Electronic structure based design of thermal shock resistant nanolaminates

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Thermal shock occurs when materials are subjected to rapid changes in temperature, leading to thermal stress buildup within an engineering component and eventually to its damage and failure. Even though this arises in many engineering applications, fundamental studies regarding the knowledge based design of these materials is at infancy. The thermal shock resistance can be described by the thermal shock parameter, containing flexural strength, thermal conductivity, Poisson’s ratio, linear coefficient of thermal expansion, and elastic (Young’s) modulus. To increase the thermal shock resistance, the thermal shock parameter must be maximized, but this is very challenging as these properties are often interconnected and there are various extrinsic factors affecting them. Using a correlative experimental and theoretical research strategy, we seek to establish the causalities between thermal shock parameter, chemical composition, and extrinsic factors for Ti2AlC, Zr2AlC, Zr3AlC2, and Ta4AlC3. Hence, we systematically investigate the influence of valence electron concentration (transition metals from IVB and VB groups), size effects (Ti2AlC vs. Zr2AlC), and stacking sequence (different amount of Zr-C and Al layers in Zr2AlC and Zr3AlC2) on thermal shock parameter. The extrinsic factors to be investigated in this project are grain size, stress, and atmosphere exposure. We will use thin film synthesis to explore the chemical variation influence on these thermomechanical properties. Applying a substrate bias potential and different substrate temperatures, the compositional influence will be decoupled from microstructure. The residual stress data will be acquired as they are of importance for thermal shock resistant materials since the residual stress competes with the thermal stress components. We will investigate the effect of venting temperature (and the associated atmosphere exposure induced changes in composition) on the thermomechanical properties. This has never been attempted for thermal shock resistant materials, but in general ambient-coating interactions modifying the surface chemistry are essential for understanding the performance. All experiments will be accompanied by density functional theory investigations, aiming at deeper understanding of the established experimental causalities on the electronic structure level. All theoretical data will be validated experimentally. Several pioneering steps will be made, such as calculation of the linear coefficient of thermal expansion for these phases within the Debye-Grüneisen theory, the thermal conductivity on the electronic and phonon level for these low symmetry systems, and explicit consideration of the extrinsic factors as well as temperature. With the fundamental causalities obtained in this project, it is expected that a solid basis will be obtained for future quantum mechanical design of thermal shock resistant materials.