NOTE The Application of Surface Analysis Techniques in the Adhesive Bonding of Oily Automotive Steel

There are fewer papers on the adhesive bonding of steel for structural applications than for aluminum and titanium alloys. However, the approach to the adhesive bonding of all three adherends has been similar, that is, the surfaces are pretreated prior to bonding. Trawinski, et al.^1,2,3 reviewed several conversion coatings or etching processes used for steel. Haak and Smith⁴ selected two surface treatments among nineteen based on minimal cost, simplicity and good durability. Smith⁵ has reported work on stainless steel-epoxy bonds under hydrothermal stress. Bischof, et al.⁶ investigated the effect of surface pretreatment of steel on bonding strength obtained with polyvinyl chloride. Ziane, et al.⁷ identified four fracture zones resulting from shear loading of epoxy bonded galvanized steel following four different surface pretreatments. But in some cases, as in the automotive industry, there is a need to bond oily steel directly without surface pretreatment. Rosty, et al.⁸ have reported a study of the role of fillers and cure temperature on the shear strength of oily steel bonded with epoxy. None of the reported research utilizes both microscopic and spectroscopic techniques to analyze the fracture surfaces.     

Novel surface and interfacial analysis techniques as aids to the development of new, high fracture toughness film adhesives

An experimental film adhesive of high fracture toughness had given a promising range of adhesive properties on pickled 2024-T3 clad aluminum alloy, but a wide variation in peel performance resulted when this adhesive was used to bond anodized aluminum adherends. High resolution scanning electron microscopy/ energy dispersive X-ray analysis of the fracture interfaces coupled with transmission electron microscopy of ultramicrotomed sections through the same specimens has shown that the problem was non-wetting of the substrate on a microscopic scale. The rheological properties of the adhesive system have been altered to overcome the problem, yielding a high fracture toughness film adhesive of consistent performance.     

Initial investigation into the effectiveness of CO2-laser treatment of aluminium for adhesive bonding

Aluminium alloy coupons were treated by CO₂-laser ablation. The degree of surface modification introduced by this treatment has been determined by a combination of Auger electron spectroscopy and scanning electron microscopy. Following laser treatment a 22% increase in initial lap shear joint strengths was observed compared with degreased-only controls.     

Preadhesion Laser Surface Treatment of Carbon Fiber Reinforced PEEK Composite

An excimer UV laser (193 nm) was used for preadhesion surface treatment of PEEK (polyetheretherketone) composite. This method presented an alternative to other limited and polluting conventional surface treatment methods. Experimental results indicated that laser preadhesion treatment significantly improved the shear and tensile adhesion strength of structural epoxy FM 300 2K bonded PEEK composite adherends compared with untreated and SiC blasted substrates. Best results were obtained with laser energies of 0.18 or 1 J/P cm.² Shear strength of laser-treated joints was improved by 450% compared with that of untreated PEEK composite and by 200% compared with SiC-blasted pretreatment at ambient and at extreme temperatures. An order of magnitude of improvement was found in the tensile strength-of laser-treated PEEK composite in a sandwich structure compared with non-treated or abraded sandwich joints. The mode of failure changed from adhesive to cohesive as the number of pulses or laser energy increased during treatment. The latter phenomenon was correlated with surface cleaning as revealed by XPS, with morphology changes as revealed by scanning electron microscopy, and by chemical modification as indicated by FTIR and XPS. The bulk of the PEEK composite adherend was not damaged by the laser irradiation during treatment as indicated by the identical flexural strength before and after laser treatment. It can be concluded that the excimer laser has a potential as a precise, clean and simple preadhesion surface treatment for PEEK composite.     

