Committee

Prof. Eric Vogel, School of MSE (advisor)

Dr. Dale Hitchcock, Savannah River National Laboratory

Prof. Mark Losego, School of MSE

Prof. Preet Singh, School of MSE

Prof. Michael Filler, School of ChBE

Abstract

MAX-phases are a class of layered, nanolaminate ceramics boasting a novel combination of metal and ceramic properties. For instance, many MAX-phases are thermally and electrically conductive, easily machinable, resistant to thermal shock, and are known to maintain many of their properties at elevated temperatures due to the thermal stability of their distinct crystal structure. Some MAX-phases are also regarded for their oxidation resistance, self-healing of cracks, hydrogen impermeability, and radiation tolerance. Consequently, MAX-phase coatings have attracted interest in a variety of contexts, including use in Ohmic contacts, heating elements, gas turbine blade coatings, and radiation-tolerant cladding in nuclear systems. However, only a few MAX-phases have been adopted in industrial applications. In particular, the high temperatures (>600 °C) typically needed to form their complex crystal structures serve as a substantial bottleneck for the widespread adoption of MAX-phase coatings on temperature-sensitive substrates. Reducing the synthesis temperature of MAX-phases is necessary to improve their viability in demanding applications, and thus improve the performance of said applications.

Due to the complex crystal structures of MAX-phases, their formation is strongly influenced by elemental diffusion, and the deposition approach may critically influence the reactions necessary for MAX-phase nucleation. This proposal seeks to enable low-temperature MAX-phase synthesis by exploiting energetic, low-temperature synthesis techniques to provide comprehensive insight into the factors influencing MAX-phase growth mechanisms. The first aim of this proposal is to reduce MAX-phase growth temperatures by modifying conventional magnetron sputtering methods. In particular, this aim will investigate underexplored effects of as-sputtered thin-film properties on MAX-phase growth temperature, such as phase composition, multi-layer film morphology, bilayer thickness in multi-layer films, and elemental composition. The second aim of this proposal is to establish plasma-enhanced atomic layer deposition (PE-ALD) as a novel means for MAX-phase thin-film synthesis. PEALD is promising: it may be capable of MAX-phase synthesis at uniquely low temperatures as it (1) relies on forms of energy other than high temperature to achieve crystallization and (2) enables short elemental diffusion distances to lower the energy necessary for crystallization of complex MAX-phase structures. Finally, the last aim of this proposal is to quantify the performance of MAX-phase films from the previous two aims. By characterizing properties such as oxidation resistance, electrical resistivity, and gas permeability, this aim serves to provide insight into the suitability of new, low-temperature synthesis methods for MAX-phase thin-films.