Atmospheric pressure plasma surface modification of titanium for high temperature adhesive bonding

In this investigation surface treatment of titanium is carried out by plasma ion implantation under atmospheric pressure plasma in order to increase the adhesive bond strength. Prior to the plasma treatment, titanium surfaces were mechanically treated by sand blasting. It is observed that the contact angle of de-ionized water decreases with the grit blast treatment time. Optical microscopy and scanning electron microscopic (SEM) analysis of untreated and atmospheric plasma treated titanium are carried out to examine the surface characteristics. A substantial improvement in the surface energy of titanium is observed after the atmospheric pressure plasma treatment. The surface energy increases with increasing exposure time of atmospheric pressure plasma. The optimized time of plasma treatment suggested in this investigation results in maximum adhesive bond strength of the titanium. Unmodified and surface modified titanium sheets by atmospheric pressure plasma were adhesively bonded by high temperature resistant polyimide adhesive. The glass transition temperature of this adhesive is 310 °C and these adhesively bonded joints were cured at high temperature. A substantial improvement in adhesive bond strength was observed after atmospheric pressure plasma treatment.     

Experimental investigation into the effect of adhesion properties of PEEK modified by atmospheric pressure plasma and low pressure plasma

High performance polymer, Polyether Ether Ketone (PEEK) (service temperature ?250°C to +300°C, tensile strength: 120 MPa) is gaining significant interest in aerospace and automotive industries. In this investigation, attention is given to understand adhesion properties of PEEK, when surface of the PEEK is modified by two different plasma processes (i) atmospheric pressure plasma and (ii) low pressure plasma under DC Glow Discharge. The PEEK sheets are fabricated by ultra high temperature resistant epoxy adhesive (DURALCO 4703, service temperature ?260°C to +350°C). The surface of the PEEK is modified through atmospheric pressure plasma with 30 and 60 s of exposure and low pressure plasma with 30, 60, 120, 240, and 480 s of exposure. It is observed that polar component of surface energy leading to total surface energy of the polymer increases significantly when exposed to atmospheric pressure plasma. In the case of low pressure plasma, polar component of surface energy leading to total surface energy of the polymer increases with time of exposure up to 120 s and thereafter, it deteriorates with increasing time of exposure. The fractured surface of the adhesively bonded PEEK is examined under SEM. It is observed that unmodified PEEK fails essentially from the adhesive to PEEK interface resulting in low adhesive bond strength. In the case of surface modified PEEK under atmospheric pressure plasma, the failure is entirely from the PEEK and essentially tensile failure at the end of the overlap resulting in significant increase in adhesive bond strength. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of plasma treatment and electron beam radiations on the strength of nanofilled adhesive‐bonded joints

This investigation highlights the adhesion performance of carbon fiber‐ and glass fiber‐reinforced polyphenylene sulfide when joined by high‐performance neat epoxy adhesive and nanofilled epoxy adhesive. A significant increase in the surface energy of these materials is observed after the surface modification with atmospheric plasma treatment. An increase in surface roughness is observed after exposing the surface to plasma. Lap shear testing of untreated and plasma‐treated joints is carried out to correlate the improvement in adhesion properties with the joint strength. A considerable increase in joint strength is observed when the surfaces of these materials are modified by atmospheric pressure plasma. There is a further increase in joint strength when the composites are joined by nanofilled epoxy adhesive, and subsequent exposure to electron beam radiations results in minor increase in the joint strength. Finally, the fractured surfaces of the joints are examined and the analysis is performed. POLYM. ENG. SCI., 50:1505–1511, 2010. © 2010 Society of Plastics Engineers     

Process optimization of solvent based polybenzimidazole adhesive for aerospace applications

