High-heat-flux loading of tungsten coatings on graphite deposited by plasma spray and physical vapor deposition

Tiles of fine-grain graphite coated with tungsten layers by different plasma spray techniques (thickness 100 to 550 mu m) and by physical vapor deposition (PVD) (thickness 20 to 100 mu m) were subjected to heat fluxes, as expected for the divertor of the Axially Symmetric Divertor Experiment (ASDEX)-Upgrade tokamak. By a step-wise increase of the applied heat flux up to 16 MW/m(2) and different pulse durations (1 to 5 s), the maximum load for disabling damage of the coating was determined. The fatigue behavior of the coatings was investigated by cyclic loading. The results show that plasma spray coatings are able to withstand heat loads up to 15 MW/m(2) for a 2-s pulse without structural changes and cyclic loading with 1000 cycles at 10 MW/m(2) and a 2-s pulse. The PVD coatings show damage by crack formation and melting at slightly lower heat loads than most of the plasma spray coatings. Under cyclic loading, the thin PVD coatings fail by extensive crack formation. The results of the tests indicate that the good performance of the plasma spray coatings is related to their higher porosity, which provides a crack-arresting mechanism, and to their mechanical strength.