Fracture Toughness Of Films Research Articles

Study of Interlaminar Fracture Toughness and Flexural Properties of Carbon Fibre Composites Reinforced with Polyethersulfone/Graphene Oxide Films

Unidirectional carbon fibre reinforced plastic (CFRP) composites were prepared by inserting polyethersulfone (PES) films or polyethersulfone/graphene oxide (GO) films into the composites and used to improve the interlaminar fracture toughness as well as the flexural properties. The effect of low-temperature cycling (35, 70, 105, and 140 cycles between −55 °C and room temperature (25 °C)) on the flexural properties of the specimens was also investigated, and the damage caused to the reinforcement layer was directly detected by comparing the interlaminar images. The results of the double cantilever beam (DCB) test and the end notched flexure (ENF) test were used to verify the optimisation of the interlaminar fracture toughness and flexural properties of the new composite film (PES/GO film). The results show that the performance of PES/GO film under both DCB test and ENF test is superior to that of PES film. Regarding the effect of low-temperature cycling on the specimens, the flexural properties of the specimens containing PES/GO films were improved. Finally, the matrix interface and failure mechanism of the interlayer reinforcement were investigated by scanning electron microscopy (SEM).

Effect of the alloying element Sc and Zr in 5B70 Al alloy on the microstructure, toughness and tribological properties of MAO ceramic film

The existence form of alloying element Sc and Zr in the micro arc oxidation (MAO) ceramic film on 5B70 Al alloy and their effects on the microstructure, fracture toughness, and tribological properties of the MAO ceramic film was investigated. The findings suggested that the Zr element primarily existed in the form of ZrO2, distributed around the Al2O3 phase. The Sc element remained spherical and dispersed in the form of Al3Sc in the ceramic film. The synergistic effect of ZrO2 and Al3Sc significantly improved the hardness, fracture toughness, and wear resistance of the dense layer in MAO ceramic film on 5B70 Al alloy. This provided theoretical basis and technical support for the development of high-performance MAO composite films on Al alloy surfaces.

Investigation of the nano and micromechanical performance of β-Ga2O3 epitaxial films on sapphire using nanoindentation

Beta-phase gallium oxide (β-Ga2O3) has attracted considerable attention because of its excellent properties as one of the emerging semiconductor materials. However, not only the fabrication of high quality β-Ga2O3 thin films on heterogeneous substrates is challenging, but there is also a lack of research pertaining to their mechanical properties. In this work, the mechanical properties of β-Ga2O3 epitaxial films of varying thicknesses on c-plane sapphire substrates fabricated via plasma enhanced chemical vapor deposition were investigated through nanoindentation and nano-scratch experiments. The β-Ga2O3 hetero-epitaxial thin films exhibit preferential orientation growth along the (−201) plane when deposited on the c-plane sapphire substrate. The elastic modulus, hardness, and fracture toughness of β-Ga2O3 films grown on sapphire substrates are 249.1 ± 14.1 GPa, 18.7 ± 0.9 GPa, and 1.822 MPa m1/2, respectively. The elastic-plastic behavior of β-Ga2O3 thin films with varying thicknesses is consistent. The critical loads for the elastic-plastic and plastic-brittle transitions are 9.4 ± 1.8 mN and 50.7 ± 2.7 mN respectively, while the scratch depths for the same are 91.8 ± 7.8 nm and 216.4 ± 10.1 nm, respectively. The critical adhesion and adhesive energy for the desquamation of the β-Ga2O3 film from the substrate are dependent on the thickness of the film. These results establish an experimental basis for understanding the mechanical behavior in the preparation and application of β-Ga2O3 thin films grown on sapphire substrates.

