Toughness Of Films Research Articles

Co‐poly(vinyl chloride‐vinyl acetate‐vinyl alcohol)‐silica nanocomposites from sol–gel process: Morphological, mechanical, and thermal investigations

AbstractOrganic–inorganic nanocomposites consisting of co‐poly(vinyl chloride‐vinyl acetate‐vinyl alcohol) and silica were prepared via sol–gel process. Two types of hybrids were prepared, one in which interactions between hydroxyl group present in the copolymer chain and silanol groups of silica network were developed. In the second set, extensive chemical bonding between the phases was achieved through the reaction of hydroxyl groups on the copolymer chains with 3‐isocyanatopropyltriethoxysilane (ICTS). Hydrolysis and condensation of tetraethoxysilane and pendant ethoxy groups on the chain yielded inorganic network structure. Mechanical and thermal behaviors of the hybrid films were studied. Increase in Young's modulus, tensile strength, and toughness was observed up to 2.5 wt % silica content relative to the neat copolymer. The system in which ICTS was employed as binding agent, the tensile strength and toughness of hybrid films increased significantly as compared to the pure copolymer. Thermogravimetric analysis showed that these nanocomposite materials were stable up to 250°C. The glass transition temperature increases up to 2.5 wt % addition of silica in both the systems. Field emission scanning electron microscope results revealed uniform distribution of silica in the copolymer matrix. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Effects of aromatic carboxylic dianhydrides on thermomechanical properties of polybenzoxazine‐dianhydride copolymers

AbstractEnhanced thermomechanical properties of bisphenol‐A based polybenzoxazine (PBA‐a) copolymers obtained by reacting bisphenol‐A‐aniline‐type benzoxazine (BA‐a) resin with three different aromatic carboxylic dianhydrides, i.e., pyromellitic dianhydride (PMDA), 3,3′,4,4′ biphenyltetracarboxylic dianhydride (s‐BPDA), or 3,3′,4,4′ benzophenonetetracarboxylic dianhydride (BTDA) were reported. Glass transition temperature (Tg), of the copolymers was found to be in the order of PBA‐a:PMDA>PBA‐a:s‐BPDA>PBA‐a:‐BTDA. The difference in the Tg of the copolymers is related to the rigidity of the dianhydride components. Furthermore, the Tg of PBA‐a:BTDA, PBA‐a:s‐BPDA, and PBA‐a:BTDA films was observed to be significantly higher than that of the neat PBA‐a owing to the enhanced crosslink density by the dianhydride addition. This greater crosslink density results from additional ester linkage formation between the hydroxyl group of PBA‐a and the anhydride group of dianhydrides formed by thermal curing. Moreover, the copolymers exhibit enhanced thermal stability with thermal degradation temperature (Td) ranging from 410°C to 426°C under nitrogen atmosphere. The char yield at 800°C of the copolymers was found to be remarkably greater than that of the neat PBA‐a with a value up to 60% vs. that of about 38% of the PBA‐a. Toughness of the copolymer films was greatly improved compared to that of the neat PBA‐a. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

The effect of Cr/Zr chemical composition ratios on the mechanical properties of CrN/ZrN multilayered coatings deposited by cathodic arc deposition system

CrN/ZrN multilayer coatings with various Cr/Zr chemical compositions ratio were fabricated by a cathode arc deposition system. The Cr/Zr atomic ratios of the CrN/ZrN multilayer coatings were controlled, ranging from 1 to 2.5. The chemical composition of CrN/ZrN multilayer thin films was determined by a field emission electron probe microanalyzer (FE-EPMA). The phase composition of multilayer coatings was analyzed by a glancing angle X-ray diffractometer. Microstructures of the thin films were examined by field emission scanning electron microscopy (FE-SEM). Nanoindentation, the Daimler−Benz Rockwell-C (HRC-DB) adhesion, microhardness, pin-on-disc wear tests and scratch tests were used to evaluate the hardness, adhesion, toughness and tribological properties of the thin films, respectively. It was found that the hardness and tribological properties were strongly influenced by the Cr/Zr chemical composition ratios of the CrN/ZrN multilayer coatings. Optimal mechanical properties and the maximum hardness of 28GPa were achieved for the coating when the Cr/Zr atomic ratio was 2.1.

