Diffusion of extreme pressure-antiwear additives in oil measured by holographic interferometry

The diffusion of extreme pressure-antiwear additives in synthetic lubricants has been studied by real-time holographic interferometry. The method and its advantages for the measurement of diffusion in oils are discussed. The diffusion coefficients of three phosphorus-containing molecules, used as model compounds, have been measured between 25 and 100 °C and very low values for the diffusion coefficients in liquid are reported. The results show that the additive structure has less influence on the diffusion coefficient than the oil viscosity. The relationship between the diffusion and the viscosity does not follow the Stokes formula and a new approximation is proposed.