TEM Microstructural Investigation of MCrAlY Coatings Manufactured by Cold Gas Dynamic Spraying

MCrAlY coatings are commonly used as bond coats in thermal barrier coating systems (TBC) for gas turbine engines.  The bond coat properties and microstructure influence its oxidation behaviour which has been shown to be directly linked to the major TBC failure mechanisms.  It is therefore desirable to have complete control of the coating microstructure and properties throughout the coating deposition process in order to optimize TBC performance.  Cold Gas Dynamic Spraying (CGDS) is a modern coating process that relies on plastic deformation resulting from the high-velocity impact of particles on a substrate to produce coatings.  Due to the absence of significant heating of the deposited material, CGDS has been shown to have the most potential in retaining a material’s microstructure throughout the deposition process and has therefore been selected as the preferred deposition technique for this study.  This paper examines the microstructure of a MCrAlY coating manufactured by the CGDS process and investigates microstructural changes that occurred throughout the deposition process.  Comparison of the coating microstructure with that of the original feedstock powder is achieved by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). As opposed to what is usually promoted as a standard feature of CGDS, noticeable microstructure and phase changes have been found.