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.

New developments in the field of thermal spraying systems (increased particle velocities, enhanced process stability) are leading to improved coating properties. At the same time innovations in the field of feedstock materials are supporting this trend. The combination of modern thermal spraying systems and new material concepts has led to a renaissance of Fe-based feedstocks. Using modern APS or HVOF systems, it is now possible to compete with classical materials for wear and corrosion applications like Ni basis (e.g. NiCrBSi) or metal matrix composites (MMC, e.g. WC/Co or Cr 3 C 2 /NiCr). The work described in this paper focuses on that combination and intends to give an analysis of the in-flight particle and spray jet properties achievable with two different modern thermal spraying systems (kerosene driven HVOF system K2, 3- cathodes APS system TriplexPro-200/-210) using Fe-based powders. The velocity fields are measured with the Laser Doppler Anemometry (LDA). Additionally, resulting coatings are analyzed metallographically with regard to their properties and a correlation with the particle in-flight properties is given. The experimental work is accompanied by computational fluid dynamics (CFD) simulations of spray jet and particle velocities, leading to a comprehensive analysis and characterization of the achievable particle properties with state-of-the-art HVOF and APS systems.

Properties and performance of coatings deposited by the Cold Gas Spraying process are affected by particle properties, which can be changed by using various spraying parameters. Different diagnostic systems can be applied to investigate these parameters. By means of the commercial SprayWatch system, particle properties (velocity, temperature, size, number density) can be measured and evaluated. In comparison the Laser Doppler Anemometry (LDA) system determines velocity and number density. Using both systems the velocity has been measured for a range of parameters, including different materials, different gas properties and nozzles thus offering the possibility to compare the two different diagnostic systems. In the present paper only the results of different gas properties will be shown.

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.

Well-adjusted processing conditions are the basics for a high quality of thermal sprayed coatings. For HVOF spraying that means super sonic particle velocities and moderate particle temperatures. To fulfil these requirements, a certain speed of the gas jet is necessary. If low fuel and gas consumption is aspired, this can only be realised with adapted nozzle designs. Some efficient ways of nozzle design optimisation are shown in this paper. Novel methods and strategies in computed fluid dynamics are explained and correlated to experimental results. For this purpose, gas jet and particle velocities are investigated by means of laser optical measurements.

The LARGE is a new generation DC-plasma torch featuring an extended arc which is operated with a perpendicular gas flow to create a wide (up to 45 cm) plasma jet well suited for large area plasma processing. Using plasma diagnostic systems like high speed imaging, enthalpy probe, emission spectroscopy and tomography the LARGE produced plasma jet characteristics have been measured and sources of instability have been identified. With a simple model/simulation of the system LARGE III-150 and numerous experimental results, a new nozzle configuration and geometry (LARGE IV-150) has been designed, which produces a more homogenous plasma jet. These improvements enable the standard applications of the LARGE plasma torch (CVD coating process and surface activation process) to operate with higher efficiency.

Coatings produced by thermal spray processes are determined by particle parameters, which can be investigated using various diagnostic systems. With the newly developed PSI (Particle Shape Imaging) four (velocity, size, shape and position) important particle parameters can be measured and evaluated. The data acquisition system consists of two separate optical setups, thus a single particle produces two pictures in the lower and upper area of an ICCD-Camera. These pictures are evaluated by assigning specially developed shape and size parameters to the particles, which in combination with the velocity can be used to predict the coating quality. This instrument can be used to determine the optimum spray parameters (i.e. find the optimal distance between substrate and thermal spray system) and to survey the coating process. A newly developed, mobile PSI-System will allow operation at industrial and research thermal spray facilities.

The number of parameters influencing the plasma spraying process is very high. Only a part of these parameters can be controlled online; some of them such as gas flows, current, voltage and spraying distance can be controlled easily, others such as particle temperature and velocity can only be controlled with substantially higher effort. As differences from parameter values preinstalled or given at the start of the process, the noise factors affect the coating properties in different ways and show big effects on the coating quality. Nevertheless there is only little knowledge about the significance of several noise factors and about the influence of small process parameter fluctuations on the coating properties. Because some of these noise factors such as plasma torch degradation cannot be avoided, the aim of this work is to determine the sets of coating parameters, where the influence of noise factors is minimized. This should be achieved by using online diagnostic tools, that afford the observation of fast and easy controllable process characteristics. On the other hand process errors shall be identified in an early process stage using appropriate diagnostic methods.