Toughness enhancement in TiN/Zr0.37Al0.63N1.09 multilayer films

The hardness and fracture toughness of high-temperature wear-resistant transition metal aluminum nitride multilayer films depend largely on the constituting layer's structure, compositional modulation, morphology, and interface coherency. We present a study on 1-micron thick multilayered films consisting of stacked layers of TiN and Zr0.37Al0.63N1.09, each layer being 10 nm thick. The films were grown using ion-assisted reactive magnetron sputtering on MgO(001) and Si(001) at substrate temperatures ranging from ambient to 900°C. By increasing growth temperature, we found that the ZrAlN layers transition from near amorphous to nanocrystalline wurtzite to decomposed c-ZrN and w-AlN domains. Concurrently, the TiN layers exhibit strong fiber texture, polycrystallinity, and epitaxial growth carried by the ZrN domains. Both hardness and fracture stress, evaluated by nanoindentation and micromechanical tests, increase with temperature from H=24 GPaMgO, 23 GPaSi to 36 GPaMgO, 30 GPaSi, and σFSi= 6.1-7.7 GPa, respectively. An improved fracture toughness of KIC=2.4-2.8 MPa√m is related to different toughening mechanisms for the various microstructures. The difference in hardness between the substrates is related to compressive stress due to the deposition conditions and thermal contraction. The superior fracture stress is attributed to dense multilayers, free from macroscopic defects due to ion-assisted growth. After being deposited at 200°C, the multilayers remained thermally stable when vacuum annealed for 15 hours at 900°C, with no significant change in phase composition or hardness. The improved hardness, toughness, and temperature stability of the otherwise brittle nitrides are promising for industrial applications.

Plane Stress Fracture Toughness Testing of Freestanding Ultra-Thin Nanocrystalline Gold Films on Water Surface.

Fracture toughness, which is the resistance of a material to crack propagation, is a critical material property for ensuring the mechanical reliability of damage-tolerant design. Recently, damage-tolerant design is introduced to flexible electronics by adopting micro-cracked ultra-thin nanocrystalline (NC) gold films as stretchable electrodes in a plane stress state. However, experimental investigation of the plane stress fracture toughness of those films remains challenging due to the intrinsic fragility from their sub-100nm thicknesses. Here, a quantitative method for systematically evaluating the plane stress fracture toughness of freestanding ultra-thin NC gold film on water surface platform is presented. After effectively fabricating single-edge-notched-tension samples with femtosecond laser, mode I stress intensity factors are measured in the plane stress state on water surface. Moreover, investigation regarding the effect of notch length, notch sharpness, and notch tip plasticity validates this method based on linear elastic fracture mechanics theory. As a demonstration, the thickness-dependent plane stress fracture toughness of ultra-thin NC gold films is qualitatively unveiled. It is revealed that the thickness confinement effect on grain boundary sliding induces a transition in fracture behavior. This method is expected to further clarify the fracture-related properties of various ultra-thin films for next-generation electronics.

Structure regulation and property correlation of Hf-B-N thin films

Hafnium nitride (HfN), as one of the most stable of transition metal nitrides, has shown great application potential in many fields. In this paper, a series of Hf-B-N films doped with different B contents were prepared. The results show that when the doping amount of B is 6.56 at.%, the hardness of Hf-B-N film is 24% higher than that of undoped HfN film, which is 35.27 GPa. And the wear rate of the film is one order of magnitude lower than that of the undoped HfN film. Solid solution strengthening and the stress generated by the coherent interface are responsible for the enhancement of strength. When the content of B continues to increase, the fracture toughness of HfN film is nearly 6 times higher than that of binary HfN film, and the highest fracture toughness is 5.98 MPa m1/2. It is found that the amorphous BN phase and the nanocrystalline HfBN phase coexist, and the synergistic effect of the ductile nanocrystalline phase and the amorphous phase improves the fracture toughness and bonding strength of the films. The films with high boron content have excellent oxidation resistance and corrosion resistance.

