J.A. Sprague

Energetic particles in PVD technology: particle-surface interaction processes and energy-particle relationships in thin film deposition

Abstract Energetic deposition techniques, including ion beam assisted deposition, ion plating, unbalanced magnetron sputtering, and cathodic-arc deposition, are playing an increasing role in the processing of coatings for mechanical, optical, and electronic applications, and for the protection of surfaces in hostile environments. Advantages of energetic film deposition include low temperature processing, improved adhesion to substrates, production of desired phases or compounds, and control of crystallographic orientation. There are certain fundamental physical processes common to these energetic deposition methods that involve the alloying of elements (ion implantation, condensate and gas adsorption), material removal (sputtering, desorption), collision-induced displacements, collision-stimulated thermal exchange, and collision-induced surface and bulk diffusion. The mechanisms by which these mechanisms affect film formation are under intense investigation. New Monte Carlo and molecular dynamics simulations of the effects of energetic atoms on surfaces as well as experimental investigations revealing relationships between deposition variables and phase formation are contributing to our understanding of thin film growth. This paper covers the above topics by first establishing the ‘ideal’ conditions for deposition of a film, indicates how different deposition processes attempt to approximate these conditions, and reviews known correlations between deposition variables and film characteristics.

Molecular dynamics simulations of film-substrate interface mixing in the energetic deposition of fcc metals

Abstract Embedded-atom-method molecular dynamic simulations have been performed to examine the interface mixing produced by deposition of fcc metals on fcc metal substrates. Atom arrival energies of 0.1, 10, 20, and 40 eV have been studied. The interface mixing initiated by atom impacts on the substrate surface was found to increase significantly with increasingly negative heats of solution of film atoms in the substrate lattice. As expected, both the total amount of interface mixing and the depth over which it occurred increased with increasing atom deposition energy. Comparison of the interface mixing results for two different temperature-control algorithms led to the conclusion that the interface mixing was very sensitive to short-lived localized substrate lattice excitations in the vicinity of atom impacts. This concept of interface mixing has some similarities to the concept of a thermal spike in bulk ion mixing, but does not involve any localized melting of the lattice. For a simulation of 0.1 eV Ni deposition on a Au substrate, a thermally-activated interface mixing process with a low activation energy was observed, driven by the difference between the surface energies of Ni and Au.

Molecular-dynamics study of film growth with energetic Ag atoms

Three‐dimensional molecular‐dynamics simulation of the growth of silver (Ag) atoms on an Ag (111) oriented substrate has been conducted as a function of incident atom energy and temperature. Incident atom energies of 0.1 eV per atom resulted in the formation of epitaxial islands that coalesced to form a continuous thin film. Increasing the incident atom energy promoted layer‐by‐layer growth, and with an incident atom energy of 10 eV per atom the film growth was nearly layer‐by‐layer. For the 10‐eV incident atom energy with a 330‐K substrate temperature a small amount of interface mixing was observed, but the mixing was eliminated by lowering the substrate to 30 K. The interface mixing process occurred by an exchange process between the energetic atom and a surface atom. Increasing the incident atom energy to 20 eV per atom increased the amount of interface mixing, but the interface mixing could not be eliminated by lowering the temperature of the substrate. This indicates that the mixing process is ballis...

Oxidation behaviour of ion-implanted NiCrAl☆

Abstract Ion implantation was used in a program to establish the oxidation characteristics of NiCrAl, a candidate bond coat alloy for use in thermal barrier coating systems. Samples of cast NiCrAl, implanted with cerium and vanadium ions to a nominal concentration of 5 at.%, were oxidized in a mixture of 80 vol.% Ar and 20 vol.% O 2 for varying durations. The oxides formed were analyzed by means of scanning transmission electron microscopy and Auger electron spectroscopy. Transient oxidation (1 min) of non-implanted NiCrAl resulted in the formation of mixtures of pure oxides, with NiO predominant. After 10 min at 900 °C the formation of spinels became predominant and the amount of Al 2 O 3 in the oxide also increased. Implantation with vanadium ions did not greatly affect the oxidation characteristics of the alloy, with the exception of increasing the oxide thickness by approximately a factor of 2 for a 1 min exposure. In contrast, implantation with cerium ions significantly altered the composition and morphology of the oxides. Distinct layers of NiO, a chromium-rich oxide and Al 2 O 3 formed. These oxides had a marked epitaxial relationship with the underlying alloy. The implications of these effects are discussed.

A molecular dynamics analysis of low energy atom-surface interaction during energetic deposition of silver thin films

Abstract A three-dimensional molecular dynamics simulation was conducted of the deposition of 500 energetic silver atoms (three monolayers) incident on a (111) oriented substrate with 1008 dynamic atoms and 1008 frozen atoms. Incident atom energies were 10, 20 and 40 eV, and initial substrate temperatures were 300 and 0 K. All of these deposition conditions resulted in layer by layer growth. During the film depositions, interface mixing between deposited atoms and substrate atoms was also observed. This mixing was found to increase with increases in either incident atom energy or substrate temperature, with energy having the largest effect. The observed interface mixing mechanism was the partial or full embedding of an incoming atom in the top substrate layer, followed by the ejection of a substrate atom from that layer and the accommodation of the deposited atom in the substrate.

