No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Effect of warm rolling and annealing on the mechanical properties of aluminum composite reinforced with boron nitride nanotubes
Abstract
The effect of rolling and annealing on boron nitride nanotube (BNNTs) reinforced aluminum- composites is investigated in this study. Composites were fabricated via conventional sintering method with 0, 2 & 5 wt% BNNT addition in aluminum matrix. Addition of 2 wt% BNNT improved hardness and elastic modulus by 23% and 18%, respectively. Rolling the same composite at 200 °C with 60% reduction in single pass improved modulus and hardness of the composite by 60% and 31%, respectively, over Al. Addition of 5 wt% BNNT led to reduced properties due to agglomeration, which on rolling developed cracks. Annealing the rolled Al-BNNT composite further led to an improvement in strength and ductility. Annealed Al-2BNNT showed highest improvement in strength of 41% and 110% over rolled and sintered condition, respectively. In addition, the same composition has recorded 157% improvement in toughness in annealed condition, as compared to rolled condition. Uniformly distributed BNNTs restricted grain growth and grain boundary migration due to pinning, leading to presence of additional grain boundaries inside the old grains thus improving the strength further. Uniform distribution of BNNT is the prime reason behind the improved properties of Al-BNNT composites.
... In a relatively recent study, Bisht et al. sintered Al-BNNT in an Ar gas environment. [61] The study reported a relatively lower green body consolidation pressure of 200 MPa, a higher sintering temperature of 600 C, and a shorter hold time of 1 h. In addition, the sintered composites were subjected to post-sintering thermo-mechanical treatments: (i) warm rolling (60% thickness reduction at 200 C), followed by (ii) annealing at 350 C for 4 h. ...
... Fabrication method, morphology, dimensions, and impurities in BNNTs and manufacturing processes employed for fabricating metal matrix composites vis-a-vis sample dimensions reported in the literature. [16,17,47,48,51,52,54,56,57,59,61,[65][66][67][68][69] BNNT-MMC ...
... Al-BNNT Strips [61] Ball milling and annealing method O, Fe Al-BNNT Strip [65] Pressurized vapor condenser Al-BNNT Sandwich [66] Pressurized vapor condenser ...
Boron Nitride Nanotubes (BNNTs), a 1D nanomaterial with extraordinary mechanical properties, are structurally like carbon nanotubes. BNNTs possess superior thermal stability of ∼900 °C, higher resistance to oxidation at elevated temperatures, and enhanced neutron shielding capacity. These benefits open a wider processing window for manufacturing BNNT-reinforced metal matrix composites (MMC) and hold promise for several structural applications, including radiation shielding. This critique presents the current status, challenges, and future scientific possibilities of BNNT-reinforced MMC. Particular emphasis is laid on the progress made in this area regarding the synthesis, manufacturing, and characterization of BNNT-MMCs to date. The challenges associated with various processing techniques, including additive manufacturing (AM), are discussed in the fabrication of BNNT-MMCs. The experimental mechanics and structure-property relationship modeling are examined in detail to establish the utilization of BNNT-reinforced MMCs. Additionally, prospective research areas with a huge untapped potential for BNNT-MMCs are suggested. The scientific framework behind these methods’ chronological development is analyzed, and a pathway for subsequent advancement is projected. By providing a comprehensive overview, this critique aims to facilitate further progress in BNNT-reinforced MMCs.
... When the content of FLM was up to 1.5 wt.%, the hardness reached the maximum at 72 HV, which was similar to the result of the tensile strength trend. A summary of tensile properties' data of composites based on the pure Al matrix in previous reports is shown in Figure 5b [4,7,8,[36][37][38][39][40][41][42]. It can be seen that FLM/Al composites exhibited a good strength-ductility balance. ...
... Besides, the dimples of the composites are smaller and shallower than those of pure Al, indicating a higher strength and lower toughness than that of pure Al. strength trend. A summary of tensile properties' data of composites based on the pure Al matrix in previous reports is shown in Figure 5b [4,7,8,[36][37][38][39][40][41][42]. It can be seen that FLM/Al composites exhibited a good strength-ductility balance. ...
In this work, few-layered MoS2 (FLM) nanosheet-reinforced Al matrix composites are developed through powder metallurgy and hot extrusion. The microstructure, mechanical properties, and strengthening mechanisms have been systematically investigated. It is found that Al12Mo and Al2S3 can be formed in-situ during the sintering process, resulting in the improvement of interfacial bonding between FLM and Al matrix. With 1.5 wt.% of FLM addition, an improved tensile strength of 234 MPa with a high elongation of 17% can be obtained. Moreover, the strengthening mechanisms are also demonstrated to be grain refinement, dislocation strengthening, and load transfer, and the calculation indicates that load transfer is the main contribution factor. This work will inspire more new designs of metal matrix composites with balanced strength and ductility.
... After this much of weight amount, the mechanical properties were started to be decreased. Bisht et al. [16] found that annealing and warm rolling of Al alloy composites reinforced with 2 wt.% boron nitride nano tubes significantly enhanced the various mechanical characteristics like tensile strength, ductility, modulus and hardness as compared to the neat Al. Rofman et al. [17] investigated the influence of thermomechanical treatment on the ductility behaviour of Al-3.0Cu-1.2 ...
... The flexural strength of the prepared composites is shown in 16 MPa is obtained corresponding to ASN-6 composition (6 wt.% Si 3 N 4 content). Similar trends were observed by Pazhouhanfar et al. [33]. ...
In this investigation, hybrid AA2024 – Si3N4 (0–6 wt.% @ 2%) – SiC (2 wt.%) – graphite (2 wt.%) alloy composites have been fabricated as per design using the stir casting method. This follows evaluation of physical, mechanical, thermal, thermo-mechanical, fracture toughness, and sliding wear performance as per standards. The density and material stability of the alloy composites improves with reinforcement content while porosity and thermal conductivity show a diminishing trend. The mechanical characteristics improve considerably with reinforcement. The alloy composite having 6 wt.% Si3N4 particulates show maximum mechanical characteristics like ultimate tensile strength (212 MPa), percent elongation (13%), flexural strength (645 MPa), micro-hardness (127 HV), and impact strength (85 J). Fracture toughness was found to improve with Si3N4 particulate content and with crack length. The storage modulus and damping capacity of the alloy composites have shown an increasing trend with reinforcement content while the loss modulus shows a reverse trend. The specific wear rate and friction coefficient tend to diminish with Si3N4 particulate content under steady state conditions. The normal load significantly influences the specific wear rate of the alloy composites under the Taguchi experimental design. Worn out surface morphology analysis shows wear mechanisms like surface fatigue, ploughing, micro-cutting, three-body abrasion, etc. responsible for surface damage. EDAX spectrum shows elemental presence and mapping and XRD analysis shows the presence and distribution of various phases of alloy composites and counter-surface disc. It could be the potential material for various components like gear, piston, cylinder liner, etc.
... Bisht et al., 16 processed the BNNTs/Al nanocomposites by conventional powder metallurgy route followed by the warm rolling. Figure 9 shows the optical microscope images of sintered, rolled, annealed neat Al and BNNTs/ Al nanocomposites. ...
Multiwalled Boron Nitride Nanotubes (BNNTs) reinforced Metal Matrix Nanocomposites (MMNCs) are discussed in this work. It is noticed that the reaction of BNNTs with Aluminum (Al) to form a compound at the interface depends on the processing condition. The processing methods include sintering, solidification, rolling, High-Pressure Torsion (HPT) and heat treatment have influenced the microstructure and mechanical properties of BNNTs filled MMNCs. The review suggests that BNNTs filled MMNCs have the potential to be used in various applications.
... The study's findings indicated that incorporating BN particles substantially enhanced the composite material's hardness, tensile strength, and yield strength. As mentioned earlier, the investigation demonstrates that incorporating BN ceramic particles into aluminum-based composites notably enhances their mechanical properties [24]. The observed enhancement can be ascribed to the reinforcing influence of the BN particles, which contribute to heightened strength and stiffness inside the composite matrix [25]. ...
