X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and scanning auger microscopy (SAM) analytical techniques have played important roles in the characterization of the surface and the interfacial chemistry governing properties and performance of materials, and material interfaces. These techniques afford spatially resolved elemental and molecular analysis of the topmost atomic layers of solid surfaces and interfaces. Currently available instrumentation provides qualitative/quantitative analysis on molecularly complex materials with detection limits in the parts-per-billion (ppb) range and spatial resolutions approaching 30 nm. Each technique is unique in the information attained, therefore necessitating a multitechnique approach to achieve a complete surface characterization. Examples of coating/interfacial characterization by XPS, TOF-SIMS, and SAM are presented illustrating the functionality of these tools and the complimentary natures of them.

Auger electron spectroscopy was used to characterize a boron-doped, CVD-deposited homoepitaxial diamond film, a well-faceted polycrystalline diamond film, cleaved highly oriented pyrolytic graphite and diamond-like carbon (DLC) films grown on Si wafers with different sample biases. This study focused on the influence of the conditions of electron beam and ion beam on the diamond and DLC Auger line shapes. The Auger results revealed that under 1 keV Ar+ ion sputtering, when the ion dose was below 1.4 × 1013 ions cm−2, the diamond Auger line shape remained similar to that of the unsputtered diamond film; with the increase of the ion dose, the line shape evolved to the one resembling graphite. Under the same ion dose, the increase or decrease of the ion beam energy also would cause the corresponding changes of the line shape. Ar sputtering appears to have similar effects on the line shape of DLC films. The increase of the electron beam current from 100 nA to 5 μA at a constant electron beam energy of 10 keV did not affect the main features of the diamond Auger line shape from the boron-doped homoepitaxial diamond film. Use of the electron and ion beam conditions determined in this study can allow accurate Auger analysis of diamond films without the introduction of artifacts.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) has been used to assess the formation of O-18-labeled photooxidation products in two multi-layer coating systems held in 20% O-18(2)/80% nitrogen atmosphere during brief Xenon are weatherometer exposure. A four-layer model coating system without hindered amine light stabilizer or ultraviolet light absorber additives was examined first. The model was exposed for 8 days in the weatherometer, and then its TOF-SIMS-O-18(-) response was recorded. This response tracks the known photooxidation resistance of the coating layers used to prepare the model system. Next, the technique was extended to a fully formulated six-layer 'repair' paint panel that had been weathered for 4 years in Florida prior to 10 additional days of weatherometer exposure in 20% O-18(2)/80% nitrogen atmosphere. The TOF-SIMS-O-18(-) response observed clearly suggests that the TOF-SIMS-O-18(-) technique can be used to assess the relative photooxidation rates of individual coating layers in fully formulated, multi-layer coating systems.

A four-factor, three-level, fractional factorial experiment design was used to investigate systematically the influences of substrate pretreatment, deposition parameters, residual stress and substrate material on the adhesion strength of diamond films produced by a microwave plasma CVD process. The experimental results revealed that pretreatment oi the substrates with diamond powder scratching was necessary for growing continuous diamond films on Mo, W and carburized W substrates. Statistical analysis showed that the substrate pretreatment had strong influences on film adhesion and, in particular, that the films on the substrates pretreated with Murakami's reagent demonstrated improved adhesion. The methane concentration in the hydrogen/methane gas source had significant effects on the diamond crystallite size and film surface morphology. Neither methane concentration nor deposition temperature in the ranges studied, however, showed significant influences on him adhesion. The total residual stress in the film was shown to be affected by substrate material and deposition temperature. The high mean residual tensile stress in the films deposited on pure W substrates was probably responsible for the poor film adhesion observed in this case. The decrease of the total stress in the films on carburized W substrates correlated well with the improved film adhesion. Carburization of the W substrates before diamond him deposition appeared to lessen the intrinsic stress in the him due to a better structure match between the diamond and tungsten carbide, thus enhancing film adhesion.

Three cold-rolled steels manufactured under different mill-processing conditions were analyzed by X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and direct oxidation CO2 coulometry (DOCC). XPS and SIMS depth profiling were also used to characterize the steels after successive DOCC oxidative treatments at 450 and 600-degrees-C in order to assess what type of carbon is consumed and what analysis depth is probed at each stage of the DOCC measurement. XPS experiments involving in situ oxidative treatments revealed that, for each steel, all detectable surface carbon is removed at 450-degrees-C. SIMS depth profile analyses determined that Fe2O3 layers formed during the 450 and 600-degrees-C DOCC combustion processes were approximately 200 and 2000 nm thick, respectively, revealing that near surface inorganic carbon is also measured during the DOCC analysis. Nevertheless, it was concluded that the DOCC analysis at 450-degrees-C measures mostly surface organic carbon, since the near surface carbon contribution is minor. The subsequent 600-degrees-C DOCC measurement is comprised entirely of near surface inorganic carbon and is reflective of the process control conditions used to manufacture the steel.

