24th International Colloquium Tribology - January 2024 99 Effects of Calcium Detergents on Micro-Pitting of Gear Metals Akira Tada 1,2,3* , Dirk Spaltmann 2 , Kazuo Tagawa 3 , Valentin L. Popov 1 1 Technical University of Berlin, Berlin, Germany 2 Federal Institute for Materials Research and Testing (BAM), Berlin, Germany 3 ENEOS Corporation, Tokyo, Japan * Corresponding author: akira.tada@campus.tu-berlin.de 1. Introduction For achievement of carbon neutrality by reducing carbon dioxide emissions, electrified vehicles (EV) such as hybrid electric vehicles (HEVs), plug-in HEVs, and battery EVs attract much attention. To increase range of EVs, reducing transmission torque loss in transaxles is effective. For reducing churning and drag loss, viscosity reduction of lubricants in transaxles is one promising method [1]. However, it generally leads to thin oil films and makes the lubrication condition severe. This sometimes results in fatigue such as micro-pitting, especially at gear contacts. Because micro-pits are known to be initiated on metal surface, lubricant additives which form tribo-films on metal surfaces are considered to affect formation of micro-pits [2]. Some of such additives are calcium detergents. Recently, it was reported that an optimal choice of calcium detergents could lead to high anti-fatigue performance, even if lubricant viscosity is low [3, 4]. However, the mechanism about how tribo-films of calcium detergents affect the formation of pits requires further investigation. In this study, to clarify influence of calcium detergents on micro-pitting, slip-rolling tests were carried out using twin disc tribometers. 2. Experimental The conditions for tribological tests were set so that they were similar to those at gear contacts, i.e., the oil temperature was set to 100-°C, slide-to-roll ratio to -9%, and the maximum Hertzian contact pressure to 2-GPa. SCM420H type of steel was chosen as substrate for the discs because it is widely used as material in transmission gears. Two types of discs were used for experiments: specimen discs and counter discs (Table 1). Counter discs had curvature on their circumference for achieving the high Hertzian pressure above. Since micro-pitting is known to be initiated by roughness asperities, roughness of counter discs was controlled to be relatively large (R a -=-0.3-mm). Furthermore, to maintain the large roughness of counter discs and to promote micro-pitting on specimen discs, softer specimen discs were used. Table 1: Specification of discs Specimen disc Counter disc Material JIS SCM 420H Curvature w/ o w/ (R = 25-mm) Ra, mm 0.04 0.3 Hardness HRC 58 HRC 61 Four kinds of lubricants were created and evaluated to investigate the effect of calcium detergents. Their formulation is shown in Table- 2. They were formulated using poly alpha-olefin (viscosity at 100-°C was 4-mm 2 / s) as base oils. All lubricants included antioxidants (AOs) for prevention of oil degradation during tests. Tricresyl phosphates (TCP), which are typical anti-wear agents, were added for Oil B so that the concentration of phosphorus was 800-ppm. Oil C and D further contained different types of calcium detergents, calcium sulfonates and calcium salicylates, respectively. The concentration of calcium for oil C and D was 600-ppm. Slip-rolling tests were carried out up to 7-million cycles. Each test was stopped at 0.1, 1 and 4-million cycles and discs were detached for surface observation with optical and confocal microscopes. After the observation, they were attached again, and test was continued. Table 2: Formulation of lubricants Oil A Oil B Oil C Oil D Base oil Poly-α-olefin Bal. Bal. Bal. Bal. Antioxidant Phenol type mass% 0.5 0.5 0.5 0.5 Anti-wear agents TCP massppm (P) 800 800 800 Detergents OB Ca Sul. massppm (Ca) 600 OB Ca Sal. massppm (Ca) 600 Kinematic viscosity 100-°C mm 2 / s 4.0 4.0 4.0 4.0 TCP: Tricresyl phosphate, OB: Over-based, Sul.: Sulfonates, Sal.: Salicylates 100 24th International Colloquium Tribology - January 2024 Effects of Calcium Detergents on Micro-Pitting of Gear Metals 3. Results and Discussion In Figure- 1, the friction coefficients of four lubricants are shown up to 7-million cycles. Comparing Oil A, B and C, it was found that friction was increased by addition of TCP and further by addition of Ca sulfonates. On the other hand, Oil D showed almost the same value as the Oil B, indicating that Ca salicylates did not increase friction. Figure 1: Friction coefficient The micrographs of the surface on the specimen discs are shown in Figure-2. In the case of Oil A, there was no micro-pit even after 7-million cycles. Addition of TCP and Ca sulfonates increased the number of micro-pits, while Ca salicylates did not significantly. Figure 2: Micrographs of surfaces of specimen discs Micro-pit area and wear volume was measured using optical and confocal microscope. The results are shown in Figure-3. Oil C, which had Ca sulfonates, was able to suppress wear, but it drastically increased micro-pit area as well. In contrast, Oil D, which included Ca salicylates also showed small wear, but micro-pit area stayed low at the same time. Figure 3: Wear volume and micro-pit area of specimen discs To clarify the influence of the additives on the running-in process, roughness change of the counter discs was measured using confocal microscope. The reduction percentage of R a value is shown in Figure 4. Oil A showed rapid decrease of R a , which could be related to the decrease of friction. TCP suppressed roughness reduction and Ca sulfonates did it further. Ca salicylates also suppressed roughness reduction, but the degree was smaller than that of Ca sulfonates. Figure 4. Roughness reduction of counter discs These results indicate that the tribo-film created by TCP/ Ca sulfonates showed high anti-wear properties, but, with respect to surface roughness, it led to an un-finished running-in process, which resulted in severe micro-pitting. On the other hand, Ca salicylates showed similar high anti-wear properties and simultaneously achieved enough running-in which prevented acceleration of micro-pitting. This difference might be related to the formation rate of tribo-films. Further analyses on the tribo-films are on-going for clarification of mechanisms. 4. Conclusion Slip-rolling tests were carried out to investigate effects of Ca detergents on micro-pitting behaviours on gear metals. The followings were found. 1. By adding Ca sulfonates, wear was reduced, but micro-pit formation was accelerated. 2. Ca salicylates did not accelerate micro-pitting, reducing wear at the same time. References [1] Iino, M. et al., SAE Technical Paper (2021) 2021-01- 1215. [2] Ueda, M. et al, Tribol. Int. 138 (2019) 342-352. [3] Tada, A. et al., SAE Int. J. Adv. & Curr. Prac. in Mobility 5(3) (2023) 1055-1062. [4] Tokozakura, D. et al., SAE Technical Paper (2022) 2022-01-1103.