STUDY OF SURFACE PREPARATION FOR ENHANCED RESISTANCE TO THERMAL SHOCKS OF PLASMA-SPRAYED TIC COATINGS

Plasma-sprayed TiC coatings are of interest for the protection of plasma limiters in fusion devices. In previous tests the maximum observed thickness which showed good resistance to thermal shocks (400-mu-m) is unfortunately not enough for long-term tokamak operation as erosion takes place. Consequently, in order to ensure the good behaviour of thicker coatings and to understand the failure mechanisms, a study has been made of the influence of different surface preparations of the metallic substrate (grit blasting, macroroughening or spraying a bond coat) combined with modified thermal shock parameters (shock duration and pre-heating of the material). The thermal shock resistance of the resulting samples was studied in an electron beam gun device where substrate and surface temperatures and video images have been monitored. Three failure modes were observed: delamination in the coating during the heating phase of the thermal shocks and segmentation or edge delamination during cooling. Coating bending and plastic deformation added to residual stresses and shear at the coating-substrate interface explain the observed failures. Hence, with a combination of surface modification and pre-heating, it has been possible to double the thickness of thermal shock-resistant coatings to up to 900-mu-m.