Benjamin M. DeKoven

XPS studies of metal/polymer interfaces — Thin films of Al on polyacrylic acid and polyethylene

Abstract A reactivity experimental study of Al with CH 2 and COOH functionalities in polymers is performed using polyacrylic acid (PAA) and low density polyethylene (LDPE). The Al metal is deposited on the polymer surfaces using in-situ sputter deposition. Using X-ray photoelectron spectroscopy (XPS), an Al oxide-carbide complex is identified at the PAA/Al interface while an Al carbide-like species is observed at the LDPE/Al interface. These conclusions are based on carefully referenced binding energy measurements of the C(1s), O(1s), and Al(2p) core electron levels. Near surface XPS studies involving solvent cast PAA films indicate that they are CH 2 rich, suggesting that a larger than statistical number of carboxylic acid groups are pointing away from the surface towards the bulk. PAA powder, however, is found to have near stoichiometric composition at the surface. The conclusions regarding Al/polymer reactivity agree well with recent literature for other metal/polymer interfaces.

A model of the dynamics of boundary film formation

Abstract The dynamics of formation and loss of the boundary films formed during sliding on steel surfaces were investigated over a range of temperature. Tests are performed on a cylinder-on-disk machine using mineral oil with various concentrations of zinc dialkyldithiophosphate (ZDP). The thickness and refractive index of the boundary films during step load test were monitored in situ with an ellipsometer, and the composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS). As temperature increases, chemical reactivity increases the film formation rate, while the film removal rate increases owing to (a) the decrease of durability of the boundary film material and (b) the reduction of hydrodynamic fluid film thickness due to decreasing viscosity of the lubricant. There is a balance between these two competing mechanisms, and this balance is reflected in the boundary film thickness. The boundary films consist of a film of oxide and metallic compound (OMM) covered by an organo-iron compound (OIC). Their relative effectiveness in preventing scuffing depends on temperature and composition. In particular, the OIC is effective in reducing wear of the opposing surfaces by covering the OMM.

A model for the boundary film formation and tribological behavior of a phosphazene lubricant on steel

The dynamics of formation and loss of the boundary films formed during the lubricated sliding of steel surfaces were investigated over a range of temperature and applied load. Tests were performed on a cyldinder-on-disk machine using a phosphazene lubricant (X-1P), a polyphenyl ether, and mineral oil with and without addition of zinc dialkyldithiophosphate (ZDP). Among these lubricants, X-1P was found to have the best high-temperature, high-load performance. The thickness and retractive index of the boundary films were monitored in situ with an ellipsometer, and the composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS). The performance of the lubricants was found to be closely associated with boundary film-forming ability. In order to achieve high load-carrying capacity, a boundary film must be formed. The films formed in X-1P grow more slowly than those in ZDP-containing mineral oil, but they remain thick even at high load and high temperature (250°C). These films are durable and effective in reducing friction and preventing scuffing. The film formed with X-1P consists of a mixture of compounds containing Fe, O, C, F, P, and N. Among the compounds formed, some form of iron fluoride appears to be most important in determining the performance of the film.

Friction studies in ultrahigh vacuum of Fe surfaces with thin films from exposure to perfluorodiethylether

The coefficient of friction for both clean and oxidized contacting polycrystalline Fe surfaces following exposure to a perfluorodiethylether (PFDE), (CF3CF2)2O, was measured in ultrahigh vacuum (UHV) using a pin‐on‐flat device (0.05–0.30 N loads). This system was chosen to examine the influence of thin films on friction using a molecule having functionalities that are present in perfluoropolyether lubricants. The friction coefficient measured for the clean (∼0.2–1.0 A oxide) surfaces in UHV was ∼3.0. The friction was measured for both oxidized and clean Fe surfaces in PFDE atmospheres (3–150 Pa). During this exposure the friction for the oxidized Fe surfaces dropped to 0.7–0.9 compared to 1.2–1.5 obtained in O2 environments. The friction for clean Fe surfaces measured in UHV following similar PFDE exposures showed two distinct friction ranges, 0.1–0.8 and 1.2–1.8. The Fe surfaces were examined in situ in the same UHV chamber using x‐ray photoelectron spectroscopy and showed the formation of a Fe fluoride ...

Cyclotriphosphazenes as Potential Lubricants for Thin Film Hard Disks

This paper explores the use of phosphazenes as alternative lubricants for thin film hard disks. Contact start/slop (CSS), stiction and drag tests were performed on five disks lubricated with a typical cyclic phosphazene lubricant, X-1p, with a thickness in the range of 0.3 nm to 0.7 nm. The coefficients of friction and stiction were found to increase initially, then level off and reach a steady-state value without a further increase after 2000 CSS cycles, ft is concluded that X-1p performs well on thin film disks with a lubricant thickness of about 0.5 nm. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Lahaina, Hawaii, October 16–20, 1994

