The main goal of this work is to improve the coating properties of three-cathode atmospheric plasma sprayed coatings with respect to porosity and residual stresses. This was done by use of numerical simulation coupled with advanced diagnostic methods. A numerical model for the triple injection of alumina feedstock, as well as acceleration and heating of the powder particles in the characteristic threefold symmetrical plasma jet cross section produced by a three-cathode-plasma torch was developed. The modeling results for the standard injector’s position “0” were calculated and experimentally verified by Laser Doppler Anemometry (LDA). Based on the criteria defined for concentrated feedstock transport and homogeneous thermal treatment of powder particles in the plasma jet, the optimal injection position was found. In the next step a previously developed, coupled CFD-FEM-simulation model was used for simulations of the coating build-up, describing flattening, solidification and deformation due to shrinkage for alumina particles on a rough substrate surface.

The goal of this research group is to homogenize properties of three-cathode plasma sprayed coatings on basis of numerical simulations and advanced diagnostics. Results of the first project phase as well as an outlook to future work are presented. A numerical model for investigation of plasma flow in the free jet, produced by three-cathode torch was developed. Modelling results are verified by plasma diagnostics (Computer Tomography). In order to include particle shrinking effects, coating formation simulation is accomplished by a newly developed model, based on Computational Fluid Dynamics coupled with the Finite Element method, whereat diagnostics carried out in the fields of particle diagnostics. During the next phase of the project, the investigation of the plasma free jet and particle injection by advanced diagnostics and simulation respectively is scheduled. In a subsequent stage the transition from conventional particles to suspensions will be considered. Coating formation simulations are scaled up to dimensions of macroscopic tensile tests. By combining these overarching investigations, appropriate process parameters for homogenized coatings will be obtained.

Powder injection parameters like gas flow, injection angle and injector position strongly influence the particle beam and thus coating properties. The interaction of the injection conditions on particle properties based on DPV-2000 measurements using the single-cathode F4 torch is presented. Furthermore, the investigation of the plasma plume by emission computer tomography is described when operating the three-cathode TriplexPro torch. By this imaging technology, the three-dimensional shape of the radiating plasma jet is reproduced based on images achieved from three CCD cameras rotating around the plume axis It is shown how the formation of the plasma jet changes with plasma parameters and how this knowledge can be used to optimize particle injection.

In the area of atmospheric plasma spraying, newly-developed triple-cathode technologies offer the potential to homogenize the coating properties with respect to porosity and residual stresses. Focused on numerical simulation, combined with advanced diagnostics, the goal of this research group is to adjust these properties systematically. A numerical model that couples fluid dynamic, electro-magnetic and thermal phenomena for a three-cathode torch was developed to investigate the plasma and the electric arc behaviour inside the torch. With help of self-developed computer tomography equipment, which is based on emission spectroscopy, combined with the solution of the Saha equation in thermodynamical equilibrium, it is now possible to reconstruct the 3- dimensional temperature distribution close to the torch outlet. This measurement allows us to confirm the torch numerical modelling. Coating formation is simulated by coupled computational fluid dynamics (CFD) and FEM simulation, so that fluid structure interaction is taken into account. This innovative approach has the advantage to predict residual stresses which occur during cooling and moreover the shrinking effects can be considered. By simulation of the individual regions, in combination with experimental results, which also include the particle velocity, diameter and surface temperature, the corresponding process parameters can be obtained for the desired coating properties.

In order to homogenize the properties of APS sprayed coatings, the spray process was investigated using numerical simulations combined with innovative diagnostic techniques. The process was subdivided into three areas: the plasma torch, the free jet, and coating formation. By simulating these areas separately and combining the results, appropriate process parameters for homogenized coatings were obtained. For a comprehensive computation of coating formation which, besides the impact, flattening, and solidification of particles, includes the mechanical properties of the coating, a volume of fluid algorithm is coupled with a finite element model. In order to verify the modeling of the plasma jet and to provide input data for the coating formation, diagnostic efforts were concentrated on measuring the gas temperature of the plasma as well as particle shape, velocity, and temperature. The results of spatially resolved 3D analysis employing an innovative tomography system are presented and compared with the numerical results.

