Application of FFT-Based Signal Analysis to Micro-Scratch Testing for Adhesion Strength Measurement of Thin Films and Measured Results Based on Different Measurement Parameters

An FFT-based signal processing method for the micro-scratch test, proposed by Baba was compared with a conventional micro-scratching method to quantitatively evaluate the suitability of the technique. Original scratch signals from a micro-scratch system were first digitized by a 16-bit analog-to-digital converter with a sampling frequency of 44.1 kHz, which were fed into a personal computer. Then, the FFT-based method and the conventional method were used to process the signals according to their individual algorithms. The output results from the two methods were compared, showing that the FFT-based method effectively reduces unnecessary signals arising from the micro-scratching mechanism, and that the output level of the unnecessary signals for the FFT-based method is about 50% lower than that observed in the conventional method. The FFT-based method was implemented into a micro-scratch tester for actual application and critical loads of various thin films were measured using various measurement parameters. The results showed that the critical load increases with increasing stylus radius, or increasing thickness of the film, whereas it decreases with increasing frequency of forced vibration. It is also shown that the amplitude of the forced vibration has only little effect on the critical load.     

A new method to measure adhesion and surface forces using a closed-loop accelerometer

A force-balanced MEMS sensor is developed to measure the forces between two surfaces with controllable distance. Its mechanical structure is like a pendulous micromachined accelerometer, and it is designed as a closed-loop system with electrostatic force feedback. The surface force on the sensor probe is balanced by the electrostatic force and the probe operates without displacement. This method avoids the displacement of the conventional cantilever, and the distance between the two surfaces is precisely controlled during the measurement. Experiments on surface force measurements between a probe surface and a ball surface are performed, and the attractive force and adhesion force between the two surfaces are measured under a precise distance controlled by a nanopositioner.     

A study of adhesion forces by atomic force microscopy

An atomic force microscope (AFM) was used to investigate Si₃N₄ tip interactions with various materials in four different liquid media (water, ethanol, ethylene glycol, and formamide). The adhesion forces calculated using surface energies and the values measured experimentally were compared. For all materials, the calculated adhesion force closely correlated with AFM measurements, except in water. In the case of water, the AFM experiments showed strong adhesion, whereas theoretically (van Oss-Chaudhury-Good model) repulsion is predicted. The difference observed is discussed in terms of the chemical interactions between Si₃N₄ and water.     

Thermoplastic film adhesives based on phenol-functional acrylic copolymers: synthesis,mechanical and adhesion properties

Acrylic polymers possessing varying proportions of pendant phenol groups were synthesized by the free radical copolymerization of N-(4-hydroxyphenyl) maleimide (HPM) with butyl acrylate (BuA) and acrylonitrile (AN) and characterized. These thermoplastics form excellent films and their mechanical and adhesion properties were evaluated as a function of the phenol content. Enhancing the HPM content increased both the tensile strength and the modulus but decreased the elongation. A nominal increase in the phenol content was found to be conducive for improving the adhesion properties of the films. At higher concentrations, the adhesion properties showed a decreasing trend due to the embrittlement caused by the rigid maleimide groups. The adhesion property at 50°C increased linearly with the HPM content due to an increased T _g, whereas a reverse trend was observed for the adhesion property measured at-196°C, due to the dominance of the embrittlement effect. The reduced flow characteristics of the high HPM-loaded systems led to a diminished honeycomb flat-wise tensile strength. Enhancing the HPM concentration in the chain promoted the adhesion properties for the vulcanization bonding of nitrile rubber to aluminium. Addition of silica filler marginally improved the lap shear strength (LSS) for the metal-metal system, but was detrimental for rubber-metal bonding; a reverse trend was observed for the carbon-filled system. The diminished performance for metal-metal bonding by carbon could be attributed to the weakening of the interphase, whereas the enhanced rubber-metal bonding could be due to possible reinforcement of the rubber phase by carbon. The fillers generally improved the high temperature adhesion. However, they impaired the flow properties of the resin and, thereby, adversely affected the flat-wise tensile strength in both cases.     

