Tribological experiments

TRIBOLOGICAL EXPERIMENTS APPLIED TO TUNGSTEN CHEMICAL MECHANICAL POLISHING... 

Our tribological experiments with the liquid crystals can be summarized as follows. LCl, which is in the nematic phase at the room temperature was subjected to a traction test. The traction of this sample is almost half of a normal liquid lubricant under the same conditions. Note that the traction did not change with die increasing speed after the initial sharp drop. These results are consistent with a quasi-solid lubricant shearing between the bounding surfaces. It should be mentioned here that a surface active material was put on the surface of the steel ball before the LCl was inserted. It was used to prevent the liquid crystal from slipping on the smooth steel surface. In any case, the traction data were unusual and would put liquid crystals of this type into a special class of lubricants. 

In the present study, we used this ATR FT-IR approach with tribological experiments to probe the lubif cant layer between sliding surfaces by measuring ATR spectra from the underside of a thin metallic film. In this way, the metal surface/lubricant interface can be continuously monitored, in situ, and in a nondestructive manner during a complete tribological sliding experiment. 

Zinc dialkyl dithiophosphates (ZnDTPs) are widely used as extreme pressure and antiwear additives in many different kinds of engine and industrial lubricants. It is known that ZnDTP forms tribological films on rubbing metal surfaces it has been proposed that these films consist of amorphous polyphosphates, but the exact chemical composition of the different polyphosphates in the ZnDTP tribofilm is not known, and a generally accepted reaction mechanism has not emerged to date. Most authors believe that thermal decomposition is the major mechanism of ZnDTP tribofilm formation as a result only tribological experiments conducted at elevated temperatures (60-200 °C) are, typically, reported in the literature. 22 All the evidence obtained so far for substantiating the amorphous polyphosphate model has been based on ex situ experiments. 

Materials. The lubricant additive used was a commercial secondary ZnDTP (Hitec 7169, Ethyl Petroleum Additives International, England), purified by hquid chromatography. The composition of the purified secondary ZnDTP has been checked by both elemental analysis and XPS quantitative analysis, and found to correspond to the molecular formula C18H40G4P2S4-Zn, suggesting amixture of diisopropyl ZnDTP and hejQrlZnDTP. The thermal and tribological experiments reported in this work were carried out in the presence of the pure ZnDTP. 

Spectra obtained with the in situ ATR FT-IR tribometer allow changes in the lubricant chemistry to be monitored upon adsorption of additives on iron, as well as the growth of reaction films to be followed as a function of sliding time and temperature in tribological experiments. To allow a correct interpretation, the spectra obtained in the in situ ATR FT-IR tribological apparatus were compared to traditional transmission spectra. 

In situ ATR thermal and tribological experiments have been performed in the presence of a pure secondary ZnDTP at room temperature, 80 °C, and 150 °C. No thermal or tribochemical reactions of ZnDTP with iron were detected during simple thermal tests at room temperature and at 80 °C. 

BSF was removed from the knee joints of cows no older than 18 months at the Hinwil slaughterhouse in Switzerland. All samples were clear, yellow, particle-free liquids, and were stabilized with protease inhibitor (Sigma-Aldrich) and 0.1% sodium azide. BSF was mixed with labeled BSA solution (5 mg/mL) to give a final total protein concentration of 12.5 mg/mL. The BSA solutions used in the tribological experiments were produced by mixing BSA in PBS (20 mg/mL) with the labeled BSA solution (5 mg/mL) to give a final total protein concentration of 12.5 mg/mL. 

As commercial products will be used in the tribological experiment, a number of physicochemical tests were conducted to confirm the presence of various types of solutions. They will form the basis for interpretation of the tribological test results. 

A more sensible way for a tribological experiment, is to routinely perform a friction loop (Figure 3) on the reference surface. If the magnitude of the friction loop remains constant at any stage of the experiment, this means that even if any molecules or polymers have been stuck on the apex there are only few and unable to produce a significant additional cantilever deflection within the range of the available sensitivity. 

Tribological Experiments. For tribological measurements, the lateral force lying in the plan of contact between the tip and the surface produces the additional vertical deflection (Figure 2b) or cantilever torsion. Lateral forces arise not only from friction but also from the local surface slope (31, 51). If the sample surface is not flat, the surface normal force will have a component directed laterally and will result in contrast in lateral image. These complications can be avoided by using flat samples whenever possible. 

Three important and related technical points remain which are not standardized but central for tribological experiments they are related to the optical detection method used for most of the AFM. First, the laser beam must be centered on the cantilever end. The centring can easily be checked by scanning parallel and perpendicular to the cantilever symmetry axis. Secondly, for each apparatus, the four photodiodes quadrants orientation must be carefully checked (one method is described in fig lb). Lastly, one has to work within the linear response domain of the photodiodes, which implies a limited variation domain for the vertical movement of the sample when the vertical difference from the photodiodes signal is used. This constraint can be detoured by using horizontal difference signal, see ref (28,52). 

For the tribological experiments, the procedure is robust the same quantitative results can be obtained with other tip-CL systems, each i value results from several hundred measurements. Nevertheless, the cost to pay is the heavy technical constraint. 

The stability of such aggregates was first pointed out by QELS experiments that reveal the absence of CMC [30]. This was confirmed by dialysis experiments through ultrathin latex membranes used to purify these nano-sized particles before their study by TEM, AFM and tribological experiments. Eor the dialysis experiment a Soxhlet apparatus (Figure 4.8) is used where the dispersion of overbased particles in mineral oil is introduced in a latex socket. Light petroleum is introduced in the boiler of the system as the extraction solvent (boiling point of 36 °C). 

Mantsot, J.L., Golabkan, V., Romana, L., and Cesaire, Th. Chemical and physical characterization by EELS of strontium hexanoate reverse micelles and strontium carbonate nanophase produced during tribological experiments, Journal, of Microscopy, 210, 2003, 110. 

Figure 13. An example of i i situ pre mortem analysis in a tribological experiment. (From Ref. 31, with permission.)...

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