One important aspect concerning the coating of surfaces using thermal spray is the improvement of the injection of material particles into the gas jet and thus the deposition efficiency. Therefore a better knowledge of the temperature distribution within the jet is relevant in order to optimize spraying conditions. Particularly interesting is the existence of a well-defined threefold finger structure in the plasma jet produced by triple electrode torches, which allows an efficient injection of coating material due to the existence of zones with higher and lower viscosity. The jet structure, however, lacks rotational symmetry and can therefore not be analyzed by systems relying on the validity of the Abel inversion, thus new systems have to be developed. In this work an innovative tomography device is described that has been designed for this purpose. By circling half around the plasma jet and taking simultaneously intensity images under different view orientations, a three-dimensional intensity distribution of the jet is generated, which can be used to determine the temperature distribution.

Increasing erosion of the anode wall of a single-cathode F4 torch changes the fluctuation pattern of the plasma jet. Trajectories and temperature history of particles injected into the plasma become accordingly modified. In this work such fluctuations are characterised through spectral analysis of the torch voltage as well as of the light intensity of the jet outside the torch. Instead of considering the evolution of the main peaks in the frequency spectrum for both voltage and optical signals, the variation with increasing torch wear of the spectrum’s higher frequency fraction (above 20kHz) is focused on. The evolution of the fractal dimension in this high frequency range exhibits a reliable correlation with the erosion of the anode wall. This is confirmed further by the development of a simple stochastic model for the motion of the arc root along the anode wall. This model yields, depending on the probability for a pronounced arc root jump, a fractal dimension and an evolution just within the range observed in the measurements. Additionally, tracking the fluctuations at different locations outside the torch enables the isolation of the fluctuation dissipation due to jet viscosity and velocity.

Wear at the electrode surfaces of a one-cathode plasma torch changes the characteristic fluctuation pattern of the plasma jet. This affects the trajectory of the particles injected into the plasma jet in a non-controllable way, which degrades the reproducibility of the process. Time-based voltage measurements and Fourier analysis were carried out on a one-cathode F4 torch at different wear conditions to determine the evolution of wear-dependent characteristics. A significant correlation is observed between increasing torch wear and decreasing voltage roughness and high frequency noise. Furthermore, by means of particle diagnostic systems, the change in the particle velocity and temperature has been measured. The variations of the particle characteristics are significant and thus an influence on the sprayed coating microstructure is to be expected.

Understanding the particle injection into the gas flow issuing from an APS torch is necessary to optimize the spraying parameters. In order to solve numerically this task, the distribution of gas velocity and temperature at the torch outlet is required. In this work this is achieved by developing a model which not only delivers the solution for the electrically charged gas flow within the torch, but also includes the thermodynamical condition of minimum entropy production. This additional condition fixes the size of the electric arc inside the torch, whose radius is particularly responsible for the form of the calculated velocity and temperature profiles at the torch nozzle. The velocity and viscosity of the gas flow near the torch outlet mainly control the trajectory of particles injected into the gas flow. For the typical gas mass flow and torch power used in APS, the resulting temperatures at the gas core are slightly above the ionization temperature of the gas species. The radial location of the viscosity maximum corresponding to the ionization temperature is calculated, since this maximum strongly influences the particle trajectory. Finally, the influence of plasma fluctuations on the heat transfer to the injected particles is discussed.

One of the most sensitive factors for the simulation of the gas flow outside the plasma torch is the determination of the velocity and temperature profiles at the torch outlet. It requires the solution of the flow and electric potential equations within the torch. In this work this problem is solved through numerical simulations, with two free parameters: the radius of the plasma core near to the cathode and the length of the electric arc (or alternatively the potential between cathode and anode). In order to reduce the number of the free parameters to only one, an analytical model is also developed, which solves the same equations as the numerical simulations but in a simplified way. The reduction of parameters is achieved in this simple model through the additional condition that the physical plasma corresponds to an extremal value in the entropy production. Since the results of the simplified analytical model are compared to more detailed numerical simulations, the free parameter can be adjusted. The effect of high hydrogen content on the gas flow is thus studied, showing that the velocity profile at the outlet displays a more pronounced peak, as expected experimentally.

