Film Fracture Research Articles

Study on the high temperature wear behavior of TiAlSiN coatings deposited on WC-TaC-Co cemented carbides

This study investigated the high-temperature wear behavior and the failure mechanism of TiAlSiN coating under thermal-mechanical coupling. The results indicate that increased temperature significantly promotes coating oxidation, forming a complete wear-resistant layer. However, coating hardness decreases significantly at elevated temperatures, deteriorating the wear property. Increased load generates higher friction heat and intensifies coating oxidation, but it accelerates oxide film fracture and spalling. Mechanical stress causes oxide film fracture, spalling and removal, while high-temperature oxidation triggers cracks healing, debris connection and oxide film re-formation. Oxide film exhibits a dynamic cycle of formation-breaking-formation in high-temperature wear.

Contact fatigue of thin hard diamond-like carbon films on M50 steels under cyclic spherical micro-indentation

Contact fatigue behaviors of thin hard diamond-like carbon (DLC) films on M50 steels during cyclic spherical micro-indentations were investigated. The stress-strain evolution of the entire system was analyzed by finite element methods. The film fracture mechanisms and interfacial bearing responses under monotonic and repeated contacts were comprehensively explored. Concentrated tensile and shear stresses were found to, respectively, govern the initiation and propagation of the surface ring cracks. Elevated tensile stresses at the initiation and epilogue of each indentation cycle enhanced bottom radial cracking. At the interfaces, the maximum tensile (σnmax) and shear (σtmax) stresses occurred at the end of unloading and loading processes, respectively. With increasing maximum indentation forces, σnmax initially increased and then gradually decreased, while σtmax increased monotonically.

Lithiation Depth Regulation of Silicon Anodes toward Excellent Stability.

Silicon (Si) is an appealing choice of anode for next-generation lithium ion batteries with high energy density, but its dramatic volume expansion makes it a tremendous challenge to achieve acceptable stability. Herein, we demonstrate that no capacity decay is observed during the testing period when the lithiation depth of Si nanoparticles is regulated at 2000 mAh g-1 or below, the fracture of Si anode films is well mitigated under suitable regulation of lithiation depth, and the cycled Si remains particulate without turning flocculent as under full lithiation. In addition, the solid electrolyte interphase (SEI) with a LiF-dominated outer region produced under lithiation regulation could better passivate the Si anodes and prevent further electrolyte decomposition than the mosaic-type SEI formed under full lithiation. Regulating lithiation depth proved to be a feasible solution to the pressing volume issues, and optimization of capacity utilization should be considered as much as materials-level optimization.

Electrified fracture of nanotube films

Electrified fracture of nanotube films

In situ characterization of stresses, deformation and fracture of thin films using transmission X-ray nanodiffraction microscopy.

The use of hard X-ray transmission nano- and microdiffraction to perform in situ stress and strain measurements during deformation has recently been demonstrated and used to investigate many thin film systems. Here a newly commissioned sample environment based on a commercially available nanoindenter is presented, which is available at the NanoMAX beamline at the MAX IV synchrotron. Using X-ray nanoprobes of around 60-70 nm at 14-16 keV and a scanning step size of 100 nm, we map the strains, stresses, plastic deformation and fracture during nanoindentation of industrial coatings with thicknesses in the range of several micrometres, relatively strong texture and large grains. The successful measurements of such challenging samples illustrate broad applicability. The sample environment is openly accessible for NanoMAX beamline users through the MAX IV sample environment pool, and its capability can be further extended for specific purposes through additional available modules.

A phase field fracture model for ultra-thin micro-/nano-films with surface effects

Surface effects usually remarkably affect the mechanical response of ultra-thin micro-/nano-structures. However, the mechanisms of surface effects on the fracture characteristics of ultra-thin films are still not fully understood. To this end, this paper develops a modeling framework to investigate the fracture of ultra-thin films at microscales or below. Such a framework couples the Gurtin–Murdoch theory with a phase-field fracture model, in which the former is adopted to introduce the surface effects, i.e., the surface residual stress and surface elasticity of a thin film, and the latter is able to model crack evolution without requiring predefined crack paths or any criteria. Furthermore, a novel crack driving force is introduced, which encompasses the tensile components of both bulk elastic energy and surface elastic energy. Several numerical examples including the biaxial tension test as well as the single-edge notched tension/shear test are performed. The simulation results indicate that the surface strain energy plays a major role in the total elastic strain energy of an ultra-thin film when its thickness is at a micro level, thus demonstrating the significance of surface effects. Moreover, the mode-I fracture test shows that the surface elasticity and surface residual stress have a remarkable influence on the displacement at failure, while for the mode-II fracture test, the surface residual stress significantly influences the fracture characteristics such as the crack path and failure displacement. The developed model paves the way for revealing the fracture mechanisms of ultra-thin micro-/nano-films and conducting their safety assessment.