Two examples of recent advances in spatially resolved surface analytical characterization of paint and adhesive chemistries are presented. The examples extend previous analytical capabilities by combining the molecular specificity and bonding information of static secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) with the ability to resolve features of interest spatially. In one example, a cross-section of two paint layers, only one of which contained a photostabilizer additive, was characterized by time-of-flight (TOF)-SIMS operated in a microscope mode. Results indicate possible migration of this additive into the bulk of the adjacent paint layer. The second example presents an XPS mapping technique which was used to study the interfacial corrosion chemistry of an epoxy adhesive applied to galvanized steel. The sample was subjected to a corrosive environment while under static load. This technique afforded spatially resolved chemical information over the entire interfacial failure surfaces and allowed localized regions of corrosion activity to be imaged. Comparisons of results and techniques are made, along with discussions of the limitations of each technique.

Epoxy adhesive/galvanized steel bonds subjected to corrosion testing show a gradual loss of strength. Bonds subjected simultaneously to a static mechanical load and corrosion testing rupture spontaneously at relatively short exposure times. The differences in interfacial chemistry that accompany these exposure conditions were studied using an XPS elemental mapping technique that allowed the interfacial composition to be resolved spatially over the entire bond failure surface. An interfacial anodic process reminiscent of crevice corrosion dominated the interfacial chemistry of specimens exposed to corrosion testing without application of a static load. Bonds exposed under high loads exhibited both anodic and cathodic corrosion sites within the bond failure area. The changes in interfacial chemistry and failure mode upon application of a load are attributed to the opening of an interfacial crack at the locus of the initial corrosive attack. The ingress of electrolyte and the formation of cathodic sites adjacent to interfacial anodic sites within the growing crack accelerate the corrosion process. The result is rapid expansion of the interfacial failure region and spontaneous rupture of the specimen.

An x-ray photoelectron spectroscopy (XPS) mapping technique was developed to define spatially the interfacial chemistry of adhesive-adherend debonding. The technique was used to identify mechanisms of corrosion-induced bond failure for an epoxy adhesive applied to electrogalvanized steel. The specimens studied were lap-shear bonds exposed simultaneously to a static load and a corrosive environment. The bond failure surfaces were examined after testing. X-ray photoelectron spectroscopy elemental maps were generated over the entire area (approximately 12.5 mm x 25.0 mm) of the bond failure on both corresponding surfaces at a spatial resolution of 0.4 mm x 0.8 mm. The mapping was accomplished using a fixed small-spot-size source in conjunction with computer-controlled sample stage movement. The XPS elemental maps, and in one case a functional group map, were generated for the constituents of the adhesive (C, O, Si), the adherend (Zn, O) and the aggressive environment (Na, Cl, O, carbonate). The raw XPS intensity data were converted into units of at.% and displayed, using a bilinear interpolation scheme, as three-dimensional chemical images with a pseudo-color scale representing variations in elemental surface composition. The images provided detailed evidence for specific sites of anodic and cathodic corrosion processes, and facilitated identification of the locus of adhesive failure.

The surfaces of tappet inserts subjected to lubricated, moving contact in a cam/tappet friction apparatus have been analyzed by IR reflection—absorption, X-ray photoelectron and Auger electron spectroscopies. The two lubricants used were similar fully formulated oils. One contained a soluble molybdenum friction modifier additive while the other did not. Thin inorganic films were formed across the contact area during the cam/tappet experiment. Regardless of the lubricant used, these films consisted of varying amounts of inorganic phosphates, sulfates, sulfides, and perhaps thiophosphates, with zinc and magnesium as cations. Molybdenum would also be a cation in the case of the friction modified oil. The films generated during the wear process differed in thickness, composition and surface coverage as a function of wear time, contact position and lubricant composition. The films formed by the friction modified oil were thinner and less continuous as compared with films formed from the unmodified oil.

Previous studies in this laboratory have shown that for the La/γ-Al2O3 system as the atomic ratio La/Al is increased from 0.05 to 2.0 the X-ray photoelectron spectroscopy (XPS) La 3d5/2 binding energy progressively decreases from 835.2 to 832.9 eV. The shirts in binding energy demonstrate a change in the coordination of lanthanum coinciding with a transformation from dispersed to particulate phases. It is of particular interest to understand more completely the chemical interaction between lanthana and alumina at lower lanthanum loadings, which is known to retard the sintering of γ-alumina and promote catalysis. In this study lanthanum-modified alumina was calcined at high temperature (1500 °C). Up to a La/Al ratio of 0.10, X-ray diffraction (XRD) identified the dominant lanthanum phase as lanthanum-β-alumina (La2O3·11Al2O3). The XPS La binding energy for lanthanum-β-alumina was 835.2 eV, which corresponds to that measured previously for dispersed phase lanthanum in γ-alumina. These results suggest that at La/Al ⩽0.10 the highly dispersed La/γ-Al2O3 species is a precursor of crystalline lanthanum-β-alumina formed at higher temperatures. At higher lanthanum concentrations XRD identified highly ordered crystalline LaAlO3 and La2O3, consistent with the phase diagram of the La/Al2O3 system. According to XPS binding energy shifts the precipitation of LaAlO3 requires a higher lanthanum concentration at 925°C (γ-alumina phase) than at 1500°C (β-alumina phase).

