Siebert, B. (Corresponding author)

1999
Forschungszentrum, Zentralbibliothek Jülich

Report No.: Juel-3669

Abstract: The present state-of-the-art enables the deposition of plasma-sprayed thermal barrier coating systems, especially in turbine construction, with the aim of improving the efficiency of turbines by higher inlet temperatures and thus increasing profitability and reducing pollutant emissions and (electricity generation) costs. To achieve this improvement, a two-layer system is used, consisting of a metallic, vacuum-plasma-sprayed NiCoCrAIY bond coat for oxidation protection and a ceramic 7-8 wt% - Y203 Zr02 thermal barrier coating produced by atmospheric plasma spraying and serving as a heat shield. The studies described in this thesis concern, on the one hand, the production of coatings using the F4 plasma torch with an established history in the industry as well as the new Triplex torch technology. Parameter variations and parameter studies for the bond coats were carried out exclusively with the F4 torch. The top coat studies were performed with both the F4 and triplex torches. For characterization, on the other hand, extensive studies were carried out using mercury analysis, image analysis, microindenter Young's Modulus and hardness measurement, thermal cycling, Raman microscopy, roughness measurements, dilatometry, SEM and optical microscopy. The factor test scheme presented here is a procedure which unambiguously defines the spraying distance as the most important factor influencing porosity and spray efficiency with the Triplex torch. For both torches, increasing wear (in particular of the cathods) is detected on the basis of different layer porosities. The Triplex torch was subjected to much stronger wear than the F4 torch. Wear reducing measures, such as using lower electrical power, contribute to very good reproducibilities for coating production. However, parameter sets cannot be directly transferred from flat specimens to cylindrical specimens or specimens of different shapes. The suitability and reproducibility of the individual investigation methods for the characterization of ceramic thermal barrier coatings is also dealt with in more detail. Thus, for example, mercury porosity measurements document a bimodal pore distribution of the ceramic which cannot be identified by image analysis with realistic magnifications and experimental effort. For the precise characterization of stress measurements of a TBC, the Raman investigations carried out proved to be too imprecise. The measurement of the Young's modulus is not trivial either, since the value depends, for example, on the coating load, the measuring method and the pore shape. In general, those methods of measuring the Young's modulus where the greatest possible portion of the microstructure has an influence on the measuring result are preferable to locally greatly restricted measurement techniques like the microindenter. In summary, it may be stated that coating production by plasma spraying requires constant control and verification due to the great complexity and the multitude of influential factors. The reproducibility of the coating morphologies and coating properties was ensured and further developed by improving spraying operations. In spite of the wear problems the Triplex torch has proved to be extremely reliable and enables homogeneous and reproducible coating production. The choice of the characterization method and its interpretation must be critically geared. That goes expecially for the porous ceramic material to be investigated which shows a high density of inhomogenities.

Keyword(s): thermal barrier coating ; spray coating ; thermal insulation

1. Werkstoffsynthese und Herstellungsverfahren (IEF-1)