Internal Diameter Coating Processes for Bond Coat (HVOF) and Thermal Barrier Coating (APS) Systems

Current developments in different industrial sectors show an increasing demand of thermally sprayed internal diameter (ID) coatings. The most recent research and development is mainly focused on commercial applications such as arc spraying (AS), atmospheric plasma spraying (APS), and plasma transferred wire arc spraying, especially for cylinder liner surfaces. However, efficient HVOF torches are meanwhile available for ID applications as well, but in this field, there is still a lack of scientific research. Especially, the compact design of HVOF-ID and APS-ID spray guns, the need of finer powders, and the internal spray situation leads to new process effects and challenges, which have to be understood in order to achieve high-quality coating properties comparable to outer diameter coatings. Thus, in the present work, the focus is on the ID spraying of bond coats (BC) and thermal barrier coatings (TBC) for high-temperature applications. An HVOF-ID gun with a N-2 injection was used to spray dense BCs (MCrAlY) coatings. The TBCs (YSZ) were sprayed by utilizing an APS-ID torch. Initially, flat steel samples were used as substrates. The morphology and properties of the sprayed ID coating systems were investigated with respect to the combination of different HVOF and APS spray parameter sets. The results of the conducted experiments show that the HVOF-ID spray process with N-2 injection allows to adjust the particle temperatures and speeds within a wide range. CoNiCrAlY bond coats with a porosity from 3.09 to 3.92% were produced. The spray distance was set to 53mm, which leads to a smallest coatable ID of 133mm. The porosity of the TBC ranged from 7.2 to 7.3%. The spray distance for the APS-ID process was set to 70mm, which leads to a smallest coatable ID of 118mm.