2.2.2. Microstructure

Scanning electron microscopy (SEM) observation was done using the Phenom-Pro microscope (Thermo Fisher Sci., Eindhoven, The Netherlands) equipped by the CeB6 thermionic cathode and working in backscattered electron (BSE) mode. The images were collected at 10 kV electron beam tension. Energy Dispersive X-ray (EDX) analysis, which is a part of the SEM, was used to determine the elemental composition of a sample. Each element emits characteristic X-rays as an interaction between the sample and a focused electron beam. Consequently, the EDX spectrum is evaluated, and based on a unique set of peaks for each element, the composition of the sample is determined. The X-ray signal used for EDX-measurements comes from an area that is at least 1 μm in diameter and around 1.5 μm depth. The element EDX maps were collected at 15 kV electron beam tension.

Polished cross sections of the coatings were prepared for microscopic observation and for microhardness measurement. Light micrographs as a base for image analysis of porosity were taken at 250x magnification via CCD camera EOS 500D (Canon, Tokyo, Japan) attached on the microscope

Neophot 32 (Zeiss, Oberkochen, Germany) and processed with the software Lucia (4.62, Laboratory Imaging, Praha, Czech Republic). For a more complex description of pores, additional criteria besides the porosity percentage (area fraction of voids, i.e., pores and cracks, in the TBC material) were introduced. "Number of Voids per unit area" (N.V.) of the cross section in combination with the porosity percentage could distinguish between porosity composed from a large number of fine voids or from a low number of coarse voids. "Equivalent Diameter" (E.D.) of voids represents their size distribution. Circularity could have values between 1 (i.e., a circle representing a globular pore) and 0 (i.e., a line representing a flat pore or a crack). All parameters were calculated for 3 images of each sample.

Selection of samples for microstructure study was done carefully to represent central parts of the planar TBC coating far enough from the edges. Polished cross-sections were prepared from as-deposited samples. Cutting was done using a diamond blade, subsequently the sections were mounted in a resin, and polishing was carried out using the Tegramin-25 automatic system (Struers, Denmark).
