Study of kinetics and thermodynamics in thin films at the limit between coherent and semi-coherent phasetransformation

• Applicant: Professorin Dr. Astrid Pundt
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2013 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 250278176

Phase boundaries and stabilities of alloy thin films differ from those of bulk systems. Nowadays, mechanical stress is considered as an important key factor. Recently, Nörthemann et al. calculated that for Niobium-Hydrogen (Nb-H) films of less than d=27 nm thickness, coherent precipitates become energetically favorable. Below this critical thickness d_c, stress release is impossible at the metal-hydride interface. This condition results from the film clamping condition, at 300 K. As a consequence, the properties of thin films should be separated in the two limiting cases: for d< d_c in a coherent high-stress regime and for d > d_c in an incoherent low-stress regime. Due to coherency and stress conditions, thermodynamics and kinetics as well as the hydride morphologies and lateral spreading should abruptly change when crossing this critical thickness.While above d_c, classical Gibbs thermodynamics can describe the film properties, below d_c coherent thermodynamics should be considered. This thermodynamics, as theoretically addressed e.g. by Schwarz et al. for the open system, predicts new features like, inter alia, phase boundaries depending on the direction of the phase transformation (two different miscibility gaps), or a thermodynamic hysteresis.Thus, we expect for dd_c we expect to find 1) incoherent hydrides and dislocations, 2) a maximum stress far below the theoretical limit, 3) classical Gibbs thermodynamics (only one miscibility gap) and a small hysteresis based on plastic deformation, 4) comparably fast phase formation kinetics especially after dislocation nucleation, 5) film properties depending on the sample history and 6) hydride growth triggered by formerly plastically deformed regions.In this project we plan to verify and cross the predicted critical film thickness d_c by using Nb-H as a model system. We plan to use a variety of complementary methods, including in-situ STM, -XRD, -resistivity and -substrate curvature measurement. Because of its model character, the results will give fundamental insights into the decomposition physics of thin alloy films when crossing the predicted d_c. They should be transferrable to other nano-sized systems fixed to stabilizers offering the possibility to tune the stress state and affect phase stabilities by crossing the critical system size. Further, since the thermodynamics of Schwarz and Khatchaturyan is rarely addressed by experimentalists, the proposed studies are also of interest for verification of the thermodynamics predictions.

• DFG Programme: Research Grants