The use of adhesive bonding for high temperature applications is becoming more challenging because of low thermal and mechanical properties of commercially available adhesives. However, the development of high performance polymers can overcome the problem of using adhesive bonding at high temperature. Polybenzimidazole (PBI) is one such recently emerged high performance polymer with excellent thermal and mechanical properties. It has a tensile strength of 160 MPa and a glass transition of 425 °C. Currently, PBI is available in solution form with only 26% concentration in Dimethyl-acetamide solvent. Due to high solvent contents, the process optimization required lot of efforts to form PBI adhesive bonded joints with considerable lap shear strength. Therefore, in present work, efforts are devoted to optimize the adhesive bonding process of PBI in order to make its application possible as an adhesive for high temperature applications. Bonding process was optimized using different curing time and temperatures. Epoxy based carbon fiber composite bonded joints were successfully formed with single lap shear strength of 21 Mpa. PBI adhesive bonded joints were also formed after performing the atmospheric pressure plasma treatment of composite substrate. Plasma treatment has further improved the lap shear strength of bonded joints from 21 MPa to 30 MPa. Atmospheric pressure plasma treatment has also changed the mode of failure of composite bonded joints.     

Plasma Treatment of Wood–Plastic Composites to Enhance Their Adhesion Properties

In this study, the adhesion properties of adhesives and paints on wood–plastic composites (WPCs) after plasma treatment at atmospheric pressure and ambient air were investigated. Surface energy determination by means of contact angle measurements according to the Owens–Wendt approach and atomic force microscopy to detect changes in surface topography were carried out. An increase in the polar component of surface energy and an increase in surface roughness after plasma treatment were detected, indicating enhanced bond strength. To confirm these results, bond strength tests were conducted. By tensile bond strength tests, increased adhesion of waterborne, solventborne and oil-based paints on plasma treated surfaces was found. Furthermore, by shear bond strength tests, an increase in bond strength of plasma treated WPCs bonded with poly(vinyl acetate) and polyurethane adhesives was ascertained.     

Strength and bonding characteristics of adhesive joints with surface-treated titanium-alloy substrates

This paper presents a study on the effect of surface treatments on the mechanical behavior of adhesively bonded titanium alloy joints. Several different treatments were selected for the preparation of Ti-6Al-4V alloy faying surfaces, and bonded joints were fabricated using surface-treated titanium alloy substrates and a film adhesive. Tensile tests were performed on single-lap specimens to evaluate the joint strength and to assess the failure mode, i.e. cohesive failure, adhesive (interfacial) failure or a mix of both. Contact angle measurements were also carried out, and the surface free energies of titanium alloys and the thermodynamic works of adhesion for the adhesive/titanium alloy interfaces were obtained. A three-dimensional finite element analysis was used to predict the strength of the specimens exhibiting cohesive failure. In addition, an expression of the relationship between the joint strength corresponding to interfacial failure and the thermodynamic work of adhesion was introduced based on the cohesive zone model (CZM) concept. It is shown that two surface treatments, Itro treatment and Laseridge, lead to cohesive failure and a significant increase in the joint strength, and the numerically predicted strength values are fairly close to the experimental values. These surface treatments are possible replacements for the traditional surface treatment processes. For degreasing, emery paper abrasion, atmospheric plasma treatment, sulfuric acid anodizing, nano adhesion technology and high-power lasershot, the specimens fail at the adhesive/substrate interface and the joint strength increases linearly with the thermodynamic work of adhesion as expected from our CZM-based expression.     

Adhesion characteristics of plasma-surface-treated carbon fiber-epoxy composite with respect to release films used during demolding

