Moses Nnaji

Advisor: Prof. Eric Vogel

will defend a doctoral thesis entitled,

Investigations into Low-Temperature Vapor Deposition of MAX-phase Titanium Aluminum Nitride Thin-Films

On

Wednesday, April 15th  at 9:00 a.m.

Marcus Nanotechnology Building, Rooms 1116-1118 

and/or

 Virtually via Microsoft Teams 

 Virtual link

Committee

 Prof. Eric Vogel, School of Materials Science and Engineering (advisor)

Dr. Dale Hitchcock, Savannah River National Laboratory

Prof. Mark Losego, School of Materials Science and Engineering

Prof. Preet Singh, School of Materials Science and Engineering

Prof. Michael Filler, School of Chemical and Biomolecular Engineering

Abstract

MAX-phases are a class of nanolaminate ceramics boasting a novel combination of metal and ceramic properties; many phases are thermally and electrically conductive, easily machinable, and resistant to thermal shock. Some phases also exhibit enhanced oxidation resistance and self-healing of cracks from thermally grown oxides based on the “A” element, such as Al2O3. As such, they have attracted interest in a variety of contexts, including for use in ohmic contacts, heating elements, gas turbine blade coatings, and radiation-tolerant cladding in nuclear reactors. However, MAX-phase thin-films are underutilized in these application spaces due to consistently high-temperature synthesis conditions (e.g., 600-1200 °C) incompatible with sensitive substrates. Thus, engineering novel processes for low-temperature MAX-phase film synthesis is necessary to improve their viability as high-performance coatings in demanding applications.

Since MAX-phase formation is greatly impacted by elemental diffusion, the chosen synthesis approach affects MAX-phase formation temperature: bulk synthesis methods such hot pressing often require temperatures of >1000 °C, but energetic vapor deposition methods such as magnetron sputtering may only require 600 °C. Even lower formation temperatures may be achieved in vapor-deposited films by engineering the film morphology and composition to facilitate elemental diffusion into the MAX-phase lattice positions. Here, two energetic vapor deposition methods were explored for low-temperature synthesis of the MAX-phase Ti2AlN: a reactive magnetron sputtering approach, and a novel plasma-enhanced atomic layer deposition (PE-ALD) approach.

First, high-purity Ti2AlN films were successfully synthesized by sputtering of composite Ti-Al-N films at ambient temperatures (≤150 °C), and post-deposition annealing at higher temperatures (≥450 °C). By annealing of as-sputtered Ti/AlN multilayers with high degrees of elemental diffusivity and stored chemical energy, trace amounts of Ti2AlN were obtained at temperatures as low as 475 °C – the lowest temperature ever reported for formation of any Ti-based MAX-phase using magnetron sputtering, which was previously 550 °C. The deuterium permeability and phase integrity of Ti2AlN films synthesized from different as-sputtered conditions was also assessed.

Next, Ti-Al-N films were grown via PE-ALD at 200-350 °C by using reactive N2 and H2 plasmas in conjunction with organic tetrakis(dimethylamido)titanium and trimethylaluminum precursors. However, formation of Ti2AlN was not observed in PE-ALD films even after post-growth annealing at 900 °C. This was attributed to poor composition control: high C and O impurity concentrations were induced by premature decomposition of the organic precursors and etching of the quartz plasma source, and further exacerbated by slow film growth rates and long exposure times to the contaminated chamber environment. Potential solutions for obtaining better composition control during PE-ALD are discussed.