Role of zirconia filler on friction and dry sliding wear behaviour of bismaleimide nanocomposites

This paper discusses the friction and dry sliding wear behaviour of nano-zirconia (nano-ZrO₂) filled bismleimide (BMI) composites. Nano-ZrO₂ filled BMI composites, containing 0.5, 1, 5 and 10 wt.% were prepared using high shear mixer. The influence of these particles on the microhardness, friction and dry sliding wear behaviour were measured with microhardness tester and pin-on-disc wear apparatus. The experimental results indicated that the frictional coefficient and specific wear rate of BMI can be reduced at rather low concentration of nano-ZrO₂. The lowest specific wear rate of 4 × 10⁻⁶ mm³/Nm was observed for 5 wt.% nano-ZrO₂ filled composite which is decreased by 78% as compared to the neat BMI. The incorporation of nano-ZrO₂ particles leads to an increased hardness of BMI and wear performance of the composites shows good correlation with the hardness up to 5 wt.% of filler loading. The results have been supplemented with scanning electron micrographs to help understand the possible wear mechanisms.     

Compressive and wear behaviors of bulk nanostructured Al2024 alloy

Compressive and wear properties of bulk nanostructured Al2024 alloy prepared by mechanical milling and hot pressing methods were investigated. Al2024 powders were subjected to high-energy milling for 30 h to produce nanostructured alloy. As-milled powders were compacted at 500 °C under 250 MPa in a uniaxial die. Consolidated sample had an average hardness and relative density values of 207.6 HV and 98%, respectively. Uniaxial compression tests at strain rates in the range of 1.67 × 10⁻⁴–1.67 × 10⁻² s⁻¹ were performed using an Instron-type machine. The wear behavior of nanostructured sample was investigated using a pin-on-disk technique under an applied load of 20 N. The compression and wear experiments were also executed on samples of commercial coarse-grained Al2024-O (annealed) and Al2024-T6 (artificially-aged) alloys, for comparison. The structure of consolidated Al2024 was characterized by X-ray diffraction (XRD). The yield strength and compressive strength of nanostructured Al2024 reached a value of 698 MPa and 712 MPa at strain rate of 1.67 × 10⁻⁴ s⁻¹, respectively, which was considerably higher than those for coarse-grained Al2024-O and Al2024-T6 counterparts. Worn surfaces and the wear debris were analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and XRD. Nanostructured Al2024 revealed a low friction coefficient of 0.3 and a wear rate of 12 × 10⁻³ mg/m, which are significantly lower than those obtained for Al2024-O and Al2024-T6 alloys. This enhanced wear resistance was mainly caused by nanocrystalline structure with high hardness value. The dominating wear mechanism of nanostructured Al2024 appeared to be delamination mechanism.     

Scanning transmission electron microscopy (STEM)-transmission electron microscopy (TEM) analysis of nitrogen ion implanted austenitic 302 stainless steel

The microstructure of nitrogen implanted AISI 302 austenitic stainless steel and the effect of long-term room temperature ageing on it have been studied. Samples were implanted in 1992 with 2.5×10²¹ N₂⁺ m⁻² at 130 keV. The characteristics of the implanted layer and the depth profile have been investigated by scanning transmission electron microscope combined with energy dispersive X-ray spectrometry. Electron diffraction patterns recorded in the implanted layer using transmission electron microscopy confirm the formation of CrN along with the presence of Cr₂N. The identification of phases by glancing angle X-ray diffraction also indicates the formation of Cr₂N and nitrogen solid solutions. The effects of ageing on the microstructure are observed to be small.     

The characterisation of PM 304 stainless steel sintered in the presence of a copper based additive

Samples of 304 stainless steel powders alloyed with a range of copper-based additive (2-8 wt.%) have been compacted and sintered at two different temperatures (1250 and 1350 °C) in an Argon atmosphere. The effect of copper-based ternary alloy on the microstructure and hardness values of a sintered stainless steel has been investigated. It was found that the quantity of copper-based additive as well as the sintering cycle has a profound impact on the microstructure and hardness values of sintered stainless steel. The residual porosity in the sintered specimens was substantially reduced in samples compacted from powder mixtures containing 2-8 wt.% of additives. The microstructural characterisation of PM 304 stainless steel alloyed with different amounts of copper addition shows that copper is homogenously diffused into the base stainless steel. The microhardness values of samples increase with an increase in the amount of additive due to the elimination of porosity.     

