Jeffrey R. Lince

Crystallinity of RF-Sputtered MoS2 Films.

The crystallinity and morphology of thin, radio-frequency (rf) -sputtered MoS2 films deposited on 440C stainless steel substrates at both ambient (∼70°C) and high temperatures (245°C) were studied by scanning electron microscopy (SEM) and by x-ray diffraction (Read thin-film photography and 0−20 scans). Under SEM the films exhibited a “ridgelike” (or platelike) formation region for thicknesses between 0.18 and 1.0 μm MoS2. X-ray diffraction was shown to give more detailed and accurate information than electron defraction, previously used for elucidating the structure of sputtered lubricant films. Read thin-film x-ray diffraction photographs revealed patterns consistent with the presence of polycrystalline films and strong orientation of the MoS2 crystallites. Correlation of those patterns with 0−20 scans of the films indicated that the basal planes of the MoS2 crystallites [i.e., the (001) planes] were perpendicular to the substrate surface plane, and that various edge planes [i.e., the (h k 0) planes] in the individual crystallites were parallel to the surface plane, in agreement with previous observations of thinner films. Sliding wear caused the crystallites to orient with their basal planes parallel to the surface plane. The crystallite lattices in all films in this study were shown to exhibit compressive stress (∼ 3%–5% with respect to natural molybdenite) in the direction perpendicular to the (h k 0) planes, and the worn films were expanded (i.e., exhibited tensile stress) perpendicular to the (001) plane. In addition, the shapes of the x-ray diffraction peaks were strongly influenced by the presence of oxygen impurities and/or sulfur vacancies in the MoS2 lattice, indicating that x-ray diffraction may provide a simple quality-control test for the production of a film with optimum lubricating properties.

A comparison of oxidation and oxygen substitution in MoS2 solid film lubricants

Abstract Significant advancements in the production of low friction, long wear life, sputter-deposited MoS 2 lubricant coatings have been made in the last decade. The introduction of multi-layered coatings, the establishment of careful controls on doping during DC and magnetron sputter deposition, and the implementation of ion assisted deposition have resulted in lubricants with substantially longer wear lives (up to a factor of ten greater than in the early 1980s) and lower sliding friction coefficients. A major research effort, designed to improve the performance of solid lubricants, involved a number of laboratories during this time period, resulting in these major breakthroughs. However, even with this concentrated effort, the typical investigation involved making an educated guess, based on previous experience, of the deposition conditions, target compositions, or post treatments that might be expected to provide improved performance of resulting coatings. One notable discovery during this time period was that typical MoS 2 films contain large quantities (up to 20 atom %) of oxygen substituted for sulfur in individual crystal lattices. In this paper we will compare the effects of this oxygen substitution with the effects of oxidation which involves a change in the oxidation number of the central molybdenum atoms within the crystals. A discussion of the relationship(s) between chemistry and coating structure and tribological performance will be presented with emphasis on defect chemistry and multiple phase interactions. Speculations on the role of coating chemistry in determining coating performance in applications such as in ball bearings will be presented.

MoS 2−x O x solid solutions in thin films produced by rf-sputter-deposition

The chemical composition and structure of MoS 2 solid lubricant films are intimately related to their friction and wear characteristics. We have conducted an x-ray photoelectron spectroscopy (XPS) study of 1-μm thick radio frequency (rf)-sputter-deposited MoS 2 films to determine the chemical state of the films, focusing on the role of oxygen impurities. Concentrations of chemisorbed and bulk species were determined from the Mo 3d, S 2 p , and O 1 s peak shapes and intensities after annealing the films to temperatures from 425 to 975 K. Films deposited on substrates that were at ∼345 K [ambient temperature (AT) films] and on substrates heated to ∼525 K [high temperature (HT) films] both had ∼10% oxygen within the bulk of the films. The relative areas and shapes of the XPS peaks for the HT films at all annealing temperatures were consistent with the formation of a MoS 2−x O x solid solution, where O atoms were probably substituted into S sites in the 2H–MoS 2 crystal lattice. In AT films, this phase composition was stable only for annealing temperatures ≥725 K, in agreement with previous studies of the changes in crystal structure of AT films with annealing. The results are discussed in terms of previous studies of the structure and composition of sputter-deposited MoS 2 films.

