Plasma Surface Treatment of Aerospace Materials for Enhanced Adhesive Bonding

The increased use of polyphenylene sulphide (PPS) and polyetheretherketone based composites for aircraft structures has highlighted the need for reliable methods of bonding these materials to metallic components such as titanium. Both composite and titanium adhesive bonds exhibit poor long-term durability when exposed to hot/wet conditions, aerospace fluids and solvents. As a result, surface treatments are employed to enhance surface energy, surface roughness and alter surface chemistry to provide better long-term durability. In this initial study the adhesive bonding of glass fibre reinforced GFR-PPS and commercially pure titanium was investigated. Prior to bonding, both materials were plasma treated using argon and oxygen gases in a RF discharge. Surface characterisation was carried out to optimise these treatments. Surface energy and wettability were examined using contact angle analysis, surface roughness was examined using scanning electron microscopy and atomic force microscopy, while X-ray photo-electron spectroscopy (XPS) was employed to study the surface chemistry. Bond strengths were determined using lap shear tests. Initial results reveal that these optimum plasma treatments produce a significant increase in bond strength.     

The Durability of Adhesively-Bonded Titanium: Performance of Plasma-Sprayed Polymeric Coating Pretreatments

Non-solution and electrochemical treatments in preparation for adhesive bonding of titanium have been studied. Polymeric materials, LaRC TPI-2000^TM, LaRC PETI-5^TM, and Aurum^TM polyimides were deposited onto titanium-6Al-4V surfaces via plasma spraying. The plasma-sprayed surfaces were characterized using infrared, solid state NMR, and surface-sensitive analytical methods. The chemical nature of plasma-sprayed polymers is equivalent to that for powdered materials. The durability of titanium, adhesively bonded with a polyimide adhesive, was investigated by immersing wedge-type specimens in boiling water. Crack length and the mode of failure were used to assess durability. The determination of the failure mode was accomplished using surface-sensitive analytical methods. Surface treatments using plasma-sprayed LaRC-type polyimides result in good durability as evidenced by minimal crack growth for wedge specimens and by failure in the adhesive (cohesive failure).     

Investigation of surface pre-treatments for the structural bonding of titanium

This paper evaluates wet-chemical pre-treatments (alkaline etching, anodising) and a plasma treatment for structural bonding of titanium (Ti₆Al₄V). The main objective of this study is the comparison of the applicability of the plasma pre-treatment to wet-chemical treatments on titanium for structural bonding. In this context, an atmospheric pressure plasma device was used to deposit thin functional films from hexamethyldisiloxane (HMDSO) precursor on titanium.X-ray photoelectron spectroscopy (XPS) was employed to assess the chemical composition of the surface after different pre-treatments on the titanium substrate, while the morphology and the film thickness were investigated with scanning electron microscopy (SEM). The adhesion properties on titanium were evaluated by means of a wedge test in hot/wet conditions. After bonding tests the fracture surface and the failure loci were analysed.Using a long arc plasma generator and HMDSO precursor almost stoichiometric SiO₂ coatings were obtained on the titanium substrate. These coatings exhibit good long-term durability and bond strength compared to an alkaline etching in the wedge test. The investigated anodising process leads to oxide layers revealing a highly porous nanostructure. In contrast to the alkaline etching, the plasma derived coatings and the oxide layer produced by the anodising process exhibit a higher micro, respectively nano roughness, and hence a better long-term durability.     

