Toughness Of Films Research Articles

Determination of residual stress and interface adhesion toughness of thin films by blisters

Circular blisters and telephone cord blisters (TCBs) can spontaneously occur in thin films under constant bi-axial compressive residual stress. In this work, new mechanical models are used in conjunction with measurements of blister morphology parameters to determine the residual stress in films and the adhesion toughness at interfaces. These new models are based on the hypothesis that pockets of energy concentration (PECs) drive the nucleation and growth of blisters instead of buckling, as in the conventional models, since the thin films are under constant compressive residual stress. Predictions from the models are in excellent agreement with independent experimental data.

PPTA-oligomer functionalized multiwalled carbon nanotubes synthesized by “one-pot” method for reinforcement of polyvinyl chloride

Multiwalled carbon nanotubes (MWNTs) are fillers of choice for composite reinforcement owing to their outstanding modulus and strength. However, the poor dispersion and load transfer between MWNTs and matrix still limit their potential and further application of MWNTs in mechanical reinforcement. In this paper, we report a facile “one-pot” method for functionalization of MWNTs with PPTA-oligomer (o-PPTA). The obtained o-PPTA functionalized multiwalled carbon nanotubes (MWNTs/o-PPTA) can be well dispersed in DMF solvent. Polyvinyl chloride (PVC) was chosen as a model polymer for investigating the reinforcement effect of MWNTs/o-PPTA. The results of tensile tests illustrated that the Young’s modulus, toughness and yield strength of composite films were obviously improved compared to those of neat PVC film, which were increased by 129%, 262% and 126%, respectively. The factors including molecular weight of loaded o-PPTA and dosage of MWNTs/o-PPTA on PVC reinforcement effect were also studied. The results showed that MWNTs/o-PPTA is promising additive for reinforcing PVC.

Fretting fatigue behaviors and surface integrity of Ag-TiN soft solid lubricating films on titanium alloy

Abstract Titanium alloys are widely used in industrial fields because of their superior characteristics, but they also have disadvantages leading to fretting fatigue (FF) damage. The silver-based composite films were deposited on a Ti6Al4V alloy substrate using ion-assisted magnetron sputtering for improving the fretting fatigue (FF) resistance of a Ti6Al4V alloy. The surface morphology was characterized by atomic force microscopy (AFM), and micro-structure was characterized by X-ray photoelectron spectroscopy measurements (XPS), X-ray diffractometry (XRD), and transmission electron microscopy (TEM), respectively. The bonding strength and toughness of films were analyzed using a repeated press–press test system that was manufactured in-house. The FF resistance of titanium alloy with the silver-based composite films was investigated using home-made FF devices. The results showed that the FF resistance of the titanium alloy was improved by the Ag-TiN composite films, which were fabricated with an Ag solid lubricating film doped with a hard TiN phase (60–80 at.% Ag). This is attributed to the fact that the composite films had better comprehensive strength-toughness behavior, friction lubrication property, and high bonding strength. However, when the atomic fraction of Ag in the composite film was larger than 90%, it became difficult to form the TiN phase, and it failed to improve the comprehensive strength-toughness behavior of the composite film.

Effect of the varied nitrogen vacancy concentration on mechanical and electrical properties of ZrNx thin films

ZrNx films were deposited with reactive magnetron sputtering in different [Ar]/([Ar] + [N2]) atmospheres and analyzed using X-ray photoelectron spectroscopy, glancing incidence X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, nanoindentation techniques, and four-point probe meter. N vacancy concentration is defined as VN = 1- x, where x is N/Zr ratio in ZrNx. Results showed that the VN ranged from 0.02, through 0.08, 0.13, 0.24 to 0.30. The glancing incidence X-ray diffraction results showed a decrease in lattice parameter a0 from 4.582, through 4.578, 4.571, 4.564 to 4.548 Å with the increase of VN from 0 to 0.30. Berkovich nanoindentation results showed that the hardness of ZrNx films increased from 18 ± 1.3 GPa to 24 ± 1.2 GPa with the increase of VN from 0 to 0.24, which is due to vacancy-induced hardening. However, further increase of VN to 0.30, a drop-in hardness was observed, which was attributed to vacancy-induced softening. Both hardening and softening proved the existing of a turning point at VN around 0.24. In addition, the fracture toughness and sheet resistance of ZrNx films also presented the same tendency. The experimental results agreed with first principle predictions that the metal-N (d-p) bonds, ZrZr (d-d) σ bonds and the valance electron density around Zr atoms had changed greatly due to the varied VN.

