Frictional coefficient films

Boundary lubrication is perhaps best defined as the lubrication of surfaces by fluid films so thin that the friction coefficient is affected by both the type of lubricant and the nature of the surface, and is largely independent of viscosity. A fluid lubricant introduced between two surfaces may spread to a microscopically thin film that reduces the sliding friction between the surfaces. The peaks of the high spots may touch, but interlocking occurs only to a limited extent and frictional resistance will be relatively low. 

The concentration of UDP also affects the friction coefficient as shown in Fig. 39. It is discovered that the friction coefficient of pure PEG also decreases gradually and reaches a somewhat reduced value due to a time effect of the film thickness [16,18]. At the speed of 2 mm/s and pressure of 174 GPa, the friction coefficient of pure PEG is the highest. That for PEG + 0.5 % UDP ranks second. Those for PEG + 0.1 % UDP and PEG+ 0.3 %UDP are almost the same and have the lowest friction coefficient among all tested oils. Therefore, there is a good concentration extent of UDP in the basic oil. If the concentration is out of such extent, the effect... 

In the past decade, effects of an EEF on the properties of lubrication and wear have attracted significant attention. Many experimental results indicate that the friction coefficient changes with the intensity of the EEF on tribo-pairs. These phenomena are thought to be that the EEF can enhance the electrochemical reaction between lubricants and the surfaces of tribo-pairs, change the tropism of polar lubricant molecules, or help the formation of ordered lubricant molecular layers [51,73-77]. An instrument for measuring lubricant film thickness with a technique of the relative optical interference intensity (ROII) has been developed by Luo et al. [4,48,51,78] to capture such real-time interference fringes and to study the phenomenon when an EEF is applied, which is helpful to the understanding of the mechanism of thin film lubrication under the action of the EEF. 

Phenomenal studies were made to observe the frictional behavior of L-B films and SAMs and its dependence on applied load and sliding velocity, which has been summarized in a review article by Zhang [33]. It has been confirmed that in comparison to the bare surface of the substrates, the friction on molecular films is significantly reduced, with friction coefficients in a range of 0.05-0.1. Friction forces are found... 

Soft metals such as In, Ag, Sn, Pb, and Au can lead to reasonably low friction coefficients, when used as solid lubricants, due to their low shear strength. The metals were generally applied as thin films prepared by the vacuum deposition process. Especially, in applications to the high temperature conditions where liquid lubricants fail due to the evaporation, the thin films of soft metals can provide effective protection to the surfaces in sliding. 

The process of transition from hydrodynamic to boundary lubrication can be described qualitatively by plotting the measured friction coefficients against film thickness, which depends on the operational conditions, such as load, sliding velocity and lubricant viscosity. A typical diagram known as the "Stribeck Curve is schematically shown in Fig. 27, in which the friction coefficients are given as a function of, ... 

At the end of last century, a near frictionless carbon (NFC) coating was reported, which is practically hydrogen contained DLC film grown on steel and sapphire substrates using a plasma enhanced chemical vapor deposition (PECVD) system [50]. By using a ball on a disk tribo-meter, a super low friction coefficient of 0.001-0.003 between the films coated on both the ball and the disk was achieved [50]. A mechanistic model was proposed that carbon atoms on the surface are partially di-hydrogenated, resulting in the chemical inertness of the surface. Consequently, adhesive interaction becomes weak and super low friction is achieved [22],... 

Table 2 lists the results of the friction coefficient derived by different researchers under different test conditions [71,73,75-80]. From the table, it can be seen that the CNx films prepared by different techniques did not demonstrate surprising frictional behavior. 

TABLE 2—Friction coefficient of CNx films—Data selected from published articles. 

Oguri, K. and Ariai, T., Friction Coefficient of Si-C, Ti-C and Ge-C Coatings with Excess Carbon Formed by Plasma-assisted Chemical Vapour Deposition, Thin Solid Films, Vol. 208,1992, pp. 158-160. 

For Au film, because it is soft and has lower shear strength than silicon, it is normal that Au film has a higher micro friction coefficient factor than silicon, and can be easily worn under the current experimental condition. 

Because PTFE has low adhesion, high lubricity, and low friction coefficient [14], it is not difficult to understand PTFE film has a lower friction coefficient factor and higher wear rate than PTFE/Si3N4 multilayers under the current experi-... 

Fig. 46—Average friction coefficient (from the 15th second to the 35th second) of the muitiiayers versus thickness of Fe-N layers. The average friction coefficient of Fe-N, Ti-N film and Si (111) wafer under the same condition is aiso shown in this figure. Load, 250 N. (a) Fe-N(40 nm)/Ti-N(2 nm) (b) Fe-N film, 450 nm (c) Ti-N fiim, 450 nm (d) Si(111) wafer.

The friction coefficient of the Fe-N/Ti-N multilayers increases with the thickness of the Fe-N layer if the thickness of the Fe-N layer is more than 10 nm. It is lower than that of Fe-N film. 

The friction and wear properties of fullerene LB fihns have been investigated. The coefficient of kinetic friction was measured using a steel ball-on-glass disk method, with the LB films deposited onto the glass disk [326,327]. The friction coefficient dropped from 0.8... 

The work on carbon nitride solids is strongly related to research on diamondlike carbon (DLC) materials [5, 6]. DLC materials are thin film amorphous metastable carbon-based solids, pure or alloyed with hydrogen, which have properties similar to that of crystalline diamond (high hardness, low friction coefficient, high resistance to wear and chemical attack). This resemblance to diamond is due to the DLC structure, which is characterized by a high fraction of highly cross-linked sp -hybridized carbon atoms. To obtain this diamond-like structure... 

Relatively few works considered the tribological properties of plasma-deposited a-C(N) H films. First, considering the coefficient of friction, nitrogen incorporation was not found to strongly modify it. Prioli et al. [93] studied the friction coefficient of a-C(N) H films against a Si3N4 probe in an atomic force microscope, in ambient air. Nitrogen incorporation up to about 10 at.% into the... 

Dekempeneer et al. [96] studied the wear behavior of a-C(N) H films deposited by RFPECVD in CH4-N2 atmospheres, with up to 13-at.% N. The wear tests were done in a ball-on-disk tribometer, under air, at fixed 50% relative humidity. The initial friction coefficient was about 0.2 for all samples, and the wear life of the... 

Figure 3 shows the friction coefficient as a function of sliding time for 2 N load in either ambient air or 10-torr H.for the DLC coating. The Argonne coating was deposited on top of a Cr bond layer, using plasma-enhanced chemical-vapor deposition with 25% CH4/ 75% H, feed gases at room temperature, to a film thickness of 1-2 pm. H content is expected to be approximately 39 at.% with a hardness of about 14,000 Hv. 

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