Process Diagnostics and Online Monitoring and Control

Abstract

Whatever maybe the spray process, the user expects that coating thickness and weight tolerances are respected, the reproducibility of coating microstructure and reliability of service properties are at least good and, if possible, excellent. Good quality control of coatings, before (powders… substrate preparation), during, and after the spray process presents many benefits: reduced rework, predictable performance and life linked to coatings, and high reproducibility with narrow variability. This chapter defines first what are coatings repeatability, reliability, and reproducibility and place the process control relatively to the other different errors observed in a production unit. A brief history of the influence of the spray process monitoring on coating quality is presented, with the spray process parameters that should and could be controlled. The high-energy jet characterizations (temperatures, velocities, turbulences, electrodes erosion), developed in laboratories, are presented. Sensors able to work in the harsh environment of spray both and developed since the nineties are then described. They include enthalpy probes, sensors for hot and cold in-flight particles, to measure their trajectories distribution, their steady or transient temperatures, velocities, and diameters either as ensemble (large measurement volume) or local measurements. Measurements of the coating under formation (hot gases flux, temperature, stress development, thickness) are then considered. The use of robots, artificial neural networks (ANN), and fuzzy logic (FL) to monitor and further control coating generation is then discussed. Finally, this chapter presents a few measurements that are used in laboratories and are very important for a better understanding of coatings generation such as particle vaporization, particle flattening and splat formation, and high-energy jet–liquid interaction.