Pre-cracking technique for fracture mechanics experiments along interface between thin film and substrate

Utilizing the difference in interface strength due to fabrication process, a technique for producing a sharp pre-crack between a thin film and a substrate is proposed. A cracked specimen for examining fracture toughness of interface between a sputtered copper (Cu) thin film and silicon (Si) is made by the method. A vacuum-evaporated Cu thin film, which has poor adhesion to Si, is inserted between the sputtered Cu thin film and the Si substrate as a release layer. The release layer debonds from the Si substrate at very low load, and the sharp pre-crack is successfully introduced along the interface. Using the pre-cracked specimen, the interface fracture toughness test is conducted and the critical J-integral, J_C, is evaluated as about 1 J/m² for the sputtered Cu/Si interface.     

Simulation of the creep damage behavior of thin film/substrate systems by bending creep tests

The purpose of this paper is to understand the creep damage properties of thin film/substrate systems by bending creep tests. To this aim, a numerical study has been performed on the creep damage development of the thin film/substrate systems by implementing the Kachanov–Rabothov damage law into finite element models. The work may shed some light on the influence of the modulus ratio of the substrate to the thin film, the thickness of the thin film and the bending load. Finite element method (FEM) results show that three obviously damaged zones are found. The first is at the edge of the loading pin, the second is at the interface between the film and the substrate ahead of the loading pin edge, the last is at the edge of the supporting pin A. The influence of the modulus ratio of the substrate to the thin film on the bending creep damage is not obvious at the preliminary stage of creep time. However, with the lapse of time, the damage rate decreases with the increase of modulus ratio of the substrate to the thin film. The change of the thickness of the thin film and the bending load also influence the creep damage behavior of the thin film/substrate systems.     

Nanoindentation and nanoscratch behaviour of reactive sputtered deposited W–S–C film

Transition metal dichalcogenides having layered structure are promising self lubricating film and can be considered as substitute for carbon based films in several varieties of environmental conditions. The macrotribological properties of these films are studied extensively and are fairly well understood. However, mechanical and tribological behaviour of these films in millinewton load range have hardly been reported. Study of mechanical and tribological properties at applied load in the millinewton range is useful for possible application related to microelectro mechanical systems or micromechanical assemblies. In view of the above, the present work is undertaken to understand the indentation behaviour and scratch behaviour under constant and low applied load of reactive sputtered deposited W–S–C thin films. Towards that purpose, W–S films containing various amount of C are deposited on 100Cr6 steel using a radio frequency magnetron sputtering unit. The load vs. displacement curves of all these films are generated for four different loads to assess the load effect, substrate effect and size effect on the hardness and the load displacement curves of these films. Curves showing the variation of load as a function of the square of displacement are also evaluated in order to understand deformation and fracture mechanisms of these films and the interface between various microstructures of these films. The scratch behaviour of these films under constant load is determined to examine the friction and wear performance. The results show that the film containing 54 at.% carbon has the maximum hardness and the minimum scratch depth. In contrast, the minimum friction coefficient is exhibited by the film containing the maximum carbon.     

Three-dimensional finite element analysis of fracture modes for the pull-off test of a thin film from a stiff substrate

A three-dimensional geometrically nonlinear finite element analysis model is presented to study the interfacial delamination for the pull-off test of a thin film strip debonded from a stiff substrate. The strain energy release rates of all three modes (mode I, mode II, and mode III) along the debond front are considered and calculated to investigate the mixed fracture modes for the entire deformation regime from bending plate to stretching membrane. These results indicate that the individual strain energy release rates and the total energy release rate vary along the width of the debond front and strong three-dimensional edge effects exist near the free edges of the film. Interestingly, residual stress also plays an important role in controlling mixed fracture modes and the variation of the energy release rates. Finally, the three-dimensional finite element model is compared with an analytical solution developed earlier. The three-dimensional finite element model is found to provide additional insights for interfacial delamination for the pull-off test.     

