Mechanical and physico-chemical study of sulfur additives effect in milling of high strength steel

Most machining operations require the presence of a cutting fluid in order to ensure their success due to the intensity of thermal and mechanical conditions generated on the cutting tools. In some cases of severe machining, the fluid contributes to lubrication in a physicochemical way thanks to the additives it contains. This study aims to analyze the tribochemical mechanisms of lubrication during milling operations and subsequently at optimizing the lubricant formulation. The objective of this study is also to identify and to compare, the performance of various extreme pressure (EP) sulfur-containing additives, and to understand their action mechanisms in metal cutting. Physico-chemical analyses are carried out by means of Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) with the purpose of detecting the nature of additive reaction products on the surfaces involved in cutting and correlating them with the milling results. The polysulfide additive is found to exhibit the best efficiency (decrease of specific cutting energy and tool wear) in the tested milling conditions. A correlation is found between the additive chemical reactivity and the milling test results.