Tribological evaluation of hydrogenated a-C:H and hydrogen-free ta-C diamond-like carbon coatings in diesel lubricated model tests

During the last few decades, diamond-like carbon (DLC) coatings have attracted considerable attention due to their interesting mechanical and tribological properties. Especially in tribocontacts subjected to high load and temperature, DLC has the function of a wear protective coating. In the diesel fuel lubricated plunger/cylinder and gear front surface/housing tribosystems, which are parts in diesel injection systems, two types of DLC coatings are generally used. The first coating is hydrogenated a-C: H, called DLC-A. DLC-B is a coating consisting of two layers: the inner layer is a ta-C and the top layer is a-C: H. Wear is observed in the field operation to arise on these mentioned tribosystems due to multiple stress conditions such as high pressure, temperature, tangential speed and contact kinematics. As model tests are more suitable when investigating the DLC and counterbody wear behaviours under multiple stress conditions, they were used in the present work. Two different model tests have been set: the skewed cylinders and the pin-on-disc model tests. The skewed cylinder model test was chosen because of its similarities to the tribosystem plunger/cylinder in a diesel pump. The test parameters in the skewed cylinders model test were load, temperature and angle between cylinders. The pin-on-disc model test was set to reproduce the kinematics of the gear front surface/housing tribosystem. The test parameters here were pressure, temperature and tangential speed v. In order to evaluate the effect of antiwear additives present in standard diesel fuel on the wear behaviour of DLC and counterbody, EN590 and GDK650 diesel fuels were used as lubricants. GDK570 was also used for some specific test conditions. EN590 was a standard diesel containing ester-based antiwear additives. GDK650, standing for German GrenzDieselKraftstoff, did not contain antiwear additives. GDK570 contained more antiwear additives than EN590. It was observed regardless of the model test that the antiwear additives had a negative effect on the DLC-A wear rate. Furthermore, it was noted that once a critical coating thickness is reached, DLC-B undergoes delamination under certain conditions. This DLC-B tendency for delamination was seen to be diesel lubricant dependent. The wear rate of the steel counterbody in both model tests was seen to be independent of the test parameters and the lubricant. The analytical results indicated the structural transformation of diamond-like carbon coating as the main mechanism responsible for wear. This structural transformation was mathematically described and a new wear law was developed. This law was seen to predict the DLC-A and DLC-B wear volumes as a function of multiple stress conditions.