Mixed-mode failure of thin films using laser-generated shear waves

A new test method is developed for studying mixed-mode interfacial failure of thin films using laser generated stress waves. Guided by recent parametric studies of laser-induced tensile spallation, we successfully extend this technique to achieve mixed-mode loading conditions. By allowing an initial longitudinal wave to mode convert at an oblique surface, a high amplitude shear wave is generated in a fused silica substrate and propagated toward the thin-film surface. A shear wave is obtained with amplitude large enough to fail an Al film/fused silica interface and the corresponding shear stress calculated from high-speed interferometric displacement measurements. Examination of the interfaces failed under mixed-mode conditions reveals significant wrinkling and tearing of the film, in great contrast to blister patterns observed in similar Al films failed under tensile loading.     

Tensile and mixed-mode strength of a thin film-substrate interface under laser induced pulse loading

Laser induced stress waves are used to characterize intrinsic interfacial strength of thin films under both tensile and mixed-mode conditions. A short-duration compressive pulse induced by pulsed-laser ablation of a sacrificial layer on one side of a substrate is allowed to impinge upon a thin test film on the opposite surface. Laser-interferometric measurements of test film displacement enable calculation of the stresses generated at the interface. The tensile stress at the onset of failure is taken to be the intrinsic tensile strength of the interface. Fused-silica substrates, with their negative nonlinear elasticity, cause the compressive stress wave generated by the pulse laser to evolve a decompression shock, critical for generation of the fast fall times needed for significant loading of surface film interfaces. By allowing the stress pulse to mode convert as it reflects from an oblique surface, a high amplitude shear wave with rapid fall time is generated and used to realize mixed-mode loading of thin film interfaces. We report intrinsic strengths of an aluminum/fused silica interface under both tensile and mixed-mode conditions. The failure mechanism under mixed-mode loading differs significantly from that observed under pure tensile loading, resulting in a higher interfacial strength for the mixed-mode case. Inferred strengths are found to be independent, as they should be, of experimental parameters.     

Mixed-mode interfacial adhesive strength of a thin film on an anisotropic substrate

The mixed-mode interfacial adhesion strength between a gold (Au) thin film and an anisotropic passivated silicon (Si) substrate is measured using laser-induced stress wave loading. Test specimens are prepared by bonding a fused silica (FS) prism to the back side of a 〈1 0 0〉 Si substrate with a thin silicon nitride (Si_xN_y) passivation layer deposited on the top surface. A high-amplitude stress wave is developed by pulsed laser ablation of a sacrificial absorbing layer on one of the lateral surfaces of the FS prism. Due to the negative non-linear elastic properties of the FS, the compressive stress wave evolves into a decompression shock with fast fall time. Careful selection of the incident angle between the pulse and the FS/Si interface generates a mode-converted shear wave in refraction, subjecting the Si_xN_y/Au thin film interface to dynamic mixed-mode loading, sufficient to cause interfacial fracture. A detailed analysis of the anisotropic wave propagation combined with interferometric measurements of surface displacements enables calculation of the interfacial stresses developed under mixed-mode loading. The mixed-mode interfacial strength is compared to the interfacial strength measured under purely tensile loading.     

激光诱导应力波测量薄膜界面强度研究进展

薄膜界面强度是影响多层薄膜装置性能的重要参数。激光诱导应力波技术是在可控制和非接触条件下定量测量薄膜界面强度最有效的技术之一。在采用高强度应力波短脉冲加载实现界面层裂的同时，通过光学测量薄膜自由面瞬态位移，并利用应力波理论计算得到临界界面强度。通过精确控制试样几何形状及尺寸，包括拉伸、剪切和复合型在内的各种界面加载模式都可以实现。本文对激光诱导应力波测量薄膜界面强度研究进展进行了综述，并特别强调了不同加载模式的实现方法，高强度超薄薄膜的界面测量技术，以及如何通过辨别薄膜自由面瞬态位移光学干涉信号中的某些特殊性征来实时判断和测量界面的层裂。     

评估TiN薄膜与基材结合的划痕试验及有限元模拟

通过有限元模型模拟划痕试验得到的结果表明∶切应力的起伏变化?膜/基界面处切应力差值?接触区附近膜层表面张应力?高载下的几种应力集中等,对膜/基体系的失效都有重要的作用.通过模型计算临界载荷下的膜/基界面处切应力差值,可用来评价膜层与基材的结合强度;提出了划痕试验中膜/基体系失效的2种机制.不同性能基材的TiN膜/基体系划痕试验结果,可验证本文有限元模拟的有效性,并表明临界载荷是膜/基结合强度?体系承载能力?内聚结合性能等的综合反映;低载往复摩擦磨损试验的结果进一步证实,用划痕试验的临界载荷评估膜/基结合强度具有局限性.     

