From Conventional to Environmentally Acceptable Additives: Tribological Behaviour in Volatile Lubricants for Punching Stamping Operations

In this study, we investigated the tribological properties of various additives (lubricity, friction modifiers, anti-wear and extreme pressure) in a highly volatile paraffinic base oil formulated for stamping applications, using a newly developed methodology for tribological testing. The investigation focused on the short-term (10 cycles) and long-term (10,000 cycles) effects of the different additive mixtures on friction and wear behaviour. It was found that the performance of the additive mixtures evolves with sliding time, which is due to changes in contact conditions: the transfer of the Fe film from the steel sheet to the WC-Co surface increases the contact area, which in turn leads to a significant reduction in contact pressure and changes the activation of tribofilm formation. The presence of tribofilms influences the amount and size of the contact area and reduces the adhesion between the contact surfaces. Among the conventional additives, sulphurised additive mixtures show stable performance under both short and long-term conditions, while more aggressive chlorinated additive mixtures perform well in the short term, but their performance decreases with prolonged sliding. Importantly, the additives with a decreasing environmental impact outperformed the conventional additives under long-term conditions: the less harmful phosphorus-based mixture outperformed the sulphurised mixtures in terms of wear properties, while the performance of environmentally acceptable polyol ester was particularly encouraging, exhibiting the lowest friction coefficient (~0.11, compared with ~0.12 for S-oil and 0.14 for S-ester) and the second lowest wear coefficient (~1.1 × 10−1 mm3/Nm compared with ~1.5 × 10−1 mm3/Nm for S-ester). Overall, the polyol ester reduced the coefficient of friction by approximately 8 to 21% compared to sulphurised additive mixtures, and its wear coefficient was also about 27% lower.