Anti-wear additives are an essential component of most lubricants including engine oils, hydraulic oils, turbine oils, or gear oils. They help us protect components by forming a sacrificial barrier at the metal surface. The most common among these are the ZDDPs, organo-molybdenum compounds, sulphur and phosphorous based additives. Antiwear action is similar to Extreme Pressure additives, and the difference between them is reasonably arbitrary.
When are antiwear additives necessary?
The Stribeck Curve describes the three major lubrication regimes. The curve describes the relationship between the coefficient of friction versus ZN/P; a non-dimensional number including factors for viscosity (Z), speed (N) and load (P).
The hydrodynamic lubrication regime encompasses the right side of the chart, but at low ZN/P values we’re specifically talking about the boundary and mixed lubrication regimes. In order to have a low value on the x-axis, either N needs to be small (slow speed) or P (load) could be very high and because it’s in the denominator it will make the term smaller.
We’re therefore talking about slow speed, high load operations or a combination of both. Under these conditions the load is shared between both the fluid and the surface. Under boundary lubrication there is full metal to metal contact and all of that load is really being taken by the metal surface. With no protection for the machine surface the situation would lead to extreme wear of components.
To alleviate the situation we must turn to additives, of which there are two large groups – solid and reactive additives.
The most common antiwear additive is ZDDP as it can be manufactured at relatively low cost that has really good at anti-wear benefits. ZDDP is a combination of thiophosphoric acid, as well as zinc oxide. Sometimes we call ZDDP a metal salt because it’s the product of a reaction of between an acid and a base. Acid plus base makes salt plus water.
Antiwear film deposition
What exactly does an additive like ZDDP do in the load zone? What happens is adhesion to the surface. The polar component (sulfur and the oxygen) is going to be drawn to metal surfaces, which have charge sitting at the surface. Under pressure the bonds between the sulphur and the zinc break. What you then get is a reaction between the sulfur and phosphorous components with the actual metal surface.
Over time this builds a glass polyphosphate. The actual anti-wear film that we talking about is extremely small; in the order of 50 to 150 nanometers.
These sacrificial films therefore form a hard barrier above the component surface, protecting it from abrasive / adhesive wear in highly loaded application.