Adhesive bonding failure surfaces

Different surface treatments such as peel ply make it possible to roughen and clean composite surfaces and consequently to improve adhesion performance. However, like many other treatments, it is limited by the cohesive failure that may occur in the material. As an alternative, laser ablation appears to be quite an interesting surface pretreatment for polymer composites since it fadhtates efficient control of the surfaces to be adhesively bonded. Furthermore, surface properties must be suitably defined by taking into account the nature of the composite material and the adhesive used. 

W. L. Baun, "Study of Adhesive Bonding and Bond Failure Surface Using ISS-SIMS," in Characterization of Metal and Polymer Surfaces. Vol. I, L. H. Lee, ed., pp. 375-390, Academic Press, New York (1977). 

H. M. Clearfield joined Martin Marietta Laboratories in January, 1985. Since then, he has primarily investigated surface and interfacial phenomena in adhesive bonding, including surface preparation of titanium alloys for structural applications at high service temperatures, mechanisms of bond failures that occur at high temperatures, and bonding of the thermal protection system to the space shuttle external tank. Additionally, he has investigated dopant depth distributions in ion-implanted and laser-annealed silicon. Dr, Clearfield is supervisor of surface analysis facilities at Martin Marietta Laboratories. Recently, Dr. Clearfield joined IBM s T. J. Watson Research Center. 

XPS images recorded from the interfacial failure surfaces of an adhesively bonded aluminum joint, prepared using an organosilane primer, with an instrument of the type shown inO Fig. 9.14. The optical mirror images are complementary views of the fracture surfaces and are mirrored along the dotted line. Visually the failure appears to be interfacial with the fracture path moving from one interface to the other at the boundary between metal and adhesive interfacial failure surfaces... 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

Surface energies are also associated with failure of an adhesive bond, because failure involves forming new surfaces and the appropriate surface energies have to be provided. The surface energy term may be the work of adhesion, VTa, or the work of cohesion, VTcoh. depending on whether the failure is adhesive or cohesive. For phases 1 and 2, these are defined as follows [lOj ... 

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

Weak boundary layer. WBL theory proposes that a cohesively weak region is present at the adhesive-substrate interface, which leads to poor adhesion. This layer can prevent the formation of adhesive bonds, or the adhesive can preferentially form bonds with the boundary layer rather that the surface it was intended for. Typically, the locus of failure is interfacial or in close proximity to the silicone-substrate interface. One of the most common causes of a WBL being formed is the presence of contaminants on the surface of the substrate. The formation of a WBL can also result from migration of additives from the bulk of the substrate, to the silicone-substrate interface. Alternatively, molecular... 

The interfacial adhesive bond surfaces generated as a result of corrosion induced failure (for adhesives C and E) have been examined using x-ray photoelectron spectroscopy. The results of these studies are shown in Table III and Figures 1 and 2. Changes in... 

The two predominant mechanisms of failure in adhesively bonded joints are adhesive failure or cohesive failure. Adhesive failure is the interfacial failure between the adhesive and one of the adherends. It indicates a weak boundary layer, often caused by improper surface preparation or adhesive choice. Cohesive failure is the internal failure of either the adhesive or, rarely, one of the adherends. 

The XPS analysis of the failure surfaces of the wedge samples primed at 51% RH and bonded with PES and immersed in DI water at 100°C are reported in Table 3. The concentration of sulfur in all of the primed samples bonded with PES was about 5% and suggests that failure occurred primarily within the adhesive. The atomic concentrations of aluminum, silicon, and titanium were below 0.5%, precluding assignment of failure within the alkoxide layer. The control sample failed at the interface between the steel and the adhesive. 

The XPS analysis of the samples primed at 18% RH and bonded with PES and immersed in DI water at 100°C are reported in Table 4. The atomic concentration of iron in these samples was about 13%. About 1-2% of sulfur was detected on these surfaces, which indicated that some adhesive was retained on these surfaces. Since iron was seen in such significant amounts on both failure surfaces, it was concluded that failure had occurred within the steel (oxide) layer. 

Secondary Ion Mass Spectrometry used as a solo Instrument or in concert with other methods has proven to be an excellent technique for studying the surface chemistry of adhesive bonding materials. The application of SIMS is shown in re.lation to pretreatments of metals and alloys, chemistry and structure of adhesives, and locus of failure of debonded specimens. 

It is seen in Table 1 that SIMS Is applicable to several areas of investigation in adhesive bonding. SIMS may be used in a variety of ways including species imaging of the surface (SUMS) which may be especially useful for clarifying mixed mode failure surfaces. 

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