Influence of wetting and thermophysical properties of diamond-like carbon coatings on the frictional behavior in automobile gearboxes under elasto-hydrodynamic lubrication

With regard to the transportation sector, an efficient powertrain technology contributes to sustainable lowering of CO2 emissions. Due to the periodic or continuous operation in boundary and mixed friction, minimization of friction losses in automobile gearboxes offers massive potential in terms of efficiency improvement and saving fossil fuels. In close cooperation between the Surface Engineering Institute (IOT) and the Gear Research Center (FZG), the aim of this work was to reduce friction losses in powertrain by diamond-like carbon (DLC) coatings on highly loaded gears under severe rolling-sliding conditions. The zirconium based DLC coatings ZrCg (a:C-H/ZrCg) and nanocomposite ZrC (a-C:H/ZrC) were deposited by physical vapor deposition (PVD) at IOT. The industrial DLC coating DLC-REF1 served as reference. Application-related tribological tests of lubricated highly-loaded rolling-sliding contacts were performed in a twin-disc test-rig and a gear efficiency test-rig at FZG. Calculations and measurements of relative lubricant film thickness confirmed that the tribological model tests covered the entire friction regime from boundary and mixed friction to fluid friction (elasto-hydrodynamic lubrication, EHL). Despite complete separation of the coated surfaces, the Coefficient of Friction was reduced by 35% using ZrCg coated discs in the twin-disc test-rig. Practical investigations of DLC coated gears in the FZG gear efficiency test-rig revealed that compared to uncoated gears friction losses in EHL were reduced by up to 25% using the industrial reference DLC-REF1 and 39% using ZrCg, especially at higher loads and higher circumferential speeds. This yet widely unknown favorable effect of DLC coatings under EHL conditions was attributed to the thermophysical properties of DLC coatings and confirmed by simulations of real rolling-sliding contacts at FZG. Wetting analyses of tribological surfaces were analyzed determining the surface properties, interfacial tension and surface energy, of the DLC coatings and the gear oils by means of contact angle measurements. The adhesion energy was calculated from contact angle data. Correlation analyses revealed a clear impact of the interfacial tension and adhesion energy on the frictional behavior under boundary and fluid friction conditions. It was found that a higher adhesion energy (good wetting) contributes to a lower CoF under boundary and mixed friction conditions as well as in the fluid friction regime (EHL).