Suspensions have shown a great potential for being employed as the spraying materials in flame spraying processes with the aim of producing thin and dense coatings. The internal axial injection of a suspension within processes like the high-velocity suspension flame spraying (HVSFS) offers the advantage of complete suspension entrainment within the gas stream, which therefore results in enhanced momentum and heat transfer to the particles. Experimental assessment of the achieved particle velocities and temperatures within the combustion chamber is nonetheless practically infeasible. A better insight into the process is attainable through employing computational simulations. Following a computational fluid dynamic (CFD) modelling approach for HVOF processes, combustion and gas flow turbulences were simulated for different combustion chamber geometries and ethene/oxygen ratios commonly used in the HVSFS process. Simulations were done with the commercial software ANSYS CFX 16.2. To account for the highly turbulent flow characteristics, the k-ε model and the Shear Stress Transport (SST) turbulence model were chosen, employing an eddy-dissipation-model for fuel gas combustion. Second-order upwind discretization was used to enable a good resolution of flow features like shock diamonds. The results of the simulation using different levels of detail of the combustion reaction were compared to experiments employing the modelled combustion chambers and gas flows. Chamber pressure and positions of the shock diamonds were monitored in order to allow a qualitative evaluation of the calculated values.

Traditional low-cost bulk materials are unable to fulfil the increasing requirements of actual technology and functional coatings – e.g. APS alumina coatings on aluminium substrates for tribological properties – are a suitable alternative. The development of new thermal sprayings is usually based in experimental procedures which involve long development times and high costs. Nowadays, numerical simulation allows the researcher a better understanding of thermal spray processes as well as reducing the time and cost for the optimization of processes, but it requires a deep insight into the physics of the phenomenon. The submodelling approach allows the researchers to work with a local model, a thin layer – just a few microns – on the metal substrate surface, and more realistic boundary conditions, which requires a little specialised knowledge. The current study involves the workflow to manage the modelling at piece scale, and the transfer and interpolation of boundary conditions in each scale. The coating is divided into several layers, which represent the successive splats deposited during the process, and the heat flow from the torch is modelled by radiation and convection. A code is implemented in order to generate the routine needed by the FEM software, in which the results are processed and interpolated for the subsequent submodel. Furthermore, the material plasticity is considered and several tests are performed in order to check the simulation results.

In recent studies, the suspension plasma spray technology (SPS) was shown as a promising method for deposition of photocatalytic TiO 2 coatings on glass substrates. However, only little information about the effects of SPS process parameters on the resulting photocatalytic activity is available. In this study, several suspensions were prepared from different powders and various dispersion mediums successively sprayed. Four titania powders with a proven photocatalytic track record, Evonik P25, Kronoclean 7050, Hombicat UV100 and Sigma Aldrich were used to study the influence of starting powder type. Furthermore, compositions of the dispersing medium were varied in order to investigate their influence on coating structure, adhesion to substrate and photocatalytic activity. Degradation of methylene blue was tested as a marker for the photocatalytic activity. At chosen torch parameters, alcohol and water in a ratio of 1:3 was identified as the best composition. Suspensions sprayed with higher alcohol content suffered from the reduction in anatase phase; higher water content reduced the coating adhesion to the substrate. Regarding the initial powders, films made of Evonik P25 and Sigma Aldrich TiO 2 had higher photocatalytic activity than others. Moreover, the photocatalytic activity of doped TiO 2 coatings illuminated by both an artificial light and sunlight were investigated.

The residual stresses present in coatings and layer composites are influenced not only by the thermal and mechanical loads generated during manufacturing, but also by the mechanical and thermophysical properties of the coating and substrate materials. In-process measurement of transient, process-induced stresses may thus enable the manufacturing of coated parts with a residual stress state that lies within a predefined application-oriented stress regime. This paper presents a quasi-nondestructive method by which such measurements may be obtained. A small amount of material is removed from the surface of a part by laser ablation, while optical interference sensors monitor surface deformation caused by stress relaxation and heating due to absorbed laser energy. The new method is evaluated by four-point bend testing using Al5754 plates coated with Al/TiO 2 by atmospheric plasma spraying.

