White Layer Formation During Nitriding/Nitrocarburizing: Function, Usefulness, and Variations for Electric Vehicles

Gas nitriding and ferritic nitrocarburizing have seen tremendous growth. Today, it continues to accelerate as more uses are being found, especially in the growing electric vehicles (EV) sector. This success is due to the ability to control protective white layers consistent with the needs of an automotive engineer. Steels and cast irons are still the materials of choice for many applications and the nitrided layer is wellknown for its tribological features (some would say even more than three) which include wear resistance, lubricity, and a low coefficient of friction. Corrosion resistance in particular has become an important advantage and depends on white layer formation and quality. The white layer (known as the compound zone) consists of two iron nitrides, epsilon (Fe_2-3[N]) and gamma prime (Fe₄N). In addition, the epsilon layer can contain varying amounts of iron carbides and/or iron carbonitrides, Fe_2-3[C]. This paper will focus mainly on the how’s and why’s of white layer: how to control its composition and properties; and how to minimize it, if required. Just as importantly, some applications of how the EV component engineers have found uses for this important steel treatment are discussed, including brake rotors.