This present study investigated the impact of incorporating boron nitride (BN) and vanadium carbide (VC) reinforcements on various properties of friction stir processed (FSP) AA6061 alloy composites, focusing specifically on grain structure, thermal conductivity, electrical conductivity, and compressive strength. The findings indicate that VC more effectively refines the grain structure of the AA6061 alloy during FSP compared to BN. The inclusion of BN particles in the metal matrix composites resulted in a decrease in both thermal and electrical conductivity. In contrast, the addition of VC particles led to an increase in both thermal and electrical conductivity. The AA6061/VC composite material exhibited the highest thermal conductivity among all composites tested. The electrical conductivity of the hybrid-composite AA6061/30%BN+70%VC showed a slight reduction, measuring only 2.8% lower than the base alloy AA6061. The mono-composite AA6061/VC exhibited a marginal decrease in thermal conductivity, with a measured value only 7.5% lower than the conventional alloy AA6061. However, the mono-composite AA6061/BN displayed a more significant decline, exhibiting a loss of 14.7% and 13.9% in electrical and thermal conductivity, respectively. The composite material comprising 30% BN and 70% VC reinforcement demonstrated the highest compressive strength compared to all other tested composites. The observed percentage enhancement in the mechanical properties of mono and hybrid composites, compared to the parent AA6061 alloy, ranged from 17.1% to 31.5%.
... The rolling process is one of the most common plastic deformation methods used for metal matrix composites in the industry, and it has also been used recently in graphene/metal composites to improve their strength and toughness [17][18][19][20][21]. Mu et al. produced a lution to remove the oxide on the Cu particle surface. ...
Rolling treatments have been identified as a promising fabrication and deformation processing technique for graphene/metal composites with high performance. However, it is still a challenge to choose appropriate rolling parameters to achieve high strength, ductility and electrical conductivity of the composite simultaneously. In this study, graphene/Cu composites were prepared with an in situ growth method and rolling treatment. The effects of rolling deformation and temperature on the microstructural evolution of graphene and Cu grains, interface bonding between graphene and the matrix, mechanical and electrical properties were systemically investigated. The cold-rolled composite with 85% deformation displayed a maximum ultimate strength of 548 MPa, a high elongation of 8.8% and a good electrical conductivity of 86.2% IACS. This is attributed to oriented graphene arrangement and matrix grain refinement. Our research provides a comprehensive understanding for the rolling behavior of graphene/Cu composites, and can promote the development of graphene-based composites with high performance.
... Large densities of dislocations were produced during cold rolling and accumulated in the sheets. The recrystallization was driven throughout the following heat treatment by the deformation stored energy, which was released through dislocation activity [32]. The dislocation motion was severely hindered by high pressure, which caused the release of deformation stored energy to be postponed. ...
Cold rolling the CC AA 5754 aluminum alloy to different reductions was followed by an hour of annealing at 537 °C under varied pressures. The changes in microstructure and texture were studied. According to the findings, high-pressure heat treatment restricts recrystallization, and the restriction effect is stronger as pressure rises. The refining impact of high-pressure heat treatment can refine recrystallized grains, and it becomes stronger as the rolling reduction gets higher. The variation of recrystallization texture is significantly affected by high-pressure heat treatment. The sheet with a 19.6% reduction is found to have a weak cube texture at atmospheric pressure and 2 GPa, but the sheet that has been annealed at 4 GPa and 6 GPa has a strong cube texture and a weak β fiber rolling texture. High-pressure heat treatment improves the intensities of the cube and R textures in the fully recrystallized sheets with the same reduction. The strengthening impact of R texture from high-pressure heat treatment improves with increasing rolling reduction.
... Haddeleme, malzemenin zıt yönde dönen iki ya da daha fazla silindir arasından sıkıştırılıp daha yoğun hale getirmesidir. Plastik şekil verilerek boyutlandırma yapılır [53,54]. ...
Endüstride ve sanayide sıkça kullanılan, metal olup hafifliği ile dikkat çeken Alüminyumun kullanımı artmaktadır. Çeşitli alaşımlandırma, kompozit bileşik yapma ve üretim teknikleriyle iyi bir mukavemet/ağırlık oranı yakalanan Alüminyum’un geliştirilmesi devam etmektedir. Bu yapılırken farklı üretim yolları ve Nanoteknolojik malzeme katkısı dikkat çekmektedir. Literatür incelendiğinde, Nano Bor Nitrat katkısının Alüminyuma mukavemet, sertlik, ağırlık, işlenebilirlik, maliyet indirgemesi gibi konularda iyileştirmeler yaptığı görülmüştür. Bu derlemede, farklı imal yollarıyla Alüminyuma eklenen Nano Bor Nitrat’ın özellikleri araştırılmıştır.
... (f) Nanohardness and elastic modulus of Al-BNNT composites [164]. aluminum (Al) [164] and titanium (Ti) [165] matrix composites through powder metallurgy techniques [166], molten fabrication [167], and physical vapor deposition [168]. The layered Al-BNNT composites were successfully fabricated by hot pressing, rolling and annealing processes. ...
Boron nitride nanotubes (BNNTs) have attracted worldwide research interest since 1995 due to their excellent properties and a wide range of applications in numerous fields. The development of efficient preparation routes for BNNTs has always been the primary obstacle, limiting their practical application. In recent years, several exciting achievements have been realized in BNNTs synthesis. This paper briefly overviews some history of BNNTs development and updates the recent advances in the synthesis of BNNTs, more focusing on the CVD method from the perspectives of experiment devices, precursors, catalyst, and temperature as well as their growth mechanisms. We also summarized some advanced applications of BNNTs in composites, water purification membranes, biological applications, and optical devices. Future challenges and opportunities in the synthesis and applications of BNNTs are discussed as well.
... Over the past few years, boron nitride nanotube (BNNT), having a structural similarity with CNT, has emerged as a potential additive in metal, ceramic, and polymer matrix composites. BNNTs possess a very high tensile strength of 25-33 GPa and own an extraordinary elastic modulus (900-1300 GPa) [17,[19][20][21][22]. Additionally, BNNTs are less curled because of strong intralayer bonding, makingtheir dispersion relatively easier [23]. All these properties impart a higher fracture strain in BNNTs and upgrade their flexibility, leading them to bend more and get inside the grain boundaries, which is quite necessary for the ceramics' bulk property improvement [24][25][26]. ...
Boron Nitride Nanotube (BNNT) has emerged as a potential reinforcement for ceramic and metal matrices due to
its excellent mechanical, chemical, and thermal properties. We have reinforced 5 vol % BNNT into the alumina
(Al2O3) matrix using the atmospheric plasma spraying technique in the present study. BNNTs have been suc-
cessfully retained after experiencing extreme temperatures of the plasma plume. Reinforcement of BNNTs has
improved the relative density of pure Al2O3 coating from 90% to 94%, whereas the hardness, elastic modulus,
and fracture toughness enhanced by 117%, 35% and 51%, respectively for the BNNTs reinforced coatings. The
critical energy release rate increased from 3.90 ± 1.18 J/m2 to 6.50 ± 1.22 J/m2 with BNNT addition. The
property enhancement reasons are attributed to improved densification, uniform distribution of BNNTs into the
matrix, and toughening mechanisms, such as BNNT truss, nanotube pull-out, and crack bridging. This BNNT
strengthened Al2O3 coating could be a successful candidate for highly durable and strong structural material for
automobile and aviation sector, high speed cutting tools and in bioimplant sectors.
... In Figure 8, the micro-hardness of the hybrid composites was measured five times, and the average results were presented. The mechanical properties of the hybrid composites depend on the interaction between the particles and matrix, microstructure, sintering temperature, and weight percentage of reinforcement materials [38]. In accordance, the different weight percentages of hybrid nanoparticles reinforcements added into aluminum matrix and an optimum amount of hybrid reinforcement required to modify the mechanical properties of HAMC were investigated. ...