The suspension plasma spraying (SPS) technique implemented on DC plasma spray guns is a complex thermal spraying process. In order to gain a better understanding of this deposition technique a systematic “splat” study using a shutter mechanism and the line-scan test was conducted varying liquid feedstock properties (viscosity and surface tension) and injection parameters (stream velocity and mass-loading). Splat morphology revealed the degree of particle agglomeration within the droplet formed from the liquid/plasma interaction, as well as their impacting velocity and heating history. The droplet formation was correlated to the liquid feedstock injection velocity and its viscosity. A simple model was developed to explain the experimental results correlating suspension properties to suspension droplet fragmentation mechanism.

The triplex-torch design providing three stable arcs in power and geometry promises a stable, quasi-laminar plasma jet leading to an uniform plasma and thus an uniform particle distribution. Additionally, the three powder injectors lead to an extended powder feed rate, resulting in a lower spraying time. This paper deals with particle properties of YSZ (SM 204NS) with respect to their injection location and powder feed rate and the resulting coating microstructure and porosity levels. Concerning the injection conditions, the three powder jets can be clearly identified. Their location depends on the spraying distance and less pronounced on the injection velocity. Interestingly, if the different powder-sizes are grouped and the jet cross-section is plotted for the different size cuts, the position of each size cut varies. Thus, a local separation of the powder takes place in the jet. The powder feed rate was varied from 30 to 320 g/min. Accordingly to this variation, the particles mean temperature was lowered from 3050°C down to 2650°C, and the velocity values dropped from 230 m/s to 155 m/s. Regarding the achieved coating thickness, up to a powder feed rate of approximately 210 g/min the thickness is increasing with no dramatic change in the porosity values. More powder does not lead to a thicker coating but results in a significantly higher porosity. Moreover, the microstructure of the coatings obtained by using a higher feed rate exhibits more segmentation cracks.

The innovative suspension plasma spraying (SPS) technique, in which the carrier gas used to inject particles (10…100 µm) into the plasma jet is replaced by a liquid feedstock, is currently under development procuring denser ceramic coatings due to the use of submicron particles. The suspension properties, as well as the most relevant injection parameters - injection angle and liquid velocity - are adjusted to improve the coating quality at acceptable deposition rates. In addition, the plasma jet instabilities are studied and correlated to the coating properties. In the present work, a feasibility study is conducted addressing the key factors influencing the coating morphological properties such as the porosity, cracks, molten-fraction and amount of over-spray. The experimental setup is adapted to different DC plasma torch architectures (Sulzer-Metco F4 and Triplex) operating under atmospheric conditions with the aim of correlating the plasma jet qualities to the properties of the feedstock under consideration. The metallographic probes and fractured surfaces of the resulting yttria stabilized zirconia (YSZ, 5 wt. % Y 2 O 3 ) coatings are analyzed by means of light microscopy and scanning electron microscopy (SEM).

The heat and momentum transfer of the plasma to the injected particles is an important issue in coating formation during plasma spraying. In this study, the plasma temperature and velocity of a Triplex-I torch was measured by means of an enthalpy probe system. Additionally, the properties of injected yttria-stabilized-zirconia powder of a fine sized fraction were recorded spatially resolved by using a DPV2000 system. The plasma temperature and velocity are decreasing by increasing the distance from 45 to 60 mm with respect to the torch exit by approximately 50% from initially 6200°C and 400 m/s, respectively. In contrast, the particles gain temperature up to the melting point at 70 mm stand-off distance as well as the velocity rises up to its highest value of 115 m/s at the maximum flow rate of the particles. Both, plasma and particle characteristics were used to obtain a deeper insight on heat- and momentum transfer of the plasma jet to the single particles.