Use of the Blister Test to Study the Adhesion of Brittle Materials. Part II. Application

Using a modified form of the blister test, where the adhesive layer was between the substrate and a massive base, instead of as a continuous sheet on top of the substrate, we determined the interfacial fracture energy F for a series of interfaces where a brittle material (ice) was adhering to various substrates. Fracture energies obtained were compared with work of adhesion values measured for water on the same substrates. Fracture energy, which contains within it both a reversible contribution due to intermolecular interactions across the interface (work of adhesion) and an irreversible contribution due to collective dissipative processes, was found to rise rapidly with modest increases in work of adhesion. The observed relation suggests that the irreversible contribution to fracture energy is influenced strongly by the intermolecular interactions at the interface.     

Use of the Blister Test to Study the Adhesion of Brittle Materials. Part II. Application

Using a modified form of the blister test, where the adhesive layer was between the substrate and a massive base, instead of as a continuous sheet on top of the substrate, we determined the interfacial fracture energy F for a series of interfaces where a brittle material (ice) was adhering to various substrates. Fracture energies obtained were compared with work of adhesion values measured for water on the same substrates. Fracture energy, which contains within it both a reversible contribution due to intermolecular interactions across the interface (work of adhesion) and an irreversible contribution due to collective dissipative processes, was found to rise rapidly with modest increases in work of adhesion. The observed relation suggests that the irreversible contribution to fracture energy is influenced strongly by the intermolecular interactions at the interface.     

Adhesion interaction in water/n-alcohol mixtures between silanized silica and polymer particles

The adhesion of polyethylene and nylon particles to silanized silica plates was investigated in water/n-alcohol (methanol, ethanol, 1-propanol, and 1-butanol) mixtures. Silica plates were treated with γ-aminopropyltriethoxysilane, methyltriethoxysilane, and perfluoroethyltrimethoxysilane. The number of particles adhering to the plate at 60 min as an apparent equilibrium adhesion value increased as a result of the silanization of silica and decreased with increasing volume ratio of n-alcohol in the water/n-alcohol mixtures. The acid-base components of the surface free energies of the substrates and liquids were decreased by the silanization of silica and by the addition of cthanol to water. The apparent equilibrium particle adhesion is discussed in terms of the total potential energies of interaction which were calculated as the sum of the electrical double layer, Lifshitz-van der Waals, and acid-base interactions, using the electrokinetic potentials and the surface free energy components. In addition, the relationship between the extent of particle adhesion and the work of adhesion was investigated. The particle adhesion in the present systems was found to be dominated by the acid-base interaction between the particle and the substrate.     

Thermal Fluctuations Limit the Adhesive Strength of Compliant Solids

For compliant solids, the stress required to separate an interface (its adhesive strength) appears to be much lower than that calculated by computing intersurface interactions. We explore the hypothesis that the adhesive strength is limited in value by thermal fluctuations. In a simple model of an interface, molecules bridging the two surfaces are represented by linear entropic springs. Asymptotic and numerical analyses are carried out to evaluate the adhesive strength and effective work of adhesion. For stiff materials, adhesive strength is found to be equal to the intrinsic strength—the maximum value of intersurface stress computed ignoring fluctuations. For compliant materials, adhesive strength is significantly reduced and is on the order of the elastic modulus. The effective work of adhesion agrees with the intrinsic work of adhesion for stiff materials and can decay slowly with increasing compliance.     

Thermal Fluctuations Limit the Adhesive Strength of Compliant Solids

For compliant solids, the stress required to separate an interface (its adhesive strength) appears to be much lower than that calculated by computing intersurface interactions. We explore the hypothesis that the adhesive strength is limited in value by thermal fluctuations. In a simple model of an interface, molecules bridging the two surfaces are represented by linear entropic springs. Asymptotic and numerical analyses are carried out to evaluate the adhesive strength and effective work of adhesion. For stiff materials, adhesive strength is found to be equal to the intrinsic strength—the maximum value of intersurface stress computed ignoring fluctuations. For compliant materials, adhesive strength is significantly reduced and is on the order of the elastic modulus. The effective work of adhesion agrees with the intrinsic work of adhesion for stiff materials and can decay slowly with increasing compliance.     

Application of the Island Blister Test for Thin Film Adhesion Measurement

The island blister test has recently been proposed as an adhesion test which allows the peel of thin, well-adhered films without exceeding the tensile strength of the film. The island blister test site is a modification of the standard blister test site, consisting of a suspended membrane of film with an “island” of substrate at the film center. The membrane support and island are secured to a rigid plate and the film is pressurized, peeling the film inward off the island. A model for this inward or “annular” peel indicates that even for systems of good adhesion, peel can be initiated at low enough pressures to prevent film failure by making the center island sufficiently small relative to the size of the film.