Hybrid reinforcement's novel composite (Al-Fe3O4-SiC) via powder metallurgy method was successfully fabricated. In this study, the aim was to define the influence of SiC-Fe3O4 nanoparticles on microstructure, mechanical, tribology, and corrosion properties of the composite. Various researchers confirmed that aluminum matrix composite (AMC) is an excellent multifunctional lightweight material with remarkable properties. However, to improve the wear resistance in high-performance tribological application, hardening and developing corrosion resistance was needed; thus, an optimized hybrid reinforcement of particulates (SiC-Fe3O4) into an aluminum matrix was explored. Based on obtained results, the density and hardness were 2.69 g/cm3, 91 HV for Al-30Fe3O4-20SiC, after the sintering process. Coefficient of friction (COF) was decreased after adding Fe3O4 and SiC hybrid composite in tribology behaviors, and the lowest COF was 0.412 for Al-30Fe3O4-20SiC. The corrosion protection efficiency increased from 88.07%, 90.91%, and 99.83% for Al-30Fe3O4, Al-15Fe3O4-30SiC, and Al-30Fe3O4-20SiC samples, respectively. Hence, the addition of this reinforcement (Al-Fe3O4-SiC) to the composite shows a positive outcome toward corrosion resistance (lower corrosion rate), in order to increase the durability and life span of material during operation. The accomplished results indicated that, by increasing the weight percentage of SiC-Fe3O4, it had improved the mechanical properties, tribology, and corrosion resistance in aluminum matrix. After comparing all samples, we then selected Al-30Fe3O4-20SiC as an optimized composite.
... As an important part of metal matrix composites, aluminum matrix composites (AMCs) provide a combination of low density, high strength, excellent wear resistance and corrosion resistance for automotive industries and aerospace applications [1][2][3][4]. Particularly, aluminum matrix composites with different microstructural design by in situ fabrication such as melting methods and spark plasma sintering (SPS) are popular owing to their advantages [5][6][7][8]. However, the purpose is always to obtain a uniform distribution of reinforcement regardless of which processing method. ...
Recently, heterogeneous structured metals have attracted extensive interest due to their exciting mechanical properties. In this work, an AlN/Al nanocomposite with heterogeneous distribution of AlN nanoparticles was successfully prepared by a liquid-solid reaction method combined with subsequent extrusion deformation, which behaves an excellent synergy of tensile strength and ductility. In order to further reveal the particle distribution evolution and the tensile property response during further deformation, a series of rolling treatments with different thickness reductions under room temperature and 300 °C was carried out. The results show that the yield strength and tensile strength of the composites increase significantly from 238 MPa, 312 MPa to 312 MPa, 360 MPa after 85% rolling reduction at 300 °C. While the elongation decreased from 15.5% to 9.8%. It is also noticed that the elongation and tensile strength of the nanocomposites increases simultaneously with increasing deformation. It is considered that the aluminum matrix strengthening effect accounts for the strength enhancement. The AlN spatial distribution in the matrix becomes more homogeneous gradually during the rolling, which is supposed to reduce the stress concentration between the particle and matrix and then promote the ductility increase.
... Addition of GNPs in Al6061 matrix restricted the grain growth during sintering up to 1 wt%GNPs (Fig. 5). Grain growth is associated with sintering temperature, as reported by Bisht et al. [47]. Another strengthening mechanism explained by Sharma, V. et al. [48] is the difference in coefficients of thermal expansion (CTE) of reinforcement and matrix. ...
Lightweight aluminium matrix composites find wide applications in automobile and aerospace sector. Aluminium alloy composites with graphene nanoplatelets (GNPs) were prepared by powder metallurgy route containing four different loadings, i.e. 0.1 wt%, 0.5 wt%, 1 wt% and 3 wt%. Two methods for exfoliation of GNPs were employed to achieve uniform dispersion in Al6061 matrix. Pressureless consolidation followed by sin-tering was carried out to obtain near-theoretical densities. Comparison of the fabricated thermal condition and artificially aged T6 samples was carried out to investigate the contribution of GNPs in the strengthening of Al6061 matrix. Microstructure evolution was confirmed by X-ray diffraction, optical and scanning electron microscopy. Electrical conductivity increased with increasing content of GNPs up to 1 wt% due to conductive network around the matrix grains. Mechanical properties were found to increase 93%, 61% and 92% in hardness , compression strength and flexural strength up till 1 wt% GNPs in matrix alloy. Agglomeration at 3 wt% GNPs resulted in deterioration of properties. The fractured surfaces were examined to determine the change in fracture morphology, associated strengthening mechanism and behaviour of GNPs.
... It showed that hardness value increased with the addition of Si 3 N 4 and decreased with addition of BN due to acceleration of precipitation behavior of Al-5%Cu alloys. Ankita et al [14] made use of aluminium-boron nitride nanotubes (Al-BNNT) composite for annealing and warm rolling studies and found that annealing the Al-BNNT composite enhanced the strength by 40% and ductility by 80%. Thirumoorthy et al [15] observed inadequate study on addition of nitrides and oxides into aluminium matrix and behavior can be improved by combination of nitrides and oxides reinforcement on aluminium matrix composite. ...
Aluminium is the most promising materials owing to its strength to weight ratio with good oxidation
resistance. The present work is intended on synthesis and study on hardness, wear resistance and
tensile strength of hexagonal boron nitride and zirconium dioxide reinforced Al7075 metal matrix
hybrid composite. The produced composite showed increased hardness of 92.6 BHN due to the
presence of uniformly distributed reinforcements within the matrix. The hybrid combination of
boron nitride (3%)with zirconium dioxide (6%)showed higher tensile strength of 187.585 MPa and
significant improvement in wear resistance was achieved as compared to unreinforced alloy. Increase
in wear resistance is due to existence of hard reinforcements which restricted the loss of material from
sample surface.
... Among the inorganic tubes developed in the past decades, boron nitride nanotubes (BNNTs) have attracted considerable attention [2] due to the extraordinary mechanical properties, high oxidation resistance properties, chemical stability, high thermal conductivity, strong ultraviolet emission and unique optical properties etc. [3][4][5][6]. Taking advantages of the high Young's modulus, high tensile strength, high thermal and oxidation stability of BNNTs, it always be considered as an effective reinforcement additive for ceramic materials [7][8][9][10][11][12]. ...
High-quality boron nitride nanotubes were successfully synthesized via a novel two-step method, including citrate-nitrate combustion reaction and catalytic chemical vapor deposition. The composition, bonding features and microstructures of as-synthesized sample were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, transmission electron microscopy and selected area electron diffraction techniques. The results show that the as-synthesized boron nitride nanotubes with smooth surface are relatively pure. The diameter ranges between 20 and 80 nm, while the length is about dozens of micrometers. During the synthesis process of boron nitride nanotubes, citric acid chelates the cobalt ions and reacts with nitrate to form the cobalt oxide, depositing on the surface of boron powder homogeneously. The catalyst content and annealing temperature have a significant impact on the composition and microstructures of the final products. Based on the experimental results and thermodynamic analysis, the possible chemical reactions are listed, and vapor-liquid-solid mechanism is proposed to be dominant for the formation of boron nitride nanotubes.
In this study, aluminium alloysAluminium alloys reinforced by aluminaAlumina (Al2O3) and silicon carbide (SiC) were investigated. Lab-scale DC cast billets with a diameter of 80 mm were cast by ultrasoundUltrasound-assisted stir-castingCasting technology. The billets were subsequently thermo-mechanically processed. The distribution of the particles in the matrix was analysed before and after thermomechanical processingThermomechanical processing. It was found that hot rollingHot rolling improved the distribution of the reinforcement particles in the matrix. Reactions between the reinforcement particles and the matrix were also investigated, and their implications studied. It was found that aluminaAlumina reinforcement reacted with magnesium (Mg) in the alloy to produce spinel and SiC reinforcement oxidised producing a layer of silicon oxide (SiO2) around the particles. The consequences of these reactions are discussed. Whilst theoretical calculations of free energies for different carbides suggested that transition metals in the alloys should substitute the silicon in SiC, no reactions were observed in the physical experimentation. This was attributed to the weight percentages of the transition metals in the alloy being too small. These results are discussed and analysed further in this study, with plans for future experimentation.
Molecular dynamics (MD) simulations are performed to study the repeated nanoindentation on aluminum (Al) substrate with different boron nitride nanosheet (BNNS) coating thickness. To reveal the strengthening mechanism of coating, the hardness, surface morphology, atomic stress, atomic strain and phase transition are analyzed. The results show that the hardness of substrate increases with the coating thickness. It is also revealed that the pressure area of Al substrate increases with coating thickness. Because of the coating force, more atoms move along the loading direction, causing the substrate to slip more and more atoms to be severely strained. However, although severely deformed, some Al atoms still recover elastically after unloading. After multiple loadings, some elastic deformations are transformed into plastic deformations. The lattice structures are destroyed, possibly with amorphous atoms.