Bioinspired interface engineering of gelatin/cellulose nanofibrils nanocomposites with high mechanical performance and antibacterial properties for active packaging

Developing an integrated biopolymer material combining favorable mechanical properties and antibacterial activity remains a great challenge. In this study, we designed a tough, hydrophobic, and antibacterial gelatin-based nanocomposites by constructing synergistic multiple coordination bonds between dopamine (DA)-grafted cellulose nanofibrils (DCNF) and montmorillonite (MMT) in a covalent polymer network. Silver nanoparticles (AgNPs) were prepared and anchored to the gelatin chains and MMT-DCNF nanohybrids using in-situ reduction and the strong catechol-metal interactions of poly(dopamine). As a result of the efficient energy dissipation ascribed to the multiple cross-linked network structure, the tensile stress and toughness of the hybrid films were simultaneously improved by 400.9% and 991.5%, respectively. The hybrid films displayed strong antibacterial activity against Staphylococcus aureus (gram-positive) and Escherichia coli (gram-negative) pathogenic bacteria because of the incorporation of AgNPs into the gelatin matrix. Compared with traditional active packaging, this gelatin-based nanocomposite film is renewable and biodegradable, it exhibited superior mechanical properties, UV-shielding capacity, water resistance, thermal stability, and antibacterial activity, which are essential characteristics for commercial applications in complex environments. This work provides a novel and facile strategy for the design of integrated gelatin-based nanocomposites with promising functional properties for application as active packaging materials.

Developing strong and tough carbon nanotube films by a proper dispersing strategy and enhanced interfacial interactions

A new approach for making carbon nanotube (CNT) films that are both strong and tough is detailed in this paper. Aqueous dispersions of long CNTs were first obtained by dispersing CNT arrays with heights of about 200 μm in deionized water with the help of polyvinylpyrrolidones (PVPs) and polydopamines (PDAs). CNT films were then obtained by vacuum-assisted filtration of CNT dispersions. The tensile strength and toughness of the composite films can be as high as 143.5 MPa and 5.0 MJ/m3, respectively, which was 330.7% and 213.8% higher than CNT films made without using PVPs and PDAs. The underlying mechanism of PVPs and PDAs induced CNT film strengthening and toughening was revealed through preliminary molecular dynamics simulations, which indicated that the hydrogen bonds between the PVPs and PDAs enhanced the strength of the film, and these bonds can cyclically break and reform, making CNT films tougher. The strategy proposed here can also be applied to making other nano-materials based strong and tough macro-assemblies.

Ultrahigh Tough, Super Clear, and Highly Anisotropic Nanofiber-Structured Regenerated Cellulose Films.

While tremendous efforts have been dedicated to developing environmentally friendly films made from natural polymers and renewable resources, in particular, multifunctional films featuring extraordinary mechanical properties, optical performance, and ordered nanostructure, challenges still remain in achieving all these characteristics in a single material via a scalable process. Here, we designed a green route to fabricating strong, super tough, regenerated cellulose films featuring tightly stacked and long-range aligned cellulose nanofibers self-assembled from cellulose solution in alkali/urea aqueous systems. The well-aligned nanofibers were generated by directionally controlling the aggregation of cellulose chains in the hydrogel state using a preorientation-assisted dual cross-linking approach; i.e., a physical cross-linking was rapidly introduced to permanently reserve the temporarily aligned nanostructure generated by preorienting the covalent cross-linked gels. After a structural densification in air-drying of hydrogel, high strength was achieved, and more importantly, a record-high toughness (41.1 MJ m-3) in anisotropic nanofibers-structured cellulose films (ACFs) was reached. Moreover, the densely packed and well-aligned cellulose nanofibers significantly decreased the interstices in the films to avoid light scattering, granting ACFs with high optical clarity (91%), low haze (<3%), and birefringence behaviors. This facile and high-efficiency strategy might be very scalable in fabricating high-strength, super tough, and clear cellulose films for emerging biodegradable next-generation packaging, flexible electronic, and optoelectronic applications.