Plasma treatment of tetrafluoroethylene-ethylene copolymers for adhesive bonding

The aim of this work was to improve adhesion to tefzel using plasma surface treatment. The plasmas used were O₂, and NH₃. Joints ,ade from the adherends using several commercially available epoxy adhesives were tested using a double lap shear configuration. Measured bond strenghts for the treated adherends were as much as 30 times greater than those for the untreated materials. Examination of the O₂ plasma-treated Tefzel by electron spectroscopy for chemical analysis indicated a surface oxidation increase of about 7NDASH;8% over the untreated material, with the oxide being primarily in the form of an ester.     

A Preliminary Study of the Use of Kynar® Piezoelectric Film to Measure Peel Stresses in Adhesive Joints

A variety of test techniques have been developed to test the performance of adhesives bonded in situ within joints. Most of these techniques measure strength, fracture toughness, or adhesive modulus of the bonded joint. Techniques to measure actual stress or strain values within a bonded joint are quite few in number. The Krieger gage¹ is able to measure the average shear displacement along a 12.5 mm. gage length of a thick adherend joint. It has been used primarily to measure in situ shear moduli of adhesives. Brinson and his colleagues² proposed bonding strain gages within adhesive joints to measure strains within the adhesive. Unfortunately, these gages are only sensitive to the lateral strains and not shear or peel strains. Because the lateral strains are dominated by the behavior of the adherends rather than the adhesive, the information which can be gained is incomplete.     

The Effect of Damage Nucleation on the Toughness of an Adhesive Joint

The intrinsic toughness of an adhesive joint has been shown to be different depending on whether the adherends remain elastic or deform in a plastic fashion. This phenomenon occurs because the different constraint imposed by the adherends results in a change in the deformation mechanisms of the adhesive layer. In the elastic geometry, damage nucleation occurs when the stresses in the adhesive layer reach a critical value before the conditions for fracture are met. Void growth then leads to large-scale bridging across the adhesive layer and an increase in the measured toughness. In contrast to this behavior, the reduced constraint associated with adherends that are thin enough to deform plastically allows the fracture criterion to be met before damage nucleation occurs. There is then no bridging contribution to the toughness. The effect of damage in an adhesive layer can be viewed either as a bridging zone behind the crack tip, or as an extended cohesive zone ahead of the crack tip. The toughness of an adhesive joint can either be increased or decreased by the nucleation of damage. The effects of a damage zone on the behavior of an adhesive joint with elastic adherends are discussed, and it is shown how numerical techniques can be used to model this behavior and to deduce the fracture parameters.     

The Effect of Damage Nucleation on the Toughness of an Adhesive Joint

The intrinsic toughness of an adhesive joint has been shown to be different depending on whether the adherends remain elastic or deform in a plastic fashion. This phenomenon occurs because the different constraint imposed by the adherends results in a change in the deformation mechanisms of the adhesive layer. In the elastic geometry, damage nucleation occurs when the stresses in the adhesive layer reach a critical value before the conditions for fracture are met. Void growth then leads to large-scale bridging across the adhesive layer and an increase in the measured toughness. In contrast to this behavior, the reduced constraint associated with adherends that are thin enough to deform plastically allows the fracture criterion to be met before damage nucleation occurs. There is then no bridging contribution to the toughness. The effect of damage in an adhesive layer can be viewed either as a bridging zone behind the crack tip, or as an extended cohesive zone ahead of the crack tip. The toughness of an adhesive joint can either be increased or decreased by the nucleation of damage. The effects of a damage zone on the behavior of an adhesive joint with elastic adherends are discussed, and it is shown how numerical techniques can be used to model this behavior and to deduce the fracture parameters.     

Preparation of metallic surfaces with microfibrous topography and their effect on adhesion of hot-melt and thermoset adhesives

Various chemical and electrochemical methods have been developed which produce acicular oxide or metal growths on the metal surface, and the adhesion of various polymers to such microfibrous surfaces has been studied. Polyethylene, which normally adheres poorly when applied to metals as a hot-melt coating unless conditions permit its oxidation, gives good adhesion to microfibrous surfaces in the presence of antioxidants. Similar surfaces give good adhesion with ethylene vinyl acetate. Comparative examination of the fracture surfaces enables the phenomenon to be related to theories of adhesion.