Oxygen substitution in sputter-deposited MoS2 films studied by extended X-ray absorption fine structure, X-ray photoelectron spectroscopy and X-ray diffraction

Abstract The morphology and crystallite structure/orientation of sputter-deposited MoS 2 solid lubricant films have been shown to be strongly affected by oxygen-containing species in the sputtering chamber. We have studied the effect of oxygen incorporation within these films using extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Films produced at The Aerospace Corporation, the National Centre of Tribology (U.K.), Hohman Plating and Manufacturing, and NASA Lewis Research Center were studied to determine common characteristics of different MoS 2 films produced by sputter deposition. In agreement with previous results, XPS of the films indicated a bulk anion:cation ratio ( i.e. (S+O):Mo) of about 2, and XRD revealed that the films consisted completely of MoS 2 -like phases that were compressed in the ( h k 0) directions as compared with pure MoS 2 . This “edge plane compression” appeared to correlate with oxygen content. EXAFS confirmed the absence of MoO 2 in the films and indicated that the films actually consist of two similar MoS 2 -like phases. Correlation of the EXAFS results with those obtained from XPS and XRD indicated that the two phases are MoS 2 and an MoS 2- x O x substitutional solution. The solid solution may be described as MoS 2 with oxygen atoms that have substituted for sulfur atoms in the MoS 2 crystal lattice. Relative peak height changes in the EXAFS radial distribution curves indicated that increasing amounts of oxygen cause a reduction in the size of crystallites in the films. The presence of the MoS 2- x O x phase may explain the superior tribological performance of MoS 2 films in some applications, which has been shown to correlate with oxygen impurities in the films.

Anisotropic oxidation of MoS2 crystallites studied by angle-resolved X-ray photoelectron spectroscopy

The oxidation behavior of the solid lubricant MoS2 was studied using X-ray photoelectron spectroscopy. MoS2-containing slip ring brushes were used to take advantage of anisotropic orientation of the plate-shaped MoS2 crystallites occurring during fabrication of the brushes. Because oxidation occurs preferentially at the edges of the crystallites, greater oxidation fractions are measured when the edges are oriented toward the photoelectron energy analyzer. These results indicate a novel method for separately probing the chemistry on the edge and (lubricious) basal surfaces of MoS2.

AuGa2 on GaSb(001): An Epitaxial, Thermodynamically Stabilized Metal/III-V Compound Semiconductor Interface.

A new type of metal/semiconductor junction has been developed. Crystalline thin films of the intermetallic compound AuGa2 have been grown by molecular beam epitaxy (MBE) on GaSb(001) substrates with the orientation (001)AuGa2∥(001)GaSb, [100]AuGa2∥[100]GaSb. The resulting films have been characterized by Auger electron spectroscopy (AES), low energy electron diffraction, electron energy loss spectroscopy, scanning electron microscopy, x‐ray microprobe analysis, and Rutherford backscattering. The growth procedure is reported, and is discussed in terms of the Au–Ga binary phase diagram. This system is of special interest for contacting technology because the compounds terminate a pseudobinary cut through the Au–Ga–Sb bulk ternary phase diagram, which minimizes the amount of chemical interaction across the metal/semiconductor interface.

