T. Shmyreva, Praxair Surface Technologies, Indianapolis, IN
Coating microstructure characteristics are dependant on the applied preparation and evaluation techniques. Three areas of microstructure characterization will be discussed.
- Nano-structure evaluation by x-ray diffraction method: The method is based on the diffraction peak broadening measurement. The factors that could influence results include the diffraction pattern scan geometry, x-ray beam wave length, the standard sample using and preparation, input of dislocation density, short range ordering and mathematic models applied for the diffraction profile characterization.
- False diffusion zones: The coating-substrate interface area is significantly influenced by the sample preparation procedure. It will be shown that the pseudo-diffusion layer could have thickness around 10-20 mm, and this artifact typically appears between coating and substrate from soft and ductile materials such as Al, Cu, Ni, and Ti.
- Microstructure. Depending on the polishing technique utilized, the same coating cross-section could reveal porosity from ~0.5 to ~4 %, the oxide inclusions could be visible or not, and number of micro-crack can vary by a factor of ten. Some examples of metallographic techniques for metal and ceramic coatings will be introduced and discussed.
Summary: Nano- and microstructure artifacts can appear in the process of thermal spray coating characterization depending on the applied sample preparation / evaluation technique. The limits of x-ray diffraction method of nano-structure identification, and different microscopy techniques to evaluate coating - substrate interface and coating microdefects (porosity, oxide inclusions, microcracks) will be discussed.