Effects of aluminum addition on microstructures and mechanical properties of NbTiVZr high-entropy alloy nitride films

In this study, the effects of Al addition on the phase evolution, morphology and mechanical properties of (NbTiVZrN)100-xAlx (x = 0, 0.7, 2.3, 3.6, 4.0) high-entropy alloy nitride films (HEANFs) were investigated. The films were prepared by co-sputtering of NbTiVZr alloy target and Al target with a constant N2/Ar flow ratio. The XRD patterns revealed FCC structure and the preferred orientation transferred from TiN(200) to (Nb, Zr)N(111) as the Al content increased. The results of the corresponding selected-area electron diffraction patterns agreed well with the XRD results. Cross-sectional SEM and TEM observations exhibited ultra-fine columnar structure. The nanoindentation test was performed to acquire the mechanical properties of the films. The hardness, reduced modulus and fracture toughness of nitride film increased with the increasing Al content and reached the maximum values of 28.1 GPa, 253 GPa and 2.08 MPa × m0.5, respectively, at x = 3.6. The significant improvement could be ascribed to solid solution strengthening, inverse Hall-Petch effect and the replacement of dominant nitride compounds. As the Al content increased from 0 to 4 at.%, the coefficient of friction decreased from 0.082 to 0.065. The relationship among the composition, structure and mechanical properties was studied for the (NbTiVZrN)100-xAlx HEANFs in this work. Comparisons with (NbTiVZr)100-xAlx HEAFs without nitridation and other nitride films were also made.

Role of Polymer–Nanoparticle Interactions on the Fracture Toughness of Polymer-Infiltrated Nanoparticle Films

Role of Polymer–Nanoparticle Interactions on the Fracture Toughness of Polymer-Infiltrated Nanoparticle Films

Exploring three-point-bending fracture toughness of thick diamond films from different directions

In this study, the effect of defects on fracture toughness in different directions was investigated. Two diamond films with a diameter of 122 mm and as-deposited thicknesses of 2.2 and 1.5 mm, respectively, were deposited by DC arc plasma jet chemical vapor deposition. To accurately obtain the fracture toughness, the films were ground and polished to 1.6 and 0.8 mm, respectively. The results indicate that many defects, including pores, are introduced into the films during the growth process, particularly on the side close to the growth surface in thicker film. The size of pores reaches micrometres, which affects the fracture toughness under loading in different directions. Due to the minimum grain size, both films exhibit a maximum toughness of 8.2 and 10.0 MPa·m1/2, respectively, with growth-surface cracks. For the thinner sample, the maximum value is followed by that of the edge-surface cracks. As the grain size of the edge surface falls between that of the growth surface and that of the nucleation surface, the results indicate that fracture toughness is affected by grain size. However, the number of pores near the growth side increases when the thickness exceeds 0.8 mm. Pores reduce the toughness, resulting in the minimum fracture toughness (6 MPa·m1/2) of the thicker diamond film with edge-surface laser cracks. The sample with an edge-surface sharp pre-crack has a similar fracture toughness of 5.6 MPa·m1/2. This study provides guidance for selecting the applied load direction.

Effects of modulation layer thickness on fracture toughness of a TiN/AlN-Ni multilayer film

A series of TiN/AlN-Ni multilayer films with different modulation layer thicknesses were prepared by magnetron sputtering. Their microstructures and mechanical properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). Therein, fracture toughness of multilayer films was determined by indentation method. The hardness and elastic modulus of all multilayers are greatly improved by introducing an AlN-Ni modulation layer. When the modulation-layer thickness is 2.4 nm, and the hardness and elastic modulus are the highest at the value of 39 GPa and 447 GPa, respectively, which is ascribed to Koehler strengthening and alternating stress hardening. Meanwhile the fracture toughness reaches the maximum, namely 5.49 MPa·m1/2. It is attributed to the indentation-induced phase transformation of c-AlN to w-AlN and the formation of a good coherent interface. With further increasing the thickness of the AlN-Ni modulation layer, the multilayer film gradually becomes an amorphous structure, resulting in the decrease of hardness and toughness.

