Composition of plasma-sprayed tungsten coatings

Mock-ups for the International Thermonuclear Experimental Reactor(ITER) were manufactured by depositing W coatings on tubular substrates of CuCrZr alloy via plasma spraying. An appropriate interlayer (of Ni, Al and Si) was optimised to increase the adhesion of W on the Cu alloy and to provide a soft interface with intermediate thermal expansion coefficient for better thermo-mechanical compatibility. The distribution of chemical elements in the layered interface and in the coating was investigated by energy dispersive spectroscopy (EDS), XPS and AES in the as-prepared material and after heat treatment of 24 h at 823 K. Moreover, the external W layer was examined by inductively coupled plasma emission spectrometry (ICP-ES). The results showed that the as-prepared material does not exhibit macro-porosity. The layers forming the interface do not exhibit any long-range diffusion of the chemical species from layer to layer during the deposition process. The heat treatment does not seem to cause long-range migration of Al and Ni into the W coating. The presence of Ni (0.4 wt%) in the as-deposited external W layer as shown by ICP-ES is only due to the low purity (commercial grade) of W powders used in spraying operations. It can be eliminated by using W powders of higher purity in the deposition process.