The effect of substrate wettability on the breakdown of a locally heated fluid film

The effect of the equilibrium contact angle of wetting on the dynamics of the dry patch propagation and on the critical heat flux upon the breakdown of a water film that is heated locally from the substrate side is studied experimentally. The equilibrium contact angle is varied from 27° ± 6° to 74° ± 9° (with no changes in the thermophysical properties of the system) through the use of different types of surface grinding. The studies are performed for three flow modes: (a) a fluid film that freely flows down along a substrate with an inclination of 5° to the horizon, (b) a film that moves along a horizontal substrate under the influence of hydrostatic pressure, and (c) a static film on a horizontal substrate. It is found that the substrate wettability has a significant effect on the dry patch propagation rate and its final size in all these cases, but has almost no effect on the threshold heat flux at which the breakdown of a film occurs.

     

Wedge indentation of a thin film on a substrate based on micromorphic plasticity

In this article, we use the small strain micromorphic plasticity (MP) to study the wedge indentation of a thin film on a substrate and find qualitative agreement with experiments. A two-dimensional plane strain finite element formulation of the entire MP theory framework is outlined. The generalization of the radial return method for modeling the elasto-plastic deformation is presented. The numerical results show that the MP theory is capable of describing the initial fall in hardness at small depth of indentation and then the rise at larger depth for a soft film on a hard substrate. The indentation force and hardness increase with decreasing film thickness for a given depth. It is also shown that the hardness falls monotonically as the indentation depth increases and never approaches a constant for a hard film on a soft substrate. Contrary to the soft film/hard substrate system, the force and hardness diminish with decreasing film thickness for a given depth. Besides, the influences of internal length scale and hardening modulus of the film on hardness predictions are investigated.     

Delocalizing strain in a thin metal film on a polymer substrate

Under tension, a freestanding thin metal film usually ruptures at a smaller strain than its bulk counterpart. Often this apparent brittleness does not result from cleavage, but from strain localization, such as necking. By volume conservation, necking causes local elongation. This elongation is much smaller than the film length, and adds little to the overall strain. The film ruptures when the overall strain just exceeds the necking initiation strain, ε_N, which for a weakly hardening film is not far beyond its elastic limit. Now consider a weakly hardening metal film on a steeply hardening polymer substrate. If the metal film is fully bonded to the polymer substrate, the substrate suppresses large local elongation in the film, so that the metal film may deform uniformly far beyond ε_N. If the metal film debonds from the substrate, however, the film becomes freestanding and ruptures at a smaller strain than the fully bonded film; the polymer substrate remains intact. We study strain delocalization in the metal film on the polymer substrate by analyzing incipient and large-amplitude nonuniform deformation, as well as debond-assisted necking. The theoretical considerations call for further experiments to clarify the rupture behavior of the metal-on-polymer laminates.     

Effect of substrate roughness on the contact damage of thin brittle films on brittle substrates

The effect of substrate and surface roughness on the contact fracture of diamond-like carbon coatings on brittle soda-lime glass substrates has been investigated. The average surface roughness (R_a) of the examined samples ranged from 15 nm to 571 nm. Contact damage was simulated by means of spherical nanoindentation, and fracture was subsequently assessed by focused ion beam microscopy. It was found that, in the absence of sub-surface damage in the substrate, fracture occurs in the coating in the form of radial, and ring/cone cracks during loading, and lateral cracks during unloading. Increasing the surface roughness results in a decrease in the critical load for crack initiation during loading, and in the suppression of fracture modes during unloading from high loads. When sub-surface damage (lateral cracks) is present in the substrate, severe spalling takes place during loading, causing a large discontinuity in the load-displacement curve. The results have implications concerning the design of damage-tolerant coated systems consisting of a brittle film on a brittle substrate.     

Equilibrium depth and spacing of cracks in a tensile residual stressed thin film deposited on a brittle substrate

The depth and spacing of cracks in a tensile residual stressed thin film bonded on a brittle substrate are analyzed thermodynamically using the minimum energy theorem on the basis that the film has the same mechanical properties as the substrate. The results show that the cracks penetrate into the substrate. Simple and approximate relationships between three dimensionless parameters, i.e., the normalized crack depth and spacing, and the cracking resistance number, are derived, which determine the fracture behavior of the film.     