Fracture mechanics of microcrystalline/nanocrystalline composited multilayer chemical vapor deposition self-standing diamond films

Though microwave plasma chemical vapor deposition (MPCVD) diamond films exhibit extraordinary strength, the toughness improvement is still a huge challenge. The catastrophic fracture of the diamond films is undesirable in many applications, especially for the application of fabricating cutting tools. In the present study, adopting the pre-notched and unnotched cantilever bending tests, fracture behaviors of monolayer microcrystalline diamond (m-M), monolayer nanocrystalline diamond (m-N), and microcrystalline/nanocrystalline composited multilayer diamond films were observed and compared. Typical fracture mechanics, including Young's modulus (E), fracture strength (σF), and fracture toughness (KIC) of different diamond films were investigated. Effects of diamond crystal structure, intrinsic stress, and tensile-side roughness were systematically analyzed. Grain size and graphite phase content dominated the E of self-standing diamond films. Tensive-side surface roughness and intergranular bonding strength affected the σF. When the growth side in tension, E of the multilayer film (modulation period Λ = 3 h) was 16.0% lower than m-M and 26.6% higher than m-N, σF of multilayer film was 16.8% higher than m-M and 8.3% lower than m-N. At fixed Λ, doubling the total deposition period, E, σF, and KIC of the multilayer film increased 72.3%, 22.3%, and 2.7%, respectively. MCD/NCD composited multilayer architecture presented significant strengthening and toughening effects on self-standing diamond films.

The combined use of wrinkling and cracking for the characterization of the mechanical properties of the laser-fabricated nanometer-thick amorphous carbon films

Direct laser writing carbonization has been recently developed to enable facile fabrication of patternable nanometer-thick two-dimensional amorphous carbon films, which may find promising applications in high-performance sensing, energy storage, catalysis, biomaterials etc. Given its nanometer-thick character and brittleness in nature, it is quite challenging to evaluate and characterize the mechanical properties of this novel material. With assistance of a sacrificial-layer transfer approach for sample preparation, herein, we rely on wrinkling metrology as a viable tool for characterizing the elastic modulus of the laser-fabricated nanometer thick amorphous carbon films. Moreover, upon combining the wrinkling metrology with channel cracking behavior investigation, the strength-related mechanical properties, including fracture toughness, fracture strength and fracture strain of the carbon thin films of varied thickness have been evaluated comprehensively. Within the experimental conditions being studied, the elastic modulus, fracture toughness, fracture strength and fracture strain of the laser-fabricated nanometer-thick carbon films respectively fall in a range of 23.1–34.9 GPa, 16.3–151.7 J/m2, 0.5–1.4 GPa, and 2.0–4.5%. The results and methodology presented in this study demonstrate the usefulness of combining wrinkling metrology and cracking technique as a simple but generally applicable approach for comprehensive characterization and evaluation of the mechanical properties of different types of brittle and nanometer-thick thin films.

Stamp-Perforation-Inspired Micronotch for Selectively Tearing Fiber-Bridged Carbon Nanotube Thin Films and Its Applications for Strain Classification.

Cracks typically deteriorate the structural and electrical properties of materials when not properly controlled. A few papers recently reported the controlling methods of crack formation in the brittle materials utilizing the lateral V-notch structure. For ductile materials, however, there have been few papers reporting cracking phenomenon, but full cracking control including predesigned initiation, propagation, and termination has not been reported yet. Therefore, we report a predesigned full cracking control in ductile conductive carbon nanotube (CNT) films by introducing inkjet-printed L-shape micronotch (LMN) structures inspired by directional stamp perforation marks. In spite of the high fracture toughness of CNT films, the LMNs determine locations of initial crack formation and guide crack propagation in a predesigned way. Selective connection of isolated cracks in the CNT film increases its resistance monotonically under tensile strain and thus tremendously well maintains high linearity (adj. R2 value > 0.99) in resistance change over record large strain ranges of 0.01-100%, which enables us to quantitatively classify strain values accurately for previously reported practical body signals for the first time. We believe that our facile printing-based crack control strategy not only provides a comprehensive solution to various stretchable sensor applications but also builds a new milestone for cracking mechanism studies in fracture mechanics.

