Unanticipated, catastrophic coating failures that occur relatively early after production are more troublesome. These are generally related to something unforeseen occurring at the interface that significantly reduces the bond strength of the coating or adhesive to the substrate. The exact cause of this type of failure is difficult to determine because so many factors contribute to bond strength.

Rather than first looking at the reasons for bond failure, it may be more instructive to look at the reasons for a successful bond. The following are vital, minimum requirements for all successful coatings and adhesives, regardless of the type of material or application:

• Cleanliness of the substrate surface and elimination of weak boundary layers;
• wetting (intimate contact or spreading of the applied material onto the substrate surface);
• solidification of the coating or adhesive;
• formation of a “joint” structure (applied material, interface region, and substrate) that is resistant to the operating stress and environment; and
• selection and control of all materials and manufacturing processes.

When unpredictable failures occur, it is often the result of insufficient appreciation of these fundamentals. Most often neglected are (1) preparation of the surfaces and (2) localized (interfacial) stresses that occur during processing or when the part is placed in service. These are the subjects of this article.

General Mechanisms of Bond Failure

There are certain common factors that contribute to the weakening of all bonds. Their relative effect on bond strength is qualitatively illustrated in Figure 1.¹ This useful illustration shows why one can never achieve theoretical bond strength values in practice.

If the adhesive or coating does not adequately wet the substrate surface, the joint strength will be degraded. (Wetting is determined by the relative surface energy of the applied material and the substrate, the topology of the substrate surface, and the viscosity of the applied material.) Most clean metal substrates provide good wetting. However, often the surface is not what we think due to the presence of weak boundary layers.

Internal stresses can occur at the interface during production because of the different physical characteristics of the applied material and the substrate. Once a bond is made and placed in service, other forces are at work, weakening the bond. These are both mechanical and environmental. The type of external stress, its orientation to the adhesive or coating, and the rate of loading are important factors. The external stress could either further reduce the measured bond strength or actually increase the measured bond strength by neutralizing internal stresses—similar to an annealing process.

Substrate Surface Condition

Above all else, one must start with a clean, strong substrate surface. Weak surface layers are sometimes common to a specific material. For example, oxide layers are standard with metallic substrates. Certain metal oxides develop rapidly (e.g., aluminum), and some are very weak (e.g., copper).

Foreign materials, such as dirt, oil, moisture, and weak oxide layers, must be removed from the substrate surface, or else the coating or adhesive will bond to these weak boundary layers rather than to the substrate. Accidental contamination of the surface is the most common source of a weak boundary layer. This can be due to grease or dust from the shop environment, fingerprints from manipulation of the substrate, or mold release over-spray from a nearby production area.

Ambient moisture can adsorb onto the surface of the substrate to form a weak boundary layer. The most common cause of this is moisture condensing onto the substrate during times of high humidity. Even less apparent ambient conditions can also affect the coating or adhesive. For example, a preassembly reaction can occur with amine-cured epoxy resins.² The amine can react with the air, resulting in bicarbonate formation. As a result, the adhesive strength can decrease dramatically when an uncured epoxy-amine is exposed to ambient air for a significant period of time. In coating systems, this reaction process results in a white “blush” forming on the coating surface.

Once the substrate is cleaned of foreign material, the adhesive or coating should be applied and cured as quickly as possible. It is the author’s experience that, in the majority of unexpected adhesive failures, improper surface condition is the culprit. Often, the coating or adhesive material or curing process is blamed, but in actuality a weak boundary layer is the root of the failure. Unfortunately, weak boundary layers are common and have many types, as shown in Figure 2.

Various pre-bond substrate surface preparation processes have been developed to strengthen and control the consistency of the surface region. If the surface is strong, the failure will not occur at the interface; rather, cohesive failure will result within the coating or adhesive. These surface treatments perform one or more of the following functions:

• Remove or strengthen any weak boundary layers;
• increase the surface energy of the substrate so that good wetting occurs;
• create functional chemical groups on the surface that can chemically bond with the adhesive; and/or
• protect the interface from corrosion, water ingress, etc., once the joint is placed in service.

These surface treatments generally involve physical or chemical processes, or a combination of both. The choice of surface preparation process will depend on the nature of the applied material, the nature of the substrate before bonding, the required bond strength and durability, and the production processes, time, and available budget.

Localized Stresses

Internal stresses are stresses that occur at the interface and can significantly reduce the inherent bond strength. Internal stresses can occur when the applied material cures or sets from a liquid to a solid, but they can also develop due to service conditions. For example, internal stresses could occur by bending forces or by dimensional change of the substrate.

There are many sources of internal stress, but the three most common are:

1. Stress concentration points at an interface due primarily to imperfections in the applied material or at the interface;
2. stress due to differences in physical properties of the substrate and applied material (e.g., thermal expansion coefficient, modulus); and
3. stress due to shrinkage of the coating or adhesive as it cures.

Localized stress concentrations may occur due to irregularities (voids, defects, etc.) within the applied material or at the interface. Such stress can generally be reduced by using a coating or adhesive that wets the substrate surface and has the ability to fill all of the surface micro-roughness. The proper degree of flow can be achieved by controlling the viscosity of the polymer or with additives. Another method of reducing stress concentration from voids and other defects is to introduce a toughening agent into the adhesive or coating formulation. The toughening agent will provide a stress relief mechanism to interrupt the growth of cracks.

