Low pressure pack sublimation: Newly developed zinc coating process

The LPPS process has been developed to coat Nd-Fe-B I,magnets and related a lloy powders. The process is similar to sherardising, except that the coati ng chamber is static and evacuated to a moderate vacuum (10(-1) torr). Comp ared to sherardising, LPPS achieves lighter coloured coatings and enhanced coating uniformity and has the ability to coat reactive ferrous materials s uch as Nd Fe B magnets. The corrosion protection provided by LPPS zinc is e quivalent to that obtained with conventional sherardising. Using SEM, XRD, and SIMS analysis, the mechanism for both sherardising and LPPS has been sh own to be the interaction of zinc vapour. with ferrous substrates to form F e-Zn intermediate phases (delta, xi, and gamma). At a given temperature, th e zinc vapour Pressure, chamber pressure, surface preparation of the compon ent, and substrate material have been found to influence the rate of layer growth and final intermediate phase mixtures. Activation energy and thermod ynamic analysis suggest that in galvanising galvannealing, sherardising, an d LPPS, Fe-Zn phase formation is dominated by the inward diffusion of zinc. For LPPS zinc coated Nd-Fe-B magnets, the same I,mechanism applies and len ds to a complex mixture of (Nd-Fe-B)-Zn phases. The 'black zinc' layers see n occasionally with sherardising and LPPS have been shown to be associated with residual oxygen? present in the deposition chamber. Black zinc surface s have high optical absorbance as a result of their finely, divided nature. Recent work on extending LPPS to coat magnet powders is also presented. The authors are in the Applied Alloy Chemistry Group, School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Contribution to the 1999 Bodycote International Paper Competition. (C) 200 0 IoM Communications Ltd.