Bcc Substrate Research Articles

Structures and adhesion of hcp thin film coating interfaces on a single-crystal bcc substrate by PVD: Ti/Mo and Zr/Mo

Polycrystalline hcp Ti and Zr thin films were developed on single-crystal bcc-Mo (110) substrate using PVD method. XRD analysis suggests the preferred facet relation of (0001)hcp//(110)bcc. Sandwich supercell models were then constructed to evaluate the interface energy, adhesion strength and bonding nature from the first principles. The Burgers and Pitsch-Schrader orientation relation (OR) were predicted as the most energy-favored for the PVD Ti and Zr thin film interfaces on the Mo (110), respectively. Interface adhesion was predicted to be fairly strong for both interfaces. The d-d orbital interaction between Ti/Zr and Mo is found closely associated with the spatial redistribution of Bader valence charges. The resulting new bonds across the interfaces are rather delocalized, showing a mixed covalent and weak ionic bond character. These results strongly suggest that single-crystal Mo (110) is a promising substrate for developing high quality hcp Ti or Zr thin films through PVD.

Microstructural characterization of Co-based coating deposited by low power pulse laser cladding

A detailed microstructural study of Stellite 6 coating deposited on a low carbon ferritic steel substrate using preplaced powder method and low power Nd:YAG pulse laser is performed. The grain structure and solidification texture of the coating are investigated by orientation imaging microscopy (OIM) and scanning electron microscopy. In addition, the effect of consecutive pulses on the microstructure of the coating is examined. The orientation relationship (OR) at coating/substrate interface and the solid state phase transformation in heat-affected zone are studied as well as the Vickers microhardness profile measurement in order to support the microstructural observations. An important conclusion is reached that the shape of solidification front during pulsed laser cladding is similar to the shape of solidification front during continuous cladding with a doubled laser beam scanning speed. Further, OIM reveals the Greninger–Troiano OR between the face centered cubic coating and bcc substrate grains. It is concluded that at the moment of solidification epitaxial growth of the grains in the coating occur on the austenitic grains of the substrate and that an austenite–ferrite transformation occurs in the heat-affected zone upon subsequent cooling.

Modeling of the deposition of Ni and Pd on Mo(1 1 0)

Recent experimental work on the deposition of fcc metals on a bcc substrate motivates this atomistic modeling analysis of Ni and Pd deposition on Mo(1 1 0). A detailed atom-by-atom analysis of the early stages of growth, focusing on the formation of surface alloys and 3D islands is presented, identifying the interactions leading to each type of behavior. Further analysis describes the growth pattern as a function of coverage. Temperature effects are studied via Monte Carlo simulations using the Bozzolo–Ferrante–Smith (BFS) method for alloys for the energetics.

Molecular Dynamics Simulation on the Modification of Crystallographic Orientation in Fragmented Particles in the Aerosol-Deposition Process

Particle fragmentation and modification of crystallographic orientation in the aerosol-deposition process were investigated by molecular dynamics simulation. Atomistic models of bcc Fe and fcc Ni particles of 10 nm in diameter were subjected to collision to at substrates, and analyzed for their structural modification due to the impact. It was found that particle fragmentation takes place depending on both crystal structure and initial orientation of incident particles. Modification of crystallographic orientation were observed in both bcc and fcc particles and in bcc substrates. Microscopic mechanisms of particle fragmentation and orientation modification were found to be classified into three types, sliding, compression, and third phase generation, which are related to slip systems and relaxation of interface structures.

Epitaxial growth of Ag on W(110)

