THERMAL SHOCK EVALUATION OF PLASMA – SPRAYED TiC THICK COATINGS BY MEANS OF THE TEST LIMITER FACILITY OF THE TOKAMAK DE VARENNES

The Tokamak de Varennes has an auxiliary limiter facility which allows various materials to be exposed to plasma discharges. Thick, plasma-sprayed TiC coatings (200–500 μm) have been evaluated The factor which limits the thickness is presently the thermal shock resistance. Samples whose stoichiometry and micro-structure were analysed have been pre-tested using a 15 kW electron beam system. The results were used to optimize the deposition of coatings on hemispherical heads which were tested in the tokamak. With plasma currents of 200 kA, the heads have absorbed 30 kW during 0.7 sec leading to a power deposition of about 15 MW/m 2 . The coatings tested so far indicated an optimum thickness of about 400 μm which is still too thin for long-term operation. For this thickness, only microcracks (segmentation of the surface) are observed. For thinner coatings, the Inconel substrate was melted, whereas for thicker ones spalling inside the coatings was observed. No contamination of the core plasma by Ti has been observed, although the production of Ti from the surface has been observed, and Ti has been detected on collector probes placed on the vessel wall. AES evaluation of the exposed surface of a TiC flake has shown no significant difference in Ti and C content when compared to the reverse (unexposed) side.