Characterization of the fracture work for ductile film undergoing the micro-scratch

The interface adhesion strength (or interface toughness) of a thin film/substrate system is often assessed by the micro-scratch test. For a brittle film material, the interface adhesion strength is easily obtained through measuring the scratch driving forces. However, to measure the interface adhesion strength (or interface toughness) for a metal thin film material (the ductile material) by the micro-scratch test is very difficult, because intense plastic deformation is involved and the problem is a three-dimensional elastic-plastic one. In the present research, using a double-cohesive zone model, the failure characteristics of the thin film/substrate system can be described and further simulated. For a steady-state scratching process, a three-dimensional elastic-plastic finite element method based on the double cohesive zone model is developed and adopted, and the steady-state fracture work of the total system is calculated. The parameter relations between the horizontal driving forces (or energy release rate of the scratching process) and the separation strength of thin film/substrate interface, and the material shear strength, as well as the material parameters are developed. Furthermore, a scratch experiment for the Al/Si film/substrate system is carried out and the failure mechanisms are explored. Finally, the prediction results are applied to a scratch experiment for the Pt/NiO material system given in the literature. The project supported by the National Natural Science Foundation of China (19891180 and 19925211) and Bai Ren Plan of CAS     

石墨烯与柔性基底界面载荷传递的拉曼光谱实验

本文研究了CVD制备的大尺寸石墨烯与柔性PET基底在拉伸变形过程中切向界面载荷传递的问题,采用原位拉曼光谱实验给出了加载过程中石墨烯的正应变、正应力以及界面切应力的分布曲线。分析表明,石墨烯与PET基底间的载荷传递存在四个阶段,分别是初始阶段、粘附阶段、滑移阶段和界面脱粘破坏阶段。在此基础上,本文对50μm、140μm、270μm和600μm四种尺寸石墨烯试件的界面力学性能进行测量,得到了不同尺寸石墨烯试件的界面力学性能参数,并初步给出了基底变形引起的石墨烯切向界面粘接能的变化,同时分析了试件尺寸对石墨烯界面力学性能的影响。实验结果表明,石墨烯材料和柔性基底最大切应力与临界脱粘切向界面粘接能等界面强度指标受到尺寸的显著影响,尺寸越小切向界面强度越高,反之,尺寸越大则越低。     

Spall Characterization in Epoxy Via Laser Spallation

Background: The strength of materials under extreme dynamic loading conditions, such as in the case of shock wave loading, is assessed from their spallation characteristics. Under laboratory conditions, flyer plate impact, or sometimes laser-induced stress waves, is employed to instigate spall in a material. These methods are often combined with velocity interferometer system for any reflector (VISAR) technique for performing transient measurements. Although the VISAR can record the velocity of extremely fast-moving surfaces, it requires a complex optical setup and a specialized data reduction technique. Objective: In this study, a simpler approach is adopted by extending laser spallation method to determine the spall strength of epoxy, while performing in situ interferometric measurements, directly on top of thick epoxy films. Methods: The glass/epoxy test samples are prepared by transferring an aluminum coating on top of epoxy layers with different thicknesses. Laser-induced stress waves transmit across the substrate/film interface and induce subsurface failure in the epoxy at sufficiently high incident laser energy. The nature and magnitude of the waves are deciphered from the out-of-plane displacement histories of the top reflective sample surfaces, which are recorded by using a Michelson interferometer. Results: The interferometric data reveal the development of two (temporally) well-separated stress waves: an ablation-induced high-amplitude short-duration longitudinal pulse, which is referred to as the primary wave, and a secondary wave, which travels at a comparatively slower speed. The complex constructive interaction of the two waves develops a high-magnitude tensile stress region in the epoxy layer. The spall strength is quantified by superimposing the two stress wave histories associated with the critical energy fluence. Conclusions: The spall depths predicted from spatiotemporal wave travel analyses are in excellent agreement with the experimental observations. The newly adopted methodology estimates the spall strength of epoxy as 260?±?20 MPa.

     

Experimental and theoretical/numerical investigations of thin films bonding strength

