Abstract
Boron carbide (B4C) is widely used as a coating material for x-ray optics in X-ray Free Electron Laser (XFEL) beamlines. In this study, single-shot damage experiments were conducted on B4C/Si-sub and B4C/Cr/Si-sub mirrors at 517 eV using the Shanghai Soft x-ray free electron laser. The damage thresholds were determined to be 0.49 ± 0.13 and 0.32 ± 0.08 J/cm2 for B4C/Si-sub and B4C/Cr/Si-sub, respectively. Morphological and microstructural characterizations reveal that XFEL-induced damage in both samples is closely related to thermal melting of the Si substrate. Based on the silicon melting criterion, the theoretical damage threshold simulated by the finite element method for B4C/Si-sub is in good agreement with the experimental result. The bump damage morphology observed at a fluence of 0.66 J/cm2 for this sample is attributed to the volumetric expansion of the molten Si substrate. In contrast, the silicon melting criterion overestimates the damage threshold for the B4C mirror with a Cr adhesive layer. The lower experimental threshold of B4C/Cr/Si-sub is attributed to the interfacial diffusion reaction and the femtosecond XFEL-induced cavitation, which leads to a more readily observable damage morphology.