LUBRICATION FUNDAMENTALS Dr. Robert M. Gresham / Contributing Editor

What’s a friction modifier? These antiwear agents play a critical role in smoothing the transition from static to dynamic motion in moving parts.

WHEN I FIRST ENTERED THE LUBRICANTS BUSINESS, I was always a little mysti- fied by so-called friction modifiers. This mystification was further inten- sified when people said these products added lubricity or were “lubricious.” Finally, I heard from people who said that while you can’t really measure this ethereal property, you can feel it between your fingers. Let’s take a look. Friction modifiers originally came Figure 1 | AW/EP additives work by chemically reacting with the metal’s surface to form a into use for applications like limited relatively durable film that’s difficult to shear off. slip gear oils, slideway lubricants, au- tomatic transmission fluids and multi- purpose tractor fluids. The idea of the friction modifier was to smooth the transition from static to dynamic motion in moving parts. Thus, when one thinks of the lubrication regimes, friction modifiers ease the transition through boundary and mixed to the hydrodynamic regime. As we know, there also are antiwear/ extreme pressure additives (AW/EP). Figure 2 | The effectiveness of friction modifiers depends on several factors, including nature People sometimes confuse the two, but of the polar head and surface temperature. in fact they are quite different. AW/EP additives chemically react with the sur- face of the metal, forming a relatively to the surface and van der Waals forces Without getting into the chemistry durable film that is difficult to shear off between molecules. The friction modi- too much, the effectiveness of friction (see Figure 1). This reaction takes place fiers typically have polar heads and hy- modifier films depends on the nature when there is sufficient heat in the sys- drocarbon tails that aggregate on the of the polar head, the chain length of tem, usually during the mixed-lubrica- surface of metals in clustered and lay- the hydrocarbon tail, the nature of the tion regime, to activate the additive. The ered matrices of relatively straight mol- chain (that is, how well it packs together resultant films also are very useful for ecules perpendicular to the metal sur- with its neighbor) and the temperature, protecting the metal under boundary face (see Figure 2). This leads to a soft, which tends to weaken the various conditions, but they exhibit relatively lubricious film of low coefficient. These bonds as it increases. Thus, we can see high coefficients of friction. films are difficult to compress, good for how friction modifiers and AW/EP ad- The films formed by friction modi- boundary and mixed regimes, easy to ditives actually can work in tandem to fiers are by contrast formed at essentially shear and good for smooth, easy start- protect and reduce friction over a wide room temperature via hydrogen bonds up from static to dynamic conditions. temperature range, from ambient tem-

24 GPS satellites are located in between geostationary and mobile satellites and circle the Earth as a constellation to provide perature start-up to a much determined with some rigor- warmer operating tempera- ous lab work. What’s done ture. In practice, friction modi- is to measure the Stribeck fiers are long chain (C-18) am- curve for a given engine oil ides or partial esters such as a but with differing friction long chain (C-18) glycerol. modifiers in the formulation. (It’s this soap-like structure The curve that demonstrates that accounts for the percep- the lowest coefficient of fric- tion of “lubriciousness” when tion, from static to dynamic rubbed between the fingers.) motion and from boundary These friction modifier regime through mixed to hy- films are vulnerable to high drodynamic, is likely the best temperatures, other additives product (see Figure 3). Wear that compete for metal sur- through the regimes also can face molecules, insufficient Figure 3 | Friction modifiers affect a lubricant’s coefficient of fric- be measured. tion. The lubricant demonstrating the lowest coefficient of friction concentration of the friction I hope friction modifiers through the various boundary regimes is likely the best choice. modifier resulting in incom- no longer seem quite so mys- plete films, contaminants that tical now. also might compete for the surface and tractor fluids. However, today we also the chemistry of the metal itself, which are seeing friction modifiers used in can affect viability of film forming. automotive engine oils to improve fuel As mentioned at the beginning, fric- efficiency to help meet CAFE standards. Bob Gresham is STLE’s director tion modifiers are used in limited slip I mentioned above that you can’t re- of professional development. gear oils, slideway lubricants, automatic ally measure lubricity, but the efficacy You can reach him at transmission fluids and multi-purpose of different friction modifiers can be [email protected].

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entire planet coverage. A group of only about 20 GPS satellites provide a service used by millions of motorists daily. 25