The global economic growth has triggered a dramatic increase in the demand for resources over the last few years, resulting in steady price increases for energy and raw materials. In the gas turbine manufacturing sector, process optimizations of cost-intensive production steps involve a heightened savings potential and form the basis for securing future competitive advantages in the market economy. In this context, the atmospheric plasma spraying (APS) process for thermal barrier coatings (TBC) has been optimized. A constraint for the APS coating process optimization is the use of the existing coating equipment. Furthermore, the current coating quality and characteristics are not allowed to change in order to avoid new qualification and testing. Using experience in atmospheric plasma spraying and empirically gained data, the process optimization plan included the variation of e.g. the plasma gas composition and flow rate, the electrical power, the arrangement and angle of the powder injectors to the plasma jet, the grain size distribution of the spray powder and the plasma torch movement procedure like spray distance, offset and iteration. In particular, plasma properties (enthalpy, velocity, temperature), powder injection conditions (injection point, injection speed, grain size distribution,) as well as the coating lamination (coating pattern, spraying distance) are examined. The optimized process and resulting coating was compared to the current situation by several diagnostics methods. The improved process provides significantly lower costs by achieving the requirement of comparable coating quality. Furthermore, a contribution was made to a better comprehension of the atmospheric plasma spraying of ceramics and a method for future process developments was defined.

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 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.

This study investigates the influence of plasma gas composition on deposition efficiency achieved during atmospheric plasma spraying and the properties of the resulting deposits. In the experiments, ternary mixtures of argon, hydrogen, and helium are used in different combinations and flow rates to spray Al 2 O 3 -TiO 2 and ZrO 2 -Y 2 O 3 powders on test substrates while measuring deposition efficiency. Several coating properties are measured, including porosity, hardness, and bond strength, and correlated with plasma gas ratios. Paper text in German.

The properties of thermal sprayed coatings depend mainly on the thermal and kinetic energy of the spray particles. Increase of thermal energy of sprayed particles can be realized using exothermic reactions between components in sprayed particles. Self propagating high temperature synthesis (SHS) is especially suitable to benefit from released energy in the spraying process. At present most commonly used spray material with exothermal reaction is Ni+Al. However, the highest amount of heat is produced in the reactions of aluminium and metal oxides. Of special interest are Cr 2 O 3 , NiO, CuO and V 2 O 5 because they obtain high reaction energies. Furthermore products of the reaction are of special, functional interest like NiAl as bonding agent or alumina as a wear resistant coating. To assure good contact between reacting substances (Al/Oxides) powders for plasma spraying were prepared by mechanical alloying. Calorimetric investigations of plasma sprayed coatings prove that during spraying Al reacts exothermically with oxides. Increase of oxide contents improves coating adhesion/ cohesion properties, hardness, and reduction of porosity. Results are discussed on the base of light microscopy, scanning electron microscopy (SEM) and X-ray structure analysis (XRD).

Wire flame sprayed molybdenum is a wide used procedure for manufacturing of wear resistance coatings. The properties of thermal sprayed coatings depend mainly on the kinetic and thermal energy of sprayed particles, i.e., a higher particle velocity causes an increase of coating quality. The now available high velocity spray system from Praxair which is used within this work is capable to realise the aim of high particle velocities. The coating properties presented in this work are analysed in comparison to conventional wire and powder plasma spray processes. HVWFS molybdenum coatings show lower porosity, higher adhesion and cohesion and better wear properties. To explain the results, particle size distribution, oxygen/carbon content and structure are analysed. Hardening mechanisms of coatings and their adhesion/cohesion properties are discussed based on light microscopy, SEM, XRD and TEM investigations.

This paper examines the potential of thermal spray coatings for manufacturing and repairing twin-screw rotors. The coatings evaluated were produced by atmospheric plasma spraying (APS). Tests show that 7% yttria stabilized zirconia (ZrO 2 7Y 2 O 3 ) coatings provide sufficient run-in characteristics and corrosion resistance. The coatings, along with a NiCrAl bond coat, were applied to various substrate materials.

Aluminum coatings reinforced with either Al 2 O 3 or SiC particles were deposited onto aluminum substrates and subjected to various tests. The coatings were made with mechanically alloyed powders via atmospheric plasma spraying (APS). Both types of coatings had uniformly distributed hard particles, porosities in the range of 4 to 5%, and bond strengths of around 20 MPa. The wear resistance of the SiC-reinforced coatings, however, was almost 35% higher than the coatings containing Al 2 O 3 . X-ray examination (XRD) showed that the Al 2 O 3 particles undergo partial phase transformation during spraying, making them more prone to wear.

Coatings with magnetic properties were produced by vaccum plasma spraying of different Nd-Fe-B powders onto stainless¬steel substrates. Microscopic examination of the coatings in a light microscope revealed a low porosity and a good bonding to the substrate. Scanning electron microscope examinations have shown different phases in these coatings. The magnetic properties of the coatings are also presented. Paper text in German.

For obtaining higher particle velocities and consequently dense coatings, a closed nozzle system for atmospheric arc spraying was developed. The proposed system is characterized by a nozzle geometry which allows an expansion of the atomizing gas only in downstream direction. First coatings, obtained with this system exhibited promising results due to dense structure and high adhesion. Paper text in German.

A high speed thermoanalytical method has been developed to determine the thermophysical properties of the powders used in thermal spraying. This method is based on heating the powder by means of an electron- or photon-beam in vessels made of materials with excellent heat conductivity. By comparing heating and melting behavior of powders with similar composition but different particle sizes and shapes, their properties can be determined. The suggested high speed thermoanalytical method makes it possible to investigate the heating and melting behavior of spray powders under high speed heating conditions. The results of the experiments, elaborated by this method can be used for further modeling work. Paper text in German.

This paper examines thermal insulation layer systems with regard to their thermal shock resistance, internal stresses, adhesive tensile strength and layer morphology. The research was conducted on a duplex coatings system consisting of zirconium dioxide/yttrium oxide ceramic coatings and CoNiCrAlY oxidizing and corrosion resisting coating. In all thermal insulation layer systems, the bonding layers were produced by vacuum plasma spraying. The thermal insulation layers were produced on the one hand by atmospheric plasma spraying and on the other hand by vacuum plasma spraying, and their properties were compared with one another. Paper includes a German-language abstract.