Markus Mutter

Investigations on the Initial Stress Evolution During Atmospheric Plasma Spraying of YSZ by In Situ Curvature Measurement

The residual stresses within plasma-sprayed coatings are an important factor that can influence the lifetime as well as the performance in operation. The investigation of stresses evolving during deposition and post-deposition cooling for atmospheric plasma spraying of yttria-stabilized zirconia coatings using in situ measurement of the samples curvature is a powerful tool for identifying the factors that contribute to stress generation. Under various spray conditions, the first deposition pass leads to a significantly larger increase in samples curvature than the subsequent passes. It is shown in this work that the amount of curvature change at the onset of spraying is significantly influenced by the spray conditions, as well as by the substrate material. More information on the origin of this steep curvature increase at the onset of spraying was obtained by single splat experiments, which yielded information on the splat bonding behavior under various conditions. A comparison of the compressive yield strength for different substrate materials indicated the influence of substrate residual stress relaxation. Residual stress measurements using the incremental hole-drilling method and x-ray diffraction confirmed that the coating deposition affects the substrate residual stress level. The yield strength data were combined with the substrate near-surface temperature during deposition, obtained by finite element simulations, and with the measured residual stress-profile. This revealed that residual stress relaxation is the key factor for the initial curvature increase.

Residual Stress Depth Distributions for Atmospheric Plasma Sprayed MnCo1.9Fe0.1O4 Spinel Layers on Crofer Steel Substrate

In solid oxide fuel cells (SOFC) for operating temperatures of 800 °C or below, the use of ferritic stainless steel can lead to degradation in cell performance due to chromium migration into the cells at the cathode side [1]. Application of a coating on the ferritic stainless steel interconnect is one option to prevent Cr outward migration through the coating. MnCo1.9Fe0.1O4 (in the following designated as MCF) spinels act as a diffusion barrier and retain high conductivity during operation [2]. Knowledge about the residual stress depth distribution throughout the complete APS coating system is important and can help to optimize the coating process. This implicitly requires reliable residual stress analysis in the coating, the interface region and in the substrate.For residual stress analysis on these specific layered systems diffraction based analysis methods (XRD) using laboratory X-ray sources can only by applied at the very surface. For larger depths sublayer removal is necessary to gain reliable residual stress data. The established method for sublayer removal is electrochemical etching, which fails, since the spinel layer is inert. However, a mechanical layer removal will affect the local residual stress distribution.As an alternative, mechanical residual stress analyses techniques can be applied. Recently, we established an approach to analyse residual stress depth distributions in thick film systems by means of the incremental hole drilling method [5, 6]. In this project, we refined our approach for the application on MCF coatings with a layer thickness between 60 – 125 μm.

Systematic Investigation on the Influence of Spray Parameters on the Mechanical Properties of Atmospheric Plasma-Sprayed YSZ Coatings

In the atmospheric plasma spray (APS) process, micro-sized ceramic powder is injected into a thermal plasma where it is rapidly heated and propelled toward the substrate. The coating formation is characterized by the subsequent impingement of a large number of more or less molten particles forming the so-called splats and eventually the coating. In this study, a systematic investigation on the influence of selected spray parameters on the coating microstructure and the coating properties was conducted. The investigation thereby comprised the coating porosity, the elastic modulus, and the residual stress evolution within the coating. The melting status of the particles at the impingement on the substrate in combination with the substrate surface condition is crucial for the coating formation. Single splats were collected on mirror-polished substrates for selected spray conditions and evaluated by identifying different types of splats (ideal, distorted, weakly bonded, and partially molten) and their relative fractions. In a previous study, these splat types were evaluated in terms of their effect on the above-mentioned coating properties. The particle melting status, which serves as a measure for the particle spreading behavior, was determined by in-flight particle temperature measurements and correlated to the coating properties. It was found that the gun power and the spray distance have a strong effect on the investigated coating properties, whereas the feed rate and the cooling show minor influence.