Effects of silica-based rheology control additives on the powder flowability and coating properties of inner diameter (ID) HVOF sprayed CoNiCrAlY bond coatings

To increase the efficiency and longevity of hot gas components, thermal barrier coatings have become crucial parts in a lot of applications requiring low thermal conductivity as well as high oxidation and corrosion resistance. Conventionally, such systems are applied with a multilayer system consisting of a MCrAlY based bond coat as well as a YSZ top coat. While such systems have been investigated extensively for conventional/outer diameter applications, respective systems for small inner diameters (ID) of less than 200 mm, which play an essential role in increasing the field of usage of TBCs, are still in their early development stage. One fundamental problem in ID HVOF spraying is the need of small particle size < 20 microns. With lower particles sizes, conventional powder feeding systems usually tend to clog, which leads to an uneven powder feeding, resulting in inhomogenous coating results. In this paper, the addition of rheology control additives is investigated to increase the flowability of small diameter powders to employ sub 10 micron particles in a conventional powder feeder. Both the flowability and effects on the coating properties were investigated at multiple levels of rheology control additives. Coatings were produced that showed no detriment over pure CoNiCrAlY coatings.