-rich environment (the debinding process) any organic components can be burned-off, resulting in ceramic powder compounds with open pore spaces, which in turn can be categorized as brown bodies. Once the debinding process is complete, left-over powder compounds can be converted into solid ceramic components, the white bodies. This can be achieved through solid-state sintering at significantly higher temperatures, e.g. at 1200 °C or above for Al2O3 ceramics. The pulp fibers within preceramic papers create a deformable support structure, being the predominant component of the organic solids. Upon debinding, the network of the almost cylindrical pulp fibers is transformed into a contiguous open pore structure with interpenetrating, channel-like pores. The porous structure within paper-derived ceramics has been shown to improve thermal shock resistance, making multilayer ceramics that incorporate such layers particularly well-suited for demanding applications that involve complex-shaped refractory ceramics. The pores can deflect or stop cracks in a thermal shock event, while a localized thermal insulation can slow down the heat exchange between the bulk material and the surface. Given their brittle nature, paper-derived ceramics are particularly susceptible to abrupt tensile stress which also occurs during warm-to-cold, or downwards thermal shocks. Simulations of thermal shock-induced fractures are becoming increasingly available. Thus, it is of interest to customize those simulations for specific ceramic microstructures, including those of paper-derived single-layer and multilayer ceramics. Notably, phase field models have emerged as highly flexible and robust types of models for predicting material failure. In phase models of ceramics undergoing thermal shock, complex hierarchical crack patterns are generally initiated at an outer surface and propagate into the bulk material. Porous ceramics are described as idealized ceramic material with uniformly distributed, spherical pores in many phase field models from literature. This idealization, however, fails to reliably represent paper-derived ceramics, for which no models of their thermal shock resistance had been published before the completion of this thesis. Consequently, the newly developed and simplified modeling approaches presented in this thesis are based on phase field models originally designed to represent a simultaneous formation of multiple cracks in low porosity ceramics. The influences of the pore structure and reinforcing ceramic fibers in paper-derived ceramics on thermal shock resistances and residual mechanical strengths were effectively captured by a multi-stage computation procedure. An initial step of this computation entailed an analysis of optical microscopy images for the sake of identifying internal pore surfaces and outer surfaces of paper-derived ceramics. The concentration of thermo-mechanical stress was expected at near-surface indentations which could be detected in microscopy images. In a distinct further step, three-dimensional finite element method (FEM) models of the temperature distribution, as well as the propagation of a singular crack, as induced by thermal shock were generated by the COMSOL Multiphysics software. The respective three-dimensional models described idealized, fully dense and flat ceramic platelet samples subjected to thermal shock, to which mechanical and thermal properties of actual paper-derived ceramics were attributed. The predictions from the three-dimensional FEM models were incorporated into simplified two-dimensional phase field models that were specifically developed in Matlab. The modeling of multiple cracks propagating simultaneously across the flat surface of a fully dense platelet-shaped ceramic yielded hierarchical crack patterns. These crack patterns were adjusted to match the real microstructures of paper-derived ceramics, with near-surface indentations that were previously identified in microscopy images. This final computation step enabled calculating the expected residual mechanical strengths of paper-derived ceramics after exposure to thermal shock. Residual mechanical strengths obtained from this approach were compared with corresponding flexural strength test results, executed both prior to and after the thermal shock. This comparison aimed to assess the predictive accuracy of the simplified phase field models employed.