A laser spallation facility has been developed to measure the strength of planar interfaces between a substrate and a thin coating. This quantity is a central requirement in contemporary thin film and protective coatings technology and its successful measurement should improve the scientific/technological potential for the design of advanced composites, protective coatings of composites that operate in hostile environments, and in joining of dissimilar materials. The technique involves impinging a laser pulse of ultra short duration on the rear surface of the substrate, which is coated by a thin layer of energy absorbing metal such as Sn and Pb. The explosive evaporation of the metallic layer, confined between a fused quartz crystal and the substrate, induces a compressive shock wave, which propagates through the substrate toward the material interface. Upon reflection from the free surface of the coating, the pressure pulse is converted into a tensile wave which, under certain conditions, can lead to spallation at the interface. It is shown by mathematical simulation that atomic bond rupture is the mechanism of separation in this experiment. Since the interaction of laser energy with matter is a complicated, highly non-linear process, our investigations, at first, were based on measurement of the pressure pulse generated by the threshold flux level that leads to spallation, by using a micro-electronics device with a piezo-electric crystal, and on computation of the tensile stress experienced at the material interface, by numerical simulation of the induced stress wave propagation. Several substrate/coating (ceramic/ceramic and ceramic/metal) systems have been investigated such as, 1–15 μm SiC by CVD, 1–4 μm TiC and TiN by PVD coatings on sapphire substrates, as well as 1–2 μm Au, Sn and Ag coatings by sputtering on sapphire, fused quartz and glass substrates. For identically prepared specimens, the measured threshold energy levels are reproducible, thus leading to reproducible bond strength values, while the spall size, as expected, is dependent on the laser pulse energy level. Finally, the bond strength values obtained are in very good agreement with similar data derived by direct experimental techniques based on Laser-Doppler-Interferometry.     

涂层/基体材料界面结合强度测量方法的现状与展望

界面结合强度是涂层/基体材料体系中的一项重要力学性能指标.而表征与评价涂层/基体材料的界面结合强度又得依靠实验 方法的测定.由于涂层/基体材料体系的多样性与复杂性, 至今还没有形成适合于测量这类材料的界面结合强度的标准方法. 目前, 常用来测量涂层/基体材料的界面结合强度的方法有：拉伸法、剪切法、弯曲法、划痕法、压入法等.本文就目前表征 与评价涂层/基体材料界面结合强度的测量方法做了综述, 讨论了它们的适用范围, 比较了它们的优势与不足.     

Interfacial stress analysis and prediction of debonding for a thin plate bonded to a curved substrate

This paper focuses on the analytical and numerical modeling of the interface between a rigid substrate with simple constant curvature and a thin bonded plate. The interfacial behavior is modeled by independent cohesive laws in the normal and tangential directions, coupled with a mixed-mode fracture criterion. The newly developed analytical model determines the interfacial shear and normal stress distributions as functions of the substrate curvature, during the various behavioral stages of the interface prior to the initiation of debonding. The model is also able to predict the debonding load and the effective bond length. In the numerical model the interface is modeled by zero-thickness node-to-segment contact elements, in which both the geometrical relationships between the nodes of the discretized problem and the interface constitutive laws are suitably defined. Numerical results and comparisons between the predictions of the two models are presented.     

EXPERIMENTAL INVESTIGATIONS OF STRENGTH,TOUGHNESS AND FAILURE MECHANISM OF THE METAL/EPOXY/METAL ADHESIVE SYSTEM

本文系统地开展了金属/环氧/金属胶结体系的强韧机理及失效行为实验研究,针对铝合金圆棒与铝合金圆棒通过环氧树脂胶层的各种斜截面方向粘结,实验观测了该体系的拉伸变形和失效行为,测量了界面失效载荷对胶层厚度和粘结界面倾斜角的依赖关系;通过引入胶结界面平均正应力、平均剪应力、平均正应变、平均剪应变等概念,可对界面失效强度进行测量,获得界面强度与界面粘结角度以及胶层厚度的关系,进而获得了铝合金/环氧胶层/铝合金体系的强度失效面以及胶结界面的断裂能和胶结体系的能量释放率.上述研究结果为深入认识金属胶结体系的强韧性能和失效机制提供了科学依据,对金属胶结体系的优化设计和性能评判具有重要指导意义.研究结果表明,铝合金/环氧胶层/铝合金体系的拉伸失效总体呈弹脆性破坏特征,失效表现为胶层粘结界面的断裂,失效强度和界面断裂能在胶层厚度为百微米量级时表现出强烈的尺度效应：界面粘结强度随着胶层厚度的减小而显著增大,临界状态的平均正应力和平均剪应力在强度破坏面上近似位于同一圆上,界面断裂能随着胶层厚度的减小而显著减小;与此同时,界面失效强度和界面断裂能也密切依赖于界面粘结角度.     

A parametric study of laser induced thin film spallation

We report parametric studies of elastic wave generation by a pulsed laser and associated spalling of thin surface films by the corresponding high stresses. Two different substrate materials, single crystal Si (100) and fused silica, are considered. Spallation behavior of Al thin films is investigated as a function of substrate thickness, film thickness, laser energy, and various parameters governing the source. Surface displacement due to the stress wave is measured by Michaelson interferometry and used to infer the stresses on the film interface. Consistent with previous studies, the maximum stress in the substrate and at the film/substrate interface increases with increasing laser fluence. For many of the conditions tested, the substrate stress is large enough to damage the Si. Moreover, the maximum interface stress is found to increase with increasing film thickness, but decrease with increasing substrate thickness due to geometric attenuation. Of particular significance is the development of a decompression shock in the fused sillica substrates, which results in very high tensile stresses at the interface. This shock enhances the failure of thin film interfaces, especially in thicker samples.     

