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.

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.

This paper presents selected research results of the DFG founded project group, consisting of four institutes focusing on diagnostic methods in thermal coating processes. The aim of this group is to characterize the Atmospheric Plasma Spraying (APS) process by means of diagnostic methods so that – based on the requirement profile of the coating – appropriate adjusting of the process parameters can be realized. For this purpose, different diagnostic tools like Particle Shape Imaging, Laser Doppler Anemometry, Schlieren Technique, Particle Image Velocimetry, Enthalpy Probe, DPV 2000 and Thermography were qualified and adjusted to each other. Most of the results presented in this article are limited to the area close to the substrate which is difficult to handle with diagnostic methods. Additionally, new achievements concerning nozzle design and system enhancements are introduced.

Modern thermal spray processes require wide use of diagnostics to gather an extensive process understanding. Today's diagnostic results provide the basis for future designs and advancements, particularly and increasingly on basis of computational approaches. Due to the measuring area in the square centimeter range and its quick and accurate results, the Particle Image Velocimetry (PIV) represents an enriching for thermal spray process diagnostics. Our experimental results obtained from PIV are in accordance to present theoretical and empirical derivations of some kinematic parameters of thermal spray processes. Beneath verification of well-known dominant parameters (for example powder fraction or carrier gas mass flow), this procedure enables the detection and characterization of ancillary influences on the process due to its high accuracy. By statistical analysis of our experiments, using multiple parameter variations per experiment according to the technique of "Design of Experiments" (DoE), we possibly found some hints on interactions between ancillary parameters which shall be analysed in further works carefully. In combination with detailed simulations on plasma - particle interactions and powder injection it should be possible to develop methods for thermal spray processes to minimize the particle flow expansion for an optimization of deposition rate and energy efficiency in the future.

The aim of the project group consisting of four research centers and founded by the DFG (German Research Society) is to characterize the plasma spraying process by means of diagnostic methods so that, based on the requirement profile of the coating, appropriate adjusting of the process parameters can be realized. For this purpose, different, partly newly-developed diagnostic tools, like Particle Shape Imaging, Laser Doppler Anemometry, Streak Technique, Particle Image Velocimetry, Enthalpy Probe, DPV 2000 and Thermography were qualified and adjusted to each other. The new results presented in this article are limited to the areas of particle injection and substrate which are difficult to handle with diagnostic methods.

This paper presents research highlights obtained over the past three years in the course of a DFG-funded project on new and emerging diagnostic methods for thermal coating. It describes the tools and techniques used, the particle and substrate variables monitored, the accuracy of each measurement, and various associations with coating properties. Paper text in German.

Arc spraying is an economical method for applying metallic layers due to its high spray rates and uniform melting of spray particles. The main disadvantage is the difficulty in achieving sufficient particle velocity to ensure good layer adhesion. This study investigates the influence of nozzle geometry, arc power, and gas pressure on the size and velocity of particles in an arc spray jet. The experiments were conducted using particle image velocimetry (PIV) to measure the spatial and velocity distribution of particles in flight. For X45Cr13 steel, particle velocities were found to be between 85 and 95 m/s at a gas volume flow of around 1 m 3 /min. Velocities of up to 150 m/s were ultimately achieved, but at the expense of higher atomizer gas consumption. Paper text in German.