Rubbed surface measurements

Similar results were reported by other investigators, [19,20], but attention was paid to investigating the effect of lubricant additives on the boundary film thickness. It is speculated that there should be no adsorbed layers formed on rubbing surfaces if purified and nonpolar lubricants are applied. The interferometer measurements show that in the case of using base oils, the relation between the film thickness and rolling speed follows the EHL power law pretty well down to 1 nm (Fig. 6(a)), or sometimes the film thickness may deviate from the Hamrock and Dowson s line and turn down quickly (Fig. 6(b)). If there is a small percentage of additives in the lubricant, on the other hand, the deviation from the power law occurs in a different way that the h-V... 

Fig. 4.3. (P) L-edge XANES spectra of ZDDP tribofilms generated at different rubbing times measured by TEY (surface) mode (left) and FY (bulk) mode (right). (A, F) 5 min, (B, G) 50 min, (C, H) 6 hr, (D, I) 12 hr, and (E, J) 0.5 hr with ZDDP+ 5.5 hr rubbing without ZDDP or the bridging oxygen (BO). ZDDP consists of the secondary iso-butyl (85%) and n-octyl (15%) groups (Yin et al., 1997a)...

Catalytic activity of rubbing surfaces (a) By reference to Table 5.5, find the metal hydroxides, oxides and nitrides that illustrate their highest exoelectrons emission intensity (I, cps), (b) Calculate the tribochemical energy (TribEn) TribEn = (AHf - WF) for the listed compounds and correlate them with measured exoelectrons emission intensity (I, cps). Explain differences. 

Figure 15-11. Measurement of "hot spot" temperatures at the rubbing surface with a lead sulfide photocell. A Glass disk. B Metal slider. C PbS photocell. D Brass shield. E Chopper. After Bowden and Thomas [27].

The described behavior of wear particles complicates the interpretation of electrochemical tribocorrosion experiments. In principle, the measurement of anodic current or charge permits one to identify the contribution of corrosion to the overall material loss from the rubbing metal surface. However, the electrochemical oxidation of metallic third body particles that are in electrical contact with the rubbing surface also contributes to the anodic current, although these particles were formed by mechanical wear [14]. To distinguish between electrochemical and mechanical metal removal mechanisms in tribocorrosion one therefore must take into account the behavior of third body particles. 

To observe chemical changes in the rubbed surface, Lee and Paek measured the polarity of a rubbed surface by using water contact angle measurements, and discussed a correlation between the pretilt angle and the polarity of the rubbed surface [44,62]. The measurement method is illustrated in Fig. 2.2. 

The second difficulty results from the non-uniform state of the rubbed surface. Behind the slider the sample surface can be laid bare for some time before some new surface layers are rebuilt. The restored surface increases gradually with the distance behind the slider along the sliding track. Even if the first difficulty was already solved and the overall impedance of the rubbed surface is obtained, it can not be used as such to characterize the non-uniform distribution of the electrochemical states behind the slider. However, it is expected that impedance measurement procedures already developed for analyzing non-uniform distributions of surface states and the electrochemical models developed for the interpretation of such measurements (Zhang et al., 1987) could be transferred to tribocorrrosion test conditions. Such a study could allow a localized characterization of dissolution and pessivation kinetics. 

The CSS friction properties of three types of lubricants, i.e., ester (Lubricant 1), amide (Lubricant 3), and carboxylic acid ammonium salt (Lubricant 5) are shown in Fig.9. These friction measurements of the synthesized lubricants revealed that the ester and the amide are far less durable than the comparable salt type. For the ester and amide lubricants, the friction coefficients ( p ) were around 0.25 for the first ten CSS operations, but rose with the increasing number of CSS operations ( n ). Especially, for the ester lubricant, i steeply increased after 20 operations, and the carbon protective layer got damaged when the p became over 0.90 at n = 138. For the amide lubricant, the slope of Ap per An decreased compared to the ester, however, a wear scar occurred at n = 3279. For the carboxylic add ammonium salt lubricant, the p value remained nearly constant at around 0.25 throughout the 104 CSS operations and the medium was scarcely damaged. The low initial value of p, 0.2-0.3, indicates that there is sufficient lubricant film to protect the rubbing surface. 

The formation of the lubricant film is a spontaneous pa-ooess caused by a decrease in the free energy of the solid surfaces and lubricant molecule adsorption. The heat of adsorption of lubricants on a rubbing surface can be taken as a measure of the strength of attachment of the lubricant molecules to the surfaces and are also shown in Table 6. 

Oils may act in two ways they provide a measure of boundary lubrication and they may exclude oxygen from the rubbing zone. However, their effectiveness is not as great as with unidirectional sliding, since there is usually sufficient oxygen present in most cases to allow oxide debris to be generated, and this tends to displace any lubricant film that was initially present between the surfaces. 

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