A novel technique and an instrument for on-line, quantitative imaging diagnostics and process control in thermal spraying have been developed and tested in laboratory and industrial conditions. In-flight spray particles are imaged by their natural luminosity with a short-exposure, digital CCD camera system. Particle images are processed using digital image processing techniques in a PC computer. The number of particles per frame and the spatially resolved particle velocities are calculated from the images. Spectrally resolved image information is further used to determine pyrometric two-color particle temperature. A number of different modes of data presentation have been developed. The developed instrument can be used to determine particle number, velocity and temperature distributions of individual in-flight particles from the imaged region of interest of the plume. Dividing the imaged area into smaller sections, spatial distributions of these parameters can be studied. SprayWatch system provides a technically simple, easy to operate, single imaging instrument, which can provide a visual overview of the spray plume in combination with quantitative evaluation of the most important spray particle parameters. In this paper examples of using the monitoring system with plasma and HVOF spraying are presented. Preliminary test results of using a semiconductor laser generated light sheet to detect cold particles is also demonstrated.

Plasma spraying is a complicated process involving many partly interdependent parameters, which are in industrial spray environments difficult to optimise without laborious and time consuming experiments. In this work a non-intensified CCD camera without any external illumination is used for in-flight particle visualisation. Particle visualisation is based purely on the spontaneous light emitted by the hot particles. The motivation for this work is to outline the possibilities to develope a CCD based, low cost and rugged in-situ measurement system suitable also for industrial use. The measurement method has been tested with Plasmatechnik A3000S plasma spraying equipment using fused and crushed Al2O3 powder. Using digital image processing techniques relative particle concentrations and particle velocities have been calculated from the acquired images. These results have been correlated with wear resistance and deposition efficiency of the coatings produced with different powder feed rate and powder port adjustments. Coatings were also produced using both new and worn electrodes. The benefits and limitations of the method are discussed and the measurement results are compared against measurements made using laser sheet illumination, which can give information concerning also the colder and/or smaller particles not visible for the passive CCD system.

The deposition efficiency (DE) of a particular powder for a particular thermal spray process is very important factor when coating economics is being considered. There are many coating applications, however, where it is also important to know how the deposition efficiency changes during a longer coating process. Normally the DE is determined as mass ratio of powder fed into the process and corresponding weight gain of the sample. In this work the deposition efficiency has been determined for aluminum oxide powder in atmospheric plasma spraying using different spray parameters and electrode wear states. The coating process and in-flight particles were monitored using a fast non-intensified CCD-camera. Using digital image analysis the relative hot particle concentrations and velocity distributions were calculated from images. The possibility to use a CCD camera based monitoring system for in-situ measurement of DE is discussed. Additional laser illumination and PTV measurements were performed to verify the cold particle flux unseen by the plain CCD camera.

The plasma spraying process is controlled by various parameters that have an influence on powder particle velocities, temperatures and trajectories just before impact to the substrate. In order to fully utilize the thermal and kinetic energy of the plasma it is important to obtain information from these powder particle properties. In this work an intensified CCD camera has been used to detect in-flight particles in an atmospheric plasma spraying process. Plasma spraying was performed using fused and crushed AI2O3 powder. The powder carrier gas flow rate was varied during the spraying experiments. All the other deposition parameters were kept constant. Coatings produced using relatively new spraygun electrodes are compared with ones produced later with the same electrodes when they were worn out. The particle concentration is determined on a relative scale by the fraction of the area of a CCD camera frame covered by particle images. Further investigations necessary to clearify the relationship between the measured relative particle concentration and the true particle concentration are identified. The coatings are analyzed for wear resistance, degree of melting, deposition efficiency, hardness and porosity. The dependence of these coating properties on the relative particle concentration and the effect of electrode wear on the relative particle concentration are studied.