There is a need to augment the mechanical strength of titanium alloys to take advantage of their low mass density for structural applications. Herein, the reinforcement potential of boron nitride nanotube (BNNT), a high‐aspect‐ratio nanomaterial with exceptional strength and thermal stability, to engineer nanocomposites based on Ti–6Al–4 V alloy, is interrogated. The stress‐transfer behavior at the matrix/filler interface is programmed by tweaking the extent of chemical reactions during sintering. A twofold improvement in indentation resistance after integrating BNNTs in the alloy due to the crack bridging mechanism is reported. There is a further 50% enhancement in hardness and stiffness as the sintering temperature is raised from 750 to 950 °C. This improvement is attributed to the escalated formation of TiN and TiB phases due to matrix–nanotube interfacial reactions at elevated temperatures. A novel indentation‐based in situ trench testing technique is employed to study the subsurface deformation mechanisms activated in the bulk of the composite specimen. The ceramic interphases promote the indentation resistance by acting as crack deflectors in the microstructure, which promotes the dissipation of mechanical work. These insights can be applied to designing Ti‐BNNT microstructures with desired mechanical properties and deformation characteristics.
Boron nitride nanosheet (BNNS) has been widely used in aluminum (Al) matrix composites because of its extraordinary mechanical properties. However, the strengthening mechanism of BNNS in the Al matrix was seldom studied. In this work, three different BNNS/Al models were constructed, and the effect of BNNS’s volume fraction on the compressive properties of BNNS/Al composites was investigated using molecular dynamics. With the increase of BNNS’s volume fraction, the ultimate strength and Young’s modulus are significantly enhanced, but the critical strain decreases gradually. The stress distribution reveals the contribution of the BNNS and Al matrix. In addition, the atomic configurations were captured to analyze the reason for the decrease in critical strain. The interaction between the Al matrix and BNNS is evident at the interface. This results in the great deformation of the Al matrix in the direction perpendicular to the BNNS. Since BNNS can transfer the compression load and block the propagation of stacking faults in the Al matrix, BNNS/Al composites have excellent compression properties in both the elastic and plastic stages.
High-purity and high-yield boron nitride nanotubes with large aspect ratio were prepared by a facile two-step process, including the synthesis of boron/nickel containing precursors by precipitation reactions and subsequent thermally catalytic chemical vapor deposition reactions. The influence of catalyst content and annealing temperature on the phase composition and microstructure of the products were investigated. The results show that it is difficult to exert the catalytic effect of nickel-based catalyst at low temperatures (<1400 °C). At appropriate temperatures (1400 ∼ 1500 °C), highly crystalline boron nitride nanotubes with a length of more than 50 microns and a diameter of 50 nanometers are formed. The content of catalyst in the precursor mainly affects the morphology of the boron nitride product. If the content is too low, it is easy to form boron nitride particles; while high catalyst content can easily lead to catalyst aggregation and form a submicron one-dimensional boron nitride with unregular structure. Based on microstructural evolutions, phase changes, and thermodynamic analysis, the vapor-liquid-solid (V-L-S) growth mechanism of the tip growth mode dominates the formation of boron nitride nanotubes has also been verified.
Boron nitride nanotubes (BNNTs) are structurally similar to carbon nanotubes (CNTs) and have comparable mechanical properties. In addition, they surpass CNTs in terms of thermal stability and inertness. This opens many additional opportunities for taking advantage of their unique properties. Contrary to the extensive research conducted on aluminum (Al)-CNT composites, few studies have focused on Al-BNNT composites. In the present work, a novel Al-BNNT dual matrix composite is investigated. Pure unmilled Al powder is mixed with an equal weight of milled Al-2wt% BNNT composite powder with the objective of forming a dual matrix composite combining strength and ductility. The bulk composite was fabricated using milling and low-energy mixing followed by uniaxial compaction, tube cold rolling, and sintering. Bi-modal pure Al samples were also prepared using the same technique and used as reference samples. The dual matrix Al-BNNT composite exhibited an enhancement in mechanical properties, namely, a (+ 35.5%) increase in tensile strength, a (+ 91%) increase in indentation hardness, and a (+ 9%) increase in modulus of elasticity compared to the bi-modal Al without any significant reduction in ductility (− 2.25%). A detailed investigation of the structure of the composite is presented.
Al/h-BN composites with high tensile and compressive strength at room and elevated temperatures, as well as enhanced ductility, were obtained by a combination of ball milling (BM) and spark plasma sintering (SPS) using Al and hexagonal BN nanopowders (0, 1, 2, 3, 4, 5, and 10 wt.% of h-BN). The use of two types of nanopowders is intended to ensure uniform distribution of the reinforcing phase and improve Al-BN chemical interaction at the manufacturing stages by increasing the surface-to-volume ratio. Due to Al with h-BN interaction, the Al/h-BN composites were simultaneously strengthened by three types of nanoparticles: Al2O3, AlN(O) and h-BN, predominantly located along the Al grain boundaries. Compared to BM+SPS aluminum, the tensile strength of Al-2wt.%BN composite increased by 82% (25°C), 64% (300°C), and 65% (500°C), and the compressive strength by 107-119% (25-500°C) while maintaining high elongation to failure in tension (13.6%, 11.6% and 10.8%) and compression (12.6%, 13.1% and 8.1%) at 25°C, 300°C, and 500°C, respectively. In terms of combination of tensile and compressive strength at room and elevated temperatures, the Al/h-BN materials are superior to many other Al-based composites. The high strength and relative elongation to fracture of the Al/h-BN composites can be explained by the formation of a heterogeneous microstructure consisting of pure Al grains surrounded by a metal-matrix composite material with fine metal grains and reinforcing ceramic nanoinclusions. The obtained results significantly expand the scope of Al/h-BN materials, since their strength at 500 oC is higher than that of pure Al at room temperature.
Sandwich type Al1060/Al6061-0.5SiC/Al1060 laminates were fabricated by hot roll bonding and then subjected to heat treatment. The microstructure, texture and mechanical properties of the laminates were investigated. The inhomogeneity of microstructure throughout the thickness of 1060 Al layer was present in the as-rolled laminates. After heat treatment, the Al6061-0.5SiC composite layer was in a partially recrystallized condition. Some grains were subdivided by low-angle grain boundaries (LAGBs), and the fraction of LAGBs increased with increasing rolling reduction. The amount of Mg2Si precipitates increased with increasing rolling reduction. The Al6061-0.5SiC composite was characterized by the α fiber and β fiber after hot rolling. After heat treatment, the texture in Al6061-0.5SiC composite was dominated by Goss and R component. The tensile strength of the as-rolled laminates increased with increasing rolling reduction, while the elongation first increased and then decreased. The heat treatment could effectively improve the tensile properties of the Al1060/Al6061-0.5SiC/Al1060 laminates.
Herein, boron nitride nanotube (BNNT)–reinforced Ti alloy composites with 1 wt% BNNT (3 vol%) are successfully fabricated by spark plasma sintering (SPS). This study addresses two challenges affecting the performance of BNNT–metal matrix composites: i) the dispersion of high‐surface‐energy BNNTs by an ultrasonic‐assisted technique and ii) the control of reactions at the metal/nanotube interface. High‐energy ultrasonic vibrations were effective in the dispersion of entangled BNNTs, and sintering at low (750 °C and 555 MPa) and high (950 °C and 60 MPa) temperature regimes resulted in relative densities of 97% and 99%. Reducing sintering temperatures by up to 35% compared to those used in conventional sintering is effective in minimizing brittle interfacial products. Also, the composites sintered at high‐temperatures/low‐pressure benefit from the rapid densification (10 min) achieved in the SPS process limiting the extent of reactions (TiB2, TiB). Compressive properties show composites sintered at low and high temperatures exhibit increases of 46% and 50% in yield strength, respectively, as compared to the Ti alloy. Strain hardening exponents of 0.8 and 0.5 characterize the capabilities of the composites to arrest plasticity in the presence of BNNTs, reaction phases, and twin boundaries pinning dislocations. This study unravels the potential of a low‐temperature/high‐pressure processing window for Ti‐BNNT composites.