Improving sliding and abrasive wear behaviour of cast A356 and wrought AA7075 aluminium alloys by plasma electrolytic oxidation

An important limitation of aluminium alloys for mechanical applications is their poor tribological behaviour. In this study, surface treatment by plasma electrolytic oxidation (PEO) has been applied to two widely used aluminium alloys: A359 (hypoeutectic Al–Si–Mg) cast alloy and AA7075 (Al–Zn–Mg–Cu) wrought alloy, in order to improve their wear resistance, under sliding and abrasive wear conditions. The main aim of this work was the comparison of the properties and wear resistance of the oxide layers grown under the same PEO treatment conditions on two different aluminium alloys which might be coupled in engineered components. Significant differences in the phase composition, microstructure and mechanical properties measured by microindentation were observed in the oxide layers grown on the two substrates, and were ascribed to the effects of the different compositions and microstructures of the substrate alloys. Abrasion tests were carried out in a micro-scale abrasion (ball-cratering) test, with both alumina and silicon carbide abrasive particles. The results demonstrated the influence of the abrasive material on wear behaviour: whereas relatively aggressive SiC particles gave comparable results for both PEO treated and untreated samples, with the less aggressive Al₂O₃ abrasive the wear rates of the PEO treated samples, for both substrates, were significantly lower than those of the untreated substrates. In unlubricated sliding the PEO treatment significantly increase the wear resistance of both the aluminium alloys, at low applied load. In this condition the wear behaviour of the PEO treated alloys is strongly influenced by the stability of a protective Fe–O transfer layer, generated by wear damage of the steel counterpart. Under high applied loads however, the transfer layer is not stable and the hardness of the PEO layer, as well as the load bearing capacity of the substrate, become the main factors in influencing wear resistance.     

Enhancement of the wear resistance of epoxy: short carbon fibre, graphite, PTFE and nano-TiO2

Enhancements of the wear resistance of epoxy using various fillers, e.g. short carbon fibre (CF), graphite, polytetrafluoroethylene (PTFE) and nano-TiO₂, have been systematically investigated in the present study. Wear properties were carried out on a block-on-ring apparatus. The best wear resistant composition was achieved by a combination of nano-TiO₂ with conventional fillers; as an example, epoxy+15 vol% graphite+5 vol% nano-TiO₂+15 vol% short-CF exhibits a specific wear rate of 3.2×10⁻⁷ mm³/Nm, which is about 100 times lower when compared to the neat epoxy. Worn surfaces were investigated using a scanning electron microscope and an atomic force microscope, from which it is assumed that a mechanism of nanoscale rolling governs this positive effect of the nanoparticles. The main concept of this paper is to strength the importance of integrating various functional fillers in the design of wear resistant polymer composites.     

Controlled growth and investigations on the morphology and mechanical properties of hydroxyapatite/titania nanocomposite thin films

In this paper, the focus is on understanding the properties of nanocomposite hydroxyapatite (HAp)/titania (TiO₂) thin films with respect to TiO₂ concentration. HAp/TiO₂ nanostructured composite thin films with different TiO₂ concentrations were successfully fabricated by a simple sol–gel dip coating method. Highly stable HAp and TiO₂ sols were prepared prior to the formation of nanocomposite thin films. The coatings were performed under controlled dipping and heat treatment processes. Phase pure HAp and TiO₂ were well developed in the nanocomposite after the heat treatment and this was confirmed by XRD. The SEM and AFM analyses of HAp/TiO₂ nanocomposite coatings show the variation in the morphology as a consequence different TiO₂ concentration. This shows a reduction in the particle size to nanoscale due to the addition of TiO₂. The mechanical strength of the coating also increased upon the addition of TiO₂ as determined by nanoindentation. The composite thin films with 50 and 80 vol.% of TiO₂ show good mechanical strength when compared to other concentrations of TiO₂.     

Surface characterization and influence of anodizing process on fatigue life of Al 7050 alloy

The present study investigates the influence of anodizing process on fatigue life of aluminium alloy 7050-T7451 by performing axial fatigue tests at stress ratio ‘R’ of 0.1. Effects of pre-treatments like degreasing and pickling employed prior to anodizing on fatigue life were studied. The post-exposure surface observations were made by scanning electron microscope (SEM) to characterize the effect of each treatment before fatigue testing. The surface observations have revealed that degreasing did not change the surface topography while pickling solution resulted in the formation of pits at the surface. Energy dispersive spectroscopy (EDS) was used to identify those constituent particles which were responsible for the pits formation. These pits are of primary concern with respect to accelerated fatigue crack initiation and subsequent anodic coating formation. The fatigue test results have shown that pickling process was detrimental in reducing the fatigue life significantly while less decrease has been observed for anodized specimens. Analyses of fracture surfaces of pickled specimens have revealed that the process completely changed the crack initiation mechanisms as compared to non-treated specimens and the crack initiation started at the pits. For most of the anodized specimens, fatigue cracks still initiated at the pits with very few cracks initiated from anodic coating. The decrease in fatigue life for pickled and anodized specimens as compared to bare condition has been attributed to decrease in initiation period and multi-site crack initiations. Multi-site crack initiation has resulted in rougher fractured surfaces for the pickled and anodized specimens as compare to bare specimens tested at same stress levels.     

