In situ micro-mechanical characterization and multi-scale modeling of thermo-mechanical properties of micro-architectured tungsten coatings
Johns Hopkins University
Micro-architectured refractory metal surfaces have been recently proposed as coatings for extremely high temperature applications and for plasma facing components in the aerospace and energy industries. While ongoing research shows that such micro-architectured coatings are capable of mitigating extreme high thermal loads and withstanding radiation damages, very little is known about their microstructure, thermo-mechanical properties, and failure modes. The gap raises severe concerns for critical usage where structural integrity is of ultimate importance and restricts them from being used in a broader array of applications. In this thesis, we aim to develop a systematic and fundamental understanding of these aspects for tungsten micro-architectured coatings, which have been proposed as potential grid material coatings for ion thrusters. A combination of advanced microstructural characterization, elevated temperature in situ scanning electron microscopy (SEM) micro-mechanical experiments, and image-based crystal plasticity finite element method (CPFEM) simulations are utilized to address these issues. It is shown that the micro-architectured tungsten coatings are composed of columnar grains with many pre-existing voids preferentially distribute along the grain boundaries. Although the voids can alleviate thermal stresses by allowing free expansion/contraction of the coating, their effect on the mechanical properties remains unknown and the mechanical performance of the coating needs to be quantified. From in situ micro-compression experiments performed from 293K to 673 K, a strong temperature dependence of strength and deformation/failure mode were observed, with a characteristic temperature between 573K and 673K for brittle-to-ductile transition. Below this range of temperature, the material is very brittle with intergranular fracture and buckling of each individual columnar grain being the predominant deformation mode, which is strongly influenced by the pre-existing voids. With increasing temperature, the strength decreases sharply, at 673 K, the catastrophic failure transitions to a steady hardening response, and the structural integrity is maintained up to 15\% of engineering strain. A physics-based model is then proposed to the predict strength of the coating as a function of temperature and grain size. The CPFEM simulations incorporated with a cohesive zone model (CZM) successfully capture the deformation mode transition and further employed to explore the microstructure-property relations. Our simulations indicate the variation in the crystallographic orientation of the tested micropillars primarily contribute to the observed scatter in the experimentally observed of flow stress and fracture behavior. This work provides experimental evidence that the micro-architectured tungsten coatings possess a combination of excellent mechanical strength, reduced internal thermal stresses, and dramatically improved defect tolerance capability, which is ideal for high temperature applications. Moreover, the computational framework presented here demonstrates great potential to identify and prioritize the promising microstructrual parameters, which eventually accelerate the future development of such coatings.
Coating, thermo-mechanical properties, in situ experiment, multi-scale modeling