Friction and wear behavior of steel-steel pair under the boundary lubrication of liquid paraffin containing complex additive of nano/micro borate and zinc dialkyldithiophosphate

The friction and wear behavior of a steel-steel pair under the lubrication of liquid paraffin containing the complex additive of nano/micrometer borate and zinc dialkyldithiophosphate (ZDDP) was examined using a four-ball machine. The worn steel surfaces were analyzed by means of scanning electron microscopy and X-ray photoelectron spectroscopy. It was found that the two types of additives showed antagonizing antiwear effect to the steel-steel pair (GCr15). The friction coefficient rose sharply at a certain extended test duration, which was preceded by a relatively smaller and more stable initial value attributed to the physical-chemical adsorption boundary lubricating film. Such a kind of sharp increase in the friction coefficient was attributed to the breakage of the adsorbed boundary lubricating film at an extended test duration. At this stage some fine wear debris was observed on the worn steel surface and the wear of the steel-steel pair became more severe. With the further extension of the test duration, the tribological reaction among the additives and the steel rubbing surfaces was initiated, with the generation of the tribochemical reaction film containing B, N, S and P. This contributed to decrease the fluctuation of the friction coefficient and abate the wear of the steel-steel pair significantly as well.