Contact geometry was 0.5 mm diameter point (~0.2 mm²). The estimated current density is approximately 50 mA/mm² at a current of 10 mA and 1 A/mm² at a current of 200 mA.
Similar results were achieved for two greases. Grease 1 showed higher ECR values during testing with AC and DC current and the CoF values were compatible.
Wear damage by AC and DC
The most important subject in this study was the damage to the surface caused by an electrical current during tribo-testing. All wear scars were investigated and measured using a 3D microscope. Figures 3 and 4 show examples of the wear scars for the tested oils, tested oil 1 (with ZDDP), at different AC current intensities.
Table 1 shows the test parameters for this study.
Figure 3: Wear scar images – oil 1 after tribo-testing under different AC current intensities.
Table 2 contains the chosen lubricants for this study.
Results The results of the 32 tests revealed the following: Friction behaviour and electrical contact resistance: • Across all tests, the CoF (Coefficient of Friction) value remained stable at between 0.12 and 0.14, independent of current type or magnitude.
• Oils and greases containing ZDDP consistently exhibited higher electrical contact resistance (ECR) values than those without.
Figure 2 shows exemplary the CoF and ECR curves during tribo-testing for oil 1 and 2.
Figure 4: Wear scar images – oil 1 after tribo-testing under different DC current intensities.
The wear area and wear depth were measured for all 32 tests. The area values are presented as bar diagram and wear depths as the line exemplary in Figure 5 and 6 for oil 1 at AC and DC currents.
Figure 2: ECR vs Current (AC) for oil 1 left and oil 2 right.
For oil 1 (with ZDDP) wear area and depth began to increase moderately at currents as low as 10 mA AC and increased significantly from 10 to 100 mA.
LUBE MAGAZINE NO.190 DECEMBER 2025 19
There were no remarkable changes in wear damage in the case of oil 2 (without ZDDP), regardless of current type or density.
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