There are many reasons why coatings fail. Often, failure can be related to an extended aging process, such as thermal, photolytic, or hydrolytic degradation. These types of failures are due to the service environment and the ability of the coating or adhesive, for that matter, to resist it. Unanticipated, catastrophic 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. 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. 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.

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. (Surface preparation methods for specific substrates will be discussed in future articles.)

For more on this story, see the February 2007 issue of Metal Finishing.

Edward M. Petrie is the sole proprietor of EMP Solutions, a consulting firm focused on solving problems in the adhesives and sealants industry. He also works as a technical expert for SpecialChem. For more information, visit www.specialchem4adhesives.com.