Friction modification and anti-wear are two properties that are commonly confused for one another or misunderstood as being a single property. However, friction modifiers and anti-wear additives are two separate classes of additives; though they do share a number of similarities. There are a few different mechanisms for friction modifiers and anti-wear additives to function. They can be surface active chemicals that bond to, react with or form a tribofilm on a surface. They can also be additives that change the properties of the lubricant itself in order to modify its fluid film to reduce friction and wear. Because friction modification and wear resistance utilize similar mechanisms, it is not uncommon for one additive to serve both functions.
The basic way of distinguishing them is this:
- Anti-wear additives reduce the surface damage caused by friction. They may or may not change the original frictional characteristics
- Friction modifiers change the frictional characteristics between surfaces. They often do, but do not necessarily reduce wear.
The need for these additives is clear once you have an understanding of the interaction between two separate surfaces and the causes of friction and wear.
At a microscopic level, bare surfaces have many asperities that cause friction and resistance when sliding against another surface.
Even honing a surface can only reduce the asperities by so much. Honing a surface only minimizes asperities, it doesn’t eliminate them. So there is no perfectly flat material to eliminate friction.
Wear occurs when a hard material literally gouges out or cracks a softer material. Even though you may have identical metals on two contacting surfaces, there are always irregularities that may result in harder and softer areas.
When wear particles are formed, and are not removed from the oil through filtration or oil changes, they become a third body in this interaction and create even more wear. Wear particles are able to embed into the surfaces creating a crack and releasing more particles. This is similar to dirt contamination which can embed into the soft metals and dislodge metal particles.
This is why anti-wear, filtration and adequate oil changes are necessary to protect motorcycle parts.
Anti-wear additives often take effect when the oil film is compromised and insufficient to keep two surfaces in a state of hydrodynamic lubrication and enter into boundary lubrication. When the fluid film is not adequate to separate surfaces, that is when anti-wear additives are crucial to protect surfaces. Friction modifiers often assist in maintaining a fluid film or coat the surface in material that has a much lower coefficient of friction than the bare metal would otherwise. However, friction modifiers often do not form as strong a barrier as anti-wear additives do.
One way that an anti-wear additive may work is by forming a sacrificial chemical layer on a metal surface. This is often done through a chemical reaction that is activated by heat and pressure. So as two surfaces are stressed against one another, these additives chemically activate and form a new layer of material that prevents the surfaces from coming into direct contact with one another.
Depending on the chemistry of the additive forming the sacrificial layer, it may also alter the friction between surfaces. That will depend on whether the new chemical layer has a lower coefficient of friction or a higher coefficient of friction than the bare metal does. Two good examples of additives that operate in this way are zinc dialkyl-dithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC). ZDDP is a very common anti-wear additive that usually does not have much, if any, effect on friction because it’s own coefficient of friction is similar to metals. Conversely, MoDTC often does have a lower coefficient of friction so it reduces wear and friction a the same time.
There are a few things to consider when you are selecting products based on their anti-wear chemistry:
- Metallic based anti-wear additives such as ZDDP and MoDTC contribute to the ash content of an oil. This means they will contribute to sludge when oxidized or they may exit through the exhaust, poison an exhaust catalyst and possibly form deposits. In 2-stroke applications this is especially important due to the ashy deposits that can form in the combustion chamber and exhaust.
- Most additives that form sacrificial wear layers have a finite life because as they react, they are depleted from the oil. Once the entire content of that additive is reacted and used, the oil may no longer have adequate anti-wear properties and will need to be changed.
- ZDDP in particular has been such a popular additive over the decades, that many people believe the best oils have the most ZDDP. In fact, there is an upper limit to the amount of ZDDP before it actually becomes destructive rather than protective. At concentrations around 1700 ppm and above, ZDDP can become corrosive and damage surfaces. The anti-corrosive additives used to counteract the corrosive nature of ZDDP only last a short time, so oils with high levels of ZDDP typically require very short change intervals.
