significantly the friction down to 0.04 followed by full organic compounds, which reduced the coefficients of friction down to 0.03. A comparison between Figures 1 to 4 shows that uncoated alternative steels in combination with additive technology compete with thin-film coatings in terms of friction reduction.
Additive response on thin films A specific DLC coating reduced the friction down to ~0.05 under slip-rolling at 120°C in factory fill oil, when both tribo-elements were DLC coated (Figure 5). Additives can achieve further reductions in friction, but are limited (Figure 5) as DLC already made a large contribution. Here, thin carbon-based films (DLC, a-C, ta-C) require anchor or adsorption points (centres) for specific additives, incorporated in the films by oxygen, sulfur, and/or nitrogen.
contact pressures of P0mean= 1.5 GPa (P0max= 2.25 GPa) to P0mean of 2.5 GPa (P0max= 3.8 GPa). Figure 6 illuminates that pre-conditioned tribofilms were either able to yield lower coefficients of friction (COF) and lower wear coefficients than for steels without pre-conditioning by a tribofilm. Moreover, stable COF values under mixed/boundary conditions approaching and even less than 0.04 were achieved, thereby rivaling DLC-coated alloy equivalents.
Figure 6: Trends on friction and wear by applying pre-conditioning (metal-organic tribofilms) in slip-rolling endurance tests at 120°C
References [1] M. Woydt
Relationships between CO2 30-34
[2] C.-A. Manier, I. Dörfel, H. Ziegele, J. Barriga and J. Goikoetxea and M. Woydt
Figure 5: Frictional response under slip-rolling at T= +120°C and P0mean = 1.5 GPa (or as P0max = 2.25 GPa) of DLC coatings by additives added to a modified factory fill engine oil “VPX”
Pre-conditioning Tribofilms formed by additives in lubricants represent a key mechanism for preventing wear and are of nano-crystalline or amorphous nature and commonly fairly thin. Tribofilms can be generated by an accelerated procedure prior usage. 10 wt.-% of the respective additive to be tested was blended with the carrier (test) formulation. This represents a high treat rate, but the pre-conditioning shall be executed fast. A stable tribofilm on the specimen was then generated with this additive/carrier lubricant at T=120°C and P0mean = 1.94 GPa (P0max = 2.91 GPa) during only 10,000 slip-rolling cycles (or within 25 minutes) prior to the endurance slip-rolling test. The selected additives were metal-organic ones [8]. The cylindrical specimen with the pre-conditioned tribofilm is mated for the endurance tests of ten million cycles at 120°C in SAE 0W-40 factory engine oil against a fresh spherical specimen without tribofilm. The arrows in Figure 6 display the trends when increasing Hertzian
Zirconium-based coatings in highly stressed rolling contacts as alternatives to DLC and ta-C coatings, WEAR 269 (2010) 770-781
[3] C-A. Manier, G. Theiler, D. Spaltmann, M. Woydt and H. Ziegele
Benchmark of thin film coatings for lubricated slip-rolling contacts, WEAR 268 (2010) 1442-1454
[4] M. Woydt, C.-A. Manier, A. Brückner and V. Weihnacht Slip-rolling resistance of ta-C and a-C coatings up to 3.000 MPa of Hertzian contact pressure, Materialwissenschaft und Werkstofftechnik, 2012, 43, No. 12, p. 1019-1028
[5] M. Woydt, Chr. Scholz, J.-T. Burbank and D- Spaltmann Alloys slip-rolling resistant up to P0max of 3,920 MPa WEAR 474–475 (2021) 203707,
https://doi.org/10.1016/j.wear.2021.203707 [6] J. Burbank and M. Woydt
Comparison of slip-rolling behaviour between 20MnCr5 gear steel, 36NiCrMoV1-5-7 hot working tool and 45SiCrMo6 spring steel, WEAR 328-329 (2015) 28–38
[7] C. Scholz, M. Woydt and H. Mohrbacher Slip-rolling resistance and load carrying capacity of 36NiCrMoV1-5-7 steel, ASTM Journal Materials Performance & Characterization, 2014, Vol. 3 (1)
http://dx.doi.org/10.1520/mpc20130022 [8] J. Burbank and M. Woydt
Friction and wear reductions in slip-rolling steel contacts through pre-conditioned chemical tribofilms from bismuth compounds, WEAR 360-361(2016)29–37
emissions and friction and the
hidden impact of wear protection on sustainability and CO2 emissions, LUBE Magazine, issue no. 163, June 2021, p.
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