Solid lubricant, bonded

Substrate Properties. It is clear from equation 5 that higher hardness of the substrate lowers friction. Wear rate of the film also is generally lower. Phosphate undercoats on steel considerably improve wear life of bonded coatings by providing a porous surface which holds reserve lubricant. The same is tme for surfaces that are vapor- or sandblasted prior to appHcation of the soHd-film lubricant. A number of typical surface pretreatments are given in Table 13 to prepare a surface for solid-film bonding (61). 

Several oxides and sulfides display the characteristics of network solids. The bond network of silica appears in Section 9-. Other examples are titania (Ti02) and alumina (AI2 O3). These two substances have extremely high melting points because their atoms are held together by networks of strong a covalent bonds. Like graphite, M0S2 is a two-dimensional network solid that serves as a solid lubricant. 

Molybdenum disulfide also forms sheet molecules. It consists of covalently bonded sheets with sulfur atoms on both sides of molybdenum atoms, as shown in Figure 17.4. These sheets are only weakly bonded. The ability of these sheets to slide over each other explains the lubricity of M02S, which is used as a high-pressure solid lubricant in metal working. 

However, Jamison has intensively studied the relationship between the crystal and electronic structures of layer-lattice solid lubricants and their frictional properties, and has shown that other aspects of its electron distribution give a particularly favourable structure to molybdenum disulphide. In its structure the molybdenum atoms in one layer do not lie directly above or below the molybdenum atoms in an adjacent layer, but are opposite holes in that layer. The sulphur atoms are directly opposite other sulphur atoms, but do not have any unpaired electrons to provide strong bonding. It is this lack of electronic interactions which leads to the high interlamellar spacing, and low interlamellar attraction. 

On the basis of their results, they put forward an "oxide interaction concept , according to which the friction and wear of bonded films, composites and simple transfer films of molybdenum disulphide are improved by the presence of low-melting oxides which either combine easily or form desirable eutectics with molybdenum oxides. Such a concept would provide valuable guidance to the development of better solid lubricants, but, as Buckley pointed out, the authors had in fact produced no evidence for the "oxide interaction" concept. The lower friction can in fact be explained by a reduction in the rate of oxidation, since the films exhibiting lower friction had all been run at some stage in air. 

The most straightforward technique for producing a thin uniform adherent coating is by the use of an adhesive binder, and bonded coatings were introduced very early in the modern development of molybdenum disulphide technology. They are probably now the most widely used form of molybdenum disulphide lubricant, and as long ago as 1968 Gresham estimated that over 95% of the solid lubricants commercially used were resin-bonded. 

The lubricating performance of the lamellar crystal structure is strongly dependent on the inter-lamellar separation and bonding, and these can be modified by the intercalation of other substances between the lamellae. The general subject of intercalation will therefore be discussed before proceeding to descriptions of the individual lamellar solid lubricants. 

Although a lamellar crystal structure is favourable for solid lubrication, the inter-lamellar spacing and the nature of the inter-lamellar bonding are of major importance in determining the resistance to inter-lamellar shear, and therefore the sliding friction, of lamellar compounds. 

Bisson, E.E. and Godfrey, D., Bonding of MoSj to Various Materials to Form a Solid Lubricating Film, NACA TN-2802, (1952). 

Hopkins, V. and Campbell, M., Film Thickness Effect on the Wear Life of a Bonded Solid Lubricant Film, Lubric. Eng., 25, 15, (1969). 

Bahun, C.J. and Jones, J.R., Influence of Load, Speed and Coating Thickness on the Wear Life of a Bonded Solid Lubricant, Lubric. Eng., 25, 351, (1969). Hopkins, V., Discussion, p. 6, on Rabinowicz, E., Variation of Friction and Wear of Solid Lubricant Films with Film Thickness, ASLE Trans., 10, 1, (1967). Whitehouse, G.D., Nandan, D. and Whitehurst, C.A., The Effect of Film Thickness on Friction Coefficients for Solid Lubricants CaF2, MoSj and Graphite, ASLE Trans., 13, 159, (1970). 

Bartz, W.J. and Xu, Jinfen, Wear Behaviour and Failure Mechanism of Bonded Solid Lubricants, Lubric. Eng., 43, 514, (1987). 

Bartz, W.J., Tribological Behaviour of Three-Component Bonded Solid Lubricant Films, Proc. JSLE Inti. Trib. Conf., Tokyo, (8-10 July, 1985). 

Benzing, R.J., McConnell, B.D. and Clow, W.L., A Methyl Phenyl Polysiloxane Bonded Lubricating Film, ASLE Inti. Conf. on Solid Lubricants, Denver, Colorado, (24-27 Aug. 1971), ASLE SP-3. 

Jones, J.R. and Hoover, G.W., Abrasiveness of MoSj in Bonded Solid Lubricants, ASLE Trans., 14, 55, (1971). 

Tsuya, Y. and Kitamura, M., Effect of Sliding Conditions on the Wear Rate of Organic and Inorganic Bonded Solid Lubricant Films, Proc.2nd. ASLE Inti. Conf. on Solid Lubrication, Denver, Colorado (15-17 Aug. 1978) ASLE SP-6 p.85. 

Davison, R.M. and Gilbert, T.I., Evaluation of MoSj Additions to a Bonded Solid Lubricant for Severe Ironing Applications, Lubric. Eng., 32, 131, (1976). 

Paxton, R.R., Carbon, Graphite and Metal-Bonded Molybdenum Disulphide Solid Lubricant Bearings, Tribology Inti., 15, 285, (1982). 

Radcliffe, S.J. and Parry, A.A., The Dispersion of Life of Bonded MoSj Solid Lubricant Coatings, Wear, 56, 203, (1979). 

Bartz, E.J., Holinski, R. and Xu, J. (1986) Wear life and frictional behaviour of bonded solid lubricants. Lubr. Eng. 42 762-769. 

TABLE 19-18. EFFECT OF TEMPERATURE ON THE DURABILITY OF BONDED FILMS OF SOLID LUBRICANTS... 

Figure 19-22. Effect of temperature on the durability of bonded solid lubricant films. Data by Campbell and Hopkins [106] with the Hohman tester at 73.15 cm/s, 2158 N load. The curves are identified in the text.

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