Thermal barrier coatings have got considerable importance for the improvement of gas turbine efficiency. These materials are applied on the surface of gas turbine blades and vanes and are based on a layer of low-oxidation material (mainly MCrAlY alloys, where M stay of Co, Ni or a combination of both) and a ceramic top layer that acts as proper thermal barrier (normally Yttria Partially Stabilized Zirconia). Coating removal is an important aspect in the production of these blades and vanes. “Decoating” or “stripping” is needed during the production of new components as well as for the reconditioning of existing ones. The present paper is dedicated to a new removal method of the ceramic Zirconia layer, based on dry ice blasting. This method will not impact on the roughness and morphology of the bond coat surface, making it suitable for re-coating with TBC, without any further operation before TBC recoating. This possibility has an important impact on the stripping costs and time, avoiding all the operations related to the bond coat. The paper presents the process tests to get the process set up and the characterization of the surfaces comparing the stripped ones with the “original ones” coated by LPPS on new components, ready to be TBC coated. Optical and SEM microscopy, 3D profilometry have been used for characterization. Finally a Thermal Cycling Fatigue test has been carried out in order to validate the procedure of stripping and re-coating.

High velocity oxyfuel flame spraying (HVOF) has a great potential for replacing the low pressure plasma spraying (LPPS) process in most applications for hot corrosion protective MCrAlY coatings for turbine hot gas path parts. Compared to LPPS coatings, state-of-the-art HVOF sprayed MCrAlY coating systems feature competitive hot corrosion protective properties. Low HVOF facilities investment costs and a stable, easy process controlling are major advantages in terms of application. Besides hot corrosion protective single layers, MCrAlY coating systems are used as bond coats for ceramic thermal barrier coatings (TBC). But HVOF sprayed MCrAlY coatings show a comparatively low surface roughness which leads to a poor adhesion of the ceramic top layer, restricting the application area of HVOF. This paper deals with a development project which aims on roughness enhancement of HVOF sprayed MCrAlY coatings in order to improve the bonding properties of the TBC. In the project’s framework, several HVOF systems and different powders were investigated. Parameter sets were developed considering both a high surface roughness and a low level of defects in the coating.

The technical and economical potential of innovative materials is gaining growing interest for technologically orientated companies. The use of lightweight materials is an extremely important consideration, especially when designing moving components (e. g. in aerospace, automotive or machine construction). Magnesium is a promising alternative to other lightweight materials, such as aluminum and titanium, due to its relatively high specific strength and stiffness. Further advantages are high thermal conductivity and good joining and machining capabilities. However, the use of magnesium alloys is restricted by relatively poor wear behavior and corrosion resistance. In order to overcome the limitations associated with magnesium alloys, a project was founded by the Materials Science Institute (MSI), at the RWTH Aachen, and the Institute of Materials Science (IW), at the University of Hannover, to deal with the application of wear and corrosion protective coatings on Mg alloys by means of thermal spraying. A variety of coating materials were applied on Mg substrates using several thermal spray processes (like Arc Spray and HVOF). The coatings were then characterized particularly with regard to their wear and corrosion properties. To further enhance the overall corrosion resistance two additional approaches were investigated. On the one hand various duplex coating systems were designed and applied. With the objective of decreasing the open porosity coatings were either densified by shot peening or sealed by applying organic sealers on the other hand.

This presentation gives an overview of a research project of the Institute of Materials Science at the University of Hannover (Germany) focussed on under water plasma spraying (UPS). The aim of this project is to qualify the process for the application of corrosion protective coatings. UPS could be used for maintenance of underwater constructions. Another field of application can be seen in an alternative for atmospheric plasma spraying (APS) in coating shops. Water is an excellent filter for noise and UV radiation. Furthermore, particle polluted water could quite easily be refined so that UPS could show environmental advantages compared to APS. The UPS process is characterized by a low standoff distance and therefore a small plasma spot. A very promising modification of the standard UPS process, which is examined in this project, is shrouded underwater plasma spraying (S-UPS). By adding the shroud to the underwater plasma spray device a large defined cavity could be build up so that spray distances comparable to APS are possible. By variation of the shroud gas in terms of air or inert gas it is possible to reach comparable or even better coating properties then using an APS process.

The high velocity oxyfuel flame spraying (HVOF) process appears to be an interesting alternative to low pressure plasma spraying (LPPS) processes for the application of MCrAlY coatings for the use of hot corrosion protection on turbine parts like blades and vanes. Lower investment costs for HVOF facilities compared to LPPS systems combined with adequate coating properties and a stable, easy controllable process can be seen as potential advantages regarding the application of this process. Several recent HVOF systems are screened concerning the application of MCrAlY coatings for hot corrosion protective coatings on turbine blades. In this research project, the Design of Experiments (DoE) is used to built up factorial experimental designs. The aim is, besides a benchmarking, to find out the potential of the HVOF systems to produce high quality hot corrosion protective coatings. The main emphasis of these preliminary investigations is on the evaluation of bonding defects in the interface, the porosity, and the oxide content of the coatings.

In this paper the development of thermal sprayed, hard phase reinforced aluminum based layers, in particular for coating light metal substrates is described. The aim of the project was to obtain wear resistant coatings for applications on light metal surfaces combined with other advantageous characteristics, especially good thermal conductivity. One possible application for these coatings can be seen in automotive light weight constructions as wear protection for brake discs or drums. Flame shock spraying as well as high velocity oxyfuel flame spraying (HVOF) were used as coating processes. As consumables mechanically alloyed powders consisting of aluminum and ferric oxide were used. Due to the high kinetic process energy developed by the selected procedures in connection with the exothermically reacting spraying material, a new light metal matrix composite was produced. One major advantage of this coating material is the in-situ synthesis of the hard particles in the aluminum matrix during the spray process resulting in good adhesion/cohesion properties. This research project includes an extensive analysis of the consumables including differential thermal analysis, SEM, and EDX. Furthermore, process parameters were optimized. This includes a characterization of the HVOF process using modern particle diagnostics. Besides good bonding properties proven coating characteristics are high thermal conductivity and thermal shock resistance as well as good wear behavior even at elevated temperatures. The results show that the developed coating system is a promising alternative for cast aluminum matrix composite materials used for wear stressed parts even at elevated temperatures.