One of our areas of specialized expertise has long been the analysis of adhesive bond failures. Adhesive bond failures are commonly said to be either cohesive failures or adhesive failures. A cohesive failure is a failure in the bulk layer of the adhesive and is usually the desired mode of failure. An adhesive failure occurs at the interface between the adhesive and the adherend, or the material to be bonded.
Adhesive failures often require sophisticated analysis to understand. The location of the failure may be in a very thin layer of a metal surface treated to promote adhesion with a high surface area surface, treated to retard corrosion with a very thin layer of solid particles, and then treated with a silane adhesion promoter which may be thinner than 10 nm thick. The adhesive itself may have numerous component ingredients and some may be at very low concentrations in the bulk of the adhesive, but may migrate to the interface to promote adhesion. Sometimes this migration process does not work right. Sometimes an impurity migrates to the interface instead. Or, the interface may have become contaminated during any of the processes used to prepare the metal surface or afterwards. Fatal contamination layers are often less than 3 nm thick! One classic example is a silicone or dimethyl siloxane release agent or lubricant. Exposure of a metal surface to humidity may also cause interfacial or adhesive bond failures in the metal surface.
Bonding to plastic, glass, ceramics, inorganic particles, or composite materials may suffer failures at or near interfaces for many reasons. Inorganic particle filler materials are often given treatments to promote polymer resin wetting. When that fails, they may agglomerate. Or, such inorganic particles may have difficulty in bonding due to excessive hydration. Glass surface bonds may fail if the glass surface was too alkaline when bonded. Metal surfaces, especially those with +2 valence states are inclined to suffer from excessively alkaline chemistries at surfaces also. Plastic bonding may fail due to migration of plasticizers to the surface or residues of mold release agents.
Determine surface and interface chemistries using XPS surface analysis:
- Surface analysis of adherend or substrate material as prepared for bonding
- Surface analysis of both sides of peeled or failed interfaces to find locus of failure, identify chemistry, and identify any contaminants
- Correlate surface chemistry of samples tested to failure with measured strength
- Aging of bondline, test to failure, determine effects of aging on chemistry
- Determine differences of bondline adhesive chemistry from the bulk adhesive chemistry, since distinct interfacial adhesive interphase layers are common
- Identify interfacial contaminants due to hydrolysis, silicones or siloxanes, plasticizers, surfactants, hand lotions, cutting oils, hard water deposits, solder fluxes, and photoresists
Prepare very clean adherend surfaces for reference to mechanically tested bond strengths of manufactured or prototype bonded materials.
Microscopy – SEM and Optical – Failure Analysis:
- Locates failure plane
- Indicates type of failure
- Reveals gas bubbling effects due to contaminants or improper 2-part adhesive mixtures
Failed lap shear tested coupon for analysis to determine the cause of adhesive failure.
Honeycomb skin with poor adhesive bond in some areas for analysis to determine the cause of failure.
- TMA to determine differences of thermal expansion of adherends and adhesive
- TMA to indicate whether thermoplastics have too much crystallinity, which may produce a large thermal expansion spike at the crystalline melting temperature
- TMA and DSC to determine maximum temperatures appropriate for curing adhesive bondlines or to prevent phase changes and degradation processes in use
- DSC to determine good adhesive curing conditions or problems due to improper mixtures of 2-part adhesives
Electrochemistry – Electrochemical Impedance Spectroscopy (EIS):
- Measures water and ionic concentration buildup in organic coatings and adhesives
- Measures effect of adhesive or coating pores, holidays, and fisheyes upon water reaching interface to cause hydrolysis attack of bondline
- Identify differences at bondline relative to in the bulk of the adhesive
- Contrast chemistry of 1 μm depth to that of the 8 nm measured by XPS
- Identify plastic, rubber, resin, and laminate materials being bonded
- Identify the adhesive and/or the primer to which adhesive is applied, as it is in aerospace adhesive bonds to aluminum surfaces