Superlubricity of tetrahedral amorphous carbon (ta-C) coatings under boundary lubrication with organic friction modifiers is important for industrial applications, but the underlying mechanisms remain elusive. Here, combined experiments and simulations unveil a universal tribochemical mechanism leading to superlubricity of ta-C/ta-C tribopairs. Pin-on-disc sliding experiments show that ultra- and superlow friction with negligible wear can be achieved by lubrication with unsaturated fatty acids or glycerol, but not with saturated fatty acids and hydrocarbons. Atomistic simulations reveal that, due to the simultaneous presence of two reactive centers (carboxylic group and C=C double bond), unsaturated fatty acids can concurrently chemisorb on both ta-C surfaces and bridge the tribogap. Sliding-induced mechanical strain triggers a cascade of molecular fragmentation reactions releasing passivating hydroxyl, keto, epoxy, hydrogen and olefinic groups. Similarly, glycerol’s three hydroxyl groups react simultaneously with both ta-C surfaces, causing the molecule’s complete mechano-chemical fragmentation and formation of aromatic passivation layers with superlow friction.

In this study, both the plasma process of filtered laser-arc evaporation and the resulting properties of tetrahedral amorphous carbon coatings are investigated. The energy distribution of the plasma species and the arc spot dynamics during the arc evaporation are described. Different ta-C coatings are synthesized by varying the bias pulse time and temperature during deposition. An increase in hardness was observed with the increased overlapping of the bias and arc pulse times. External heating resulted in a significant loss of hardness. A strong discrepancy between the in-plane properties and the properties in the film normal direction was detected specifically for a medium temperature of 120 °C during deposition. Investigations using electron microscopy revealed that this strong anisotropy can be explained by the formation of nanocrystalline graphite areas and their orientation toward the film’s normal direction. This novel coating type differs from standard amorphous a-C and ta-C coatings and offers new possibilities for superior mechanical behavior due to its combination of a high hardness and low in-plane Young’s Modulus.

Vacuum environments provide challenging conditions for tribological systems. MoS₂ is one of the materials commonly known to provide low friction for both ambient and vacuum conditions. However, it also exhibits poor wear resistance and low ability to withstand higher contact pressures. In search of wear-resistant alternatives, superhard hydrogen-free tetrahedral amorphous carbon coatings (ta-C) are explored in this study. Although known to have excellent friction and wear properties in ambient atmospheres, their vacuum performance is limited when self-paired and with steel. In this study, the influence of the paired material on the friction behavior of ta-C is studied using counterbodies made from brass, bronze, copper, silicon carbide, and aluminum oxide, as well as from steel and ta-C coatings as reference materials. Brass was found to be the most promising counterbody material and was further tested in direct comparison to steel, as well as in long-term performance experiments. It was shown that the brass/ta-C friction pair exhibits low friction (µ < 0.1) and high wear in the short term, irrespective of ambient pressure, whereas in the long term, the friction coefficient increases due to a change in the wear mechanism. Al₂O₃ was identified as another promising sliding partner against ta-C, with a higher friction coefficient than that of brass (µ = 0.3), but considerably lower wear. All other pairings exhibited high friction, high wear, or both.

Boron doping of diamond-like carbon coatings has multiple effects on their tribological properties. While boron typically reduces wear in cutting applications, some B-doped coatings show poor tribological performance compared with undoped films. This is the case of the tribological tests presented in this work in which an alumina ball is placed in frictional contact with different undoped and B-doped amorphous carbon coatings in humid air. With B-doped coatings, a higher friction coefficient at a steady state with respect to their undoped counterparts was observed. Estimates of the average contact shear stress based on experimental friction coefficients, surface topographies, and Persson's contact theory suggest that the increased friction is compatible with the formation of a sparse network of interfacial ether bonds leading to a mild cold-welding friction regime, as documented in the literature. Tight binding and density functional theory simulations were performed to investigate the chemical effect of B-doping on the interfacial properties of the carbon coatings. The results reveal that OH groups that normally passivate carbon surfaces in humid environments can be activated by boron and form B-O dative bonds across the tribological interfaces, leading to a mild cold-welding friction regime. Simulations performed on different tribological pairs suggest that this mechanism could be valid for B-doped carbon surfaces in contact with a variety of materials. In general, this study highlights the impact that subtle modifications in surface and interface chemistry caused by the presence of impurities can have on macroscopic properties, such as friction and wear.

Surface acoustic wave spectroscopy has been established as non‐destructive and fast method for characterization of mechanical properties of surfaces and bulk materials in both research and industry. The present work shows that by application of a novel and robust aluminum nitride (AlN) coated piezoelectric contact sensor the advantages of the method can be extended from room temperature to at least 600°C. An overview of sensor concepts and applications of the method is discussed first, followed by theoretical and practical considerations for design and coating of a novel temperature stable contact sensor. After fabrication of such a sensor using magnetron sputtering, it was tested in a modified surface acoustic wave spectroscopy setup with an incorporated heating table concerning signal amplitude and frequency range. The AlN coated sensor was found to perform well up to 600°C, with temperature limited by the specification of the heating table. At room temperature, performance was acceptable when compared with a conventional contact sensor using a PVDF piezoelectric foil. Application of the high temperature capabilities of the setup was demonstrated by measuring temperature stability of hydrogen‐free amorphous carbon coatings (a‐C and ta‐C) depending on their sp ³ carbon ratio. In another example, high precision temperature dependent measurement of Young's modulus for ultrasonic fatigue test specimen was taken, achieving an accuracy better than 1%. Use of the developed sensor opens up new possibilities in material science for in situ study of temperature depending mechanical properties for coatings and surfaces.