Carbon Deposition on Beryllium Substrates and Subsequent Delamination

Beryllium and carbon are foreseen as materials for plasma facing components of future fusion devices. Erosion, re-deposition and thermal annealing events arising from heat-loads during reactor operation will produce mixed material layers and compounds on the plasma facing surfaces, leading to changes in the material properties. In order to mimic the erosion/deposition and compound formation processes, carbon layers have been evaporated onto beryllium plates and annealed in the 373 to 1073 K range for 90 min. Ion beam measurements revealed a smooth beryllium and carbon interdiffusion at the samples interface up to 773 K. A carbide formation reaction front became apparent for higher temperatures in scanning electron microscopy observations, with the volume fraction of Be2C crystals resulting also evident in X-ray diffraction patterns. The annealing route induced delamination of large surface areas of the samples through telephone cords blistering, attributed to strain energy release. At 973 and 1073 K, cracking occurred preferentially along the blisters boundaries and evolved in time, leading to their final removal. This fracture behaviour seems caused by the different thermal expansion coefficients of the phases. The superficial films remain unchanged at lower temperatures. The results prove that the compound formation promotes by itself the occurrence of dust emission events in tokamaks.