Preparation of electrospun siloxane-poly(lactic acid)-vaterite hybrid fibrous membranes for guided bone regeneration

A nonwoven fabric scaffold for guided bone regeneration (GBR) consisting of siloxane-poly(lactic acid) (PLA)-vaterite hybrid material (SiPVH) was prepared by an electrospinning method. To improve the cellular compatibility of the fabric, the fibers were completely coated with hydroxyapatite (HA) by soaking in simulated body fluid. The HA-coated SiPVH nonwoven fabric contained large-sized spaces and showed the ability of releasing soluble silica and calcium species, which have previously been reported to stimulate osteogenic cells at the genetic level. A new type of GBR membrane was prepared by bonding SiPVH fabric with PLA nonwoven fabric with small-sized pores for preventing the intrusion of soft tissue. The resultant bi-layered membrane was expected to be effective not only for having an open structure for bone formation and a barrier to soft tissue, but also for enhancing bone growth by the release of ionic species.     

Synthesis and electrochemical performance of LiV3O8/carbon nanosheet composite as cathode material for lithium-ion batteries

To improve the rate capability and cyclability of LiV₃O₈ cathode for Li-ion batteries, LiV₃O₈ was modified by forming LiV₃O₈/carbon nanosheet composite. The LiV₃O₈/carbon nanosheet composite was successfully achieved via a hydrothermal route followed by a carbon coating process. The morphology and structural properties of the samples were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). TEM observations demonstrated that LiV₃O₈/carbon composite has a very flat sheet-like morphology, with each nanosheet having a smooth surface and a typical length of 400-700 nm, width of 200-350 nm, and thickness of 10-50 nm. Each sheet was surrounded by a thick layer of amorphous carbon. Electrochemical tests showed that the LiV₃O₈/carbon composite cathode features long-term cycling stability (194 mAh g⁻¹ at 0.2 C after 100 cycles) and excellent rate capability (110 mAh g⁻¹ at 5 C, 104 mAh g⁻¹ at 10 C, and 82 mAh g⁻¹ at 20 C after 250 cycles). Electrochemical impedance spectra (EIS) indicated that the LiV₃O₈/carbon composite electrode has very low charge-transfer resistance compared with pristine LiV₃O₈, indicating the enhanced ionic conductivity of the LiV₃O₈/carbon composite. The enhanced cycling stability is attributed to the fact that the LiV₃O₈/carbon composite can prevent the aggregation of active materials, accommodate the large volume variation, and maintain good electronic contact.     

Modeling of DC conductivity for ethylene vinyl acetate (EVA)/polyaniline conductive composites prepared through insitu polymerization of aniline in EVA matrix

Ethylene vinyl acetate (EVA)/polyaniline (Pani) composites were prepared by insitu polymerization technique. DC and AC conductivity of the composites have been investigated. Different theoretical models like Voet, Scarisbrick, Bueche, and McCullough have been applied to predict DC conductivity of the composite systems. Scarisbrick model exhibits somewhat similarity between experimentally observed and theoretically predicted conductivity. The limitations of the models as well as the deviations between the theoretically predicted and experimentally observed results have been discussed. A new model for conductivity has been proposed which fit well with the experimentally observed results.     