Thermal spraying of suspensions containing particles of submicron or nano-size offers new possibilities in functional coating development and enables new application fields. Spraying of suspensions containing bioceramic materials by hypersonic flame spraying (HVSFS), result in coatings with a refined microstructure. A layer thickness ranging from 10 - 50 µm can be achieved. Thermally sprayed HAp coatings are widely used for various biomedical applications due to the fact that HAp is a bioactive, osteoconductive material capable of forming a direct and firm biological fixation with surrounding bone tissue. Bioceramic coatings (e.g. Hydroxyapatite HAp, Tricalcium Phosphate TCP or Bioglass) were thermally sprayed on Ti plates by high-velocity suspension flame spraying. The deposited coatings were mechanically characterized. The bond strength of the layer composites was analyzed by the pull-off method and compared for different spraying conditions.

Thermal spraying of oxide ceramic suspensions containing fine and ultrafine powder particles is a new approach for manufacturing ceramic coatings exhibiting a refined microstructure. Suspension sprayed coatings clearly differ from conventionally sprayed coatings regarding microstructure phase composition and resulting mechanical properties. Several industrial applications may take advantage in future; among these are thermal barrier structures, thermal shock protection, solid electrolytes, catalytically active surfaces and wear resistant coatings. Two methods, namely Suspension Plasma Spraying (SPS) and High Velocity Suspension Flame Spraying (HVSFS) are suitable to process suspensions but lead to rather different coating structures due to differences in the achievable particle velocities and temperature. Generally, HVSFS can lead to more dense coatings with low porosity values. With SPS on the other hand, coatings with a high volume fraction of porosity featuring a homogeneous pore structure are achievable. The presentation will compare SPS and HVSFS regarding the spray process, achieved properties of the oxide coatings and potential applications.

Industrial robots are widely used in thermal spraying operations in order to assure process accuracy and reproducibility, as well as to increase worker´s safety. Robot path planning for the coating of free form geometries keeping constant the kinematic parameters can lead to complex robot paths and in some cases to unreachable robot configurations. An external rotational axis in coordinated movement with the robot can be used to manipulate complex shaped components during the spraying operation and to simplify the robot movement. Moreover, the robot kinematics, which define the spraying path, spray angle, spray distance and spray velocity, are key parameters to control the heat and mass transfer to the substrate during the coating deposition, and they influence therefore the development of residual stresses. The present work describes the off-line planning of robot trajectories for thermal spraying operations using an external rotational axis taking into consideration the heat transfer to the component.

High-velocity suspension flame spraying (HVSFS) is used for direct processing of submicron and nano-scaled particles to achieve dense surface layers in supersonic mode with a refined structure, from which superior properties are expected. The application of solutions as a carrier fluid for nano-particles in thermal spray systems is a new approach that requires some thermo-physical and chemical optimization. Three dimensional modeling and analysis of the combustion and gas dynamic phenomena of the three-phase HVSFS process is performed in this study for an industrial TopGun-G torch, based on a numerical model for a conventional HVOF process. Parameter analysis of the solution mixture (proportion between aqueous and organic solvent) in a suspension is performed as well as analysis of the variation of the combustion chamber depending on the torch design, leading to more homogeneous flow properties for an improved HVSFS torch.

Efficient transport is a key requirement for a powerful economy and will continue to gain importance leading to the necessity for high performance transport vehicles. A highly loaded major component of trains is the chassis. With increasing load, new materials and concepts have to be developed in order to increase the performance of these components. One approach to improving chassis components like bearings for chassis components is the deployment of thermally sprayed coatings by means of HVOF and HVSFS, which can significantly improve their wear resistance, prolong their life time and simultaneously reduce frictional losses. Using the example of spherical bearings, several HVOF sprayed metallic, cermetic and ceramic coatings have been tested for their ability to improve the properties of the components. In addition to metallographic investigations, extensive mechanical tests on test rigs have been carried out to determine the performance of the coatings under realistic conditions.