EVA Film Reinforced with Acid Functionalized Graphene Nanoplatelets as a Transparent Barrier Layer to Enhance the Durability of Solar Cells

This research work concerns the development of transparent barrier films based on ethylene vinyl acetate copolymer (EVA) reinforced with the acid functionalized graphene nanoplatelets (GNP). GNP was melt-compounded with EVA in a micro-compounder via two types of mixing schemes. The effect of concentration of GNP on the barrier, thermo-mechanical and optical properties of the EVA composite films were investigated. Water vapour transmission rate of the EVA film decreased by more than 70% after 0.001-0.01 wt% of the GNP was applied. The visible light transmittance in the range of 84-91% was maintained. SEM images showed a random distribution of GNP in the EVA matrix with intimate contact between phases. FTIR spectra imply a polar-polar interaction between the two materials. Tensile toughness of the composite films increased with the concentration of GNP. These demonstrated that the acid functionalized GNP is compatible with EVA. The degree of swelling of the cured EVA films also decreased with the GNP content. The above effects were less obvious when the mixing scheme was changed to the master-batch method. Finally, by applying the EVA/GNP film to encapsulate dye-sensitised solar cells, the lifetime of the device was prolonged by at least three weeks, with the normalised PCE value of 0.74.

Adiabatic effects on the temperature and rate dependency of the fracture toughness of an ethylene-fluoroethylene film

In this work, the essential work of fracture (EWF) test has been applied to determine the temperature and strain-rate dependency of the fracture toughness of ethylene-fluoroethylene (ETFE) thin films.EWF tests were conducted at different temperatures (23, 50 and 80 °C) and different cross-head rates (1, 10, and 100 mm/min). At 23 °C, the specific essential work of fracture (we) values slightly increase as the strain rate increases, while at 50 °C and 80 °C we values decrease. This trend is associated to an increase of the specific essential work for crack propagation (we,prop) while the specific essential work for crack initiation (we,init) decreases with strain rate with a magnitude related to the testing temperature. Thermal observations with an infrared camera revealed an increase of the temperature at the crack tip for the highest strain rate, thus suggesting, for the sample tested at the lowest temperature, the concurrent effects of strain-rate embrittlement and adiabatic heating. Digital image correlation (DIC) analysis revealed an ovoid shape of the outer plastic zone (OPZ) prior to the start of the crack propagation. DIC also indicated that the extension of the OPZ increases linearly with the square of the ligament length.

Effect of bias on structure mechanical properties and corrosion resistance of TiNx films prepared by ion source assisted magnetron sputtering

Titanium nitride (TiNx) films were deposited on 304 stainless steel and silicon (100) wafers by ion source assisted magnetron sputtering. The effect of substrate bias voltage on the preparation and properties of TiNx films was investigated. The results show that as the substrate bias voltage increases from 0 V to −400 V, the deposition rate of TiNx film decreases significantly and the density increases, which can be attributed to the re-sputtering effect at high ion bombardment. The texture of TiNx films varies gradually from (111) to (200) with the increase of energy delivered into the growing film. The hardness of the film deposited under −200 V bias voltage reaches the maximum value, 12.9 GPa. Meanwhile, the toughness and corrosion resistance of the film reach a maximum at a bias voltage of −400 V. In conclusion, the density, hardness, toughness and corrosion resistance of TiNx films can be increased by changing the substrate bias voltage.

