Abstract
Coating production with reproducible properties within a range of values acceptable for the specific application requires a on-line control of the thermal spray process. Sprayed coatings present very often reproducibility problems due to spray parameter variations. In fact the quality of coatings is strongly linked to the temperature and velocity of particles at impact, and the temperature evolution of substrate and coating, before (preheating), during (spraying) and after (cooling). The particle temperature and velocity measurements require sophisticated devices rather expensive and not well adapted to work in the harsh environment of spray booths. Moreover in HVOF spraying, the particle surface temperature is generally below 2200K which make the measurements trickier than in plasma conditions. That is why, the Spray and Deposit Control (SDC) has been developed (collaboration between SPCTS laboratory of the University of Limoges and SNECMA Services), which allows to follow continuously both the light emitted by the hot particles (maximum intensity, mean trajectory and trajectory dispersion) and the substrate and coating temperature evolution during spraying. This simple and light system, fixed on the torch, has been tested in flame, plasma and HVOF spraying. The SDC measured parameters allow to develop easily process windows to obtain good working areas corresponding to specific coating parameters. In this study, the SDC on-line system is used in industrial conditions to optimize and control the HVOF spraying of carbide (WC-Co 17% wt) powders. A special attention is given to the possibilities of process parameters adjustment to regain the specific particle jet parameters and the good substrate temperature, in case of measured SDC parameters shifting. This study demonstrates the SDC system ability for an industrial on-line control of HVOF spraying, using process maps, to increase the consistency of coating properties. It is a step to a closed-loop control of the process through both the particle spray jet properties (mean trajectory and radiated flux distribution) and the target surface temperature.