Novel Poly(benzoxazole-etherimide) Copolymer for Two-Layer Flexible Copper-Clad Laminate

A new random co-polyimide (co-PI), poly(benzoxazole-etherimide), was prepared via the poly(amic acid) from the reaction of 5-amino-2-(4-aminobenzene)-1,3-benzoxazole (BOA) and 4,4′-oxydianiline (ODA) with 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA) at a diamine molar ratio of 6:4. The BOA-type and ODA-type homo-PIs were also prepared for comparison. The aggregation structure and properties of the PI films were analyzed. The co-PI film showed a good balance in thermal properties, tensile strength, film toughness, and water absorption. The two-layer flexible copper-clad laminates made from these PIs were produced by coating poly(amic acid) solutions onto copper foils and then imidizing thermally. The adhesion properties and relative thermodimensional stability of the laminates were evaluated. The laminate based on the co-PI had higher relative thermodimensional stability as well as better adhesion property than those based on the two homo-PIs, which is due to the improvement in both the bonding strength of PI/copper at the interface and the cohesive strength of the co-PI itself in the boundary layer near the interface.

Preparation and characterization of antibacterial poly(vinylidene fluoride)-silver composites

Poly(vinylidene fluoride)-silver (PVDF-Ag) composite films were synthesized by in situ reduction of silver nitrate using N, N-dimethylformamide as solvent and reductant simultaneously. The effects of silver on the thermal property and crystal structure of PVDF were characterized using differential scanning calorimetry and X-ray diffraction. The mechanical properties and morphology of the composite membrane were characterized using a material testing machine and scanning electron microscopy. The toughness of the composite film was proved to increase sharply by adding 10% silver nitrate. The composite films were verified to be effective to kill Escherichia coli.

Preparation and characterization of regenerated cellulose/poly (vinylidene fluoride) (PVDF) blend films

Regenerated cellulose/poly (vinylidene fluoride) (PVDF) blend film was successfully prepared through coagulating their N,N-dimethyl acetamide (DMAC)/LiCl solution with water. The toughness of the blend films, compared with virgin regenerated cellulose film, was significantly improved when the content of PVDF in blend films was no more than 20wt.%. The elongation at break increased from 12% to 34%, and the tensile strength was also improved from 89 to 106MPa with the addition of PVDF till 20wt.%. The mechanical properties of the blend films became worse when the content of PVDF in blend films was more than 20%. The oxygen permeability was firstly decreased from 2.3×10−10 to 0.036×10−10cm3cm/cm2sPa when the PVDF content increased from 0 to 5wt.% in the blend films. Afterwards, the oxygen permeability would increase with the increasing of the PVDF content. The film showed some pores when the loading level of PVDF was more than 50%. The pore size of blend film became larger with the increasing of PVDF content. These phenomena were well correlated to the crystallinity, hydrogen bonds and morphologies of the blend films.

Size-dependent rupture strain of elastically stretchable metal conductors

Experiments show that the rupture strain of gold conductors on elastomers decreases as the conductors are made long and narrow. Rupture is caused by the irreversible coalescence of microcracks into one long crack. A mechanics model identifies a critical crack length ℓ(cr), above which the long crack propagates across the entire conductor width. ℓ(cr) depends on the fracture toughness of the gold film and the width of the conductor. The model provides guidance for the design of highly stretchable conductors.

