Shear and thermal effects in boundary film formation during sliding

A prerequisite for understanding mechano- and tribochemical reaction pathways is that the interface be in thermodynamic equilibrium and that the temperature be well defined. It is suggested that this occurs in two regimes: when the surfaces are only slightly perturbed during sliding, leading to negligible frictional heating, and when the surface temperatures are very high (1000 K), in the so-called extreme pressure regime. The tribochemistry occurring in each regime is discussed in terms of the elementary steps leading to tribofilm formation, namely (i) a reaction of the additive or gas-phase lubricant on the surface to form an adsorbed precursor, (ii) decomposition of the molecular precursor, (iii) a process that causes the formation of a tribofilm that (iv) regenerates a clean surface that allows this tribochemical cycle to continue to form a thicker film. These steps are thermally driven in the extreme-pressure regime, while under milder conditions, they are induced by interfacial shear. In intermediate situations, the processes are likely to be a combination of those occurring at the extrema.Fil: Furlong, Octavio Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Luis. Instituto de Física Aplicada; ArgentinaFil: Miller, Brendan Paul. Chevron Oronite Company LLC; Estados UnidosFil: Kotvis, Peter V.. University Of Wisconsin; Estados UnidosFil: Adams, Heather. University Of Wisconsin; Estados UnidosFil: Tysoe, Wilfred T.. University Of Wisconsin; Estados Unido

Kinetic Monte Carlo theory of sliding friction

The sliding friction as a function of scanning velocity at the nanometer scale was simulated based on a modified one-dimensional Tomlinson model. Monte Carlo theory was exploited to describe the thermally activated hopping of the contact atoms, where both backward and forward jumps were allowed to occur. By comparing with the Monte Carlo results, improvements to current semiempirical solutions were made. Finally, experimental results of sliding friction on a NaCl(100) as a function of normal load and scanning velocity where successfully simulated. © 2009 The American Physical Society.Fil: Furlong, Octavio Javier. University of Wisconsin Milwaukee; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich". Universidad Nacional de San Luis. Facultad de Ciencias Físico Matemáticas y Naturales. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich"; ArgentinaFil: Manzi, Sergio Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich". Universidad Nacional de San Luis. Facultad de Ciencias Físico Matemáticas y Naturales. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich"; ArgentinaFil: Pereyra, Victor Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Matemática Aplicada de San Luis "Prof. Ezio Marchi". Universidad Nacional de San Luis. Facultad de Ciencias Físico, Matemáticas y Naturales. Instituto de Matemática Aplicada de San Luis "Prof. Ezio Marchi"; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich". Universidad Nacional de San Luis. Facultad de Ciencias Físico Matemáticas y Naturales. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich"; ArgentinaFil: Bustos Giunta, Victor Angel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich". Universidad Nacional de San Luis. Facultad de Ciencias Físico Matemáticas y Naturales. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich"; ArgentinaFil: Tysoe, Wilfred T.. University of Wisconsin Milwaukee; Estados Unido

Influence of dimple shape on tribofilm formation and tribological properties of textured surfaces under full and starved lubrication

Lubrication conditions have significant influences on the formation of tribofilms and then affect tribological behavior. In this work, the influence of tribofilm formation on the tribological behavior of textured surfaces with oval shapes was measured using a pin-on-plate tribometer. The results show that, under full lubrication, the adsorbed oil film controlled the friction and wear behavior of steel/steel tribopairs but under starved lubrication, the formation of a tribofilm significantly influenced the tribological behavior. The appropriate textured surfaces with oval-shaped dimples contribute to obtaining excellent antifriction and antiwear behavior. However, excessively high ratios of the major to the minor axis of the oval can result in high contact stresses which can destroy the tribofilm.Fil: Xu, Yufu. Hefei University of Technology; República de ChinaFil: Zheng, Quan. Hefei University of Technology; República de ChinaFil: Abuflaha, Rasha. University of Wisconsin; Estados Unidos. Al al-Bayt University; JordaniaFil: Olson, Dustin. University of Wisconsin; Estados UnidosFil: Furlong, Octavio Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich". Universidad Nacional de San Luis. Facultad de Ciencias Físico Matemáticas y Naturales. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich"; ArgentinaFil: You, Tao. University of Wisconsin; Estados UnidosFil: Zhang, Qiangqiang. Hefei University of Technology; República de ChinaFil: Hu, Xianguo. Hefei University of Technology; República de ChinaFil: Tysoe, Wilfred T.. University of Wisconsin; Estados Unido

EMSL and Institute for Integrated Catalysis (IIC) Catalysis Workshop

Within the context of significantly accelerating scientific progress in research areas that address important societal problems, a workshop was held in November 2010 at EMSL to identify specific and topically important areas of research and capability needs in catalysis-related science

