Cyclic heat load experiments of W/Cu and W/C coatings

The W/Cu functionally graded materials(FGMs) and W/C coatings were made by plasma spraying or heat pressing process, in which W/C coatings have multi-layer W and Re diffusion barrier. In order to investigate their cyclic heat load properties under plasma disruptions, a cyclic heat load experiment was performed by using high power ND: YAG laser. Results showed that W/Cu FGMs sustained the mechanical integrity after 200 to 700 times of cyclic heat loads with 100-400 MW/m''2 power density and 4 ms pulse width. Surface recrystallization was found by SEM observation after 700 thermal shocks with 123 MW/m''2 power density. Recrystallized grains have columnar structure, which is normal to the surface, and no obvious growth due to rapidly cooling. But severe intergranular corrosion and cracks were observed. The annealing experiment of W/C coatings indicated that the recrystallization temperature of W coatings is slightly higher than 1400 degree. Under much higher heat load (398 MW/m''2), an obvious erosion crater occurred on the surface and a porous beehive-like structure was observed in the crater. A re-deposition zone occurred on the edge of the crater, which consisted of tungsten and a great deal of metal impurities. SEM observations showed that the intergranular fracture and cracks for the specimen made by plasma spraying were more easily formed than those made by heat pressing. The loss of weight of W/Cu FGM was lower than that of graphite under the same heat load