Hydrogen influence on material interaction with ZDDP and MoDTC lubricant additives

In this paper the interaction of lubricant additives with hydrogen at the frictional interface has been investigated. Three different states of the base AISI 52100 alloy steel have been tested: untreated, nitrided and sulphonitrided, with different combinations of PAO6 base oil and ZDDP/MoDTC (Zinc DialkylDithioPhosphate and Molybdenum Dialkyldithiocarbamate) additives. Experiments have been carried out on pin-on-plate reciprocating tester, immersed in the lubricant heated to 100 °C. In the boundary lubricated regime the results showed the best friction behaviour for treated surfaces tested in presence of PAO6 with additives suggesting some interaction at the frictional interface of nitrided and sulphonitrided surfaces with lubricant additives. The minimum recorded value of coefficient of friction was as low as 0.05 for the sulphonitrided sample with PAO6+MoDTC oil. In the case of the treated surfaces a characteristic “low friction phase” has been observed when tested with PAO6 with additives. After a given time, the coefficient of friction was increasing to a higher steady-state value and the duration of this low friction phase varied from sample to sample. This can be explained by the mechanism of hydrogen interaction in the boundary lubrication regime, which was postulated for the base oil case by some of the authors in their previous papers. To validate the hypothesis, an experiment has been carried out where the test was stopped at the end of the “low friction phase” and during the hold period the sample was re-saturated with hydrogen. After resuming the experiment the low friction regime was again observed. The effect of a potential synergistic mechanism between hydrogen and ZDDP or MoDTC lubricant additives on frictional behaviour of nitrided and sulphonitrided surfaces is discussed in this paper.