Effect of skin pass rolling on the primer adhesion and corrosion resistance of hot-dip galvanized (HDG) steel

By using three different skin pass reductions, 0%, 0.75%, and 1.5%, the influence of skin pass rolling on the primer adhesion and corrosion resistance of primed hot-dip galvanized (HDG) steel has been studied. The corrosion resistance of primed panels was determined by a cyclic prohesion test, and the primer adhesion was examined with a combined cross-cut and impact test. Surface roughness was determined for untreated and pretreated skin passed panels and the samples were also studied using an optical microscope and a scanning electron microscope. Electron spectroscopy for chemical analysis (ESCA) and glow discharge optical emission spectroscopy (GD-OES) were used to characterize the chemical surface composition of the panels. According to the prohesion test results, the roughest samples showed the best corrosion resistance and also slightly improved adhesion test results. The ESCA and GD-OES results showed that the outermost surface was enriched by aluminium in the zinc coating. During skin pass rolling, the aluminium oxide-rich surface is broken and zinc is revealed to the surface. An increase in the skin pass reduction resulted in an almost linear increase in the surface roughness. Mechanical removal of the surface aluminium also affected the amount of aluminium dissolved during the chemical pretreatment. The best results obtained for the roughest samples are mainly due to the most homogeneous skin pass pattern obtained with the highest skin pass reduction.     

Wettability and surface free energy of corona-treated biaxially-oriented polypropylene film

Several methods for the determination of both the surface free energy of polymer materials and the conditions necessary to perform contact angle measurements are discussed. The effects of the corona-treatment energy on the surface free energy and on the adhesion of acrylic adhesive were studied using a biaxially-oriented polypropylene film. The surface free energy was determined by the Owens-Wendt, and van Oss-Chaudhury-Good approaches, as well as with the wettability method, using different liquids. The presented results confirm that the surface free energy value depended on both the method used and the nature of probe liquids. Thus, it cannot be considered as a parameter characterizing unambiguously the surface layer of a corona-treated film. The values of the surface free energy for different film samples can be compared with one another only if determined using the same method and the same liquids. The variations of particular components of the surface free energy with the corona-treatment energy depend on e.g. the nature of probe liquids, which makes interpretation of the observed effects difficult.     

Study of tack properties of uncrosslinked natural rubber

The purpose of this study was to better understand the effect of non-rubber substances (mainly proteins and lipids) on adhesion (against glasses) and self-adhesion tack properties of uncrosslinked natural rubber (NR). Various types of NR, with or without non-rubber constituents, were prepared. Synthetic polyisoprene rubber was also used as a reference material. All the rubbers were first characterized by many techniques (FT-IR, DSC, GPC, etc.). Two experimental tests were specially utilized to measure the level of adhesion and self-adhesion: (i) at very short contact times (from a few milliseconds up to 0.1 s) the impact of a pendulum and (ii) for longer contact times (from 0.1 s to a few h) the contact of a probe using a tensile testing machine. The tack energy increased with contact time for all the rubbers studied. Natural rubber which did not contain proteins and lipids exhibited the highest adhesion and self-adhesion tack abilities. In contrast, whole natural rubber, containing both proteins and lipids, showed the lowest tack property. In each case, self-adhesion levels are higher than those of adhesion, presumably due to interdiffusion of macromolecular chains or chain segments at the interface.     

Adhesion measurement of thin films to a porous low dielectric constant film using a modified tape test

In the microelectronics industry, a simple tape test (ASTM 3359) is typically used to qualitatively study the adhesion of dielectric films. In this work, a novel approach to conduct the tape test is proposed. This new method eliminates the inconsistency in the results encountered in the traditional tape test, and, at the same time, it provides both qualitative and quantitative results. This approach employs a spring-loaded mechanism and eliminates the subjective interpretation of the results of the traditional tape test. It also provides quantitative measures of the peel rate and force, factors that are relevant in the characterization of the interfacial strength of thin films. We illustrate the use of this new, modified tape test (MTT) in the study of the adhesion of multi-layered thin film structures. This study is of significance to the understanding of the damascene interconnect process integration. Thus, the adhesion of various films (e.g. oxide, insulator and metallic films) on the spun-on porous carbon-doped silicon dioxide low dielectric constant (k) material is examined. Also, the effects of surface treatments (ion implantation and N₂ plasma) and curing temperature of the porous low-k films on the adhesion of the overlying oxide films are also described. An explanation of the adhesion strength with respect to the thickness of the cap layer (overlayer) is presented as well.     

