Elemental and chemical-state mapping of boron carbide fracture surfaces

Abstract

Samples of commercial B 4C and of reactive hot-pressed B 9C were fractured in situ in a scanning Auger microscopy (SAM) system. The B 4C fracture surfaces showed small areas of pore structures, as well as larger relatively smooth areas. Impurities (Cu, Si, S, Ca, N, O and Cr) were found in trace amounts in the pore areas. SAM maps were made of most of these impurities. The nitrogen was found by Auger lineshape in point Auger electron spectroscopy scans to be in the compound BN. Carbon was found on the surface in both boron carbide and as graphite. SAM was used to seperately map B in B 4C and B in BN, as well as C in B 4C and C in graphite, in a pore region. In the smooth areas of B 4C fracture surfaces fewer impurities were found. The carbon occured in B 4C and in localized carbon-rich areas which had graphite surface layers. In contrast, only B and C were found on the B 9C fracture surfaces; but the distributions of these elements were not homogenous. Carbon was found both in elemental form as graphite and chemically bound to boron as a carbide. In comparison with the graphite regions on the B 4C surfaces, the graphite areas on the B 9C fracture surfaces were not localized. The total surface area was nearly balanced between graphite and carbide regions. Argon ion bombardment revealed the graphite areas to be less than 100 nm thick.