In previous work, a thermal spray multilayer system consisting of ZrO 2 and an MCrAlY top coat showed promising results regarding oxidation behavior of the γ-TiAl substrates tested, which encouraged further research activities. Diffusion of substrate material was successfully inhibited by a ceramic ZrO 2 coating. A building up of a dense and stable oxide layer could be achieved by additional application of an MCrAlY top coat, leading to improved oxidation resistance and thus showing feasibility. In this work the main focus for development was put on enhancing adhesion and lowering residual stresses of the coatings in order to allow long term and cyclic testing without delamination taking place. Being a very brittle material, Gamma Titanium Aluminides require special surface treatment to enable roughening which is crucial for a strong mechanical bond between substrate and coating. Alternatives to conventional grit blasting as a standard preparation method were investigated. These were micro-abrasive blasting and blasting at elevated temperature (≈300-550 °C) to allow a more ductile behavior. The paper will highlight the implications by means of these measures and will also show the present development status of the multilayer system.

Due to excellent mechanical properties and low density compared to super alloys (e.g. Ni-based alloys) Titanium Aluminide is often used as base material in the aerospace industry. But the thermodynamic conditions within turbines limit the capabilities of the material. At the moment γ-TiAl is used for parts, which have to withstand temperatures up to 700 °C. Above this temperature oxidation kinetics cause a thick oxide layer consisting of several oxides, which tend to fast chipping. Therefore the surface of the γ-TiAl is being destroyed and the material loses its excellent mechanical properties. To enable the use of this material at higher temperatures, the development of an oxidation protection coating is necessary. Several coating techniques e.g. EB-PVD were tried in the last years, but the oxidation behaviour of the γ-TiAl could not be significantly improved. Protective thermal spray coatings so far seem to be a promising technology in order to protect γ-TiAl components against oxidation. Therefore this technique was used within this work, which aims for the development of new oxidation protection coatings. A multilayer system was developed. The multilayer consists of a ceramic ZrO 2 -7Y 2 O 3 coating with a NiCoCrAlY top coat. In this case the ceramic coating avoids the diffusion of Ti or Al of the γ-TiAl into the MCrAlY coating or the other way around. The NiCoCrAlY coating improved the oxidation behaviour of the Titanium Aluminide by building a dense oxide layer on top of the multilayer. The paper will give an overview about the results of the oxidation tests with the new developed multilayer concept for protection of the γ-TiAl against oxidation.

This paper reports on the development of NiZn-ferrite powders and their deposition by air plasma and high-velocity oxyfuel spraying. The microstructure and phase composition of the powders and coatings are analyzed and the influence of process parameters on coating development is assessed for sprayed layers up to 500 μm thick. Particular attention is paid to the degradation of the spinel crystal structure, the formation of iron oxide phases, and elemental loss during spraying. The results show that a degree of ferrite decomposition occurs with the loss of zinc and formation of wüstite and that zinc loss is very dependent on the surface-to-volume ratio of the powder.

This work demonstrates the capabilities of oxide dispersion strengthened MCrAlY coatings and the process used to produce the feedstock powders. Mechanically alloyed powder mixtures were prepared in a high-energy ball mill using commercial NiCoCrAlY and YPSZ powders combined in different amounts. A high-velocity oxyfuel torch was used to deposit the powders on Inconel substrates that were then heat treated for 6 h at 1050 °C. Samples were also produced from unaltered NiCoCrAlY powder to serve as a reference. In high-temperature erosion tests, the strengthened coatings had a mass loss of 47 mg/cm 2 after 30 sec compared to 38 mg/cm 2 for the unstrengthened sample. High-temperature corrosion tests showed that the strengthened coatings were not dense enough to protect the substrate from oxidation. The hard phases inhibited sintering and diffusion effects during heat treatment. As a result, dislocations, defects, and voids could not move and were used by oxygen for diffusion. In further investigations, the strengthened coatings will be treated at a higher temperature to achieve a better sintering effect.

MCrAlY materials are widely used as bond coats for thermal barrier coatings on turbine blades. The aim of this work is to improve mechanical properties and wear resistance of thermal sprayed NiCoCrAlY-coatings by strengthening the coating with hard phase particles. In order to retain the effect of the dispersion reinforcement at high temperatures, the use of temperature-stable oxide hard phases such as ZrO 2 is necessary. To realise this new material structure, the high energy ball milling process is applied and analysed. With this process it is possible to achieve a homogeneous distribution of the oxide hard phases in the NiCoCrAlY matrix. The mixture ratio between NiCoCrAlY and ZrO 2 was varied between 5 wt-% and 10 wt-% ZrO 2 . The influences of the milling time of the high energy ball milling process on the distribution of the hard phases in the metal matrix were analysed. After spraying with a HVOF system the mechanical properties of the coatings are measured and compared with conventional NiCoCrAlY coatings.

Demands on functional coatings with high dimensional accuracy and high surface quality has led to increasing interest in processing of very fine powder grades in a particle size range < 25 µm. Fine powders are not only showing a distinct potential for application of thin and dimensionally accurate coatings, but are also very promising for the production of dense and homogeneous coatings with improved mechanical properties. The large specific surface of fine powders is allowing for relatively low thermal energy levels that are introduced into the process. Nevertheless this also requires a very sensitive temperature control, to prevent overheating of the particles. The reduction of the thermal energy level is resulting in significant advantages particularly for the usability of the HVOF process for coating of inner diameters. Within this work in-flight particle properties of ultrafine carbide powders were analyzed. The studied HVOF process allows the adjustment of a broad parameter range by utilization of a hydrogen stabilized liquid fuel combustion process. A conventional straight nozzle type as well as a curved nozzle for internal spraying was studied. For a further assessment of the potential of ultrafine carbide powders also spray trials with a plasma spraying system have been made.