Driven by the search for an optimum combination of particle velocity and process temperature to achieve dense hard metal coatings at high deposition efficiencies and powder feed rates, the high velocity air-fuel spraying process (HVAF) was developed. In terms of achievable particle velocities and temperatures, this process can be classified between high velocity oxy-fuel spraying (HVOF) and cold gas spraying (CGS). The particular advantages of HVAF regarding moderate process temperatures, high particle velocities as well as high productivity and efficiency suggest that the application of HVAF should be also investigated for the manufacture of MCrAlY (M = Co and/or Ni) bond coats (BCs) in thermal barrier coating (TBC) systems. In this work, corresponding HVAF spray parameters were developed based on detailed process analyses. Different diagnostics were carried out to characterize the working gas jet and the particles in flight. The coatings were investigated with respect to their microstructure, surface roughness and oxygen content. The spray process was assessed for its effectiveness. Process diagnostics as well as calculations of the gas flow in the jet and the particle acceleration and heating were applied to explain the governing mechanisms on the coating characteristics. The results show that HVAF is a promising alternative manufacturing process.

In thermal spray processes, the interaction between the gas jet and the particulate feedstock can affect the coating build-up mechanisms considerably. Especially under high-kinetic and low-pressure conditions, small particles are subjected to rapid deflection and velocity changes close to the substrate. In this work, numerical studies were performed to investigate the particle trajectories in the substrate boundary layers (BL). Typical conditions for suspension plasma spraying (SPS) and plasma spray-physical vapor deposition (PS-PVD) were taken as a basis. Particular importance was attached to the consideration of rarefaction and compressibility effects on the drag force. The particle impact conditions in the different thermal spray processes are compared. Possible effects on the resulting coating build-up mechanisms and microstructure formation are discussed.

The residual stresses within plasma sprayed coatings are an important factor which can influence the lifetime of the coatings. The investigation of the evolving stresses during deposition and post-deposition cooling of YSZ coatings by measuring in-situ the specimen’s curvature with the so-called ICP sensor is a powerful tool to identify the different stress generation factors. Under certain spray conditions one can observe that the first torch pass leads to a significantly higher increase in specimen´s curvature than the following deposition passes, which indicates significantly higher stresses within the interface coating region. The reason for this steep curvature increase was investigated. It is suggested to be a combination of a stronger bonding of those splats being connected directly to the substrate and the relief of compressive stress within the substrate. The slope of this increase depends on the spray parameters as well as on the substrate conditions, which was investigated also.

Partially yttria stabilised zirconia (YSZ) approaches limits of performance in long-term operation at 1200°C as a thermal barrier coating (TBC). For the next generation of TBCs, higher temperature capability and longer lifetime of the coatings are essential to boost efficiency of gas turbine engines. In this work, gadolinium zirconate (Gd 2 Zr 2 O 7 ) (GZO)/YSZ double ceramic layer TBCs are presented enabling to withstand higher temperatures. Processing of the GZO layers by atmospheric plasma spraying with TriplexPro TM spray torch, which is complicated due to the different vapour pressures of the feedstock constituents, is described. Furthermore, the influence of varied GZO microstructures in terms of porosity on lifetime and failure modes of the double layers tested at surface temperature of 1400°C is reported.

A new method for fabricating microsensors which can provide accurate real-time temperature monitoring of thermal barrier coatings on gas turbine engines was developed. A high temperature K-type thermocouple sensor for hostile environments was deposited using a coaxial pulsed laser cladding process with optimized process parameters giving minimal intrusive features to the substrate and afterwards embedded in typical ceramic layers. The dimensions of the cladded thermocouple were about one hundred microns in thickness and width. The thermal and electrical response of the cladded thermocouple was tested before and after embedding over temperatures ranging from ambient up to approximately 500 °C in a furnace with flowing argon as protective gas. The results were compared to that of a commercial standard K-type thermocouple, which indicate that laser cladding is a promising technology for manufacturing microsensors for in-situ monitoring in harsh operation environments.

It is generally known that the porosity of thermal barrier coatings is essential to guarantee a sufficiently high strain tolerance of the coating during thermal cycling. However, much less is known about the influence of the specific kind of porosity such as micro cracks and typically larger pores on the performance of the coatings. Both features are usually formed during plasma spraying of yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). In this investigation the influence of micro cracks on the thermal cycling behavior was studied. The amount of micro cracks within YSZ thermal barrier coatings was changed by changing the powder feed rate. However, only small changes of the total porosity were observed by changing powder feed rate. Mercury porosimetry served as a tool to investigate both the amount of micro cracks and pores in the coating. Additionally, micro crack densities were determined from metallographical investigations. A linear dependence between the amount of fine pores determined by Hg porosimetry and the crack density was obtained for one set of coatings. Thermal cycling specimens with thermal barrier coatings having different micro crack densities were produced and tested in a gas burner test facility. At high surface temperatures above 1300°C failure occurred in the ceramic close to the surface. Under these conditions the samples with increased horizontal micro crack densities showed a significant increase of thermal cycling life.