Lubricant Additives

Organic Friction Modifi ers 201

mild conditions (temperature and load) of the ML regime. These conditions require a fairly high
level of chemical activity as refl ected by the phosphorus and sulfur chemistry applied.
An exception to this is stearic acid. Theoretically, the friction-reducing effect of stearic acid
should decrease with increasing temperature due to desorption of the molecule from the metal sur-
face. However, stearic acid experimentally shows a remarkable drop of friction with increasing tem-
perature, which can only be explained by the formation of chemically reacted protective layers.

7.4.1.2 Formation of Absorbed Layers

The formation of absorbed layers occurs due to the polar nature of the molecules. FMs dissolved in
oil are attracted to metal surfaces by strong absorption forces, which can be as high as 13 kcal/mol.
The polar head is anchored to the metal surface, and the hydrocarbon tail is left solubilized in the
oil, perpendicular to the metal surface (see Figure 7.4). Next the following steps occur:

1. Other FM molecules have their polar heads attracted to one another by hydrogen bonding
   and Debye orientation forces, resulting in dimer clusters. Forces are ∼15 kcal/mol.


2. Van der Waals forces cause the molecules to align themselves such that they form multi-
   molecular clusters that are parallel to one another.


3. The orienting fi eld of the absorbed layer induces further clusters to position themselves
   with their methyl groups stacking onto the methyl groups of the tails of the absorbed
   monolayer [17,18].

As a result, all molecules line up, straight, perpendicular to the metal surface, leading to a multi-
layer matrix of FM molecules (see Figure 7.5).
The FM layers are diffi cult to compress but very easy to shear at the hydrocarbon tail interfaces,
explaining the friction-reducing properties of FMs. Owing to the strong orienting forces, mentioned
earlier, sheared-off layers are quite easily rebuilt to their original state.
The thickness and effectiveness of the absorbed FM fi lms depend on several parameters, four
of which are explained here.

1. Polar group. Polarity itself is not necessarily suffi cient for adsorption; the polar group
   must also have hydrogen-bonding capability. Molecules with highly polar functional
   groups that are not capable of forming hydrogen bonds, such as nitroparaffi ns, do not adsorb.

Van der Waals

forces

Van der Waals

forces

Adhesive

hydrogen bonding

Long, nonpolar chains

Polar heads

Dipole−dipole

interactions

Oxidized and hydroxylated metal surface

FIGURE 7.4 Organic FMs—formation of adsorbed layers.

FIGURE 7.5 Multilayer

matrix of FM molecules.

Metal surface

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H H H H H = Polar head

T T T T T T = Hydrocarbon tail

T T T T T T

H H H H H

H H H H H

T T T T T T

OIL OIL OIL

OIL OIL OIL

OIL OIL OIL

T T T T T T

H H H H H

H H H H H

T T T T T T

T T T T T T

H H H H H

////////////////////

Metal surface