The load capabilities of carbon fiber-epoxy composite adhesive joints are affected by surface characteristics of the composite adherends such as surface free energy and chemical composition, which can be altered by plasma surface treatment and the type of release film for demolding carbon fiber-epoxy composites from metal molds. In this paper, suitable plasma surface treatment conditions for carbon fiber-epoxy composite adherends were investigated to enhance the strength of carbon fiber-epoxy composite adhesive joints using dielectric barrier discharges of atmospheric pressure plasmas. The effects of plasma surface treatment on the surface free energy and adhesion strength of carbon fiber-epoxy composites were experimentally investigated with respect to surface treatment time. Also, the surface and adhesion characteristics of carbon fiber-epoxy composites were investigated with respect to release films such as fluorinated ethylene propylene (FEP), high density polyethylene (PE) and Nylon 6.6. Quantitative chemical bonding analysis with X-ray photoelectron spectroscopy (XPS) was also performed to understand the load capabilities of composite adhesive joints with respect to plasma treatment time and release films. From the experimental results, it was found that plasma treatment of carbon fiber-epoxy composites did enhance its adhesion strength, irrespective of the type of release film. Regarding adhesion strength, Nylon 6.6 was found to be the most suitable release film for these composites when no plasma treatment could be applied. From the XPS measurements on carbon fiber-epoxy composites, it was found that the carbon bond ratio of C=O to C-C and C-H reached a maximum at around 10 s treatment time, which corresponded well with the load transmission capability of the composite adhesive joint.     

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.     

Effect of different surface preparations on the tensile strength of adhesively bonded metal joints

In this in vitro study, the effects of different surface preparations and resins on the strength and durability of adhesively bonded joints were evaluated. Disk-shaped cobalt-chromium substrate samples of the first group were treated by the Silicoater MD® system. Samples of the two subgroups were bonded with two different bisphenol-A glycidyl methacrylate (Bis-GMA) adhesives. Samples of the second group were treated by the Rocatec®) system and bonded with a Bis-GMA adhesive. Alumina-blasted samples of the third group were bonded with two different types of Bis-GMA adhesive modified with a phosphate monomer. Samples were stored in water for 3 days, or thermocycled and stored in water for 6 months. The joint samples were then tested for tensile bond strength. When the alumina-blasted samples were bonded with Panavia Ex® or Panavia 21® adhesive the highest bond strength was obtained, regardless of the storage conditions. The Silicoater MD method in combination with the Bis-GMA adhesive yielded high initial bond strengths comparable to those obtained with the Panavia systems, but also recorded the highest drops in bond strengths with both types of adhesive after thermal stressing and water storage. The Rocatec system in combination with Nimetic Grip adhesive produced a low but stable bond strength even after thermocycling and water storage.     

Ultrasonic cleaning of SBR rubber to improve the performance of subsequent plasma torch treatment

Paraffin wax and other moieties in sulfur vulcanized styrene-butadiene rubber formulations may migrate to the surface, reducing the adhesive strength in joints produced with polyurethane adhesive. In this study, with the aim to remove paraffin wax and other anti-adhesion moieties on the rubber surface to improve adhesion, prior to plasma torch treatment, a methyl ethyl ketone (MEK) cleaning in an ultrasonic bath has been carried out. The surface modifications produced on the rubber surface have been analyzed by contact angle measurements, ATR-IR spectroscopy, XPS and SEM. The adhesion properties have been evaluated by T-peel strength of treated rubber/polyurethane adhesive joints. Ultrasonic cleaning in MEK resulted in partial removal of paraffin wax on the rubber surface and, thus, lower contact angle values, decreased relative intensity of the infrared bands due to hydrocarbon moieties and lower percentage of carbon assessed from XPS spectroscopy were obtained. The ultrasonic cleaning in MEK of the rubber increased the effectiveness of the atmospheric pressure plasma torch treatment, and surface oxidation was produced. However, the oxidation degree decreased with time after plasma torch treatment, likely due to ageing of the surface treated rubber.     

Surface Free Energy and 7075 Aluminium Bonded Joint Strength Following Degreasing Only and Without Any Prior Treatment

