Abstract
The LPPS process has been developed to coat Nd–Fe–B magnets and related alloy powders. The process is similar to sherardising, except that the coating chamber is static and evacuated to a moderate vacuum (10-1 torr). Compared to sherardising, LPPS achieves lighter coloured coatings and enhanced coating uniformity and has the ability to coat reactive ferrous materials such as Nd–Fe–B magnets. The corrosion protection provided by LPPS zinc is equivalent to that obtained with conventional sherardising. Using SEM, XRD, and SIMS analysis, the mechanism for both sherardising and LPPS has been shown to be the interaction of zinc vapour with ferrous substrates to form Fe–Zn intermediate phases (δ, ξ, and γ). At a given temperature, the zinc vapour pressure, chamber pressure, surface preparation of the component, and substrate material have been found to influence the rate of layer growth and final intermediate phase mixtures. Activation energy and thermodynamic analysis suggest that in galvanising, galvannealing, sherardising, and LPPS, Fe–Zn phase formation is dominated by the inward diffusion of zinc. For LPPS zinc coated Nd–Fe–B magnets, the same mechanism applies and leads to a complex mixture of (Nd–Fe–B)–Zn phases. The ‘black zinc’ layers seen occasionally with sherardising and LPPS have been shown to be associated with residual oxygen present in the deposition chamber. Black zinc surfaces have high optical absorbance as a result of their finely divided nature. Recent work on extending LPPS to coat magnet powders is also presented.
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