Very low pressure plasma spraying has been intensively studied in recent years especially the properties of plasma jet. These properties are affected by plasma generating and working conditions. These operating parameters such as arc power, plasma gas flow rate and chamber pressure have influences on specific enthalpy and temperature of plasma jet. In this work, the measurements under very low pressure were performed using enthalpy probe which was previously modified (increase of the internal diameter and depositing TBC coating (Ni/Al and ZrO 2 + Y 2 O 3 ) on the head). Different parameters, for instance, current intensity, hydrogen gas flow rate and detecting distance were changed in order to point out their effect on the characteristics of plasma jet. The specific enthalpy, temperature and quantity of heat in this situation were obtained.

As a promising thermal spray technology, the very low pressure plasma spray (VLPPS) process has been significantly used to deposit thin, dense and homogenous ceramic coating materials for special application needs in recent years. In this study, a home-made transferred arc nozzle was mounted on a low power F100 plasma torch for enhancing the plasma jet energy under very low pressure conditions. Thin and dense alumina (Al 2 O 3 ) and yttria-stabilized zirconia (YSZ) ceramic coatings with an average thickness of 30-40 µm were successfully elaborated by VLPPS process below 1 mbar. Optical emission spectroscopy (OES) was used to analyze the properties of the plasma jet with or without powder injection. The microstructures of the coatings were observed by means of scanning electron microscopy (SEM). It was found that the YSZ coatings displayed a bimodal microstructure that was composed of splats formed by melted particles and a little amount of vapor condensation from evaporated particles. However, vapor condensation could not be observed in the Al 2 O 3 coatings formed by lamellar splats only. The Vickers microhardnesses of both coatings were also evaluated.

Cu coatings were obtained by the VLPPS (Very Low Pressure Plasma Spray) process using a torch F4-VB. The tank pressure was varied from 1 mbar to 5 mbar: these specific conditions can allow obtaining a higher vapor condensation fraction in the coating. Different sizes of powders are used to compare the vaporization level. The other possible influencing factors for obtaining compact film-like coating are also considered such as the distance between the torch and substrate, the orientation of the vapors and also the substrate temperatures. Microstructures of coatings are analyzed and combined with the results of plasma diagnostics. Jobin-Yvon spectrometer (type TRIAX190, UK) and Plasus Specline Spectroscopy software are both used for detecting and analyzing plasma spectrum data. The value of plasma electronic excited temperature Te was calculated through choosing Hα and Hβ two atom spectra. The results showed that the plasma belongs to cold plasma in the local thermodynamic equilibrium situation in VLPPS.

Magnesium doped lanthanum silicate with apatite-type structure was prepared by solid state sintering, as a solid electrolyte for intermediate-temperature solid oxide fuel cells. The electrolyte layers were fabricated by APS, followed by post heat treatments, and their structures and phase were characterized by SEM and XRD. It is showed that an amount of amorphous oxides exist in as-sprayed electrolyte layer, and then disappear after a post heat treatment in air furnace at temperature up to 1000°C. The gas permeation of electrolyte layers was measured by a specific instrument with pure H 2 and O 2 at room temperature. The conductivity of plasma sprayed electrolytes was studied by impendence spectroscopy in the range of 500-900°C in air.