Interface Flaws Research Articles

Nucleation of Voids in Thin-Film Interconnects Through Crystallographic Slip

ABSTRACTA two-dimensional void nucleation model in thin-film metal interconnects is proposed. The model is based on the evolution of stress and deformation fields obtained from numerical modeling. Interface flaws between the metal and the surrounding dielectric are assumed to exist. A unique pattern of shear stress resolved on the slip systems of the metal is found. A model of dislocation slip is constructed in accord with the evolution of the shear mode. The mechanism of crystallographic slip is such that lateral thinning of the metal line at the debond region, together with the slip steps produced at the edges of debond lead to a net transport of atoms away from the interface defect, and a physical void is thus formed.

The interaction between a crack and a particle cluster

A numerical method has been developed to study the interaction between a crack and second phase particles in a discontinuously reinforced composite material. The simulation is achieved using a ‘dual’ boundary integral method, coupled with a maximum energy release rate criterion for determining the direction of crack propagation. The method has been applied to a composite material composed of components having the elastic properties of Aluminium (matrix) and Silicon Carbide (reinforcement). In particular, the method is used to investigate the crack trajectory and energetics as it interacts with a single particle and with clusters consisting of two particles or a random distribution of ten particles. It is found that although the energy release rate is affected by the particle(s) at relatively large distances, the crack trajectory is not substantially altered until the crack is very close to the particle(s). A pre-existing interface flaw is observed to attract the crack and substantially increase the energy release rate.

Effects of stress ratio on fatigue life of carbon-carbon composites

Cyclic loading causes cumulative damage and therefore degrades the inelastic properties of composite materials. Present work investigates the damage development under tension-tension fatique, and the effect of stress ratio on the fatigue life of carbon-carbon ( C C ) composites at room temperature at a frequency of 3 Hz. The fatigue damage has been identified through ultrasonic non-destructive technique, optical microscopy and scanning electron microscopy. From the S- N curve it has been observed that the endurance strength of C C composite is quite high; approximately 85% of the ultimate tensile strength. The fatigue life of C C composites has also been observed to increase with the stress ratio. Matrix cracks, filament splitting within the yarns, complete delamination and the nucleation of the interfacial flaws have been identified as the failure mechanisms during the fatigue tests. On the other hand, the failure modes during the static test were found to be complete fiber fracture accompanied by partial delamination. A statistical fatigue life distribution for carbon-carbon composites has also been presented in this paper.

The effect of flaws on the propagation of cracks at bi-materials inferfaces

This study uses the boundary element method to investigate the effects of interfacial flaws on the propagation of a crack at or near an interface between two elastic, isotropic materials. These calculations reveal that flaws tend to deflect cracks approaching the interface from their original trajectory if the distance between the flaw and crack tip is small in relation to the flaw size. It is found that the higher the elastic mismatch parameter, the more crack trajectories are attracted to flaws. Other calculations show that materials with interfacial flaws have a significantly increased tendency to deflect cracks along the interface as compared to defect-free materials.

Interface formation and strength in ceramic-metal systems

Optimizing the mechanical integrity of ceramic-metal bonds requires an understanding of the influence of numerous variables on strength. These include processing, interfacial chemistry and morphology, thermal and elastic mismatch, and metal yield strength and thickness which, in turn, dictate considerations of residual and applied stress distributions, flaw populations in the ceramic and at the interface, the interfacial fracture resistance, and the resultant crack path. The influence of metal plasticity is especially strong when low yield strength metals are used. Localized plasticity may blunt interface flaws and limit stress concentrations that develop at and near the interface and can induce premature brittle fracture of either the interface or ceramic. The driving force to form a ceramic-metal interface is the yield in free energy when intimate contact is achieved between the ceramic and metal, and is characterized by the work of adhesion, W[sub ad]. The interfacial fracture energy is typically much larger owing to dissipative processes, including metal plasticity, but the extent of these may scale with W[sub ad]. It has been widely considered that a low metal contact angle, as observed in sessile drop experiments, is a prerequisite for the formation of a strong ceramic-metal bond. But recent experience has shownmore » that bonds between ceramics and ductile metals can be strong despite characteristic contact angles > 90[degree]. Some correlation between low contact angle and good bond strength may be, in part, a result of more easily obtaining a high fraction of bonded area, and low stress concentrations. Therefore, it is anticipated that a fundamental correlation exists between good interfacial bonding and high strength. This paper makes a preliminary attempt to distinguish the relative importance of the various primary and secondary ways such bonding can be important, a distinction which rarely has been articulated clearly.« less