Thermonuclear fusion is a promising source of clean energy for the future. Max-Planck-Institute für Plasmaphysik (IPP, Greifswald, Germany) is currently working on the new type of fusion reactor, the stellarator Wendelstein 7-X. The extreme operating conditions of fusion reactor devices have lead to an increasing interest in the field of high performance materials. The present work describes the development of coating systems acting as efficient absorbers for 140 GHz radiation, which is the microwave frequency to which the analyzed components of Wendelstein 7-X are subjected. Several types of oxide ceramic coatings were applied by Atmospheric Plasma Spraying. Different powders were used as feedstock material for the coating operation. The influence of the process parameters on the coating properties and microwave absorbing capability was analyzed. The coatings microstructure and mechanical properties were characterized in terms of porosity, microhardness, roughness, adhesion and residual stresses. XRD and SEM were carried out. It was found that thickness and microstructure of the coatings have a significant influence on microwave absorption behavior. For Al 2 O 3 /TiO 2 coatings, absorption values over 90% were obtained. After optimization of the coating structure, the coating process was adapted to several real reactor components that will work in Wendelstein 7-X.

The image-based measurement method 2D Continuous Particle Image Velocimetry (2D Continuous PIV) is commonly used for measuring particle velocities in thermal spraying processes due to its basic measurement setup and its large measurement volume compared to methods based on point sensors. The accuracy of such image-based measurement techniques depends on the measurement algorithm, the process environment, such as distributions of particle characteristics, and the error of the imaging system. However, in the case of 2D measuring, accuracy might also depend on the fact that 2D methods measure only two of the three velocity vector components while ignoring the third component. In this paper, the impact of measuring only two of the three velocity vector components on the accuracy of a closed-source 2D Continuous PIV algorithm is investigated. The analysis is based on a virtual measuring instrument that includes optical aberrations of the imaging system and it is shown that this error contribution of measuring only in 2D is within acceptable limits for typical thermal spraying processes.

The combination of thermally sprayed hard coatings with a polymer based top coat leads to multilayered coating systems with tailored functionalities concerning wear resistance, friction, adhesion, wettability or specific electrical properties. The basic concept is to combine the mechanical properties of the hard base coating with the tribological or chemical abilities of the polymer top coat suitable for the respective application. This paper gives an overview of different types of recently developed multilayer coatings and their application in power transmission under dry sliding conditions. State of the art coatings for dry sliding applications in power transmission are mostly based on thin film coatings like DLC or solid lubricants, e.g. MoS 2 . A new approach is the combination of thin film coatings with combined multilayer coatings. To evaluate the capability of these tribological systems, a multi-stage investigation has been carried out. In the first stage the performance of the sliding lacquers and surface topography of the steel substrate has been evaluated. For this purpose case-hardened steel substrates were laser textured and coated with different sliding lacquers. In the following stage thermally sprayed hard coatings were tested in combination with different sliding lacquers. For this test stage steel samples were coated with oxide ceramics, metal alloys and hard metals by high velocity flame spraying (HVOF) and high velocity suspension flame spraying (HVSFS). After a grinding process several types of sliding lacquers were applied by air spraying on the coated specimens. Wear resistance and friction coefficients of combined coatings were determined using a twin disc test-bed.

In HVSFS (High Velocity Suspension Spraying), a suspension is axially fed into the combustion chamber of a modified HVOF torch. The particle formation is strongly influenced by the phenomena occurring during the short dwell time in the combustion chamber and expansion nozzle, i.e. break up and evaporation of the liquid jet, particle formation, sintering and melting. Optimization of the spray torch has already led to an improved coating process but is still an important topic for the future. A deeper understanding of the suspension liquid / flame interaction is still necessary to control coating properties. It turns out, that among other process parameters, rheological properties and agglomeration behaviour of the suspension have a strong influence of the particle formation and hence resulting coating properties. As a result of using nano particles HVSF sprayed coatings show partly different microstructures compared to HVOF sprayed coatings. To gain a deeper understanding of the process particles were collected in-flight to get information about the size distribution and melting degree during the spray process. The paper will give an overview of the present state of HVSFS development at the IFKB and present experimental results.