Friction studies of ‘‘clean’’ and oxygen exposed Fe surfaces in ultrahigh vacuum

The coefficient of friction for both clean and oxygen exposed contacting polycrystalline Fe surfaces was measured in ultrahigh vacuum (UHV) using a pin on flat device. This system was chosen in order to examine the influence of very thin oxide films (≤20 A) on friction. The surfaces were then examined in situ in the same UHV chamber using both x‐ray photoelectron spectroscopy (XPS) and scanning Auger microprobe (SAM). The oxide thickness was determined using O (1s) XPS intensities. The loads used were 0.12–0.16 N. The Fe pin (radius=0.005 m) motion was linear over 0.004 m at a speed of 5×10−6 m/s. The friction coefficient measured for the ‘‘clean’’ (∼0.2–1.0 A oxide) surfaces was 3.0–5.8. Large stick slips were seen during these measurements. The friction dropped monotonically for increasing oxide thickness (≤25 A) to 1.7–2.0. Auger line scan studies of the O/Fe ratio in several scars showed a large depletion of O in the contacted area. Even though the Fe flat surface is deformed up to depths of 3 μm duri...

HREELS, XPS and in-situ friction studies of thin polyphenyl ether films on steel surfaces

Abstract The surface chemistry of thin films of polyphenyl ether (5P-4E) lubricant on clean and oxidized M-50 steel surfaces were studied using high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS) coupled with in-situ friction measurements. Polyphenyl ether films were applied in-situ by evaporation. Liquid film thicknesses were obtained using both XPS and ellipsometry. HREELS and XPS of evaporated films of 5P-4E onto both clean and oxidized M-50 steel surfaces indicate reaction of the lubricant with the substrate occurs at room temperature. XPS results suggest C-O bond cleavage occurs producing metal oxide. HREELS results suggest loss of aromatic character of the adsorbed 5P-4E occurs as well. Friction measurements for varying 5P-4E film thicknesses show that ∼ 100 A films are needed to drop the friction from 2.0 (clean surface value) to 0.3 XPS studies of scratched areas show changes due to reaction which are similar to those measured in the thermal reactions. Wetting of surfaces by 5P-4E was found to be a strong function of surface cleanliness. XPS and ellipsometry show that the degree of film continuity depends upon the amount and/or nature of carbon on the surface. The implications of these results to lubrication are discussed.

Surfaces of Semi-Fluorinated Block Copolymers Studied Using Nexafs

The molecular orientation within a surface liquid crystalline layer made up of semi-fluorinated side-groups [-CO-(CH{sub 2}){sub x{minus}1}-(CF{sub 2}){sub y}F] (SF groups) attached to the isoprene block of a styrene-isoprene diblock copolymer was determined by analyzing the partial electron yield C-edge NEXAFS signal. The results show that in contrast to the bulk, where the SF groups lie parallel to the diblock copolymer lamellae and thus parallel to the surface, the surface SF groups make an average angle with the surface normal of between 29 and 46{degree} depending on x and y.

Hardness and friction of N2+ and Al+ implanted B4C

Surfaces of boron carbide (B4C) were implanted with 200 keV N2+ or 100 keV Al+ ions to a dose of 8 × 1017 cm −2. Ultralow load indentation via the Nanoindenter was used to determine the changes in surface hardness and elastic modulus following implantation. A pin-on-disk testing device in ultrahigh vacuum was used for measuring changes in friction and wear of the implanted disks. Results of hardness measurements show that the hardness of the nitrogen-implanted sample drops to about 58% of the unimplanted hardness, and that for the aluminum-implanted sample drops to about 52% of the unimplanted value. The elastic modulus also falls to approximately 62% of its unimplanted value for both implantations. Friction and wear studies conducted in UHV show an increase in friction of both implantations from 0.5–0.6 after one cycle to 0.8–1.1 after ten cycles. This is larger than the 0.17–0.25 values measured for pure B4C. The wear is significantly reduced in the early cycles of the implanted samples relative to the unimplanted ones. The dramatic softening and improved wear behavior are probably due to the formation of an amorphous surface layer.

Formation of a BN layer on the surface of B4C using nitrogen implantation

Abstract The surface of B 4 C was irradiated using nitrogen ions and then examined to determine the alterations in the surface chemical and mechanical properties following implantation. Pure boron was also implanted to compare uptakes and chemistry. The implantations were performed in situ at 10 keV in an ultrahigh vacuum system and at 100 keV using a commercial implanter. For doses up to 5.0 × 10 17 atoms cm -2 all implanted nitrogen reacted. The nitrogen implanted (10 keV) into either B 4 C or boron was chemically in the form of BN. 100 keV nitrogen implantation into boron resulted in BN formation while a boron carbide-nitride species was formed using B 4 C. Nitrogen doses of about 6.0 × 10 17 cm -2 at 100 keV for B 4 C produced chemical halos, 2 – 6 μm in diameter, around a crater owing to expulsion of a TiB 2 particulate impurity. The B 4 C surface, modified at 100 keV, (dose less than or equal to 5.5 × 10 17 cm -2 ) showed a 60% reduction in the pin wear scar diameter and a 200% increase in the coefficient of friction compared with the unimplanted surface. These changes in the surface chemical and mechanical properties suggest that a hard BN layer forms on B 4 C following nitrogen ion implantation.