Stresses in coated fiber-reinforced composites with interface flaw

Stresses in coated fiber-reinforced composites with interface flaw

Adhesion of electrolessly deposited Ni(P) layers on alumina ceramic. I. Mechanical properties

The adhesion mechanism of electrolessly deposited Ni(P) on alumina ceramic substrates has been investigated. The adhesion was measured by direct pull-off tests and by 90° peel tests, which provided information on adhesion strength and fracture energy, respectively. An assessment is made of quantitative aspects of both adhesion measurement techniques. The observed mechanical behavior is rationalized using the Griffith–Irwin theory. Two types of alumina substrates with different roughnesses were used. Ni(P) was deposited from two types of electroless Ni(P) solutions, one with glycine and one with acetate as the complexing agent. The fracture surfaces were analysed with scanning electron microscopy, combined with energy dispersive analysis of x rays. The adhesion strength of the glycine-type Ni(P) was much higher and the fracture energy was lower than that of the acetate-type Ni(P), for both substrate types. This implies that the difference in adhesion strength is not caused by differences in interfacial chemical bonding, but rather by differences in flaw sizes. Since high adhesion strength was measured on smooth substrates, along with low peel strength, it is concluded that strong adhesion can be obtained without making use of mechanical interlocking. Further research should be aimed at controlling the interfacial flaw sizes.

Study of cracking and spalling of Cr2O3 scale formed on Ni-30Cr alloy

Acoustic emission (AE) was used to monitor the scale cracking and spalling of Ni-30Cr alloy samples that had been oxidized at 1000°C for 2 hours or 20 hours then cooled slowly to room temperature in two different ways: natural furnace cooling or constant-rate cooling at a rate of about 19°C/ hour. The morphology of the remaining scale and spalled areas was observed by scanning electron microscope (SEM). The AE results show that the scale started to crack and spall continuously during cooling when a certain temperature was reached. The cracking and spalling continued over a temperature range, indicating that there is distribution of the critical-fracture stresses. The SEM analysis shows that the interface between the Cr2O3 scale and the Ni-30Cr alloy substrate is relatively weak. Under this condition, a mathematical model proposed by A. G. Evans can be used to estimate the size of the spalled areas and the critical size of the interfacial flaws required to produce buckling. Measurements of the diameter of spalled areas from SEM images yielded a range of values that were well described by a normal distribution. By applying the Evans model to the AE data, normal distributions of the diameter of spalled areas were estimated, which were 2–3 times larger than the measured sizes. This lack of agreement may be because the model did not take into account stress-concentration effects of other defects, such as oxide grain boundaries, small porosity in the scale, or small V-notches between individual oxide crystals at the scale surface. It is suggested that the distribution of the interfacial-flaw sizes causes the distribution of the critical-fracture stresses that are inferred from the AE experiments.

Numerical analysis of scattering by interface flaws: 44721 Xu, Y.; Achenbach, J.D. Northwestern University, Evanston, Illinois (United States), AD-A212 996/3/GAR, 10 pp. (15 Aug. 1989)

Numerical analysis of scattering by interface flaws: 44721 Xu, Y.; Achenbach, J.D. Northwestern University, Evanston, Illinois (United States), AD-A212 996/3/GAR, 10 pp. (15 Aug. 1989)

Numerical analysis of scattering by interface flaws: 45934 Xu, Y.; Achenbach, J.D. Review of Progress in Quantitative Nondestructive Evaluation, Brunswick, Maine (United States), 23–28 Jul. 1989. Vol. 9A, pp. 93– 100. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1990)

Numerical analysis of scattering by interface flaws: 45934 Xu, Y.; Achenbach, J.D. Review of Progress in Quantitative Nondestructive Evaluation, Brunswick, Maine (United States), 23–28 Jul. 1989. Vol. 9A, pp. 93– 100. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1990)

Using Weibull Statistics to Analyze Ion Beam Enhanced Adhesion as Measured by the pull Test