Improved wear imbalance with multilayered nanocomposite nanocrystalline Cu and tetrahedral amorphous carbon coating

Tetrahedral amorphous carbon (ta-C) has a relatively high hardness, and it can be used to enhance film properties such as wear resistance. However, the high hardness of ta-C can adversely affect a counterpart and accelerate its wear, and the resulting wear imbalance between the film and its counterpart can cause vibrations. This issue may be resolved by improving the wear of the counterpart. This study aimed to reduce the hardness and improve the fracture toughness of ta-C films to enhance the durability of a tribosystem, which was achieved by toughening a composite and ductile phase. A multilayered nanocrystalline (nc)-Cu/ta-C nanocomposite film was fabricated that allowed for reductions in the wear of the film and its counterpart of more than 88% and 99%, respectively.

Deposition and mechanical properties of δ-TaNx films with different stoichiometry by DC magnetron sputtering

δ-TaNx films with different stoichiometry were deposited by DC magnetron sputtering on Si (100) substrate. Their microstructures and compositions were investigated by XRD, XPS, Raman and HRTEM. The hardness (H), elastic modulus (E), fracture toughness and tribological properties of films were evaluated by the ultra-micro indentation and a ball-on-disk tribometer. The correlation between H/E, H3/E2 and wear rate was discussed. As indicated by the results, the H, toughness of δ-TaNx films can be modified by controlling the variation of stoichiometric (N/Ta ratio). The H decreases from 39.5 to 24.7 GPa with N/Ta ratio increasing from 0.95 to 1.21. Meanwhile, the wear resistance deteriorates with the increase of N/Ta ratio. The film with a sub-stoichiometric shows a higher H/E, H3/E2, a better resistance of cracking and an improved wear resistance. Sound mechanical properties such as high hardness (H ~ 39.5 GPa), toughness (H/E ~ 0.12), resistance to cracking, lower wear rate (4.6 × 10−6 mm3/Nm) are simultaneously obtained from δ-TaN0.95 film.

First-principles surface energies for monoclinic Ga2O3 and Al2O3 and consequences for cracking of (AlxGa1−x)2O3

Crack formation limits the growth of (AlxGa1−x)2O3 epitaxial films on Ga2O3 substrates. We employ first-principles calculations to determine the brittle fracture toughness of such films for three growth orientations of the monoclinic structure: [100], [010], and [001]. Surface energies and elastic constants are computed for the end compounds—monoclinic Ga2O3 and Al2O3—and used to interpolate to (AlxGa1−x)2O3 alloys. The appropriate crack plane for each orientation is determined, and the corresponding critical thicknesses are calculated based on Griffith’s theory, which relies on the balance between elastic energy and surface energy. We obtain lower bounds for the critical thickness, which compare well with available experiments. We also perform an in-depth analysis of surface energies for both relaxed and unrelaxed surfaces, providing important insights into the factors that determine the relative stability of different surfaces. Our study provides physical insights into surface stability, crack planes, and the different degrees of crack formation in (AlxGa1−x)2O3 films for different growth orientations.

Effects of Hf addition on the structure, mechanical and tribological properties of CrN film

Alloying elements in binary nitride is considered as a promising method to design hard and tough films. Hf atoms are proposed to be incorporated in CrN nitrid films to form ternary nitrides, which may possess the improved mechanical and tribological properties. In this study, Cr1-xHfxN [x = Hf/(Cr + Hf)] films were sputtering deposited by incorporating Hf into CrN film in an mixed atmosphere of Ar and N2. The influences of Hf addition on the microstructure, mechanical and tribological properties of Cr1-xHfxN films were investigated. The results show that the hardness and fracture toughness of CrN film is significantly improved due to solid solution strengthening by incorporating Hf atoms. Thanks to the improved hardness and toughness, the wear resistance of CrN film is enhanced. A high H/E and H3/E2 values, which are regarded as the reflection of toughness of hard films, are obtained from Cr0.69Hf0.31N film with higher hardness (30.49 GPa). The lowest wear rate of about 1.03 × 10−6 mm3/Nm is also presented for Cr0.69Hf0.31N film, which corresponds to higher H/E(~0.125) and H3/E2(~0.481 GPa).