Toughness enhancement in TiAlN-based quarternary alloys

Improved toughness in hard and superhard thin films is a primary requirement for present day ceramic hard coatings, known to be prone to brittle failure during in-use conditions. We use density functional theory calculations to investigate a number of (TiAl)1−xMxN thin films in the B1 structure, with 0.06≤x≤0.75, obtained by alloying TiAlN with M=V, Nb, Ta, Mo and W. Results show significant ductility enhancements, hence increased toughness, in these compounds. Importantly, these thin films are also predicted to be superhard, with similar or increased hardness values, compared to Ti0.5Al0.5N. For (TiAl)1−xWxN the results are experimentally confirmed. The ductility increase originates in the enhanced occupancy of d-t2g metallic states, induced by the valence electrons of substitutional elements (V, Nb, Ta, Mo, W). This effect is more pronounced with increasing valence electron concentration, and, upon shearing, leads to the formation of a layered electronic structure in the compound material, consisting of alternating layers of high and low charge density in the metallic sublattice, which in turn, allows a selective response to normal and shear stresses.

Conductive composite particles synthesized via pickering emulsion polymerization using conductive latex of poly(3,4-ethylenedioxythiophene) (PEDOT) as stabilizer

In this research, poly(3,4-ethylenedioxythiophene) (PEDOT) nanoparticles less than 100 nm were synthesized first and applied as the solid stabilizer for producing PEDOT-polystyrene (PEDOT-PSt) composite latex by Pickering emulsion polymerization. The results showed that most PEDOT nanoparticles adhered to the PSt core particles having the size from 100 to 250 nm. By casting the latex, the obtained PEDOT-PSt film had a surface resistance of 4–5 kΩ/□, almost the same as the pure PEDOT film, though its PEDOT content was only 6.2 wt%. Those PEDOT nanoparticles in the outer layer could contact with one another, forming a continuous network as the conductive passageway. Furthermore, soft latex particles of poly(styrene-co-butyl acrylate), P(St-BA), were synthesized and mixed with the conducting rigid PEDOT-PSt latex for improving the toughness and transparency of the casting film. A critical point at about 2 wt% of PEDOT content in the PEDOT-PSt/P(St-BA) film was observed in the surface resistance measurement.

The role of specimen thickness in the fracture toughness and fatigue crack growth resistance of nanocrystalline platinum films

Most empirical studies and numerical simulations of fatigue damage accumulation mechanisms for metals with nanoscale grains usually invoke grain boundary-mediated processes. However, our recent studies of ∼500nm platinum films showed that when the grain morphology is stable in the presence of high stresses and/or elevated temperatures the slip of dislocations is of primary interest. These thin film metals exhibit low fracture toughness and inferior resistance to fatigue crack growth when compared to conventional microscale grained bulk forms of the materials. In this manuscript we explore how sample form (thickness) contributes to the fracture and fatigue crack growth behavior of platinum thin films. When the thickness of the films was doubled to ∼1μm the tensile yield and ultimate strengths were slightly depressed (yield strength ∼1.3GPa, ultimate tensile strength ∼1.45GPa). However, the fracture toughness markedly improved (Kq value 25.4MPa√m) and the fatigue crack growth resistance was very similar to bulk forms (power law exponent m≈3.9). The progression and type of damage shows that sample form makes a critical and distinct contribution to the fatigue crack growth mechanisms in nanograined metals.

Residual stress in as-deposited Al–Cu–Fe–B quasicrystalline thin films

Abstract

Metallized nanotube polymer composites via supercritical fluid impregnation

Although many metal decorated nanotubes and nanowires appear in the literature, well-dispersed metal decorated nanotube polymer composites have rarely been reported because of the excessive density mismatch between the decorated nanotubes and polymer matrix. Here, we report a novel method to prepare well-dispersed, highly functional, metallized nanotube polymer composites (MNPCs) that possess remarkably improved electrical conductivity and mechanical toughness. The MNPCs are prepared by supercritical fluid impregnation of an organometal compound into a premade well-dispersed single wall carbon nanotube-polymer composite film. The infused precursor preferentially migrates towards the nanotubes to undergo spontaneous reduction and form nanometer-scale metal particles leading to an increase in the conductivity of the MNPC films. The environmentally friendly supercritical fluid impregnation process significantly improved the toughness of the composite films, regardless of the presence of metal. Additional functionality can be imparted into the resulting MNPC by infusing other precursors such as magnetic and catalytic metal compounds. © 2011 Wiley Periodicals, Inc.* J Polym Sci Part B: Polym Phys, 2012