Aluminium matrix composites with high specific strength are attracting attention for use in automobile and aerospace applications. Graphene nano-platelets (GNPs) were added in 0.1, 0.5, and 1 weight fractions to an Al6061 matrix. Spark plasma sintering was used with a combination of solution sonication and ball milling to disperse the GNPs in the Al6061 matrix. The evolution of the microstructure was studied using optical and scanning electron microscopy. The uniformity of the GNP distribution is discussed in light of selected ball milling parameters. Electron backscattered diffraction analysis was used to measure the grain size and misorientation. X-ray diffraction analysis and transmission electron microscopy revealed neat and clean interfaces between the matrix and GNPs. Hardness and tensile testing revealed a considerable increment in the strength of the final composite after addition of GNPs. Traces of GNP clusters were found in the 1 wt.% composite as well as premature failure at lower strain due to the insufficient load transfer capability of the Al6061-T6 matrix. An illustrative two-dimensional model was developed to explain the load transfer behavior and the deterioration of the mechanical properties.
Over the years, high performance lightweight aluminium matrix composites have, mainly due to their excellent mechanical properties, found wide applications in automobile and aerospace applications. In the present study, Graphene NanoPlatelets based reinforced aluminium alloy AA6061 composites have been prepared through spark plasma sintering with four different loadings, i.e. 0.1, 0.5, 1 and 3 wt%. Through property characterisation of the obtained composites, it was established that a higher content of nano reinforcement had a substantial effect on the resulting microstructure, as well as on the electrical conductivity that proved to increase due to uniform distribution of the graphene network around the grains. An increase in the hardness and compressive strength was also obtained. The associated strengthening mechanism is discussed. A 2-dimensional physical model based on the bridging effect of graphene nanoplatelets in a composite matrix is presented, and its correlation with experimental results is discussed.
Boron Nitride Nanotube (BNNT) is a promising reinforcement for developing strong and lightweight metal matrix composites due to its brilliant mechanical properties and excellent high‐temperature oxidation resistance. In this study, layered composites of aluminum and BNNT are fabricated by a multi‐step process comprising of hot pressing, rolling, and annealing. Less than 0.1 wt% of ultra‐long nanotubes (up to 200 μm in length) are used to induce superior strengthening. For achieving enhanced mechanical properties, nanotubes are de‐agglomerated and chemically dispersed in an aqueous solution before introducing between the layers of aluminum sheets. Application of pressure (up to 10 MPa) and temperatures (up to 300 °C) during rolling and hot pressing induces a bonding between aluminum sheets and nanotubes. Scanning electron microscopy reveals that the nanotubes survive these conditions, suggesting superior thermal and mechanical endurance of BNNT. The nanohardness and elastic modulus of the composites are found to improve by 52% and 17%, respectively, by merely 0.045 wt% BNNT addition. BNNT reinforced composites exhibit up to 13% improvement in ultimate tensile strength. The improvement in mechanical properties is ascribed to effective load transfer from the aluminum matrix to the long nanotubes via interfacial shear stress and enhancement of dislocation density.
The influence of reinforcing AlN, AlB2 and BN phases, either individually or in combination, on the mechanical properties of Al-based composites fabricated via reactive ball milling of Al/BN, Al/B, and Al/Li3N powder mixtures, followed by their spark plasma sintering (SPS) was studied. An increase in tensile strength by 135% (25 °C) and 185% (500 °C) compared to pure Al was achieved in the Al/BN composites where BN, AlN, and AlB2 phases simultaneously formed. Density functional theory (DFT) calculations of the interface strength between Al and formed phases supported the experimental data. The obtained results demonstrated that the combination of reactive ball milling and SPS is a promising technique allowing for the formation of reinforcing phases from source materials.
Morphological and chemical transformations in boron nitride nanotubes under high temperature atmospheric conditions is probed in this study. We report atmospheric oxygen induced cleavage of boron nitride nanotubes at temperatures exceeding 750 °C for the first time. Unzipping is then followed by coalescence of these densely clustered multiple uncurled ribbons to form stacks of 2D sheets. FTIR and EDS analysis suggest these 2D platelets to be Boron Nitride Oxide platelets, with analogous structure to Graphene Oxide, and therefore we term them as "White Graphene Oxide" (WGO). However, not all BNNTs deteriorate even at temperatures as high as 1000 °C. This leads to the formation of a hybrid nanomaterial system comprising of 1D BN nanotubes and 2D BN oxide platelets, potentially having advanced high temperature sensing, radiation shielding, mechanical strengthening, electron emission and thermal management applications due to synergistic improvement of multi-plane transport and mechanical properties. This is the first report on transformation of BNNT bundles to a continuous array of White Graphene Oxide nanoplatelet stacks.
The mechanically-measured Young’s modulus of metals is consistently lower than the physically measured one, particularly after plastic straining. Furthermore, the nominally elastic loading and unloading behavior is not linear; it shows significant curvature and hysteresis. While many reports of this so-called “modulus effect” have appeared, the consistency of the behavior among grades of steel, or within a single grade produced by alternate methods and suppliers, is unknown. That is, there is little information on whether it is necessary for manufacturers to measure and control the mechanical modulus for every coil of steel in order to guarantee accurate simulations, consistent forming, and reliable in-service behavior. In order to address these issues, 12 steels (4 diverse grades: IF, HSLA, DP600, DP980; 3 producers per grade) were subjected to high-precision modulus measurements using mechanical testing, resonant frequency damping analysis, and ultrasonic pulse-echo techniques. All of these measurements show remarkable consistency among not only suppliers but also among grades. The primary determinant of hysteresis/curvature of the stress–strain response was found to be the nominal flow stress of the alloy. Other variations of overall mechanical modulus are minor compared with hysteresis/curvature. The following conclusions were reached: 1) there is no significant difference among suppliers of a single steel grade, 2) there is very little difference between grades of steel, except for that attributable to differing strengths, 3) mechanical unloading and reloading after pre-strain are similar, 4) cyclic loading and unloading cycles have no accumulated effect except through a minor change of flow stress, and 5) the initial loading or unloading modulus is very similar to the physical modulus, but the mechanically measured slope degrades very rapidly as loading or unloading proceeds, and plateaus at even small strain (<2%). The measured unloading and reloading behavior is more consistent and reproducible than that during initial loading, and unloading behavior is more consistent and reproducible than reloading behavior. Therefore, it is recommended that unloading after pre-strain is used to represent all of nominally elastic nonlinear behavior most accurately.
Current discoveries of different forms of carbon nanostructures have motivated research on their applications in various fields. They hold promise for applications in medicine, gene, and drug delivery areas. Many different production methods for carbon nanotubes (CNTs) have been introduced; functionalization, filling, doping, and chemical modification have been achieved, and characterization, separation, and manipulation of individual CNTs are now possible. Parameters such as structure, surface area, surface charge, size distribution, surface chemistry, and agglomeration state as well as purity of the samples have considerable impact on the reactivity of carbon nanotubes. Otherwise, the strength and flexibility of carbon nanotubes make them of potential use in controlling other nanoscale structures, which suggests they will have a significant role in nanotechnology engineering.
In recent years, graphene has attracted considerable research interest in all fields of science due to its unique properties. Its excellent mechanical properties lead it to be used in nano-composites for strength enhancement. This paper reports an Aluminum–Graphene Nanoplatelets (Al/GNPs) composite using a semi-powder method followed by hot extrusion. The effect of GNP nano-particle integration on tensile, compressive and hardness response of Al is investigated in this paper. It is demonstrated that 0.3 wt% Graphene Nanoplatelets distributed homogeneously in the matrix aluminum act as an effective reinforcing filler to prevent deformation. Compared to monolithic aluminum (in tension), Al–0.3 wt% GNPs composite exhibited higher 0.2% yield strength (+14.7%), ultimate tensile strength (+11.1%) and lower failure strain (−40.6%). Surprisingly, compared to monolithic Al (in compression), Al–0.3 wt% GNPs composite exhibited same 0.2% compressive yield strength and lower ultimate compression strength (−7.8%), and lower failure strain (−20.2%). The Al–0.3 wt% GNPs composite exhibited higher Vickers hardness compared to monolithic aluminum (+11.8%). Scanning electron microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS) and X-ray diffraction (XRD) were used to investigate the surface morphology, elemental percentage composition, and phase analysis, respectively.
