The High Resolution Imaging Atom-Probe with Application to Nickel Platings.

This work concerns the development of a high resolution imaging atom-probe time-of-flight mass spectrometer, its performance, and its application to electro- and electroless nickel, nickel-phosphorus platings, and to hydrogen adsorbed on rhodium. An imaging atom-probe displays the distribution over a samples surface of a constituent mass species selected from the samples time-of-flight spectra. Imaging of the sample surface is at atomic resolution by field-ion, desorption, and gated desorption microscopy. The mass resolution of this 13.5-cm flight-path instrument was significantly increased by minimizing dispersion in the initial energy of field evaporated ions while improving timing and gating electronics. A preliminary application to the chemistry of hydrogen adsorbed on metallic solids displayed crystallographically and atomically specific variations in the distribution of adsorbed hydrogen over an undisturbed rhodium surface in an applied electric field. The principal application is the atomic level metallurgical analysis of stripped Ni and Ni-P electro- and electroless deposited plates. The mass spectra of as-deposited Ni electroplate show it to be 99 pure, while field-ion micrographs show boundaries of atomic width between impinging crystals. Micrographs, after annealing, show an ordered overlayer rich in phosphorus indicating the segregation of phosphorus to the surface during the transformation to Ni3P-Ni. The underlying structure is a mixture of ordered phases, Ni3P and Ni, with boundaries that coincide with those present in Ni-P prior to the anneal. The existence and coincidence of the boundaries indicate Ni-P is not totally amorphous, in the sense of a liquid, as was previously thought.