7 Boundary Lubrication and Additives
7.1.2 Adsorption Films and Their Lubrication Mechanisms
Lubricating oils usually contain some polar substances, such as 1–2% fatty acids (CnH2n+1+COOH) as shown in Figure 7.2a. The end with -COOH is called a polar group. A fatty acid is of a long-chain structure. The straight lines are used to represent the non-polar molecules, and the circles represent the polar groups, as shown in Figure 7.2b. Relying on the Van der Waals force between molecules or atoms, polar groups can be firmly adsorbed on the metal surface to form a monolayer or multi-molecular directional and orderly film.
k k Figure 7.2 Polar molecular structure of fatty acid and adsorption film structure.
Such an adsorption is referred to as the physical adsorption. Except for some individual peaks of roughness, the adsorption film separates the two friction surfaces, providing a low shear resistance interface so that its friction coefficient is low and the metal surface adhesion can be avoided. Although the long-chain hydrocarbon has a physical adsorption capacity, its physical adsorption film is relatively weak. Furthermore, the physical adsorption film can be reversibly desorbed.
As shown in Figure 7.2b, an adsorption film usually consists of three or four molecular layers.
Each molecular layer is closely arranged by the cohesion force between molecules so that the molecular film has a load-carrying capacity to be able to separate the two friction surfaces.
Friction occurs between neighboring adsorption layers during sliding.
When the temperature is high, the polar molecules adsorbed on the metal surface may form metal soap films, such as CnH2n+1+COOM. They are also polar molecules to be adsorbed on the metal surface through chemical combinations, called chemical adsorption. The metal ions in a chemical adsorption film do not stray from the original metal lattice, and the lubricant molecules are also retained in their original physical properties. A metal soap film is of a high melting point and has thermal stability. A chemical adsorption film is not reversible, and its friction coefficient is low. Compared with the physical adsorption film, the chemical adsorption film can work under heavy load, high speed and high temperature conditions.
The number of adsorbed molecules per unit area is called the adsorption ratio, which is a measure of the adsorption capacity. The adsorption ratio increases with increase of the concen- tration or the adsorbed polar molecule number inside the base fluid. The maximum adsorption capacity is called the saturation quantity. Figure 7.3 gives adsorption curves of several polar
Figure 7.3 Adsorption curves of some polar molecules. I: stearic acid; II: palmitic acid; III: laurate; IV:
octadecanol; V: stearic acid acetate.
k k molecules. It can be seen that the highest adsorption capacity is the stearic acid, and the lowest
is the stearic acid acetate. It can also be seen that a turning point can be found in the curve of the octadecanol. It is generally believed that the point is caused by phase transformation.
Experimental results show that at the beginning, adsorption is quick. For example, in the first five minutes, about 90% polar molecules may have been adsorbed. However, it will take a very long time to reach saturation. This is because the adsorption capacity decreases with increase of the distance from the metal surface. The adsorption of the second layer mainly depends on the adsorption force of the first layer. At saturation, the chemical adsorption film has good lubrication properties. For example, its friction coefficient stays low and stable. Therefore, in order to get good lubrication, the adsorption film should have enough adsorbed layers, as shown in Table 7.1.
The adsorption capacity and adsorption intensity of a lubricant on metal is called oiliness.
Oiliness is a composite indicator related to the nature and state of lubricants and metal surfaces. Animal oils are of good oiliness, vegetable oils are fairly good, but mineral oils are less oily because they contain few fatty acids, but usually contain some unsaturated hydrocarbons, which also have some adsorption capacities. Active metals, such as copper, iron and vanadium have good adsorption capacity, while the adsorption capacities of nickel, chromium and platinum are poor.
7.1.2.2 Structure and Property of Adsorption Films
The viscosity of a boundary film can be determined by the air flow method, as shown in Figure 7.4a. Under the action of an air flow, a liquid in the narrow gap presents a certain shape.
According to the shape and Newton’s viscosity law, the viscosity can be obtained. Figure 7.4b gives some measured results.
For Vaseline oil with non-polar molecules, its curve is a straight line in 40 nm away from the solid wall. This indicates that the viscosity of this non-polar lubricant is a constant, while for the amyl sebacic acid with polar molecules, there is a turning point at 10 nm away from the wall.
Its viscosity is much larger when close to the wall than that away from the wall. This shows that the properties of an adsorption film being of an alignment structure are quite different from the original liquid.
Table 7.1 Adsorbed layers for good lubrication.
