Electronic structure based design of thermal shock resistant nanolaminates
Final Report Abstract
Thermal shock resistance can be described by the so-called thermal shock parameter (RT), which is defined as the ratio between the product of flexural strength, thermal conductivity and 1 minus the Poisson’s ratio and the product of the linear coefficient of thermal expansion, and elastic (Young’s) modulus. Hence, for improved thermal shock resistance, the thermal shock parameter should be maximized, which can be achieved by maximizing the nominator and/or minimizing the denominator. This project aimed to provide pathways for material design based on combined experimental and computational approaches. Both the influence of the material system (Ti3AlC2 vs Cr2AlC) and the effect of the microstructure (columnar vs equiaxed vs bulk) on the properties which affect the thermal shock parameter were investigated. The following parameters were evaluated by theory and experiment: thermal expansion coefficient and Young’s Modulus. The thermal conductivity of Ti3AlC2 and Cr2AlC was calculated, while experimental values were obtained only for the Cr2AlC thin film samples of different microstructures. The Poisson’s ratio was calculated, and the flexural strength was approximated by measuring and calculating the hardness. The major outcomes of this project were: • The thermal shock behavior of ionically-covalently bonded materials can be evaluated by ab initio calculations as the thermal shock parameter of both Ti3AlC2 and Cr2AlC obtained through quantum-mechanical predictions is in very good agreement with the experimentally obtained thermal shock parameter. • Ti3AlC2 has a superior thermal shock resistance in comparison with Cr2AlC as indicated by both measurements and quantum-mechanical predictions. This is attributed primarily to the higher coefficient of thermal expansion of Cr2AlC. • Comparing the thermal shock parameters obtained by experiments and calculations to the bulk literature data, the difference is caused mainly by the larger flexural strength due to the higher hardness of thin films, which was rationalized based on the Hall-Petch relationship. • The outcomes of the microstructural study allowed for isolating the determinant parameters and their impact on the thermal shock parameter. The largest differences were obtained for the thermal conductivity (lowest~7 W/m.K, highest ~24 W/m.K) and the flexural strength (lowest ~0.6 GPa, highest ~1.3 GPa).
Publications
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On the determination of the thermal shock parameter of MAX phases: A combined experimental-computational study. Journal of the European Ceramic Society, 43(13), 5484-5492.
Fekete, Matej; Azina, Clio; Ondračka, Pavel; Löfler, Lukas; Bogdanovski, Dimitri; Primetzhofer, Daniel; Hans, Marcus & Schneider, Jochen M.
