Nitridation of iron with ammonia: Growth and microstructure of nitrides and pore formation in zeta-Fe2N

Layers of gamma'-Fe4N, epsilon -Fe3N, and zeta -Fe2N on ferrite were produc ed by nitridation of single crystals and rolled sheet of iron in flowing am monia at 430 and 475 degreesC. The nitride layers on ferrite were prepared in cross section and were characterised using X-ray diffraction, light micr oscopy as well as scanning and transmission electron microscopy. The growth rate of the nitride layers on the single crystals were found to be similar as that on the rolled sheet. The size of the nitride grains on the single crystal are significantly larger, which is identified as the reason for the different pore structure in the two materials. Pores only occur in the out er zeta -Fe2N layer. The formation of pores at the interface epsilon -Fe3N/ zeta -Fe2N and the crystallographic orientation relationship between epsilo n -Fe3N and zeta -Fe2N were studied and determined using electron microscop y and electron diffraction. The pore formation is explained by the existenc e of a two-phase region at the phase boundary epsilon -Fe3N/zeta -Fe2N and by an indifferent equilibrium which is caused by local instabilities in the nitrogen flux.