We have fabricated island blister test sites using micromachining techniques and have used them to measure the debond energy of polymer films on various substrates. The peel data obtained from these island sites match well to the behavior predicted by a simple fracture mechanics analysis. This paper reports the fabrication of the island test sites, the experimental verification of the test, and the results of application of the test to polyimide films on metallic and polymeric substrates.     

Adhesion-Induced Interactions between Micron-Sized Zirconia or Carbon Spheres and Melamine-Cross-Linked Polyester Surfaces

The thermodynamic work of adhesion between micron-sized zirconia or carbon spheres and polyester–melamine surfaces was determined using the Johnson-Kendall-Roberts (JKR) relationship, after first validating the JKR approach for this system. The calculated works of adhesion for both zirconia and carbon were similar for any given substrate composition and were found to be approximately 35 mJ/m² at low melamine concentrations, close to theoretical predictions. The apparent work of adhesion decreased with increasing melamine concentration, most likely due to the presence of a glassy melamine-rich surface layer, which is not representative of the bulk. The value found for low melamine concentration was assumed to be a true value for cocondensed polyester–melamine, and this was used to estimate surface modulus and the amount of excess melamine in the surface as a function of bulk composition.     

Adhesion-Induced Interactions between Micron-Sized Zirconia or Carbon Spheres and Melamine-Cross-Linked Polyester Surfaces

The thermodynamic work of adhesion between micron-sized zirconia or carbon spheres and polyester-melamine surfaces was determined using the Johnson-Kendall-Roberts (JKR) relationship, after first validating the JKR approach for this system. The calculated works of adhesion for both zirconia and carbon were similar for any given substrate composition and were found to be approximately 35 mJ/m² at low melamine concentrations, close to theoretical predictions. The apparent work of adhesion decreased with increasing melamine concentration, most likely due to the presence of a glassy melamine-rich surface layer, which is not representative of the bulk. The value found for low melamine concentration was assumed to be a true value for cocondensed polyester-melamine, and this was used to estimate surface modulus and the amount of excess melamine in the surface as a function of bulk composition.     

A peel adhesion study of electroless Cu layers on polymer substrates

The peel adhesion between two different electroless-plated Cu layers and polymer substrates was studied. Cu was electroless-plated onto polymer substrates using two different commercial solutions with different compositions. The adhesion strength between the electroless Cu layers and polymer substrates was measured with the 90° peel test. The adhesion was influenced by the coverage, grain size, and the thickness of the electroless Cu layer. Poor coverage of the electroless Cu layer increased the density of the pores at the interface between the Cu layer and the substrates, thereby degrading the adhesion strength because of a decrease in the contact area. In addition, the electroless Cu layers with larger nodules and larger grains were softer and had higher peel adhesion since the soft and ductile Cu layer promoted a greater amount of plastic deformation during the peel test. This led to enhanced peel adhesion. Finally, as the thickness of the electroless Cu layer increased, the peel adhesion decreased. The thicker Cu layers are not easily bent. Poor bending of the Cu layer induced less plastic deformation, causing a decrease in the peel adhesion. In conclusion, soft and thin electroless Cu layers with greater coverage are preferred in order to obtain good adhesion.     

Changes in the Peeled Surfaces of Copper Thin Films Deposited on Polyimide Substrates After Heat Treatment

Peel strength between a copper (Cu) thin film and a polyimide (pyromellitic dianhydride-oxydianiline, or PMDA-ODA) substrate is reduced by heat treatment at 150°C in air. In this work, we investigated the peel strength, the morphology of the interface between Cu films and polyimide substrates using optical microscopy and electron microscopy, and chemical change of the interface using Auger electron spectroscopy (AES) and micro X-ray photoelectron spectroscopy (XPS). The analysis showed that CuO “lumps” were present on the peeled surface of PMDA-ODA after heat treatment at 150°C in air. The peeled surfaces of other polyimide substrates were also analyzed: biphenyl dianhydride-para phenylene diamine (BPDA-PDA) and biphenyl dianhydride-oxydianiline (BPDA-ODA). CuO lumps were present on the peeled surface of BPDA-ODA after the heat treatment, but not that of BPDA-PDA. Compared with the adhesion strength for the Cu thin film, the adhesion strength was high for the Cu/PMDA-ODA and Cu/BPDA-ODA laminates, but the adhesion strength was very low for the Cu/BPDA-PDA laminate. This low strength is the reason that CuO lumps were not detected on the peeled surface of the BPDA-PDA substrate. These CuO lumps were related to the adhesion degradation of the Cu/polyimide laminates after the heat treatment.     

