Ion Mixing in the Ni-Sn System
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ION MIXING IN THE Ni-Sn SYSTEM + ++ + u++ AND C. TOSELLO + G. PRINCIPI, L. GUZMAN, L.M. GRATTON, L. CALLIARI, +Istituto per la Ricerca Scientifica e Tecnologica, Povo (Trento), Italy; ++FacoltA di Scienze, UniversitA di Trento, Italy; olstituto di Chimica Industriale, UniversitA di Padova, Italy
ABSTRACT Nickel substrates were coated with a thin Sn film and implanted with 100 keV Xe to fluences between 3 and 7.1015 2
ions/cm . Surface chemistry changes were monitored using Auger and Mfssbauer spectroscopies. By comparing between profiles of un-implanted and implanted specimens, it was found that ion implantation through the film caused Ni to move into the Sn film and Sn to penetrate into the Ni substrate. The Mdssbauer analysis disclosed the presence of new surface phases. A substantial increase in the tarnishing resistance of Ni was found by using this treatment.
INTRODUCTION Ion beam mixing provides a valuable alternative to high dose ion implantation for surface alloying, because the main limitation imposed by sputtering can be overcome and a whole class of stable or metastable phases may be produced. Ion mixing studies have been reported recently in many metal systems EU, including systems in which constituents alloy readily as well as those where no alloys are known to exist under normal thermodynamic conditions. Considerable attention has been directed towards the nature of ion mixed surfaces. In some systems, terminal solubilities can be greatly extended [2,33. In other cases, where the two constituents have different crystalline structure, it has been found as a rule for multilayered samples, that an extension of the terminal solubility limits from both sides of the phase diagram becomes structurally incompatible and an amorphous alloy is formed [4,5,6). The formation via ion mixing of new phases with favourable properties appears to be an important mechanism for improvements in surface sensitive properties of metals, such as wear and corrosion resistance. One of the most interesting occurrences is the possible presence of an amorphous surface layer. In particular, the excellent corrosion resistance of amorphous metals prepared by otherthan ion implantation techniques is well known [7) . From a technological point of view, it is interesting to improve the surface properties of nickel, which is very much affected by corrosion in hydrogen suiphide environments. Tarnishing of Ni occurs basically because the sulphide-containing surface film that forms is not protective, in contrast to the protective layers that form for example on stainless steel. Conventional alloying techniques have been used in an attempt to develop tarnish-resistant nickel alloys. Among them, electrodeposited Ni-Sn alloys are known to be excellent coatings E[8 from the point of view of their hardness and corrosion resistance in various atmospheres and aqueous solutions E9), including those Mat.
Res.
Soc. Symp.
Proc. Vol.
27 (1984)(Elsevier Science Publishing Co.,
Inc.
86
which readily attack the pure component metals. The corrosi
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