Microstructure,adhesion strength and failure path at a polymer/roughened metal interface

Metals and polymers are extensively used in microelectronics packaging where they are joined together. Since both the yield and reliability of packages are strongly affected by the interfacial adhesion between polymers and metals, extensive studies have been performed in order to improve the resistance to debonding of many resulting interfaces. In the present work, the interfacial fracture energy of representative polymer/metal interfaces commonly encountered in micoroelectronics packaging was characterized. A copper-based alloy leadframe was used as the metal and an epoxy molding compound (EMC) was used as the polymer. The leadframe surfaces were roughened by chemical oxidation in a hot alkaline solution and molded with the EMC. In general, roughening of metal surfaces enhances their adhesion to polymers by mechanical interlocking, yet often produces a cohesive failure in the polymer. Sandwiched double-cantilever beam (SDCB) specimens were employed to measure the adhesion strength in terms of interfacial fracture energy. After the adhesion test, the microstructures of metal surfaces before molding with the EMC were correlated to the adhesion strength, and the fracture surfaces were analyzed using various techniques to determine the failure path.     

Surface characterization and adhesion of black-oxide-coated copper substrate: effect of surface hardening processes

The effects of surface-hardening processes on the changes in surface characteristics and adhesion of black copper oxide substrate with epoxy resins are studied. Various techniques, namely SEM, XPS, AFM, XRD, Auger electron spectroscopy, contact angle goniometry, D-SIMS and RBS, were used to identify the changes in surface characteristics. Dense, fibrillar cupric oxide crystals characterized the as-deposited oxide coating with high surface roughness. The surface-hardening process flattened and consolidated the fibrils without changing the compositional and thermodynamic characteristics of the coated surface. The surface-hardening process reduced the total thickness of copper oxide by approximately 50–150 nm. The reduction in oxide thickness was not a predominant factor for the reduced bond strength of the surface-hardened coating. The bond strengths of both the as-deposited and surface-hardened black oxide coatings increased with oxidation time, until saturation at about 120–150 s. For the as-deposited oxide coating, mechanical interlocking, high wettability and resistance to surface contamination are the three major sources for improved adhesion, amongst which the enhanced mechanical interlocking provided by the fibrillar cupric oxide is the most important. Surface hardening reduced the efficiency of mechanical interlocking mechanism. There was close functional dependence between the button-shear strength and surface characteristics, such as surface roughness, coating thickness and surface free energy.     

Characteristics of Acoustic Emission Signal Specific to Isolated Cracks Triggered by Scratch Tests in Stainless Steel Coatings

The tribological process during contact between two surfaces involves mechanical and tribochemical changes as well as material transfer. PVD coatings have a complex structure, and there is a need to characterize the failure of such coatings and this has been carried out by a non-destructive method, namely acoustic emission. In this paper simple scratch tests are described in order to study some characteristics of nitrogen-doped stainless steel coatings on 40CrMo4 construction steel. Scratch tests were performed in order to induce a mechanical failure in the coatings. The aim of this paper is to show that the nucleation of isolated cracks mechanism that leads to failure can be monitored by certain characteristics of the acoustic emission signal.     

The effects of urine and temperature on the physicochemical surface properties and adhesion behaviour of uropathogenic bacteria

Bacterial adhesion in relation to urinary-tract infections has gained importance in the last years because of the increasing catheterization in hospitals to assist post-surgery flow of urine. Since the initial adhesion of bacteria to biomaterials is governed by physicochemical forces emerging from the physicochemical properties of both interacting phases, we have investigated the physicochemical surface changes of uropathogen Enterococcus faecalis ATCC29212 bacteria due to the presence of urine in its growth medium and to the differences in the environmental temperature. Urine-grown cells were found to be less hydrophobic based on water contact angles at 22°C, while no changes were detected at 37°C. In addition, they exhibited higher acid-base surface energy component than urine-free cultured cells. These changes in surface properties were also reflected in thermodynamic predictions of the adhesion to glass and silicone, which were experimentally compared with the in vitro adhesion curves obtained in a parallel plate flow chamber. The shapes of the adhesion graphs indicated that interaction free energies should be used to describe only the initial adhesion stages. Adhesion to silicone was always enhanced by urine-grown cells, while the adhesion to glass did not seem to be affected by the urine constituents. Despite the fact that the interaction free energies were not able to explain the adhesion process in some cases, changes in the electron-donor and electron-acceptor parameters of their surface free energy due to urine addition seemed to have a relation with initial adhesion rates.     

