Failure, adhesive substrate

In most cases, failure cracks arise in the weak bormdary layers. Bikerman distinguished many groups of weak boundary layers according to their origin. Similar to the formation of the adhesion joint, three phases usually participate in failure adhesive, substrate and air (or other medium). Various admixtru-es form a weak boundary layer, being concentrated at the phase border. The defects in the structure of the substrate and adhesive and admixtures determine... 

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... 

Not only is low tensile shear strength noticed on moisture aging, but also the mode of failure changes from one of cohesion to adhesion. Table 7.6 shows the effect of humidity and water immersion on an epoxy-nylon adhesive compared to a nitrile-phenolic adhesive. Substrate primers have been used with epoxy-nylon adhesives to provide improved moisture... 

The adhesive-substrate bond may also be subject to thermal stresses resulting from differences in the coefficients of thermal expansion of the hard tissue and the resin. Ideally the values of these two coefficients should be the same (in practice the coefficient of thermal expansion of the adhesive is usually much higher) to avoid the build-up of stresses which eventually may lead to bond failure. This is especially important for dental adhesives since the temperature range in the mouth may vary from 2 C to 55 C. 

One of the principal reasons for failure of the adhesion bonds is a specific adsorption reaction of the medium with the material to be cemented at the boundary with the adhesive. There is an adsorption substitution of adhesive-substrate bonds by medium-substrate bonds. Surface structural defects that are present in each solid are the first to be subjected to adsorption. It is to be expected that the probability of appearance of such defects is higher at an interface of two materials with different properties. The rate of penetration of the medium along the polymer-substrate interface frequently substantially exceeds the rate of diffusion of the medium in pure polymer [212]. Adsorption substitution of the polymer macromolecules by water molecules on the metal surface explains the low water resistance of such adhesive-bonded joints as fluoroplastic-steel or polyethylene-steel [34]. The adhesion strength, which decreases during hold-up of adhesive-bonded joints in water, is frequently reestablished after the joints are dried [213]. 

An attempt is made to justify the belt and braces approach as a logical outcome of experience of practical applications of sealants and adhesives in construction work. Consideration is given to the probability of failure/ success, adhesive/substrate compatibility, workmanship/ quality control, problem prevention and where not to use adhesives. 

The above comments are seen to be reinforced by observations on the failure path in joints before and after environmental attack. The locus of joint failure of adhesive joints when initially prepared is usually by cohesive fracture in the adhesive layer, or possibly in the substrate materials. However, a classic symptom of environmental attack is that, after such attack, the joints exhibit some degree of apparently interfacial failure between the substrate (or primer) and the substrate. The extent of such apparently interfacial attack increases with time of exposure to the hostile environment. In many instances environmental attack is not accompanied by gross corrosion and the substrates appear clean and in a pristine condition, whilst in other instances the substrates may be heavily corroded. However, as will be shown later, first appearances may be deceptive. For example, to determine whether the failure path is truly at the interface, or whether it is in the oxide layer, or in a boundary layer of the adhesive or primer (if present), requires the use of modern surface analytical methods one cannot rely simply upon a visual assessment. Also, the presence of corrosion on the failed surfaces does not necessarily imply that it was a key aspect in the mechanism of environmental attack. In many instances, corrosion only occurs once the intrinsic adhesion forces at the adhesive/substrate interface, or the oxide layer itself, have failed due the ingressing liquid the substrate surface is now exposed and a liquid electrolyte is present so that post-failure corrosion of the substrate may now result. 

Failure modes indicate that moisture affects adhesive bulk instead of the adhesive-substrate interface. 

Locus of failure Where weak boundary layers exert an effect on adhesion behavior, failure typically occurs within the weak layer, close to the adhesive-substrate interface. The boundary layer will often be very thin, in macroscopic terms, and there are large numbers of examples where workers have reported the adhesive (i.e., interfacial) failure of a joint, when in fact it could be shown that failure was cohesive within a boundary layer. Bikerman s insistence (2) that adhesive failure could not occur had the beneficial effect of concentrating attention on the locus of failure. Careful examination of locus of failure is an important technique — still sometimes neglected - in the study of adhesive bonds. Modem methods of surface analysis are appropriate here. They have established that, although adhesive failure maybe rare, it does occur sometimes. 

In considering locus of failure, it must be emphasized that observation of cohesive failure close to the adhesive-substrate interface does not, itself, prove the presence of a weak boundary layer. The idea that an adhesive joint fails at its weakest link is naive. The locus of failure of a joint depends in a complex way on the chemical and physical properties of the materials in the joint, and on the way the stress is applied to them. Cohesive failure close to an interface is entirely possible in the complete absence of weak interfacial layers (Good 1972). Dillard has demonstrated how failure can change between cohesive, interfacial, and oscillating for exactly the same joint, depending on how the stress is applied. (Chen and Dillard 2002)... 

Another important aspect of testing the adhesive as part of an adhesive-joint system is that the joint presents a number of options for the location of the failure path. Failure may be cohesive in approximately the center of the adhesive layer. It may be cohesive but near the interface as is often seen in peel testing. It may be interfacial along the adhesive-substrate interface or it may run entirely within an interphase, for example, within a metal oxide/ adhesive interphase region. The failure path could run cohesively through the substrate, for example, the crack could run in the interlaminar region of a fiber-reinforced polymer composite substrate (Kinloch et al. 1992). Finally, some combination of the above could occur. Each of these options for the failure path may lead to a different fi-acture resistance being measured and thus adhesive-joint tests and their interpretation are necessarily more complex than bulk adhesive studies. 

In order to analyze the effects of rate and temperature on the interfacial strength of bonded joints, one can initially use a simple energy approach in the following maimer A critical energy level, Wc, is used to represent interfacial failure. In the presence of adhesive, substrate, and interphase (a distinct material layer between the substrate and the adhesive layer which transmits the rigid substrate s energy to the adhesive layer), the combined elastic energy can be written as ... 

Failure can either occur due to cohesive fracture of the adhesive (a), due to interfacial sliding (b), or due to adhesive debonding at or close to the adhesive substrate interface (c) as illustrated inO Fig. 34.11. 

A set of creep experiments at different load levels can be used to build creep-strength time curves indicating at which time- and load-related limits a failure under static load condition is to be expected for a certain adhesive-substrate combination. The construction of a so called creep rupture envelope by connecting the points of the onset of the tertiary creep stage is shown in Fig. 34.12. 

Film Adhesion. The adhesion of an inorganic thin film to a surface depends on the deformation and fracture modes associated with the failure (4). The strength of the adhesion depends on the mechanical properties of the substrate surface, fracture toughness of the interfacial material, and the appHed stress. Adhesion failure can occur owiag to mechanical stressing, corrosion, or diffusion of interfacial species away from the interface. The failure can be exacerbated by residual stresses in the film, a low fracture toughness of the interfacial material, or the chemical and thermal environment or species in the substrate, such as gases, that can diffuse to the interface. 

---