Influence of plasma light scattered by in-flight particle on the measured temperature by high speed pyrometry

In thermal spraying, temperature, size and velocity of particles before they impact on the substrate are among the main characteristics influencing the microstructure of the resulting coating. Individual in-flight particle characteristics can be measured by using various optical techniques developed in the laboratory or available on the market. Some of these techniques aim to estimate the individual particle thermal radiation by forming an image of the particles when they pass in front of a collecting lens. In thermal spraying, and more specifically in plasma spraying, measuring the thermal radiation of individual sprayed particles can be a challenging task because of the intense radiation coming from the plasma. The aim of this work is to quantify the error made in estimating particle thermal radiation from measurement of visible and near-infrared radiation coming from individual particles. To achieve this goal, two kinds of measurements were made. First, optical radiation emitted, or reflected, by a jet of sprayed particles is analyzed by an optical emission spectrometer. Then, after triggering by laser light scattering on individual cold particles, the near-infrared radiation collected from those particles is measured as it is usually done by two-color pyrometry on hot sprayed particles. Analysis of these measurements allows us to estimate the accuracy of thermal radiation measurement and then the error made on particle surface temperature in thermal spraying diagnostic.