Improving fuel economy and tribological properties in automotive applications is a continuous quest, where only a few percent decrease in friction and emissions can make a huge difference. Energy savings and environmental influence are today very important factors when designing new tribological systems for automotive applications.
DLC coatings are often seen as a homogenous group of hard amorphous carbon coatings, being chemically inert with great wear resistance in most lubricated environments. However, the hardness can vary within almost an order of magnitude between different DLC variants and combined with large variations in chemical composition, the wear and friction properties may vary significantly, depending on the test conditions.
In this study, an experimental coating of amorphous carbon deposited with PA-CVD is compared with three commercially available and optimized DLC coatings, frequently used in automotive applications. The coatings have different hardness, chemical composition and structure.
The coatings are tested in a reciprocal sliding test rig, at contact pressures and temperature representative of real engines. The tests are performed at low speeds and are designed to operate in the boundary friction regime, where the need for coatings is the highest. An uncoated reference material is tested with the same parameters to highlight the benefits of the coatings under the different test conditions. Five oils are tested, including four special oils designed with different amounts of additives and one used dully formulated engine oil taken from a real engine.
Results show that the DLCs do not behave as a homogenous group, but have different tribological properties depending on the hardness and composition. Interestingly, the ranking of the coatings varies for the different oils. Special attention is put on friction and wear behavior and on how the surfaces have changed during the test. Further, deeper surface analysis of some selected systems is performed using EDS, XPS and TEM.