Molecular-level Design of Heterogeneous Chiral Catalysts

It has been shown previously that the adsorption of a chiral 2-butanol template on Pd(111) leads to enantioselective adsorption of chiral propylene oxide probe molecules. Enantioselectivity is expressed over a narrow coverage range where the maximum value of enantioselectivity ratio (ER defined as Θ(R-propylene oxide)/Θ(S-propylene oxide), where Θ is the coverage) reaches ~2. Probe coverages in this case were measured using either reflection-absorption infrared spectroscopy (RAIRS) or temperature-programmed desorption (TPD) [1,2]. The enantioselectivity disappears when the 2-butanol-covered surface was heated to ~200 K since the adsorbed butoxy species decomposes by a β-hydride elimination reaction to yield a non-chiral ketone. Monte Carlo calculations of the effect of chiral modifiers have yielded results that are consistent with these experimental observations [3,4]. Similar experiments using 2-methyl butanoic acid as a template, where the chiral center is identical to that in 2-butanol but is now anchored by a carboxylate group rather than by an alkoxide, shows no enantioselectivity. In this case, propylene oxide coverages were measured using the King and Wells method. RAIRS experiments and density functional calculations suggest that the 2-butyl group of the 2-butoxy species is oriented parallel to the surface. A possible origin for the lack of enantioselectivity of a 2-methyl butanoic acid-covered surface may be that the 2-butyl group is farther from the surface, allowing it to rotate more freely, averaging out any asymmetry, resulting in a loss of chirality. In order to test this idea, the alkyl group on the carboxylic acid was functionalized with an amine to anchor the chiral center to the surface. Using the amino-acids alanine and 2-amino butanoic acid as templates restored the enantioselectivity and yielded ER values of 2.0 ± 0.2 and 1.75 ± 0.15 respectively. These results suggest that a two-point attachment of the chiral template is required, one for surface adsorption and the other to allow the enantioselectivity to be expressed. Low-energy electron diffraction (LEED) intensity versus energy (I/E) measurements are used to measure the structure of templates and probes on the Pd(111) surface, where these results will be compared with calculations carried out by the Sholl group. Since the aminoacids are relatively large, initial experiments were carried out to determine the structure of carboxylates on the surface to determine the carboxylate group anchoring site. Since carboxylates do not form ordered structures on Pd(111), we have exploited a method recently developed in collaboration with Professor Saldin to measure structures of disordered overlayers [5]. Results show that the formate OCO plane is oriented perpendicular to the surface with the oxygen atoms located across a short bridge on the (111) surface. The effect of the size of the functional group on the amino acid template (RCH(NH2)COOH) was also investigated where the maximum ER values obtained using propylene oxide were 2.0 ± 0.2 (R=CH3), 1.75 ± 0.15 (R=C2H5), 1.65 ± 0.15 (R=C3H6) and 1.30 ± 0.15 (R=CH2CH(CH3)2) thus showing a decreasing trend with increasing size of the side chain. The enantioselectivity of S-(1-naphthyl) ethylamine-covered surfaces have been explored using propylene oxide as a probe, but these systems showed no enantioselectivity. However, using 2-butanol as a probe lead to enantioselective chemisorption implying that one-to-one modification requires a direct hydrogen-bonding interaction between the probe and modifier. 1. Enantioselective Chemisorption on a Chirally Patterned Surface in Ultrahigh Vacuum: Adsorption of Propylene Oxide on 2-butoxy-Covered Pd(111), D. Stacchiola, L. Burkholder and W.T. Tysoe, J. Am. Chem. Soc., 124, 8984 (2002) 2. Enantioselective Chemisorption on a Chirally Modified Surface in Ultrahigh Vacuum: Adsorption of Propylene Oxide on 2-butoxide-Covered Pd(111), Darío Stacchiola, Luke Burkholder and Wilfred T. Tysoe, J. Mol. Catal A: Chemical, 216, 215 (2004) 3. Theoretical Analysis of the Coverage Dependence of Enantioselective Chemisorption on a Chirally Patterned Surface, F. Roma, D. Stacchiola, G. Zgrablich and W. T. Tysoe, Journal of Chemical Physics, 118, 6030 (2003) 4. Lattice-gas Modeling of Enantioselective Adsorption by Template Chiral Substrates, F. Romá, D. Stacchiola, W.T. Tysoe and G. Zgrablich, Physica A., 338, 493 (2004) 5. Structure Determination of Disordered Organic Molecules on Surfaces from the Bragg Spots of Low Energy Electron Diffraction and Total Energy Calculations, H. C. Poon, M. Weinert, D. K. Saldin, D. Stacchiola, T. Zheng and W. T. Tysoe, Phys. Rev. B., 69, 35401 (2004