ABSTRACTThe adhesion of iron films to single crystal Al2O3 substrates was investigated using a pull test. Chromium (300 keV) or nickel (340 keV) ions were implanted to a fluence of 1 × 1015 ions-cm-2 after film deposition. The adhesion test results were widely scattered due to a random distribution of interfacial flaw sizes controlling the failure nucleation. Because Weibull statistics were developed to describe the failure probability due to a population of flaw-initiated cracks, the Weibull distribution was chosen to analyze the data. Modifications in the adhesion strength due to the ion implantation were reflected in the failure distributions. It was found that the chromium ions improved the adhesion of the Fe/Al2O3 system while the implantation of nickel did not.

New developments in the modelling of the hardness and scratch adhesion of thin films

Abstract Both hardness testing and scratch adhesion testing are now routinely used for coating evaluation, although the fundamental understanding of both processes is somewhat limited. To establish the hardness of thin coatings independent of substrate, the volume-law-of-mixtures hardness model has been applied to a large number of coating/substrate systems, and extensions to an earlier model have been developed including better deforming volume calculations and the ability to obtain best-fit coating parameters. In order to extend the understanding of the scratch test, the statistical nature of coating failure has been investigated by counting the number of individual failures at a given load for a fixed scratch length. The results are analysed in terms of the Weibull distribution and these are related to the surface roughness of the substrate prior to coating. With increased surface roughness both the critical load and Weibull parameters are reduced, reflecting a change in the distribution of interfacial flaws.

A theory for creep by interfacial flaw growth in ceramics and ceramic composites

The nucleation and growth of flaws along grain boundaries and interfaces are known to cause significant reductions in elastic moduli and to play an important role in determining the deformation characteristics of ceramic materials at elevated temperatures. This paper presents an analysis of the creep behavior of deteriorating elastic solids where the principal mechanism of deformation is the growth of intergranular or interfacial flaws. The changes in elastic moduli induced by the growth of internal damage are used to derive the stress exponent in the power-law creep regime. When the flaws advance at a rate which is proportional to the local normal stress or normal strain, a power-law creep exponent of 2 is predicted for short time, steady-state creep for a population of aligned slit cracks and randomly oriented penny-shaped cracks. For long-time creep, the variation of nonsteady state creep strain rate as a function of the far-field stress and time is explicitly determined. General solutions for creep strain rates are also presented for situations where the microcrack growth rate has a power-law dependence on the local normal stress or stress intensity factor. The predicted dependence of creep strain rate on the far-field stress, the progression of damage and the consequent reduction in elastic moduli, overall creep ductility, and implications pertaining to microstructural and temperature effects on creep are found to be in accord with a wide variety of experimental observations for ceramics and ceramic composites. The temperature, stress and material conditions for which the proposed mechanism is applicable are discussed and a general theory of creep damage in progressively microfracturing elastic brittle solids is developed.

High‐Temperature Failure of an Alumina‐Silicon Carbide Composite under Cyclic loads: Mechanisms of Fatigue Crack‐Tip Damage

Experimental results are presented on the mechanisms of tensile cyclic fatigue crack growth in an A1203‐33‐vol%‐SiC‐whisker composite at 1400°C. The ceramic composite exhibits subcritical fatigue crack propagation at stress‐intensity‐fator values far below the fracture toughness. The fatigue characterized by the stressintensity‐factor range, ΔK, and crack propagation rates are found to be strongly sensitive to the mean stress (load ratio) and the frequency of the fatigue cycle. Detailed transmission electron microscopy of the fatigue crack‐tip region, in conjunction with optical microscopy, reveals that the principal mechanism of permanent damage ahead of the advancing crack is the nucleation and growth of interfacial flaws. The oxidation of Sic whiskers in the crack‐tip region leads to the formation of a silica‐glass phase in the 1400°C air environment. The viscous flow of glass causes debonding of the whisker‐matrix interface; the nucleation, growth, and coalescence of interfacial cavities aids in developing a diffuse microcrack zone at the fatigue crack tip. The shielding effect and periodic crack branching promoted by the microcracks result in an apparently benefcial fatigue crack‐growth resistance in the A1203—SiC composite, as compared with the unreinforced alumina with a comparable grain size. A comparison of static and cyclic load crack velocities is provided to gain insight into the mechanisms of elevated temperature fatigue in ceramic composites.

