The impact of usage on the performance of lubricating emulsions in micro-forming

Lubricants are known to change over time, both in terms of aging of the oil and additives, and additive depletion through use. Samples were extracted at intervals from an industrial micro-deep drawing process line to evaluate how the lubricant changes over time. Four-ball testing showed that the tribological performance degraded quickly but stabilised after one million strokes. Measuring of the pH-level and correlation with the oil concentration showed that the emulsion initially becomes more acidic but stabilises after one million strokes, despite changes in oil concentration. Elemental analysis using water inductively coupled plasma (ICP) analysis standard showed that particles in the lubricant could be related to an increasing formation of wear debris. Fourier-transform infrared (FT-IR) analysis was used to evaluate changes in chemical composition of the base oil. Lubricant properties were found to degrade in samples obtained from the onset of the production, while they stabilised for samples that had been in use for a period of more than 1 week. The performance of the used lubricant is therefore different from that of the fresh lubricant, with four-ball wear scars increasing by approximately 30%. These results show that when tribological tests are tailored to industrial tribo-systems, the lubricant that is used in the test should be the steady state lubricant found in the real system rather than fresh lubricant.