Effect of curing time on phase morphology and fracture toughness of PEK-C film interleaved carbon fibre/epoxy composite laminates

The phase morphology developed by thermosetting and thermoplastic resin is one of the main concerns in interleaved thermosetting composites by thermoplastic resin, which directly affects the toughening efficiency. The carbon fibre reinforced epoxy matrix composite laminates were interleaved by Polyetherketone-cardo (PEK-C) films and the effect of curing time on the phase morphology spectrum and fracture toughness was investigated in the study. The microstructure-property correlation was explored. The results showed that the PEK-C film interlayer was efficient in the interlaminar toughening and the fracture toughness (GIC) was enhanced by 90.65% at curing time of 1 h. Nevertheless, increasing curing time weakened toughening effect of PEK-C films and GIC was only increased by 9.37% under the curing time of 3 h. The characteristic morphology spectrum in the interlaminar region, consisting of homogeneous PEK-C resin, nodular dualphase and sea-island dualphase, contributed to the improvement of toughness. The homogeneous PEK-C layer was decreased with the curing time increasing due to the more fully permeation of epoxy, resulting in the crack path change from the center of interlaminar region and the interface between interlaminar region and composite layer to merely the interface, which might be the main reason for the weakening of measured crack propagation resistance.

Fracture toughness of amorphus SiC thin films using nanoindentation and simulation

Fracture toughness of SiC on Si thin films of thicknesses of 150, 750, and 1500 nm were measured using Agilent XP nanoindenter equipped with a Dynamic Control Module (DCM) in Load Control (LC) and Continuous Stiffness Method (CSM) protocols. The fracture toughness of the Si substrate is also measured. Nanovision images implied that indentations into the films and well deep into the Si caused cracks to initiate at the Si substrate and propagate upward to the films. The composite fracture toughness of the SiC/Si was measured and the fracture toughness of the SiC films was determined based on models that estimate film properties from substrate properties. The composite hardness and modulus of the SiC films were measured as well. For the DCM, the hardness decreases from an average of 35 GPa to an average of 13 GPa as the film thick increases from 150 nm to 1500 nm. The hardness and moduli of the films depict the hardness and modulus of Si at deep indents of 12 and 200 GPa respectively, which correlate well with literature hardness and modulus values of Si. The fracture toughness values of the films were reported as 3.2 MPa√m.

Fracture Behaviour of Collagen: Effect of Environment

Collagen forms one-third of the human-body proteome and finds a wide range of applications in a biomedical field thanks to its mechanical stability, biocompatibility and biodegradability. Collagen can be produced in a form of films suitable for scaffolds, tissue regeneration, flexible electronics etc. significant differences in the mechanical properties were observed for collagen films tested in-aqua environment. Considering this and potential biomedical applications of collagen films, their mechanical testing should be performed in aqua to mimic the in-vivo conditions. Hence, this study reported the fracture behaviour of collagen in-aqua compared with that at ambient (in-air) loading conditions. Single-edged notched tension (SENT) specimens of collagen films demonstrated completely different stress-strain curves in-aqua conditions. A reduction in their tensile strength (by 90%) and fracture energy (by 40%) accompanied with an increase in the failure strain (by 1600%) was observed for such conditions. Crack propagation was rapid for in-air specimens, with a brittle failure, while for in-aqua specimens the crack opening was rather slow and accompanied with by crack blunting, leading to large plastic deformation (ductile failure). These behaviours encouraged the quantification of the fracture toughness of collagen films using different fracture toughness parameters: KIC (linear elastic fracture mechanics) for in-air specimens and JC-integral (elastic-plastic fracture mechanics) for in-aqua specimens.