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B. Kohlhauser:
"Phase transformation derived protective effects of PVD coatings";
Supervisor, Reviewer: P.H. Mayrhofer, H. Riedl, A. Erdemir, C. Gachot; E308, 2018; oral examination: 2018-12-17.

Phase formation and phase transformations are of high interest for the development of thin films. Thin film materials regularly experience phase transformations, whereat the transformation occurs either at the coating to atmosphere interface (e.g. corrosion, oxidation, catalytic/mechanochemical effects and many more) or within the coating itself (e.g., decomposition, structural changes or chemical changes). This work focuses on two effects: the influence of W or Mo based coatings on the tribofilm formation for wear-protective applications, as a surface phase transformation, while the thermally induced transformation of metastable γ-Al2O3 thin films to α-Al2O3 represents solid state coating phase transformations. To gain a better understanding of the in-situ formation of transition metal disulfide based tribofilms, detailed studies, uncovering the reaction pathway in uncoated metallic W and Mo contacts, have been carried out. The conclusions were utilized to design a coating that allows for exceptional wear protection via the formation of a crystalline WS2 phase. The lubricating nature of the tribofilm was established in ball on disk tests and the formation of WS2 confirmed by Raman spectroscopy and transmission electron microscopy. In terms of solid-state phase transformation of the coating, the influence of heavy element micro-alloyed Al targets on the deposition process of alumina thin film was investigated. While the dopants (Cr, Mo, Nb or W) generally lead to a stabilization of the deposition process, they have very different influences on the deposition rate, structure and mechanical properties of alumina thin films. Low levels of W show the overall most beneficial effect, and a particularly high increase of the deposition rate as a result of the rebound sputtering effect. Doping alumina coatings with W also showed the most favorable influence on the γ- to α-Al2O3 transformation in differential scanning calorimetry and non-ambient temperature X-ray diffraction measurements, delaying the transformation to higher temperatures. This delay is contributed to a solute drag effect as well as the minimization of surface energy. It therefore opens alumina thin films, produced by physical vapor deposition, to applications at high temperatures.

PVD thin films; WS2/MoS2 in-situ formation