|Author||: Trent Mitchell Borman|
|Release Date||: 2020|
|Available Language||: English, Spanish, And French|
A resurgence of interest in hypersonic flight has led to an increased demand for new refractory materials that possess a complex blend of physical, thermal, chemical, and mechanical properties. The selection of materials for use at extreme temperatures (>3000 °C) is dominated by the Group IVB and VB carbides, diborides, and nitrides. While these ultra high temperature ceramics (UHTCs) provide an excellent basis from which to start, new compositions are necessary for the envisioned applications. As complexity increases from binary carbides, diborides, and nitrides to ternary, quaternary, and high entropy compositions, the breadth of the compositional space grows exponentially. These new and vast, multi-dimensional phase diagrams pose a few important questions: what are the metal stoichiometries of interest? and how do the property-chemistry trends observed in binary systems translate to these complex compositions? Studying these new materials systems and answering these questions is not a trivial undertaking. Throughout the history of UHTC synthesis, the intrinsic properties of these ultra refractory materials have been convoluted with extrinsic factors, such as microstructure, phase purity, and defects. A valid study of the roles of metal and anion stoichiometry in these materials requires synthesis of UHTCs over broad compositional ranges while limiting the impacts of extrinsic characteristics. Physical vapor deposition has been widely used to study high entropy systems including alloys, oxides, carbides, and nitrides. This work expands on previous studies and focuses on understanding and improving the sputter deposition process for multicomponent carbides. The advantages and limitations of conventional sputtering techniques were investigated; avenues to improve the process, ranging from gas flows to pulsed power techniques, were explored; and finally, the benefits of high power impulse magnetron sputtering inspired the development of new co-sputtering techniques. (HfNbTaTiZr)C has received significant research interest in the UHTC community, as it combines 5 of the most refractory carbide systems; however, researchers had not studied the influence of carbon stoichiometry in this, or other, high entropy compositions. In this work, (HfNbTaTiZr)C films were synthesized over a broad range of carbon stoichiometries with reactive RF sputtering. These films exhibited broad crystallographic and microstructural transitions from metallic to carbide and finally nanocomposite films, simply by changing carbon content. Carbon vacancies were observed to cluster into stacking faults in substoichiometric films, despite the chemical disorder of the metal sublattice. A near-stoichiometric film with a hardness of 24 ± 3 GPa was synthesized, closely matching the rule of mixtures for the binary constituents. Additionally, ab-initio calculations validated the experimental mechanical property findings. Overall, the synthesis and property trends of (HfNbTaTiZr)C closely mirrored those of binary counterparts. Unfortunately, as with other carbides, excess carbon rapidly precipitated at methane flow rates slightly (2.5%) higher than the stoichiometric flow rate. The sudden onset of excess carbon precipitation stymied the rapid and facile synthesis of near-stoichiometric multicomponent carbides. Consequently, the deposition process needed to be improved before studying other compositions. A study of gas flows and pressures determined that operating with a modest fixed argon pressure (5-10 mT) increased deposition rate and could reduce target poisoning and carbon precipitation. Additionally, the results indicated that most of the methane was being consumed by the growing carbide film; however, partial pressure control was not feasible with the chamber's configuration. As a result, the best carbon control strategy was determined to be a combination of carefully regulated methane (flow rate) and metal (sputter rate) fluxes. Conventional temperature and pressure based microstructural development strategies were not feasible for use with reactively sputtered high entropy carbides. Fortunately, tunable high energy ion bombardment was demonstrated to be a viable alternative, influencing the microstructure, stress, and crystallography of the growing carbide films. The increased plasma densities, fixed energetics, and consistent energetics of high power impulse magnetron sputtering (HiPIMS) produced carbide films which were more microstructurally and crystallographically consistent than conventionally sputtered films. Simultaneous power and voltage regulation of the HiPIMS process resulted in more consistent deposition rates than the power regulation of conventional sputtering processes. Furthermore, films deposited with HiPIMS exhibited a much more gradual onset of excess carbon precipitation than RF sputtered counterparts. Asynchronously patterned pulsed sputtering (APPS) was developed based on the flux and energetic decoupling of HiPIMS. Conventional co-sputtering is rife with tedious calibrations and changing energetics. With conventional sputtering techniques, flux is changed by power which changes the sputtering voltage and the energetics of the deposition, resulting in inconsistent film quality. During HiPIMS, the flux is controlled by the frequency, while the energetics are dominated by the pulsing parameters (width and voltage). Asynchronously patterned pulsed sputtering consists of two HiPIMS supplies operating at the same frequency but phase shifted so the plasmas don't interact. One supply skips a fraction of the pulses, changing the time average flux and thus controlling the stoichiometry independently of energetics. APPS was demonstrated to produce linear compositional trends, consistent deposition energetics, and uniform film qualities across the entire stoichiometry range. The development of APPS and reactive APPS enabled the rapid synthesis of ternary systems, facilitating the search for properties of interest such as ductility in (NbW)C.