Ductile substrate

Molybdenum disulphide adheres readily to most substrates. As a result, when sliding takes place between molybdenum disulphide and a solid surface, the phenomena of adhesion and possibly junction growth will take place, and high frictional forces will be generated. This adhesion will be augmented by the action of burnishing (see Chapter 6). However, while adhesive forces between molybdenum disulphide and solid substrates are usually high, the cohesive forces between lamellae of molybdenum disulphide are low. It follows that the coefficient of friction between lamellae will be much lower than that between a lamella and a ductile substrate, and slip will take place preferentially between lamellae. 

When considering a ductile substrate coated with a brittle film and subjected to a uniaxial strain during loading and before debond of the film (perfect adhesion), the displacement is assumed to be continuous at the interface. Then, the substrate deformation is entirely transmitted to the film through the interface. When exceeding the critical cracking strain of the film, a network of transverse cracks develops. At the interface, each crack tip will be surrounded by a... 

The brittle film cracking with plastic deformation of the ductile substrate at the interface has been described by using the shear lag model. " This model, which was proposed in the analysis of the fragmentation of fiber composites," " develops a relation for the critical stress producing the steady-state cracking of the film. It assumes that the interfacial shear stress, on the one hand, is activated at each crack tip along the characteristic slip length r, and, on the... 

Coating thickness can interact with other factors in a complex manner to influence the critical load. Thicker coatings commonly have lower critical loads because of accumulated residual stresses. However, hard thicker films can also provide greater load-bearing capability if residual stresses are not too high. This is particularly important for hard, brittle coatings on soft, ductile substrates where considerable substrate deformation can occur before delamination failure. Under these conditions, the critical load for spallation may increase with coating thickness. 

M. D. Drory and J. W. Hutchinson, Measurement of the adhesion of a brittle film on a ductile substrate 1 indentation, Proc. R. Soc. Land.,... 

Delannay, F. and Warren, P. (1991), On crack interaction and crack density in strain-induced cracking of brittle films on ductile substrates, Acta Metallurgica et Materialia 39, 1061-1072. 

In this study thin ceramic coatings have been deposited onto both brittle and ductile substrates to assess the fracture behaviour of the coating/substrate system. Niobium... 

Cladding may be less expensive than selective electro deposition when coatings greater than 1 p.m of a noble metal are required, but may be more expensive than electro deposition for thinner coatings. Selective techniques are most easily used for sheet metal substrates that are to be machine stamped and formed into contacts. Clad noble metals are considerably more ductile (and less hard) than comparable electro deposits and, therefore, are better suited to forming operations. Contacts that are made into separate parts from rod by screw machining are usually coated on all exposed surfaces by barrel electroplating. 

Detachment of a particle from a substrate when plastic deformations have occurred is somewhat a more complicated problem than in the case of simple elasticity. Essentially, one has to first determine if the detachment occurs because of a cohesive or interfacial failure and whether the failure is ductile or brittle. For... 

The use of the 99-8% and 99-99% grades is usually confined to those applications where very high corrosion resistance or ductility is required. The chemical industry can advantageously use these purities for handling some products, but because of their low mechanical strength they are sometimes used as a cladding material for a stronger substrate. 

Continuous plating of wire and strip is, unlike the preceding techniques, a prefabrication process. The production of tinplate is the largest scale continuous operation, but any electrodeposit may be applied this way. Subsequent fabrication processes arc likely to damage the coating, so that pre-coating is best reserved for ductile coatings which are anodic to the substrate in service, as is the case for tin. 

Under micro-discontinuous chromium coatings, copper undercoats improve corrosion resistance. On non-conductors, especially on plastic substrates, copper is often applied before nickel-chromium plating over the initial electroless copper or nickel deposit in order to improve ductility and adhesion, e.g. as tested by the standard thermal-cycling test methods ... 

Figure 35 shows the optical microscopic images of the first crack point on the sample surface. The scratch scar of monolayer Sample 1 has the feature of brittleness. However, there is an obvious crack along the scratch scar of Sample 2 before the coating delamination. This indicates that mono-layer Sample 2 has the feature of ductility, and the adhesion between the film and the substrate is poor. However, there is no obvious crack before the delamination in the scratch scars of other samples. The feature of multilayer Samples 3 and 4 is different from monolayer Samples 1 and 2. There are no obvious cracks in the scratch scars of Samples 5 and 6, except several small cracks along the edge of the scars. These... 

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