Effects of sandblasting distance and angles on resin cement bonding to zirconia and titanium

ObjectivesThe aim of this study was to evaluate effects of sandblasting distance and angles resin to zirconia and titanium bonding.MethodsDensely sintered zirconia and cp2 titanium specimens were prepared and randomly divided into groups, and then sandblasted with various distance (5 mm, 10 mm and 15 mm) and angles (45°, 60°, 75° and 90°). After surface treatment, each specimen surface underwent a silane primer application (RelyX, 3M ESPE), followed by bonding of a resin cement (RelyX Unicem Aplicap, 3M ESPE). Then, each cylindrical resin stub (diameter 3.6 mm×2 mm) underwent a shear adhesive (bond) strength test and surface roughness evaluation. SEM evaluation and EDX analysis were used to observe surface properties of both zirconia and titanium samples. Results were statistically analyzed using analysis of variance (ANOVA) and Turkey test (α=0.05).ResultsSurface roughness showed a significant difference amongst the different distances and angles for both the zirconia and titanium materials and these changes in surface roughness were evident in the SEM imaging photos. As for the adhesive strength, there was a significant difference in the adhesive strength for the titanium and zirconia with different angles. In general, 75° gives the best results although this is not significantly different from 90°. However, no significant difference was observed in changes of sandblasting distance for both materials. EDX analysis at the surface revealed elements carbon, oxygen, silicon, aluminum, and zirconia on the surface.ConclusionsSandblasting at various distance and angles contributes differences in surface roughness when it comes to both zirconia and titanium materials. Despite both 75° or 90° sandblasting angle could yield a sufficiently high adhesive strength for resin to titanium or zirconia bonding, sandblasting at 75° seems to be optimal to increase the adhesive strength.     

The influence of mechanical and chemical treatments on the environmental resistance of epoxy adhesive bonds to titanium

In this investigation, the effect of mechanical and chemical treatments on titanium for bonding to a rubber toughened epoxy adhesive has been examined. Titanium alloy was roughened using either abrasion or grit-blasting techniques. An air-plasma treatment of the titanium prior to organosilane treatment also improved uniformity of the resultant thin film as determined by AFM and XPS. Grit-blasting significantly increased the titanium surface roughness and increased the alumina character of the surface layers. The fracture toughness of the epoxy-titanium bonds, as measured by the wedge test, was affected by the degree of surface roughness at short humid-exposure times. Further, the addition of a thin organosilane film to the titanium slowed degradation rates and led to higher fracture toughness at longer humid-exposure times.     

In Memoriam

The surface modification and adhesive bonding of a carbon fiber reinforced plastic (CFRP) composite has been investigated. Wettability studies showed that plasma-treated specimens provide a significant increment in the surface energy, relative to untreated material. The surface modification resulted in significantly improved adhesion between the composite and an applied toughened acrylic adhesive; a considerable increase in fracture energy was observed following grit blasting and grit blasting plus silane treatments. Specimens treated with atmospheric plasma showed a slight increment in fracture energy, usually failing adhesively. The durability was tested using a wedge test. Specimens degreased and treated with atmospheric plasma showed the greatest crack growth and failed in an adhesive mode.     

Surface treatment of AM355 stainless steel for adhesive bonding

Surface treatments for preparing AM355 stainless steel for adhesive bonding have been studied. Lap shear, wedge test and stress durability data are presented for bonds prepared using 19 different surface treatments. The oxides formed on the stainless steel as a result of the surface treaments have been characterized using Auger electron spectroscopy, X-ray photoelectron spectroscopy, ellipsometry, and surface potential difference, photoelectron emission and water contact angle methods.     

The influence of hydroxyl group concentration on epoxy–aluminium bond durability

The influence of hydroxyl group (OH) concentration on the durability of adhesive bonds formed between an epoxy resin and aluminium adherend has been examined. Initially, surface analysis in combination with chemical derivatisation was employed to characterise the OH and epoxy functional groups present in FM-73, a structural epoxy adhesive. Bulk FM-73 indicated a higher degree of cure than the surface of FM-73 present at the interface of an epoxy–aluminium adhesive joint. Plasma and water treatment of the aluminium adherend was employed to alter the metal oxide's surface OH concentration. Despite a several-fold difference of aluminium surface OH concentrations for the different metal pre-treatments, there was no significant variation in the adhesive joint fracture toughness in a humid environment, G _Iscc. In contrast, grit-blasting the aluminium prior to bonding increased G _Iscc almost 15-fold. Simple calculations indicate that the aluminium surfaces used in the bonding experiments would have a large excess of OH groups available to react with a standard epoxy resin and that the influence of surface roughness on adhesion durability is not insignificant.     

