6 Lubrication Transformation and Nanoscale Thin Film Lubrication
6.2.2 Time Effect of Thin Film Lubrication
The authors observed that the lubricant film thickness gradually rises to a stable value after a continuous shearing process in thin film lubrication conditions [3, 11]. The thixotropic property of a lubricant cannot be used to explain the time effect of thin film lubrication, because the thixotropic effect gradually reduces the thickness to achieve stability.
The experimental results show that the time effect has relationships with the load, the entrain- ment velocity and the viscosity of the lubricant.
Figure 6.10 shows that under the conditions of the steel ball diameterd=20 mm, temperature T=27∘C, entrainment velocityu=4.49 mm/s, loadW=4 N and 7 N respectively, the central lubrication film thickness of liquid paraffin varies with the run-time. By comparison, it can be seen that when the load is 4 N and other parameters remain unchanged, the film thickness increases about 6 nm after 80 min of continuous running. The thickness is 7 nm after stopping, so it increases 2 nm more than that at the beginning. When the load is increased to 7 N, the film thickness increment is more than 13 nm after 70 min running and 7 nm after stopping. Thus, the load increases the film thickness increment.
Further experimental studies showed that the time effect of the entrainment velocity on the lubricant film thickness is more complex. The lower the entrainment velocity, the stronger the
k k Figure 6.10 Film thickness vs. running time.
Figure 6.11 Time effect on three lubricants.
time effect. However, the static contact film thickness does not change with time so there is no time effect, nor is there any time effect with high entrainment velocity. Therefore, we can conclude that the time effect only exists at a certain range of velocity.
Figure 6.11 gives the time effect of different lubricants, with the loadW=4 N, the entrain- ment velocityu=3.12 mm/s, the steel ball diameterd=20 mm, the central film thicknesses of 10#, 30# and 40# mechanical oils, the continuous operation timet=70 min. The film thickness of 10# rises significantly, but that of 40# hardly rises at all.
Luo Jianbin summarized a large number of experimental results and concluded that the rela- tionship between the load and the time effect is as shown in Figure 6.12 and the relationship between the velocity times viscosity and the time-effect is as shown in Figure 6.13 [9].
This study showed that in a certain velocity region, the time-effect is stronger for small vis- cosity and large load. That means that the lubricant film thickness will increase as the running time increases. However, according to the theory of hydrodynamic lubrication, these changes of the working condition parameters for viscous fluid are unfavorable factors in reducing the film thickness. So the film-forming mechanism of the thin film lubrication is different from that of hydrodynamic lubrication because the main factors are surface energy and structure of lubricant film molecular ordering.
Experiments show that during operation the increase in thin film thickness slowly drops after the thickness rise to about 30 nm. This thickness stabilizes, and is related to the surface force range. This shows that the time effect is related to the action of surface energy in the friction interface.
k k Figure 6.12 Time effect under different loads.
Figure 6.13 Time effect with differentu𝜂0.
The thin lubricant film is confined in a narrow gap between the friction surfaces. Because of shearing of molecules, the lubricant film will be restructured. First, the liquid molecules near the surface are arranged vertically to form an adsorption film. Then, the adsorption film passes the force and the adsorption potential to the neighboring molecules, gradually in an orderly arrangement. As the running time increases, the orderly arrangement of the molecules increases, so that the film thickness increases until it reaches the range where the surface force is balanced. Then, the ordered film thickness becomes stable.
The above conclusions can be verified by the research of Alsten and Granick (1990), and Thompson et al. [12]. They used molecular dynamics to simulate the shearing process of molecular behaviors of thin film lubrication, and found that the structure of the original phase will be broken, so the molecule phase will be transited or recrystallized. The spherical molecules being squeezed between the two surfaces are much faster than the unconstrained crystallization of molecules. The chain-like molecules in the liquid state are much longer than the spherical molecules, and it is much easier to form an ordered arrangement. The simulation also showed that high pressure contributes to the occurrence of the phase transition.
The above analysis shows that in the thin film lubrication state, the time effect changes with the molecular structures. With decrease of the lubricant viscosity, increase of the load and decrease of the velocity, the time effect will be strengthened such that the film thickness increases with shearing time and then stabilizes at a certain value. Furthermore, in the static contact, the time effect does not appear, and the time effect is connected with the history of shearing.