Fracture of films caused by uniaxial tensions: a numerical model

Fracture of films caused by uniaxial tensions: a numerical model

Tailored nanocrystalline Niobium coatings on steel substrates for superior resistance to micro-crack initiation

Fracture of metallic thin films during deformation hampers extensive applications of coated engineering components. In this study, the resistance to micro-crack initiation of Niobium (Nb) coatings deposited on stainless steel substrates is improved remarkably by tuning the substrate bias voltages during unbalanced magnetron sputtering. With the tailored nanocrystalline microstructure, the failure strain for coating fracture is increased to 30 %, which is far greater than that of previous metallic coatings (below 10 %). Microstructure characterizations reveal that intergranular nano-pinholes and amorphous phases can be eliminated effectively with a moderate ions bombardment effect to avoid brittle film fracture. At the same time, compact and homogenous nanocrystalline grains with appropriate intragranular defects are obtained via enhanced atom diffusivity during deposition. Therefore, large plastic deformation can be accommodated through both stress-driven grain coarsening and continuous dislocation plasticity, avoiding fracture of the Nb coatings.

High thermoelectric performance of aluminum-doped cuprous selenide thin films with exceptional flexibility for wearable applications

Flexible thin-film generators are a great promising energy source for wearable device applications. In this work, Al-doped Cu2Se thin film with highly (0l0) preferred orientation was achieved by magnetron co-sputtering deposition. It possessed a high ZT value of 0.76 at 275 °C, and further surface organic component coating treatment resulted in its exceptional flexibility. The Al dopant was proven to be successfully introduced into the lattice of Cu2Se samples, which can decrease carrier concentration and result in the enhancement of Seebeck coefficient. Meanwhile, a significant decline in lattice thermal conductivity was achieved after Al doping. The organic component coating can prevent the fracture of inorganic thin films after bending. The resistance difference (∆R/R0) after 1000 cycles of bending was lower than 10%, which is a great improvement compared with that in pristine thin film (over 15%). The thermoelectric device prepared by the Al-doped Cu2Se obtained a stable power density of ∼5.7 W m−2 at a temperature difference of 20 °C.

A new route to fabricate high-quality double-side-polished freestanding diamond film

Freestanding diamond films are widely applied, but available fabrication techniques present obvious flaws. A new route to fabricate high-quality double-side-polished freestanding diamond film is proposed in this study. Typical MCD (Micro-crystalline diamond) or NCD (Nano-crystalline diamond) film is firstly deposited on pretreated SiC substrate by HFCVD technique. As-grown surfaces are smoothened by mechanical polishing. Diamond films are then separated from substrates by laser cutting. Finally, bottom surfaces, which are known as nucleated surfaces, are also polished. Compared with the Mo substrate used in the conventional route based on the natural peeling off of the diamond film, the thermal deformation and residual stress of the diamond film on the SiC substrate are smaller, mainly due to the reduced difference between thermal expansion coefficients of the diamond film and substrate. This is beneficial for avoiding the fracture of freestanding films in the cooling stage, and also beneficial for the subsequent polishing. Consequently, the interruptible repeated growth strategy with constant growth rates (1.0–1.2 μm/h for MCD and 1.3–1.5 μm/h for NCD) is feasible, and the long-duration operation of HFCVD device is dispensable. Compared with the Si substrate used in the route based on the Si etching, the SiC substrate is much more reusable in the present route.

Концепция микромоделирования процесса соединения разнородных материалов пластической деформацией

Introduction. Bond strength between dissimilar materials is the most important characteristic of laminated composites, which determines the success of its development for industrial use. In order to develop the theory of joining materials by plastic deformation, it is proposed to perform computer simulation of joint deformation of representative volumes of dissimilar materials on a microscale and compare the parameters of the stress-strain state with the previously presented theoretical mechanism. The aim of this work is to analyze the stress-strain state of dissimilar materials under plastic deformation on a microscale and to establish the location of the onset of fracture of surface oxide films. To achieve this aim, the following tasks of the work are formulated: 1) to study the surface profiles of dissimilar materials to be bonded by plastic deformation; 2) to simulate by the finite element method (FE) the plastic deformation of contact surfaces of dissimilar materials on a microscale; 3) to study the stages of joint deformation of dissimilar materials on a microscale and verify of the theoretical mechanism. Research methodology. The study of three-dimensional topography and roughness was carried out on a Veeco Wyko NT1100 Optical Profiling System. Deform-3D FE simulation package was chosen as the main research tool. Aluminum alloys AMg3 and D16 were chosen as the materials under study. Results and discussion. In this work, computer FE simulating of the joint deformation of the surface layers of AMg3 and D16 alloys on a microscale was performed, an analysis of the surface profiles of materials after various types of processing was carried out, the parameters of the stress-strain state were studied and compared with the parameters of the theoretical mechanism. Based on the results of the comparison, the adequacy of the proposed theoretical mechanism was assessed, and the practical difficulties of theoretical simulation of the joint deformation of dissimilar materials on the microscale were noted. Microscale FE simulation made it possible to study the flow of plastic deformation in the near-surface layers of materials, as well as to identify areas of the most probable fracture of surface oxide films and, consequently, areas of primary bonding of dissimilar materials.