The effect of microfibrous surfaces on steel, copper and zinc on the adhesion of epoxy resins has been studied using fracture mechanics tests giving values for G_c fracture energy. In some cases these strong engineering adhesives give problems with the strength of bond between the metal and microfibrous surface layer. However, in many cases changing from a smooth to a microfibrous surface alters the mechanical response of the adhesive from a brittle to a more ductile one, thus increasing the value of joint toughness. With a rubber-toughened epoxy, the microfibrous surface enables much more of the potential toughness of the resin to be realized.     

Note: The Optimum Profile for a Lap Joint.

IN LAP JOINTS, stress concentrations in the adhesive layer can arise from differences in elastic moduli and abrupt variations in thickness of the adherends and the adhesive layer¹. Various attempts have been made to design joints in which these stress concentrations are minimised. Mylonas and de Bruyne² suggested that the stress concentrations could be reduced by tapering the ends of the adherends, so that a more even distribution of strain along the joint could be obtained. Hennig³ suggested that the same objective would be attained by using a high modulus adhesive in the centre of the joint and a lower modulus adhesive at the ends of the joint; he reported that an increase in joint strength of 20% could be obtained by this method. Segerlind⁴ discussed the variation of the magnitude of the stress concentrations in a lap joint with the dimensions of the joint and showed that since the increase in joint strength with increase in overlap effectively fell to zero above a given length of the overlap (dependent upon the geometry of the joint and the physical properties of adhesive and adherend), then it was possible to specify an optimum overlap for the joint.     

Preadhesion Laser Treatment of Aluminum Surfaces

An excimer laser may be used for preadhesion treatment of aluminum alloys. This method presents an alternative to the use of ecologically unfriendly chemicals involved in conventional anodizing pretreatments.

Experimental results indicate that preadhesion laser surface treatment significantly improved the shear strength of modified-epoxy bonded aluminum specimens compared with untreated and anodized substrates. The best results were obtained with laser energy of about 0.2 J/Pulse/cm² where single lap shear strength was improved by 600-700% compared with that of untreated Al alloy, and by 40% compared with chromic acid anodizing pretreatment.

The mode of failure changed from adhesive to cohesive as the number of laser pulses increased during treatment. The latter phenomenon has been correlated with morphology changes as revealed by electron microscopy, and chemical modification as indicated by Auger and infrared spectroscopy.

It can be concluded that the excimer laser has potential as a precise, clean and simple preadhesion treatment of Al alloys.     

Bonding characteristics and corrosion resistance of silane–cerium-treated aluminum adherend

A silane–cerium treatment was applied on an aluminum adherend to simultaneously improve the bonding performance and corrosion resistance of the adhesively bonded aluminum joint in cryogenic applications, such as with liquefied natural gas containment tanks. The lap shear strengths and corrosion performances of the adhesively bonded joints composed of treated aluminum adherends were measured with respect to the silane–cerium treatment and the surface pretreatment on the aluminum adherend. The bonding characteristics of the aluminum adherend were investigated by measuring the water contact angle and conducting the potentiodynamic polarization test after the aluminum adherends with different surface treatments of silane–cerium were immersed in a 0.5?M NaCl solution. In addition, the surfaces were analyzed with scanning electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy to characterize the chemical compositions of the silane–cerium-treated aluminum adherend. The experimental results show that an appropriate silane–cerium treatment on the aluminum adherend produces an effective corrosion-resistant layer and that it has a highly reliable bonding characteristic for the adhesive joint at a cryogenic temperature of ?150?°C.     

