24th International Colloquium Tribology - January 2024 83 The Running-In of a DLC-Metal-Tribosystem - A Study on Multiple Scales Matthias Scherge, Joachim Faller Fraunhofer/ KIT MikroTribologie Centrum, Rintheimer Querallee 2b, 76131 Karlsruhe 1. Introduction This work deals with the running-in behavior of amorphous carbon coatings in a lubricated tribosystem with a metallic counterbody. The extent to which friction and wear develop as a function of stressing sequence and final machining of the coatings was investigated. For this purpose, experiments were carried out in a pin-on-disk tribometer with online wear measurement (RNT) in the mixed lubrication regime. The measurements show a pronounced running-in, comparable to a metal-metal-tribosystem, which benefits from high, initial loading. For the final machining, there is a narrow corridor in which the smallest friction coefficients and wear rates occur. By means of several analysis methods, it could be shown that the running-in leads to an enrichment of the sp 2 content in the near surface zone. 2. Results The goal of this work was to generate a fundamental understanding of the process of running-in of an amorphous carbon (ta-C) coating paired with an iron-sprayed layer (LDS, 13Mn6). For this purpose, systems with different coatings were run in, systematically tested for their sensitivity and the chemical change investigated (XPS, EELS). The tribological system is subject to a topographical as well as tribochemical running-in, which was quantified on the sprayed layer using RNT and on the DLC coating using in situ topography measurements. Fig. 1: Running-in with optimized stressing sequence. The running-in behavior was studied by means of a designated test program and optimized by skilful sequencing of load and speed in the direction of smaller friction values and wear rates, see Fig. 1. Furthermore, by varying the finishing of the ta-C coatings, a narrow roughness corridor in which the smallest friction values occur as a function of the initial peak roughness was discovered, see Fig. 2. This was accompanied by the realization that the friction is largely determined by the operating roughness. Fig. 2: Roughness corridor. The coefficients of friction shown here were taken from the minimum of a Stribeck test run. In addition to the change in topography, a change in the microstructure and chemistry of the near-surface area could be observed on the sprayed layer as a function of the running-in behavior and the counterbody. The amorphous carbon coatings are also subject to a mechanochemical running-in, in which lubricant constituents are incorporated into the near-surface area and the uppermost nanometers are enriched with sp 2 hybridized carbon. Thus, it can be confirmed for these investigated tribosystems that the running-in behavior is largely determined by the formation of a third body, see Figs. 3 and 4. Fig. 3: Chemical composition of the near-surface area of the sprayed metal layer, determined by XPS depth profiling. The first five nanometers consist mainly of CH x contamination. At a depth of ten nanometers, the iron oxide content is 84 24th International Colloquium Tribology - January 2024 The Running-In of a DLC-Metal-Tribosystem - A Study on Multiple Scales maximum with over 20 at.%. This drops below the detection limit over the next 30 nm. Phosphorus and zinc are only present on the first ten nanometers in significant amount, one and two atomic percent. Sulfur and molybdenum have their maximum at a depth of about 20 nm with 5, respectively 3 at.%. Also sodium, silicon and calcium are present on the first 30nm in detectable amounts. Fig. 4: Chemical composition of the near-surface area of the DLC coating, determined by XPS depth profiling. For the DLC coatings q tribochemical change can only be detected on the first three nanometers. Oxygen, nitrogen, calcium, sodium and zinc are present there. The composition of the ta-C coatings influences the tribology significantly via the sp 3 content. This again forms a corridor, which, however, is sharply limited only in the range of high sp 3 contents. The optimum lies between 60 and 75- at.% sp 3 . The wear rates of both bodies run parallel, albeit in different orders of magnitude, see Fig. 5. Fig. 5: Wear rate as function of sp 3 content for DLC and the 13Mn6 sprayed layer. It should be noted that the hardness of the amorphous carbon coating, determined by means of nanoindentation, leads to a corridor-like distribution of the friction coefficients as well. This exhibits a minimum between 30 and 50 GPa, which is a sufficiently wide corridor for a practical application, see Fig. 6. Fig. 6: Coefficient of friction as function of hardness of the DLC coatings. 3. Summary What all the tribosystems investigated have in common is that - particularly due to the narrow roughness corridor the sensitivity is high and, as a result, a running-in with the smallest friction values and wear rates did not occur in all cases. This makes a practical application, where a certain friction value has to be achieved, a challenge. However, if a coefficient of friction below 0.1 is sufficient for the practitioner, a DLC-metal system is a favorable choice from a wear perspective. For applications requiring stable, lowest friction values, pure ta-C pairings should be used. 4. Outlook On the basis of the results obtained, further investigations are required in several areas. On the one hand, this includes deepening the understanding of the tribochemical running-in of the amorphous carbon coatings. For this purpose, further EELS measurements on the identical material with smaller windowing and on coatings with other sp 3 contents are necessary. Furthermore, the transfer film on the counterbody has to be investigated in more depth. The analysis of the carbon hybridization by means of EELS and the characterization of locally-confined effects by EDX on TEM lamellae are suitable for this purpose. On the other hand, the gained knowledge should be used for the production of new coatings, rendering mechanical preconditioning obsolete.