Another way that an anti-wear additive may function is through a bonded layer onto the surfaces. This layer will generally bond directly to the metal surface because of polar attraction rather than reacting with the surface. The polar molecules will be attracted to the metal surface and displace the bulk oil film, thereby adding a stronger layer of protection than the less polar oil would alone. These types of additives will often consist of a highly polar chemical with a positive end and a negative end (commonly referred to the head and tail).
The polarity of the molecules make them orient identically across the surface with the polar “head” on the surface and the polar “tail” facing outward, becoming the new contact point between surfaces. Since opposites attract and vice versa, the surfaces maintain a separation due to both the resistance of the similarly charged “tails” and by forming a physical barrier. However as you can demonstrate by forcing two magnets together, polarity can be overcome by sufficient force. This is why polar tribofilms alone are rarely adequate for protection in extreme pressure applications.
So in applications such as transmissions, polar additives will not significantly lower the wear potential. Because boundary lubrication is much more prevalent in these systems, these types of friction modifiers are not strong enough to prevent contact and they do not perform as well as stronger anti-wear and extreme pressure additives. Additives commonly used for this purpose are glycerol mono-oleate (GMO) and high viscosity ester compounds.
Here are a few things to consider when selecting products based on their friction modifying properties:
- Friction modifiers are very often not compatible with wet clutches. Their effects are not selective to specific parts and what is good for the engine and gearbox is going to be bad for the clutch. Oils that contain friction modifiers often cause clutch slippage and overheating.
- Certain friction modifiers are somewhat unstable and will decompose into their base components quicker than other additives in the oil. This means the friction modification may not last as long as the other properties of the oil, therefore decreasing the oil life prematurely.
The other way that wear and friction can be reduced is through physical means derived from the lubricant’s properties. If a high viscosity component is added to the oil, this component will serve as a fluid film booster because its large molecules will help to maintain a thicker film between surfaces. A similar but more extreme way to form a film such as this is to suspend solid particles in oil. Certain insoluble solid additives can be dispersed within a liquid. Wherever the lubricant travels, it carries these solid particles with it. A common physical form of these solids is platelets which can slide over one another.
Platelet interactions often have significantly less friction than either the metal surfaces or an oil film will exhibit. These solid particles are often much stronger than liquid additives, so it is more difficult to generate the high pressures required to break through this layer compared to other additive types. Common chemistries that are used for this purpose are molybdenum disulfide (MoS2) and calcium carbonate (CaCO3).
Here are a few things to consider when you are selecting products containing suspended solid additives:
- Suspended solids have the potential to be caught in and plug the oil filter if the particle size is too large. This hurts the filter’s efficiency.
- Because of their tendency to settle over time, these particles can create a sludge-like accumulation at the lowest point of the oil. This can eventually clog the sump as well as clog narrow oil passages throughout a machine.
- Solid films often do not conduct heat as well as liquids and may insulate surfaces; causing excessive heat issues.
One result of using friction modifiers can be seen in the heat generated. Decreased friction will result in less heat produced and lower temperatures. This gives the benefit of extending both machine and lubricant life. Less heat in the lubricant will reduce the oxidation rate of the oil and prolong lubricant life. For the machine, less heat reduces overheating and stress on components.
The main effects of anti-wear additives are fairly straight forward. By reducing machine wear, proper clearances are maintained between surfaces, and surface integrity is kept at an optimum level. It also means fewer wear particles will be introduced to contaminate the oil. This again will increase the life of the machine and prolong drain intervals.
By tailoring these effects for a specific application, properly formulated lubricants optimize protection based on the types of friction and wear that occur in their intended application. There are many different ways to customize performance and combine these additives and properties to optimize a lubricant’s performance for specific applications.
Every manufacturer will have some level of these additives in their products; it is simply a matter of discovering which is the best in your particular application. Differences between engines and transmissions, bearings compared to pistons, and even different engine configurations and sizes will each have their own optimum product. One particular formula is unlikely to perform the best in every application due to the differences in loads, friction regimes and temperatures. However, most products are well balanced enough to provide adequate protection in a variety of applications.