High-Speed Cobalt Film Fracture under the Action of Loads Created by a Picosecond Laser Pulse

High-Speed Cobalt Film Fracture under the Action of Loads Created by a Picosecond Laser Pulse

Parameters influencing the fracture of Mo films and their wider significance

Fragmentation testing has been used for decades to assess thin film fracture and delamination. Hooke’s law is generally used to determine a film fracture stress from the crack onset strain observed in micrographs or measured as an electrical resistance increase. While this method is in theory suitable in the elastic regime, it neglects important film characteristics, such as residual stress, microstructure, or film architecture. Thus, there is a need to improve fracture analysis using fragmentation to avoid significant errors in measuring fracture stress or apparent fracture toughness of thin films. In-situ X-ray diffraction fragmentation experiments can measure the film fracture stress even for individual layers being part of a multilayer. Which characteristics influence the apparent fracture behavior will be demonstrated on Mo thin films on polyimide.Graphical abstract

Numerical and experimental studies on crack nucleation and propagation in thin films

The prediction of damage and cracking patterns in ceramic thin films plays a vital role in the optimal design thereof. In this study, we focus on developing a numerical framework to predict fracture in ceramic thin films. For accurate and efficient modeling, the fracture energy and the material strength (ultimate stress) are taken into account by the cohesive phase-field damage model. Moreover, we argue that the orientation of the grain morphology induces a preferential direction for the crack which serves as a weak spot for crack initiation and propagation. A novel equivalent fracture energy is introduced into the formulation to account for the effects of microstructure on the cracking behavior of thin films. On the experimental side, tensile tests on (V,Al)N and (V,Al)(O,N) thin films deposited on ductile substrates are performed. It has been shown that this approach is a fast and efficient tracking tool for determining the mode-I fracture properties. To evaluate the accuracy of the anisotropic cohesive phase-field damage model proposed in this study, the crack patterns and crack density values of two chemically different thin films are compared for both numerical and experimental setups. This work provides new insights into the effect of different material parameters on the damage behavior of thin films. The key parameters can be summarized as the ultimate strength, fracture energy as well as the existing residual stress within the film.

Lattice Boltzmann Study of Microdroplet Evaporation Process in Pixel Pits

Inkjet printing has the advantages of high material utilization, low cost, and large-area production and is a promising manufacturing technology for organic light-emitting diode (OLED) displays. However, the droplet evaporation in micron-size pixel pits is highly influenced by the pit wall. Such a process is extremely difficult to control, leading to the appearance of defects such as the coffee ring in the printing process of OLED displays. In this work, a multiphase thermal lattice Boltzmann (LB) model based on multiple distribution functions is established to study the evaporation process of micron-size droplets in pits. According to the characteristics of the largest number of the three-phase contact line (TCL) appearing in the evaporation process, the evaporation modes can be divided into three types, i.e., one, two, and three TCLs. In the 1-TCL mode, the droplet stays in constant contact radius (CCR) for the shortest time; in 2-TCL and 3-TCL modes, the liquid film fracture behavior of evaporating droplets in the pit is well captured. The effects of the pit height and the contact angle on the droplet evaporation mode are investigated in detail. The phase diagrams of evaporation modes with different parameters are also established. The revealed evaporation mechanism is supposed to be useful for regulating the droplet evaporation behavior and controlling the cured film shape in the OLED printing process.

Effect of texturing region on friction properties of journal bearings at different rotational speeds

The effect of texturing area on the friction properties of partially textured bearing at different rotational speeds was investigated theoretically. Four texturing configurations are limited on the bearing surface from the oil film fracture zone of the smooth bearing (200º to 360º) in the rotational direction with different eccentricity ratios. The results show that the friction coefficient of the partially textured bearing can be reduced compared with the smooth bearing. The friction coefficient decreases greater with the increasing texture depth at the fixed rotational speed. When the texturing configuration starts from the oil film fracture zone at the eccentricity ratio of 0.4 or 0.6, the partially textured bearings perform better at different speeds.

