Lubricity and Adsorption of Castor Oil Sulfated Sodium Salt Emulsion Solution on Titanium Alloy

Effective water-based lubricants for titanium alloys have been investigated. The results reveal that castor oil sulfated sodium salt (CSS) solution can decrease the friction coefficient and adhesive wear compared with water, polyalkylene glycol (PAG) aqueous solution, or castor oil. The surface morphology of the tested discs and balls were measured, and the mechanism investigated. The rubbing surface was analyzed by using time-of-flight secondary-ion mass spectrometry (ToF–SIMS), indicating that a layer of physically adsorbed film remained on the surface, the main ingredient of which was CSS molecules. The mass and morphology of the adsorbed film were explored by quartz crystal microbalance measurements with dissipation monitoring and atom force microscopy, respectively. An electrical voltage, which could change the electrostatic force of the lubricants with the metal surface, was then applied to the frictional pair to explore the mechanism driving the adsorption. The physically adsorbed film played an important role in lubrication. The results of this study indicate that CSS could represent a good water-based lubricant base stock for the design of cutting fluids for titanium alloys.