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.

The life time of some thermal sprayed coatings is limited. Flaking or partial removal of the coating reduces its protective effect. To allow further use of the component the coating has to be reworked before the part gets heavily damaged. The damaged coating must be removed with minimum impact on the surface of the component. Many removal methods as chemical, electrochemical or waterjet techniques have to be used with great care to prevent damaging of the substrate or are environmentally hazardous. An alternative technique which removes the coating selective with minimum impact on the substrate is the use of dry ice. Pellets of solid carbon dioxide are accelerated by means of compressed air. During the impact the carbon dioxide sublimates at a temperature of –78°C. The sublimation generates a large expansion (~600 times) of the carbon dioxide. The coating is strongly cooled while the shock wave of the sublimation destroys the cohesion. To support this process and to enlarge the thermal stresses in the coating a laser can be used. In this contribution the pellet velocity is measured and compared for three different dry ice jet devices by particle image velocimetry. Analysed parameters which are supposed to have an influence to the particle velocity are the mass flow of the compressed air, nozzle geometry and the working principles of the devices. The pellet velocity and the resulting kinetic energy are important process parameter. The results are used to optimise the laser assisted dry ice jet process.

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.

Wire flame sprayed molybdenum is a wide used procedure for manufacturing of wear resistance coatings. The properties of thermal sprayed coatings depend mainly on the kinetic and thermal energy of sprayed particles, i.e., a higher particle velocity causes an increase of coating quality. The now available high velocity spray system from Praxair which is used within this work is capable to realise the aim of high particle velocities. The coating properties presented in this work are analysed in comparison to conventional wire and powder plasma spray processes. HVWFS molybdenum coatings show lower porosity, higher adhesion and cohesion and better wear properties. To explain the results, particle size distribution, oxygen/carbon content and structure are analysed. Hardening mechanisms of coatings and their adhesion/cohesion properties are discussed based on light microscopy, SEM, XRD and TEM investigations.