Structure of Burnished or Run-in Films

Brudnyi and Karmadonov described the degree of crystal orientation at the surface of a burnished film in terms of a reflection or texture coefficient C, such that 

The reflection coefficient of a fully burnished film was found to be the same as that of the MoSj (001) plane and their reflectivities were also the same. They therefore concluded that the surface of the film consisted of an assembly of (001) basal planes. They also inferred that a highly reflective surface occurs when the coefficient of friction is a minimum. 

Yuko Tsuya also used electron diffraction in studying the progress of running-in with an unconsolidated floated film initially about 0.5/ym thick, using an oscillatory friction tester with a stroke of 20 mm and load 8 kg. She found a gradual transition from a layer of randomly-oriented particles with little adhesion or cohesion to a smooth cohesive layer with the molybdenum disulphide crystallites almost fully oriented with the (001) basal planes parallel to the substrate surface. The coefficient 

Gamulya and co-workers also concluded on the basis of electron microscopy and micro-electron diffraction that the production of a highly reflective surface occurs when the coefficient of friction reaches a minimum level. The full orientation appears to be limited to a very thin surface layer, which they found to be about 0.1 tjm thick in their tests, while Brudnyi and Karmadonov described it as being only one crystal thick, regardless of the force used for burnishing. On the other hand, a fully densified, fully-oriented layer without discontinuities may not be distinguishable in practice from an extended single crystal. 

One interesting aspect of these and other studies which may be worth emphasising is that the same ultimate surface condition may arise with either burnished powder or certain bonded films. It appears that the same orientation mechanism can occur with or without the presence of a binder. On that basis the function of at least the softer binders would be simply to retain the molybdenum disulphide in position while the running-in or burnishing is taking place. This subject is further discussed later. 

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