MCrAlY coatings (where M stands for Co, Ni, or CoNi) are used as a single overlay coating or as bond coat for thermal barrier coating (TBC) systems. Low Pressure Plasma Spray (LPPS) or Vacuum Plasma Spray (VPS) MCrAlY coatings are considered nowadays the state of the art, nevertheless High Velocity Oxygen Fuel (HVOF) MCrAlY coating is gaining more and more importance due to its quality and to the possible cost reduction in comparison to the state of the art LPPS/VPS coatings. On the other hand the HVOF process tends to have a relatively low surface roughness, which hinders the possibility to act as bond coat for a TBC coating layer. In this work, a HVOF deposition process was developed to obtain a rough MCrAlY coating which can be used as bond coat for yttria partially stabilized zirconia (YPSZ) top coat. Developed parameters were used to apply a TBC system on a stage 2 blade. TBC sections on the same blade have successfully passed thermal fatigue testing according to the specification of an important OEM.

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 today to be a state of the art. 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 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 first part of two and addresses the development of a surface preparation method, in order to obtain a completely pollution free surface of the substrate with an adequate roughness to be coated afterwards with HVOF. A new surface preparation process, named eXclen , was set up and HVOF was used to apply MCrAlY coating on the prepared base material. Optical and electronic microscopies were used to evaluate the quality of the interface Base Material (BM)/Bond Coat (BC). It was possible to observe the absence not only of foreign entrapped particles, but also of any chemical contamination or alteration of both coating and substrate materials. The BM/BC interface quality and MCrAlY coating adhesion resulted to satisfy the Original Equipment Manufacturer (OEM) requirements.

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 MCrAlY bond coat. TBC coated blade and vane sections successfully passed thermal fatigue tests following the procedure of an important original equipment manufacturer (OEM).

The most commonly used structural materials for blades and other high temperature components of gas turbines are nickel base superalloys. A TBC protection coating system consists of a top coat of yttria partially stabilized zirconia and an underlying bond coat, usually MCrAlY (where M stands for Ni, Co or a combination of both). MCrAlYs are normally deposited by the thermal spray processes: air plasma spray (APS), vacuum plasma spray (VPS/LPPS) or high velocity oxygen fuel (HVOF). The adhesion between the bond coat and the substrate, and therefore of the whole thermal barrier system, strongly depends upon the surface roughness. A high level of roughness generally denotes better adhesion, especially with the HVOF thermal spray process where it is a necessity. Generally the roughness is reached by means of grit blasting with an abrasive media; this results in a certain level of surface contamination due to the entrapment of abrasive particles. The aim of this work was to set up a new surface preparation process in order to obtain a completely clean surface with a suitable roughness, which can be coated afterwards with HVOF or VPS/LPPS thermal spray technology. The tests carried out by this process on turbine blades, coated with a HVOF system, led to obtaining a coating/base material interface without any contamination caused by the surface preparation.