The study of regenerated cellulose films toughened with thermoplastic polyurethane elastomers

Regenerated cellulose blend film with thermoplastic polyurethane (TPU) was successfully prepared by coagulating cellulose/TPU solution with water in the presence of a thermoplastic polyurethane elastomer (TPU). Compared with pristine regenerated cellulose film, the toughness and thermal stability of the blend film was significantly improved. For example, the elongation at break was increased from 11% of pristine cellulose film to 51% of blend film with 20 wt. % TPU. The 50% weight loss temperature of this blend film was increased by 33 °C compared to neat cellulose. The relaxation transition temperature of cellulose was decreased with the addition of TPU through dynamic mechanical thermal analysis. The oxygen permeability was decreased from 2.3 × 10−10 cm3 cm/cm2 s Pa of pristine cellulose film to 0.08 × 10−10 cm3 cm/cm2 s Pa of the blend film with 20 wt.%. TPU The X-ray diffraction spectra showed that the crystallinity of cellulose decreased with incorporation of TPU. The images of scanning electron microscope discovered that there was good compatibility between cellulose and TPU. TPU was nano-dispersed in cellulose matrix. The blend film still maintained quite good transparency.

A Microbridge Method in Tensile Testing of Substrate for Fracture Toughness of Thin Films

A new microbridge method is introduced in this paper in tensile testing of substrate for fracture toughness of thin films. Measurement of fracture toughness for bulk materials is a routine but extremely difficult and still not standardized for thin films (or coatings). The difficulties in clamping a freestanding thin film and the requirement of a critical delicate loading system are the main obstacles. In this paper, the film to be tested is deposited on a rectangular silicon wafer on which an edge crack is fabricated beforehand. The film is patterned and made into microbridges perpendicular to and ahead of the initial substrate crack. A displacement controlled tensile force is applied to propagate the substrate crack and fracture the microbridges. The critical fracture strain of the microbridge is measured through measuring that of the substrate at the respective location of the bridge. The fracture toughness of the film is thus obtained. This method successfully turned the tensile testing of the film into tensile loading of the substrate, avoided the delicate clamping and loading on the film itself. Two case studies are also presented.

Study on Film of Polyvinyl Acetate Emulsion Modified by Starch

The use of starch on polyvinyl acetate emulsion blend modified in order to achieve improved film toughness, which aims to improve water resistance and film-forming emulsion adhesive performance. Dynamic mechanical properties (DMA), scanning electron microscopy (SEM) method was used to analyze the dynamics of film emulsion blend composite mechanics, the distribution of latex particles and glass transition temperature. The results show that: with pure polyvinyl acetate (PVAc) compared to starch-modified film-forming emulsion film distribution, glass transition temperature of the homopolymer emulsion up to 30°C.

Mechanical and tribological properties evaluation of cathodic arc deposited CrN/ZrN multilayer coatings

Five nanostructured CrN/ZrN multilayer coatings were deposited periodically by cathodic arc evaporation. The bilayer periods of the CrN/ZrN multilayer coatings were controlled in the range of 5 to 30nm. The structures and bilayer period of the multilayer coatings were characterized by an X-ray diffractometer. The microstructures of thin films were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. Nanoindentation, scratch tests, Daimler–Benz Rockwell-C (HRC-DB) adhesion tests, microhardness and pin-on-disk wear tests were used to evaluate the hardness, adhesion, indentation toughness and tribological properties of thin films, respectively. It was found that the hardness and tribological properties were strongly influenced by the bilayer period of the CrN/ZrN multilayer coatings. An optimal combination of mechanical properties and excellent tribological behavior was found for a coating with a critical bilayer period of 30nm.