Preliminary Examination of the Thermophysical Bond of Insert-molded Poly (Carbonate)/C Fiber Composites

The adhesive strength of a thermophysical bond between two polymers has been examined using fracture mechanics. Bimaterial composite specimens were constructed by injecting C fiber poly(etheretherketone) (PEEK) into a mold containing one-half of a pre-molded poly(carbonate) (PC) dogbone. The resulting specimens were notched at the interface and tested in tension. Adhesion of the two materials was reasonably good, as demonstrated by fracture surfaces that showed a mixture of PC and C fiber PEEK fragments. Interfacial fracture energy of the composite was approximately 1.5kJ/m².     

Dynamic strength of single lap joints with similar and dissimilar adherends

The increased use of adhesives for joining structural parts demands a thorough understanding of their load carrying capacity. The strength of the adhesive joints depends on several factors such as the joint geometry, adhesive type, adherend properties and also on the loading conditions. Particularly polymer based adhesives exhibit sensitivity to loading rate and therefore it is important to understand their behavior under impact like situations. The effect of similar versus dissimilar adherends on the dynamic strength of adhesive lap joints is addressed in this study. The dynamic strength is evaluated using the split-cylinder lap joint geometry in a split Hopkinson pressure bar setup. The commercial adhesive Araldite 2014 is used for preparing the joints. The adherend materials considered included steel and aluminum. The results of the study indicated that the dynamic strength of the lap joint is influenced by the adherend material and also by the adherent combination. Even in the case of joints with similar adherends, the strength was affected by the adherend type. The strength of steel–steel joints was higher than that for aluminum–aluminum joints. In the case of dissimilar adherends, the strength was lower than that of the case of similar adherends. The results of this study indicate that the combination of adherend material should also be accounted for while designing lap joints.     

Adhesion of Copper and Alumina from First Principles

Structures and energetics of various, plausible realizations of the Al₂O₃(0001)/Cu(111) interface have been studied by density-functional theory. The oxygen-terminated interface is found to be relatively strong with a work of separation ( W _sep) of 7.03 J/m². For hydrogen-rich (e.g., moist) conditions, we have determined that the interface accommodates about 1/3 of a monolayer of hydrogen. This lowers W _sep to 4.72 J/m², which is, however, greater than the corresponding value for bulk copper, which implies that fracture in such a system occurs in the copper region, consistent with fracture toughness experiments. We also have studied the initial stages of copper and aluminum growth on the hydrogen-stabilized oxygen-terminated Al₂O₃(0001) surface. Although a monolayer of aluminum already completely dissociates the interfacial O-H group, a monolayer of copper leaves it intact. If, for a thick copper film, such a full hydrogen layer were maintained, the resulting metastable interface would be very weak, with W _sep= 0.63 J/m².     

Plasma-Sprayed Aluminum and Titanium Adherends: I. Characterization of Coatings

The morphological and surface chemical properties of plasma-sprayed coatings on metals have been investigated using surface characterization techniques. Organic polymeric and inorganic powders were plasma-sprayed on aluminum and titanium. Organic-polymeric coatings were prepared using epoxy, polyester, polyimide, and cyanate ester components. Inorganic coatings were obtained by plasma-spraying Al₂O₃, AlPO₄, MgO, and SiO₂ on aluminum adherends, and TiO₂, TiSi₂, MgO, and SiO₂ on titanium adherends. The organic-polymeric coatings were prepared at one thickness while the inorganic coatings were sprayed to obtain two different thicknesses. SEM photographs reveal various morphological differences in the sprayed specimens. The surface morphology ranged from smooth to nodular among the plasma-sprayed specimens. Surface chemical analysis of the plasma-sprayed coatings indicated that little or no chemical degradation of the components occurred as a result of plasma-spraying. However, plasma-sprayed TiSi₂ appeared to be a mixture of silica and a titanium silicate.     

