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The inevitable increase of torque or load, because of lightweight approaches, pushes the oil-film temperatures on the teeth flanks of gears above 200°C, and requires alloys with higher strength and toughness properties. Furthermore, the real temperature on surfaces during the physical vapor deposition (PVD) or chemical vapor deposition (CVD) of thin films can exceed the annealing temperature of the state-of-the-art bearing and gear materials, such as AISI 52100 (1.3505), 16MnCr5 (1.7131), 21NiCrMo (1.6523), and 30CrMoV9 (1.7707). Such trends call for slip-rolling-resistant, alternative alloys suited for contact stresses above P0max of >2.2 GPa. The tribological profile under slip-rolling conditions at 120°C in a factory engine oil (SAE 0W-30, ACEA A3/B4, ν120°C = 5.33 mPa, HTHS150°C = 3.0 mPa) until 10 × 106 load cycles between P0max of 2.25 GPa to 3.92 GPa of uncoated 100Cr6 (1.3505, PBQ), 102Cr6 (1.2067), Cronidur 30 (1.4108, DESU), 45SiCrMo6 (1.8062, CVEM), 40SiNiCrMo1 [vacuum arc remelting (VAR)], CSS-42L (AMS 5932, vacuum induction melting (VIM)-VAR, and carburized), 36NiCrMoV1-5-7 (CAB), ASP2012 (ASP), and 20MnCr5 (1.7131, carburized) was established in twin disk machines (Amsler-type and 2Disk). The present benchmark illuminates the tribological impact of uncoated bearing steels in comparison to uncoated alternative steel alloys on friction, wear and slip-rolling resistance, as well as load carrying capacity. Iron-based steel alloyed with silicon or molybdenum reduces alloying costs and avoids thermo-chemical treatments. This benchmark suggests considering an extension to fracture toughness from hardness and strength as a relevant property for slip-rolling alloys. If slip-rolling resistant, the wear rates of the metallurgical different steels were more or less within one order of magnitude, but the influence of increasing contact stresses on the tribological profile differed significantly.
The approaching CO2 targets have now enforced automotive OEMs to engage R&D efforts also in powertrain to reduce friction, increase lifetime properties, and load carrying capacities of running systems. Martensitic steels such as 36NiCrMoV1-5-7 have a great potential to be used in automotive powertrain applications due to its favorable mechanical properties. In order to realize lightweight strategies, it is essential that steels with improved fracture toughness values and higher annealing temperatures with regard to higher contact pressures and possible thin film coating application be considered. State-of-the-art steels, like 16MnCr5, 21NiCrMo, 30CrMoV9, and 100Cr6 are not able to sustain a further increase in torque or load during operating time as well as annealing effects under higher oil or deposition temperatures. Therefore, the recent research on the slip-rolling resistance, frictional, and wear behavior of steels such as 36NiCrMoV1-5-7, Cronidur 30 (AMS 5898), 20MnCr5 (SAE 4820 or 5120), and 100Cr6H (SAE E52100) showed that possible lean alloying concept alternatives with promising performance characteristics are already available.