To meet new regulations and specifications for internal combustion engines, new approaches to significantly decrease fuel consumption and emissions are needed. The deployment of tribologically functional coatings applied by supersonic flame spraying represent a promising technology for achieving these targets. Thermally sprayed coatings can help in improving efficiency of internal combustion engines by reducing the internal friction and improving the durability and wear resistance of the engine’s cylinder wall thereby facilitating extreme engine downsizing concepts. Thermal spraying is also capable of processing highly corrosion resistant materials like alloys and ceramics to enable the safe utilization of biofuels in modern combustion engines. In addition, specific surface structure of thermal spray coatings, including their intrinsic porosity, shows the benefit of reducing the friction by sustaining hydrodynamic friction even in spots with low relative movement, e.g. top and bottom dead center. On top, the open surface porosity can reduce the oil consumption and thereby decrease the polluting emissions of internal combustion engines. The thermally sprayed coatings were applied using HVOF and HVSFS processes deploying various materials, including novel nanostructured powders. The coated cylinders and engines have been compared to state-of-the-art components with respect to friction coefficient, wear and oil consumption.

This paper provides an overview of the high velocity suspension flame spraying (HVSFS) process that covers spray gun design, suspension and substrate preparation, and process optimization. Examples are given showing how the process is used to produce tribofunctional coatings for engine applications, electrolyte layers for SOFCs, and netshape chromia for high-end scissor blades. The substrates in the three examples are AlSi 9 Cu 3 , Crofer nickel cermet, and low carbon steel.

In this study, a TiO 2 (anatase) nanopowder suspension was processed by high velocity suspension flame spraying (HVSFS). The resulting coatings were characterized and compared to conventional HVOF and atmospheric plasma sprayed layers. It is shown that the HVSFS operating parameters can be adjusted to achieve dense titania with a near nanostructure and homogeneous distribution of anatase and rutile phases. These coatings have lower pore interconnectivity and higher wear resistance than the APS and HVOF layers. Alternatively, large unmelted agglomerates of anatase nanoparticles can be embedded in the coating, increasing the porosity and anatase content for enhanced photocatalytic efficiency.

The aim of this work is to develop an analytical methodology for the analysis and prediction of high-velocity suspension flame spraying (HVSFS) under various operating conditions, to determine the effect of individual parameters on the process, and to aid and promote the design of HVSFS torches. A key aspect of the work is the development of a model that accounts for fuel gas combustion, evaporation of organic solvents, and heat, momentum, and mass transfer between the flame and suspension droplets.

This paper presents a new approach for computer aided generation of optimized trajectories in thermal spray systems. Through a combination of CFD and FEM modeling, the influence of torch trajectory and speed profile on heat and mass transfer during deposition is assessed along with its effect on residual stress and coating properties. Coating experiments with WC-Co as the spray material were performed on real components in order to validate the developed programming and simulation tools.

Coating operations over glass ceramic substrates represent a new field for thermal spray applications. Due to the unique thermal and mechanical properties of glass ceramics, especially the low or even negative CTE, coating processes must be adapted to reduce the distribution of thermal stresses in the system and to not damage the substrate. This study investigates the deposition of a complex-shaped ceramic-metallic multilayer coating system that could potentially serve as a heating element in a glass ceramic cooking plate. To ensure coating adhesion, the substrates are preheated and their surfaces are grit blasted. In order to minimize stresses associated with the deposition of metal, the movement of the spraying mechanism was automated with robot control and new masking concepts were developed to ensure the accuracy of the shape and placement of the coating. The influence of spraying parameters on coating properties and residual stress distribution is analyzed as well.

The production of light metal matrix composites (MMCs) using coated fiber prepregs processed by thixoforging offers several advantages over well-established technologies like squeeze casting and diffusion bonding. In order to obtain the required globular microstructure prior to thixoforging, reinforcement fibers are coated with the matrix material by twin wire arc spraying. Damage to the sensitive fibers is avoided by reducing the thermal load via optimized cooling. This study analyzes the influence of spraying parameters on the microstructure and mechanical properties of MMCs. An innovative method for automated the coating of reinforcement fibers is presented.