Measurement Of Interfacial Fracture Toughness Research Articles

Cr-containing diamond-like carbon coatings deposited on 316 stainless steel substrates: Characterization and interfacial fracture toughness measurements

A series of Cr-containing hydrogenated amorphous carbon (a-C:H:Cr) coatings were deposited onto 316 stainless steel (316SS) substrates using inductively coupled plasma assisted reactive sputter deposition. Elemental Cr interlayers with different thicknesses of 100, 200, and 300 nm were deposited between a-C:H:Cr and 316SS. Detailed composition, structure, and mechanical behavior characterization of deposited a-C:H:Cr/Cr/316SS specimens were performed. Fracture toughness values of the a-C:H:Cr layer, the Cr layer, and the a-C:H:Cr/Cr/316SS interfacial regions were measured by bending of microcantilever beams with focused ion beam milled pre-notches in-situ a scanning electron microscope. Measured fracture toughness of a-C:H:Cr/Cr/316SS interfacial regions exhibits an approximately linear correlation with the area fraction of the fracture surface occurring in the Cr interlayer, indicating that the interfacial fracture toughness depends on the detailed path of crack propagation and suggesting that interfacial toughness can be engineered through materials design of the interfacial region.

Interfacial fracture toughness measurement in both steady state and transient regimes using four-point bending test

The paper describes some extensions of four-point bending tests for determining the interfacial fracture toughness $$\Gamma $$, in both steady-state and transient regimes, as a function of the fracture mode mixity. Two sets of multimaterial systems were studied: (i) Aluminum alloy (ASTM 2017)/epoxy/PMMA polymer and, (ii) Aluminum alloy (ASTM 2017)/stainless steel (ASTM 301) obtained by bonding and thermal spray coating techniques respectively. The interfacial fracture toughness was investigated by means of analytical and Finite Element Analysis using ABAQUS software. The numerical trend solution of both interfacial fracture toughnesses as function of crack length, and friction coefficient has been obtained and compared to an analytical one. We will propose a method on (i) how both reversed notch position in multimaterial systems and crack delamination beyond the inner loading points (transient regime) are explored to extend the measurement of interfacial fracture toughness, (ii) how the numerical analysis is used to determine the interfacial fracture toughness through an experimental compliance measurement in transient regime, and (iii) we attempt to reveal why the interfacial toughness has strong phase angle dependence.

Measurement of Interfacial Fracture Toughness of Surface Coatings Using Pulsed-Laser-Induced Ultrasonic Waves

This research develops a new technique for the measurement of interfacial fracture toughness of films/surface coatings using laser-induced ultrasonic waves. Using pulsed laser ablation on the bottom substrate surface, strong stress waves are generated leading to interfacial fractures and coating delamination. Simultaneously, a laser ultrasonic interferometer is used to measure the normal (out-of-plane) displacement of the top surface coating in order to detect coating delamination in a non-destructive manner. We can thus determine the critical laser energy for delamination, yielding the critical stress (that is, the interfacial strength). Subsequently, to examine the interfacial fracture toughness, additional pulsed laser irradiation is applied to a pre-delaminated specimen to show that the delamination area expands. This type of interfacial crack growth can be visualized using laser ultrasonic scanning. Furthermore, the calculation of elastic wave propagation was carried out using a finite-difference time-domain method) in order to accurately estimate the interfacial stress field. In this calculation, the stress distribution around the initial delamination is calculated to obtain the stress intensity factor. Based on the experimental and computational results, interfacial fracture toughness can be quantitatively evaluated. Since this technique relies on a two-laser system in a non-contact approach, it may be useful for a quantitative evaluation of adhesion/bonding quality (including both interfacial fracture strength and toughness) in various environments.

Measurement of underfill interfacial and bulk fracture toughness in flip-chip packages

Flip-chip package reliability is greatly improved by encapsulating the solder interconnections between a polymeric encapsulant or underfill. However, thermo-mechanical stresses within such packages often lead to failures initiating in the vicinity of chip and underfill interface. In this study, we present experimental results geared towards measuring and understanding such failure mechanisms. We provide the bulk fracture toughness of the underfill material and interfacial fracture toughness between the underfill material and the silicon die. The bulk and interfacial fracture toughness measurements are performed as a function of temperature. We use the single edge notch bending test to calculate the bulk fracture toughness of the underfill and to measure the interfacial fracture toughness, we use a novel technique referred to as the wedge delamination method. The wedge delamination method provides substantial advantage in measuring the interfacial fracture toughness for brittle materials over traditional methods. Using the wedge delamination method we compare the fracture strength between the underfill and silicon at the front-face and side-wall interfaces. Additionally, the influence of dicing technique on fracture toughness is also investigated.