Nanoindentation studies of thin film coated systems

In this paper we report the results of ultralow load indentation (“nanoindentation”) tests carried out on a range of thin film coated systems. In addition to characterizing the properties of each system at high resolutions, a further principal aim has been to explore the extent to which it is possible to generalize the nanoindentation response of the various systems in terms of the relative mechanical properties of the coating and the substrate. In this case we classify the substrate and coating in terms of the relative elastic and plastic responses (i.e. the ratio of yield stress to Young's modulus). Some of the different possible responses are shown together with the transitions in response (elasticity-dominated to plasticity-dominated and vice versa) which can occur. We also report on the various steps which can occur in loading curves and our attempts to relate them, via high resolution scanning electron microscopy, to the fracture patterns (through-thickness cracks and interfacial cracks) within and around the indentations.     

X-ray section topographs of a vapour-grown diamond film on a diamond substrate

X-ray section topographs of a diamond thin film deposited onto a diamond substrate were taken by using a four-circle diffractometer constructed for synchrotron radiation use. The topographs of the 111 reflection revealed a flattened hexagon and parallel spikes at the edges of the hexagon. The former corresponds to the section of the diamond substrate and the latter to the deposited thin film. In the image of the diamond substrate, many triangular contrasts were observed. These contrasts were attributed not to any inner texture of the substrate but to narrow cracks running in the deposited thin film, because of the identity of the patterns in the topograph with that observed on the surface of the substrate by optical microscopy and scanning electron microscopy. The parallel arrangement of the spikes, i.e. the image of the deposited thin film, may indicate the presence of residual strain in the thin film owing to the coherent contact between the thin film and the substrate.     

Chromium silicide film on ceramic substrate for pressure measurement

In this paper we present a device, based on amorphous silicon technology, fabricated on ceramic substrate able to perform pressure measurement with both good linearity and sensitivity. The active material of the sensor is a thin film (below 5 nm) of chromium silicide formed at room temperature on an n-type amorphous silicon layer. Sensors with different shapes (square and rectangular) and positions on the ceramic membrane have been characterized. Sensitivity in the order of 400 μV/kPa has been achieved.     

Evaluation of the elastic modulus of thin film considering the substrate effect and geometry effect of indenter tip

A method using finite element method (FEM) is proposed to evaluate the geometry effect of indenter tip on indentation behavior of film/substrate system. For the nanoindentation of film/substrate system, the power function relationship is proposed to describe the loading curve of the thin film indentation process due to substrate effect. The exponent of the power function and the maximum indentation load can reflect the geometry effect of indenter and substrate effect. In the forward analysis, FEM is used to simulate the indentation behavior of thin film with different apex angles of numerical conical indenter tip, and maximum indentation load and loading curve exponent are obtained from the numerical loading curves. Meanwhile, the dimensionless equations between the loading curve exponent, the maximum load, elastic properties of film/substrate system and apex angle of indenter are established considering substrate effect. In the reverse analysis, a nanoindentation test was performed on thin film to obtain the maximum indentation load and the loading curve exponent, and then the experimental data is substituted into the dimensionless equations. The elastic modulus of thin film and the real apex angle of indenter can be obtained by solving the dimensionless equations. The results can be helpful to the measurement of the mechanical properties of thin films by means of nanoindentation.     

Hydrogen effect on fracture toughness of thin film/substrate interfaces

The effect of hydrogen on the interface fracture toughness of two nano-film/substrate structures, Ni/Si and Cu/Si, were evaluated using four-point bend specimens with and without hydrogen charging. Hydrogen typically decreases the fracture toughness of materials. However, we found in this study that the interfacial toughness between the Ni film and the Si substrate increased due to the presence of hydrogen, while that of Cu/Si decreased. Nanoindentation experiments for the Ni and Cu films revealed that local plasticity in the Ni and Cu films is promoted by the charged hydrogen. The critical stress intensity at the Ni/Si interface crack considering the plasticity of Ni, namely the true fracture toughness, is scarcely influenced by the existence of hydrogen. The apparent increase in fracture toughness of the Ni/Si interface is due to the large stress relaxation near the crack tip caused by softening due to the presence of hydrogen. Although the promotion of plastic deformation of Cu relaxes the stress intensity at the Cu/Si interface crack, the apparent interfacial toughness still decreases because of the significant decrease in the true toughness due to the presence of hydrogen.     

