Substrate Interface

Fracture mechanics (qv) affect adhesion. Fractures can result from imperfections in a coating film which act to concentrate stresses. In some cases, stress concentration results in the propagation of a crack through the film, leading to cohesive failure with less total stress appHcation. Propagating cracks can proceed to the coating/substrate interface, then the coating may peel off the interface, which may require much less force than a normal force pull would require. 

When the nucleus is formed on a solid substrate by heterogeneous nucleation the above equations must be modified because of the nucleus-substrate interactions. These are reflected in the balance of the interfacial energies between the substrate and the environment, usually a vacuum, and the nucleus-vacuum and the nucleus-substrate interface energies. The effect of these terms is usually to reduce the critical size of the nucleus, to an extent dependent on... 

As mentioned earlier, if there is a large disparity in sttiicture at the film-substrate interface, such as a crystalline phase growing on an amorphous, glassy, substrate, the film may detach and grow a separate morphology. 

Figure 3.27 shows the depth profile of such a layer. Enrichment of Ti and A1 at the layer-substrate interface is visible. The Si signal in the layer increases with depth. It will subsequently be shown that it is not possible to determine from the depth profile alone whether there is diffusion of Si into the CrN layer. 

When infrared radiation with electric field amplitude Eo impinges on the film-covered substrate, some is reflected from the ambient/film interface while some is transmitted into the film and then reflected at the film/substrate interface. Some of the radiation reflected at the film/substrate interface is reflected back into the film at the film/ambient interface. However, some is transmitted into the ambient (see Fig. 4). The reflection coefficient (r) for the film/substrate system is calculated by summing the electric field amplitudes for all of the waves reflected into the ambient and then dividing by the electric field amplitude Eo) of the incident radiation. 

The presence of a third peak in the sulfur profile, near the film/substrate interface, indicates that some sulfur diffused through the plasma polymer films and reacted with iron, probably forming FeS at the interface. 

The surface of the substrate, the silicone/substrate interface, and the bulk properties of silicones all play significant and influential roles that affect practical adhesion and performance of the silicone. The design of silicone adhesives, sealants, coatings, encapsulants or any products where adhesion property is needed requires the development chemist to have a thorough understanding of both silicone chemistry and adhesion phenomena. 

It is noteworthy that an important industrial application is based on pure silicone network [9]. This is the organic PSA release technology where an uncured silicone is deposited as a thin coating to a flexible substrate. Strong adhesion develops at the silicone-substrate interface whilst the coating cures. 

PDMS based siloxane polymers wet and spread easily on most surfaces as their surface tensions are less than the critical surface tensions of most substrates. This thermodynamically driven property ensures that surface irregularities and pores are filled with adhesive, giving an interfacial phase that is continuous and without voids. The gas permeability of the silicone will allow any gases trapped at the interface to be displaced. Thus, maximum van der Waals and London dispersion intermolecular interactions are obtained at the silicone-substrate interface. It must be noted that suitable liquids reaching the adhesive-substrate interface would immediately interfere with these intermolecular interactions and displace the adhesive from the surface. For example, a study that involved curing a one-part alkoxy terminated silicone adhesive against a wafer of alumina, has shown that water will theoretically displace the cured silicone from the surface of the wafer if physisorption was the sole interaction between the surfaces [38]. Moreover, all these low energy bonds would be thermally sensitive and reversible. 

Loss of adhesion occurs at the silicone substrate interface and two main mechanisms can be outlined the formation of a weak boundary layer (WBL) and the breaking of adhesive bonds. 

Weak boundary layer. WBL theory proposes that a cohesively weak region is present at the adhesive-substrate interface, which leads to poor adhesion. This layer can prevent the formation of adhesive bonds, or the adhesive can preferentially form bonds with the boundary layer rather that the surface it was intended for. Typically, the locus of failure is interfacial or in close proximity to the silicone-substrate interface. One of the most common causes of a WBL being formed is the presence of contaminants on the surface of the substrate. The formation of a WBL can also result from migration of additives from the bulk of the substrate, to the silicone-substrate interface. Alternatively, molecular... 

For satisfactory operation, the rubber lining must be adequately bonded to the substrate it is protecting. BS 6374 part 5 gives definitive load to peel levels for various elastomers, but it is usually required that, on separation, the rubber should tear rather than part either at the primer/rubber interface or the primer/substrate interface. 

Half-Bubbles or Domes at Elastomer-Substrate Interfaces... 

Internal bubble growth occurs because flaws at an elastomer-substrate interface act as nucleation sites where gas which is coming out of solution in the mbber can collect, following a reduction in pressure. If there are flaw sites, based on energy considerations, gas will take the easiest option— which is to collect at an already preexisting bubble and cause it to grow. 

Correlation has been suggested between the morphology, electronic structure and catalytic properties of supported gold nanoparticles with special attention to the role of the substrate/interface behaviour. 

Figure 8. TEM and optical absorption of the sample implanted with 5 x 10 Au /cm (a) TEM cross-sectional micrograph (dashed lines represent the free surface and film-substrate interface) (b) nanoparticles size distribution (c) simulated optical spectra (1) Au cluster in a non-absorbing medium with n = 1.6 (2) Au cluster in polyimide (absorbing) (3) Au(core)-C(shell) cluster in a nonabsorbing medium with n = 1.6 (4) the experimental spectrum of Au-implanted polyimide sample, (d) X-ray diffraction patterns as a function of the implantation fiuence.

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