Dry sliding tribology of dioctyldithiophosphoric acid deposited on steel at 150 °C

Dioctyldithiophosphoric acid was deposited on 7 nm (RMS) rough steel surfaces at 150 °C to create a densely packed (area-95%) deposit of glassy phosphate islands of maximum 500 nm height. This standard substrate was subjected to dry lubricated ball-on-disc tribology using a 2 mm diameter steel ball in the 25-500 mN normal load range at a sliding velocity of 1 mm/s. The experiments recorded friction coefficient of the order of 0.18 but the life of the film as it wears out with sliding distance was found to decline with increasing load. The life-load characteristics that we record in these experiments, we argue, can only be explained if there is a load induced growth of the deposited film during sliding. We record this load induced growth to yield phosphate islands which are significantly smaller but harder than those grown by thermal activation alone.