Efficacy of uncommon surface treatment methods on titanium in order to improve bond strengths for adhesive cementation

The aim of this study is to comparison of effects of uncommon surface treatments, especially new alternatives on the adhesive strength between resin and titanium surfaces. Fifty-five titanium disks were prepared and they were separated into 5 groups as follows: (1) Control group; (2) Tribochemical treatment group in the laboratory; (3) Tribochemical treatment group in the clinic; (4) Acid etch group; and (5) Nd:YAG Laser-irradiated group. Surface roughness of the specimens was measured using a profilometer, and the topographic patterns were observed by scanning electron microscope. After these tests, resin cement was applied to the titanium samples. Shear bond test was performed via a universal testing machine at a crosshead speed of 0.5 mm/minute. In addition, the correlation between the surface roughness and bond strength was checked using Spearman correlation test (0.01 level). The highest surface roughness value was observed for the acid etch group (1.53 μm). The highest mean shear bond strength was recorded with the tribochemical procedure group in the laboratory (13.74 MPa) and the lowest with the control group (3.69 MPa). A positive correlation was found between the bond strength and surface roughness for all groups. All of the surface treatment methods that were used in present study increased the bond strength between resin and titanium except for the laser group.     

A Comparison of Chromate-Phosphate and Chromate-Free Conversion Coatings for Adhesive Bonding

Three different conversion coatings have been evaluated. One of these is an established chromate-phosphate treatment (BONDERITE 705) whilst the others are chromate-free (BONDERITE 777 and EP2472) and not widely used for adhesive bonding. In the present study, the degree of surface modification introduced by these treatments has been determined using Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). Both initial single lap shear and stressed durability results have been obtained using a single part epoxide adhesive. Degreased-only and grit-blasted adherends were used as controls. Overall, the conversion coatings provided better durability performance than the mechanical treatments. The developmental treatment EP2472, a chromate-free conversion coating, out-performed the established chromate-phosphate process at low applied loads (?0.5 kN). All three conversion coatings performed similarly at the higher loads (? 1 kN).     

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.     

Bonding and Failure Mechanisms in Aluminium Alloy Adhesive Joints

X-ray photoelectron spectroscopy and scanning electron microscopy have been used to investigate the surface chemistry of an aluminium-alloy which has been pretreated by various industrial methods commonly employed prior to adhesive bonding. The fracture surfaces of butt joints, consisting of the pretreated alloy bonded with an epoxy adhesive, have also been studied. The analyses have been conducted before and after exposure of the specimens to water, the main hostile environment that structural adhesive joints usually encounter. It is suggested that the concentration of magnesium, in the form of magnesium oxide, on the pretreated aluminium-alloy surface may be an important factor in influencing the durability of adhesive joints.     

A Comparison of Chromate-Phosphate and Chromate-Free Conversion Coatings for Adhesive Bonding

Three different conversion coatings have been evaluated. One of these is an established chromate-phosphate treatment (BONDERITE 705) whilst the others are chromate-free (BONDERITE 777 and EP2472) and not widely used for adhesive bonding. In the present study, the degree of surface modification introduced by these treatments has been determined using Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). Both initial single lap shear and stressed durability results have been obtained using a single part epoxide adhesive. Degreased-only and grit-blasted adherends were used as controls. Overall, the conversion coatings provided better durability performance than the mechanical treatments. The developmental treatment EP2472, a chromate-free conversion coating, out-performed the established chromate-phosphate process at low applied loads (≤0.5 kN). All three conversion coatings performed similarly at the higher loads (≥ 1 kN).     

