ELECTRON ^MICROSC.OI'Y 



Fig. 7. Electron micrograph of particles in a 

 used (10,000 miles) heavy duty detergent crank- 

 case oil. GOOOX 



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P'iG. 8. Reversed print of an electron micro- 

 graph of a shadowed "Formvar" replica of a worn 

 steel surface lubricated with a mineral oil contain- 

 ing an E. P. additive. 13,500X 



Iron microscope has been successfully used to 

 study the solubility and dispersivity of de- 

 tergent additives (17, 18) and also, by em- 

 ploying special specimen techniques, to 

 compare contaminants in used plain and 

 detergent oils (19) (Fig. 7). 



The so-called extreme pressure (E.P.) 

 additives are principally lead or other heavy 

 metal soaps and organic compounds of 

 sulfur, phosphorus and chlorine used in 

 various combinations to prevent destruc- 

 tive metal-to-metal contact between rela- 

 tively moving surfaces. E.P. additives react 



chemically with the metal of the surface to 

 form a svu'face film which has a lower shear 

 strength than the metal itself, and acts as a 

 boundary lubricant. The electron microscope 

 has provided experimental evidence of this 

 (4) (Fig. 8). _ 



Conventional lubricating greases contain 

 soap fibers or other particles as thickening 

 agents. As the structure of the thickening 

 agents largely determines the physical char- 

 acteristics of the bulk grease it is often im- 

 portant to examine them microscopically. 

 The optical microscope using polarized light 

 and the phenomenon of birefringence has 

 been used to show the orientation of fibers in 

 grease due to shear (20) as fibers are crystals 

 in the true sense of the word. Knowing how 

 they orientate helps to elucidate grease 

 struct lu-e. 



As the finer fibers of lubricating greases 

 are beyond the resolving power of optical 

 microscopy the electron microscope is the 

 only tool at present available for studying 

 the changes which soap fibers undergo when 

 a grease is subjected to mechanical work, 

 heating, etc. (21). The successful application 

 of electron microscopy to the study of grease 

 structure depends upon a satisfactory 

 method of specimen preparation which can 

 give reproducible results free from artefacts. 

 The oil phase must be removed and the 

 method of removal must not disturb the 

 fiber arrangement. Solvent techniques (20, 

 22), bulk freezing and slicing (23) and 

 vacuum evaporation (21) have been suc- 

 cessfully applied to achieve this (Fig. 9). 



Solid lubricants are used where other 

 lubricants are unsuitable due to tempera- 

 ture limitations or inaccessibility. The 

 mechanism of lubrication is similar for all 

 lubricants, a thin film supporting the sur- 

 faces is sheared during relative movement. 

 Solid lubricants usually have a layer lattice 

 structure and present planes on which shear 

 may easily occur but which are resistant to 

 compression at right angles to the plane. 

 Their success depends on the size and shape 



312 



