Frank S. Honecy

Surface segregation in multicomponent systems: Modeling of surface alloys and alloy surfaces

Abstract The study of surface segregation, though of great technological importance, has been largely restricted to experimental work due to limitations associated with theoretical methods. However, recent improvements in both first-principles and semiempirical methods are opening the doors to an array of new possibilities for surface scientists. We apply one of these techniques, the BFS method for alloys, which is particularly suitable for complex systems, to several aspects of the computational modeling of surfaces and segregation, including alloy surface segregation, structure and composition of alloy surfaces, and the formation of surface alloys. We conclude with the study of complex NiAl-based binary, ternary and quaternary thin films (with Ti, Cr and Cu additions to NiAl). Differences and similarities between bulk and surface compositions are discussed, illustrated by the results of Monte Carlo simulations. For some binary and ternary cases, the theoretical predictions are compared to experimental results, highlighting the accuracy and value of this developing theoretical tool.

Study of copper on graphite with titanium or chromium bond layer

Improvement of copper to graphite adhesion by thin interfacial films of titanium and chromium was investigated. Graphite fibers and highly oriented pyrolytic graphite flats were sputter-coated first with 10 nm of titanium or chromium and then with 50 nm of copper. After annealing to 970 °C in argon/5%-hydrogen at atmospheric pressure for 5 min, copper without an interfacial bond layer agglomerated into nearly spherical particles, copper with the chromium bond layer agglomerated into particles with a contact angle less than 90°, indicating improvement in adhesion, and copper with the titanium bond layer exhibited a continuous metal film. In the latter case, most of the interfacial titanium was observed to have migrated into the copper and to the free surface, where the titanium reacted with contaminants in the annealing ambient.

Tribological properties of Ag/Ti films on Al2O3 ceramic substrates☆

Abstract Silver solid lubricant films with a thin titanium interlayer for enhanced adhesion, were sputter deposited on Al 2 O 3 substrate disks to reduce friction and wear. The dual Ag/Ti films were tested at room temperature in a pin-on-disk tribometer sliding against bare, uncoated Al 2 O 3 pins under a load of 4.9 N at a sliding velocity of 1 m s -1 . The Ag/Ti films reduced the friction coefficient by 50% to about 0.42 compared to unlubricated baseline specimens. Pin wear was reduced by a factor of 140 and disk wear was reduced by a factor of 2.5 compared to the baseline. These films retain their good tribological properties, including adhesion, after heat treatment in argon at 850°C and thus may be able to lubricate over a wide temperature range. This lubrication technique is applicable to space lubrication, advanced heat engines and advanced transportation systems.

A Vacuum (10−9 Torr) Friction Apparatus for Determining Friction and Endurance Life of MoSx Films

The first part of this paper describes an ultrahigh vacuum friction apparatus (tribometer). The tribometer can be used in a ball-on-disk configuration and is specifically designed to measure the friction and endurance life of solid lubricating films such as MoSx in vacuum at a pressure of 10−7 Pa, 10−9 torr. The sliding mode is typically unidirectional at a constant rotating speed. The second part of this paper presents some representative friction and endurance life data for magnetron sputtered MoSx films, 110 nm thick, deposited on sputter-cleaned 440C stainless-steel disk substrates, which were slid against a 6-mm-diameter 440C stainless-steel bearing ball. All experiments were conducted with loads of 0.49 to 3.6 N, average Hertzian contact pressure, 0.33 to 0.69 GPa, at a constant rotating speed of 120 rpm, sliding velocity ranging from 31 to 107 mm/s due to the range of wear track radii involved in the experiments, in a vacuum of 7×10−7 Pa, 5× 10−9 torr, and at room temperature. The results indicate ...

BFS simulation and experimental analysis of the effect of Ti additions on the structure of NiAl

The Bozzolo–Ferrante–Smith (BFS) method for alloy energetics is applied to the study of ternary additions to NiAl. A description of the method and its application to alloy design is given. Two different approaches are used in the analysis of the effect of Ti additions to NiAl. First, a thorough analytical study is performed, where the energy of formation, lattice parameter and bulk modulus are calculated for a large number of possible atomic distributions of Ni, Al and Ti. Substitutional site preference schemes and formation of precipitates are thus predicted and analyzed. The second approach used consists of the determination of temperature effects on the final results, as obtained by performing a number of large-scale numerical simulations using the Monte Carlo–Metropolis procedure and BFS for the calculation of the energy at every step in the simulation. The results indicate a sharp preference of Ti for Al sites in Ni-rich NiAl alloys and the formation of ternary Heusler precipitates beyond the predicted solubility limit of 5 at. % Ti. Experimental analysis of three Ni–Al–Ti alloys confirms the theoretical predictions.

Surface segregation in ternary alloys: a BFS study of Ni–Al–Cu(100) surface composition

Abstract Atomistic simulations of surface segregation constitute an essential tool in determining the surface composition and structure of binary or higher order alloys. The presence of metastable states, where widely different surface composition profiles are energetically possible, are severe tests for the ability of experimental techniques for accurately determining the composition of the surface. A clear example of such phenomenon is analyzed in this paper, where the Bozzolo–Ferrante–Smith (BFS) semiempirical method for alloys is applied to the study of surface composition of Ni-rich Ni–Al–Cu alloys. A wide range of Cu concentration is considered, showing distinct segregation trends characterizing the low-Cu and high-Cu concentration regimes. The existence of metastable states with energy close to that of the ground state is identified and shown to drive the resulting segregation pattern. The results of several Monte Carlo/Metropolis simulations are shown, highlighting the role of the initial state used in the simulation and the temperature treatment of the sample.

Auger analysis of a fiber/matrix interface in a ceramic matrix composite

Auger electron spectroscopy (AES) depth profiling was used to characterize the fiber/matrix interface of an SiC fiber, reaction bonded Si{sub 3}N{sub 4} matrix composite. Depth profiles of the as received double coated fiber revealed concentration oscillations which disappeared after annealing the fiber in the environment used to fabricate the composite. After the composite was fractured, the Auger depth profiles showed that failure occurred in neither the Beta-SiC fiber body nor in the Si{sub 3}N{sub 4} matrix but, concurrently, at the fiber coating/matrix interface and within the fiber coating itself.