Synthesis and characterization of well-dispersed multi-walled carbon nanotube/low-bandgap poly(3,4-alkoxythiophene) nanocomposites

In this study, we prepared nanocomposites of multi-walled carbon nanotubes (MWCNTs) and low-energy-bandgap conjugated polymers incorporating 3,4-alkoxythiophene monomers. Poly(3,4-dihexyloxythiophene) (PDHOT) and poly(3,4-dimethoxythiophene-co-3,4-dihexyloxythiophene) [P(DMOT-co-DHOT)] have relatively low-energy-bandgaps (ca. 1.38 and 1.34 eV, respectively), determined from the onsets of absorbances in their UV–Vis spectra, because of the electron-donating effects of their alkoxy groups. MWCNTs have poor solubility in common organic solvents; after surface modification with alkyl side chains using the Tour reaction, however, the p-hexylaniline modified MWCNT derivative (MWCNT-HA) was readily dispersed in CHCl₃ and could be mixed with the low bandgap polymers. Scanning electron microscopy images revealed that MWCNT-HA was dispersed well in each polythiophene derivative; only a few MWCNT-HA bundles could be observed at a high MWCNT-HA content (≧20 wt.%). The electrical conductivities of the MWCNTs/PDHOT composites were dependent on their MWCNT content, reaching 16 S/cm at 30 wt.% MWCNT-HA. We suspect that the two hexyloxy chains of PDHOT enhanced its solubility and allowed it to wrap around the surfaces of the MWCNTs more readily.     

Dielectric properties of Poly(vinylidene fluoride)/CaCu3Ti4O12 composites

The possibility of obtaining relatively high dielectric constant polymer–ceramic composite by incorporating the giant dielectric constant material, CaCu₃Ti₄O₁₂ (CCTO) in a Poly(vinylidene fluoride) (PVDF) polymer matrix by melt mixing and hot pressing process was demonstrated. The structure, morphology and dielectric properties of the composites were characterized using X-ray diffraction, Thermal analysis, scanning electron microscope, and impedance analyzer. The effective dielectric constant (ε_eff) of the composite increased with increase in the volume fraction of CCTO at all the frequencies (100 Hz–1 MHz) under study. The dielectric loss did not show any variation up to 40% loading of CCTO, but showed an increasing trend beyond 40%. The room temperature dielectric constant as high as 95 at 100 Hz has been realized for the composite with 55 vol.% of CCTO, which has increased to about 190 at 150 °C. Theoretical models like Maxwell’s, Clausius–Mossotti, Effective medium theory, logarithmic law and Yamada were employed to rationalize the dielectric behaviour of the composite and discussed.     

Use of lignin as a compatibiliser in hemp/epoxy composites

This study was designed to ascertain if the addition of lignin to hemp-epoxy composites was beneficial to their mechanical properties. Composites were made using a VARTM method with a two-part epoxy resin and a non-woven hemp fibre mat. Lignin was added to the resin before infusion at concentrations varying between 0 and 10% w/w. Samples were then tested according to the relevant ISO standards. There was an increase in impact properties of the fabricated composites with the energy absorbed by the composite containing 5% w/w lignin being 145% higher than the composite with no lignin added. Both flexural and tensile modulus showed an increase when lignin was added up to 2.5% w/w, although there was a drop in both when the lignin was increased to 5% w/w, attributed to poor mixing and infusion due to the increased viscosity of the resin. In all cases, the addition of lignin increased the structural properties of the composites to some degree when compared with composites with no additional lignin.     

Fabrication and characterization of aluminum nitride polymer matrix composites with high thermal conductivity and low dielectric constant for electronic packaging

Polymethyl methacrylate (PMMA) composites filled with Aluminum Nitride (AlN) were prepared by powder processing technique. The microstructures of the composites were investigated by scanning electron microscopy techniques. The effect of AlN filler content (0.1–0.7 volume fraction (vf)) on the thermal conductivity, relative permittivity, and dielectric loss were investigated. As the vf of AlN filler increased, the thermal conductivity of the specimens increased. The thermal conductivity and relative permittivity of AlN/PMMA composites with 0.7 vf AlN filler were improved to 1.87 W/(m K) and 4.4 (at 1 MHz), respectively. The experimental thermal conductivity and relative permittivity were compared with that from simulation model.     

Formulation optimization of unreinforced and lignin nanoparticle-reinforced phenolic foams using an analysis of variance approach

Formulations of unreinforced and lignin nanoparticle-reinforced phenolic foams were optimized using an analysis of variance approach. The variables studied in the formulation of phenolic foams were stirring speed (650–850 rpm) and blowing agent amount (1.5–3.5 wt.%). For lignin nanoparticle-reinforced phenolic foams, the variables were lignin nanoparticle weight fraction (1.5–8.5 wt.%) and blowing agent amount (1.5–3.5 wt.%). The responses measured for both foams were density, compressive modulus, and compressive strength. In addition, the morphology of the foams was observed using scanning electron microscopy (SEM) to determine cell size distributions. The results showed that the variables studied exhibited a strong influence on the responses and the cell size distribution of the foams. Statistical models allowed for prediction of the properties of the foams and for comparison of the properties of unreinforced and lignin nanoparticle-reinforced phenolic foams. The incorporation of lignin nanoparticles in phenolic foams results in a compressive modulus and compressive strength that has up to 128% and 174%, respectively, of the values for unreinforced foams. The amount of blowing agent saved to produce a reinforced foam was up to 31% of the amount necessary to produce an unreinforced foam of the same density.     