Dentin Bonding Testing Using a Mini-interfacial Fracture Toughness Approach

Measurement of interfacial fracture toughness (iFT) is considered a more valid method to assess bonding effectiveness as compared with conventional bond strength testing. Common fracture toughness tests are, however, laborious and require a relatively bulky specimen size. This study aimed to evaluate a new simplified and miniaturized iFT (mini-iFT) test. Four dentin adhesives, representing the main adhesive classes, and 1 glass ionomer cement were applied onto flat dentin. Mini-iFT (1.5 × 2.0 × 16 to 18 mm) and microtensile bond strength (µTBS; 1.5 × 1.5 × 16 to 18 mm) specimens were prepared from the same tooth. For the mini-iFT specimens, a single notch was cut at the adhesive-dentin interface with a 150-µm diamond blade under water cooling; the specimens were loaded until failure in a 4-point bending test setup. Finite element analysis was used to analyze stress distribution during mini-iFT testing. The correlation between the mean mini-iFT and µTBS was examined and found to be significant; a strong positive correlation was found (r2 = 0.94, P = 0.004). Weibull data analysis suggested the mini-iFT to vary less than the µTBS. Both the mini-iFT and the µTBS revealed the same performance order, with the 3-step etch-and-rinse adhesive outperforming the 2-step self-etch and 2-step etch-and-rinse adhesive, followed by the 1-step SE adhesive and, finally, the glass ionomer cement. Scanning electron microscopy failure analysis revealed the adhesive-dentin interface to fail more at the actual interface with the mini-iFT test, while µTBS specimens failed more within dentin and composite. This finding was corroborated by finite element analysis showing stress to concentrate at the interface during mini-iFT loading and crack propagation. In conclusion, the new mini-iFT test appeared more discriminative and valid than the µTBS to assess bonding effectiveness; the latter test nevertheless remains more versatile. Specimen size and workload were alike, making the mini-iFT test a valid alternative for the popular µTBS test.

On the specimen for interfacial fracture toughness evaluation of foam-core sandwich structures

Foam-core sandwich specimens for interfacial fracture toughness measurement are designed based on the prediction method of steady-state crack location in foam-core sandwich beams using the crack deflection model developed previously. This paper derives the necessary condition that the face sheet debonding grows along the interface between the face sheet and the core during the fracture toughness tests. The recommended specimen thickness ratios for several testing methods are presented. Finally, the physical meaning of the derived condition is explained, and the effect of residual thermal stresses on the condition is discussed.

Mode I Fracture Toughness of a Tri-Layered Beam with Interfacial Crack

A tri-layered cracked beam under opening loading is developed for the interfacial fracture toughness measurement. Determination of the mode I strain energy release rate along the second and third layers of the tri-layered beam is carried out analytically. The analytical prediction of the strain energy release rate is validated with the finite element results. The influences of the layer thickness and Young’s modulus on the strain energy release rate are examined through a parametric study.

A non-contact, thermally-driven buckling delamination test to measure interfacial fracture toughness of thin film systems

Interfacial fracture toughness measurements of thin film-substrate systems are of importance in many applications. In the microelectronics industry, the interfacial adhesion between the dielectric-barrier-metal layers on a semiconductor chip is critical for chip reliability. In this paper, we propose a thermally-driven patterned buckling delamination test that does not use a pre-existing weak interface. The test relies on causing a patterned film to debond from its substrate by inducing a compressive stress due to heating of the film on a thick silicon substrate. The compressive stress causes the film to buckle and debond from the substrate. A model for the propagation of the buckling-induced debond is then developed to estimate interfacial fracture toughness. The efficacy of the thermally-driven buckling test is demonstrated on a model Al/SU8/Si film-substrate system wherein the Al film debonds along its interface to SU8. The interfacial toughness of the Al/SU8 interface is estimated using the proposed test and is compared to the toughness for the same system obtained using an alternative test with a weakened interface to validate the developed elastic–plastic model for buckling-induced debond propagation.

Silicon and Nanoscale Metal Interface Characterization Using Stress-Engineered Superlayer Test Methods

Thin film layers are utilized in emerging microelectronics, optoelectronics, and microelectromechanical systems (MEMS) devices. Typically, these thin film layers are composed of different materials with dissimilar properties. A common mode of failure for thin films is delamination caused by external loading or intrinsic stress present in the materials. To characterize bonded thin film material systems, it is necessary to measure the interfacial fracture toughness. When material thicknesses approach micro- and nanoscales, interfacial fracture toughness measurement is a challenging task. Accordingly, innovative test techniques need to be developed to study interfacial fracture parameters. The ongoing research at Georgia Institute of Technology is developing fixtureless delamination test techniques that can be used to measure interfacial properties of micro- and nanoscale thin films. The single substrate decohesion test (SSuDT) and the single-strip decohesion Test (SSDT) are such fixtureless tests under development. In these tests, a thin film interface material of interest is deposited on a substrate. Then, delamination is driven by a superlayer material on top of the interface material. This superlayer material is sputter deposited and has high intrinsic stress. A deposited release layer material allows for the contact area between the interface material and the substrate to be controlled. These tests differ in geometry, but share the same generic methodology and can be used for a number of material systems over a wide range of mode mixities. This paper presents the methodology and implementation of the SSuDT and SSDT tests and compares results to better understand their scope. A case study of the interfacial fracture toughness as a function of mode mixity for titanium and silicon interface was performed.