Hexagonal BN nanoparticles (BNNPs) and boron nitride nanotubes (BNNTs) were determined to fabricate the composites of 75wt% SiO2–10wt% Al2O3–5wt% BNNPs (S-A-BNNPs) and 75wt% SiO2–10wt% Al2O3–5wt% BNNTs (S-A-BNNTs) via hot pressing. Comprehensive investigations on mechanical properties of the two composites were conducted, respectively. The two composites had high flexural strength and fracture toughness. The flexural strength of S-A-BNNPs and S-A-BNNTs composites are approximately 216.1% and 279.1% higher than that of the unreinforced SiO2, and the fracture toughness increases from 0.58MPam1/2 to 1.23MPam1/2, and to 1.39MPam1/2, respectively. Based upon detailed examination on microstructures, the strengthening and toughening mechanism was discussed.
Multiwalled boron nitride nanotubes (BNNTs) have very attractive mechanical and thermal properties, e.g., elasticity, tensile strength, and high resistance to oxidation, and may be considered as ideal reinforcing agents in lightweight metal matrix composites. Herein, for the first time, Al-BNNT ribbons with various BNNT contents (up to 3 wt.%) were fabricated via melt spinning in an argon atmosphere. BNNTs were randomly dispersed within a microcrystalline Al matrix under ribbon casting and led to more than doubling of room-temperature ultimate tensile strength of the composites compared to pure Al ribbons produced at the similar conditions.
Introducing carbon nanotubes (CNTs) into polymer or ceramic matrices has been a promising approach to obtain ultra-strong, extra-toughened materials as well as multifunctional composites. Most of the previous work on CNT composites has focused on strengthening and toughening of matrix materials at ambient conditions. However, so far there is a lack of information on the mechanical behavior of these composites at elevated temperature. Recently, single-walled CNTs were found to undergo a superplastic deformation with an appealing 280% elongation at a high temperature (Huang et al 2006 Nature 439 281). This discovery implies the high probability for the potential usage of CNTs as reinforcing agents in engineering high-temperature ceramics with improved ductility. Here, for the first time, we demonstrate that a small addition of boron nitride nanotubes (BNNTs) can dramatically enhance the high-temperature superplastic deformation (SPD) of engineering ceramics. More specifically, 0.5 wt% addition of BNNTs leads to an inspiring brittle-to-ductile transition in Al 2 O 3 ceramics even at a moderate temperature (1300 • C). For Si 3 N 4 ceramics, 0.5 wt% addition of BNNTs could also decrease the true stress by 75% under the same deformation conditions. In contrast, addition of micro-sized or nano-sized BN powders has no or a negative effect on the superplasticity of these ceramics. The underlying SPD-enhancement mechanism is discussed in terms of the inhibition of static and dynamic grain growth of the matrix and the energy-absorption mechanism of BNNTs. The unraveled capability of BNNTs to enhance the SPD behavior will make BNNTs promising components in cost-effective complex ceramics with good comprehensive mechanical properties.
Carbon nanotubes reinforced aluminum matrix composites were fabricated by isostatic pressing followed hot extrusion techniques. Differential scanning calorimetric, X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy has been carried out to examine the reaction condition of nanotubes and aluminum, and to analyze the composites structure. The effects of nanotubes content on mechanical properties of composites were investigated. Experimental results showed that nanotubes are homogeneously distributed in the composites. Some nanotubes act as bridges across cracks, others are pulled-out on fracture surfaces of composites. However, nanotubes react with aluminum and form Al4C3 phases when the temperature is above 656.3 °C. The nanotubes content affects significantly mechanical properties of composites. Meanwhile, the 1.0 wt.% nanotube/2024Al composite is found to exhibit the highest tensile strength and Young's modulus. The maximal increments of tensile strength and Young's modulus of the composite, compared with the 2024Al matrix, are 35.7% and 41.3%, respectively.
Packaging of organic photovoltaic (OPV) devices is an important issue which has been rarely addressed in the past. With the recent reports of high efficiency organic photovoltaics (6%), the need to produce materials which can effectively protect the device from degradation due to exposure to oxygen, moisture and radiation is pressing. We report a novel Saran (a co-polymer of vinylidene chloride and acrylonitrile) based polymer nanotube composite, which shows high transparency in the visible region, good barrier properties and thermal stability, for use as an encapsulant for OPV devices. Different loadings of Saran and boron nitride nanotubes were taken and the composites were prepared to optimize the composition of the composite. UV-visible spectroscopy, infra-red spectroscopy and thermal analysis were used to characterize the composite. The barrier properties of the composite were tested on poly(3-hexylthiophene), which is used in high efficiency OPV devices.
The recent observation of high flexibility in buckled boron nitride nanotubes (BNNTs) contradicts the pre-existing belief about BN nanotube brittleness due to the partially ionic character of bonding between the B and N atoms. However, the underlying mechanisms and relationships within the nanotube remained unexplored. This study reports for the first time the buckling mechanism in multi-walled BNNTs upon severe mechanical deformation. Individual BNNTs were deformed inside a transmission electron microscope (TEM) equipped with an in situ atomic force microscopy holder. High-resolution TEM images revealed that bent BNNTs form multiple rippling upon buckling. The critical strain to form the first ripple was measured as 4.1% and the buckling process was reversible up to 26% strain. As opposed to carbon nanotubes, the BNNTs buckled into V-shaped ripples rather than smooth wavy shapes. The rippling wavelength was quantified in terms of the outer diameter and thickness of the nanotubes. The BNNTs showed a larger rippling wavelength compared to that of CNTs with the same number of walls. This difference was explained by the tendency of BN structures to reduce the number of thermodynamically unfavorable B-B and N-N bonds at the sharp corners in the rippling regions. The BNNTs' structure also exhibited a higher fracture strain compared to their counterpart.
Boron nitride (BN) nanotubes have the same nanostructure as carbon nanotubes but are found to exhibit significant resistance to oxidation at high temperatures. Our systematic study has revealed that BN nanotubes are stable at 700 °C in air and that some thin nanotubes (diameter less than 20 nm) with perfect multiwalled cylindrical structure can survive up to 900 °C. Thermogravimetric analysis reveals an onset temperature for oxidation of BN nanotubes of 800 °C compared with only 400 °C for carbon nanotubes under the same conditions. This more pronounced resistance of BN nanotubes to oxidation is inherited from the hexagonal BN and also depends on the nanocrystalline structure. This high level of resistance to oxidation allows promising BN nanotube applications at
high temperatures<br /
Interactions between long boron nitride nanotube (BNNT) fibrils and molten aluminum (Al) pool are probed in this study to assess the feasibility of fabricating composite materials by solidification route. BNNTs were found to survive high temperature and reactive conditions present in molten aluminum. Very limited interfacial reaction was observed, resulting in the formation of AlN, AlB2 and AlB10 in trace amounts. AlN was the principal reaction product, resulting in improved interfacial wetting. Calculations based on surface energies revealed improved work of interfacial adhesion due to AlN formation. BNNTs were found to be well integrated in the aluminum matrix, signifying AlN induced excellent wetting. We also report capillarity-induced high temperature filling of BNNT by molten Al. The filling was promoted by AlN formation. In addition, formation of B-rich AlB10 phase inside the nanotube was observed. Nanotube filling by metal and subsequent reaction to form nano-ceramic phases is expected to alter mechanical properties of the cast Aluminum-BNNT composites. This study establishes the suitability of solidification route for developing high strength Al-BNNT composites in future.
