Locus of failure

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. 

The primary drawback to the application of XPS in adhesion science is associated with the limited spatial resolution of the technique. This can make it difficult to study processes that are highly localized, such as corrosion, or to accurately characterize certain types of failure surfaces where, for example, the locus of failure may pass back and forth between two phases. 

Another factor that can contribute to the low release force provided by a release material is the presence of a mechanically weak boundary layer at the surface of the release coating [40,41]. Upon peeling the PSA from the release coating, the locus of failure is within this mechanically weak layer, resulting in transfer of material to the adhesive and a subsequent loss in adhesion of the PSA. Although the use of a weak boundary layer may not be the preferred method of achieving low adhesion for PSA release coatings, it can be useful if the amount of transfer is consistent and kept to a minimum [42]. However, in many cases the unintentional or uncontrolled transfer of a weak boundary layer to a PSA results in an undesirable loss in readhesion. 

Since the locus of failure can clearly distinguish between adhesive and cohesive failures, the following discussion separates loss of adherence into loss of adhesion and loss of cohesion. In the loss of cohesion it is the polysiloxane network that degrades, which can be dealt with independently of the substrate. The loss of adhesion, however, is dependent on the cure chemistry of the silicone, the chemical and physical properties of the substrates, and the specific mechanisms of adhesion involved. 

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... 

On the interfacial lacquer surface, the concentration of the ester group decreased toward the outside (the fracture interface), suggesting the diffusion of DOS into the lacquer film. SEM observation of the fracture surface of metal side is shown in Fig. 6. It can be seen that some lacquer remains as an island state. The apparent disagreement with XPS data seems to be due to the presence of invisible lacquer by this magnification (x 550). According to XPS results and SEM observation the locus of failure may be schematically represented as shown in Fig. 7. 

Figure 7. Schematic model showing a locus of failure...

Failure occurred primarily in the adhesive for up to 5 days exposure in these aging environments. After 5 days, a mixture of failure sites could be identified with more than one interface often exposed on a given sample. This indicated that during extended exposure to humid environments, any non-uniformity or interfacial weakness could be attacked and eventually become the locus of failure. 

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. 

In a specific example of adhesive bonds between cold rolled steel and SMC adherends (Table II) an adhesive based on hydrolysis resistant epoxy chemistry (i.e., adhesive E) was compared with an adhesive based on hydrolysis prone urethane chemistry (i.e., adhesive C) in composite to cold rolled steel bonds. After corrosion testing, a significant difference in both retention of initial bond strength and locus of failure was observed. For bonds prepared with adhesive E, little if any reduction of the initial bond strength was observed after corrosion testing. The locus of failure for both the tested and untested bonds was largely in the... 

Locus of failure studies 75 80) on metal/epoxy joints that had been exposed to water indicate that corrosion of the metal substrate does not occur until after interfacial failure has occurred. This suggests that corrosion itself does not play a primary role in the loss of adhesion strength mechanism of metal/epoxy joints, but rather is a post-failure phenomenon. However, for the case of metal/epoxy protective coating systems, Leidheiser and coworkers 88-91 -92) and Dickie and coworkers 5 87-89-90> have proposed that localized corrosion processes are part of an important delamination mechanism. 

In order to accurately determine the locus of failure of adhesion systems, the chemisti y of the fracture surfaces must be analyzed using surface-sensitive characterization techniques. Many surface analysis techniques are presently available and each technique is based on an intrinsic property of the surface atoms or molecules. Lee155 , Czanderna 156) and Park 157) have reviewed these techniques. However, they suggest that one be aware that new techniques and applications are continually being introduced. 

Table 3. Comparison of primary elemental surface characterization techniques used to determine the locus of failure in adhesion systems 159). (Reprinted from Ref. 59, p. 136, by courtesy of Plenum Press)...

The electron beam used as a probe in AES can be focused to analyze a very small area on the sample surface (diameter l-50p) 62 172). On the other hand, the spatial resolution that can presently be achieved with XPS is relatively poor since it is very difficult to focus the X-ray beam. Therefore, since AES and XPS techniques exhibit complementary strengths, they are often employed together to achieve an accurate determination of the locus of failure in adhesion systems. 

The recorded bond strengths clearly show that MAMS and ECMS were totally ineffective as adhesion promoters on glass. MPS, APES and AAMS were all effective and their use resulted in a marked improvement in the bond strength of both adhesives. All three silanes resulted in a change of the site or mode of failure, the locus of failure transferring from the glass surface to the aluminium test specimen or within the adhesive. 

Bond failure may occur at any of the locations indicated in Fig. 1. Visual determination of the locus of failure is possible only if failure has occurred in the relatively thick polymer layer, leaving continuous layers of material on both sides of the fracture. The appearance of a metallic-appearing fracture surface is not definite proof of interfacial failure since the coupling agent, polymer films, or oxide layers may be so thin that they are not detectable visually. Surface-sensitive techniques such as X-ray photoelectron spectroscopy (XPS) and contact angle measurements are appropriate to determine the nature of the failure surfaces scanning electron microscopy (SEM) may also be helpful if the failed surface can be identified. 

The factors controlling the mechanical behavior of polymer-coupling agent-metal oxide systems have been discussed in terms of the weakest link in a chain concept. Determination of the locus of failure and thus the weak link is not usually reliable by visual inspection, and surface roughness can cause misleading spectroscopic results if failure is near an interface. 

The locus of failure after peel testing was determined using XPS analysis of both the silicon wafer and the polyimide failure surfaces. This analysis was also done as a function of the humidity. 

The locus of failure was determined for samples 2 and 4 after peeling at low and high RH. The data are presented in Table 6. 

One should consider, however, that siloxane (like any polymeric matrix) will not create a hermetic interface. Even though it is hydrophobic in nature, it may still allow water penetration. The second possible explanation involves the change in the relative number of Si—O bonds that would need to be broken to create weakness in the system to the point of changing the peel locus of failure. It may be considered that the APS-siloxane network on the plasma-treated F-contaminated Si02 surface effectively brings another layer of Si—O bonds, the number of which may be too high to be effectively broken during the peel test [21]. 

PMDA-ODA on Si02. It is clear from Fig. 3 that the adhesion of PMDA-ODA to SiO, surface is significantly improved by the application of APS. This is not only seen initially but also after exposure to extended times at T H conditions, i.e. the reliability of the interface has been improved. Notice the spontaneous delamination (zero peel strength) of the PMDA-ODA film from non-APS treated silica surface after only 100 h in T H. It should be pointed out here that the 100 h exposure was the first point at which the samples were removed from the T H test chamber. It is possible that the delamination may have occurred much earlier than the 100 h reported here. Table 2 shows the locus of failure analysis results for the interfaces after initial peel and after exposure to T H for 100 (no APS only) and 700 h. 

No significant difference is observed in the locus of failure data for the APS treated samples presented in Table 2. A closer look at the XPS high resolution scans as shown in Fig. 4 supports the interpretation that the failure is within the polyimide close to the PI-APS interface, but not at it. This is in particular verified by the presence of C=0 C(ls) peak at about 288.9 eV, ji-to-jr transition peak, and the N( 1 s) peak at 400.8 eV due to imide nitrogens. 

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