Plasma-Sprayed Aluminum and Titanium Adherends: II. Durability Studies for Wedge Specimens Bonded with Polyimide Adhesive

The durability of plasma-sprayed metals bonded with a polyimide adhesive has been studied. Metal adherend surfaces were prepared for adhesive bonding by plasma-spraying inorganic powders on aluminum and titanium. The plasma-sprayed materials included Al₂O₃, AlPO₄. MgO, and SiO₂ on aluminum, and TiO₂, TiSi₂, MgO, and SiO₂ on titanium. The coatings were sprayed at two different thicknesses. Durability studies of samples prepared in a wedge-type geometry were carried out. Bonded specimens were maintained in an environmental cycle that included exposure to the conditions; low temperature, - 20°C; relative humidity at elevated temperature, 70% RH at 66°C; elevated temperature (160°C) in air, high temperature (160°C) in vacuum (130 torr, 0.2 atm.), and room temperature. Crack growth rate and mode of failure were determined. The results of the durability tests indicate that thin coatings (25 μm) of plasma-sprayed materials perform better than thicker (150 μm) coatings. The crack growth rate for thin coatings (25 μm) of Al₂O₃, AlPO₄, SiO₂, and MgO plasma-sprayed on aluminum was equivalent to that for phosphoric acid anodized aluminum. Similarly, the durability performance for titanium samples prepared with a 25 μm-thick TiO₂, TiSi₂, and SiO₂ plasma-sprayed coatings was equivalent to that for a Turco®-prepared titanium surface. Although the evaluation of durability as a function of surface chemistry was an objective of the study, it was not possible to evaluate the effect, since most failures occurred within the adhesive (cohesive failure) during the environmental tests. That failure occurred in the adhesive indicates that the coating-adherend and the coating-adhesive interactions are sufficiently robust to prevent interfacial failure under the experimental conditions investigated.     

Development of improved polypropylene adhesive bonding by abrasion and atmospheric plasma surface modifications

The present work deals with the problematic adhesive bonding of substrates with low surface energy. Different approaches have been explored with the aim of creating adequate adhesive joints based on polyolefinic substrate and polyurethane adhesive. The selected material under study was polypropylene (PP) as adherend, and a commercial Sikaflex^®-252 polyurethane one component based structural adhesive (PU) as joint fluid. Among the diverse pre-treatments typically used to prepare surfaces prior to bonding, mechanical abrasion with emery paper of 80 grain size, the use of a chemical primer and atmospheric pressure air plasma torch (APPT) were the selected methods to facilitate the application of the PU by means of surface energy enhancement as well as to create a correct mechanical interlocking of the adherent–adhesive interface. Changes in the wettability of the polymer were evaluated by contact angle measurements following the UNE EN 828:2010. Surface energy was calculated both in terms of Owens approximation and acid–base considerations, leading to the possibility of determining a relationship between changes in surface energy and adhesion. Changes in the chemical composition of the surface were studied by X-ray photoelectron spectroscopy (XPS), electron diffraction X-Ray (EDX) probe and attenuated total multiple reflection mode infrared spectroscopy (ATR-FTIR). Morphological modifications were investigated with scanning electron microscopy (SEM). Variations in the strength of single-lap PP–PP joints with the treatments were evaluated by lap shear tests following the UNE-EN 1465:2008 standard. Experimental evidence supports the superiority of the APPT treatment to increase wettability and adhesion of polyolefinic surfaces, especially when combined with the use of a primer.     

Durability of carbon fibre reinforced polyetheretherketone–polyurethane adhesive joints in aqueous environments

Abstract

As part of a project to develop a biomimetic artificial joint, it was necessary to bond carbon fibre reinforced polyetheretherketone (APC2) to thermoplastic polyurethane. As no qualified adhesives were available, adhesive bonding using hot compression moulding was the chosen methodology. The human body is one of the most aggressive environments that an adhesive bond has to endure. The durability of these APC2–polyurethane joints has been investigated in various aqueous environments including Ringer's solution and distilled water. Prior to bonding, the APC2 specimens were exposed to plasma etching and oxidising flame surface treatments to improve adhesion properties. Peel testing revealed that strong and durable bond strengths were recorded and no aging effects were observed when the adhesive joints were stored in a dry atmosphere for 1 year. However, when stored in the aqueous media the bonds rapidly deteriorated to unacceptable levels after 7 days. It was found using a variety of techniques including mass uptake analysis, surface force microscopy, and X-ray photoelectron spectroscopy, that the primary mechanism of bond degradation was due to ingression of water by wicking and diffusion.     

