Ultra-low friction achieved by diluted lactic acid solutions
Notable lubrication performance was found using lactic acid solutions between two solid surfaces, silica glass against silicon nitride. The sliding friction coefficient was as low as 0.02 under a Hertzian contact pressure up to 400 MPa with a quite low concentration of 0.085 mol L−1. The results in the present work also exhibit the significant effect of concentration on the running-in process. A shortened running-in period, which could be contributed to the effective adsorption of the lactic acid molecules on the solid substrates, could be obtained by increasing the concentration of the lactic acid solution. In addition, surface hydroxyl groups gained by piranha solution were suggested to effectively reduce the friction. The work suggests that the excellent lubricity of lactic acid can be mostly contributed to the surface adsorption combined with the surface hydroxylation.