Residual stresses in the graded interlayer between W and CuCrZr alloy

One of the big challenges for realizing future nuclear fusion reactors is the development of materials able to withstand the high thermal loads and particle fluxes typical of these reactors. This work investigated the residual stresses in a W coating deposited through plasma spraying on a substrate of CuCrZr alloy used in the active cooling system of ITER (International Thermonuclear Experimental Reactor). To this purpose, a graded interlayer consisting of a mixture of W and Cu whose composition gradually changes was deposited between coating and substrate. The W coating-interlayer-CuCrZr substrate system showed good adhesion and no cracks were observed. Residual stresses were investigated by X-ray diffraction (XRD) and nanoindentation tests. Residual stresses in the interlayer have different sign in W (compressive) and in Cu (tensile) and reach their maximum value in the inner part of the interlayer where the relative amounts of the two deposited metals are similar. The main contribution to residual stresses comes from thermal stresses arising when deposited metal and substrate cool together from deposition temperature to ambient temperature owing to the different coefficients of thermal expansion (CTE). The residual stresses measured in W are always below the critical value for crack formation (275 ± 50 MPa) but not negligible. Based on present results, the optimization of deposition temperature seems to be the key for reducing residual stresses in the interlayer.