Ultra-long Boron Nitride Nanotubes (100–200 mm) based layered Al–BNNT–Al composites are fabricated by spark plasma sintering, followed by cold rolling. The BNNT mat is sputter coated with Al to engineer strong metal-nanotube interface. The BNNTs exhibit perfect alignment along the cold rolling direction. The tensile strength of the composite is found to be 200 MPa, which is 400% greater than the strength of pure Al (%40 MPa). Young's modulus of this sandwich composite (%134 GPa) is found to be double the modulus of pure Al (%70 GPa) (with standard deviations less than 10%). Strengthening is explained by three major mechanisms: superior load transfer for long BNNT reinforcement, improvement in matrix-nanotube bonding due to trace amount of interfacial product formation, and crack bridging by directionally aligned long nanotubes.
Carbon nanotube (CNT) reinforced A356 aluminum alloys cast nanocomposites containing lower CNT contents were successfully fabricated where the way of introducing diluted Al–8 wt% CNT master nanocomposite in A356 melts was used. The differential thermal analysis and x-ray diffraction results showed that aluminum carbide phases (Al4C3) were formed before Al melting. The formation of Al4C3 was then proved to improve the wettability of CNTs during Al melting. Effect of CNT addition on microstructure and mechanical properties of CNTs/A356 nanocomposites were investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy, and universal tensile testing machine. The results showed that CNTs (<0.4 wt%) were well distributed in the CNTs/A356 nanocomposites. CNTs could greatly refine the microstructure of A356 alloy. The mechanical properties of CNTs/A356 nanocomposites were also enhanced by CNT addition. Fractography analysis revealed that CNTs were distributed uniformly throughout the fracture surface.
Uniaxial stress-strain curves are known to exhibit significant curvature and hysteresis even in the
nominally elastic regime, i.e. before the standard yield stress is attained. In order to probe the
nature of this behavior, hundreds of high-precision loading-unloading-loading tensile tests were
performed using 26 commercial sheet alloys exhibiting a wide range of strength, ductility and
crystal structure. Corresponding analysis shows that:
1. There is no significant linear elastic region, that is, the proportional limit is 0 MPa. While
the first increment of deformation shows a stress-strain slope equal to Young’s modulus,
progressive deviations of slope start immediately.
2. The shape of the transitional stress-strain curve can be represented by a simple oneparameter
equation representing the “modulus reduction rate.” It captures ~80% of the
measured variation and can be determined from a single test. This approach reduces the
error inherent in standard Young’s modulus or chord modulus approximations by a factor
of 3 to 6.
3. A “Universal Law” having no independently-determined parameters, i.e. no testing or
fitting required, was developed. It captures ~90% of the variation represented by the oneparameter
representation for the materials tested.
The practical and theoretical implications of these results are discussed. On the practical side,
the results provide an immediate path to improving applied constitutive models in the transitional
regime. An example of an application and results is provided. On the theoretical side, the
consistency of the effect for a wide range of metals suggests answers to questions about the
governing deformation mechanisms.
Aluminum matrix composites loaded with various fractions of multi-walled, well-structured boron nitride nanotubes (BNNTs), up to 5 wt.% fractions, were fabricated using powder constituents by means of a high pressure torsion technique (HPT) at room temperature under 5 GPa pressurization. Transient ultrathin amorphous-like layers, with a thickness of 2–5 nm, composed of Al(BNO) phases, which formed under severe plastic deformation and developed under further heat treatments of the composites at 350 °C and 450 °C, were detected at the interfacial regions between Al grains and tightly embracing them BN layers. Room temperature hardness and tensile tests on fabricated composites before and after heat treatments were conducted. The highest value of room temperature tensile strength was obtained on Al-5 wt.% BNNT samples annealed at 450 °C, that reached up to ~420 MPa, thus exhibiting more than a doubled increase in strength compared to HPT-fabricated pure Al samples under identical compacting conditions.
Melt-spun aluminum ribbons with up to 5 wt.% of embedded nano/micro boron nitride (BN) phases, namely multiwalled BN nanotubes (BNNTs) or BN microparticles (BNMPs), were fabricated by melt-spinning in an argon atmosphere. The comparative structural characteristics were analyzed using X-ray diffraction, scanning and transmission electron microscopy, and internal friction measurements as a function of temperature within an 80-800 K range. Room temperature tensile tests were carried out on ribbons. These revealed reinforcing effects on pure Al-matrices after nano/micro-BN embedment for both added phases with the notably higher numbers peculiar to the BNNT-containing samples. The intruastructural interactions between BN additions and Al-matrices are discussed based on the structural analysis and the internal friction data.
Boron nitride nanotubes ( BNNT s)/alumina composites were fabricated by hot pressing. The effect of BNNT s on the mechanical properties, including ambient and high‐temperature flexure strength and fracture toughness was investigated. As a result of sufficient physical and chemical bonding between BNNT s and alumina, the pullout and fracture of BNNT s concurrent with the suppression of BNNT s on abnormal growth of grains contributes to the excellent ambient and high‐temperature mechanical properties.
This study compares the damping behavior of boron nitride nanotubes (BNNTs) and carbon nanotubes (CNTs) as reinforcement in PLC, a biodegradable copolymer. The damping behavior of PLC composites reinforced with 2 wt % or 5 wt % nanotube filler is evaluated by nanodynamic mechanical analysis (NanoDMA). The addition of 2 wt % CNT leads to the greatest enhancement in damping (tan δ) behavior. This is attributed to pullout in CNTs because of lower interfacial shear strength with the polymer matrix and a more effective sword-in-sheath mechanism as opposed to BNNTs which have bamboo-like nodes. BNNTs however have a superior distribution in the PLC polymer matrix enabling higher contents of BNNT to further enhance the damping behavior. This is in contrast with CNTs which agglomerate at higher concentrations, thus preventing further improvement at higher concentrations. It is observed that for different compositions, tan δ values show no significant changes over varying dynamic loads or prolonged cycles. This shows the ability of nanotube mechanisms to function at varying strain rates and to survive long cycles.
Recent progress in boron nitride nanotube (BNNT) synthesis has opened new possibilities for utilization of the attractive combination of the excellent mechanical characteristics and superb thermal and chemical stabilities of BNNTs for the creation of new structural and functional reinforced materials. In this work we have applied metal ion implantation to prepare novel BNNT/metal matrix composites. The resulting structures have been thoroughly studied by high-resolution transmission electron microscopy and Raman spectroscopy.
Boron nitride nanotube (BNNT)/aluminum matrix composite nanohybrids were fabricated through magnetron sputtering of Al onto dispersed multiwalled BNNTs with average external diameters of 40–50 nm. Aluminum phase coating tightly wrapped the BNNTs after the deposition. The coating thickness in the range of 5–200 nm was controlled by changing sputtering time. Using imaging techniques and electron diffraction analysis in a transmission electron microscope, the Al phase was found to create nanocrystalline shields around individual BNNTs. The chemical states of the hybrid nanomaterials during the initial stages of sputtering were analyzed by X-ray photoelectron spectroscopy. Direct in situ bending and tensile tests on individual BNNT–Al nanocomposites were carried out by using a dedicated transmission electron microscope-atomic force microscope holder. In parallel, high-resolution TEM images and video recordings were taken for the analysis of deformation kinetics and fracture mechanisms. The nanohybrids with a suitably thick aluminum coating (∼40 nm) withstood at least nine times higher stresses compared to a pure non-armed Al metal. This pioneering work opens up a prospective pathway for making ultralight and superstrong “dream” structural materials for future automotive and aerospace applications.
In the present work, thermal expansion/contraction of armchair boron nitride nanotubes (BNNTs) with various diameters was investigated. The coefficient of thermal expansion (CTE) of BNNTs in axial, circumferential and radial directions were determined at temperatures from 100 K to 700 K, using molecular dynamics simulation. Two CTE in axial direction were defined; real axial CTE and apparent axial CTE. All BNNTs showed real axial thermal expansion, which was due to the thermal expansion of B–N bond length. However, large diameter BNNTs exhibited apparent axial thermal contraction at low temperatures. Although the apparent axial thermal contraction was not observed in small diameter BNNTs, their apparent axial CTE was smaller than their real axial CTE. In circumferential direction, all BNNTs were thermally expanded. In spite of thermal expansion in circumferential direction, BNNTs with large diameters exhibited thermal contraction in radial direction. The above observations were explained by considering both the effect of dynamic thermal vibration of BNNTs and the effect of thermal expansion of B–N bond length.
