Novel dispersions of thermally reduced graphite oxide (TRGO) in ester oil were prepared by means of high-pressure homogenization. Due to the versatility of the homogenization process, TRGO dispersions with a wide concentration range of 0.01-0.5 wt% could be prepared both in ester oil that contains additives (FGL46 E) and in an additive-free ester oil formulation. The obtained dispersions were characterized with respect to their sedimentation stability and their tribological properties. Therefore, highly resolved Stribeck curves were measured in the boundary region. Compared to benchmark dispersions of carbon black and graphite, our TRGO-containing formulations showed considerably lower friction and wear. The results of this study show the positive influence of TRGO on the friction behavior of ester oil formulations, thereby presenting TRGO-containing ester oils as a promising new family of liquid-based lubricants.

Dry milling of graphite in a ball mill represents a versatile one-step mechanochemical process for fabricating mechanochemically functionalized multilayer graphene (MG) bearing different functional groups. The variation of the milling parameters enables to control particle size, shape, functionality, specific surface area and dispersability of the MG functional fillers. In this study, MG was used as functional nanofiller for the production of SiC/MG nanocomposites. The nanocomposites exhibit significantly improved tribological behavior. The results of rotating pin on disc sliding tests show that with SiC/MG a noticeable improvement of friction and wear behavior under water-lubricated conditions like in slide bearings and face seals can be achieved. Sliding friction systems with the variant SiC+2% MG-CO 2 -120h appear to have the most promising tribological properties, due to the reduced size of the homogeneously distributed graphite particles, which promote the formation of advantageous surface states.

Stiction, run-in wear and friction of lubricated polyoxymethylene homopolymer (POM)- and aliphatic polyamide (PA46)-steel tribosystems were investigated for mild-loaded mixed lubrication conditions with and without thermal conditioning of the polymers in the lubricant prior to testing. Macroscopic oscillatory tribometry and standard gliding experiments were carried out. The hypothesis that sorption of a lubricant into a thermoplastic polymer and partial solving of the surface by the lubricant can change wear rate and friction was tested. It was found that for POM-lubricant-pairings, the tribological behavior is dominated by the sorption of the lubricant into the polymer; it is not influenced by the spreading energy. For the PA46-lubricant pairings, no mass uptake by sorption was measured, and the tribological behavior is influenced by spreading and changes in hardness due to thermal aging. For mild loading in mixed lubricated conditions, friction and wear properties seem to be primarily determined by the hardness-dependence of abrasive contact and less by adhesion or hysteretic mechanisms.

Even though hard, low friction coatings such as diamond like carbon (DLC) would be beneficial for the performance and longevity of rubber seals, a crucial challenge remains. The elastic mismatch of rubber substrate and DLC coating prevents a fracture free coating application. In this work, a nature inspired approach is applied to render the stiff coating flexible and resilient to delamination at the same time by direct patterning. Rubber substrates were laser structured with tile patterns and subsequently DLC coated. Tensile and tribology tests were performed on structured and unstructured samples. Unstructured DLC coatings showed a crack pattern induced by the coating process, which was further fragmented by tensile stress. Coatings with tile patterns did not experience a further fragmentation under load. During continuous tribological loading, less heterogenous damage is produced for tile structured samples. The findings are ascribed to the relief of induced coating stress by the tile structure, meaning a more resilient coating.

Polyether ether ketone is a highly resilient thermoplastic that can withstand as an unfilled or filled polymer tribological high loads in high-temperature applications, as it has a high glass and melting temperature. Currently, it attracts interest as a lubricated material for drive systems to minimize noise, as a lightweight solution improving the CO2 balance, or in combination with novel lubricants. We have produced friction and wear maps with pin-on-disk tests on steel axial bearing rings and investigated the interaction and running-in behavior with different lubricants, such as different polyalkylene glycols, polyalphaolefin, and various esters. In general, the behavior of polyether ether ketone is hardly influenced by the frictional heat when loaded at room temperature. It shows a lubricant-specific polymer transfer and surface softening. This running-in impact is enhanced if a loading strategy with initial high friction power is applied. The beneficial transfer forms on the track in mixed lubrication on the rough counterpart until an increasing adhesive contact limits this effect. The appearance of a minimal wear coefficient was found below the glass temperature with lubricants strongly sorbing in polyether ether ketones, such as polar polyalkylenglycol or trimellitic acid ester-based lubricants. Using the interfacial energy of the system to derive an energy term describing the tendency to form a stable transfer, systematic trends in the tribological behavior of polyether ether ketone and polyketone could be identified that can be projected on other systems.