Structural evolution of protective coatings for gamma-TiAl based alloys

Martin Moser

Research output: ThesisDoctoral Thesis

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Abstract

gamma-TiAl based alloys are considered for high temperature applications in aerospace and automotive industry. However, the onset of rapid oxidation at temperatures above 750 °C limits a wider utilization. Consequently, coating systems have to be developed which provide protection against oxidation as well as high temperature erosion and corrosion. Therefore, the structural evolution in dependence of coating composition and its influence on thermal stability, oxidation resistance, mechanical properties, and interaction with the gamma-TiAl substrate material was investigated by two model PVD coating systems, metallic films based on the intermetallic Al2Au phase and transition-metal nitride films based on (Ti1-xAlx)1-yYyN. Al-Au coatings with Al/Au at.% ratios between 1.85 and 4.32 were developed by unbalanced magnetron sputtering. Despite the fact that the majority phase in all films is the Zintl-type intermetallic Al2Au, the microstructure of the coatings differ significantly. Within this thesis the microstructural development of the various films is described and it is demonstrated that single-phase Al2Au coatings have superior mechanical and thermal properties as compared to films with higher Al or Au contents. Further, single-phase Al2Au provides excellent oxidation protection up to temperatures of 850 °C. The influence of yttrium on microstructure, mechanical properties, and thermal stability of metastable solid-solution Ti0.45Al0.55N coatings is investigated. The addition of 6 at.% Y leads to a decrease of the metastable solubility limit of the desired cubic phase by 23 % from 0.69 to 0.56 AlN mole fractions. Consequently, alloying only 1 at.% Y to single-phase cubic Ti0.45Al0.55N results in the formation of a binary phase composition of cubic- and hexagonal-(Ti1-xAlx)1-yYyN. As a consequence of the hexagonal phase fraction, these coatings have decreased hardness and oxidation resistance. However, by influencing the plasma and growth conditions with the use of bipolar pulsed magnetron sputtering for the preparation of the (Ti1-xAlx)1-yYyN films, single-phase cubic films can be grown which combine the excellent mechanical properties of Ti0.45A0.55N with improved oxidation resistance by the addition of Y.
Translated title of the contributionStrukturentwicklung von Schutzschichten für gamma-TiAl Basislegierungen
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Dehm, Gerhard, Assessor B (internal)
  • Mayrhofer, Paul Heinz, Assessor A (internal)
Publication statusPublished - 2008

Bibliographical note

embargoed until null

Keywords

  • gamma-TiAl
  • Al-Au
  • Ti-Al-N
  • Ti-Al-Y-N
  • structure
  • STA
  • DSC
  • TGA
  • MS
  • TEM
  • XRD
  • phase transformation
  • decomposition
  • growth process
  • bipolar pulsed magnetron sputtering

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