Molybdenum shearing

A number of substances such as graphite, talc, and molybdenum disulfide have sheetlike crystal structures, and it might be supposed that the shear strength along such layers would be small and hence the coefficient of friction. It is true... 

More recendy, molecular molybdenum-sulfur complexes and clusters have been used as soluble precursors for M0S2 in the formulation of lubricating oils for a variety of appHcations (70). Presumably, the oil-soluble molybdenum—sulfur-containing precursors decompose under shear, pressure, or temperature stress at the wear surface to give beneficial coatings. In several cases it has been shown that the soluble precursors are trifunctional in that they not only display antifriction properties, but have antiwear and antioxidant characteristics as weU. In most cases, the ligands for the Mo are of the 1,1-dithiolate type, including dithiocarbamates, dithiophosphates, and xanthates (55,71). 

Molybdenum disulhde (M0S2), graphite, hexagonal boron nitride, and boric acid are examples of lamella materials commonly applied as solid lubricants. The self-lubricating nature of the materials results from the lamella crystalline structure that can shear easily to provide low friction. Some of these materials used to be added to oils and greases in powder forms to enhance their lubricity. Attention has been shifted in recent years to the production and use of nanosize particles of M0S2, WS2, and graphite to be dispersed in liquid lubricants, which yields substantial decreases in friction and wear. 

Non-stoichiometric compounds are found for the higher oxides of tungsten, molybdenum, and titanium—WOs-, MoOs-, and Ti02- f, respectively. The reaction of these systems to the presence of point defects is entirely different from what has been discussed previously. In fact, the point defects are eliminated by a process known as crystallographic shear (CS). 

Fig. 14. Schematic representation of the way spillover oxygen inhibits the formation of shear structures leading to molybdenum suboxidcs during ally tic oxidation (1,27,28).

By increasing the value given to n, the Mo03 structure is ultimately obtained in all these series. It is evident that, at least in theory, topochemical reduction of MoOs should be made possible by such a shear mechanism initially appearing along very widely separated dislocation lines this may be the clue to the structure of the discolored molybdenum trioxide crystals which are the main product when Mo03 is heated strongly in vacuo (12). 

Empty perovskite lattices can also form oxygen deficient phases by a process known as Crystallographic Shear, which introduces edge-sharing octahedra in addition to comer sharing. Examples include the reduced molybdenum oxides M08O23, M09O26, and VeOis. The latter is a metallic phase with substantial reversible capacity for electrochemical lithium intercalation between 2.8 and 2.2 V with respect to lithium metal. 

The low inter-lamellar attractive forces in molybdenum disulphide consist only of weak Van der Waals forces. In addition the separation distance between the sulphur layers of adjacent lamellae is 3.49A, and is larger than the 3.1 A thickness of an individual lamella. Cleavage or shear of molybdenum disulphide crystals between adjacent lamellae is therefore inherently likely to be easy. 

A number of investigations have been made into the influence of contact load on the frictional properties of molybdenum disulphide. Puchkov and Pashkov used a technique which they claimed to differentiate between shear stress and surface friction. They studied the effect of varying compressive stress on the resistance to... 

Figure 5.2 Change of Shear Stress with Load for Bonded Molybdenum Disulphide Film (Ref. 107)...

This exactly follows the relationship found experimentally by several investigators, and provides strong support for the generally-accepted view that the low friction of molybdenum disulphide is due to easy shear between lamellae. 

It can therefore be accepted that whether it is in the natural crystalline form or in a consolidated film, the friction of molybdenum disulphide is adhesive friction, and its low magnitude is due to the easy shear between adjacent lamellae which is made possible by the unusually favourable crystal and electronic structure. However, it must be remembered that the correct orientation of the crystallites is essential for the maintenance of low friction. The shear strength is low only parallel to the basal plane of the lamellae. In other directions the shear strength is high, so that the coefficient of friction will also be high . ... 

