SURFACE COSEGREGATION AND SURFACE PRECIPITATION ON FE-3-PERCENT V-C,N(100) SINGLE-CRYSTALS

Surface cosegregation and surface precipitation has been studied on (100) oriented surfaces of Fe-3%V-C,N and Fe-3%V-C single crystals by means of AES and LEED. Between 450 and 750-degrees-C cosegregation leads to the formation of the two-dimensional surface compounds V(C,N) and VC, which are both epitaxially arranged on the substrate surface as indicated by LEED. The stoichiometry of the surface compounds, determined by quantitative evaluation of Auger spectra, is VC1.2 and V(C(x)N(y)), x + y = 1.2, the composition of the latter varies between V(C0.6N0.6) at 450-degrees-C and V(C0.2N1.0) at 650-degrees-C. Saturation is observed up to 650-degrees-C, at higher temperatures the total surface coverage decreases gradually. Ar+ sputtering and room temperature oxidation experiments indicate that the surface compounds form a homogeneous layer on the substrates, the layer thickness is estimated to be less than 2 atomic layers. Upon quenching V(C,N) and VC surface precipitates are growing on clean and sulphur saturated substrate surfaces to a thickness of the precipitates of about 50 angstrom. Sulphur does not prevent the formation of surface precipitates upon quenching but effectively suppresses the formation of V(C,N) and VC surface compounds.