Controlled crack arrest in brittle thin films: The effect of embedded voids

Controlled crack arrest is useful for controlling self-assembled crack paths in micro-fabrication, as well as limiting crack length in toughening composites. Compliant inclusions and voids can effectively attract and arrest cracks, thereby controlling the crack pattern. Analytical stress solution indicates that when the inclusion is about 10 times more compliant than its matrix, it may be effectively modeled as a void. The crack arrest capability is expressed in terms of the critical angle of the initial crack path, as the inclusion size, shape, crack origin and film properties are varied. Simple criteria are established to maximize the crack arrest ability and to determine the critical crack angle. The effectiveness of using different void patterns to arrest cracks is also explored. The results are useful for both controlling and restricting cracks in brittle thin films, through the utilization of compliant or void-like inclusions as crack arrestors.     

Strain gradient effect in nanoscale thin films

Compared to uniform deformation, gradient-dominant deformation experiments at the micro-scale have consistently shown remarkable strengthening effect. It is proposed in the literature that pronounced strain gradient, on the order of the inverse of a characteristic length scale, adds to the materials strength. The literature contains several formulations that account for the strain gradient in the constitutive equation. However, the role of microstructures in these formulations remains to be investigated. Due to the difficulties in micro/nano scale experimentation, attempts to investigate this mechanism so far have been confined to specimens with dimensions more than 10 microns, or to nano-indentation experiments. In this study, we use MEMS-based testing techniques to explore the effect of strain gradient in 100, 150, 200 and 485 nm thick freestanding Aluminum specimens with average grain size of 50, 65, 80 and 212 nm respectively. Strain gradient plasticity analysis show that the characteristic length scale for Aluminum is about 4.5 μm, which is similar to the values for copper and nickel reported in the literature. Experimental results suggest that the strain gradient effect is fundamentally related to dislocation-based mechanisms, and is absent at extremely small length scales (< 100 nm for Aluminum), where accommodation of dislocations in the crystal grains is not energetically favorable.     

Repeated frictional sliding properties of copper containing nanoscale twins

Bulk dynamic plastic deformation (DPD) materials comprise a composite structure of nanoscale twin bundles and nanoscale grains. The tribological properties of DPD-processed pure nano-Cu have been investigated in this study and compared with conventional coarse-grained (CG) Cu under both monotonic and repeated frictional sliding. We demonstrate that DPD nano-Cu and CG Cu exhibit steady-state mechanical characteristics after repeated frictional sliding that are similar to those seen in nanotwinned (NT) Cu produced by pulsed electrodeposition.     

High strength-ductility of thin nanocrystalline palladium films with nanoscale twins: On-chip testing and grain aggregate model

The mechanical behaviour of thin nanocrystalline palladium films with an ∼30 nm in plane grain size has been characterized on chip under uniaxial tension. The films exhibit a large strain hardening capacity and a significant increase in the strength with decreasing thickness. Transmission electron microscopy has revealed the presence of a moderate density of growth nanotwins interacting with dislocations. A semi-analytical grain aggregate model is proposed to investigate the impact of different contributions to the flow behaviour, involving the effect of twins, of grain size and of the presence of a thin surface layer. This model provides guidelines to optimizing the strength/ductility ratio of the films.     

Strength, strain-rate sensitivity and ductility of copper with nanoscale twins

We present a comprehensive computational analysis of the deformation of ultrafine crystalline pure Cu with nanoscale growth twins. This physically motivated model benefits from our experimental studies of the effects of the density of coherent nanotwins on the plastic deformation characteristics of Cu, and from post-deformation transmission electron microscopy investigations of dislocation structures in the twinned metal. The analysis accounts for high plastic anisotropy and rate sensitivity anisotropy by treating the twin boundary as an internal interface and allowing special slip geometry arrangements that involve soft and hard modes of deformation. This model correctly predicts the experimentally observed trends of the effects of twin density on flow strength, rate sensitivity of plastic flow and ductility, in addition to matching many of the quantitative details of plastic deformation reasonably well. The computational simulations also provide critical mechanistic insights into why the metal with nanoscale twins can provide the same level of yield strength, hardness and strain rate sensitivity as a nanostructured counterpart without twins (but of grain size comparable to the twin spacing of the twinned Cu). The analysis also offers some useful understanding of why the nanotwinned Cu with high strength does not lead to diminished ductility with structural refinement involving twins, whereas nanostructured Cu normally causes the ductility to be compromised at the expense of strength upon grain refinement.     

In situ TEM observations of fast grain-boundary motion in stressed nanocrystalline aluminum films

Free-standing nanocrystalline Al thin films have been strained in situ in a transmission electron microscope at room-temperature. Extensive grain-boundary migration accompanies the in situ loading and has been observed to occur preferentially at crack tips and only in the presence of the applied stress. This grain growth precedes dislocation activity, and measured boundary velocities are greater than can be explained by diffusive processes. The unambiguous observations of stress-assisted grain growth are compatible with recently proposed models for stress-coupled grain-boundary migration. The growth occurs in a faceted manner indicative of preferential boundaries. The fast collapse of small grains with sizes of 30–50 nm demonstrates the unstable nature of a nanocrystalline structure. Clearly observable shape changes testify to the effectiveness of grain-boundary migration as a deformation mechanism, and preferential grain growth at crack tips resulted in efficient crack tip blunting, which is expected to improve the films’ fracture toughness.     

