Page 225 - Handbook of Adhesives and Sealants

194 Chapter Five

tion it is sometimes evident why adhesive failure occurred. Evidence
may suggest improper wetting at the original interface or some new
interface.
In comparing surface features after bond failure with the original
adherend surface, the maximum resolution of about 1 10 8 m (100
˚
A) for scanning electron microscopes may not always be sufficient to
detect a thin film of adhesive closely reproducing the original surface
8
profile. Optical and staining methods described by Brett to determine
the presence of such films are mainly applicable to fairly thick films
since optical techniques use interference phenomenon. Films a few
angstroms thick are still largely undetected.
The sciences of microphotography and holography have also been
used for NDT of adhesive bonds. Magnification and photography of
the failed substrate will often lead to useful clues to the cause of fail-
ure. Holography is a method of producing photographic images of
flaws and voids using coherent light such as that produced by a laser.
The major advantage of holography is that it photographs successive
‘‘slices’’ through the scene volume. A true three dimensional image of
a defect or void can then be reconstructed.
The use of highly specialized surface characterization tools has
greatly improved the opportunity for deducing the surface chemical
composition. These tools have been developed for the purpose of ana-
lyzing both the adherend and the adhesive. For adherends, analytical
examination generally centers on either:
(1) The surface chemistry by elemental analysis, chemical species, or
analysis of contaminates and boundary layers, or
(2) On analysis of failed surface for evidence of interfacial failure, fail-
ure within the adherend (e.g., metal oxide, composite matrix or
fiber, etc.) or within a primer or other boundary layer. Analysis of
the adhesive generally consists of characterizing the cured film,
curing agents, and failed specimen surface chemistry.

Over the last 20 years, analytical tools have become available that
allow for the characterization of the elemental and chemical compo-
sition of solid surfaces. The application of these analytical tools has
increased our understanding of surface properties and successfully
characterized surface layers. Table 5.6 shows a comparison of analyt-
ical capabilities of some surface sensitive analytical tools. The most
popular of these are secondary ion mass spectroscopy (SIMS), ion scat-
tering spectroscopy (ISS), and Auger electron spectroscopy (AES). 10–14
These tools have proven practical even when the surface films are only
on the order of atomic dimensions or when the failure occurred near
the original interface and included parts of both the adhesive and the
adherend.