David B. Asay

Nanotribology and MEMS

The tribological phenomena of adhesion, friction, and wear arise when solid objects make contact. As the size of devices shrinks to micro- and nanoscales, the surface-to-volume ratio increases and the effects of body forces (gravity and inertia) become insignificant compared with those of surface forces (van der Waals, capillary, electrostatic, and chemical bonding). In microelectromechanical systems (MEMS), tribological and static interfacial forces are comparable with forces driving device motion. In this situation, macroscale lubrication and wear mitigation methods, such as the use of bulk fluids and micrometer thick coatings, are ineffective; new nano-engineering approaches must be employed for MEMS devices with moving structures. We review fundamental tribological problems related to micro- and nanoscale mechanical contacts and developments in MEMS lubrications.

Effects of adsorbed water layer structure on adhesion force of silicon oxide nanoasperity contact in humid ambient

The origin of the large relative-humidity (RH) dependence of the adhesion force in the single-asperity contact between silicon oxide surfaces is elucidated. As RH increases, the adhesion force measured with an atomic force microscopy (AFM) initially increases, reaches a maximum, and then decreases at high RH. The capillary force alone cannot explain the observed magnitude of the RH dependence. The origin of the large RH dependence is due to the presence of an icelike structured water adsorbed at the silicon oxide surface at room temperature. A solid-adsorbate-solid model is developed calculating the contributions from capillary forces, van der Waals interactions, and the rupture of an ice-ice bridge at the center of the contact region. This model illustrates how the structure, thickness, and viscoelastic behavior of the adsorbed water layer influence the adhesion force of the silicon oxide nanoasperity contact.

Experimental and Density Functional Theory Study of the Tribochemical Wear Behavior of SiO2 in Humid and Alcohol Vapor Environments

This paper investigates the reaction steps involved in tribochemical wear of SiO(2) surfaces in humid ambient conditions and the mechanism of wear prevention due to alcohol adsorption. The friction and wear behaviors of SiO(2) were tested in three distinct gaseous environments at room temperature: dry argon, argon with 50% relative humidity (RH), and argon with n-pentanol vapor pressure 50% relative to the saturation pressure (P/P(sat)). Adsorbed gas molecules have significant chemical influences on the wear of the surface. The SiO(2) surface wears more readily in humid ambient compared to the dry case; however, it does not show any measurable wear in 50% P/P(sat) n-pentanol vapor at the same nominal contact load tested in the dry and humid environments. The tribochemical wear of the SiO(2) surface can be considered the Si-O-Si bond cleavage upon reactions with the impinging vapor molecules under tribological stress. DFT calculations were used to estimate the apparent activation energy needed to cleave the Si-O-Si bond at beta-cristobalite (111) and alpha-quartz (001) surfaces by reactions with impinging water and alcohol vapor molecules. The alkoxide termination of the SiO(2) surfaces increases the energy barrier required to cleave the Si-O-Si bonds when compared to hydroxyl-terminated SiO(2) surfaces.

Average molecular orientations in the adsorbed water layers on silicon oxide in ambient conditions

The average molecular orientation in the adsorbed water layers formed on amorphous SiO(2) in ambient conditions was determined as a function of relative humidity using polarization attenuated total reflectance infrared spectroscopy (ATR-IR). The silicon oxide surface was prepared by chemically cleaning in aqueous solution, washing with water, and drying with argon. After drying, this produced a SiO(2) surface with hydroxyl groups, giving rise to a water contact angle < 5 degrees. Primarily two types of vibrational peaks that correspond to liquid water and solid-like water were observed in the adsorbed water layers formed on this surface at room temperature. The average orientation of the water molecules was determined from the dichroic ratio of s- to p-polarization absorbances. At low relative humidities, the highly hydrogen bonded solid-like structure exhibits a dichroic ratio as low as approximately 0.4, while the liquid water structure exhibits a dichroic ratio close to approximately 1.0. As the relative humidity increases, the dichroic ratio of both water structures approaches a dichroic ratio of 0.7 approximately 0.8, which is consistent with the random orientation of molecules of bulk water and ice.

Direct force balance method for atomic force microscopy lateral force calibration

A new and simple calibration method for atomic force microscopy (AFM) is developed. This nonscanning method is based on direct force balances on surfaces with known slopes. The lateral force calibration is performed during force-distance measurements for normal force calibration. This method requires a substrate with known slopes, the z motion of the piezocalibrated, and the normal spring constant known. This technique determines not only the lateral detector sensitivity (N/V) but also the detector offset (V/m) and off-centering angle (α) for asymmetric cantilever-tip geometries. Because it is nonscanning, the AFM cantilever can be calibrated without dulling the tip.

