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.

When plasma spraying operations require high throughput, three-cathode guns may be specified due to their stationary plasma jet and elevated power characteristics, higher feeding rates, and adequate deposition efficiencies compared to one-cathode guns. A new three-anode gun system has been introduced to the market that offers a combination of high power inputs into the plasma as well as stable process conditions. These new guns feature a narrower nozzle outlet diameter compared to multi-cathode designs and they can be used with hydrogen as secondary plasma gas. Both of these characteristics result in higher plasma velocities and net powers. The conceptional designs for two such guns are discussed as well as their suitability for suspension and shrouded plasma spraying.

Up to now no coating systems are marketable in the field of direct hot extrusion, which provide both surface protection of the parts being in contact to the billet (i.e. container and die), and a significant reduction of the frictional losses being induced by the billet passing along the container walls. To dispense the use of lubricants and to enhance the usable forming capacity of the process, different oxide ceramics were given in one suspension and plasma sprayed. The aim is to reach a mixing of the feedstock to obtain deterministic solid solutions of the oxide phases which show a reduction of their coefficient of friction under dry sliding conditions. To reach this goal the high surface-to-volume ratio of feedstock with primary particle sizes below 100 nm was used. By means of X-ray diffraction it could be proven, that the desired phases could be synthesized. The coatings showed a considerable lowering of their frictional coefficient in tribological testings against 100Cr6 in the region of the operation temperatures for the hot extrusion of aluminium alloys. Besides the experimental work the fundamentals of the mixing process of different oxides regarding crystallographic aspects are discussed.

In this study, suspension plasma spraying is used to produce self-lubricating titanium oxide coatings. Certain nonstoichiometric titanium oxide phases, called Magneli phases, exhibit a reduction in friction under dry sliding conditions at elevated temperatures. These phases, however, tend to undergo crystal changes during thermal spraying, resulting in the loss of their good friction behavior. In this work, the goal is to stabilize these phases with suitable lattice substitutions for Ti 4+ . The resulting phases are shown to be homologous to Ti n O 2 n -1 , but have the advantages of a three-component system, making them more thermally stable with a broader area of formation.

In this work several powder materials typically used for wear and corrosion protection (WCCoCr, Cr 3 C 2 NiCr, Cr 2 O 3 ) were considered for developing near net shape coatings. In contrast to grain sizes commonly used in thermal spray processes the grain sizes of all examined powders were specified with maximum 25 µm (-15+5 µm, -20+5 µm, -25+5 µm). During the coating experiments the HVOF process was used to apply carbide based powder materials (WCCoCr, Cr 3 C 2 NiCr) whereas the APS process was used to deposit Cr 2 O 3 coatings. Taguchi techniques were utilized in order to reduce the number of experiments and to evaluate and to adjust main process variables. The effectiveness of these techniques could be verified by spraying validation samples successfully. The coatings were examined in terms of deposition efficiency, surface roughness, hardness, porosity, wear and corrosion resistance. The results showed that improvements in terms of porosity, surface roughness and corrosion resistance could be reached by introducing fine powder materials in the spray process. Reducing the influence of effects like decarburization or oxidation caused by the overheating of small spray particles should be considered in future work.

Near net shape coating is a trend in thermal spraying being aimed at for several years to lower costs through shortened spray time and reduced after treatment work. Because of better microstructure compared to conventional coatings, the thickness is often also reduced. To characterise the quality of those thin layer systems the standardized tensile adhesive test is not suitable, as the adhesion outweighs the cohesion by far due to reduced quantities of coating defects. To characterize the coatings behaviour under tensile stress, three-point bending tests were performed. As especially thin cermet coatings do not show abrupt catastrophic failure in bending tests, ultrasonic signals being emitted from cracks propagating through the coatings were taken during the tests. Three coating systems (Cr 2 O 3 , Cr 3 C 2 -NiCr and WCCoCr) were investigated in this work using fine grained feedstock powders with three different size distributions per system. The tests showed the negative influence of stress moments in case of the oxide ceramic and the embrittlement of the cermets, especially the Cr 3 C 2 -NiCr, with increasing spraying temperature regime due to increased formation of solid solutions and decarburization. It can be said, that the analysis of acoustic emission during bending tests gives valuable information about how to achieve thin wear and corrosive protecting coatings being qualified for high operational demands.