Improvement in antibacterial and functional properties of mussel-inspired cellulose nanofibrils/gelatin nanocomposites incorporated with graphene oxide for active packaging

Inspired by the natural mussel chemical engineering, we proposed a novel and biomimetic strategy for the fabrication of strong integrated gelatin/cellulose nanofibrils (CNF) nanocomposite materials reinforced with functional graphene oxide (GO) nanohybrids. GO nanosheets were chemically reduced and dip-coated to yield poly(dopamine) (PDA)-functionalized GO (PGO) via self-polymerization of catechol-containing dopamine. Furthermore, Ag nanoparticles (NPs) were further prepared and immobilized by the catechol groups of the functional PDA layers through in-situ reduction and strong catechol-metal interaction. The fracture surface morphology clearly revealed that the interfacial adhesion of gelatin-based nanocomposite was remarkably improved via the synergistic effect and multiple cross-linking interactions of the CNF-PGO nanohybrids and Ag NPs. The tensile strength and toughness of the nanocomposite film were simultaneously increased to 35.99 MPa and 13.49 MJ/m3, respectively. The water vapor permeability of the nanocomposite was remarkably reduced by 51.60%. The nanocomposite film showed strong antibacterial activity against both S. aureus and E. coli pathogenic bacteria. In addition, the film also exhibited significant improvement in the water resistance, UV-shielding capacity, and thermal stability. Hence, the bio-inspired strategy for constructing functional PGO nanohybrids in renewable natural gelatin/CNF nanocomposites provides a promising approach to the fabrication of active packaging materials for a wide range of industrial applications.

Experimental investigation on the dilatational interfacial rheology of dust-suppressing foam and its effect on foam performance

Foam dust suppression is an effective technique for controlling dust at its source. Dilatational interfacial rheology is an important property of aqueous foam. It characterizes the strength and toughness of liquid film and acts on the foaming process and drainage, affecting foaming capacity (FC) and foam stability (FS). The interfacial viscoelastic properties of anionic and nonionic surfactants commonly used were studied by testing the viscoelastic properties with different foaming agent concentrations (FAC) and their effects on foam performance were investigated. The results show that increasing FAC from 0.1 wt. ‰ to 1.0 wt. ‰ generally decreased the viscoelastic modulus, which was negatively correlated with FC: 6501 had a highly linear relationship between FC and viscoelastic modulus. The viscoelastic modulus of K12 and AES were positively correlated with FC in the low-modulus range, then negatively correlated at higher viscoelastic modulus values; the turning point was defined as the optimal viscoelastic modulus. The viscoelastic modulus had a significant positive effect on FS. The surface viscoelasticity contributed greatly to improving FS before the critical micelle concentration (CMC) was reached. After reaching CMC, micelle properties of bulk phase mainly affected the foam life.

Cellulose Nanocrystals (CNCs) Induced Crystallization of Polyvinyl Alcohol (PVA) Super Performing Nanocomposite Films.

This study is aimed to explore the properties of cellulose nanocrystals (CNC)/polyvinyl alcohol (PVA) composite films with and without 1,2,3,4-butane tetracarboxylic acid (BTCA), a nontoxic crosslinker. CNC and CNC-PVA nanocomposite films are prepared using solution-casting technique. Differential scanning calorimetry (DSC) analyses show that crosslinking increased the glass transition temperature but reduced the melting temperature and crystallinity. Furthermore, high CNC concentrations in the PVA matrix interfere with PVA crystallinity, whereas in specific ratio between CNC and PVA, two different crystalline structures are observed within the PVA matrix. Film surfaces and fracture topographies characterized using scanning electron microscope indicate that at certain CNC-PVA ratios, micron-sized needle-like crystals have formed. These crystalline structures correlate with the remarkable improvement in mechanical properties of the CNC-PVA nanocomposite films, that is, enhanced tensile strain and toughness to 570% and 202MJm-3 , respectively, as compared to pristine PVA. BTCA enhances the tensile strain, ultimate tensile stress, toughness, and modulus of CNC films compared to pristine CNC films. Water absorption of crosslinked CNC and CNC-PVA nanocomposite films is significantly reduced, while film transparency is significantly improved as a function of PVA and crosslinker content. The presented results indicate that CNC-PVA nanocomposite films may find applications in packaging, and though materials applications.

