Composition of the Transfer Source

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. 

Compact with Compaction Pressure (Based on data from Ref.201) 

Lancaster has suggested that because transfer is an inefficient process, the concentration of solid lubricant in a reservoir composite must be at least 25%. Successful results have been claimed for composites with much lower concentrations, but comparisons are difficult because different workers have used different criteria for successful operation. Certainly most of the successful composites which consist only of molybdenum disulphide in a strong matrix have contained at least 20% of the solid lubricant. 

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 

Neumann and Sethna ° reviewed the general subject of solid lubricant composites in polymers and metals. They pointed out that the reduction in mechanical properties with higher concentrations of solid lubricant can be offset by the use of fibre reinforcement. Glass fibre is probably the most commonly used reinforcing fibre, with carbon fibre as a second choice. Metal and ceramic fibres have been used experimentally to reinforce polymers, but have not apparently been used commercially. To some extent powders such as bronze, lead, silica, alumina, titanium oxide or calcium carbonate can be used to improve compressive modulus, hardness and wear rate. 

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