ToF-SIMS investigation of tribochemical surface reaction films built up by specially added cooling lubricants

Abstract

Cooling lubricants based on polyalphaolefin (PAO) are used in finishing processes for metallic components, but their influence on the formation of lifespan increasing tribochemical surface reaction films (boundary films) is poorly understood. The binding characteristics of different additives during finishing processes have been investigated using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Steel substrates heated to selected temperatures between 150 and 700 °C were dipped in four different cooling lubricants based on PAO to study fundamental binding processes. The boundary films built up on the steel surfaces were analyzed using ToF-SIMS via acquiring surface spectra as well as depth profiles. The results of these studies reveal the temperatures at which additives bind to steel surfaces and provide information regarding the thickness of the boundary films formed during the finishing. They also show that the temperature at which the boundary film formation starts is at around 400 to 450 °C. The starting temperature, as well as the thickness of the boundary films formed, depends on the composition of the cooling lubricants and on the concentration of additives. Thus, an increase in additive concentration leads not only to delayed starting temperatures, but also to greater film thicknesses.