Melt-blended PLA/curcumin-cross-linked polyurethane film for enhanced UV-shielding ability

Abstract Biomass films with ultraviolet (UV)-shielding ability have attracted considerable attention. Curcumin was introduced into castor oil-based polyurethane (CCPU) as a chain extender, which was melt with polylactic acid (PLA) as a reinforcement to obtain biomass UV-shielding film. The excellent UV absorption and antioxidant qualities of curcumin contributed to the impressive UV-shielding capacity (97.6% UV radiation absorption) and antioxidant (51% free radical scavenging) of PLA/CCPU-20 film. In the scanning electron microscopic images of film fracture, the mixing of CCPU elastomer into the PLA matrix caused the blend films to exhibit significant toughening fracture characteristics compared to the pure PLA film. The excellent thermal stability, low water swelling degree, and low water solubility of PLA/CCPU blend films were maintained after CCPU was added to the PLA matrix. Therefore, the PLA/CCPU blend films can be considered as a potential packaging material because of its favorable UV-shielding properties and film stability.

Controlling and visualizing fracture of 2D crystals by wrinkling

Shaping two-dimensional (2D) crystals with well-defined edges along specific directions remains technically challenging due to the intrinsic lattice discreteness and anisotropy in their fracture toughness. The patterns of crystal cleavage are determined by the fracture resistance and loading conditions, and usually display kinking features. We show that the fracture of single-crystalline 2D films (MoS2, MoTe2, graphene) can be controlled by mechanically-induced wrinkles, and the distortion of their profiles can visualize the distorted local strain state near the crack tip. In thin samples with stretch-induced wrinkles, the elastic driving force with effective anisotropy steers crack propagation, resulting in straight edges following the corrugation profile. In contrast, thick samples remain flat under stretch for their elevated bending resistance, and the cracks advance with sawtooth patterns following the zigzag motif. The experimental findings are explained by wrinkling-induced elastic anisotropy, which dominates over fracture anisotropy. The control of fracture in 2D crystals can be applied to fabricate micro- and nanostructures for their applications in electromechanical and optoelectronic devices.

Experimental Investigations on the Chemo-Mechanical Coupling in Solid-State Batteries and Electrode Materials

Increasing attention has been paid to the safety and efficiency of batteries due to the rapid development and widespread use of electric vehicles. Solid-state batteries have the advantages of good safety, high energy density, and strong cycle performance, and are recognized as the next generation of power batteries. However, solid-state batteries generate large stress changes due to the volume change of electrode materials during cycling, resulting in pulverization and exfoliation of active materials, fracture of solid-electrolyte interface films, and development of internal cracks in solid electrolytes. As a consequence, the cycle performance of the battery is degraded, or even a short circuit can occur. Therefore, it is important to study the stress changes of solid-state batteries or electrode materials during cycling. This review presents a current overview of chemo-mechanical characterization techniques applied to solid-state batteries and experimental setups. Moreover, some methods to improve the mechanical properties by changing the composition or structure of the electrode materials are also summarized. This review aims to highlight the impact of the stress generated inside solid-state batteries and summarizes a part of the research methods used to study the stress of solid-state batteries, which help improve the design level of solid-state batteries, thereby improving battery performance and safety.

Capping layer enabled controlled fragmentation of two-dimensional materials by cold drawing.

Cold drawing, a well-established processing technique in the polymer industry, was recently revisited and discovered as an efficient material structuring method to create ordered patterns in composites consisting of both cold-drawable polymers and brittle target materials. Such a high-yield and low-cost manufacturing technique enables the large-scale fabrication of micro-ribbon structures for a wide range of functional materials, including two-dimensional (2D) layered materials. Compared to the abundant phenomenological results from experiments, however, the underlying mechanisms of this technique are not fully explored. Here, supported by experimental investigation, finite element calculations, and theoretical modeling, we systematically study the effect of a capping layer on the controlled fragmentation of 2D materials deposited on polymer substrates during the cold drawing. The capping layer is found to prevent the premature fracture of the 2D thin films during elastic deformation of the substrate, when a specific requirement proposed by the theoretical model is satisfied. Controlled fragmentation is enabled in the necking stage due to the protective effect of the capping layer, which also influences the size of the resulting fragments. Flexible and stretchable electrodes based on 2D material ribbons are fabricated to demonstrate the effectiveness of the proposed roadmap. This study gives an accurate understanding of interactions between 2D materials, polymer substrates, and capping layers during cold drawing, and offers guidance for potential applications such as flexible electronics.