Thermal effects influencing stability and performance of coatings in automotive applications

• Investigation of the temperature stability of DLC in non-lubricated conditions
• Measurement of coating hardness by nanoindentation at different indentation depths
• Structural analysis of DLC coatings with Raman spectroscopy after tribotests
• Analysis of the influence of thermal properties of coatings on tribological systems
• Measurement of thermal conductivities of different coatings

Carbon based coatings are well established in the automotive industry to solve tribological problems in automotive applications. In systems under high load and/or high contact pressures, these coatings are used to reduce friction and wear. Other coatings like chromium nitride are useful to prevent scuffing. However, a real challenge in evaluating coatings for their suitability in automotive applications is the broad range of conditions which defines the load collective of the tribological system.In this work, we focused on the thermal effects that might influence the stability and performance of coatings in tribological applications. In a car engine, the temperature range might be much broader than the − 20 °C to 120 °C as given by the average oil temperature. In the tribological contact zone the temperature can be higher up to several hundred degrees Celsius, especially if it comes temporary dry running due to starved lubrication. These locally high temperatures can affect the properties of a coated surface, but the coated surface itself might also have an influence on the temperature in the contact zone. This is especially the case, if coatings with low thermal conductivities like carbon based materials are used.Therefore, we focused our here presented work on the investigation of the influence of the temperature on the wear performance, the friction coefficient and the thermal stability of a DLC coating in unlubricated conditions. The DLC coating was submitted to dry running reciprocating sliding wear tests in a broad temperature range and then the thermal stability of the coating has been analyzed by means of hardness measurements (nanoinentation) and a structural approach (Raman spectroscopy). In addition, we analyzed the effect of a DLC coating on the tribological properties of a lubricated contact on a two-disc tribometer. These efforts were completed by measurements of the thermal conductivity of the DLC coating.