Raman spectra of oxidation induced defects in sphalerite

Many of the anomalies associated with the properties of mineral sulphides can be traced to their response to oxidative attack, the processes of which are not at present well understood. During the process it is postulated that the increase in cation vacancy concentration which occurs on oxidation of ionic crystals is followed by the collapse of the vacancies to a diatomic sulphur molecule, allowing sulphur rich non-stoichiometry of the crystal

Mechanistic and Kinetic Measurements of Elementary Surface Reactions Using Temperature-Programmed X‑ray Photoelectron Spectroscopy

This paper describes a method by which surface-reaction kinetics can be measured by slowly and precisely ramping up the surface temperature at a constant rate while simultaneously collecting X-ray photoelectron spectra (XPS). This approach results in the collection of a large amount of data over relatively small temperature steps to produce quasi-continuous kinetic data. The method is illustrated for the desorption and reaction of diethyl disulfide (DEDS) on a Au(111) substrate in ultrahigh vacuum, where the results can be compared with previous conventional temperature-programmed desorption (TPD) data from Au(111). Experiments were carried out using a double-pass cylindrical-mirror analyzer with a channeltron detector to demonstrate how this approach can be implemented in a routine, multitechnique vacuum chamber. The approach will be even more effective in a more modern, specialized XPS apparatus with high-transmission hemispherical analyzers with multichannel array detectors, which will enable the spectra of several elements to be measured simultaneously. The results yielded an activation energy for multilayer desorption of DEDS of 41 ± 1 kJ/mol, with a pre-exponential factor of 8 ± 7 × 1012 s–1, an activation energy of 53 ± 6 kJ/mol and pre-exponential factor of 9 ± 8 × 1013 s–1 for monolayer desorption and an activation energy of 90 ± 6 kJ/mol with a prefactor of 1.0 ± 0.3 × 1015 s–1 for the reaction of adsorbed ethyl thiolate species to adsorbed DEDS. While these results were collected for a system for which the kinetic data could have been obtained using conventional TPD, this method can be more usefully applied to those surface reaction processes that do not rely on the formation of desorption products. This system, having been previously studied by TPD, facilitates a comparison with results obtained by conventional methods

The Kinetics of Shear-Induced Boundary Film Formation from Dimethyl Disulfide on Copper

The kinetics of the shear-induced surface-to-bulk transport of methyl thiolate species formed from dimethyl disulfide (DMDS) on a copper surface are explored. It is found that the loss of surface species as a function of the number of rubbing cycles can be modeled by assuming that the adsorbed layer penetrates the subsurface a distance of ~0.7 nm per scan. Adding wear to this model does not improve the fit to the experimental data providing an upper limit for the wear rate of ~0.06 nm/scan. This model is applied to analyzing the depth distribution of sulfur within the subsurface region as a function of the number of rubbing cycles, measured by Auger depth profiling when continually dosing the copper sample with DMDS. It is found that the shape of the experimental depth profile is in agreement with the model developed to analyze the surface-to-bulk transport kinetics of the adsorbed layer. However, the profiles are almost identical for surfaces that have been rubbed 130 and 360 times, so that the surface-to-bulk transport kinetics are self limiting.Fil: Miller, Brendan. University of Wisconsin-Milwaukee. Department of Chemistry and Laboratory for Surface Studies; Estados UnidosFil: Furlong, Octavio Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Luis. Instituto de Física Aplicada; ArgentinaFil: Tysoe, Wilfred T.. University of Wisconsin-Milwaukee. Department of Chemistry and Laboratory for Surface Studies; Estados Unido

Local and Extended Structures of d‑(−)-Tartaric Acid on Pd(111)

The structures of d-(−)-tartaric acid on the Pd(111) surface are studied by scanning tunneling microscopy (STM) supplemented by density functional theory (DFT) calculations as a function of coverage and sample temperature. When low coverages of d-(−)-tartaric acid were dosed with the sample at 300 K, and then cooled to 120 K for imaging, doubly dehydrogenated tartrate species (C₂H₄O₆^2–) are observed, while at higher coverages, singly deprotonated bitartrate species (C₂H₅O₆^1–) are prevalent. STM images show that the tartrate species are isolated on the surface, while the bitartrate species oligomerize by extensive hydrogen-bonding interactions. DFT calculations show that the adsorption of tartrate species locally disrupts the Pd lattice thereby imparting strain to the surface. The interaction between the two tartrate species in a dimeric pair is suggested to be the contribution of three factors: the adsorption-induced stress to the surface, Coulombic repulsion between the tartrate species, and the intermolecular hydrogen-bonding interactions. Occasionally, two different kinds of kinetically controlled ordered structures are observed after long times. The first consists of intermolecularly hydrogen-bonded bitartrate species and the second comprises tartrate species interacting through the substrate. DFT calculations suggest that the ordered tartrate domains form as a result of minimizing the local strain energy of the surface and hydrogen-bonding interactions