W/EUROFER functionally graded coatings for plasma facing components: Technology transfer to industry and upscaling

The tritium breeding blanket is a vital component of future fusion reactors, providing the fuel for the fusion reaction. Tungsten is a viable coating material to protect its first wall from erosion by plasma. The feasibility of applying tungsten coatings by vacuum plasma spraying has already been demonstrated, using a functionally graded material of mixed tungsten and EUROFER97 steel as connecting layer to mitigate a thermal expansion mismatch. This technology was now transferred to industrial level to enable further upscaling. Samples of three different sizes were coated, the largest ones measuring 500 x 250 mm2 and containing mock-up cooling channels. The sample distortion was found to be small. An ultrasonic analysis did not reveal any delamination but indicated potential weaker spots in the corners that may have been subjected to faster cooling. Both the coating's thickness of 2 mm and linear chemical gradation over five interlayers met well with specifications, as verified by scanning electron microscopy, energy-dispersive X-ray spectroscopy as well as image thresholding analysis. The microstructure consisted of typical splat-shaped particles of tungsten and EUROFER, with minor porosity only. In total, these first results indicate that good coating quality can be achieved even in dimensions approaching fusion-relevant size.