锌漆薄膜/2Cr13钢基体系统薄膜裂纹试验与模拟

本文采用试验和数值模拟方法研究锌漆薄膜/2Cr13钢基体系统薄膜开裂和裂纹扩展情况。创新性地使用三点弯曲试验结合声发射技术监测到薄膜裂纹萌生时刻,并计算薄膜断裂韧性。采用扩展有限元法研究三点弯曲作用下薄膜裂纹扩展过程,发现模拟得到的荷载-位移曲线与试验曲线相吻合。模拟结果表明,薄膜裂纹尖端区域存在应力峰值,当该值达到损伤判据临界应力时,裂纹发生扩展。同时,对多种因素影响下薄膜周期裂纹无量纲能量释放率进行分析,发现薄膜厚度一定时,薄膜相对于基体刚度越大,半无限基体上薄膜裂纹前缘达到稳定状态时对应的基体厚度与裂纹间距越大。      

Deterministic positioning of three-dimensional structures on a substrate by film growth

A process to fabricate three-dimensional crystalline structures at controlled locations on a substrate during film growth and annealing is demonstrated. Low-energy electron microscopy reveals that silver is transported to regions on a tungsten surface with closely spaced atomic steps. By controlling the substrate topography using a focused ion beam to machine small holes, this general mechanism produces an array of cylinders as a silver film dewets the substrate.     

Shear-horizontal piezoelectric waves in an aluminum nitride film on a silicon substrate

The propagation of shear-horizontal waves in a piezoelectric film of aluminum nitride on a silicon substrate is studied. Three different electrode configurations are considered for thin film acoustic wave resonator application. A theoretical analysis is performed. The equations of linear piezoelectricity and anisotropic elasticity are used for the film and the substrate, respectively. Real and imaginary dispersion curves as well as electromechanical mode shapes are obtained. The effects of electrode configuration on the distribution of the electromechanical fields and the dispersion curves of long thickness-twist waves as well as energy trapping are examined.     

Interface fracture toughness evaluation for MA956 oxide film

Microelectronics, optoelectronics, and thermal barrier coating technologies are dependent on a thin or thick film of one material deposited onto a substrate of a different material. Fabrication of such a structure inevitably gives rise to stress in the film due to lattice mismatch, differing coefficients of thermal expansion, chemical reactions, and/or other physical effects. Therefore, the weakest link in this composite system often resides at the interface between the film and substrate. In order to assume the long-term reliability of the interface, the fracture behavior of the material interfaces must be known. A new approach of using a spiral notch torsion fracture toughness test system for evaluating interface fracture toughness is described. This innovative technology was demonstrated for oxide scales formed on high-temperature alloys of MA956. The estimated energy release rate (in terms of J-integral) at the interface of the alumina scale and MA956 substrate is 3.7 N-m/m², and the estimated equivalent Mode I fracture toughness is 1.1 MPa √m.     

Cyclic heating of a film on a substrate

A mathematical model is presented for cyclic heat treatment of a film on a substrate, and heating curves have been constructed for a thin silicon wafer heated by an infrared source and cooled by forced convection.     

A numerical fracture analysis of indentation into thin hard films on soft substrates

In this paper, finite element simulations of spherical indentation of a thin hard film deposited on a soft substrate are carried out. The primary objective of this work is to understand the mechanics of fracture of the film due to formation of cylindrical or circumferential cracks extending inwards from the film surface. Also, the role of plastic yielding in the substrate on the above mechanics is studied. To this end, the plastic zone development in the substrate and its influence on the load versus indentation depth characteristics and the stress distribution in the film are first examined. Next, the energy release rate J associated with cylindrical cracks is computed. The variation of J with indentation depth and crack length is investigated. The results show that for cracks located near the indenter axis and at small indentation depth, J decreases over a range of crack lengths, which implies stability of crack growth. This regime vanishes as the location of the crack from the axis increases, particularly for a substrate with low yield strength. Finally, a method for combining experimental load versus indentation depth data with simulation results in order to obtain the fracture energy of the film is proposed.