纳米薄膜X射线吸收光谱的鉴定

采用X射线吸收光谱研究了热丝化学气相沉积(CVD)合成的纳米金刚石薄膜和脉冲激光沉积的纳米SiC薄膜.结果表明:纳米金刚石薄膜的碳K边X射线吸收精细结构光谱显示的激发峰相当于微米金刚石薄膜的蓝移,是量子效应的显著特征,证明制备的是纳米金刚石薄膜,与高分辨透射电镜的结果完全吻合;纳米SiC薄膜的硅K边X射线吸收精细结构光谱和扩展X射线吸收精细结构光谱也显示了纳米薄膜短程有序的结构特征,表明获得的是纳米SiC薄膜.     

In situ measurement of corrosion on the nanoscale

Measurement of the electrochemical properties phase-by-phase on the nanoscale in real (commercial) alloys is critical to understanding the microstructure-corrosion relationship and subsequently controlling it. This work presents a novel AFM based in situ corrosion probing methodology (for the first time) that is capable of resolving the electrochemical activity (impedance response) into the nanometer range; the method subsequently having major ramifications in the study of aluminium alloy corrosion, the interpretation of corrosion propagation, and the subsequent development of corrosion resistant aluminium alloys.     

In situ oxidation studies on (001) copper-nickel alloy thin films

The (001)-oriented, single crystalline thin films of Cu-3% Ni, Cu-4.6% Ni, and Cu-50% Ni alloy were prepared by vapor deposition onto (001) NaCl substrates. The films were subsequently annealed at around 1100°K and oxidized at 725°K at an oxygen partial pressure of 5×10^–1 N · m^–2 (5× 10^–6 atm). High-resolution transmission electron microscopy was employed to observe the changes in situ. For all the alloy concentrations Cu₂O and NiO were observed to nucleate and grow independently; no mixed oxides were noted. The shape and growth rates of Cu₂O nuclei were similar to those found in previous work on pure copper films. For low-nickel alloy concentrations the NiO nuclei were larger and the number density of NiO was less when compared to the oxidation of pure nickel films. For the Cu-50% Ni films the shape and growth rates of NiO were identical to those for the oxidation of pure nickel films. Low nickel concentrations exhibited a reduced induction period for Cu₂O when compared to pure copper films. Cu-50%Ni films showed surface precipitation and growth of NiO first, followed by Cu₂O in a typical through -thickness growth after a prolonged induction period. The results are consistent with the previously established oxidation mechanisms of pure copper and pure nickel films.This work was performed at the Ames Research Center and funded by NASA Grants NCA2-OP390-403 and NSG-2025.     

Constrained diffusional creep in UHV-produced copper thin films

The thermomechanical behavior of metallic thin films on stiff substrates is relevant for thin-film devices, but its mechanisms are not fully understood. In this investigation, the mechanical properties of pure Cu and Cu–1 at.% Al films on diffusion-barrier coated Si substrates were studied with the wafer-curvature technique. In ultra-pure films, which were sputtered and annealed under ultra-high vacuum conditions, characteristic stress relaxation at high temperatures was measured, which could be clearly attributed to diffusional creep. Good quantitative agreement with a recent model of diffusional creep constrained by a substrate was obtained. These features were absent in the Cu–Al alloy films, in which Al surface segregation and oxidation had produced a “self-passivating” effect, and in films produced in less clean environments. Based on these results, we propose a model of thin-film deformation based on dislocation glide and constrained diffusional creep.     

Kinetics and mechanism of high-temperature oxidation of copper covered by bismuth thin films

The oxidation kinetics of copper covered by thin films of bismuth were studied by TGA, X-ray diffraction, X-ray micro-elemental, coulombmetric methods, and by electron and optical microscopy. At 1003 K catastrophic oxidation of copper coated by bismuth thin films was observed. The parabolic rate constant of copper oxidation (Kp) depends markedly on the thickness of the bismuth film and is more than 1000 times greater than that of bare copper. The mechanism of catastrophic copper oxidation in contact with bismuth is discussed.     

Cu掺杂对Be薄膜微结构的影响(英文)

利用离子束溅射法在硅基底上制备高纯Be薄膜并实现Cu元素的可控掺杂,利用X射线能谱、扫描电镜、X射线衍射以及透射电镜等对Cu掺杂Be薄膜进行表征分析。研究结果表明:Cu元素在Be膜内分布均一,且Cu掺杂量对Be薄膜的微观结构有显著影响。Cu掺杂能抑制Be晶粒生长,Be晶粒随着薄膜中Cu含量的增多而减小,并且尺寸分布更加均匀;Cu掺杂影响Be晶粒的生长取向,使其形成更为紧凑的薄膜结构。这些因素使得掺杂Cu的Be薄膜的表面粗糙度明显降低。     

On the atmospheric corrosion of thin copper films

Sputtered and electrodeposited copper specimens were exposed to the laboratory atmosphere for 18 months, after which the corrosion products were analysed by electrochemical methods and reflectance spectra. The formation of corrosion products takes place in three stages: initially cuprous oxide is formed, followed by the growth of cuprous sulphide, and finally cuprous oxide is converted to cuprous sulphide. The presence of water is essential for these corrosion reactions to occur.     

