Strength measurement of thin lubricating films

A new method has been developed to measure the film-failure maximum shear stress at surface τsm of thin films in sliding. A ball-on-inclined plane scratch test was employed in these measurements. The tested balls were 52100 steel and the tested plane samples were 52100 steel, sapphire and a silicon wafer. Films on the plane samples were dip-coated to different thickness (0.5–200 nm). Paraffin, oleyl alcohol, octadecanol, octadecamide, n-octadecyl mercaptan, 1,2-octadecanediol, octadecylamine, oleic acid, stearic acid, ZDDP, etc. were used to deposit different films. The test results indicate that the film-failure maximum shear stress at surface τsm increases with an increase of film thickness. Roughness and substrate differences are also found to have an influence on the measured τsm. A smoother surface gives higher τsm. Among the studied films, stearic acid possesses the highest τsm while octadecanol has the lowest.