The effect of CdO–ZnO nanoparticles addition on structural, electrical and mechanical properties of PVA films

Cadmium oxide doped zinc oxide (CdO–ZnO) nanoparticles were synthesized by solution combustion technique. CdO–ZnO nano particles were used as fillers and their concentration ranged between 0.5% to 2.5% by weight in polyvinyl alcohol (PVA) films. The effect of nano filler concentration on structural, electrical and mechanical properties was investigated. Dielectric constant values decreased upto 1.24 GHz frequency, between 1.24 to 3 GHz frequency range a slight increase in dielectric constant was observed. At frequencies above 500 kHz increase in AC conductivity and loss tangent of the polymer nanocomposites were observed. Enhanced mechanical properties were observed with increase in dopant concentration. A significant increase of 152.65% in film toughness, 113.58% increase in elastic modulus and 83.10% increase in ultimate tensile strength was observed at 2.5 wt% concentration of CdO–ZnO in PVA film as compared to pristine PVA film, which makes CdO–ZnO a desirable reinforcing material for nanocomposites.

Effects of surface oxide layer on fracture toughness of submicrometer-thick copper film

Effects of surface oxide layer on fracture toughness of submicrometer-thick copper film

Toughening Nanoparticle Films via Polymer Infiltration and Confinement.

Disorderednanoparticle filmshave significant technological applications as coatings and membranes. Unfortunately, their use to date has been limited by poor mechanical properties, notably low fracture toughness, which often results in brittle failure and cracking. We demonstrate that the fracture toughness of TiO2 nanoparticle films can be increased by nearly an order of magnitude through infiltration of polystyrene into the film. The fracture properties of films with various polymer volume fractions were characterized via nanoindentation pillar-splitting tests. Significant toughening is observed even at low volume fractions of polymer, which allows the nanoparticle packing to be toughened while retaining porosity. Moreover, higher-molecular-weight polymers lead to greater toughening at low polymer volume fractions. The toughness enhancement observed in polymer-infiltrated nanoparticle films may be attributed to multiple factors, including an increase in the area and strength of interparticle contacts, deflection and blunting of cracks during failure, and confinement-induced polymer bridging of nanoparticles. Our findings demonstrate that polymer infiltration is a highly effective route for reinforcing nanoparticle packings while retaining porosity.

Evaluation of the fracture toughness of Ti1-xZrxN hard coatings: Effect of compositions

The objectives of this study were to evaluate the fracture toughness of Ti1-xZrxN hard coatings using the internal energy induced cracking (IEIC) method and to investigate the compositional effect on the fracture toughness, from which the optimum composition for fracture toughness could be attained. Ti1-xZrxN was selected to be the model system, because Ti1-xZrxN remained single phase structure in the entire compositional range at deposition temperature below 500 °C. Three compositions of Ti1-xZrxN coatings, x = 0.25, 0.55 and 0.85, were deposited by unbalanced magnetron sputtering. The parameters of IEIC method included the residual stress determined by the laser curvature method, Young's modulus obtained from nanoindentation and the film thickness measured from SEM cross-sectional image. The residual stress and film thickness before specimen fracture were used to calculate the elastic stored energy (Gs), from which the fracture toughness (Gc) could be derived. The resultant Gc of the Ti1-xZrxN coatings varied with Zr fraction, ranging from 26.0 to 48.7 J/m2, and reaching a maximum at a composition of Ti0.15Zr0.85N. The results showed that adding Zr atoms into TiN could effectively increase the fracture toughness. The maximum increase of fracture toughness lay in the intermediate range of composition (Zr = 0.55–0.85), suggesting that different properties of Ti and Zr atoms may play an important role on the fracture toughness of single phase Ti1-xZrxN thin films. The increase of Gc with Zr composition may be correlated with the change of configurational entropy of the Ti-Zr-N system. The atomic size difference of Zr and Ti may be crucial on increasing fracture toughness. The increase of fracture toughness for Zr-dominant Ti0.15Zr0.85N coating was higher than that for Ti-dominant Ti0.75Zr0.25N coating. This asymmetrical behavior could be attributed to the difference in lattice constants between Ti-rich and Zr-rich compounds, where the capability of increasing Gs may be higher for a smaller Ti atom incorporated into a larger Zr site in ZrN lattice.

