In this investigation, alumina powders prepared by different methods were sprayed on carbon steel substrates using a conventional plasma torch with radial injection. The spraying process and powder injection parameters were varied and the injection behavior of the powder was studied. Changes in particle acceleration, deceleration, and impact were measured with novel spray diagnostic equipment and are correlated with the structure and properties of the coatings obtained. Dense coatings were achieved with several agglomerated-and-sintered Al 2 O 3 powders, although higher impact velocities and temperatures were recorded for the larger and denser fused-and-crushed particles.

In order to produce plasma sprayed coatings with best economical, technical and environmental qualities several factors need to be taken into consideration. These include properties of the powder feedstock, equipment reliability, optimized process parameters and stability of the spray process. In case of plasma spray guns with conventional and frequently used radial powder feeding configurations, proper and constant powder feeding during spraying plays an important role. Once optimized spray parameters need to be preserved and optimal spray conditions for best coating quality and highest deposition efficiency should also be kept constant during possible changes of the spray parameters, e.g. due to electrode wear, powder quality variations etc. Various spray process monitoring tools are available and useful for this purpose; especially if they are cost-effective to be adopted by industrial thermal sprayers. In the present work, a conventional 40 kW plasma spray process equipped with a typical radial single point powder feeding was studied with the robust and industrially viable “SprayWatch G” system with an on-line monitoring CCD camera. The plasma spray torch was run under slightly varying spray conditions typical to normal industrial spraying; the spray process was monitored, and various actions, e.g. powder carrier gas flow rate adjustment, were realized in order to restore the most optimum conditions. Special attention was paid on finding correlations between the temperature, brightness, shape, and direction of the spray plume and the properties, e.g. deposition efficiency and physical properties of the sprayed alumina coatings.

Imaging techniques have gained popularity in thermal spray diagnostics over the last decade. They provide large amounts of data for research, development and process monitoring. The most important measured spray parameters are particle temperature and velocity. The biggest advantage of imaging techniques compared to optical point measurements is that they provide instantaneous distribution measurements. We present the application of high-power diode laser illumination to detect weakly emitting particles in thermal spaying. This extends the feasibility of imaging diagnostics to several demanding environments, including cold spray, powder mixing region of a plasma spraying torch and low-pressure plasma spray.