Investigation of long term stability of W/B4C multilayer structures

The long term stability of as-deposited, annealed, and synchrotron radiation exposed W/B4C multilayer structures was investigated. The multilayer structures were studied using hard X-ray reflectivity, grazing incidence Xray diffraction (GIXRD), extended X-ray absorption fine structure (EXAFS), and atomic force microscopy (AFM). The structural degradation in annealed and synchrotron radiation exposed samples occurs at faster rate as compared to untreated samples. The structural degradation rate clearly depends on the thickness of B4C layer in multilayer structures. GIXRD measurements confirm the formation of tungsten carbide and boride phases. From EXAFS measurements at the W L-3-edge, we find that the increase in the diffusion of oxygen is responsible for the observed structural degradation at faster rate in annealed samples consisting of thinner B4C layer. AFM images reveal that the surface modifications occur at slower rate in untreated samples as compared to annealed samples. Moreover, B4C proves to be an efficient cap layer to protect the as-deposited and annealed multilayer structures from oxidation.