Innovative solutions for roughness problems related to MCrAlY coatings manufacturing by HVOF II: Bond Coat/Top Coat Interface
C. Giolli, D. Russo, A. Lanzi, M. Spagnoli, G. Rizzi, A. Giorgetti, A. Scrivani
MCrAlY (where M is Ni, Co or NiCo) coatings are used as single overlay coating or as bond coat for thermal barrier coating (TBC) systems. Vacuum plasma spray (VPS) McrAlY coatings are considered state of the art today. Nevertheless high velocity oxygen fuel (HVOF) sprayed MCrAlY coating is gaining in popularity due to its quality and cost effectiveness. On the other hand HVOF process has some limits: It requires heavy sand blasting of the substrates to obtain an adequate base metal roughness for the good adhesion of the deposited McrAlY coatings and this causes a high interface pollution. Moreover, HVOF process allows to obtain relatively low surface roughness of the MCrAlY coatings, which leads to a poor adhesion of the ceramic yttria partially stabilized zirconia (YPSZ) top coat. Studies performed by the authors tried to resolve these two problems by providing an adequate pollution free interface between substrate and coating and a rough surface of the HVOF coating in order to be used as bond coat for overlaying TBC coating. This paper is the second part of two and addresses the developments of new McrAlY HVOF deposition process, in order to obtain a rough MCrAlY coating to be used as bond coat for YPSZ top coat. Optical and electronic microscopies were used to evaluate the quality of the obtained coating, in comparison with the standard dense one. Air plasma spray (APS) was used to apply TBC systems on blades and vanes coated by the enhanced HVOF McrAY bond coat. TBC coated blade and vane sections successully passed thermal fatigue tests following the procedure of an important Original Equipment Manufacturer (OEM).