Thin film degradation testing of lubricants for gas generator, diesel engine and hydraulic applications

Lubricant stability is often tested at temperatures above those of normal operating regime to reduce testing durations. Increase in viscosity and acidity frequently constitutes a failing mark, although lubricant still remains liquid. Thus thin film tests gain popularity, because they can show the ability of a lubricant to remain liquid without decomposing into volatiles or polymerizing to solids [1].
Several commercial lubricants were obtained from BM Energy (Latvia) and CJC (Denmark), representing different manufacturers of diesel engine, gas generator and transmission lubricants. Thin film degradation was performed on low carbon steel coupons [1]. Films of initial 500 μm thickness were heated at 130 °C for up to 1815 hrs and then at 140 °C in a Memmert forced draft oven. From 3 to 5 coupons were used for each lubricant and periodically volatile emissions were measured gravimetrically. Typical plots for each lubricant appear in Fig. 1 (i). Time and Temperature Superposition (TTS) was utilized to normalize testing durations at 130 °C to those at 140 °C by applying van’t Hoff’s rule, i.e. doubling the duration with 10 °C drop in temperature [2], e.g.1815 hrs at 130 °C into 907.5 hrs at 140 °C.
When suspecting film solidification, the coupon was placed into a beaker with 20 mL of the same fresh lubricant and held for 180 min at 90 °C, stirring it several times. Afterwards the coupon was rinsed with heptane and dried. Most samples eventually produced insoluble residues when degreasing in chloroform for 180 min. Coupons were photographed before and after degreasing or heating. Results show that commercial lubricants produce very different trends of volatile emissions and formation of residues insoluble in fresh oil. Lab observations are likely to relate to field performance