TRIBOLOGY
Five pathways to reduce friction under slip-rolling or highly concentrated contacts
Dr. Mathias Woydt, Managing Partner, MATRILUB, Berlin
Introduction There is an undisputed link between friction losses and CO2
emissions [1]. So-called “highly
concentrated contacts” or slip-rolling contacts represent key tribosystems in green technologies, like wind turbines (upstream) and electrified powertrains (downstream). In consequences, friction reduction transduces to better economics, climate protection and/or extended ranges. The technological question is: How friction can be reduced and longevity extended?
There are the following pathways: 1. Thin film coatings, 2. Alloying steels without using thin film coatings, 3. Additive response on uncoated alloys 4. Additive response on thin films, 5. Pre-conditioning as well as combinations of thereof, which illuminate significant, but practicable reductions in coefficients of friction without compromising longevity.
Another aspect for improving sustainability is downsizing (power-to-weight ratio) in the frame of lightweight approaches, which reduces the material consumption and its associated carbon footprint of materials usages, but inevitably lead to a rise in Hertzian contact pressures with an associated increase in surface or oil film temperatures.
The slip-rolling tests under the regime of mixed/ boundary lubrication were executed using a 2disk test rig of Optimol Instruments under initial Hertzian contact pressures from P0max of 2.25 GPa to 3.92 GPa at an oil temperature of 120°C. Two disks with the same outer diameter slip-rolled against each other
on their circumferences. The lower, cylindrical and uncoated disk with super polished surface (Ra ~ 0.005 μm) was mated against a fine grinded and uncoated (Ra = 0.150–0.300 μm) spherical counterpart. The calculated value of minimal oil film thickness hmin is 0.0252 μm at T = 120°C and FN = 2,000 N for the fully formulated engine oil SAE FF 0W-30 (“VP1”; HTHS = 3.0 mPas, ACEA A3/B4; η120°C
= 5.33 mPas).
The Figures 1-5 highlight the coefficients of friction under mixed-boundary lubrication regime (Tallian parameter λ calculates to ~0.144) at 120°C under slip-rolling with 10 % slip.
Thin-film coatings
Thin-film coatings, as per diamond-like coatings (DLC) or tetrahedral amorphous carbon (ta-C), are today well established friction reduces in mechanical engineering, but they have to be slip-rolling resistant and retain the ability to reduce friction. DLC and carbon-based coatings have as features low surface energies, minimising the reactivity with oils and some additives. In consequences, other thin film metallurgies, like Zr(C,N) or ZrCg, can overcome this issue, but they have deposition temperatures above the annealing temperature of common bearing and gear steels. The application of thin-film coatings [2,3] only on the polished cylindrical test sample remarkably reduced the friction values of a given engine oil (see Figure 1). The lowest friction was achieved by a ta-C-type film [4]. Figure 2 displays a benchmark of different DLC-type coatings deposited on one specimen (disk) and on both specimen (disks). The safest approach to reduce friction down to 0.04 is to coat both surfaces (Figure 1) unless a specific thin film coatings is identified, which offers the same friction reduction only by coating one tribo-element.
Continued on page 18 LUBE MAGAZINE NO.173 FEBRUARY 2023 17
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