Investigation of accumulated laser fluence and bondline thickness effects on adhesive joint performance of CFRP composites

In the present study, both the effects of accumulated laser fluence as surface treatment and bondline thickness on adhesive bonding of carbon fiber reinforced polymer (CFRP) composite materials were investigated. Proper CFRP composite surfaces for adhesive bonding were obtained by a laser treatment process using pulsed CO₂ laser. Laser treatments were obtained with different accumulated laser fluences and then surfaces were analyzed with roughness and contact angle measurements. Adhesive bonding was performed with various bondline thicknesses ranged between 30–500 µm using two component structural epoxy based paste adhesive (Loctite Hysol ® EA 9396^TM). Adhesive bonding strength of bonded samples was determined with single lap shear tests. It is worthy to note that if the accumulated laser fluence which has significant effect on shear strength does not optimize, it causes ineffective adhesion.     

Air plasma treatment of aluminium trihydrate filled poly(methyl methacrylate)

Air plasma treatments of aluminium trihydrate filled poly(methyl methacrylate) polymer (PMMA) composites were carried out in a dielectric barrier discharge. X-ray photoelectron spectroscopy (XPS) and attenuated total reflection – infrared spectroscopy have been employed to analyse the changes in chemical composition. Confocal laser-scanning microscopy and atomic force microscopy yielded the impact on surface structure and morphology. Surface free energy (SFE) measurements and adhesive bond strength tests were used to correlate the results to possible applications like gluing and lacquering. The plasma treatments resulted mainly in an etching of the matrix polymer and a minor chemical modification. An apparent contradiction of XPS, ATR and SFE results was attributed to a re-polymerization and re-deposition of the etched PMMA material as debris back onto the surface. This effect, which is seldom taken into account, annihilated the positive impact of the plasma treatment due to the debris acting as rated break point.     

Surface properties of glass plates activated by air,oxygen, nitrogen and argon plasma

Glass surface properties were investigated after air, nitrogen, oxygen and argon plasmas treatment. The samples were treated by low pressure plasma for 30 s with the gas flow 22 sccm. After modification kinetics of water spreading was measured. Surface topography was determined using optical profilometry, scanning electron microscopy and X-ray photoelectron spectroscopy. It was found that using all types of gases plasma treatment leads to decrease of the surface roughness. The kinetics of water spreading depends on gases type used for glass plates modification. Analyzing the photoelectron spectra the increase of oxygen amount on the surface was observed. For the increase of wettability and adhesive properties of plasma treated glass, the introduction of new polar functional groups on the surface has greater influence than changing the surface roughness.     

Enhanced Cell Adhesion to Helium Plasma-Treated Polypropylene

Materials used for biomedical applications are required to have suitable surface properties since they depend more on the surface properties than on the bulk properties. Surface properties greatly influence the cell adhesion and its behavior either directly by guiding cell spreading or indirectly by controlling proteins adsorption and their structural rearrangement on the material. Modulation of physical and chemical properties of polymers by various treatments can render the substrates adhesive for cells in a culture. In the present study, polypropylene surface was modified using helium plasma to enhance cell adhesion to its surface. The experiments were run according to the central composite design of response surface methodology to optimize the process conditions. The effects of the process variables, namely, RF power, pressure, flowrate and treatment time on surface energy and percentage weight loss were studied through central composite design (CCD). A statistical model relating the process variables and the responses was developed. The improved hydrophilicity of polypropylene through helium plasma treatment was observed from its surface energy data. Changes in surface chemistry and surface morphology were studied by Fourier transform infrared spectroscopy and scanning electron microscopy, respectively. Enhanced cell adhesion to polypropylene treated with helium plasma at the optimum conditions, obtained from the statistical design, was observed from cell adhesion test and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay with L929 mouse fibroblast cells.