First-principles study of fracture toughness enhancement in transition metal nitrides

Due to its superior thermophysical properties, ZrN thin film has been studied as a diffusion barrier for a variety of applications. However, cracks and delamination of ZrN coating takes place because of its brittleness. To identify new diffusion barrier materials with improved fracture toughness, we systematically studied the mechanical properties, surface energies and fracture toughness of IVB, VB and VIB transition metal mononitrides using first-principles calculations. TiN and HfN show slightly better toughness than ZrN. To improve the toughness of ZrN, we investigated the fracture toughness of ZrN alloyed with 25% IVB, VB and VIB transition metal nitrides. TiN, HfN and TaN was found to be able to increase the fracture toughness of ZrN through alloying. Meanwhile, we predicted the interfacial fracture toughness of ZrN/TiN and ZrN/HfN interfaces to evaluate the fracture toughness of ZrN/TiN and ZrN/HfN multilayer structures. Both ZrN/TiN and ZrN/HfN films exhibit higher interfacial fracture toughness than that of ZrN single film. Therefore, alloying and multilayer structures are promising approaches to enhance the toughness of brittle ZrN film.

Tough and conductive bio-based artificial nacre via synergistic effect between water-soluble cellulose acetate and graphene

Inspired by the hierarchical structure of natural nacre, a binary artificial nacre with excellent mechanical performance, tensile strength and toughness was successfully constructed based on water-soluble cellulose acetate (WSCA) and reduced graphene oxide (rGO). The tensile strength and toughness of the hybrid films were markedly improved by the introduction of rGO, and the tensile strength and toughness of the rGO/WSCA composite films reached 499.3 MPa and 9.6 MJ/m3 (rGO-WSCA-III), respectively, which are approximately 5.0 and 5.3 times that of natural nacre. In addition, the hybrid rGO/WSCA films showed better flame retardancy and stability in water, and high electrical conductivity. In this study, we provide a strategy to design and fabricate exceptionally strong and tough artificial nacre, which has potential application in tissue engineering and as a humidity sensor.

Crack on a chip test method for thin freestanding films

Fracture mechanics has been applied for more than two decades to various configurations of cracks in films on substrate. Fracture toughness data are indeed needed for the design and integrity assessment of many coatings and microelectronics devices. Nevertheless, it is sometimes complicated to deconvolute the constraint exerted by the substrate on the cracking process especially in the presence of viscoelastic or plastic dissipation. Here, a new on chip test method has been developed to determine the fracture toughness of freestanding submicron films. Beside the advantage of avoiding the constraint induced by the substrate, freestanding films allow, if sufficiently thin, direct observation of the fracture mechanisms by transmission electron microscopy. The design of this new nano(micro)-testing consists of two long actuator beams undergoing large internal stress. A specimen is attached to these two actuators, incorporating a notch produced by lithography. Two types of geometries are addressed, one being a double cantilever beam type configuration, while the other is a center cracked panel. Both actuators and specimen are deposited on a sacrificial layer. The etching of the sacrificial layer induces the release of the test structure, with the actuators then contracting and pulling on the test specimen. A crack is initiated from the notch tip, propagates and finally stops when the energy release rate has decreased down to its critical value. This crack arrest measurement avoids the problem of introducing a sufficiently sharp precrack. Analytical equations that describe the stress intensity factor as a function of the geometrical characteristics of the test structures are worked out to guide the dimensional analysis. Extensive finite element analysis provides the full parameter variations necessary to quantify the fracture toughness from experimental data and to capture the process of initiation, unstable cracking and arrest followed by possible further stable propagation. For the sake of a proof of concept, ∼50 and ∼100 nm-thick silicon nitride films produced by low pressure chemical vapor deposition were tested, leading to a mean fracture toughness equal to ∼2 MPam.