The impact of thermal fatigue and carbidization on the W coatings deposited on CFC tiles for the ITER-like Wall project at JET

Abstract

Since August 2011 JET operates with the ITER-like wall comprising bulk Be tiles, bulk W tiles and W coated CFC tiles with a thickness of 10–15μm and 20–25μm. In order to evaluate behavior of the W coatings to a cyclic thermal loading relevant to JET operation, high heat flux (HHF) tests have been carried out up to 5100 pulses with an electron beam facility at peak temperatures of 1000°C, 1250°C and 1450°C. The pulse duration was 24s. Optical inspections of the W layer performed periodically by interrupting the test revealed small delaminations with the size of 50–500μm. The dependence of the delamination percentage on the number of pulses can be seen as a degradation curve for each particular W coating. In this way the thermo-mechanical properties of the W coatings can be characterized quantitatively. Thermal fatigue and carbidization of the tungsten due to the diffusion of the carbon from the substrate have been recognized as mechanisms for degradation of the coatings. Tungsten carbides have been identified by using TEM (transmission electron microscopy) diffraction analysis on FIB (focused ion beam) prepared cross-section samples subjected to HHF tests. Nano-pores developed at the CFC–Mo and Mo–W interfaces during the tests might be also responsible for the degradation of the coating.