Tribochemical polymerization of adsorbed n-pentanol on SiO2 during rubbing: when does it occur and is it responsible for effective vapor phase lubrication?

The origin and role of tribochemical reaction products formed while sliding silicon oxide surfaces in the presence of adsorbed alcohol molecules in equilibrium with the vapor phase were studied. Wear and friction coefficient studies with varying contact loads and n-pentanol vapor environments were used to determine under what operating conditions the tribochemical reaction species was produced. Imaging time-of-flight secondary ion mass spectrometry and microinfrared spectroscopy found that hydrocarbon species with a molecular weight higher than the starting vapor molecules are produced when there is wear of the SiO(2) surface. When the n-pentanol vapor lubrication is effective and the silicon oxide surface does not wear, then the tribochemical polymerization products are negligible. These results imply that the tribochemical polymerization is associated with the substrate wear process occurring due to insufficient adsorbate supply or high mechanical load. The tribochemical reactions do not seem to be the primary lubrication mechanism for vapor phase lubrication of SiO(2) surfaces with alcohol, although they may lubricate the substrate momentarily upon failure of the alcohol vapor delivery to the sliding contact.

Equilibrium Vapor Adsorption and Capillary Force: Exact Laplace–Young Equation Solution and Circular Approximation Approaches

The capillary adhesion force of an asperity of radius R as a function of vapor partial pressure is calculated using exact and approximate methods assuming a continuum model. The equilibrium between the capillary meniscus at the asperity and the adsorbate film on the surface is discussed through a disjoining pressure term. It is found that the two methods agree very well over a wide partial pressure range. Without taking into account the effect of the adsorbate film, the theoretical calculation results do not show the experimental partial pressure dependence of the capillary force except near the saturation vapor condition. The experimental capillary force trend with partial pressure can be explained when the presence of the adsorbate film is included in the calculation.

Solid state water motions revealed by deuterium relaxation in 2H2O-synthesized kanemite and 2H2O-hydrated Na+-zeolite A.

Deuterium NMR relaxation experiments, low temperature deuterium NMR lineshape analysis, and FTIR spectra are consistent with a new model for solid state jump dynamics of water in (2)H(2)O-synthesized kanemite and (2)H(2)O-hydrated Na(+)-Zeolite A. Exchange occurs between two populations of water: one in which water molecules are directly coordinated to sodium ions and experience C(2) symmetry jumps of their OH bonds, and a population of interstitial water molecules outside the sodium ion coordination sphere that experience tetrahedral jumps of their OH bonds. For both samples the C(2) jump rate is much faster than the tetrahedral jump rate. (2)H NMR relaxation experiments match well with the fast exchange regime of the model over a wide range of temperatures, including room temperature and above. For hydrated Zeolite A, the kinetic activation parameters for the tetrahedral and C(2) symmetry jumps are Delta H tet++=+17 kJ/mol, Delta S tet++=-109 J/(mol K), Delta H C2++=+19 kJ/mol, and Delta S C2++=-20 J/(mol K). For kanemite, Delta H tet++ =+23 kJ/mol, Delta S tet++=-69 J/(mol K), Delta H C2++ =+23 kJ/mol, and Delta S C2++ =-11 J/(mol K).

Effects of adsorbate coverage and capillary on nano-asperity friction in atmosphere containing organic vapor

The influence of alcohol adsorption on the nano-asperity friction of silicon oxide surfaces under equilibrium conditions was studied with atomic force microscopy (AFM). In the intermediate regime of the relative partial pressure (P/Psat) of alcohol, the friction versus applied load (F-L) curve deviates from the expected DMT behavior, while the F-L curve in dry and near saturation vapor conditions follows the DMT contact mechanics. The full analysis of the observed P/Psat dependence of the F-L data with theoretical models reveals clearly that the shear stress of the contact is governed by the coverage of the adsorbed alcohol on the surface while the friction near the critical snap-off is governed by the capillary meniscus formed at the nano-asperity contact.

Corrected direct force balance method for atomic force microscopy lateral force calibration.

This paper reports corrections and improvements of the previously reported direct force balance method (DFBM) developed for lateral calibration of atomic force microscopy. The DFBM method employs the lateral force signal obtained during a force-distance measurement on a sloped surface and relates this signal to the applied load and the slope of the surface to determine the lateral calibration factor. In the original publication [Rev. Sci. Instrum. 77, 043903 (2006)], the tip-substrate contact was assumed to be pinned at the point of contact, i.e., no slip along the slope. In control experiments, the tip was found to slide along the slope during force-distance curve measurement. This paper presents the correct force balance for lateral force calibration.