Effects of the Substrate on Film Formation

No information seems to exist about the formation and performance of films on very soft materials, but down to the lowest hardnesses which are of interest for bearing use, i.e. about 200 VPN, the wear life of the molybdenum disulphide tends to improve as substrate hardness decreases, although Tsuya s findings were less 

The evidence for more specific physical and chemical influences of substrate composition is less clear. Chemically, molybdenum disulphide is very inert. The sulphur atoms which form the surface layer of a lamella are strongly bonded to the molybdenum atoms, and their valency electrons are fully occupied in those bonds. Although molybdenum disulphide is highly polarised in its hexagonal crystals, the free energy at the lamellar surfaces is very small. 

Fleischauer has carried out a detailed analysis of the electronic structure of molybdenum disulphide. This analysis showed that all the accessible orbital electrons for both molybdenum and sulphur are used in intralaver bonding, leaving only high-energy antibonding orbitals available for bonding between layers or for basal surface adhesion to substrates. There are no accessible orbital electrons on either 

He concluded that the undisturbed (001) basal surface of molybdenum disulphide has no ability to form bonds or to react unless its molecular orbital structure is altered by physical or chemical means. 

The role of the surface oxide was mainly in influencing the accessibility of the metal to the molybdenum disulphide. With copper, for example, the oxygen is not strongly bonded to the metal and can be displaced. On the other hand, with titanium the oxide is strongly held, but lattice vacancies are present which expose titanium metal to the molybdenum disulphide. 

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