Continued from page 17


3 shows that uncoated alternative steels achieve the same friction coefficients as thin-films coated on one tribo-element.


Figure 1: Evolutions in coefficients of friction of thin film coatings slip-rolling endurance tested at T = 120°C in SAE 0W-30 engine oil (P0mean = 1.5/1.94 GPa)


Figure 3: Evolutions in coefficients of friction until 10 million slip-rolling cycles at P0mean = 1.5 GPa and T = 120°C in SAE 0W-30 engine oil with uncoated steels


Figure 2: Evolutions in coefficients of friction of thin film coatings slip-rolling endurance tested at T = 120°C in SAE 0W-30 engine oil (P0mean = 1.5 GPa)


Alloying steels without using thin-film coatings Alloying steels with specific elements is a hidden option in order to reduce friction, improve toughness and strength as well as annealing temperature [5]. A “monolithic” solution avoids the additional step of deposition and functional risks associated with delamination and fatigue of coatings. Martensitic steel 100Cr6 (AISI 52100) and the martensitic nitrogen alloyed steel Cronidur 30®


are references


for ball/roller bearings. Same for gear steels with carburised 20MnCr5 (AISI 5120H). Figure 2 and Figure 3 illuminate for P0mean = 1.5 GPa (or FZG >14), that uncoated steel couples offer a clear potential for friction reduction by metallurgical measures, such as the silicon-alloyed spring steel 45SiCrMo6 (1.8062, V300) [6] and the molybdenum-alloyed 36NiCrMoV1-5-7 [7]. Both inexpensive metallurgies have annealing temperatures above 400°C. The heat-resistant and hot working steel 36NiCrMoV1- 5-7 offers high fracture toughness KIC of ~120 MPa√m far above of those for 100Cr6 (AISI 52100) or Cronidur30®


. A comparison between Figures 1 to 18 LUBE MAGAZINE NO.173 FEBRUARY 2023


Additive response on uncoated alloys The frictional responses of individual additives blended by 1 wt.-% to a carrier (test) oil lubricating self-mated couples of uncoated, silicon alloyed spring steel (40SiNiCrMoV10 (UNS K54015, NC310YW; ~57 HRC, KIC ~52 MPa√m) are shown in Figure 4, right. The carrier (test) oil “VPX” was derived from the associated fully formulated SAE 0W-30 engine oil and was a synthetic hydrocarbon (PAO), which only contained the same additives for corrosion protection and antioxidants. Each test was executed with new test couples. The evolutions of the coefficients of friction underline, that 500,000 cycles [12] are sufficient for the tribofilm formation and to identify the initial frictional response.


Figure 4: Frictional response of additives under slip-rolling in a modified factory fill SAE 0W-30 engine oil “VPX” at +120°C and P0mean = 1.5 GPa (P0max = 2.25 GPa)


Due to the test conditions, all additives displayed a high load carrying capacity. Some organo- molybdenum and organo-bismuth compounds reduce


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