The operating environment in typical automotive planetary gearsets leads to harsh tribological conditions. Relatively low operating speeds combined with low viscosity lubricants hinder the formation of protective oil films. As power density increases, gear surface fatigue (pitting) limits gear life and the life of the transmission as a whole. Coatings have been found to be effective in extending pitting fatigue life of gears. The authors have tested a number of different coatings over the last ten years. Some of them increased gear life by fifteen times in bench tests. The most successful coatings have been hard, ceramic coatings such as B_4C and Diamond-Like Carbon (DLC) that smooth the mating surface and improve the lubrication conditions in the contact. Recently CrN, a widely available and affordable coating, has been studied and found to be a good alternative to other coatings.
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