Epitaxial growth of Ag on W(110) at room temperature was studied by scanning tunneling microscopy (STM) and polarization-dependent photoemission. At coverages far below one monolayer Ag atoms populate bcc sites of the substrate and form close-packed islands of monolayer thickness. With increasing coverage geometrical misfit between Ag(111)-like layers and W(110) generates surface stress along $\mathrm{W}[1\overline{1}0]$. This is released by formation of domain walls parallel W[001] which are observed with a distance between about $25\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and $30\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, depending on the details of the growth process. At one monolayer coverage most of the Ag atoms still reside in or very near to bcc substrate positions, but now the strain release pattern is changed: solitons aligned along $\mathrm{W}[\overline{1}12]$ are formed at an average distance between $35\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and $50\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. The details of the soliton arrangement depend critically on the degree of equilibration and the presence of holes in the monolayer film which allow an additional stress release. This is evident from a comparison with results of STM studies performed at the closed and carefully annealed Ag monolayer [Kim et al., Phys. Rev. B 67, 223401 (2003)]. Further deposition of Ag starts growth of a second monolayer by formation of islands which increase in size with coverage. At a nominal coverage of 1.5 monolayers the strain relieve pattern changes again: some corrugation lines are oriented along W[001] as in the submonolayers, but other orientations related to Ag(111) directions appear as well. This indicates that several possibilities are available at similar energy costs and that the transition from the W substrate potential to a Ag potential seen by the second layer is very soft. Finally at a nominal coverage of several monolayers, Stranski-Krastanov growth is observed producing Ag(111)-like terraces with one of the dense-packed Ag rows oriented parallel to $\mathrm{W}[1\overline{1}1]$.

Structural stability and lattice defects in copper:Ab initio, tight-binding, and embedded-atom calculations

We evaluate the ability of the embedded-atom method (EAM) potentials and the tight-binding (TB) method to predict reliably energies and stability of nonequilibrium structures by taking Cu as a model material. Two EAM potentials are used here. One is constructed in this work by using more fitting parameters than usual and including ab initio energies in the fitting database. The other potential was constructed previously using a traditional scheme. Excellent agreement is observed between ab initio, TB, and EAM results for the energies and stability of several nonequilibrium structures of Cu, as well as for energies along deformation paths between different structures. We conclude that not only TB calculations but also EAM potentials can be suitable for simulations in which correct energies and stability of different atomic configurations are essential, at least for Cu. The bcc, simple cubic, and diamond structures of Cu were identified as elastically unstable, while some other structures (e.g., hcp and 9R) are metastable. As an application of this analysis, nonequilibrium structures of epitaxial Cu films on (001)-oriented fcc or bcc substrates are evaluated using a simple model and atomistic simulations with an EAM potential. In agreement with experimental data, the structure of the film can be either deformed fcc or deformed hcp. The bcc structure cannot be stabilized by epitaxial constraints.

Ordering at Solid-Liquid Interfaces Between Dissimilar Materials

In an earlier report we explored structural correlations at a liquid-solid interface with molecular dynamics simulations of a model aluminium system using the Ercolessi-Adams potential and up to 4320 atoms. Substrate atoms were pinned to their equilibrium fcc crystalline positions while liquid atoms were free to move. A direct correlation between the amount of ordering in the liquid phase and the underlying substrate orientation was found. In the present paper we extend this study to the case of a fixed bcc substrate in contact with liquid aluminium. We find surprisingly similar results for the density profiles of both (100) and (110) substrates. However, there is a far greater in-plane ordering in the (100) than for the (110) system. For the (100) substrates we observe adsorption of liquid atoms into the terminating plane of the bcc (100) substrate, effectively transforming the bcc (100) plane into an fcc (100) plane.

A tight-binding study of hydrogen chemisorption on Pt7/Ni(111) and Pt/Ni(111) systems

We study the electronic structure of a Pt overlayer and Pt7 adatom cluster, respectively, on the Ni(111) surface by using a self-consistent tight-binding model. Together with epitaxial geometries, a mean-field 'non-epitaxial' model is also employed. For these structures, calculations for hydrogen adsorption are performed. Correlation between the geometry, surface reactivity, core-level shifts and local surface electronic structure has been recently discussed for Pd and Pt ultrathin films on BCC substrates. Similar correlation is also found in the present study. Nevertheless, its origin is now traced to the change of the local d-band width due to overlayer compression rather than to a specific hybridization between orbitals on different atoms. An analogy with PtNi alloy surfaces and their catalytic properties is pointed out.

A grand ensemble Monte Carlo study of metal adsorption on a (110) bcc substrate

The multilayer adsorption of a metal on a (110) bcc substrate has been studied by a grand-canonical Monte Carlo simulation in continuous, three-dimensional space. The obtained values of the critical parameters are compared with the results of a similar 2D Monte Carlo calculation and are interpreted in the spirit of the theory of Asada. A sharp transition from monolayer to multilayer adsorption is observed. The transition is accompanied by a substantial structural rearrangement in the adlayers immediately at the substrate. Generally it appears that the inclusion of the third dimension in the simulation reveals some important features of the phase transition and of the structure of the adsorbate which are absent in the 2D calculation.