Abstract

Adhesion is a surface phenomenon occurring in many processes, e.g., bonding, painting or varnishing. Knowing the adhesion properties is critical for evaluating the usability or behaviour of materials during these processes. Good adhesion properties favour the processes of bonding, resulting in high strength of adhesive joints. Adhesive bonded joints are used in many industries, and the subject of this study was 7075 aluminium alloy sheet bonded joints as typically used in the aviation or construction industry. Surface free energy (SFE) can be used to determine the adhesion properties of the materials. The SFE of the tested sheets was determined with the Owens–Wendt method, which consists in determining the dispersion and polar components of SFE. The purpose of this work was to correlate the bonded joint strength of selected aluminium alloy sheets to the surface free energy of the sheets that had been subjected to degreasing only and no other prior treatment was used. Single-lap bonded joints of 7075 aluminium alloy sheets were tested. Higher joint strength was measured for the thinner sheets, while the lowest strength was measured for the thickest sheets. This suggests that the thickness of the joined parts is an important factor in the strength of bonded joints. The comparison of adhesion properties to the strength of adhesive joints of tested materials shows that there is no direct relation between good adhesion properties (i.e., high SFE) and joint strength. As for degreasing, the highest joint strength was observed for aluminium alloy sheets with the lowest SFE; the sheets which were not degreased gave the highest SFE and highest joint strength.     

Comparative Analysis of Energetic Properties of Ti6Al4V Titanium and EN-AW-2017A(PA6) Aluminum Alloy Surface Layers for an Adhesive Bonding Application

The purpose of this article is to present energetic properties of surface Ti6Al4V titanium as well as surface EN-AW-2017A(PA6) aluminum alloy layers. Values of surface free energy after selected mechanical operations and ozonation were compared. In addition, the influence of different values of ozone concentration on surface layer energetic activation was analyzed. Dispersive and polar components of surface free energy were of particular concern. Comparative evaluation of shearing strength of Ti6Al4V titanium and EN-AW-2017A(PA6) aluminum alloy single-lap adhesive bonded joints were presented. Results can be used as pro-ecological methods of titanium and aluminum alloys preparing for applications where adhesive phenomenon is important.     

Adhesion characteristics of plasma surface treated carbon/epoxy composite

The load transmission capability of adhesive joints can be improved by increasing the surface free energy of the adherends with surface treatments. In this paper, suitable plasma surface treatment conditions for carbon/epoxy composite adherend were investigated to enhance the strength of carbon/epoxy composite adhesive joints using a capacitively coupled radio-frequency plasma system. Effects of plasma surface treatment parameters on the surface free energy and adhesion strength of carbon/epoxy composite were experimentally investigated with respect to gas flow rate, chamber pressure, power intensity, and surface treatment time. Quantitative chemical bonding analysis determined with XPS (X-ray photoelectron spectroscopy) was also performed to understand the load transmission capabilities of composite adhesive joints with respect to surface treatment time.     

Adhesion characteristics of carbon/epoxy composites treated with low- and atmospheric pressure plasmas

Although an adhesive joint can distribute load over a larger area than a mechanical joint, requires no holes, adds very little weight to structures and has superior fatigue resistance, it requires careful surface preparation of adherends for reliable joining and low susceptibility to service environments. The load transmission capability of adhesive joints can be improved by increasing the surface free energy of the adherends with suitable surface treatments. In this study, two types of surface treatment, namely the low pressure and the atmospheric pressure plasma treatment, were performed to enhance the mechanical load transmission capabilities of carbon/epoxy composite adhesive joints. The suitable surface treatment conditions for carbon/epoxy composite adhesive joints for both low and atmospheric pressure plasma systems were experimentally investigated with respect to chamber pressure, power intensity and surface treatment time by measuring the surface free energies of the specimens. The change in surface topography of carbon/epoxy composites was measured with AFM (Atomic Force Microscopy) and quantitative surface atomic concentrations were determined with XPS (X-ray Photoelectron Spectroscopy) to investigate the failure modes of composite adhesive joints with respect to surface treatment time. From the XPS investigation of carbon/epoxy composites, it was found that the ratio of oxygen concentration to carbon concentration for both low and atmospheric pressure plasma-treated carbon/epoxy composite surfaces was maximum after about 30 s treatment time, which corresponded with the maximum load transmission capability of the composite adhesive joint.     

