Numerical study of tensile tests conducted on systems with elastic-plastic films deposited onto elastic-plastic substrates

In this work, a series of two-dimensional plane-strain finite element analyses was conducted to further understand the stress distribution during tensile tests on coated systems. Besides the film and the substrate, the finite element model also considered a number of cracks perpendicular to the film/substrate interface. Different from analyses commonly found in the literature, the mechanical behavior of both film and substrate was considered elastic-perfectly plastic in part of the analyses. Together with the film yield stress and the number of film cracks, other variables that were considered were crack tip geometry, the distance between two consecutive cracks and the presence of an interlayer. The analysis was based on the normal stresses parallel to the loading axis (σ_xx), which are responsible for cohesive failures that are observed in the film during this type of test. Results indicated that some configurations studied in this work have significantly reduced the value of σ_xx at the film/substrate interface and close to the pre-defined crack tips. Furthermore, in all the cases studied the values of σ_xx were systematically larger at the film/substrate interface than at the film surface.     

Edge effects in thin film delamination

Thin films bonded to a substrate often sustain large in-plane residual stresses that are transferred to the film via shear stresses on the interface near their edges. These edge zones play a significant role in film delamination. A new method is introduced to analyze both the residual stress distribution in a film near its edge and the energy release rate and mode mix for an interface delamination crack emerging from, or converging upon, an edge. Two two-dimensional configurations are considered: (a) a film whose edge lies in the interior of the substrate and (b) a film whose edge is aligned with the edge of the substrate (i.e. the film/substrate geometry is a quarter-plane). There are significant differences between the two cases. For the former, (a), the energy release rate approaches the steady-state, limiting rate for a long interface crack when the crack has extended less than one film thickness. By contrast, the energy release rate in case (b) remains far below the steady-state rate until the crack extends to ten or more film thicknesses from the edge. In case (b), the edge effect provides a significant protection against edge delamination, whereas in case (a) it does not. Elastic mismatch between the film and the substrate is significant in case (b), but not in case (a). A second set of behaviors is investigated wherein the interface crack approaches the edge of the film from the interior. For both types of edges, the energy release rate drops well below the steady-state rate at remaining ligament lengths that are very large compared to the film thickness, approaching zero as the delamination converges on the edge. Analytic features which account for the various behaviors will be highlighted, and practical implications for thin film delamination will be discussed.     

The effect of temperature and strain rate on the periodic cracking of amorphous AlxOy films on Cu

The high temperature properties of metal/ceramic interfaces play an important role in the operation of microprocessors and coated products. Determination of the interface properties over a range of testing conditions is critical in understanding and improving the performance of such systems. Periodic cracking of ceramic films on metal substrates provides a direct measure of the interface strength. A model Al_xO_y/Cu system is investigated over temperatures of 25–650 °C and at strain rates of 3.5 ⋅ 10^{− 2} s^{− 1} and 1.7 ⋅ 10^{− 5} s^{− 1}. For this system it is found that temperature does not significantly affect the spacing of film cracks in the steady state, at a given strain rate. However, the high strain rate tests broaden the measured crack spacing distributions compared to the low strain rate tests. This broadening causes an increase in the average crack spacing at high strain rate. The minimum crack spacing was increased 15–20% by both increasing strain rate and increasing temperature.     

Microstructure and Electron Energy-Loss Spectroscopy Analysis of Interface Between Cu Substrate and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Film Formed by Aerosol Deposition Method

Aerosol deposition method is a technique to form dense films by impacting solid particles on a substrate at room temperature. To clarify the bonding mechanism between AD films and substrates, TEM observation and electron energy-loss spectroscopy (EELS) analysis of the interface between Al₂O₃ AD films and Cu substrates were conducted. The Al₂O₃ film was directly adhered to the Cu substrate without any void or crack. The film was composed of randomly oriented α-Al₂O₃ crystal grains of about 10-20 nm large. At the Al₂O₃/Cu interface, the lattice fringes of the film were recognized, and no interfacial layer with nanometer-order thickness could be found. EELS spectra near O-K edge obtained at the interface had the pre-peak feature at around 528 eV. According to previously reported experiments and theoretical calculations, this suggests interactions between Cu and O in Al₂O₃ at the interface. It is inferred that not only the anchoring effect but also the ionic bonding and covalent bonding that originates from the Cu-O interactions contribute to the bonding between Al₂O₃ AD films and Cu substrates.     

