Vacuum plasma spraying of functionally graded tungsten/EUROFER97 coatings for fusion applications

As structural materials for future fusion power plants, reduced activation ferritic martensitic steels as EUROFER97 can be used. Unfortunately, the interaction of the plasma with the steel would result in a limited lifetime, so protective layers are investigated. An excellent protective material is tungsten, as it shows unique properties with respect to low sputtering, high melting points and low activation. However, the mismatch of thermo-physical properties between tungsten and EUROFER97 can lead to large stress levels and even failure.A possible way to overcome this problem is the use of functionally graded material (FGM). The paper will describe the manufacture of these FGMs by vacuum plasma spraying and their characterization. First of all, two different feeding lines have been used to produce the coatings. A major problem lies in different melting points of tungsten and steel. So the particle size distribution has to be adjusted to achieve sufficient melting of both materials during the spray process. In a second step, the feeding rates were optimized to obtain the wanted amount of tungsten and steel phases in the graded structures. In a thermal spray process, the gradient cannot be made continuously, however it has to be applied in a step-wise manner. In this investigation, samples with 3 and 5 different concentrations (excluding the pure steel and tungsten part) have been produced. The microstructures of these layers have been investigated. In addition, hardness was measured and the residual stress state was determined by the hole drilling method