Evidence of Acid-Base Interfacial Adducts in Various Polymer/Metal Systems by IRAS: Improvement of Adhesion

The physical interactions of polymers with inorganic substrates are determined by two major contributions: Van der Waals forces and acid-base interactions, taken in the most general “Lewis” electron acceptor-donor sense. The present work shows that the work of adhesion can be very appreciably increased by the creation of interfacial acid-base interactions. Practically, polymers such as poly(ethylene-co-vinyl acetate) (EVA), terpene-phenol resins (TPR), and their blends, were solution cast on basic and acidic substrates. The nature of the interfacial bonds and the enthalpy of adduct formation through electron exchange are evidenced by Fourier transform infrared reflection-absorption spectroscopy (IRAS). Moreover, it is shown that, on the one hand, modification of the electron donor ability of the polymer functionalities reveals the amphoteric character of the substrate and, on the other hand, modification of the electron donor ability of the substrate changes the nature of the species involved in interfacial adduct formation. Then, practical adhesion tests were carried out in order to correlate the nature and strength of interfacial acid-base bonds with simultaneous increases in adhesive strengths. Thermodynamic considerations allowed us to propose estimated values of the acid-base work of adhesion, W^ab, and of the density of acid-base sites, n^ab.     

Adhesion of elastomers: Dwell time effects

The strength of adhesion of elastomers to rigid substrates generally increases with time of contact. This effect has been studied for samples of butyl and chlorobutyl rubber adhering to some rigid substrates. The peel strength increased continuously over long periods of contact until in some cases failure became cohesive within the elastomer layer. At higher temperatures the strength increased more rapidly, consistent with the WLF relation governing molecular motions. It is postulated that slow molecular rearrangements occur at the interface and increase the bond strength. A criterion for the observed transition from interfacial to cohesive failure is suggested.     

Adhesive Failure of Model Acrylic Pressure Sensitive Adhesives

An axisymmetric adhesion apparatus was used to characterize the adhesive and viscoelastic properties of acrylic block copolymer layers that behave as model pressure sensitive adhesives. The mechanisms of deformation were summarized and related to the structure and linear viscoelastic response of each model adhesive. In cases where the area between the adhesive layer and adhering surface remained circular and shrunk uniformly during detachment, the adhesive failure criterion can be quantified and compared to predictions from linear elastic fracture mechanics. The nature of adhesive failure can not be reconciled with these traditional, low-strain approaches, but is consistent with models of large strain elasticity, provided that the finite thickness of the adhesive layer is taken into account. A dimensionless ratio involving the adhesive strength, elastic modulus and adhesive layer thickness can be used to define the regime in which the adhesive failure criterion can be quantified with linear elastic fracture mechanics.     

Adhesive Failure of Model Acrylic Pressure Sensitive Adhesives

An axisymmetric adhesion apparatus was used to characterize the adhesive and viscoelastic properties of acrylic block copolymer layers that behave as model pressure sensitive adhesives. The mechanisms of deformation were summarized and related to the structure and linear viscoelastic response of each model adhesive. In cases where the area between the adhesive layer and adhering surface remained circular and shrunk uniformly during detachment, the adhesive failure criterion can be quantified and compared to predictions from linear elastic fracture mechanics. The nature of adhesive failure can not be reconciled with these traditional, low-strain approaches, but is consistent with models of large strain elasticity, provided that the finite thickness of the adhesive layer is taken into account. A dimensionless ratio involving the adhesive strength, elastic modulus and adhesive layer thickness can be used to define the regime in which the adhesive failure criterion can be quantified with linear elastic fracture mechanics.     

Reversible and Irreversible Adhesion of Polymers: Possibilities for Measurement and Calculation

Comparative analysis of existing direct and indirect techniques for estimation of the work of adhesion (W_A) between a polymer and a solid surface - inverse gas chromatography (IGC), wetting, direct adhesion forces measurement - has been carried out. The work of adhesion was calculated from experimental data obtained using different techniques for identical polymer/solid systems. The relationship between the work of adhesion and the bond strength was analyzed, including possible W_A estimations from destructive micromechanical tests. For non-polar polymers, whose adhesion is due to dispersion interaction only, all techniques are in good agreement with each other. However, the estimates of work of adhesion obtained by different techniques considerably differ for polar polymers. The reason for this obviously consists in deficiency of theoretical knowledge about non-dispersion interactions at interfaces. Each of the considered approaches has its own advantages and shortcomings. The problems concerning the estimation of non-dispersion component of the work of adhesion can be solved only by comprehensive use of several different techniques.     