Effects of gradient interlayer on residual stress and cracking in TiN thin films

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Effects of electroplated Cu thickness and polyimide plasma treatment conditions on the interfacial fracture mechanics parameters in the Cu/Cr/polyimide system

The interfacial fracture energies of Cu/Cr/polyimide systems are deduced by subtracting work expenditure from the peel strength. Two methods were used to obtain the work expenditure: on based on the X-ray measurement of the plastic strain of peeled metal film and the other based on a theoretical analysis of the foundation model for the attached part of the metal film. These two methods yield reasonably consistent work expenditure in most cases, imparting validity to the experimental and theoretical methods used here. The interfacial fracture energies of the Cr/polyimide interface were shown w.r.t., the film thickness and rf plasma power density activating the polyimide surface. The phase angle of the interface crack between the Cu and polyimide is determined by using elasto-plastic finite element analysis and the path independence of theJ-integral. The presence of the polyimide interlayer increased mode mixity, and the phase angles were virtually independent of the metal film thickness, which is consistent with the nearly constant interfacial fracture energy with Cu film thickness.     

Tetragonal phase stability in ZrO2 film formed on zirconium alloys and its effects on corrosion resistance

A thermodynamic model is developed to understand the origin of variation in the microstructure of ZrO₂ film formed on zirconium alloys and its effects on corrosion resistance. The correlation among the tetragonal phase fraction, the stress (macroscopic and internal one), the ZrO₂ grain size and the microstructural change of oxide film is formulized, and then analyzed. The results show that many complicated factors simultaneously govern the microstructure of oxide film. The tetragonal phase content near the oxide/metal interface, the macroscopic compressive stress near the interface, the decline gradient of macroscopic compressive stress and the internal stress induced by the transformation from the tetragonal to the monoclinic phase have very important influences on the transition from columnar grains to equiaxed grains, the crack formation and the degradation of oxidation resistance. The presence of intermetallic precipitates in oxide film may effectively relax the internal stress caused by transformation strain, stabilize the columnar-grain structure and reduce the probability of crack formation. How to reduce the transformation stress in the oxide film is a key to improve the corrosion resistance of zirconium alloys.     

Mechanical properties of the coating/substrate composite system: Nanostructured copper films on a LiF substrate

The study of a series of composite structures (CSs) such as CS-1, CS-2, and CS-3 being “soft film/soft substrate” systems revealed much new information on the mechanical properties of these materials. The general and distinctive properties of CSs both within the series of СS-1, CS-2, and CS-3 and the properties of raw materials of Cu and LiF are considered. It is found that the deformation process passes through three main phases in a wide range of loads at the nanomicroindentation of Cu/LiF CSs: (1) when β = h/t < 0.5 (h is the indentation depth, t is the coating thickness) the plastic deformation is mainly concentrated in the film, and only a small elastic deformation can take place in the substrate; (2) at β ≈ 1.0 the deformation occurs in the film and in the interface zone; (3) when β > 1.0 the plastic deformation extends into the substrate bulk, capturing all the typical levels of the system (film–interface zone–substrate) naturally becoming more complex as the load increases. It is shown that the “film/substrate” CSs are complex systems with their highly individual properties even possessing the same chemical composition with the same production method, differing by only one parameter (the film thickness t).     

In Situ SEM Observations of Fracture Behavior of Laser Welded–Brazed Al/Steel Dissimilar Joint

Laser welding–brazing of 6061-T6 aluminum alloy to DP590 dual-phase steel with Al-Si12 flux-cored filler wire was performed. The microstructure at the brazing interface was characterized. Fracture behavior was observed and analyzed by in situ scanning electron microscope. The microstructure of the brazing interface showed that inhomogeneous intermetallic compounds formed along the thickness direction, which had a great influence on the crack initiation and propagation. In the top region, the reaction layer at the interface consisted of scattered needle-like Fe(Al,Si)₃ and serration-shaped Fe_1.8Al_7.2Si. In the middle region, the compound at the interface was only serration-shaped Fe_1.8Al_7.2Si. In the bottom region, the interface was composed of lamellar-shaped Fe_1.8Al_7.2Si. The cracks were first detected in the bottom region and propagated from bottom to top along the interface. At the bottom region, the crack initiated and propagated along the Fe_1.8Al_7.2Si/weld seam interface during the in situ tensile test. When the crack propagated into the middle region, a deflection of crack propagation appeared. The crack first propagated along the steel/Fe_1.8Al_7.2Si interface and then moved along the weld seam until the failure of the joint. The tensile strength of the joint was 146.5 MPa. Some micro-cracks were detected at Fe(Al,Si)₃ and the interface between the steel substrate and Fe(Al,Si)₃ in the top region while the interface was still connected.     