X-ray photoelectron spectroscopy was employed to examine the reducibility of oxides possessing multiple oxidation states (titania, niobia, and ceria) in the presence of palladium and a valence-invariant oxide (lanthana). The materials consisted of sputter-deposited ultra-thin films containing palladium with either titania, niobia, or ceria, with and without lanthana, on a sapphire substrate. For lanthana/palladium films, no reduction of the lanthana surface was observed after in situ treatment in hydrogen up to 600°C. After similar treatment, in the absence of lanthana a partial reduction to the lower oxidation states of niobia and titania and a total reduction to the lower oxidation state of ceria was observed. The presence of lanthana in the films inhibits the reduction of the titania completely and that of the niobia and ceria partially. Also, in a complementing set of films containing ceria an overlayer of high-surface-area γ-Al2O3, from the decomposition of an aqueous film of aluminum propoxide, was put over the smooth sapphire surface before the deposition of the other film components. In this case, the inhibiting effect on ceria-reducibility was attenuated. This points to the redistribution of lanthana over the available oxide surfaces. The preponderant portion of the lanthana accommodated on the porous γ-Al2O3 has rendered a significant part of the ceria unprotected and, in the presence of Pd, susceptible to reduction by H2 at 600°C. The results of these studies are a further manifestation of the known surface reactivity of lanthana, which while remaining itself irreducible, may strongly affect the behavior of other irreducible oxides such as alumina, or as presently shown, of reducible oxides such as titania, niobia, and ceria.

It was demonstrated by X-ray photoelectron spectroscopy (XPS) that oxidized palladium supported on gamma-alumina can exist in two chemically different entities, distinguishable by their observed electron binding energies and by selective reduction. Larger oxidized particles am characterized by a binding energy attributed to PdO. This oxide is reduced to metallic palladium at room temperature in hydrogen, as would be expected for bulk PdO. On the other hand, at low palladium loadings of 0.5 wt.-% and less, the majority of the palladium is present at a binding energy shifted approximately 1.6 eV higher than that observed for PdO. The shift appears to be associated with a metal-support interaction at concentrations where the palladium is highly dispersed. Contrary to that observed at loadings greater than 0.5 wt.-%, this palladium remains mostly oxidized when exposed to hydrogen at room temperature. The results are compared with literature data reported for platinum/gamma-alumina.

The chemical interaction of dispersed lanthanum with γ-alumina was investigated by combining X-ray photoelectron spectroscopy (XPS) and carbon dioxide absorption. In order to establish the saturation concentration of lanthanum dispersed in the alumina surface layer, a series of materials was formulated at lanthanum concentrations corresponding to La/Al atomic ratios in the range from 0.05 to 2.0. The samples were prepared by impregnation of γ-alumina with aqueous lanthanum nitrate solutions, followed by calcination in air at 925 °C. Upon exposure to carbon dioxide, particles of La2O3 reacted to form bulk La2 (CO3)3, while lanthanum interacting with alumina did not. The reacted carbon dioxide was quantified by observing the carbonate in the C 1s spectra. The La/Al atomic ratio observed by XPS at low loadings from 0.05 to 0.15 was in good agreement with the bulk ratios. The carbon dioxide uptake remained constant at these loadings, while the La 3d5/2 spectral envelope appeared approximately 1.8 eV higher than that expected for La2O3. This shift to higher binding energy and the agreement of theoretical and XPS measurements is consistent with a monolayer dispersion previously reported. As the La loading increased above La/Al =0.15, the ratio measured by XPS increased first slightly, and then substantially over the bulk La/Al ratio, while the amount of carbon dioxide absorbed increased at the same time. In addition, the O 1s, Al 2p and La 3d5/2 spectral envelopes gradually shifted to lower binding energies. As the La/Al bulk ratio approached 2.0, the La 3d5/2 and O 1s spectral envelopes were shifted to what is expected for La2O3, while the Al 2p peak shifted to a binding energy observed for LaAlO3. The differences observed spectrally and quantitatively at low and high loadings imply a change in the coordination of lanthanum.

The frictional behavior of two oils was measured in a cam/tappet friction apparatus using a direct acting bucket tappet geometry. One oil contained a soluble molybdenum friction modifier. The tappet was equipped with a removable friction disk, which allowed the frictional surface to be easily removed for surface characterization using surface profilometry, reflection infrared and XPS spectroscopies. The films consisted of varying amounts of inorganic phosphates, sulfates and sulfides with zinc, magnesium, and possibly molybdenum as cations. MoS2, expected because additives containing molybdenum and sulfur were present, was not detected. The detection limit was less than 0.1 atom percent Mo. The film formed by the friction modified oil was thinner, smoother and contained small amounts of molybdenum. While the smooth surface finish could reduce friction by improving microelastohydrodynamic lubrication, friction reduction could not be tied to any of the chemical structures detected in these experiments.

This comment summarizes the historical development of a new high-spatial and high-mass resolution SIMS instrument developed in our laboratories. Reference is given to the lack of published material on similar developments which may have taken place in other laboratories.