Peripheral roughening in a monolayer: A molecular-dynamics study.

We have performed a molecular-dynamics simulation of the peripheral roughening of a two-dimensional triangular lattice on a bcc substrate. The model in which the peripheral atoms exhibit a weakened bond to the inner atoms shows a smooth increase in the roughness as a function of temperature. The dependence of the roughness on the strength of the interaction is in agreement with a theoretical spin model but, however, disagrees with earlier claims based on thermodynamic measurementsn of epitaxial films. The implications for epitaxial growth are also discussed.

Chemisorption on a contaminated BCC metal

The presence of an impurity atom within a metal substrate affects the change in density of states (CDOS) upon chemisorption, and these effects are investigated via the Green's function method. A tight-binding model is employed for the (100) surface of a bcc substrate, and the impurity is described by the Koster-Slater model. The CDOS is computed as a function of different impurity and adatom-substrate coupling parameters for on-site and centered-site adsorption.

Ion channeling and TEM studies of pulse melted Fe(Pd) alloys

Metastable Fe(Pd) surface alloys formed by deposition of layers of Pd on (100) Fe and subsequent pulsed (≈70 ns, 1.9 J/cm 2) electron beam melting into the bcc substrate have been investigated by ion backscattering/channeling and TEM. Repeated pulsed melting allowed examination of the epitaxial growth of the bcc substrate phase into the alloyed region for successively lower Pd concentrations. This procedure permitted detailed examination of the conditions for solidification with the fcc or bcc crystal structure and for metastable phase formation during rapid solidification. Bcc epitaxy is absent at Pd concentrations ≳30 at.%, where an fcc surface layer forms. Channeling of the Pd and Fe signals was observed at Pd concentrations ≲16.5 at.%, where a single-phase, supersaturated bcc alloy is observed. At an intermediate Pd concentration of 22–23 at.%, both epitaxial bcc and fcc materials are formed. The observed maximum substitutional Pd concentration in the bcc phase is about a factor of 5 greater than the maximum equilibrium solid solubility.

Textures in diffusion-processed superconducting Nb3Sn and V3Ga layers

The texture studies of the Nb3Sn and V3Ga superconducting tapes produced by the surface diffusion process (SDP) and by the composite diffusion process (CDP) were made by pole figure works using an x-ray diffractometer. The textures are described as (100)[011]+(112)[110] for the SDP-Nb3Sn and V3Ga layers and (100)[011] for the CDP-Nb3Sn and V3Ga layers, although exceptionally weak for the CDP-V3Ga layer. The results suggest that the A-15 grains grow into the bcc substrate phase with a parallel or nearly parallel orientation relationship. It is found that the weak texture of the CDP-V3Ga yields larger grain-boundary pinning force than the sharp texture of the SDP-V3Ga does.

Study of adsorption of titanium on tungsten and rhenium by field electron emission

The deposition and layer growth of titanium, evaporated on to tungsten and rhenium field emitter tips has been studied, and field emission average work-functions measured at varying titanium layer thicknesses from less than a monolayer to several monolayers. Titanium evaporated on to a clean tungsten tip at 293 K in the submonolayer region tended to collect preferentially on atomically rough planes. With further deposition, a “pseudo-clean” emission pattern was obtained at about a monolayer coverage when the titanium atoms were concluded to be in epitaxial registry with the BCC substrate; at about this coverage the work-function for the system passed through a minimum. Further titanium deposition in excess of three monolayers resulted in a progressive roughening of the surface due to microcrystallinity: brief heating (~ 1 min, 1300 K) of such a surface resulted in a well ordered layer identified as ß-titanium (BCC) presumably epitaxed to the tungsten. This epitaxed layer remained indefinitely stable at 293 K. In the deposition of titanium on to a clean rhenium emitter tip at 293 K, titanium again appeared to collect on atomically rough planes in the submonolayer region. Further deposition in excess of two monolayers resulted in the formation of a well ordered layer identified as α-titanium (HCP) probably epitaxed to the rhenium. The field emission average work-functions of α-titanium and ß-titanium were determined as 4.00 ± 0.05 and 3.65 ± 0.05 eV respectively.