Spark anodizing behaviour of titanium and its alloys in alkaline aluminate electrolyte

Spark anodizing of titanium, Ti-6Al-4V and Ti-15V-3Al-3Cr-3Sn in alkaline aluminate electrolyte produces highly crystalline anodic films consisting mainly of Al₂TiO₅ with α- and γ-Al₂O₃ as minor oxide phases, irrespective of substrate composition. However, the apparent efficiency for film formation decreases in the following order: Ti-6Al-4V, titanium and Ti-15V-3Al-3Cr-3Sn. A large amount of aluminium species are incorporated from the electrolyte, probably by plasma-chemical reaction, and become distributed throughout the film thickness. This distribution indicates that the electrolyte penetrates near to the film/substrate interface through the discharge channels. Thus, the outwardly migrating aluminium ions under a high electric field can be present even in the inner part of the anodic films. Voids are developed at the film/substrate interface, particularly on the vanadium-containing alloys, reducing the adhesion of the anodic film to the substrate.     

Stresses from Oxide Film Imperfections During Metal Dusting

When metals or alloys protected by oxide films are exposed to a carburizing atmosphere, carbon may diffuse into the metal substrate because of imperfections in the film. Stresses from the change in carbon concentration in the metal substrate may arise and can result in spallation of the oxide film, crack initiation on the exposed surface and acceleration of metal dusting. In this paper, the stresses at a corner point, where the channel or flaw intersects the interface between the oxide film and metal substrate, are studied by a coupling model between carbon diffusion and elastic deformation. The results show that the stresses of the corner point depend on the oxide film thickness and channel width and are more sensitive to the former. The dependency varies with the time for which the metal substrate is exposed to the carburizing atmosphere. It is expected that the present study may lead to the comprehension of the metal dusting corrosion with regard to the effect of diffusion induced stress.     

Cracking behaviour of PVD tungsten coatings deposited on steel substrates

Tungsten coatings have been deposited on steel substrates by magnetron sputtering. For the same processing conditions, the increase of the coating thickness enhances the (111) component of the crystallographic texture whereas the residual stress level decreases. Tensile and four-point bending tests, associated with an acoustic emission analysis, have been performed inside a SEM chamber in order to study the cracking mechanisms. When the residual stresses are taken into account, an intrinsic critical cracking stress and the associated energy release rate can be determined; the obtained values suggest an intergranular cracking mechanism. No debonding has been observed at the interface despite the large plastic deformation of the substrate at the crack tips. The observed strain localisation modes in the substrate near the interface have been shown to have a major influence on the limit crack density.     

大气常压等离子弧清洗的能量耦合作用有限元分析

大气压等离子弧清洗是一种新型的表面预处理清洗技术,了解等离子弧与工件间的能量耦合作用是研究大气压等离子弧清洗技术的关键。通过建立有限元模型,并对分析结果进行回归计算,得到了界面清洗力和界面清洗温度的经验公式,进一步揭示了等离子弧功率,基板厚度和清洗速度等主要参数对能量耦合的影响,并对回归分析结果进行了验证。结果表明:等离子弧输出功率与清洗速度的相互作用对界面清洗力和界面温度影响最大,其中,等离子弧输出功率对界面清洁力和界面温度的影响最大,清洗速度次之,基板厚度对它们的影响最小。     

三维多粗糙峰涂层等效应力分析与表征

利用有限元方法建立了刚性平面与多粗糙峰涂层的弹性接触模型,研究了刚性平面分别与二维涂层粗糙峰、三维涂层粗糙峰的接触状态,揭示了涂层/基体弹性模量比、涂层厚度、粗糙峰问距、刚性平面压下深度对涂层粗糙峰表面、涂层/基体界面等效应力分布及涂层基体变形的影响规律.计算结果表明:压下深度对涂层粗糙峰表面最大等效米塞斯应力的影响最大,涂层厚度和涂层/基体弹性模量比的影响次之,粗糙峰间距的影响最小;增大涂层厚度,减小压下深度、粗糙峰间距和低弹性模量比,会使得最大等效应力值显著降低.     

酸,碱两步催化合成聚钛硅氧烷铝基材涂膜的研究

聚钛硅氧烷底涂液是用Ｎ－［３－（三乙氧基硅烷］－４，５二羟咪唑和钛烷氧化物在甲醇、水溶液中以酸和碱两步催化水解、缩聚而成，该底涂液涂复在铝基材上经热解，脱碳化物转化成准无机耐蚀涂膜，下述因素对涂膜的耐蚀性起到了重要作用：（１）涂膜的厚热解温度；（２）膜与基材之间的界面结合；（３）介质对涂膜的润湿性。     

Structural changes during the reaction of Ni thin films with (1 0 0) silicon substrates

