Vacuum thermal spray processes are widely used in industrial domains such as aeronautics, gas turbines and medical industries. They are a well established coating process for fast deposition of dense oxide free layers in a controlled atmosphere. Low Pressure Plasma Spraying (LPPS), including LPPS-Thin Film (LPPS-TF), cover a large range of work pressure from typically 200 mbar down to a few millibars. The broad operation range of these processes leads to unconventional plasma jet characteristics which can be used to obtain specific coating properties, such as different Thermal Barrier Coatings (TBC), multilayer systems, EB-PVD-like coatings with columnar microstructure and functional layers for Solid Oxide Fuel Cell (SOFC) components, amongst other.

 Compared to conventional thermal processes, the LPPS-Thin Film technique is characterized by a high power plasma source and a low ambient pressure. These conditions allow partial vaporizing of the material, injected as powder, giving unconventional layers properties in the domain of thermal spray. As a consequence, specific coatings only seen in PVD processes (EB-PVD) can be reproduced using the LPPS-Thin Film technology which evidences the presence of vapor phase in the plasma jet. Therefore, the study of low pressure plasma spraying processes is limited when using existing particle in-flight diagnostics such as the DPV 2000 or CCD cameras because they rely on the presence of visible or hot emitting particles. In some conditions of the LPPS-TF process, the injected material can be partially or completely evaporated which remains undetected by these conventional techniques. The addition of a diagnostic tool, for example optical emission spectrometry, allows studying the different vapor phases during the spray process. Moreover, the combination of particle in-flight measurements and optical spectroscopy allows quantifying the vapor and splat phase proportions in the plasma jet as a function of the process parameters. Optical emission spectroscopy also allows monitoring changes of the gas composition which could result from a reaction of the injected raw material with the plasma jet gases.

 This paper presents a qualitative study of the injected material properties in the plasma jet for vacuum thermal spray processes, in particular LPPS-Thin Film, using DPV 2000 measurements and optical emission spectroscopy. The results from the combined optical diagnostics contribute to improve the process reliability by understanding the interaction of the injected material with the plasma jet.