Lubrication

M O O

C CH2 H2CCH2 CH2 CH2 CH2

H2C H2C H2C H3C

H2C

Figure 2.2.: Schematic structure of surfactant molecules of the carboxylic acid kind cov-ering a metal oxide surface, protecting it from wear. The acid shown here is dodecanoic acid, also called lauric acid. Carboxylic “heads”, the carboxy groups of the acid, are displayed as circles, the alkyl chain, the “tail”, is rep-resented as solid lines. M is a metal atom of the metal oxide surface, O is oxygen, C is carbon, H is hydrogen.

lipophilic and thus has favorable interactions with a standard alkyl-based lubricant like mineral or synthetic oils. The lipophilic interactions of the alkyl chains of the surfactant improve the wetting with the lipophilic oils, because a pristine metal oxide surface is oleophobic. The improved wetting in turn reduces the friction coefficient. The strong chemical binding of the surfactant makes the squeeze-out of the molecules difficult, in-creasing boundary lubrication performance of the lubricant mixture.

Dodecanoic or lauric acid (C₁₁H₂₃COOH) is a common additive and it is the parent com-pound of a surfactant creating one of the least wettable surfaces ever found: Perfluoro-dodecanoic acid (C₁₁F₂₃COOH) on a metal oxide [2]. However, is not a wettable surface optimal for the application of a lubricant? That depends on the desired application.

Regarding a bearing that has to function in the higher rotation speed regime, a closed lubricant film is vital, because a disrupted lubricant film leads to bearing failure very soon. On the other hand, the Stribeck curve of a bearing has a minimum (see figure 1.4 on page 10) in the region of mixed lubrication. This fact is utilized in hard disk drives (HDD) in a computer, where the disk head flies over the disk at immense rotation speeds, and the friction is minimal, because there is a sub-monolayer coverage of a special surfactant.

The coverage is not sufficient to span the whole disk area, resulting in the formation of lu-bricant islands. Consequently the head is subject to rapid change of physical interactions with the surface, preventing that the system reaches equilibrium. Hence the disk head maintains a distance from the disk that is close enough to scan the data but far enough to avoid frictional interaction, which would instantly destroy the HDD. Perfluorinated

Chapter 2. Contact 2.3. Lubrication

CF₂ CF₂CF₃ CF₃

F CF O n

Figure 2.3.: Chemical structure of a perfluoro-polyether of the DuPont Krytox^® family.

C is carbon, F is fluorine, O is oxygen, n is the number of repeating units.

lubricants, regardless of the application as surfactant or as lubricant molecules, reduce the wettability of the surfaces with water- or oil-based lubricants, because perfluorinated molecules are even autophob and do not interact favorably with other perfluorinated molecules. In addition to that, the friction on perfluorinated molecules is reduced, which led to the development of so-called perfluorinated self-assembled monolayers (SAMs).

SAMs spontaneously form once a solution of the surfactant molecules is poured on top of a surface, while the resulting monolayer exhibits a structure that is in principle identical to the surfactant scheme shown in figure 2.2. In contrast to the formation of a SAM, a monolayer coverage of surfactant is usually achieved by employing special dipping tech-niques, which are also used to get selective coverages of more than one monolayer. The surfactant molecules are spread out over a fluid the surfactant is immiscible with, and then the surface is dipped into the basin slowly. Resultantly the surfactant layer is put over the surface like a sheath, for more than one monolayer the process is repeated.

The chemical interaction of a surfactant with the underlying material is resilient enough to persist under normal shearing conditions. However, as has been shown by molecular dynamics (MD) simulations of perfluorinated SAMs under shearing condition by Lorenz et al. [21], by the external lateral force the structure of the monolayers does not only get tilted: After a short period, the alkyl chains entangle, which will result in SAM wear at first, but finally the coating by the surfactant molecules will be destroyed, leading to wear of the hence as pristine surface. This disadvantage of SAMs regarding high load situations regularly occurring in a bearing may be overcome by an even more resilient, perfluorinated coating of the bearing surface, which will be discussed in the section

“Plasma-Enhanced Chemical Vapor Deposition” (section 3.3).

In addition to the stable coverage of the surface by surfactant molecules and SAMs, perfluorinated substances also serve as lubricant molecules that almost do not interact with the surfaces at all. Hence these molecules do not mitigate interaction of the surfaces, which e.g. is the case of water molecules between hydrophilic and even protic surfaces.

Between laterally sliding asperities the formation of hydrogen bonds within the water layer and between the water molecules and the surfaces leads to the formation of capillary bridges [2]. These capillary bridges increase the cohesion of the asperities and thus increase the friction. Furthermore perfluorinated molecules offer an increased chemical stability up to inertness compared to alkyl-based lubricants, which is the reason, why

dry state (a) lubricated state (b) trapped

lubricant

Figure 2.4.: Deformation of a ball on a flat in the dry state (a) and in the lubricated state (b).

the U.S. Food and Drug Administration (FDA) officially allowed a special lubricant class for direct food contact: Perfluorinated Polyethers (PFPE), like e.g the PFPE high performance lubricant, whose chemical formula is shown in figure 2.3. Resulting from the ether bridges, their molecular structure renders PFPEs flexible, yet PFPEs are chemically inert, because of the perfluorinated alkyl segments. Perfluorinated alkyl segments are highly stable because of the very strong carbon-fluorine bond on the one hand, and the high electron density at the fluorine atoms on the other. The high electron density at the fluorine atoms in connection with the rather short carbon-fluorine bonds protects the carbon atoms from nucleophilic attacks. In addition to the chemical inertness, PFPE molecules are advantageous for lubrication because PFPEs are longer chain polymers.

The technical data sheet of the Krytox^® family lubricants states that the number of repeating unitsn starts from ten for low viscosity lubricants and ranges up to 60. The longer the chain of a lubricant molecule the more difficult the molecules are to squeeze-out, as will be discussed in the next section. Molecules difficult to squeeze out are high-performance boundary lubricants.