Ping Huang

The Effect of the Electric Double Layer on a Very Thin Water Lubricating Film

This paper discusses the effect of the electric double layer on a very thin water lubricating film with and without consideration of the elastic deformation of the opposing surfaces. A modified Reynolds equation that considers the electric double layer is used in a numerical analysis. The effect of zeta potential on the film thickness and pressure is numerically calculated. For both hydrodynamic and elastohydrodynamic cases, the electric double layer significantly increases the lubricating film thickness. The pressure is also marginally increased, as illustrated in the hydrodynamic analysis. However, the effect on pressure is almost unnoticeable in the elastohydrodynamic analysis. Overall, the electric-double-layer effect is only significant for a water-film thickness of less than approximately 100 nanometers.

Effect of geometry change of rough point contact due to lubricated sliding wear on lubrication

The geometry change of a single asperity due to lubricated wear was studied by an experimental simulation with a ball‐on‐disc set up. The wear leads to the formation of a tilted section at the tip of the ball, which is proved to be due to the presence of oil during the process. The effect of the geometry change of rough surface contacts due to wear was examined by a micro‐EHL analysis. A non‐Newtonian visco‐plastic fluid model which includes the effect of a limiting shear strength was used.

Logarithmic decrease of adhesion force with lateral dynamic revealed by an AFM cantilever at different humidities

ABSTRACT Adhesion forces between a tipless cantilever and an Au film were determined to investigate the influence of lateral velocity by recording force curves with an atomic force microscope at 20%–90% relative humidities. The sample was moved laterally, forth and back, with a frequency of 0.001–100 Hz and scan distances of 0.8, 8, and 80 μm to achieve a velocity ranging over 7 orders of magnitude. Experimental results show that at low lateral velocities (between 1.6 nm/s and 1–10 μm/s), the adhesion force either increases or decreases or remains stable with the lateral velocity without a certain characteristic trend. However, after a critical velocity, the adhesion force decreases logarithmically with the lateral velocity (between 1–10 and 16,000 μm/s). The decreasing magnitude can be as large as 97.3% of the maximum adhesion force. This decrease is well-explained by the contact time dependence of water bridges formed by capillary condensation.

Experimental and Numerical Analysis of Soft Elastohydrodynamic Lubrication in Line Contact

This paper describes an experimental setup that allows the observations of the stress distribution under soft elastohydrodynamic lubrication (EHL) in line contact by means of photoelasticity. This experimental method is rarely used in the EHL field. The photoelastic fringe patterns in the circular polarization are collected under different rolling speeds and loads. Numerical calculations are carried out to analyze the oil film thickness, pressure, and shear stress distribution. Compared with those results, numerical and experimental stress distribution are basically consistent. Some beneficial conclusions are obtained. These are helpful to investigate further thexa0oil pressure distribution and directly observe the stress distribution under lubrication conditions through experiment method.

The Effect of the Electric Double Layer on Very Thin Thermal Elastohydrodynamic Lubricating Film

The effect of the electric double layer (EDL) of friction surface on lubrication is significant under the condition of very thin lubricating film. This article presents a theoretical evaluation concerning the effect of the EDL on the film thickness and the pressure distribution of the elastohydrodynamic lubricating water film. These numerical analyses are based on the modified Reynolds equation that considers the EDL. Owing to the temperature risen readily in elastohydrodynamic lubrication (EHL) contact area, the influence of the temperature rise on the EDL effect was also investigated. The analysis results show that the EDL leads to a noticeable increase in the film thickness but has few influences on the pressure. Further, the analytical comparisons between isothermal and thermal conditions reveal that the temperature rise in the contact area weakens the effect of the EDL on the EHL film. Overall, consideration of the EDL effect gives a thicker EHL film, but once the temperature rise in the EHL regime is taken into account, the film thickness is correspondingly decreased.

