Adhesion interphase material

The early adhesives were natural products (e.g. glues, starch, natural resins) but most modern adhesives are based on synthetic polymers (e.g. polyacrylates). In adhesion, two materials come sufficiently dose for strong interaction to occur. The interface is considered as the zone between the interacting substances, which is sometimes referred to as the interphase. 

The depositing film material may diffuse and react with the substrate to form an interfacial region . The material in the interfacial region has been called the interphase material and its properties are important to the adhesion, electrical, and electronic properties of film-substrate systems. In particular, the development of ohmic contacts to semiconductor materials is very dependent on the interface formation process, The type and extent of the interfacial region can change as the deposition process proceeds or may be modified by post-deposition treatments. Interfacial regions are categorized as ... 

Generally, the interfacial region and the interfacial (interphase) material are difficult to characterize since they usually consist of a small amount of material buried under a relatively thick film. Figure 10.4 shows the RBS analysis of tungsten metallization of a Si-Ge thermoelectric element as deposited and after a furnace treatment, which diffused material at the interface. Before diffusion, the interface has no features discernible by RBS. Interdiffusion rejects the germanium and reacts to form a tungsten siUcide. After extensive diffusion the interface is weakened and the adhesion fails. 

Loss of adhesion at the interface, in the interfacial (interphase) material, or in nearby material can occur as a result of a number of effects. These include mechanical stress, chemical corrosion, diffusion of material to or away from the interface, or fatigue effects. Sometimes several factors are involved at the same time, such as stress and corrosion. In some cases, film properties influence the failure mechanism. For example, residual film stress can add to the applied mechanical stress and can even stress the interface to such an extent that adhesion failure occurs without any externally applied stress. 

Good adhesion of metal films to metallic substrates is typically attained by utilizing surface preparation techniques that remove surface contamination and surface barrier layers, then depositing a material that will readily alloy with the substrate material. Elevated surface temperatures aid in interfacial diffusion and often increase the adhesion, but overdiffusion can decrease adhesion by generating a weak interphase material. Non-soluble metal-metal couples such as Ag-Fe, Au-Ir, and Au-Os should be avoided. However, good adhesion can... 

Interfacial flaws (film formation, adhesion) Flaws, such as microcracks or voids, that reduce the fracture strength of the interphase material. 

Interphase material (adhesion, film formation) The material at the interface that is formed by diffusion, reaction, or co-deposition at the interface between the film and the substrate. The properties of this material are an important consideration in adhesion. Also called Interfacial material. 

Weak surface layer (adhesion) When the surface layer is weak either due to a low molecular weight layer (polymer) or surface flaws (brittle solid). During film deposition this weak region becomes part of the interphase material, resulting in poor apparent adhesion. 

The interphase is the volume of material ia which the properties of one substance gradually change iato the properties of another. The iaterphase is useful for describiag the properties of an adhesive bond. The interface contained within the iaterphase, is the plane of contact between the surface of one material and the surface of another. Except ia certain special cases, the iaterface is imaginary. It is useful ia describiag surface eaergetics. 

Fig. 1. (a) Adhesive vs. cohesive failure, (b) Close-up view of adhesive failure in the pre.sence of an interphase. The locus of failure may be adjacent to or within the interphase (as shown), and particles of material may be ejected during the debonding process. 

Once it is recognized that particles adhere to a substrate so strongly that cohesive fracture often results upon application of a detachment force and that the contact region is better describable as an interphase [ 18J rather than a sharp demarcation or interface, the concept of treating a particle as an entity that is totally distinct from the substrate vanishes. Rather, one begins to see the substrate-particle structure somewhat as a composite material. To paraphrase this concept, one could, in many instances, treat surface roughness (a.k.a. asperities) as particles appended to the surface of a substrate. These asperities control the adhesion between two macroscopic bodies. 

Diffusion theory involves the interdiffusion of macromolecules between the adhesive and the substrate across the interface. The original interface becomes an interphase composed of mixtures of the two polymer materials. The chemical composition of the interphase becomes complex due to the development of concentration gradients. Such a macromolecular interdiffusion process is only... 

Jones, F.R., Interfacial aspects of glass fibre reinforced plastics. In Jones, F.R. (Ed.), Interfacial Phenomena in Composite Materials. Butterworths, London, 1989, pp. 25-32. Chaudhury, M.K., Gentle, T.M. and Plueddemann, E., Adhesion mechanism of poly(vinyl chloride) to silane primed metal surfaces. J. Adhes. Sci. Technol, 1(1), 29-38 (1987). Gellman, A.J., Naasz, B.M., Schmidt, R.G., Chaudhury, M.K, and Gentle, T.M., Secondary neutral mass spectrometry studies of germanium-silane coupling agent-polymer interphases. J. Adhes. Sci. Technol., 4(7), 597-601 (1990). 

Evidently, the critical pressure to cause failure decreases with a stiffer interphase modulus, E, or a reduced interlayer thickness, h, or both. This hypothesis has been tested on several simulation systems, which confirm that increased adhesion is possible with a negative transversal modulus gradient at the material interface. 

Cho, C.R. and Jang, J. (1990). Adhesion of ultrasonic high modulus polyethylene fiber-epoxy composite interfaces. In Controlled Interphases in Composite Materials, Prod. ICCI-III, (H. Ishida ed.), Elsevier Sci. Pub., New York, pp. 97 107. 

The primer chosen for this investigation consisted of an equimolar mixture of phenyl- and amino-functional silanes, suggested as a potential superior primer for aluminum/epoxy adhesive joints [7], The amino-functional silane is known to be effective as an adhesion promoter for fiber-reinforced composite materials [1, 2] as well as for epoxy/metal adhesive joints [8, 9] and provides for strong chemical interaction between the adhesive and primer, while the phenyl functional silane should reduce the overall concentration of polar, hydrophilic functional groups in the interphase region and at the same time maintain or improve the ability of the resin and primer to interpenetrate due to its structural similarity to the adhesive resin. 

Both fiber-matrix interphase-sensitive mechanical tests (interlaminar shear strength, 90° flexure) and interphase-insensitive tests (0° flexure) were conducted on high volume composite samples fabricated from the same materials and in the same manner as discussed above to see if the interphase and its properties altered the composite mechanical properties and in what manner. A summary of the data is plotted as a bar graph in Fig. 7. The first set of bars represents the difference in fiber-matrix adhesion measured between the bare fibers and the sized fibers by the ITS. The composite properties plotted on the figure also show increased values for the epoxy-sized material over the bare fiber composite. 

The modulus and toughness of this interphase combined with the increased fiber-matrix adhesion can be used to explain the resulting mechanical properties of these composite materials. For both interphase-sensitive and -insensitive mechanical properties, it has been concluded that the strength of the interphase and the failure mode initiated by the interphase properties can be responsible for composite mechanical properties. 

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