Towards autonomously self-reporting thin films and phase formation in YTaO$_{4}$-ZrO$_{2}$

Stelzer, Bastian; Schneider, Jochen M. (Thesis advisor); Scheu, Christina (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2021)
Book, Dissertation / PhD Thesis

In: Materials Chemistry Dissertation 37
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020

Abstract

Extreme temperatures and oxidizing atmospheres are well known to degrade materials and hence limit the service time of equipment operating under these conditions. In this work, the possible enhancement of operating times of coatings applied under these challenging environments is pursued by two approaches. In the first part, a novel concept of autonomously self-reporting materials is introduced in which an unmodified thin film itself is acting as a sensor to monitor the “material’s health”. It is shown that in situ resistance measurements are useful to track phase transformations as well as the oxidation behavior, both affecting the integrity or the functionality of a component during application and hence allow to maximize operating times while ensuring safe operation. In a first study, the feasibility to track phase changes by in situ resistivity measurements was studied on the example of Cr2AlC. To this end, resistivity changes of magnetron sputtered, amorphous Cr2AlC thin films were measured during heating in vacuum. Based on correlative X-ray diffraction (XRD), in situ and ex situ selected area electron diffraction measurements as well as differential scanning calorimetry data from literature it is evident that the resistivity changes at 552 ± 4 and 585 ± 13 °C indicate the phase transitions from amorphous to a hexagonal disordered solid solution structure and from the latter to MAX phase, respectively. Hence, it was shown that phase changes in Cr2AlC thin films can be revealed by in situ measurements of thermally induced resistivity changes. This may allow the estimation of amorphization due to irradiation in materials employed in nuclear applications. In a second study, the temporal oxidation behavior of TiN thin films is tracked by in situ sheet resistance measurements. Correlative film morphology, structure, and local composition data show that observed resistance changes are caused by oxidation of TiN. Ex situ thickness measurements of the remaining TiN under the oxide layer are in very good agreement with thicknesses deduced from in situ sheet resistance measurements. Hence, the in situ measured sheet resistance is an autonomous self-reporting property useful for tracking the temporal oxidation behavior of TiN coatings.In the second part of this thesis, the need to improve sustainability and lifetimes of gas turbine components is addressed by the evaluation of ZrO2 alloyed YTaO4 as a new promising material for thermal barrier coatings, surpassing thermal stability and corrosion resistance of to date employed Y2O3 stabilized ZrO2 while matching its fracture toughness and thermal conductivity. To this end, YTaO4 coatings alloyed with up to 44 mol% ZrO2 have been synthesized by reactive magnetron sputtering for the first time and have been subsequently investigated regarding the phase formation and mechanical properties. Phase pure monoclinic M’-YTaO4 coatings were obtained at a substrate temperature of 900 °C. Alloying with ZrO2 resulted in the growth of M’ along with tetragonal t-Zr(Y,Ta)O2 for ≤ 15 mol%, while for ≥ 28 mol% ZrO2 XRD phase pure metastable t was formed, which may be induced by small grain sizes and kinetic limitations during growth. The former phase region transformed into M’ and M and the latter to an M’ + t and M + t phase region upon annealing to 1300 °C and 1650 °C, respectively. In addition to M and t, tetragonal T-YTa(Zr)O4 phase fractions were observed at room temperature for ZrO2 contents of 28 mol% and higher after annealing to 1650 °C. T phase fractions increased during in situ heating XRD at 80 °C, while a simultaneous decrease in M phase fractions was observed. At 1650 °C a reaction with the α-Al2O3 substrate resulted in the formation of orthorhombic AlTaO4 and an Al-Ta-Y-O compound.

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