Platinum Stainless Silver Nickel Mo
Stearate >10 3 7 3 3
Stearic acid soap 7–9 1 3 3 3
Figure 7.4 Boundary layer viscosity.
k k Figure 7.5 Surfaces sliding with an adsorption film.
The polar molecules of the adsorption film are parallel to each other and perpendicular to the solid surface. Such an arrangement results in the maximum number of adsorbed molecules.
During sliding, the adsorbed molecules tilt and bend to form molecular brushes in order to reduce the resistance of friction so that the friction coefficient is low and this arrangement also effectively prevents direct contact of the two friction surfaces, as shown in Figure 7.5.
Molecules of a fatty acid are easily adsorbed. However, because the lengths of molecular chains are different, the lubrication abilities are different. The molecular chain of acetic acid is short, but that of the stearic acid is long. Usually, a long molecular chain forms a thicker adsorption film so that the two friction surfaces are separated easily. Usually, the friction coef- ficient of a boundary film decreases with increase of the chain length of the polar molecules and tends to be more stable. The number of carbon atoms determines the chain length. There- fore, the greater the number of carbon atoms of the polar molecules, the lower is the friction coefficient.
Molecules of an adsorption film are oriented and hierarchically structured to be capable of carrying loads. Because the molecules in the same layer are independent of the other layers so as to form many easily sliding interfaces, the boundary friction is the outer friction between the layers, and the film thickness presents in a ladder-style, as shown in Figure 7.6.
The effect of boundary lubrication is closely related to the quantity of lubricants, as shown in Figure 7.7. The solid surface is covered by an adsorption film which reduces the surface free energy. If lubricant is lacking, the adsorption film on the surface is only a single molecular layer and the corresponding surface free energy is the minimum – see shapeAin Figure 7.7. If lubricant is abundant, the film is as shapeCin Figure 7.7. If the quantity of lubricant is between AandC, the film is shapeBin Figure 7.7. At this time, because friction mostly occurs on the peaks, the lubricant has no significant influence on the friction coefficient. During friction, once the adsorption film on the peaks is destroyed, the lubricant in the valley will be rapidly applied to Figure 7.6 Velocity distribution.
k k Figure 7.7 Adsorption film forms.
recover the film quickly. When the lubricant is less thanA, it cannot be applied again. Therefore, if the film on the peaks is destroyed, dry friction occurs. When lubricant exceeds shapeC, the friction coefficient will be unstable.
A polar molecular lubricant has two ways of influencing friction and wear. One is to form an adsorption film to prevent metals from directly contacting so as to reduce friction and wear.
The other way is that when a metal surface has some cracks, the polar molecule will expand the cracks. As shown in Figure 7.8a, in order to form an adsorption film, the polar molecules are forced to enter the crack tip from the outside so as to expand the crack. Such a phenomenon is called the wedge effect.
The wedge effect is also the reason that an adsorption film of a peak can carry a load. In Figure 7.8b, the polar molecules are forced to enter the contact area of two surfaces during adsorption. Because of the wedge effect, the increasing pressure separates the two surfaces away to prevent them from directly contacting.
It is very difficulty to obtain a sufficiently thick lubricant film if the load is too heavy, the impact is too large, starting or stopping is too frequent, the speed is too low, the surface is too rough, or in an intermittent swinging operating condition. Under these circumstances, the polar substances can be added into a lubricant to form an adsorption film on the metal surface to prevent from tear and to reduce wear. For the effectiveness of an adsorption film, the following notes are important.
1. Carefully select the surface materials and the lubricants as well as strictly controlling the surface roughness;
2. Add some necessary additives into the basic lubricant, because a pure mineral oil is non-polar and a mineral oil not highly refined contains few fatty acids, only 1–2%. There- fore, their adsorption capacities are poor. Common oily additives include high fatty acids, esters, alcohols and metal soaps, such as oleic acid, dimer acid, aluminum stearate, oleic acid ester, dimer acid glycol monoester, zinc dialkyl dithio phosphate (ZDDP), formic acid phosphate ester and chlorinated paraffins. Oily additive is usually added at less than 10%. In
Figure 7.8 Wedge effect of adsorption film.
k k addition, although the animal and vegetable oils have good adsorption capacities, they are
easily oxidized.
3. Strictly control the working conditions. An adsorption film usually exists below 200∘C and under slow relative velocity. In the light and middle load conditions, an adsorption film can effectively reduce the adhesive, fretting and oxidized wears of the surfaces, but its effective- ness for abrasive wear depends a lot on the sizes of the abrasive particles.