Use of stress-engineered material layers for the measurement of interfacial fracture toughness of nanoscale thin films

A new technique employing stress-engineered material layers for the measurement of interfacial fracture toughness of nanoscale thin films has been developed. This technique has been implemented to measure the interfacial fracture toughness of patterned Titanium (Ti) film on a Silicon (Si) substrate. This test employs a stress-engineered super layer deposited on Ti to provide the energy for the delamination propagation of Ti thin film from the Si substrate. The amount of energy available for the delamination propagation is varied by depositing a selectively-etchable thin release layer between the film strips and the substrate. By designing a decreasing area of the release layer, it is possible to arrest the delamination at a given location, and the interfacial fracture toughness or the critical energy release rate can be found at the location where the delamination ceases to propagate. For Ti film with thickness of 90 nm, the results show that the interfacial fracture toughness of Ti/Si ranges from 3.45 to 5.70 J/m² when the mode mixity increases from 6.8 to 38.4°. The methodology presented in this paper is generic in nature, and can be used to measure the process-dependent interfacial fracture toughness of various nanoscale thin film interfaces.

Interfacial Fracture Toughness for Film on Substrate Determined by Indentation Method

Indentation method was used to determine the interfacial fracture toughness of epoxy coating on aluminum substrate. Tensile testing followed by finite element analysis was also performed to determine the interface fracture toughness. Fracture toughness values determined by two methods were consistent, giving reliability to indentation method for interfacial fracture toughness measurement.

Adhesion properties of polymethylsilsesquioxane based low dielectric constant materials by the modified edge lift-off test

Delamination occurring during the chemical and mechanical planarization process or wire bonding steps in packaging is a fundamental issue in integrating of low dielectric constant (low-k) materials into the multilayer structures of semiconductor chips. Since it is known that low adhesion strength is mainly attributed to the failure phenomenon, the measurement of interfacial fracture toughness is critical to provide a quantitative basis in the choice of the materials. In this study, a modified edge lift-off test was adopted to measure the fracture toughness of polymethylsilsesquioxane based low-k materials with various chemical and physical structures. Interfacial fracture toughness was improved by adding multi-functional monomers to methylsilsesquioxane monomers or by increasing the percentage of functional end groups inside the prepolymers. In addition, the change in curing conditions and thickness influenced the adhesion performance presumably by changing the morphology of low-k materials.

Interfacial Fracture Toughness Measurement of Thick Ceramic Coatings by Indentation

The basic requirement for the use of a ceramic coating is sufficient adhesion to its substrate. A measure of the adhesive properties of a coating is the interfacial fracture toughness. The test method applicable for interfacial fracture toughness measurements depends on the mechanical properties of the material system and the geometry of the test piece. In this work, indentation methods have been evaluated for the estimation of the fracture toughness of ceramic thermal barrier coatings on metallic substrates. Coatings of 100 to 300 µm thickness were applied by electron beam – physical vapour deposition. The performed test types were Vickers indentation at the interface of polished cross sections of the coating system and Rockwell indentation with a brale C indenter, penetrating the coating perpendicular to the surface. Both tests generate delamination, in which the delamination crack length corresponds to the interfacial fracture toughness. Fracture surfaces and cross sections of the fractured coatings were investigated by optical and scanning electron microscope. Determined fracture toughness values are discussed with respect to the loading conditions in the test and the fracture process – i.e. interaction between indenter and coating system and the crack propagation path.