Temperature influence on the physicochemical surface properties and adhesion behaviour of Enterococcus faecalis to glass and silicone

The interest in studies on the physicochemical surface properties of bacteria has increased because they are related to the causes of the initial adhesion of microorganisms to biomaterials and the subsequent biofilm formation on indwelling medical devices. The determination of physicochemical parameters such as hydrophobicity or surface tension is usually done at room temperature, not taking into account the real temperature at which bacteria cause infection inside the human body. In this work, the influence of the experimental temperature on the surface physicochemical characteristics and adhesion behaviour of Enterococcus faecalis ATCC29212 to glass and silicone has been studied. Water, formamide and diiodomethane contact angles on bacterial lawns changed when the experimental temperature was increased from 22°C to 37°C. Moreover, hydrophobicity, as determined by water contact angle, increases with temperature, in agreement with the higher and lower adhesion to silicone and glass, respectively, observed at 37°C, with respect to the results at 22°C. Also, when the formamide and diiodomethane contact angles are considered, the changes in the adhesion behaviour to glass and silicone are predicted by the sum of Lifshitz-van der Waals and acid-base interaction free energies if the measurement temperature is the same as the bacterial growth temperature, i.e. 37°C.     

The effect of surface properties on the adhesion of modified polychloroprene used as adhesive

The adhesion properties of polychloroprene can be improved by addition of such materials as piperylene–styrene co-polymer (PSC), VeoVa-10 polymer, VeoVa-11/methyl methacrylate/2ethylhexyl acrylate co-polymer (VeoVa-11/MMA/2EHA) and poly(vinyl acetate) waste (wPVAc). Here, the relationship between adhesion properties and surface tension of polychloroprene was investigated. Contact angle measurements have been used to study the effects of nature and content of polymeric additives on the adhesion and surface properties of polychloroprene. Low-surface-tension VeoVa-10 polymer has the tendency to migrate to the surface of polychloroprene; thus, adhesion is determined mainly by this additive property. Enrichment of polychloroprene film bottom layer by the additive was observed using high-surface-tension PSC and wPVAc. In this case, the adhesion properties of polychloroprene depend on the interactions at the interface. Adhesion properties of polychloroprene were found to depend not only on compatibility between adhesive components, but also on compatibility between the adherend and the adhesive.     

Influence of glass mechanical strengthening on the adhesion properties of poly(vinyl butyral) films to a float glass surface

The effects of various methods of mechanical strengthening of glass on the adhesion properties of poly(vinyl butyral) (PVB) film to a float glass surface were investigated. The mechanisms of the influence of the strengthening processes on the adhesion properties were analyzed. The influence of different types of pretreatment of the glass surface on the adhesion of the polymer films was also considered. It was shown that ion-exchange strengthening followed by treatment with an alkaline water solution provided the best combination of high mechanical strength of glass and good adhesion of the PVB films to the glass surface. Metal-oxide coatings on float glass increased the mechanical strength of glass but decreased the adhesion strength between the polymer and glass. The adhesion of PVB to the metal-oxide layers was determined not only by the chemical composition of the layers, but also by the method of layers formation, the type of glass surface pretreatment, and the nature of the intermediate layer between the metal-oxide layer and the glass surface.     

Interfacial adhesion between polyurethane binder and poly(tetrafluoroethylene) powder in bonded solid lubricant films

Poly(tetrafluoroethylene) (PTFE) powder was irradiated with ⁶⁰Co γ-rays to improve its dispersing ability in polyurethane (PU) as a binder. The bonded solid lubricant films of the irradiated PTFE were prepared on an AISI 1045 steel block by spraying and curing at ambient temperature, with PU as the binder. The tribological properties of bonded solid lubricant films with the PTFE pigment volume fraction were examined on a ring-on-block friction and wear tester. The interfacial adhesion between the PU binder and PTFE powder was investigated using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), immersion heat, and X-ray photoelectron spectroscopy (XPS). It was found that γ-ray irradiation increases the activity of the PTFE powder surface and improves the interfacial adhesion between the PTFE powder and the PU binder, which is helpful for improving the wear resistance of the corresponding bonded solid lubricant films.     

Characterization and tribological properties of two polyimide thin films sputtered onto a copper substrate

Two kinds of polyimide (PI), pyromellitic dianhydride-oxydianiline (PMDA-ODA) and biphenyl dianhydride-p-phenylene diamine (BPDA-PDA), thin films were sputtered onto a copper substrate by conventional RF sputtering with argon. These PI thin films were characterized, and their adhesion and tribological properties were evaluated. Elemental compositions and chemical bonding states of these thin films were analyzed with X-ray photoelectron spectroscopy (XPS). Oxygen and nitrogen concentrations in these thin films were less than those in bulk PIs. In addition, the amounts of C—O and C—N moieties in these PI thin films decreased as compared to the bulk PIs. Contact angles of water and methylene iodide on these PI thin films were higher than those on the bulk PIs. Surface energies of these PI thin films were calculated by measuring contact angles of water and methylene iodide. Surface energies of these PI thin films were lower than those of the bulk PIs and the polar components of these PI thin films were one-third of those of the bulk PIs. Friction coefficients of these thin films were almost the same as those of the bulk PIs. The abrasion durability of PMDA-ODA thin film was higher than that of BPDA-PDA thin film. The adhesion strength between the PMDA-ODA thin film and copper substrate was also higher than that between BPDA-PDA thin film and copper substrate.     

