It is well known that legacy one-cathode/anode thermal spray guns are sensitive to aging. One reason is the large power density in particular at the arc roots on the cathode tip and the anode wall. Anode wear was studied in showing that it leads to a thinner boundary layer and a reduced motion of the arc root, which increases the local thermal load. This also results in a voltage drop, and thus in power level reduction if the power source is operated in a constant current mode. In this case, it is widely practiced to increase the secondary plasma gas flow, which, however, can only be of some help.

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 plasma spraying, hydrogen is widely used as a secondary working gas. Under low-pressure conditions, even small amounts of hydrogen can have a significant effect on the plasma jet as mechanisms such as diffusion and recombination come into play. This study investigates the influence of Ar-H 2 mixtures on electron densities, temperature distributions, and local composition in the plasma jet using optical emission spectroscopy. Several mechanisms reported in the literature are consulted to explain the observed phenomena.

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 use of plasma generators for thermal surface coating has grown enormously in the field of thermal insulation and wear-resistant coatings, especially with regard to deposition of refractory oxide ceramic layers using powders such as aluminum oxide, alumina / titania, zirconia and chromium oxide. Nonetheless, innovative plasma sources are still being developed to provide more-efficient, tailor-made solutions due to their specific characteristics. A brief overview of the DC-plasma generator Mettech “Axial III” are shown, which generator are allowed to use the central injection (advantage for suspension applications). In this paper first diagnostic results characterized the physical behaviour and special plasma symmetry, which can be described by three partial plasma jets. Thereby computer tomography (CT), high speed camera investigations coupled with time-adjusted current-voltage measurements, particle parameter detections and first resulting coatings are presented and discussed. The complete paper concludes with an outlook for future diagnostic measurements and possible future applications for this DC-plasma torch.

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.