The roles of toughness and cohesive strength on crack deflection at interfaces

In order to design composites and laminated materials, it is necessary to understand the issues that govern crack deflection and crack penetration at interfaces. Historically, models of crack deflection have been developed using either a strength-based or an energy-based fracture criterion. However, in general, crack propagation depends on both strength and toughness. Therefore, in this paper, crack deflection has been studied using a cohesive-zone model which incorporates both strength and toughness parameters simultaneously. Under appropriate limiting conditions, this model reproduces earlier results that were based on either strength or energy considerations alone. However, the general model reveals a number of interesting results. Of particular note is the apparent absence of any lower bound for the ratio of the substrate to interface toughness to guarantee crack penetration. It appears that, no matter how tough an interface is, crack deflection can always be induced if the strength of the interface is low enough compared to the strength of the substrate. This may be of significance for biological applications where brittle organic matrices can be bonded by relatively tough organic layers. Conversely, it appears that there is a lower bound for the ratio of the substrate strength to interfacial strength, below which penetration is guaranteed no matter how brittle the interface. Finally, it is noted that the effect of modulus mismatch on crack deflection is very sensitive to the mixed-mode failure criterion for the interface, particularly if the cracked layer is much stiffer than the substrate.     

Glass-modified stress waves for adhesion measurement of ultra thin films for device applications

Laser-generated stress wave profiles with rarefaction shocks (almost zero post-peak decay times) have been uncovered in different types of glasses and presented in this communication. The rise time of the pulses was found to increase with their amplitude, with values reaching as high as . This is in contrast to measurements in other brittle crystalline solids where pulses with rise times of and post-peak decay times of were recorded. The formation of rarefaction shock is attributed to the increased compressibility of glasses with increasing pressures. This was demonstrated using a one-dimensional nonlinear elastic wave propagation model in which the wave speed was taken as a function of particle velocity. The technological importance of these pulses in measuring the tensile strength of very thin film interfaces is demonstrated by using a previously developed laser spallation experiment in which a laser-generated compressive stress pulse in the substrate reflects into a tensile wave from the free surface of the film and pries off its interface at a threshold amplitude. Because of the rarefaction shock, glass-modified waves allow generation of substantially higher interfacial tensile stress amplitudes compared with those with finite post-peak decay profiles. Thus, for the first time, tensile strengths of very strong and ultra thin film interfaces can be measured. Results presented here indicate that interfaces of 185-nm-thick films, and with strengths as high as , can be measured. Thus, an important advance has been made that should allow material optimization of ultra thin layer systems that may form the basis of future MEMS-based microelectronic, mechanical and clinical devices.     

基于弹性-塑性内聚力模型的纤维拔出界面宏观行为分析

用解析法分析了单纤维从聚合物基体中的拔出过程,采用弹性—塑性内聚力模型模拟裂纹的扩展和界面失效,确定了临界纤维埋入长度,该值区分两种不同长度的纤维拔出过程. 在纤维拔出过程,界面经历不同的阶段. 纤维埋长小于临界长度时,界面的脱粘载荷与纤维的埋长成正比;超过临界长度后,界面的脱粘载荷近似为常数. 分析了界面参数对脱粘载荷的影响:增加界面的剪切强度和界面的断裂韧性,或减小界面裂纹萌生位移,均能提高界面的脱粘载荷;界面脱粘后无界面摩擦应力时,拔出载荷—位移曲线的峰值载荷等于界面的脱粘载荷;界面摩擦应力存在时,使峰值载荷大于脱粘载荷,需要较长的纤维埋入长度和较大的界面摩擦应力.      

A periodic unit-cell simulation of fiber arrangement dependence on the transverse tensile failure in unidirectional carbon fiber reinforced composites

The effect of fiber arrangement on transverse tensile failure in unidirectional carbon fiber reinforced composites with a strong fiber-matrix interface was studied using a unit-cell model that includes a continuum damage mechanics model. The simulated results indicated that tensile strength is lower when neighboring fibers are arrayed parallel to the loading direction than with other fiber arrangements. A shear band occurs between neighboring fibers, and the damage in the matrix propagates around the shear band when the interfacial normal stress (INS) is sufficiently high. Moreover, based on the observation of Hobbiebrunken et al., we reproduced the damage process in actual composites with a nonuniform fiber arrangement. The simulated results clarified that the region where neighboring fibers are arrayed parallel to the loading direction becomes the origin of the transverse failure in the composites. The cracking sites observed in the simulation are consistent with experimental results. Therefore, the matrix damage in the region where the fiber is arrayed parallel to the loading direction is a key factor in understanding transverse failure in unidirectional carbon fiber reinforced composites with a strong fiber/matrix interface.