Ferroelastic Domain Switching in Polydomain Tetragonal Zirconia Single Crystals

As-received, yttria-doped (4.2 wt% Y₂O₃) single crystals of zirconia were heated to ≥2100°C in air. Cube-shaped samples with faces perpendicular to 〈100〉 axes on the basis of the pseudocubic symmetry were cut from the crystals. X-ray and electron diffraction indicated that the crystals are polydomain with [001] axes, on the basis of the tetragonal symmetry, in three mutually orthogonal directions (perpendicular to the cube faces). The cube-shaped crystals were tested in compression at temperatures as high as 1400°C. X-ray diffraction indicated that ferroelastic domains underwent reorientation (switching) in compression. Subsequently, notched samples with the long direction of the beams along 〈100〉 on the basis of the pseudocubic symmetry were fractured in three-point bending at temperatures as high as 1000°C. X-ray diffraction from fracture surfaces showed that domain reorientation had occurred and that no monoclinic phase was observed on fracture or ground surfaces. The fracture toughness at room temperature and at 1000°C was ∼12 and ∼8 MPa · m^1/2, respectively. Preliminary experiments on polycrystalline tetragonal zirconia samples containing 5.4 wt% Y₂O₃ and sintered at ≥2100°C also showed no evidence of the monoclinic phase on fracture or ground surfaces. The toughness of the polycrystalline samples was typically 7.7 MPa · m^1/2. These results indicate that ferroelastic domain switching can occur during fracture and may contribute to toughness.     

Adhesive bonding of aerospace materials — surface characterization of metallic adherends

Adhesively bonded structures are widely used in the aerospace industry. Unfortunately, adhesion is a macroscopic property depending specifically upon numerous sub-microoscopic parameters so that producing a strong and durable bond is a difficult task which needs many fundamental and applied research investigations. In this context, the surface composition of adherends plays an extremely important role in the overall performance of a bonded structure: it is well known that surfaces of ‘as-received’ materials are chemically very different from the bulk composition and are usually unsuitable for adhesive bonding. Consequently, surfaces of such materials hav to be submitted to appropriate chemical or electrochemical prebonding treatments in order to modify both their morphology and chemistry.

This paper discusses the surface characterization of aluminium, titanium and their alloys at various prebonding stages. Practical applications of X-ray fluorescence spectroscopy, low-energy electron-induced X-ray spectroscopy and glow discharge optical spectroscopy are described.     

Fracture-Mode Change in Alumina-Silicon Carbide Composites Doped with Rare-Earth Impurities

Al₂O₃-5 vol% SiC particle composites doped with 800 ppm rare-earth impurities (Y³⁺, Nd³⁺, and La³⁺) were fabricated by hot-pressing at a temperature of 1550°C. Doping of rare-earth impurities in Al₂O₃-SiC composites led to a fracture- mode change from transgranular in dopant-free composites to intergranular in rare-earth-doped composites. The fracture mode change obviously increased the crack deflection so that the fracture toughness of rare-earth-doped composites was higher than that of the composites without dopants, especially for the Nd³⁺- and La³⁺-doped composites. It was found that the fracture-mode change originated from a weak grain-boundary bonding caused by co-segregation of the rare-earth dopants and Si⁴⁺ ions dissolved from the SiC particle surfaces.     

Plasma-Sprayed Aluminum and Titanium Adherends: III

The durability of aluminum and titanium adherends, plasma-sprayed with polymeric coatings, and bonded with an epoxy and a polyimide adhesive has been investigated. Organic-polymeric coatings were plasma-sprayed using epoxy, polyester, polyimide, and cyanate ester components. Durability was investigated using a wedge-type specimen by exposing the specimens to an environmental cycle that included low temperature, high relative humidity at elevated temperature, high temperature at atmospheric pressure in air, high temperature in a vacuum, and room temperature. The systems exhibiting durability comparable with that for adherends treated using standard solution methods, included aluminum or titanium coated with a bis-maleimide/cyanate ester (B-CE) or a bis-maleimide-LaRC TPI-1500^® (B-TPI) mixture and bonded with an epoxy or a polyimide adhesive. For these B-CE- and B-TPI-coated specimens, failure during exposure to the environmental cycle occurred in the adhesive, indicating a favorable adherend/plasma-sprayed coating interaction.