Ultrathin films of nickel deposited onto (1 0 0) Si substrates were found to form kinetically constrained multilayered interface structures characterized by structural and compositional gradients. The presence of a native SiO₂ on the substrate surface in tandem with thickness-dependent intrinsic stress of the metal film limits the solid-state reaction between Ni and Si. A roughly 6.5 nm thick Ni film on top of the native oxide was observed regardless of the initial nominal film thickness of either 5 or 15 nm. The thickness of the silicide layer that formed by Ni diffusion into the Si substrate, however, scales with the nominal film thickness. Cross-sectional in situ annealing experiments in the transmission electron microscope elucidate the kinetics of interface transformation towards thermodynamic equilibrium. Two competing mechanisms are active during thermal annealing: thermally activated diffusion of Ni through the native oxide layer and subsequent transformation of the observed compositional gradient into a thick reaction layer of NiSi₂ with an epitaxial orientation relationship to the Si substrate; and, secondly, metal film dispersion and subsequent formation of faceted Ni islands on top of the native oxide layer.     

Fracture resistance evaluation of RuO2-based thick film resistor material by in situ crack extension observation in a scanning electron microscope

Parameters for the fracture mechanics of thick film materials are scarce in the literature. One reason is that for many such materials it is very difficult to produce a bulk specimen as required for most standard tests. This paper describes an alternative method for measuring the fracture resistance of a ruthenium dioxide (RuO₂)-based thick film resistor material for electronic applications. The method is based on an in situ investigation of crack propagation in the loaded material. The investigated material is printed as a thick film on a substrate of low-temperature-co-fired-ceramic. An initial crack in the film is introduced with a Vickers indenter. The crack is subsequently loaded with a four-point bending equipment in a scanning electron microscope, which allows for in situ crack length measurement. The crack growth measurements reveal that once a certain crack length is achieved the load required to extend the crack becomes independent of the crack length. Beyond this length, the crack propagates in the so-called steady-state region, which is used in the present method to estimate the fracture resistance of the film. Both tensile stresses resulting from bending and tensile residual stresses are taken into account. Although a brittle substrate was used, the crack did not penetrate into the substrate. The measured fracture resistance of 0.69 ± 0.14 MPa√m is found to be realistic for the investigated thick film material with high silicate glass content.     

双涂层界面接触应力分析

目的研究在接触应力作用下,双涂层的总厚度及厚度比对其界面应力的影响规律。方法采用有限元方法研究两个界面,即涂层与基底界面和涂层间界面的Mises应力突变量和界面最大剪应力。结果总厚度一定时,涂层厚度比变化对涂层/基底界面应力的影响较小,而对涂层间界面应力的影响明显。对于薄涂层,涂层间界面Mises应力突变量和最大剪应力随着中间层厚度比例的增大而减小;对于厚涂层,涂层间界面Mises应力突变量和最大剪应力随着中间层厚度比例的增大而减小。涂层厚度比一定时,随着涂层总厚度增加,涂层间界面Mises应力突变量和最大剪应力先增大,后减小。结论双涂层的总厚度及厚度比对其界面应力有显著影响。     

Stress induced delamination methods for the study of adhesion of Pt thin films to Si

Adhesion of Pt films to Si substrates with a native oxide has been investigated using two methods of quantitative adhesion characterization. The nanoindentation induced delamination method uses an impression to store compressive strain in an overlayer film to induce delamination at the Pt/SiO₂ interface. Likewise, the telephone cord delamination method involves sputtering a thick compressively stressed overlayer onto the Pt/SiO₂ films to induce telephone cord delamination patterns in the Pt film. Crack extension forces and interface toughnesses are calculated from the dimensions of the circular blister or the telephone cords using currently available models. Focused ion beam (FIB) observations show that the nanoindentation method is difficult to implement because of extensive crack formation in the substrate beneath the indentation, causing interface toughnesses from this test to be gross overestimates. The telephone cord measurements, by comparison, give realistic interface toughnesses, allowing us to show that decreasing the argon pressure during Pt sputtering significantly increases the adhesion of the films to the substrate.     

Fe-Co薄膜与Al2O3 基体相互扩散和相互作用相关的微观结构演变

利用先进分析透射电子显微镜,检测并系统地研究了磁控溅射Fe-Co薄膜与Al₂O₃基体之间的相互扩散。结果表明,扩散会形成尖晶石相FeAl₂O₄,并导致界面层的形成。微观结构表征表明,在界面附近的Fe-Co薄膜中形成了与相互扩散相关的非公度结构。本研究不仅检测到Fe-Co薄膜与蓝宝石基体之间的相互扩散和伴随的新相形成,而且揭示了界面区域相应的微观结构演变,这些结果可能对薄膜的磁学性质有很大的影响。