Evolution and level behavior of adhesion force by repeated contacts of an AFM colloid probe in dry environment

ABSTRACT Adhesion forces between a colloid probe and some samples were consecutively measured at a single location with an atomic force microscope (AFM) in a dry environment. The outcomes show that the adhesion force depends considerably on material, contact history, and number and distribution of asperities within the contact zone. Generally, there are four different stages for the adhesion force with increasing measurement number: random behavior for the first several or tens of contacts, then increasing monotonically, later remaining stable, and finally decreasing. Usually, the measured data points are grouped into several levels. Moreover, the adhesion force jumps frequently between different levels and is more inclined to jump to a neighboring level. The level behavior was attributed to the fact that the real contact region is not exactly the same between successive measurements. The differences in the adhesion force between two neighboring levels are almost the same for one location. The adhesion force in some levels usually increases or decreases discontinuously after jumping to other levels and then jumping back. The magnitude and fluctuations of the adhesion force and the number of levels depend on the number and distribution of asperities in the contact zone.

Influence of electric double layers on elastohydrodynamic lubricating water film in line contact

The influence of the electric double layers of two friction surfaces on nanoscale lubrication is significant. Based on the previous mathematical model of electro-viscosity considering the asymmetrical electric double layers, and by using a viscosity–pressure model of water, a theoretical evaluation concerning the effect of the electric double layers on the film thickness and the pressure distribution of the elastohydrodynamic lubricating water film of the line contact is carried out. The analysis results show that the electric double layers lead to a noticeable increase in the film thickness, but the influence of the electric double layers on the pressure distribution is minimal.

Instantaneous Plane Stress Observation and Numerical Simulation During Wear in an Initial Line Contact

The photoelastic technique was applied to capture in situ plane stress in a block-on-cylinder contact. This method allows researchers to qualitatively visualize the distribution of principal stress difference during wear processes. Additionally, numerical simulation of the evolution of wear was conducted. The conjugate gradient method together with fast Fourier transform was employed to solve the pressure distribution. Based on the computed pressure, the block wear was solved with Archard’s wear law. The subsurface stress was computed using the influence coefficient method. The simulated results of wear and stress were compared with the experimental results; the similarities and differences between the results highlight the usefulness and limitations of the numerical method. However, the photoelastic technique together with the numerical method casts light on the nature of wear.

Effect of water thin film on the adhesion force between two silica surfaces using AFM

ABSTRACT The adhesion force between two silica surfaces was measured with different experimental parameters to investigate the effect of water thin film adsorbed on the surfaces using atomic force microscopes (AFM) at various relative humidities (RH). Results show that the adhesion force behavior largely depends on a water film. Under extremely dry condition, the adhesion force is independent of contact time without a water film. With a small (large) film thickness, the adhesion force is time-dependent (time-independent) and decreases (increases) with retraction velocity, and vice versa. For repeated contacts at one location, the time-dependent adhesion force shows larger fluctuations than the time-independent one. There may be a threshold thickness of water film to distinguish time-dependent and time-independent ones. The thickness can be changed to reach a certain value via RH adjustment, with which the adhesion force is almost independent of retraction velocity, and which may be very close to the threshold value. The adhesion force behavior by adjusting RH is closely related to water film thickness instead of RH itself, since the equilibrium of vapor-film interface may be difficult to reach. These results may help facilitate the anti-adhesion design of small-scale silicon-based systems under different conditions.

An Improved Interference Method for Measuring Lubricant Film Thickness Using Monochromatic Light

Optical interferometry is one of the most important methods used to measure lubricant film thickness. Based on the principle of relative optical interference intensity and using monochromatic light, this paper proposes an improved interference method to measure lubricant film thickness. First, a universal formula for calculating lubricant film thickness is deduced according to the basic principle of relative optical interference intensity. Then, based on the actual curve describing the relationship between the light intensity and film thickness, the accuracy of the interference method for monochromatic light is improved. The methods used to calibrate the inference order are also discussed. This paper uses the proposed method to measure the lubricant film thickness of base oil. The proposed method was validated through measurements and compared to the calculation results from the Hamrock–Dowson formula and another measurement method. Finally, a fitting formula for calculating the film thickness was derived for the light loading and high-velocity conditions.