Processing of cellulose nanowhiskers/cellulose acetate butyrate nanocomposites using sol-gel process to facilitate dispersion

Biobased nanocomposites based on cellulose nanowhiskers (CNWs) and cellulose acetate butyrate (CAB) were prepared using solvent exchange of CNWs to ethanol by sol-gel method followed by casting. The strong flow birefringence of the solutions indicated evenly dispersed cellulose nanowhiskers in the dissolved polymer CAB. Scanning electron microscopy of the nanocomposites confirmed well dispersed CNWs in the CAB matrix, which was further supported by the high transparency exhibited by the nanocomposites. The results of tensile tests indicated significant improvements in the mechanical properties of nanocomposites by increasing the CNWs contents. The Young’s modulus and strength increased 83% and 70%, respectively, for nanocomposites with 12 wt% of CNW, and the strain was not suppressed compared to the neat CAB. The dynamic mechanical thermal analysis demonstrated significant improvement in storage modulus with increasing CNW contents, and the tan δ peak position was moved towards higher temperature when CNW was added. It is expected that solvent exchange by the sol-gel route followed by casting of nanocomposites from the same solvent will provide a promising route for obtaining cellulose nanocomposites with well dispersed CNW, leading to improved mechanical properties, even with low nanowhisker contents.     

Effect of additive content on transient liquid phase sintering in SiC nanopowder infiltrated SiCf/SiC composites

SiC nanopowder infiltrated SiC_f/SiC composites with a high fiber volume fraction above 50 vol.% were prepared at a relatively low fabrication temperature of 1800 °C by transient liquid phase sintering using Al₂O₃-Y₂O₃-SiO₂ additives. The effects of additive content with 6-18 wt.% were investigated, based on densification, microstructure, mechanical properties and fracture behaviors of the composites. The results showed that the densification and mechanical properties of the composites were greatly improved with the additive content. Microstructural observation indicated that the infiltration of SiC nanopowder inside fiber-bundles were enchanced with the increase of additive content due to the effectively widen space by the reaction between pyrocarbon (PyC) interface and the additives especially with the addition of SiO₂. It proven that the enchanced matrix-PyC interface bonding by the high densification inside fiber-bundles played a key role in the improved mechanical properties and fracture behaviors of the composites.     

Determination of particle size distributions and the degree of dispersion in nanocomposites

Objective of this study was the investigation of measurement techniques to determine the quality of the dispersion process of nanoparticles in polymer composites. In order to prepare the matrix suspension, alumina nanoparticles were dispersed applying shear mixing techniques in a high performance laboratory kneader. The product quality in liquid state was determined by means of dynamic light scattering (DLS) and centrifugal sedimentation analysis (CSA). However, particle measurements in carrier fluids like epoxy resin are complex and challenging. Measuring values like particle size distribution and grade of homogeneousness are strongly influenced by the sample preparation and adjustments of the measuring device. Within this study the machine settings and the formulation was analysed systematically. Hereby an identification of the key parameters and an optimisation of the measuring process were possible. Additionally, the composite was cured and analysed by scanning electron microscopy (SEM). Finally all measuring techniques were evaluated and compared among each other. Thus, DLS is the fastest method to measure spherically particles in the liquid matrix, CSA allows a certain deviation from the spherical shape and SEM gives a qualitative impression of the final particle size in cured composite condition.     

Intraply fracture of fiber-reinforced composites: Microscopic mechanisms and modeling

The fracture behavior parallel to the fibers of an E-glass/epoxy unidirectional laminate was studied by means of three-point tests on notched beams. Selected tests were carried out within a scanning electron microscope to ascertain the damage and fracture micromechanisms upon loading. The mechanical behavior of the notched beam was simulated within the framework of the embedded cell model, in which the actual composite microstructure was resolved in front of the notch tip. In addition, matrix and interface properties were independently measured in situ using a nanoindentor. The numerical simulations very accurately predicted the macroscopic response of the composite as well as the damage development and crack growth in front of the notch tip, demonstrating the ability of the embedded cell approach to simulate the fracture behavior of heterogeneous materials. Finally, this methodology was exploited to ascertain the influence of matrix and interface properties on the intraply toughness.