High performance polyimide composite films prepared by homogeneity reinforcement of electrospun nanofibers

Highly aligned polyimide (PI) and PI nanocomposite fibers containing carbon nanotubes (CNTs) were produced by electrospinning. Scanning electron microscopy showed the electrospun nanofibers were uniform and almost free of defects. Transmission electron microscopy indicated that the CNTs were finely dispersed and highly oriented along the CNT/PI nanofiber axis at a relatively low concentration. The as-prepared well-aligned electrospun nanofibers were then directly used as homogeneity reinforcement to enhance the tensile strength and toughness of PI films. The neat PI nanofiber reinforced PI films showed good transparency, decreased bulk density and significantly improved mechanical properties. Compared with neat PI film prepared by solution casting, the tensile strength and elongation at break for the PI film reinforced with 2 wt.% CNT/PI nanofibers were remarkably increased by 138% and 104%, respectively. The significant increases in the overall mechanical properties of the nanofibers reinforced polyimide films can be ascribed to good compatibility between the electrospun nanofibers and the matrix as well as high nanofiber orientation in the matrix. Our study demonstrates a good example for fabricating high performance and high toughness polyimide nanocomposites by using this facile homogeneity self-reinforcement method.

Toughing of ZrAlN Film through Superlattice Design

Two kinds of magnetron sputtering ZrAlN films containing 23%atAl were deposited. The first was multicomponent ZrAlN film, the second was structure gradient and mulilayer film, named superlattice ZrAlN film. The microstructure was studied by FESEM, TEM , XRD, its mechanical properties were evaluated by nano-indentation method. The fracture toughness of films were determined from the length of ‘radial cracks’ on the applied diamond identer load 1.96N. the results show that, multicomponent ZrAlN film has hardness value of 35GPa, fracutre toughness value of 1.97MPa·m-0.5; while superlattice ZrAlN film has coresponding value of 40.1GPa and 3.17MPa·m-0.5. TEM image illustrates the superlattice ZrAlN film period is 2.5nm, nanoidentation test shows superlattice film has higher elastic recovery parameter and lower plastic work.

Microstructure, mechanical and tribological properties of TiCN nanocomposite films deposited by DC magnetron sputtering

TiCN nanocomposite films are prepared by direct current magnetron sputtering from Ti C combined target under different nitrogen flow rates. The composition, morphology and microstructure of the nanocomposite films are characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, X-ray diffraction and transmission electron microscopy. Hardness and tribological properties are tested by nanoindentation measurement and ball-on-disk tribometer, respectively. With the increase of nitrogen flow ranging from 0 to 30 sccm at a work pressure of 0.3 Pa, both crystallinity and sp 2 carbon content in the TiCN films increase. In addition, the ratio of TiN to Ti(C, N) increases as the nitrogen flow rate increases. The friction coefficient and wear rate could be greatly reduced due to the increase of sp 2 carbon and better toughness in the composite films. TiCN nanocomposite film with high hardness and good wear resistance is obtained under a nitrogen flow rate of 30 sccm.

In situ electro-mechanical experiments and mechanics modeling of tensile cracking in indium tin oxide thin films on polyimide substrates

Indium tin oxide (ITO) thin films supported by polymer substrates have been widely used as transparent electrodes/interconnects in flexible electronics. Understanding the electro-mechanical behaviors of such material system is crucial for reliable operation of flexible devices under large deformation. In this paper, we performed in situ mechanical and electrical tests of ITO thin films with two different thicknesses (200 and 80 nm) deposited on polyimide substrates inside a scanning electron microscope. The crack initiation and propagation, crack density evolution and the corresponding electrical resistance variation were systematically investigated. It was found that cracks initiated at a higher tensile strain level and saturated with a higher density in thinner ITO films. Integrated with a coherently formulated mechanics model, the cohesive toughness and fracture strength of ITO thin films and the ITO/polyimide interfacial toughness were quantitatively determined. The experimentally observed thickness dependence of the saturated crack density in ITO thin films was also quantitatively verified by the model.