다구찌 방법에 의한 유리-실리콘 양극접합 계면의 파괴인성치 측정 및 양극접합공정 조건에 따른 접합강도 분석

Anodic bonding process has been quantitatively evaluated based on the Taguchi analysis of the interfacial fracture toughness, measured at the interface of anodically bonded silicon-glass bimorphs. A new test specimen with a pre-inserted blade has been devised for interfacial fracture toughness measurement. A set of 81 different anodic bonding conditions has been generated based on the three different conditions for four different process parameters of bonding load, bonding temperature, anodic voltage and voltage supply time. Taguchi method has been used to reduce the number of experiments required for the bonding strength evaluation, thus obtaining nine independent cases out of the 81 possible combinations. The interfacial fracture toughness has been measured for the nine cases in the range of 0.03-6.12 J/m². Among the four process parameters, the bonding temperature causes the most dominant influence to the bonding strength with the influence factor of 67.7%. The influence factors of other process parameters, such as anodic voltage and voltage supply time, bonding load, are evaluated as 18%, 12% and 2.3%, respectively. The maximum bonding strength of 7.23J/㎡ has been achieved at the bonding temperature of 460℃ with the bonding load of 45gf7㎠, the applied voltage of 600V and the voltage supply time of 25minites.

Experimental evaluation of anodic bonding process based on the Taguchi analysis of interfacial fracture toughness

Anodic bonding process has been quantitatively evaluated based on the Taguchi analysis of the interfacial fracture toughness, measured at the interface of anodically bonded silicon–glass bimorphs. A new test specimen with a pre-inserted blade has been devised for interfacial fracture toughness measurement. A set of 81 different anodic bonding conditions has been considered and included three different conditions for each of four process parameters: bonding load, bonding temperature, anodic voltage and voltage supply time. The Taguchi method has been used to reduce the number of experiments required for the bonding strength evaluation, thus obtaining nine independent cases out of the 81 possible combinations. The interfacial fracture toughness has been measured for the nine cases in the range of 0.03∼6.12 J/m 2. Among the four process parameters, the bonding temperature causes the most dominant influence to the bonding strength with the influence factor of 67.7%. The influence factors of other process parameters, such as anodic voltage and voltage supply time, bonding load, are evaluated as 18%, 12% and 2.3%, respectively. The maximum bonding strength of 7.23 J/m 2 has been achieved at the bonding temperature of 460°C with the bonding load of 45 gf/cm 2, the applied voltage of 600 V and the voltage supply time of 25 minites.

Measurement of Interfacial Fracture Toughness Under Combined Mechanical and Thermal Stresses

The interfacial fracture resistance of a polyimide passivation/underfill interface was measured using an asymmetric double cantilever beam technique. A thin layer of polyimide was coated on a standard epoxy as one of the beams and an underfill was flowed over the polyimide film and cured to form the second beam. The relative contributions of the thermal and mechanical stress were systematically varied by varying the thickness ratio of the two beams. The results indicate that the effect of thermal residual stress must be taken into account in the calculation of critical strain energy release rate to obtain the interfacial fracture energy.

Modifying a Polystyrene/Poly(methyl methacrylate) Interface with Poly(styrene-co-methyl methacrylate) Random Copolymers

Joints of polystyrene (PS) and poly(methyl methacrylate) (PMMA) modified with ∼50 nm of poly(styrene-co-methyl methacrylate) random copolymer [P(S-ran-MMA)] were investigated. Copolymers having styrene compositions of fS = 0.48 and fS = 0.73 were used. Transmission electron microscopy reveals that the copolymers phase separate to form a distinct layer at the joint such that there is an interface with each homopolymer. Interfacial fracture toughness measurements, using the asymmetric double cantilever beam geometry, show a strong effect of the PS to PMMA sheet thickness ratio; that is, the phase angle influences the measured interfacial toughness. Reflection infrared spectroscopy on fracture surfaces indicates that the crack propagates at or near the PS/copolymer interface for all thickness ratios and for both copolymers. In-plane crazing was not observed in front of the crack tip for these systems. Rather, strengthening appears to be exclusively a consequence of oblique crazes in the more compliant PS sheet which form at 45° or 135° relative to the crack direction. Joints modified with P(S0.73-ran-MMA) exhibit denser oblique crazes than those modified with P(S0.48-ran-MMA), resulting in a higher measured fracture toughness at all sheet thickness ratios or phase angles.

Reactive Reinforcement of Polystyrene/Poly(2-vinylpyridine) Interfaces

The effect of reactive compatibilization on adhesion at the interface between the immiscible polymers polystyrene and poly(2-vinylpyridine) was investigated. Reactivity was incorporated into the system by lightly sulfonating the polystyrene component, providing acid functionality which is coreactive with the basic pyridine nitrogen. The interaction between sulfonic acid and pyridine groups generates interchain cross-links and therefore compatibilizing graft copolymers. The reinforcing effects of in-situ copolymer generation on the interface were quantified by evaluating interfacial adhesion through measurement of interfacial fracture toughness using the asymmetric double cantilever beam test. Interfacial properties were assessed as a function of the concentration of sulfonic acid groups present. The results show that the toughness of the interface is dramatically improved by the incorporation of reactivity, increasing from ∼2 J/m2 in the unreactive system up to >150 J/m2 in the reactive system. At long re...