Environmental aging of the Ti-6Al-4V/FM-5 polyimide. adhesive bonded system: implications of physical and chemical aging on durability

Ti-6A1-4V/FM-5 polyimide adhesively bonded double cantilever beam (DCB) specimens were aged for 12 months at elevated temperatures (177°C and 204°C) in one of three different environments: ambient atmospheric air pressure and reduced air pressures of 2 psi (13.8 kPa) and 0.2 psi (1.38 kPa), to assess bond durability. The FM-5 polyimide adhesive (Tg~ 250°C) is based on a polyimide developed by NASA Langley Research Center and is produced by Cytec Industries, Inc. Bonds aged for different times were tested to measure the critical strain energy release rate as a function of the temperature and environment. The greatest loss in bond strength occurred after aging in air at 204°C. Following thermal rejuvenation of the aged bonds at 300°C for 2 h, part of the strength loss could be recovered. This strength recovery was attributed to the reversal of physical aging in the adhesive resin. Further evidence for physical aging, which is a thermo-reversible phenomenon, was obtained from tests conducted on neat resin specimens using DMA (dynamic mechanical analysis) and DSC (differential scanning calorimetry). The unrecovered portion of the loss in bond strength following longer-term aging was attributed to chemical aging/degradation of the bonded 'system'. The 'system' in this study includes the adherends, the adhesive, the surface pretreatment (chromic acid anodization, CAA), and their respective interphase/interface regions. Evidence for chemical aging was also seen from weight loss, and Soxhlet extraction data on neat resin specimens.     

Investigation of optimal surface treatments for carbon/epoxy composite adhesive joints

Although an adhesive joint can distribute the load over a larger area than a mechanical joint, requires no holes, adds very little weight to the structure and has superior fatigue resistance, but it not only requires a careful surface preparation of the adherends but also is affected by service environments. In this paper, suitable conditions for surface treatments such as plasma surface treatment, mechanical abrasion, and sandblast treatment were investigated to enhance the mechanical load capabilities of carbon/epoxy composite adhesive joints. A capacitively coupled radiofrequency plasma system was used for the plasma surface treatment of carbon/epoxy composites and suitable surface treatment conditions were experimentally investigated with respect to gas flow rate, chamber pressure, power intensity, and surface treatment time by measuring the surface free energies of treated specimens. The optimal mechanical abrasion conditions with sandpapers were investigated with respect to the mesh number of sandpaper, and optimal sandblast conditions were investigated with respect to sandblast pressure and particle size by observing geometric shape changes of adherends during sandblast process. Also the failure modes of composite adhesive joints were investigated with respect to surface treatment. From the peel tests on plasma treated composite adhesive joints, it was found that all composite adhesive joints failed cohesively in the adhesive layer when the surface free energy was higher than about 40 mJ/m², because of high adhesion strength between the plasma treated surface and the adhesive. From the peel tests on mechanically abraded composite adhesive joints, it was also found that the optimal surface roughness and adhesive thickness increased as the failure load increased.     

Surface preparation for adhesive bonding of polycyanurate‐based fiber‐reinforced composites using atmospheric plasma treatment

In this article, the effects of atmospheric plasma treatment on the microstructural, chemical, and mechanical behavior of epoxy‐bonded polycyanurate composites are investigated. Adhesive bond strength of plasma‐treated specimens exhibited strength increases of over 35% to that of peel‐ply and solvent‐wiped surface preparation techniques. The improvements were as much as 50% greater than those obtained using abrasive surface preparation techniques. X‐ray photoelectron spectroscopy analysis showed an increase in the surface concentration of oxygen as a function of plasma treatment passes. However, the levels were substantially lower than that of epoxy composites treated under identical conditions. In addition, the concentration of carboxyl groups (O CO), which have been associated with improved adhesive strength in epoxy‐based composites, was shown to saturate in cyanate ester composites after a much lower exposure period than what was observed when treating epoxies. The effect of plasma surface treatment on the surface morphology of the cyanate ester composite was also studied using scanning electron microscopy and atomic force microscopy. Atomic force microscopy analysis showed a progressive increase in surface roughness with treatment; however, this increase only translated into a marginal increase in surface area and is not believed to contribute significantly to adhesive strength. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Optimization of Surface Treatment and Adhesive Selection for Bond Durability in Ti-15-3 Laminates