The strength of ceramics bonded with metals

The strength of Al2O3 materials bonded using thin Pt layers is investigated. The measured strength levels are interpreted by conducting elastic/plastic stress analysis in conjunction with weakest link statistics. Comparisons between theory and experiment includes bond thickness effects, coupled with the role of interfacial flaws and the flow stress of the Pt.

The Role of Fracture Mechanics in Adhesion

AbstractThe problem of adhesion cannot be completely comprehended by consideration of interfacial properties alone. Other parameters such as geometry, applied and residual stresses, and the interfacial flaw population are equally important. The well-established field of fracture mechanics permits these aspects to be considered. One of the most important features of fracture mechanics is that it can provide a means of characterizing interfacial properties which does not depend upon the method of testing. It may also allow a predictive ability in determining failure and a rational explanation of what is known as “adhesive” or “cohesive” failure. These issues are addressed in conjunction with a discussion of the current limitations in the application of fracture mechanics to the topic of adhesion.

Structure, chemistry and diffusion bonding of metal/ceramic interfaces

A special technique is used for studying the mechanisms of diffusion bonding between metals and ceramics, particularly at Nb/Al 2O 3 interfaces. The Nb surfaces are prepared so that arrangements of well defined linear defects, about 10 μm wide and 1 μm deep, are included. Subsequently, bonding is performed under well defined conditions (temperature, time, vacuum, loading, cooling rate, …) and the change of the structure of the flaws is analyzed after decohesion of the partially bonded interface. It can be shown experimentally that a solute solution process of Al 2O 3 occurs at the interface when sufficiently high temperatures are used. The closing of the flaws depends on (i) the chemical reaction rate, (ii) the diffusion mechanisms in Nb, and (iii) the Al 2O 3 condensation at interface flaws. The chemical compositions of near-interface regions in Al 2O 3 and in Nb are determined by analytical electron microscopy. Direct lattice imaging is used for studying the atomic structure near the interface. Facets and dislocations at and near the interface can be identified.

Interface cracks in adhesively bounded lap-shear joints

A study on the elastic behavior of interface cracks in adhesively bonded lap-shear joints is presented. The problem is investigated by using a recently developed method of analysis based on conservation laws in elasticity for nonhomogeneous solids and fundamental relationships in fracture mechanics of dissimilar materials. The formulation leads to a pair of linear algebraic equations in mixed-mode stress intensity factors. Singular crack-tip stress intensity solutions are determined directly by information extracted from the far field. Stress intensity factors and associated energy release rates are obtained for various cases of interest. Fundamental nature of the interfacial flaw behavior in lap-shear adhesive joints is examined in detail.

Rapid interface flaw extension with friction—I. Basic analysis

If the bonding failure is in shear and large compressive stresses are present initially, the extension of an interface flaw in a laminate may be characterized by relative slip between the flaw surfaces resisted by sliding friction. As an introduction to a study of this problem, the basic mathematical analysis for the case of a non-symmetric, uniformly extending flaw at the interface of perfectly bonded half-planes under an initially uniform in-plane shear and compression stress field is considered. Several results which place restrictions on this initial field and raise questions for future work are noted. In particular, the dynamic stresses at the flaw edges generally do not have the standard square-root singularity. However, a critical flaw edge speed may exist at which the singular behavior partially vanishes and square-root singularities are present.

Combined fracture and delamination: A simple idealized example

U nder static pre-stress, small voids located between constituent layers in a laminated composite may extend both as interface flaws and as cracks into the layers themselves. Thus, the material may fail due to the combined effects of delamination (separation) and fracture of the layers. As a first step in understanding this complex process, while minimizing mathematical difficulties, a simple idealized example of such a process is examined. A cruciform brittle crack-flaw combination is assumed to initiate at the interface of perfectly-bonded half-spaces under a uniform anti-plane shear field and to extend with constant speeds. When the crack and wave speeds satisfy a simple relation, straight-forward solutions to the resulting wave propagation problem can be obtained and several crackflaw interaction effects noted. In particular, the stress intensities and energy flux rates at the crack and flaw edges are studied.