From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical bulletin on the properties and application of colloidal graphite particles for lubrication and the concept of oiliness.
| Identifier | ExFiles\Box 145\3\ scan0011 | |
| Date | 1st January 1932 guessed | |
| TECHNICAL BULLETIN No. 200.6 Sodium and chlorine are known as monovalent elements or elements possessing a combining power of one. Langmuir¹ calls attention to the fact that if sodium and chlorine possessed only their primary valence of one, a crystal of rock salt could not exist, as the entire combining power of the two atoms would be utilized in the formation of individual salt molecules. These would be wholly free and unattached because of the absence of additional binding forces. Inasmuch as effort is required to break a salt crystal, binding forces are certainly existent within the mass. Langmuir reasons that every sodium atom must have affinity for each of the six chlorine atoms which surround it, and in a similar manner, every chlorine atom has like affinity for its sodium neighbours. This affinity is described as secondary valence. In view of these conditions, Langmuir concludes that a molecule is as large as the crystal, as it is evident that no particular affinity exists between any specific pair of sodium and chlorine atoms. It is only on vaporization that the atoms reveal their usually accepted, or primary, valence to form what is generally defined as a molecule. Since the atoms, during the process of evaporation or condensation, have different valency relations and have altered their “molecular” state, Langmuir considers these to be chemical processes. The distribution of secondary valence forces in a metal may be shown as illustrated in Fig. 3. It will be noted that the surface atoms must necessarily create a stray field for their excess secondary valence or cohesion bonds. It is on this ground that adsorption and surface tension have been looked upon as chemical, rather than physical phenomena. Some startling data² verifying the chemical nature of adsorption have been published. M = Metal Atoms Figure 3 COLLOIDAL GRAPHITE PARTICLES READILY ADSORBED The particle size of colloidal graphite in oil has been determined by Freundlich³, in many cases, to be much less than 500 millimicrons. These minute particles, possessing a huge surface as compared with their mass, are capable of being readily adsorbed by metal surfaces seeking to satisfy their surface energy. Adsorbed graphite, while retaining its ability to lubricate, as tests later to be cited will show, possesses one of the fundamental properties of an efficient lubricant—the ability to saturate the surface forces of the bearing. Hardy⁴ is the authority for the statement that “the function of a lubricant is to reduce the energy of the surface and thereby to reduce the capacity for cohesion and the resistance to slip when two composite surfaces are applied the one to the other.” He⁵ further states that the friction of two clean surfaces is due not to the surfaces but to the adhesion between them. A film of oxide or sulphide on a surface of copper, for example, acts as a very effective lubricant owing to the lowering of the surface energy of the copper. Hardy⁶ concludes that the adsorption of the lubricant by the solid appears to be a controlling factor. The better lubricant is the one more adsorbed by the solid. Wells and Southcombe⁷ found that by adding a small amount of a fatty acid to a mineral oil a much improved lubricant was obtained. Archbutt and Philip,⁸ in a discussion on the “germ process” as Wells and Southcombe term their methods, endeavoured to explain the action of the fatty acid in terms of adsorption. Archbutt suggested that Deeley, in a communication to the Physical Society, expressed the probable effect when he said that the unsaturated molecules of the lubricant on the solid metal surface and formed a new surface which opposed less resistance to shear than the unlubricated metallic surface. Philip expressed the belief that metallic soaps were formed by the action of the fatty acids on the metal bearings, and being formed at the interface of oil and metal were adsorbed by the latter. Philip found in practice that after substituting mineral oil for a pure sperm oil, in a certain instance, that flakes of oleates and stearates of copper and iron were freed from the bearing surfaces. Upon the removal of these soaps an increase in the coefficient of friction resulted. The importance of adsorption is thus emphasized. In the discussions pertaining to graphite-oil suspensions, the adsorbed film is usually referred to as the “graphoid surface.” In practice, employing the usual 0.20% suspension, a suitable surface, however, may not be formed for several hours. A graphoid surface is essential to reduced friction and may be obtained more quickly by painting the friction surfaces with a 10% graphite suspension. After the graphoid surface has been formed, a 0.20% suspension of colloidal graphite in oil may thereafter be substituted, as its graphite content is sufficient to maintain the graphoid surface. CONCERNING “ OILINESS ” “ Oiliness ” is an essential property of a good lubricant. Many attempts have been made to determine just what is necessary to promote “ oiliness.” Wells and Southcombe feel that by the “germ process” the physical rationale of the property of oiliness is now explained “ and give as grounds for their belief, the decrease in interfacial tension between the fatty acid bearing oil and the metal surface. They say “the permanency of films is dependent upon a diminished interfacial tension between the oil and the metal in contact therewith. If such a film is broken the possibility of its uniting again to form an unbroken layer depends entirely upon the interfacial tension being low. Any substance which lowers the interfacial tension causes the liquid to spread over a larger area of the solid. It follows, therefore, that if a substance be added to an oil which brings about a lowering of interfacial tension, such addition will act favourably as far as lubrication is concerned by preventing a rupture of the liquid film, and preventing, in turn, the metals coming into direct contact.” If the spread of liquid over a surface is indicative of interfacial tension, the tension at the interface of graphite and oil is lower than at the interface of metal and oil. A simple test to prove this consists of placing drops of oil on both a clean metal surface and a similar surface carrying an adsorbed graphite film and noting the difference in the rate at which they spread. Permitting the drops to run on these surfaces when placed at an angle will further prove that the graphoid surface is more freely wetted by oil. With the proper use of oil carrying colloidal graphite in suspension, it is quite impossible for the metal surfaces to come into contact and seize. The metal surfaces are isolated from each other by two graphoid surfaces, each of which is an excellent lubricant in itself. These surfaces in turn are protected by a graphite charged oil which, because of the low interfacial tension existing at the adsorbed film, insures the thorough “wetting” which is so necessary for efficient lubrication. Mabery⁹ made a series of interesting tests with graphite suspensions which very convincingly show the superiority of colloidal graphite lubrication. Mabery states : “Of all known materials the substance graphite alone possesses the qualities of a normal lubricant. In its ordinary natural condition it is not possible mechanically to subdivide it so completely that it can penetrate the fine interstices of metallic surfaces and at the same time form a persistent coherent lubricated surface ; but in a form of complete purity, free from the mineral constituents of natural graphite and in a condition of minute sub-division, such as is formed by the conversion of Acheson’s electric furnace graphite into its colloidal condition, there is available a solid lubricant that fulfils the requirements of economic lubrication. It is so finely divided that it readily permeates metals and by reason of its unctuous quality its own friction is reduced to a practically negligible quantity, thus escaping the internal friction of oil lubricants that is an important factor in the losses of power.” “ Fig. 4 shows the coefficients of friction extending through the period of the test (two hours) and also that the oil film broke seventeen minutes after the supply was shut off. “ Fig. 4 also presents the curve for the same oil carrying 0.35% graphite under the same elements of pressure, speed, and supply of oil to the bearing. The low coefficient of friction is apparent which, with a corresponding lower | ||