LIQUID JET AND PLASMA JET INTERACTION: A FIRST STEP IN THE STUDY OF FINE STRUCTURED COATINGS ELABORATION

Classical plasma spraying is nowadays industrially used to achieve thick layers, up to few millimeters, of a wide range of materials. However, new applications, such as solid oxide fuel cells require fine structured layers or very thin coatings, which can't be achieved by classical plasma spraying. Actually, ultrafine powders (d < 5μm) can't penetrate inside the plasma jet, the carrier gas being unable to give enough momentum to very small particles, without perturbing drastically the plasma jet. This paper presents a new process using a d. c. plasma torch where a liquid precursor is injected in order to obtain layers which thickness is between l00nm and 10μm together with a fine structure involving grains of about 1μm and below. That precursor consists of ceramic particles in suspension in a carrier liquid. The presented work is a first step of the spraying process elaboration. It is devoted to the study of the liquid injection in the plasma jet. Firstly, a liquid injector was designed and characterized in order to produce liquid droplets penetrating into the plasma core. Secondly, the interaction between the liquid and the plasma was studied through the injection process. For such an investigation, an emission spectroscopy system was used to determine the plasma temperature together with its composition, with and without water injection.