In this test done by a major...
In this test done by a major OEM manufacturer, WPC outperformed these various types of coating in friction reduction by a large amount.
WPC
WPC is a Japanese process closely related to shot peening except it's done on a much smaller scale. WPC is a well-kept motorsport secret used extensively at high levels of the sport, like JGTC. Recently WPC has been gaining popularity in North America in venues like Top Fuel and Pro Stock drag racing, IRL, and NASCAR. WPC is also gaining acceptance from the OEM auto manufacturers as a cheaper and better performing alternative to coatings for wear resistance and friction reduction. WPC, like many Japanese companies, is very secretive about their exact process, but we've been able to glean a few important details from indirect observation. As we mentioned before, WPC is very much like shot peening in that it involves hitting a part with spherical projectiles to produce surface compressive stress, plastic deformation, and grain refinement. However, WPC differs from shot peening because the peening media is much smaller and harder and the part is hit with much higher velocities. To the casual observer, the WPC blasting media resembles baby powder.
We conjecture that the material is some sort of ultra-hard ceramic, like silicon nitride in the low micron range, and the velocities are over sonic in speed. We also feel that sometimes additives like zinc and moly are added to the mix to add some lubricity or extreme pressure characteristics to the surface. At high velocities, the additives are probably embedded into the surface of the metal at a molecular level, making their effects permanent and long lasting. We asked the folks at WPC to confirm this but they sort of just smiled and said nothing.
WPC offers many advantages over shot peening. Due to the small, light projectiles and high velocities, WPC offers a higher amount of compressive stress and an increase in the degrees of grain refinement to the base metal. The velocities are so high that in addition to cold working there is also a melting phenomena going on at a very micro level as well. The result is an extremely fine-grained, slip plane-less nano crystalline structure with a high degree of surface hardness, unlike shot peening, which has only a slight affect on surface hardness and strength. Because of this ultrafine-grained surface, WPC has superior fatigue strength gains and stress corrosion fracture resistance to shot peening.
Due to the small mass of the media, the affected zone of WPC treatment is less than 0.001 of an inch, much shallower than shot peening. The WPC media is so fine that the surface dimpling can't be seen with the naked eye and can only be observed with a microscope where the final finish is about the same surface roughness as micro polishing. Due to this smoothness, WPC can be done everywhere and can be applied straight to areas where dimensional control and surface finish is critical, like bearing bores, pistons, cylinder bores, camshafts and cam followers, and seal surfaces. Due to the smallness of the media, WPC, for the most part, doesn't affect the dimensions of a part. Due to the light mass of the media, WPC can also be used for fragile dimensionally critical parts like piston rings and even bearings. Nearly every engine part can benefit from WPC treatment.
Another big advantage that WPC has is a big reduction in friction. WPC's micro-dimpled surface helps reduce friction in three ways. First, the dimpling helps maintain an oil film, which reduces metal-to-metal contact. Second, the dimpled surface helps reduce contact area in general to reduce friction. Third, the hard surface with anti-friction and extreme pressure materials embedded into it is a slippery and longwearing surface in its own.