Surface-modification technologies for improving rolling-contact fatigue strength of automotive-gear steel

摘要

Remarkable developments of production technologies have been carried out to address the requirement for increasing the rolling contact fatigue strength in terms of pitting resistance. For example, carbonitriding with high nitrogen concentration followed by hard-shot peening has already been put to practical use to manufacture automotive transmission gears. Recently, process technologies combined with plasma carbide-dispersion carburizing or vacuum carbonitriding and plasma-enhanced chemical-vapor-deposited (CVD) or ion-beam-deposited diamond-like-carbon (DLC) coating, in which 0.19 %C-0.51 %Si-0.36 %Mn-1.44 %Cr-0.43 %Mo steel is used, have been studied to meet future demands for high functionality. Roller-pitting-fatigue tests show that these combined treatments improve the pitting lifetime of the steel significantly under high contact pressure up to 4.5 GPa. This excellent pitting endurance may be explained by reduction in friction coefficient between both sliding surfaces due to DLC films in addition to excellent adhesion of the DLC films. The compound process technologies are therefore expected to be used for the next-generation automotive applications such as gears and pullys.