DIFFUSION IN THE B-ALPHA-FE SYSTEM AND COMPOUND FORMATION BETWEEN ELECTRON-GUN DEPOSITED BORON THIN-FILMS AND STEEL SUBSTRATE

Boriding of steels can be performed by electron gun deposition of boron films and subsequent low temperature annealing. Already for annealing at 973 K for 900 s borides are formed as indicated by X-ray diffraction. The borides are Fe2B and FeB. The formation of FeB contrary to expectation on the basis of phase relations in the Fe-B system can be explained by the fact that Fe2B acts as a diffusion barrier. Hence the supply of boron for the continued growth of Fe2B is reduced. Boron atoms accumulating behind the layer of Fe2B react with it, forming FeB at the outside region. At the lower temperatures interdiffusion between boron and iron occurred, and effective diffusion coefficients were determined from Auger depth profiles. The effective diffusion parameters derived on the basis of the Auger analysis are 218.4 kJ mol-1 and 3 x 10(-5) m2 s-1 for the activation energy and the pre-exponential factor respectively.