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Microwave plasma chemical vapor deposition (MPCVD) was used to diffuse boron into tantalum using plasma initiated from a feedgas mixture containing hydrogen and diborane. The role of substrate temperature and substrate bias in influencing surface chemical structure and hardness was investigated. X-ray diffraction shows that increased temperature results in increased TaB2 formation (relative to TaB) along with increased strain in the tantalum body-centered cubic lattice. Once the strained tantalum becomes locally supersaturated with boron, TaB and TaB2 precipitate. Additional boron remains in a solid solution within the tantalum. The combination of precipitation and solid solution hardening along with boron-induced lattice strain may help explain the 40 GPa average hardness measured by nanoindentation. Application of negative substrate bias did not further increase the hardness, possibly due to etching from increased ion bombardment. These results show that MPCVD is a viable method for synthesis of superhard borides based on plasma-assisted diffusion.
A series of atom probe and transmission electron microscopy (TEM) studies have been performed to quantify minute compositional fluctuations in Fe55Pt45 thin films during the A1 to L10 phase transformation. The atom probe specimens were analyzed in an Imago Local Electrode Atom Probe (LEAP®) at a target evaporation of 0.5%, a pulse fraction of 20% and a temperature of 120K. We noted a propensity of fracture failures in the LEAP with this material at lower temperatures. The atom probe reconstruction showed small levels of Pt segregation at grain boundaries in the as-deposited films. Fresnel-contrast TEM imaging confirmed high density fluctuations in these boundaries. Upon annealing at 600°C for 10 minutes, the film transformed from A1 to L10 and the grain boundaries become Fe enriched as compared to the as-deposited film.
The first dedicated local electrode atom probes (LEAP [a
trademark of Imago Scientific Instruments Corporation]) have been
built and tested as commercial prototypes. Several key performance
parameters have been markedly improved relative to conventional
three-dimensional atom probe (3DAP) designs. The Imago LEAP can operate
at a sustained data collection rate of 1 million atoms/minute. This
is some 600 times faster than the next fastest atom probe and large
images can be collected in less than 1 h that otherwise would take many
days. The field of view of the Imago LEAP is about 40 times larger than
conventional 3DAPs. This makes it possible to analyze regions that are
about 100 nm diameter by 100 nm deep containing on the order of 50 to
100 million atoms with this instrument. Several example applications
that illustrate the advantages of the LEAP for materials analysis are