NRL Materials Testing Facility

The Naval Research Laboratory performs basic research on high power railgun electric launchers. The program uses a 1.5-MJ, 2.5 km/s launch velocity railgun located in NRLs Materials Testing Facility. The railgun consists of an 11-MJ capacitive energy store configured as 22, 0.5-MJ modules. Each bank module has an independently triggered thyristor switch, series inductor, and crowbar diode and is joined to the railgun breech with coaxial cables. Individual bank timing and charge levels can be set to produce up to 1.5 MA peak current and 4-5 ms long current pulses. The 6-m long railgun used a nominally 5 cm bore diameter with steel or copper rails and epoxy laminate insulators. The muzzle contains a Tungsten-Copper arc horn to minimize damage from residual drive current upon launch. Aluminum armatures with acrylic bore riders are used for the launch package. Launch data is recorded digitally and analyzed using in-house computer codes. The system design and operation will be discussed.

Initial stages of oxide formation on a CoCrAlY coating alloy at 700°C☆

Abstract A cast Co22Cr11Al0.5Y alloy and the oxide films formed on it at 700°C for times of between 3 and 50 min were studied by analytical electron microscopy. Prior to oxidation, the alloy microstructure consisted of an ordered b.c.c. β phase, densely distributed h.c.p. α phase precipitates and widely distributed yttride precipitates at some α-β phase boundaries. The oxide films consisted of thin regions of mostly γ-Al 2 O 3 formed over the β phase and thicker regions containing mostly spinels formed over the α phase. In all the oxide films, yttrium-rich protrusions, or “pegs”, were observed extending approximately 2 μm from the films into the alloy. The pegs apparently formed on the yttride precipitates at the alloy surface. The X-ray spectra from the pegs were consistent with a model of aluminum- and yttrium-rich oxide sheaths surrounding the yttrides.

EM Gun Bore Life Experiments at Naval Research Laboratory

The Naval Research Laboratory (NRL) performs basic and applied research on high power railguns as part of the US Navy EM Launcher program. The understanding of damage mechanisms as a function of armature and barrel materials, launch parameters, and bore geometry is of primary interest to the development of a viable high power railgun. Research is performed on a 6-m, 1.5-MJ railgun located at NRL. Barrel studies utilize in situ diagnostics coupled with detailed ex situ analysis of rail materials to provide clues to the conditions present during launch. Results are compared with coupled 3-D electromagnetic and mechanical finite element analysis models of railgun operation. Results of several experiments on rail wear will be discussed.

Interface mixing of energetic metals deposited onto metals

Abstract Experimental studies of ion beam mixing have indicated that the extent of mixing is sensitive to the heat of mixing of the two atom types. Analytical studies of this phenomenon have utilized an initial short time random ballistic mixing combined with a longer time diffusional process that is a function of the heat of mixing. We have utilized molecular dynamics simulation with potentials of the embedded atom method to study low energy (0.1 to 40 eV per atom) deposition of metals onto metal substrates. We have observed that even the short term ballistic mixing process is sensitive to the heat of solution. For example, Pt and Au have similar masses as do Ni and Cu, and the heat of solution for the potentials we utilized are ∮.54 eV per Pt atom into Cu and +0.30 eV per Au atom into Ni. For 1 monolayer (200 atoms) of Pt deposited onto Cu with 10 eV kinetic energy per Pt atom, we observed that 85 of the Pt atoms mixed into the Cu substrate during a simulation of 200 ps. Under the same conditions 28 Au atoms mixed into the Ni substrate. Detailed study of individual mixing events shows that most of the mixing occurs during the ballistic part of the deposition process. We report on the deposition as a function of incident atom energy, substrate temperature, heat of solution of the atom pairs, and relative mass of the pairs. We discuss the relationship of these results to theoretical analysis of energetic atom mixing at very low energies.

A molecular dynamics study of transient processes during deposition on (001) metal surfaces

Strong reflection high‐energy electron diffraction oscillations have been observed during growth of Cu onto (001)Ag substrates at 77 K [W. F. Egelhoff and I. Jacob, Phys. Rev. Lett. 62, 921 (1989)]. This result indicates layer by layer film growth and surface mobility of the deposited atoms. This unexpected result was explained by proposing a ‘‘transient mobility’’ to account for layer growth. We have utilized molecular dynamics simulations with the embedded atom method to study transient processes that occur over hundreds of picoseconds. Transient processes were observed that promoted the formation of clusters, and other processes were observed that resulted in spreading atoms across the surface.