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 sample's surface of a constituent mass species selected from the sample's 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. The mass resolution, m/(DELTA)m, at an acceptance angle of 5.4(DEGREES) is 150 at 20% above the base line. This is equivalent to the performance of one- to two-meter straight flight-path atom-probes, and is shown by isotopic resolution W('+++) and Ni('++) mass spectra. Mass determinations remain accurate at the 31(DEGREES) full acceptance angle as the low mass side of spectral peaks is sharp; however, resolution is reduced to 50 by "tailing" ions resulting from premature evaporation induced by adsorbed residual hydrogen and longer off-axis flight times. The included apertured and full angle spectra of Ni('++) and Rh('++) compare in the above manner. The maximum time resolution of gated images at full acceptance angle has been increased to 100 at a representative flight time of one microsecond and a gate width of 10 nanoseconds. The included gated micrographs show the spatial distributions of W('3+) and W('4+), Ni('++) and P('++) from Ni-P electroless plate, and H('+) adsorbed on rhodium. 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 variations in coverage observed in gated images of H('+) have since been confirmed by energy-compensated atom-probe measurements. In addition, results indicate that adsorption occurs from the gas phase during both atom-probe analysis and hydrogen promoted imaging. 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 filed-ion micrographs show boundaries of atomic width between impinging crystals. Overall reorientation at depths of 30-150 atomic layers indicate a grain size of 60-300(ANGSTROM), while nearly uniform desorption patterns confirm extended perfect crystals are not present. The mass spectra of as-deposited Ni-P(2.7%) electroless plate and the annealed Ni(,3)P-Ni phase mixture show phosphorus content in agreement with expected values, while expanded Ni-P spectra show oxygen and a trace amount of chlorine. Field ion, desorption, and gated desorption micrographs show boundaries of atomic width in the otherwise amorphous Ni-P. Micrographs, after annealing, show an ordered overlayer rich in phosphorus indicating the segregation of phosphorus to the surface during the transformation to Ni(,3)P-Ni. The underlying structure is a mixture of ordered phases, Ni(,3)P 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.