Application of freeze-fracture technique for analyzing the structure of lubricant greases

Abstract

The freeze-fracture technique has been developed to study solid-liquid dispersions. In this technique, a sample of dispersion is frozen, then fractured. A replica of the fracture surface is made and observed using a scanning electron microscope. The physical form of the solid thickener in dispersion can be studied, and the structure of the thickener can be revealed as well as a combination of rupture properties of the solid and the frozen liquid. The application of this technique is on lubricant greases. Greases are two-phase dispersed systems composed of mineral oil gelled with solid particles of soap. Three lithium 12-hydroxystearate greases are examined. Two different kinds of washing of the replica are used, the first eliminating the liquid phase. The configuration of soap particles and the network formed by them are observed, and artifacts such as twisting soap fibers are avoided. The second washing eliminates the solid and liquid phase. Examination of the fracture surface of the soap provides information on crystalline arrangement. The observed structures by freeze fracture are related to the thermal treatments during grease manufacture. The results are compared with the observations obtained from an electron transmission microscope analysis after preparation by the usual dispersion technique.