The promising mechanical performance of a baseline Hybrid Titanium Composite Laminate (HTCL) inspired an investigation into maximizing the strength and environmental performance of this new aerospace material. This research focused upon finding the strongest and most durably combination of three commercially-available titanium surface treatments (i.e., Pasa-Jell 107^TM, Boeing's Sol-Gel, and Turco 5578^R) and two polyimide adhesives (i.e., LaRC^TM-IAX and FM5^R) for use in HTCL. The tests employed the cracked-lap shear (CLS) specimen geometry for fatigue crack growth measurements and also for fracture toughness analyses of the bonded specimens. The CLS geometry models several bonded applications found in the aerospace industry, and it also represents the debonding characteristics of a cracked titanium foil in HTCL.

The environmental performance of these six material combinations has been evaluated after 5,000 hours of continuous exposure to either a Hot/Wet environment that subjected the bonded specimens to 160°F (71°C) with relative humidity in excess of 95%, or to a Hot/Dry environment of 350°F (177°C) with a relative humidity of less than 5%. These two exposure environments utilized in this study are the most aggressive long-term environments that the HTCL is projected to experience while in service.

Test results showed that the best combination of the titanium surface treatments and the polyimide adhesives in the FM^R adhesive used in conjunction with Boeing's Sol-Gel titanium surface treatment. Though the FM5^R/Sol-Gel system was the strongest of all combinations, its performance dropped to less than 50% of its original strength after exposure to the Hot/Dry environment. An important finding is that this bonded system did not significantly degrade after exposure to the Hot/Wet environment. The only other material combination that showed substantial bond strength was the FM5^R/Pasa-Jell 107 system, though its strength also dropped to less than 50% of its original strength after exposure to the Hot/Dry environment.     

The Durability of Adhesively-bonded Ti-6Al-4V

The durability of chromic acid-anodized Ti-6Al-4V alloy, adhesively-bonded with FM-5 supported polyimide adhesive has been studied. The performance tests compared titanium samples that had been thermally treated and bonded, and samples that were bonded and thermally treated. Following the thermal treatment, the durability was examined (1) by immersing wedge-type specimens in boiling water and measuring the crack growth and (2) by measuring the lap shear strength for single lap specimens. In the wedge tests, failure occurs within the adhesive for specimens treated at temperatures below 371°C for less than one hour. For treatments at higher temperatures and for longer periods of time, failure occurs within the anodic oxide. From the lap shear tests, the principal finding is that the lap strength decreases with increasing treatment time at constant temperature and with increasing temperature at a fixed time. For the lap specimens, failure occurs to a greater extent within the oxide as the treatment time and temperature increase. Surface analysis results indicate the formation of an aluminum fluoride species. It is reasoned that the formation of fluorine-containing materials weakens the oxide and promotes failure within the anodic oxide.     

The Durability of Adhesively-bonded Ti-6Al-4V

The durability of chromic acid-anodized Ti-6Al-4V alloy, adhesively-bonded with FM-5 supported polyimide adhesive has been studied. The performance tests compared titanium samples that had been thermally treated and bonded, and samples that were bonded and thermally treated. Following the thermal treatment, the durability was examined (1) by immersing wedge-type specimens in boiling water and measuring the crack growth and (2) by measuring the lap shear strength for single lap specimens. In the wedge tests, failure occurs within the adhesive for specimens treated at temperatures below 371°C for less than one hour. For treatments at higher temperatures and for longer periods of time, failure occurs within the anodic oxide. From the lap shear tests, the principal finding is that the lap strength decreases with increasing treatment time at constant temperature and with increasing temperature at a fixed time. For the lap specimens, failure occurs to a greater extent within the oxide as the treatment time and temperature increase. Surface analysis results indicate the formation of an aluminum fluoride species. It is reasoned that the formation of fluorine-containing materials weakens the oxide and promotes failure within the anodic oxide.