A corrosion study of nanocrystalline copper thin films

Thin nanocrystalline, compact films, based on the copper–nitrogen system, up to 2.5 μm thickness and 3.5% nitrogen, were deposited by magnetron sputtering at different partial pressure ratios of N₂ and Ar, without formation of Cu_xN compounds, the nitrogen concentration influencing grain size (down to 30 nm) and film homogeneity. Electrochemical corrosion properties were investigated using polarization curves and electrochemical impedance spectroscopy in 0.5 M NaCl aqueous solution, and compared with pure bulk copper; morphology was examined by scanning electron microscopy. Significant variations in corrosion currents between samples were attributed to grain size and structural defects on the grain boundaries.     

In situ TEM straining of single crystal Au films on polyimide: Change of deformation mechanisms at the nanoscale

In situ transmission electron microscopy straining experiments were performed on 40, 60, 80 and 160 nm thick single crystalline Au films on polyimide substrates. A transition in deformation mechanisms was observed with decreasing film thickness: the 160 nm thick film deforms predominantly by perfect dislocations while thinner films deform mainly by partial dislocations separated by stacking faults. In contrast to the 160 nm thick film, interfacial dislocation segments are rarely laid down by threading dislocations for the thinner films. At the late stages of deformation in the thicker Au films prior to fracture, dislocations start to glide on the (0 0 1) planes (cube-glide) near the interface with the polymer substrate. The impact of size-dependent dislocation mechanisms on thin film plasticity is addressed.     

In situ observations and crystallographic analysis of martensitic transformation in steel

The development of a martensitic structure in a low-carbon and low-alloy steel was characterized using in situ confocal laser microscopy, high-speed photography and crystallographic analysis, including the nature of variant selection. The initial stage of transformation involves the partitioning of the austenite grain into packets, after which the rate of transformation is gradual. The crystallographic orientation of the plates that form is not random, but involves selection determined by the relationship between the shape deformation direction and the free surface. The vicinity of austenite grain and twin boundaries, and martensite/austenite interfaces also affect variant selection.     

Concepts for the design of advanced nanoscale PVD multilayer protective thin films

Technological challenges in future surface engineering applications demand continuously new material solutions offering superior properties and performance. Concepts for the design of such advanced multifunctional materials can be systematically evolved and verified by means of physical vapour deposition. The classical multilayer coating concept today is well established and widely used for the design of protective thin films for wear and tribological applications. It has proven great potential for the development of novel thin film materials with tailored properties. In the past decade, the emerging new class of nanoscale coatings has offered to the material scientists an even more powerful toolbox for the engineering thin film design through a combination of the multilayer concept with new nano-coatings. Some examples are the use and integration of low friction carbon-based nanocomposites in advanced multilayer structures or the stabilization of a specific coating in another structure in a nanolaminated multilayer composite. This paper reviews the latest developments in hard, wear-resistant thin films based on the multilayer coating concept. It describes the integration of nanocrystalline, amorphous and nanocrystalline/amorphous composite materials in multilayers and covers various phenomena such as the superlattice effect, stabilization of materials in another, foreign structure, and effects related to coherent and epitaxial growth. Innovative concepts for future, smart multilayer designs based on an extremely fine structural ordering at the nanoscale are presented as well.     

In situ conductivity and photoconductivity measurements of polyaniline films

In situ conductivity measurements on a polyaniline film with a fourprobe electrode are performed in NH₄F, 2.3HF and in propylene carbonate in the dark and under illumination. The conductivity is also determined on a pellet of chemically prepared polyaniline mixed with carbon black as for a battery electrode. The upper values of conductivities are 120 ohm⁻¹ cm⁻¹ in the hydrofluoric medium, 25 ohm⁻¹ cm⁻¹ in propylene carbonate and 8 ohm⁻¹ cm⁻¹ in the solid dry state. For a pellet with 20% carbon black it is 0.5 ohm⁻¹ cm⁻¹. The increase in conductivity of the polyaniline film in the solid state upon illumination is about 0.7 ohm⁻¹ cm⁻¹ (10%).     

Deformation behavior of thin copper films on deformable substrates

The role of strain hardening for the deformation of thin Cu films was investigated quantitatively by conducting specialized tensile testing allowing the simultaneous characterization of the film stress and the dislocation density as a function of plastic strain. The stress–strain behavior was studied as a function of microstructural parameters of the films, such as film thickness (0.4–3.2 μm), grain size and texture. It was found that the stress–strain behavior can be divided into three regimes, i.e. elastic, plastic with strong strain hardening and plastic with weak hardening. The flow stresses and the hardening rate increase with decreasing film thickness and/or grain size, and are about two times higher in (111)-grains compared to the (100)-grains. These effects will be discussed in the light of existing models for plastic deformation of thin films or fine grained metals.