Polymer coatings in race engines
This month's RET-Monitor article on materials discusses some of the uses of polymer materials for manufacturing race engine components. Polymers, in comparison to most engineering metals, generally lack strength and stiffness. They are also often more limited than metals in terms of temperature resistance and corrosion. They do have some useful properties though, which can be put to good use in a racing powertrain. In general they are electrically non-conductive and have low thermal conductivity compared to metals. They also have low coefficients of friction.
While the use of polymers in race engines for the manufacture of entire components has been sporadic and pretty limited, the use of polymer coatings on components has remained reasonably popular over a the past 20 years or more. Much of their popularity as a coating comes from two main properties - low coefficient of friction and high wear rate.
The attraction of a low coefficient of friction is easy to understand: wherever loaded sliding contacts occur, less energy is converted to heat and lost to atmosphere. Thin polymer coatings on metallic components can reduce the coefficient of friction in sliding contacts. Such coatings remain popular on racing pistons for many applications, having been used widely for series such as Formula One and IndyCar racing in the past. While DLC is now sometimes preferred by some engine builders, polymer piston skirt coatings are well known and are more forgiving. As we shall see below, it is not only for friction that polymer coatings are used.
Whether high wear rate is an attractive trait in a race engine is an interesting debate, and the answer depends on the application to a certain extent, but even more relevant is the portion of a component's life during which the wear occurs. Polymer coatings tend to wear heavily where contact pressures are high and sliding is present; where contact pressures are low, wear rates are minimal. The fact that material is easily lost at points of high contact pressure means that polymer coated components easily 'wear in' to an optimal profile.
Again, the main application is piston skirts. The ideal skirt profile for pistons in identical engines can be different based on how and where the engine is used. A skirt profile that can wear to form a profile which is close to ideal is very attractive. Polymer coatings have actively been used to determine piston skirt profiles for metallic pistons in the past.
The same principle of components wearing in means that polymer coatings find widespread use in oil pumps on pump housings and on some pump elements. Polymer coatings also allow small particles to become harmlessly embedded in the coating, where the same situation with close-running pump components can lead to damage when small, hard particles pass through the pump.
In recent years there has also been a trend towards giving shell bearings a polymer coating. There are mixed views on the effectiveness of this; some people are convinced of the benefits while others cope without the coatings.
Some engine builders equate low friction to an ability to 'shed' oil from the surfaces of components. Occasionally I have seen engines where all manner of components, from con rods to the inside of crankcases, have been polymer coated.
Written by Wayne Ward