The mechanism by which water vapour increases the coefficient of friction has not been established. The effect can arise with well run-in and burnished films in which the exposed surfaces consist for practical purposes entirely of crystallite basal planes, and can typically result in an increase in the coefficient of friction from 0.05 to 0.15. Lancaster pointed out that the higher friction is comparable with that which occurs between a molybdenum disulphide film and a metal substrate during the initial formation of a transferred film. He therefore inferred that the increased friction on exposure to moisture must be due to the replacement of interfaciai sliding by subsurface shear. He postulated that this could only be due to one of the following mechanisms -... 

It is now generally accepted that the very low sliding friction of molybdenum disulphide is due to the very low shear strength parallel to the basal plane of the crystal lamellae, compared with the high strength or hardness perpendicular to the basal plane. The low shear strength is caused by the wide separation distance... 

As a result there is no clear detailed picture of the way in which transfer takes place, although there is a broad understanding of the nature of the process. It is generally accepted that transfer takes place by the movement of crystallites rather than at the molecular level. Where the source of the molybdenum disulphide is a single crystal, a crystallite in transferring to a counterface must be detached from the source crystal. This can take place by cleavage, fracture or shear, or a combination of one or more of these mechanisms. 

Molybdenum disulphide alone can be used as the reservoir material, either in the form of single crystal or as a compact. It is difficult to define the structural strength of single-crystal molybdenum disulphide. Because of its anisotropic nature, the ultimate stress in shear, tension or compression varies critically with the direction of the applied stress in relation to the crystal orientation, as discussed in Chapter 4, but some indication is given by the hardness values on the crystal faces and edges of 1.5 and 8 Mohs respectively. 

The upper limit for the concentration of molybdenum disulphide in such a simple composite is imposed by the low friction and low limiting shear stress of the... 

The situation is quite different when moiybdenum disulphide powder is used in a liquid. As has been shown, friction reduction and film formation only arise when the geometry permits particles of the powder to be trapped between bearing surfaces, and probably sheared. Such break-up of particles within a non-polar liquid is directly comparable with the procedure used by Goszek for the production of oleophilic molybdenum disulphide, so that the resulting fractured particles will presumably also be oleophilic. 

In the case of the equivalent niobium compounds, the same electronic effects are not present. He postulated that in pure stoichiometric niobium disulphide this results in poor lubrication. When good lubrication behaviour is observed, it is probably caused by additional niobium atoms intercalated between the lamellae, which contribute non-bonding electrons. On the basis of this theory, non-bonded atoms intercalated between the lamellae can increase the inter-lamellar spacing, whereas bonded intercalated atoms increase the resistance to inter-lamellar shear, and therefore the friction. However, an alternative interpretation is that certain intercalated atoms alter the interaction between the niobium atoms, allowing rearrangement to the 2H structure of molybdenum disulphide, and it is the favourable structure which provides good lubrication performance. 

Their crystal structures have been mentioned briefly in connection with intercalation in Section 14.2. All five compounds can be obtained in the layered hexagonal crystal form, and most are also found in rhombohedral or trigonal form. The compounds of the Group 6 metals, molybdenum and tungsten, as well as niobium diselenide, have a hexagonal form similar to that of molybdenum disulphide, in which the metal atoms in one layer are displaced sideways from those in the layers immediately above and below. This structure results in the widest interlamellar spacing, the easiest interlamellar shear, and the lowest friction. 

Briscoe, B.J. and Smith, A.C., The Interfacial Shear Strength of Molybdenum Disulphide and Graphite Films, ASLE Trans. 25, 349, (1982). 

Duffy TS, Shen GY, Shu JF, Mao HK, Hemley RJ, Singh AK (1999b) Elasticity, shear strength, and equation of state of molybdenum and gold from X-ray diffraction under nonhydrostatic compression to 24 GPa. J Appl Phys 86 6729-6736... 

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