Adhesive oxide interface

As already mentioned, the weakest component of an aluminum joint is the oxide itself rather than the adhesive/oxide interface. The use of coupling agents would not be expected to provide an enhancement in durability under these circumstances. However, Brockmann has found that priming aluminum with 2,4,5,7-tetrahydroxyflavonene can substantially improve the durability of joints prepared with phenolic or nitrile-epoxy adhesives. This compound may be acting as a hydration inhibitor rather than as a coupling agent. 

In cases in which surface treatment produces a smooth oxide (intentionally or unintentionally) bond performance is controlled by chemical bonds across the oxide-epoxy interface. This situation can arise, for example, when an FPL adherend is rinsed in fluorine-contaminated water or is exposed to fluorine vapor. ( 8) The oxide-epoxy bonds are relatively weak and are readily disrupted by moisture.(, 50) As a result, bond failure is rapid upon exposure to humid conditions, and the crack propagates along the adhesive-oxide interface. In cases in which a smooth oxide is formed intentionally, coupling agents such as silanes can be used to improve durability. This is discussed in detail in Chapter 9 by E. Pleuddemann in the accompanying volume, Fundamentals of Adhesion. 

She et al. [128] used rolling contact to estimate the adhesion hysteresis at polymer/oxide interfaces. By plasma oxidation of the cylinders of crosslinked PDMS, silica-like surfaces were generated which could hydrogen bond to PDMS r olecules. In contrast to unmodified surfaces, the adhesion hysteresis was shown to be larger and proportional to the molecular weight of grafted polymer on the substrate. The observed hysteresis was interpreted in terms of the orientation and relaxation of polymer chains known as Lake-Thomas effect. 

In practice, thermal cycling rather than isothermal conditions more frequently occurs, leading to a deviation from steady state thermodynamic conditions and introducing kinetic modifications. Lattice expansion and contraction, the development of stresses and the production of voids at the alloy-oxide interface, as well as temperature-induced compositional changes, can all give rise to further complications. The resulting loss of scale adhesion and spalling may lead to breakaway oxidation " in which linear oxidation replaces parabolic oxidation (see Section 1.10). 

Moisture acts as a debonding agent through one of or a combination of the following mechanisms 1) attack of the metallic surface to form a weak, hydrated oxide interface, 2) moisture assisted chemical bond breakdown, or 3) attack of the adhesive. (2 ) A primary drawback to good durability of metal/adhesive bonds in wet environments is the ever present substrate surface oxide. Under normal circumstances, the oxide layer can be altered, but not entirely removed. Since both metal oxides and water are relatively polar, water will preferentially adsorb onto the oxide surface, and so create a weak boundary layer at the adhesive/metal interface. For the purposes of this work, the detrimental effects of moisture upon the adhesive itself will be neglected. The nitrile rubber modified adhesive used here contains few hydrolyzable ester linkages and therefore will be considered to remain essentially stable. 

While a non-phosphated topcoat/adhesive interface provided an excellent, moisture resistant, occlusive seal even under the most severe cycle testing, phosphated ZM adherends did not prove to be as durable in comparison (Figure 11). The reason for this lies in the fact that phosphate coverage on Zincrometal is incomplete. Partially crystalline phosphates are non-uniformly interspersed on randomly exposed zinc dust spheres at the surface. Consequently, the moisture resistance normally provided at the adhesive/topcoat interface was reduced due to the incomplete sealing between the topcoat/ adhesive surfaces. This became apparent as most of the failures examined after aging in these environments were concentrated at the adhesive/phosphate/paint interface. Results obtained on these samples were similar to those obtained for phosphated CRS joints, indicating that the locus of failure occurred at phosphate crystal sites. Note, however, that the durability of these joints was still considered to be very good in comparison to other metallic oxide/ adhesive interfaces. 

The results of semi-quantitative charge spreading tests suggests that the lateral conductance of polyimide-field oxide interfaces can be sufficiently low to permit reliable device operation. This topic must be addressed in the context of the overall processing of the interface, including any adhesion promoters used. 

Failure can occur in metal/epoxy adhesion systems in any one or more of a number of different regions. The fracture may propagate through the bulk metal or epoxy, the metal oxide layer, the metal oxide/epoxy or metal/metal oxide interfaces, or through weak boundary layers (WBL s) very near the interfaces. Some workers -78,153) be]ieve that most failures that have been claimed to be interfacial have actually... 

In comparing the acido-basicity of the untreated and electrochemically (or anodically) treated fibers in Table 11, it is observed that the amine-treatments increase significantly the basicity of the fibers (Samples II and III), while commercial oxidation increases the acidity (IV), as expected. This behavior is probably explained in terms of the presence of any physisorbed species (such as, amine and oxygen functional groups for the fiber surfaces amined and oxidized, respectively). This can improve the degree of adhesion at interfaces of the composites for fiber surfaces incorporated with epoxy matrix for playing an important role in forming the fiber-matrix bond. 

A more dramatic failure results in peel strengths of 0-10 g/mm and is characterized as an adhesive failure at the polyimide/metal oxide interface.This was the only failure mode observed in Ti and Zr films. Isotopically tagged water used with SIMS analysis shows that on annealing water reacts with the Ti with oxygen segregating to the metal/polyimide interface and hydrogen penetrating into the bulk of the Ti, in these samples. 

With the help of complementary surface analysis techniques such as XPS, Static SIMS and AES, we have been able to show how a short (23 msfilms leads to a slight oxidation of the surface as well as to the formation of N2 containing species. These modifications are necessary for the improvement of the adhesion observed with a scotch-tape test. However, the presence of oxygen is not the only factor responsible for a good adhesion, since the AES profiles of die deposited aluminium, show the same oxidized interface in the case of the non treated metallized polymeric film. The films are pretreated in a corona discharge configuration (hollow electrode-grounded cylinder) and the aluminium is deposited onto the film in situ. 

In these models, the method consists of (i) assuming that a certain type of interaction such as van der Waals dispersion forces (Didier and Jupille 1994, Naidich 1981) or image forces (Stoneham et al. 1995) is predominant at metal/oxide interfaces, (ii) calculation of the adhesion energy resulted from these... 

Despite their great technical importance, there are few reported studies of segregation at solid metal/oxide interfaces. Hondros (1978) used Auger spectroscopy to examine interfaces formed between sapphire and Fe containing Cr or Ni and found a high interfacial enrichment of Cr over a zone of a few nanometers. The results were used to explain the strong mechanical adhesion of Fe on A1203 induced by Cr additions. 

The scanning Kelvin probe, which measures the Volta potential difference between a specimen and the calibrated sensing probe, is introduced as the only electrochemical technique which allows nondestructive, real-time measurements of electrode potentials at adhesive/metal oxide interfaces in situ, even if they are covered with an adhesive layer. 

At elevated temperatures where titanium alloys would be the adherend of choice, a different failure mechanism becomes important. Because the solubility of oxygen in titanium increases with temperature, the oxygen in a CAA or other oxide diffuses or dissolves into the metal, leaving voids or microcracks at the metal-oxide interface and embrittles the metal near the interface (Fig. 7). Consequently, stresses are concentrated over small areas at the interface and the joint fails at low stress levels [75,77]. Such phenomena have been observed for adherends exposed to 600°C for as little as 1 hour or 300°C for 710 hours prior to bonding [75] and for bonds using a high-temperature adhesive cured at 371°C [78] or 400°C [75]. 

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