Nowadays, it is required to reduce in size of automobiles to increase fuel economy and to lighten the adverse impact on the environment. To fulfill the requirement, it is needed to strengthen automotive components in a powertrain. Gears should be operated with durability at high-speed friction and/or high load, since gears are the major parts of the powertrain. Recently, the fatigue of tooth surface, especially pitting, has been a focus of attention as a serious problem. Tooth surface is always running under a rolling contact fatigue by a local high stress with slip. To improve the gears' lifetime, it is necessary condition to harden the tooth surface and to provide a lower friction coefficient.
In the present study, DLC film was coated by a plasma-CVD process on JIS-SCr420 steel, which was quenched and tempered followed carburizing. Influence on DLC film of its hardening and friction coefficient was studied. TiN film was also deposited by the PVD process on the same material to compare with the properties of DLC's. Lifetime of pitting was timed by the roller-pitting test after the observation of the microstructure by optical microscope and determination of Vickers hardness profile on a cross-section of a sample. To characterize the peculiarities of DLC film, hydrogen content was analyzed by ERDA, and the combined state of carbon was analyzed by Raman spectroscopy. The hardness of DLC film was determined by nanoindenter, and the friction coefficient was also measured by the ball-on-disk. Furthermore, detailed microstructure in the vicinity of the interface between the DLC film and the martensite was observed by TEM. The roller-pitting test was carried out in the range of Hertzian contact stress of 2.5GPa with the lubrication of ATF.
There is no influence on Vickers hardness profile on the cross section of a specimen by the coating process of DLC film. There is no heterogeneous carburized structure. The hardness of DLC film was 21.1GPa and the elasticity coefficient was 192.3GPa. It is confirmed a typical DLC (a-C: H), of which the hydrogen content of DLC film was about 20%. The surface fatigue resistance of DLC-coated specimen corresponds to 100 times life of carburized and quenched one even under Hertzian contact stress of 3.0GPa. |