24th International Colloquium Tribology - January 2024 87 Assessment of Different Coatings on the Friction and Wear Behavior of Differential Shafts for Electric Vehicles Etienne Macron 1* , Johnny Dufils 1 , Christophe Héau 1 1 IREIS, HEF Group, ZI Sud, Rue Benoit Fourneyron, 42162 Andrézieux-Bouthéon, France * Corresponding author: emacron@hef.group 1. Introduction Electric engines for vehicles generate high torques. Compared to ICE engines, this leads to higher torque transmitted to the differential and therefore to higher tribological stress on the components. In particular, the normal load applied to the contact between the differential shaft and the planet gear is increased. Conventional surface treatments are showing some limitations. Therefore, a study combining numerical and experimental approaches was conducted to qualify the performances of hydrogenated amorphous carbon (a-C: H) coating in terms of friction and wear behaviour for this specific application. 2. Preliminary simulations In order to evaluate the influence of the different contact parameters on the lubrication regime, a preliminary numerical simulation work was undertaken. Figure 1: Simplified illustration of a differential assembly First, the normal load between the differential gear and the shaft was calculated on the basis of the characteristics of a dual motor electrical vehicle especially max torque T motor (967 Nm), reduction ratio λ (9.73: 1) and lever arm between the middle of the differential shaft and the middle of the oil cut set to 38 mm. Assuming that torques applied to the two differentials are equal for this dual motor configuration and that the load was equally split between the two pinions, the normal load was estimated to 62 kN max. using the following formula: The estimated oil film thickness was calculated using the Dowson-Higginson formula for a 2D line contact. Various velocity, load and oil temperature conditions were computed using the components characteristics described in section 3. The data were plotted as L parameter combining the oil film thickness with the initial composite roughness criteria of the contact as a function of the differential rotation speed of the wheels (see Figure 2). Figure 2: Lambda ratio as a function of the differential rotation speed of the wheels (oil temp. 50-°C) From the data available and the assumptions made, these calculations showed that: i) the lubrication regime was mainly affected by the differential rotation speed; ii) the pinion/ shaft contact seemed to be initially stressed in the boundary and mixed lubrication regime except at very low motor torque and high differential rotation speeds. These theoretical outputs provided also some useful data in order to define the experimental test protocols. 3. Experimental set up on a Shaft/ Bearing tribometer A specific test setup (Figure 3 & Figure 4) was designed in order to assess the tribological behaviour of hydrogenated amorphous carbon (a-C: H) coatings applied on the shafts. The shafts were rubbed against a steel ring which inner surface simulating the gear inner surface. Rotation speed up to 10 000 rpm Normal load from 500 to 20 000 N Shaft diameter up to 30 mm Oil temperature up to 120-°C Oil volume ~1.2 L Controlled parameters Possible control of test sequence with variation of speed and/ or normal load 88 24th International Colloquium Tribology - January 2024 Assessment of Different Coatings on the Friction and Wear Behavior of Differential Shafts for Electric Vehicles Figure 3: Shaft bearing tribometer and its main features (IREIS Design) Figure 4: Specific test set up for differential shaft-gear contact (IREIS Design) Test specimens were made out of real differential shafts re-designed to fit into the experimental set up presented in Figure 4 . The shafts have an outer diameter of 22.2 mm. The rotating rings standing for the pinion were designed to obtain an oil clearance ≈ 0.14 mm and a line contact length of 16 mm for the short-term test to determine friction coefficient and 2 mm for the endurance test to evaluate wear behaviour. The ring material was a typical gear-type steel (20MnCr5) with a hardness of ≈ 60 HRC and an initial surface roughness of Ra ≈ 0.3 mm (inner diameter). Finally, a commercial lubricant (Gearbox oil 75W80 Tranself NFP) was used at a temperature of 50-°C at the beginning of each test sequence. The oil temperature at the end of each test sequence was dependent on how much power was dissipated by friction in the contact. 4. Experimental results To quantify the friction coefficient, a test sequence at a constant load of 15 kN and a decreasing rotation speed from 300 rpm to 50 rpm by 50 rpm steps of 2 min each was repeated until friction stabilization and at least 5 times for shafts with two surface treatments: e-nickel (reference) and a-C: H DLC coating. The results are presented in Figure 5 with data plotted as a function of (viscosity × entrainment speed)/ (contact pressure × roughness) in order to consider the variability between the different material combinations in terms of i) final roughness of the steel ring (1st order of magnitude parameter) and ii) oil temperature and contact pressure (both being friction and wear dependent). Figure 5: Comparison of the friction coefficient between e-nickel and a-C: H DLC coated differential shafts Complementary tests with a longer duration (8h) were undertaken to better compare the wear behaviour of the electroless nickel treatment and the a-C: H DLC coating (see Figure 6 ). Figure 6: Wear rate obtained after a 8-hour test at constant speed (100 rpm) and constant load (8 kN) with an oil temperature of 50-°C 5. Conclusion In terms of friction coefficient, the entrainment product and the composite roughness of the rubbing surfaces turned out to be first order parameters in order to determine whether a transition from mixed lubrication to hydrodynamic lubrication was achieved or not. Both for a-C: H and e-nickel coated pins, a transition from mixed lubrication to EHL was observed because a low composite roughness is achieved. Out of the endurance test protocol, DLC-coated shafts (a-C: H) showed almost no wear when the electroless nickel was subject to abrasive wear despite a stabilised friction coefficient equivalent in both cases. Well known for their low friction properties, DLC coatings (a-C: H in this case) demonstrated here under realistic test conditions a strong durability performance in this differential assembly application for electric vehicles compared to the standard electroless nickel solution.