Application of freeze-fracture technique for analyzing the structure of lubricant greases
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J. Mater. Res., Vol. 4, No. 4, Jul/Aug 1989
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thickener, such as the characteristic length, diameter, configuration, and interaction, which determine the properties of the grease. Many papers have been devoted to microscope examinations and methods of grease specimen preparation. Amongst these, transmission electron microscopy is the most frequently used. Many methods of grease specimen preparation are examined in Refs. 2 and 4. In most of them, the oil which obstructs the view of the underlying structure of soap is eliminated. Only soap can be observed. With most of these methods oil is eliminated without paying special attention, causing destruction of the continuous soap structure existing in the original grease. With lithium 12-hydroxystearate greases, artifacts such as twisted fiber structures, broken fibers, and structure distortion can be observed. To avoid altering the soap structure and production of artifacts, great care is required in the preparation of grease specimens and deoiling. Of these methods, Kistler's aerogel technique5'6 or Anderson's method4 ought to be singled out because they use gentle deoiling processes and give more satisfying results. To make a proper examination of the lubricant greases, the components of the system should be minimally disturbed. Elimination of oil is always likely to create distortions. So this operation can be avoided by studying the initial material. Freezing the liquid phase enables this to be accomplished. Void, Coffer, and Baker7'8 froze grease samples at - 7 0 °C and observed, by transmission microscopy, a thin section (about 0.1 /xm) of the frozen grease obtained using their sectioning technique and a special microtome. 1989 Materials Research Society
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A. Magnin and J.M. Piau: Freeze-fracture technique
Very quick freezing is used in the freeze-fracture technique developed in the present study and described in detail further on. Then the frozen sample is fractured. A replica of surface fracture is made and examined under high magnification by a scanning electron microscope. By proceeding in the latter manner when preparing, as well as cleaning the replica in two ways, in order to eliminate either the liquid phase or both phases, an easy and informative observation of the material's structure is obtained. In the first part of this paper, the freeze fracture procedure is described in detail. Then, in the second part, observations of three different greases, using the latter method, are analyzed. II. FREEZE-FRACTURE PREPARATION Figure 1 shows the procedure used for the preparation of the samples. A small amount of grease (about 1 mm3) is mounted in gold specimen holders. The set to be frozen is immersed in a bath of liquid nitrogen. The very small amount of grease combined with the very low temperature allow bulk freezing of the specimen within about one tenth of one second. In this way the structure transformation or crystallization process is avoided. In vacuum and at very low tempe
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