Stresses due to differences in thermal expansion must especially be considered when the coating or adhesive solidifies at a temperature that is different from the normal operating temperature. The thermal expansion coefficients for some common polymeric materials and metal substrates can be more than an order of magnitude apart. This means that the bulk adhesive will travel more than 10 times as far as the substrate when the temperature changes, thereby causing stress at the interface.

The stresses produced by thermal expansion differences can be significant. Such stress is evident by noting the test results of a typical elevated temperature-cured adhesives joint as a function of test temperature (Figure 3). Notice that the bond strength actually increases with temperature to a maximum, and then it falls-off with further increasing test temperature. At some elevated temperature, the internal stresses are completely relieved and the bond strength reaches a maximum. The test temperature at which this occurs is usually very close to the curing temperature. At higher test temperatures, additional stresses develop or the effects of thermal degradation become evident, and the bond strength then decreases with increasing test temperature.
The above example illustrates why the external forces exerted on the bond joint by the test procedure can be either positive or negative, as shown in Figure 1. When the external stress acts to neutralize the internal stress, the measured bond strength can increase. Of course, external stress can also add to the internal stress and result in a total stress level that results in catastrophic failure.

There are several possible solutions to the expansion miss-match problem. One can use a resilient coating or adhesive that moves with the substrate during temperature change, thus relieving the internal stress. Another approach is to adjust the expansion coefficient of the applied material to a value that is near to the substrate. Coatings and adhesives can be formulated with various fillers to modify thermal-expansion and limit internal stresses.

Nearly all polymeric materials (including coatings) shrink during solidification. Sometimes they shrink because of escaping solvent, leaving less mass in the bond line. Even 100% reactive polymers, such as epoxies and urethanes, experience some shrinkage because their solid polymerized mass occupies less volume than the liquid reactants. The result of such shrinkage is, again, internal stresses at the interface and the possible formation of cracks and voids within the bond line itself.

A possible solution to the shrinkage problem may be to, again, flexibilize the adhesive or coating. It is also especially important that voids and gas bubbles be eliminated from adhesives that have a high degree of shrinkage. Small voids can grow due to the applied material’s shrinkage and significantly degrade the cohesive strength of the film. Vacuum degassing of a mixed two-part adhesive is common practice to remove any entrapped air.

Avoid These Deadly Sins

Ancient philosophers have been able to distill all of the problems of mankind into seven deadly sins: pride, envy, anger, sloth, avarice, gluttony, and lust. It may be possible to distill most of the problems associated with coatings or adhesives into distinctive sins as well. For our purposes, notice that most are sins of omission.

1. Assuming that solid surfaces are smooth, pure, and static: It is often believed that surface roughness improves adhesion via mechanical interlocking and increased bond area. However, if the adhesive does not wet the surface well, the roughness may cause air pockets to be trapped at the interface. Substrates could also change with time before application of the coating or adhesive (e.g., oxidation, contamination, moisture condensation) or after being placed in service (e.g., corrosion, thermal expansion, or contraction).
2. Forgetting that coatings and adhesives shrink during cure: All polymeric materials shrink on hardening, either due to loss of solvent or to the polymerization process. This contributes to internal stresses at the interface, and cracks and voids could form. A mistake sometimes is to accelerate cure via heat or catalysts. This often results in greater shrinkage and higher internal stress (see 4 below).
3. Assuming that bond strength is due only to adhesion: Not only must the coating or adhesives form strong bonds to the substrate, but it also must be strong internally. The bulk properties of the applied material are as important as the bond properties in determining strength and durability.
4. Forgetting about thermal expansion coefficients: Differences in thermal expansion provide sufficient internal stress to degrade the bond strength. Stresses could develop when an elevated-temperature-cured joint is brought down to room temperature or during thermal cycling when in service.
5. Creating a joint design that maximizes the strength of the bond: Stress distribution within a bond under load is not uniform. Peel- and cleavage-type forces concentrate all of the stress at the edge of the bond. Shear loads, however, distribute the load over the entire bonded area. Adhesive joints should be designed, if possible, so that peel and cleavage forces are minimized.
6. Not controlling the surface preparation process: Quality control must attend to the surface preparation of the substrates. Cleaning solution contamination, high relative humidity, dirty shop environment, etc., all can contribute negatively to the strength of bond.
7. Underestimating the need for education: Education is important due to the many disciplines required (materials science, surface chemistry, manufacturing engineering, etc.). Everyone associated with the bonding process needs to be trained to understand the factors that can contribute to bond failure.

References

1. Reinhart, F.W., “Survey of Adhesion and Types of Bonds Involved,” in: Adhesion and Adhesives Fundamentals and Practices, J.E. Rutzler and R.I. Savage (eds.), Society of Chemical Industry, London; 1954.
2. Bell, J.P., et. al., “Amine Cured Epoxy Resins: Adhesion Loss Due to Reaction with Air,” Journal of Applied Polymer Science, 21(1095–1102); 1977.

Bio

Edward M. Petrie is the sole proprietor of EMP Solutions, a Cary-N.C.-based consulting firm focused on solving problems in the adhesives and sealants industry. He also works as a technical expert for SpecialChem.