One of the most effecting parameters for the wire arc spraying with the regard to the properties of the sprayed coatings is the flow characteristic of the atomizing gas. Depending on the nozzle inner contour at the entrance of the torch, the atomizing gas is emanating the nozzle as a non-adjusted supersonic gas jet, either under- or over expanded. Thus, a convergent-divergent nozzle with a bell-shaped divergent exit is designed in order to generate a completely expanded atomizing gas flow inside the nozzle without any shock nodes in the emanating gas jet. The influence of the flow characteristic of the atomizing gas (air) on the droplet velocity is investigated by Laser- Doppler-Anemometry measurements. The coating quality is characterised with regard to the surface roughness and porosity.

For constant conditions concerning substrate state, feedstock and environment properties of thermal spray coatings depend only on temperature and velocity of particles at impact on the substrate. Two different HVOF spraying guns, the kerosene fuel system Tafa JP5000 with radial powder injection and the ethylene fuel system Sulzer Metco Diamond Jet Hybrid 2700 with axial powder feeding, are characterized concerning the evolution of space resolved velocity and density of particles by LDA. Also influence of process parameter variations is examined. The region of shock diamonds is studied specifically. The influence of different characteristics concerning impact velocity and trajectories on the coatings microstructure is determined by means of optical microscopy and microhardness testing.

An optimised de-Laval-type-contour for the Atmospheric Plasma Spraying (APS) with respect to the thermal and deposition efficiency as well as to the arc voltage fluctuations and the sound level of the plasma torch is presented. Investigations of the plasma gas dynamics and two different feedstock materials (Al 2 O 3 (fused and crashed) and Cr 2 O 3 (agglomerated and sintered)) have been done to promote the industrial utilization of convergent-divergent nozzles for the APS.

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.

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.

As a new plasma gun technology the TRIPLEX system has been introduced in the industrial field two years ago. The core of the TRIPLEX technology is a plasma gun with three cathodes and a long cascaded nozzle consisting of several insulated rings. Only the last ring with a relatively long distance to the cathode is operated as anode. Because of the equal and constant lengths of the three independent arcs, stretching from the three cathodes to the common anode, a stationary plasma jet is generated. Compared to conventional torches, the improved stability of the plasma jet allows a more uniform powder treatment and a higher deposition efficiency as well as the powder feed rate can be increased using a triple injection system. A significantly longer life time of the electrodes reduces the cost for quality control in the coating process. The characteristic properties of oxide ceramic coatings are improved in comparison with the coatings produced by conventional plasma torches. The results of two years industrial application of the innovative torch system TRIPLEX are presented in the paper.

The quality and the deposition efficiency of coatings produced in thermal spraying processes are influenced by many parameters. Therefore normally the process optimization is laborious and time consuming. To reduce these efforts the low cost In-situ diagnostic method PFI was developed. A stationary CCD camera records the radiation of the hot propellant carrier-medium and of the luminous particle flux. A connected PC immediately converts the brightness distributions to a false colour image corresponding to the temperature and density profiles of the carrier-medium and of the particle flux. Utilizing a reference image the parameters can be adjusted for a prompt and successful optimization of the spray process. The performance of the PFI- method is demonstrated in some examples. Paper text in German.

A project group consisting of four institutes and funded by the DFG has started a new approach to investigate the complicated processes (phenomena) involved in thermal coating in order to control these methods and here especially the plasma spraying in a better way and to make use of them more efficiently. This approach starts from the desired quality of the layer to be produced and derives the required conditions and features of plasma and spray material prior to deposition to get this quality. To obtain well-defined interrelationships several modern and specially modified diagnostic methods have to be used and theoretical considerations performed as well. These and the state of the work are described. Paper text in German.