P. Pallav, PhD

Hydrodynamic Lubrication

The circumstances in sliding contacts are often favorable for a so-called 'self-generating hydrodynamic lubrication film'. This may occur when the two sliding surfaces are wetted with a fluid of sufficient viscosity (Reynolds, 1886).

The fluid may be motor oil, but also saliva, possibly mixed with food. As the contact moves, there is not enough time for the fluid to escape from under the contact. Consequently the surfaces will not touch and there will be hardly any wear or friction. This works best with smooth surfaces. These conditions resemble aquaplaning with a car driving very fast on a road with a few centimeters of water with no profile on the tires.

The force, which loads the contact is often borne completely by the pressure, which builds up in the film. When the fluid is very clean, hardly any wear at all may occur. But with the presence of food, this will be different; the CFA type of wear will be more intense.

Boundary Lubrication

Although saliva is an excellent lubricant there is often direct sliding contact between the hard surfaces of the upper and lower teeth. In the oral cavity the surfaces are covered with an adsorbed layer, the pellicle, which lubricates the contacts. In this way it reduces wear and friction, especially on smooth surfaces.

I don't know if it has ever been tested on dental materials, but when wear tests are performed under absolute vacuum, i.e. absolutely no adsorbed gasses, water, or any other molecules, the wear is generally devastating, maybe even a thousand or more times the wear you would get otherwise.
If you would let air in, I would guess the wear rate will decrease. But I am not sure, you never know with oxygen.

Adding water decreases the wear rate. It will still be considerably more say up to ten times than 'clinical' wear and depend highly on the circumstances, but not a thousand times.

This wear rate and friction decreasing effect of a surface layer is called boundary lubrication. Apparently there are still molecules between the surfaces. As you might have guessed, the pellicle is an excellent boundary lubricant.

The smoother the surfaces (Ra-value), the narrower the gap around the micro contacts, which make up the actual contact area. The narrower the gap is with respect to the diameter of the apparent contact area, the less shear strength is needed in the adsorbed layer to withstand the contact pressure. It is basically the low shear strength in the adsorbed layer which is responsible for the decrease in friction, S_s in the equations on friction.

Note that the pressure on the boundary layer at the actual contacts is about equal to the surface hardness. The hardness of enamel or materials to restore it is in the order of 100 MPa and far more. This is a very high pressure.

Under mild conditions the wear at the micro high spots on the surface may polish the surface and improve the circumstances for boundary lubrication.

Because of the balance of damage and restoration of the surface film, the general wear equation, (p x v) often applies only roughly to boundary lubrication conditions. It is as if p should be raised to a power somewhat greater than 1 and v to a power a bit smaller than 1. This may be explained with an example.

Example

Original conditions: Saliva and two hypothetical cusps, which slide over each other during one second and the motion is repeated each four seconds. This results in a certain wear rate.

Variation A: The cusps slide each two seconds, twice as often, but with the same sliding speed. When the state or quality of the adsorbed layer would remain the same, the wear rate would be twice as great. However, in the new situation only half the time is available for the saliva to restore the damage to the adsorbed layer, which will decrease its effectiveness. The result is that now each single sliding motion will generate more wear and the wear rate will have more than doubled.

Variation B: The cusps slide with twice the contact force. Again, when the quality of the adsorbed layer would stay the same, the wear rate would be twice as great. But with twice the force there will be more actual contact area and more damage to the adsorbed layer. Therefore the wear rate will have more than doubled.

Variation C: The cusps slide the same length with twice the sliding velocity, still each four seconds. When the quality of the adsorbed layer would remain the same, the wear rate would still be the same, because the average sliding velocity (sliding length divided by interval time) is still the same. However, because of hydrodynamics and its deformation behavior the adsorbed layer will be better and the wear rate will be less.

It depends greatly on the circumstances how much 'more' or 'less' will turn out to be.

The more than proportional way in which the wear rate responds to pressure with boundary lubrication brings about that there is an end to lubrication; in cases of heavy clenching, bruxism, grinding etc. the wear can be devastating.

Wear Mix

Material loss at direct contacts is mainly a combination of abrasive and adhesive wear with surface fatigue phenomena caused by contact stresses. The relative influence of each varies with the load.

At light loads the lubrication will be more efficient and the wear will be mild and may be exclusively abrasive.
As explained above, when the load increases, the damage to the adsorbed layer increases as well and direct contact between the substrate materials is more likely to occur. Depending on the affinity of the surface materials to each other (friction) and to the adsorbed layer, this is where an adhesive component may come into play.

Reynolds O (1886): On the Theory of Lubrication and its Application to Mr. Beauchamp Tower's Experiments, Including an Experimental Determination of the Viscosity of Olive Oil. Phil Trans R Soc London 177: 157-234.