An adhesive must be able to effectively wet the substrate acted upon, harden, and withstand stress. Various internal and external forces such as heat, stress or chemical solvents can weaken an adhesive. When the strength of the adhesion weakens, the bonded surfaces may separate, causing failure.
Bonding failures can be classified as:
- Cohesive – the failure originates in the bulk bonding material and may be caused by degradation of the adhesive.
- Adhesive – the failure occurs at the interface of the two materials being bonded and may be caused by improper surface preparation. Even very thin contaminating films, only a few molecular layers thick, can prevent proper adhesion and cause failure.
- Mixed – a combination of cohesive and adhesive failures.
Techniques Used to Determine Adhesion Failure
SEM Analysis with EDS
Scanning electron microscopy (SEM) with energy dispersive analysis (EDS) identifies the elemental microstructure of a sample and generates high resolution images. This is often the first step in determining particle characterization in any general failure analysis. However, the analysis depth of SEM/EDS below the surface is relatively deep. Thin surface layers or films can easily be obstructed by the signal from below the surface (typically 1-2 micrometers or 1000-2000 nanometers). When sampling the chemistry of the very top surface layer of 10 to 30 nanometers is necessary, instrumentation specifically designed for this purpose is required since even a very thin film can result in adhesion failure.
XPS and Auger
X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) are excellent surface analysis techniques perfectly suited for adhesion failures, surface staining, surface contamination, etc. Both instruments detect chemical species on the surface of the sample and are complementary to each other. The resultant spectra are “fingerprints” of the analyzed material. If the fingerprint cannot readily be determined by one instrument, the other sometimes may show a clearer fingerprint. XPS is used to analyze inorganic and organic compounds and various composite materials, such as metal alloys, polymers, and ceramics. It is an excellent technique to determine how the surface of a material was prepared for bonding or to determine the point of failure at the surfaces’ interface.
AES is an efficient and straightforward characterization technique for examining the chemical and compositional elements of the top surface of a solid. This technique is known for its sensitivity and quantitative detail.
Comparison of XPS and AES
- XPS has a larger spatial zone (or diameter) of analysis compared to Auger. It is typically 0.05 to 1.0 mm2, making it more suitable for sampling a relatively large, diffuse surface layer.
- In contrast, Auger has a small, focused high-resolution beam which can be used to identify the composition of small particles ~5 micrometers in size.
- In addition to detecting the elements themselves, XPS can detect the bonding states of elements as well as their chemical bonds, making it the technique of choice for identifying compounds on a surface.
- While Auger is more sensitive to lower elemental concentration levels, it is generally not sensitive to bonding parameters or identifying compounds.
- Both instruments isolate surface chemistry from the bulk, being sensitive to ~1 to 2 nanometers depth (3 to 10 atomic layers).
- Both instruments can sputter away the surface layer, allowing a composition depth profile to be generated.
FTIR and Raman
Both Fourier Transform Infrared spectroscopy (FTIR) and Raman spectroscopy are used to characterize and identify organic materials and adhesive, and/or the surface preparation and cleaning materials used in bonding. They require different sample sizes for analysis and have varying sampling zone (diameter) or spatial resolution of the spectra generated. As with the XPS and Auger techniques, these analytical methods complement each other by filling in the gaps to offer a complete picture of why materials, such as polymers and rubber, lubricants and liquids, fail to bond.
FTIR can identify organic material and contaminants, including polymers, powders, and films, but provides somewhat limited inorganic data. Raman is used to characterize and identify both organic and inorganic materials and can determine the chemical structure of a material to identify its compounds.
Raman and FTIR can be used for probing the bulk chemical composition of an adhesive for properties such as additive concentration, cross link density, oxidation, and other factors that would affect the cohesive strength of an adhesive.
Comparison of FTIR and Raman:
- FTIR requires a sample of at least 15µm in diameter, but Raman has a minimum analysis area of approximately 0.5µm providing better spatial resolution.
- FTIR is used typically for investigations consisting of organic materials but cannot be used for aqueous analyses; Raman is used to identify organic, inorganic and aqueous samples.
- Raman also identifies crystalline structure which makes it easier to determine stress failures.
- Raman imaging and confocal depth profiling can be used to produce maps that identify how components are dispersed in mixtures in three dimensions.
- FTIR and Raman are both qualitative and quantitative however quantitative Raman analyses can be more difficult.
- Both have access to spectral libraries to assist in identification.
- Both techniques are non-destructive and can be used under ambient conditions.
For more information on failure analysis for adhesives and binding, please contact us at 1.800.860.1775.