Influence of Steel Surface Chemistry on the Bonding of Lubricant Films

摘要

Virtually all moving mechanical assemblies on spacecraft contain steel componentsthat must be effectively lubricated to ensure optimum performance and long life. This report details the surface chemical composition of 440C stainless steel and its interaction with two lubricant species: the solid lubricant MoS2 and the liquid lubricant extreme pressure (EP) additive, lead naphthenate (Pbnp). X-ray photoelectron spectroscopy (XPS) studies show that the 440C surface has a layered oxide structure following polishing and solvent cleaning; a 2.5 nm thick iron oxide layer exists on top of a thinner 1.5 nm chromium oxide underlayer. A region of high metal carbide concentration is present at the chromium oxide-bulk steel interface. Therefore, a layer of iron oxide (not the corrosion barrier formed by the chromium oxide layer) is the top surface layer with which lubricant species interact under conditions of low stress. The surface chemical composition of 440C can be altered by chemical or physical means. Chemical treatments such as acidic and alkaline etches selectively remove the iron oxide overlayer, leaving behind a surface enriched in chromium oxide. Furthermore, the alkaline treatment proved to be more controllable, while the acid treatment can cause surface damage by breaking through the corrosion barrier and severely oxidizing the bulk steel. Extreme Pressure Additive, Molybdenum Disulfide, 440C Stainless Steel, Solid Lubrication, Lead Naphthenate, Surface Chemistry.