PCMO Lubricant Friction Modifier Performance Durability-Extended Tribology Studies
Designing friction modifiers (FM) resistant to performance degradation is important to meet future demands for increased fuel economy, by using low viscosity oils and longer drain intervals. Whether studying organic or organometallic FM-additives, tribological research can help increase our understanding of friction modifier surface-adsorption and performance retention, focusing on how the coefficient of friction (COF) changes with contact time. In this study we measured the performance characteristics of glycerol monooleate (GMO) industry standard organic FM, Molybdenum dithiocarbamate (MoDTC), common known FMs [1, 2], and an experimental organic friction modifier (EXP-OFM) with performance retention. For these studies passenger car motor oil (PCMO) was formulated with Group III oil base stocks and a full range of additives meeting the viscometric requirements for a 5W-30 and 5W-20 finished oil. These additives include: viscosity and pour point improvers, calcium sulfonate detergents, polyisobutylene succinimide dispersant, amine and phenolic antioxidants, zinc dialkyldithiophosphate (ZDDP) anti-wear, silicone anti-foam, and the friction modifiers. To monitor changes in the COF with time (performance retention), the Cameron Plint TE-77 tribological testing instrument and PCS Instruments Mini-TractionMachine were utilized in non-routine extended tribology test methods. Two types of friction modifier tribology studies were then carried out to measure retention of performance. The first study measures initial FM performance (COF-decrease) in oil held for an 60 minutes (after standard testing 50 o C to 160 o C) to insure tribofilm formation. Following this hold stage and after draining and refilling test cell with nonfriction modifier oil (heated to 160 o C), the COF is monitored for ninety minutes. Finally, after draining and refilling the test cell with the original friction modifier containing oil, the COF (at 160 o C), is monitored for an additional ninety minutes. An example of the results is given in the figure below in Figure 1. These tests measured the memory of the tribofilm initially formed and ability for it to be refreshed to its original state with subsequent FM containing PCMO. The results indicated the 5W-30 oil with GMO at (1 % wt.) treat rate gave an initial friction reduction of -29% COF and retained a reduction of -18% COF on switching to non-FM oil. This could then be completely refreshed to -29% COF after the second oil switch back to GMO containing oil. The same base formulated 5W-30 with Exp-OFM at (1 %wt), had an initial friction reduction of -45% COF, retained -31% COF with non-FM oil, and was then refreshed back to a – 44% COF reduction. In contrast, although MoDTC at (1 %wt) shows a stronger initial friction reduction of -70% COF, during the first hold period, before oil switches, this degrades up to -45% COF. In addition, after the non-FM oil switch, it degrades further all the way up to only -13% COF reduction. Finally, following the second MoDTC (1 %wt) oil switch, the COF again first drops to -70% COF and then degrades up to a -44% COF reduction.