ADHESION OF ELASTOMERS: DWELL TIME EFFECTS

The strength of adhesion of elastomers to rigid substrates generally increases with time of contact. This effect has been studied for samples of butyl and chlorobutyl rubber adhering to some rigid substrates. The peel strength increased continuously over long periods of contact until in some cases failure became cohesive within the elastomer layer. At higher temperatures the strength increased more rapidly, consistent with the WLF relation governing molecular motions. It is postulated that slow molecular rearrangements occur at the interface and increase the bond strength. A criterion for the observed transition from interfacial to cohesive failure is suggested.     

Polyvinylpyrrolidone adhesion layer for increased uniformity and optical transmittance of silica nanoparticle antireflective coatings

The uniformity of silica nanoparticle antireflective coatings deposited from aqueous solutions on glass substrates is limited by the high surface tension and low evaporation rate of water. In this work, thin films of polyvinylpyrrolidone (PVP) were utilized as an adhesion layer to increase the uniformity and optical transmittance of silica nanoparticle coatings. The increase in adhesive force caused by the presence of the PVP layer was measured using atomic force microscopy (AFM). The micro- and nanoscale uniformities of silica nanoparticle films with and without PVP adhesion layers were characterized using scanning electron microscopy and AFM. It was found that a thin PVP adhesion layer provides the adhesion required to form uniform films of silica nanoparticles. Solar weighted transmittance of 97.6% over a wavelength range of 330–1000 nm was achieved with soda-lime glass substrates coated on both sides.     

Changes in the adhesion strength between copper thin films and polyimide substrates after heat treatment

The adhesion strength between a copper (Cu) thin film and a polyimide [pyromellitic dianhydride-oxydianiline (PMDA-ODA)] substrate is reduced by heat treatment at 150°C in air. In this work, we determined the changes in adhesion strength between Cu films and polyimide substrates using Auger electron spectroscopy (AES), attenuated total reflection Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The analysis showed that the weak boundary layer (WBL) shifted towards the Cu interface as the heat treatment time was increased. To confirm this shift, we looked at two other polyimide substrates: biphenyl dianhydride-p-phenylene diamine (BPDA-PDA) and biphenyl dianhydride-oxydianiline (BPDA-ODA). Comparing the adhesion strength for the Cu thin film, the adhesion strength was high for the Cu/PMDA-ODA and Cu/BPDA-ODA laminates, but very low for the Cu/BPDA-PDA laminate. One of the possible reasons for this behavior could be that the ether moiety between the two benzene rings in ODA is related to the adhesion between a Cu film and an 0₂-plasma-treated polyimide (PI) substrate. The relationship between the adhesion strength and chemical bonding states is also discussed. We conclude that a Cu thin film sputtered onto a PI substrate is apt to peel at the oxidized interface, due to the heat treatment.     

New and Improved Tests for Adhesion

Three new methods are discussed for measuring the work G_a, required to detach unit area of an adhering material from a substrate. The first is a simple modification of the Outwater double-torsion test for long rectangular plates, bonded together. This method is suitable for evaluating aluminum-epoxy bonds, for example, or the transverse strength of fibrous composites. The second is a pull-off test for long strips adhering to a rigid surface. It seems suitable for adhesive tapes and laminates. The third is a reconsideration of the “blister” test for films and coatings, in which a circular debond at the interface is made to grow by internal pressure. The relation obtained between pull-off force F for a strip, or blow-off pressure P for a layer, takes the unusual form:

F⁴ (or P⁴) ∞ KG³_a

where K is the tensile stiffness of the detaching layer. This dependence arises from the non-linear (cubic) relation between load or pressure and deflection in these configurations. Nevertheless, the product Fθ, where θ is the angle of detachment of a strip, or Py, where y is the height of a “blister”, give direct measures of the strength of adhesion G_a, independent of the stiffness of the adhering material and of the extent of detachment.     

Reversible and Irreversible Adhesion of Polymers: Possibilities for Measurement and Calculation

Comparative analysis of existing direct and indirect techniques for estimation of the work of adhesion (W_A ) between a polymer and a solid surface - inverse gas chromatography (IGC), wetting, direct adhesion forces measurement - has been carried out. The work of adhesion was calculated from experimental data obtained using different techniques for identical polymer/solid systems. The relationship between the work of adhesion and the bond strength was analyzed, including possible W_A estimations from destructive micromechanical tests. For non-polar polymers, whose adhesion is due to dispersion interaction only, all techniques are in good agreement with each other. However, the estimates of work of adhesion obtained by different techniques considerably differ for polar polymers. The reason for this obviously consists in deficiency of theoretical knowledge about non-dispersion interactions at interfaces. Each of the considered approaches has its own advantages and shortcomings. The problems concerning the estimation of non-dispersion component of the work of adhesion can be solved only by comprehensive use of several different techniques.