Surface and adhesion changes of atmospheric barrier discharge-treated polypropylene in air and nitrogen

Oriented polypropylene treated by atmospheric barrier discharges in air and nitrogen was investigated using several techniques: contact angle measurements, ATR-FT-IR spectroscopy and two adhesion tests based on the stripping of an applied ink layer. The activation in an air discharge was found to be much weaker compared to the activation in industrial grade nitrogen, particularly with respect to adhesion. The adhesion was found to be much better in nitrogen in spite of the common use of air in industrial 'corona discharges'. A new 'abrasive shear-stripping' (AS) test for ink coating adhesion was designed and performed. It was shown that the AS test was much more sensitive than the classical adhesive tape test and was sensitive enough to monitor ageing and overtreatment. The contact angle measurements did not correlate completely with the adhesion properties and could not monitor the overtreatment, while the ATR-FT-IR technique indicated changes just for overtreated foils.     

Surface reactivity of polyimide and its effect on adhesion to epoxy

Polyimides are commonly used as organic passivation layers for microelectronic devices due to their unique combination of properties such as low dielectric constant, high thermal stability, excellent mechanical properties and superior solvent resistance. Unfortunately, polyimides are well known to be difficult to bond to other materials, especially to epoxy resins. Many surface treatments have been developed to increase epoxy–polyimide adhesion. These treatments include exposure to ion beams, plasmas and chemical solutions. The goal of our research was to relate surface reactivity of epoxy and polyimide resins to the strength of epoxy–polyimide interfaces. The surface reactivity of four polyimides was studied and quantified using contact angle measurements, flow microcalorimetry (FMC), Fourier transform infrared (FT-IR) spectroscopy (using an attenuated total reflection (ATR) accessory) and X-ray photoelectron spectroscopy (XPS). Several ways of analyzing contact angles were tried and only a weak correlation between the polar component or the acid–base components of the surface free energy with the critical interfacial strain energy release rate (i.e., the interfacial fracture strength) was observed. FMC results suggest that the strength of epoxy–polyimide interfaces is related to the molecular interactions between the curing agent and polyimide. The molecular interactions between the curing agent and polyimide surfaces were found to be either greater than epoxy and polyimide interactions or more irreversible. Therefore, the curing agent (2,4-EMI) is thought to play a critical role in controlling adhesion strength.     

Modification of epoxy resin by polysulfone to improve the interfacial and mechanical properties in glass fibre composites. II. Adhesion of the epoxy-polysulfone matrices to glass fibres

Adhesion of epoxy-polysulfone (PSF) matrices to glass fibres of 12–30 μm in diameter was studied under both quasi-static and cyclic loadings. A pull-out technique was used for adhesion measurement. It was shown that incorporation of PSF into epoxy resin changed its adhesion to fibres. A maximum was observed in the adhesion strength vs. PSF content dependence at 10 wt% thermoplastic concentration. The results obtained were compared with the data on the epoxy-PSF matrices adhesion to thick steel wire (d = 150 μm) and Nylon-6 fibres (d = 250 μm). Similar values of the adhesion strength increase (22–25%) confirmed that all the changes at the interface were connected primarily with the matrix. A new preferably non-destructive cyclic loading technique was used to test the systems under cyclic loading at varying force amplitudes, frequencies and displacement amplitudes. In this technique the interphase behaviour is characterised by two variables: by the phase angle between the deformation applied to the matrix and the force transferred by the matrix to the fibre, and also by the amplitude of this force. Minimal force amplitudes were observed for the joints with 10 wt% polysulfone. Moreover, phase-angle values for epoxy-10% polysulfone joints were minimal among all the systems investigated. Increase in the number of loading cycles caused much more damage to unmodified epoxy matrix than that to epoxy-polysulfone matrices. Thus, modification of epoxy resin by polysulfone enhanced its adhesion to fibres under both quasistatic and cyclic loadings, especially for epoxy-10% polysulfone matrix. The possible mechanism of the phenomenon observed is discussed.