EXAFS of sputter-deposited MoS2 films

Abstract The composition and structure of sputter-deposited MoS2 solid lubricant films are difficult to analyze by diffraction techniques because the films typically exhibit poor crystallinity. The extended X-ray absorption fine structure (EXAFS) technique is ideal for analyzing these properties because it probes short-range (local) order parameters such as interatomic bond lengths. EXAFS data for the Mo K X-ray absorption edge were analyzed, including least-squares fitting, to investigate films produced under a number of conditions, as well as those produced with varied amounts of oxygen incorporated in the films. The results were compared with X-ray photoelectron spectroscopy and X-ray diffraction data of the same films. Analysis showed that the films, which contained 9–40 at.% oxygen, consisted predominantly of a MoS2-xOx phase, with x continuously variable. The MoS2−xOx phase exhibited a MoS2-like structure, where oxygen atoms had substituted for sulfur atoms in the MoS2 crystal lattice. In addition, smaller quantities of pure MoS2 (10–;25 times lower) were detected in the films. Increasing oxygen content correlated with increasing values of x in the MoS2−xOx phase, as well as with decreasing relative amounts of the pure MoS2 phase. The presence of the MoS2−xOx phase may explain the decrease in friction with increasing oxygen content that has been observed for MoS2 films. In addition, the poor order (high defect level) in this phase may explain the density enhancement caused by crystallite size reduction; higher densities can be correlated with beneficial wear behavior of the films.

Chemical effects of Ne+ bombardment on the MoS2(0001) surface studied by high-resolution photoelectron spectroscopy

The effect of 1 keV Ne+ bombardment on the clean MoS2(0001)-1 × 1 surface with fluences between 4 × 1014 and 4 × 1016 Ne+/cm2 was studied using high-resolution photoelectron spectroscopy excited with synchrotron radiation. Spectra of the Mo 3d and S 2p core levels were measured with photon energies that ensured that the kinetic energy of the photoelectrons was the same, resulting in the same depth being probed for both core levels. For lower fluences (i.e., ≲2 × 1015 Ne+/cm2), S vacancy defect formation occurs in the MoS2 lattice, with the concurrent formation of a small amount (< 10%) of dispersed elemental molybdenum [Mo(0)]. For fluences greater than ∼l × 1016 Ne+/cm2, the Mo(0) is the predominant species in the surface region, while the remaining species consist of amorphous MoS2−x and polysulfide species. Valence band spectra taken with photon energies of 152 and 225 eV were consistent with the core level results. The movement of the valence band maximum toward the Fermi level indicated the formation of a metallic surface region. Annealing the sample to temperatures up to 1000 K resulted in the formation of metallic Mo coexisting, in approximately equal amounts, with reformed MoS2 in a surface with no long-range order as determined by LEED. Finally, a qualitative depth distribution of the chemical species present after Ne+ bombardment was determined by varying the photon energies used for the core level spectra. The results indicate that the preferential sputtering of sulfur over molybdenum occurs predominantly through a mechanism involving chemical bonding effects, specifically, through the preferential emission of polysulfide ions over other species in the bombarded region.

Epitaxial growth of Ag films on Ge(001)

The deposition of thin films of Ag on clean Ge(001) substrates has been investigated using low energy electron diffraction, Auger electron spectroscopy, and scanning electron microscopy. Ag was deposited from an effusion source onto Ge at both 325 and 600 K substrate temperatures. Deposition on high temperature substrates resulted in the formation of Ag islands with no apparent crystalline order, whereas films deposited at temperatures near 325 K onto clean Ge(001) 2×1 surfaces were oriented. The deposition of the equivalent of one Ag(001) monolayer on the Ge surfaces resulted in a Ge(001) 1×1‐like LEED pattern. As deposition was continued (up to 500 monolayers), a pattern evolved which is consistent with two (011) 1×1 Ag domains oriented at 90° to one another. This behavior is interpreted in terms of a predominantly Stranski–Krastanov growth mechanism for the initial deposition of Ag onto near room temperature Ge(001) substrates. Annealing of a sample with a thin, ordered film revealed the original Ge(00...

Chemical interaction of thin Cr films with the MoS2(0001) surface studied by x‐ray photoelectron spectroscopy and scanning Auger microscopy

The growth of chromium films on the MoS2(0001) surface has been studied with core‐level x‐ray photoelectron spectroscopy and scanning Auger microscopy. The films grown on room temperature surfaces result in a limited chemical reaction to produce metallic Mo and chromium sulfide(s). The surfaces of the films are relatively S‐rich, while the bulk of the films contains smaller amounts of S. Mo is found only at the film/MoS2 interface and is not incorporated into the bulk of the overlayer. Annealing the Cr covered sample to 650 °C and higher further drives the reaction between Cr and MoS2, and causes partial coalescence of the film.