PEER REVIEWED Online Diagnostic System to Monitor Temperature of In-Flight Particles in Suspension Plasma Spray A. Akbarnozari 1 • F. Ben-Ettouil 1 • S. Amiri 1 • O. Bamber 2 • J.-D. Grenon 2 • M. Choquet 2 • L. Pouliot 2 • C. Moreau 1 Submitted: 25 October 2019 / in revised form: 29 April 2020 / Published online: 17 May 2020 Ó ASM International 2020 Abstract Suspension plasma spray (SPS) is going through a transition phase from research and development to daily use on production lines. Improving repeatability and reproducibility of coating elements and parameters makes SPS a replacement of former well-developed processes. This transition can be achieved by using in-flight particles diagnostic systems to monitor and control key parameters that influence the coating microstructure. Temperature and velocity of the in-flight particles are among the most crit- ical parameters that should be monitored. However, accu- rately characterizing the in-flight particles in SPS is particularly challenging due to the small particle size of coating materials, harsh spray conditions, considerably shorter spray distances compared to APS, possible inter- ference from the solvent, and limitations of previous measurement systems. In this study, different strategies were investigated to improve the accuracy of temperature measurements of in-flight particles in SPS. For this pur- pose, two light collection configurations (double-point and single-point measurement) were investigated along with the influence of plasma radiation. The results were evalu- ated by collecting and studying splats. The size and shape of splats were correlated with the temperature of in-flight particles in order to confirm the accuracy of the sensor’s temperature measurements. In addition, the sensitivity of temperature measurements to the optical filter used for two-color pyrometry, reflection of plasma radiation from surrounding objects, and direct radiation from plasma were investigated. The results showed that the single-point measurement configuration was well adapted for SPS. Keywords diagnostic system Á online measurement Á in- flight particle temperature Á suspension plasma spray Á thermal emission Á two-color pyrometer Introduction Suspension plasma spray (SPS) produces coatings with unique microstructures achieved by injecting submicron ceramic particles through a liquid carrier in a high-tem- perature plasma jet. The SPS coatings display superior chemical, mechanical, and thermal properties which opens opportunities for numerous applications (Ref 1). Temper- ature and velocity of in-flight particles in SPS are among the main parameters controlling the coating microstructure and, consequently, its overall properties of the surface. The condition of the in-flight particles before impingement on the substrate has a direct impact on the coating character- istics (Ref 2). Therefore, to control hardness, thermal conductivity, and other properties of the final coating, temperature and velocity of in-flight particles are key parameters that should be monitored and controlled (Ref 3). Regarding the importance of online monitoring sys- tems, on the one hand, these tools have become an important component for developing, understanding, and optimizing new processes in research, and on the other This article is an invited paper selected from presentations at the 2019 International Thermal Spray Conference, held May 26-29, 2019 in Yokohama, Japan and has been expanded from the original presentation. & A. Akbarnozari christian.moreau@concordia.ca; anozari007@gmail.com 1 Department of Mechanical, Industrial, and Aerospace Engineering, Concordia University, Montreal, QC H3G 1M8, Canada 2 Tecnar Automation Lte ´e, Saint-Bruno-de-Montarville, QC J3V 6B5, Canada 123 J Therm Spray Tech (2020) 29:908–920 https://doi.org/10.1007/s11666-020-01045-2