In order to overcome intrinsic brittleness and poor mechanical properties of SiO2, two kinds of hexagonal boron nitride (h-BN) (boron nitride nanotubes (BNNTs) and boron nitride nanoparticles (BNNPs)) were employed to reinforce SiO2 matrix. The mechanical properties, relative density and dielectric constant of the composites were investigated detailedly. Compared to the monolithic SiO2, 5wt% BNNTs/SiO2 and 5wt% BNNPs/SiO2 composites exhibited excellent mechanical properties and low dielectric constant. Furthermore, phase composition and microstructure of the composites were analyzed thoroughly by X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscopy.
Mechanical alloying has been widely utilized to break down the clustered CNTs for incorporation in the metal matrix composites. However, the breakage of CNTs during the ball milling process degrades their effectiveness. Due to the challenges in collecting the CNTs for measurement, quantitative study of CNT breakage has been difficult. In this study, the CNTs with Al6061 powder were mechanically alloyed with high energy milling equipment. The CNTs from the surface of mechanically alloyed particles were collected and measured. Due to the difficulty in obtaining the CNTs embedded inside the particles, a mathematical model has been developed to predict the overall CNT length distribution in the composite. Significant CNT breakage occurred during the initial phase of the mechanical alloying due to the crushing of the clusters. The model predicted that no further change occurred inthe overall CNT length during time greater than one hour of mechanical alloying because most of the CNTs had already become embedded within the particles and were thus protected from further milling media impacts. A faster dispersion of CNTs and lower particle fracturing rate may help preserve the original CNTs.
Nanotubes are considered to be unique structural and electronic materials. The possibility of boron nitride nanotubes as a nanoinsulating shield for any conducting material is also considered. Multiwalled boron nitride nanotubes were synthesized under heating of chemically-vapor-deposited multiwalled carbon nanotubes and boron trioxide to 1773 K over 30-60 minutes by induction currents in a flowing nitrogen atmosphere. An HRTEM micrograph of a representative rope synthesized using a PbO-promoter was described.
Boron nitride nanotubes (BNNT) reinforced aluminum based composites are synthesized by spark plasma sintering (SPS). The concentration of BNNT is varied as 0, 2 and 5 vol% in the aluminum matrix. Micro-pillar compression testing revealed that Al–5 vol% BNNT has yield strength and compressive strength as 88 MPa and 216 MPa respectively, which is more than 50% improvement over unreinforced Al. BNNT play an active role in strengthening Al matrix through effective load bearing and transfer by crack bridging and sword in sheath mechanisms. Cold rolling of Al–5 vol% BNNT with 75% thickness reduction in a single pass exhibited high deformability without cracking or disintegration. The deformation is dominated by slip signifying ductile behavior in sintered Al with and without BNNT. BNNT survives the extreme temperature and pressure conditions during SPS processing and heavy deformation during cold rolling.
The dual role of carbon nanotubes (CNTs) in strengthening roll bonded aluminum composites has been
elucidated in this study. An increase in the elastic modulus by 59% has been observed at 2 vol.% CNT
addition in aluminum, whereas tensile strength increases by 250% with 9.5 vol.% CNT addition. CNTs
play a dual role in the strengthening mechanism in Al–CNT composite foil, which can be correlated to
the degree of dispersion of CNTs in the matrix. Better CNT dispersion leads to improvement of elastic
properties. In contrast, CNT clusters in the aluminum matrix impede dislocation motion, causing strain
hardening and thus improvement in the tensile strength. Dislocation density of the composites has been
computed as a function of CNT content to show the effect on strain hardening of the metal matrix–CNT
composite.
Nature and mechanism of interfacial reactions between boron nitride nanotubes (BNNTs) and aluminum matrix at high temperature (650 °C) are studied using high-resolution transmission electron microscopy (HRTEM). This study analyzes the feasibility of the use of BNNTs as reinforcement in aluminum matrix composites for structural application, for which interface plays a critical role. Thermodynamic comparison of aluminum (Al)-BNNT with analogous Al-carbon nanotube (Al-CNT) system reveals lesser amount of reaction in the former. Experimental observation also reveals thin (;7 nm) reaction-product formation at Al-BNNT interface even after 120 min of exposure at 650 °C. The spatial distribution of the reaction-product species at the interface is governed by the competitive diffusion of N, Al, and B. Morphology of the reaction products are influenced by their orientation relationship with BNNT walls. A theoretical prediction on Al-BNNT interface in macroscale composite suggests the formation of strong bond between the matrix and reinforcement phase.
Boron nitride nanotube (BNNT)/polystyrene (PS) composite films were fabricated for the first time using high-quality BNNTs synthesized via a chemical-vapor-deposition method. The composite films exhibited good transparency. Tensile tests indicated that the elastic modulus of the films was increased by ∼21% when a ∼1 wt% soluble BNNT fraction was in use. Dispersion of BNNTs in PS and interfacial interactions between them were investigated using transmission electron microscopy. The film thermal properties, such as stability to oxidation and glass transition temperatures were measured. The experimental results and simple theoretical estimates indicate that BNNTs is a promising additive material for polymeric composites.
Boron nitride nanotube (BNNT)/polyvinyl alcohol (PVA) composite fibers (<5 vol % BNNTs) were fabricated via electrospinning so that all BNNTs became aligned in the fiber casting direction. A several-fibers-thick ensemble of parallel-arranged contacting fibers made a single polymer sheet. Numerous sheets were then stacked in different ways with respect to the BNNT orientation (all fibers in adjacent sheets were either parallel or alternately rotated 90°) to make multilayer films that were finally hot-pressed. Various BNNT textures were reflected by the corresponding differences in the measured thermal conductivities of the resultant films due to anisotropy of thermal transport in the nanotubes. The highest values (0.54 W/mK) were obtained along the long axes of aligned BNNTs. Somewhat lower values (0.38 W/mK) were documented in films with alternately stacked fibers/tubes. The theoretical thermal conductivity values were estimated using the Nielsen’s model. These show good match with the experimental data. The control of high-fraction BNNT (>10 vol %) alignment within the polymeric composites is proposed to be a promising way to further increase the polymeric film thermal conductivities toward wide practical applications.
Carbon nanotube (CNT) reinforced aluminum (Al) matrix composite materials were successfully fabricated by mechanical ball milling followed by powder hot extrusion processes. Microstructural analysis revealed that the CNTs were well dispersed at the boundaries and were aligned with the extrusion direction in the composites obtained. Although only a small quantity of CNTs were added to the composite (1 vol%), the Vickers hardness and the tensile strength were significantly enhanced, with an up to three-fold increase relative to that of pure Al. From the fractography of the extruded Al-CNT composite, several shapes were observed in the fracture surface, and this unique morphology is discussed based on the strengthening mechanism. The damage in the CNTs was investigated with Raman spectroscopy. However, the Al-CNT composite materials were not only strengthened by the addition of CNTs but also enhanced by several synergistic effects. The nanoindentation stress-strain curve was successfully constructed by setting the effective zero-load and zero-displacement points and was compared with the tensile stress-strain curve. The yield strengths of the Al-CNT composites from the nanoindentation and tensile tests were compared and discussed. We believe that the yield strength can be predicted using a simple nanoindentation stress/strain curve and that this method will be useful for materials that are difficult to machine, such as complex ceramics.
Alumina-matrix composite containing 2.0 wt% boron nitride nanotubes and monolithic alumina were fabricated by hot pressing, and their thermal and mechanical properties were investigated. Thermal shock resistance was evaluated by water quenching and subsequently by measuring the residual flexural strength by three-point bending test. Though the residual flexural strength of the composite is higher than that of the monolith at temperature differences lower than 280°C, the thermal shock of the composite is more sensitive to temperature change, exhibiting no significant improvement in thermal shock resistance. The variation in thermal shock resistance for the composite is resulted from the introduction of BNNTs.
Ultilizing boron nitride nanotubes (BNNTs) as fillers, composites are fabricated with poly(methyl methacrylate), polystyrene, poly(vinyl butyral), or poly(ethylene vinyl alcohol) as the matrix and their thermal, electrical, and mechanical properties are evaluated. More than 20-fold thermal conductivity improvement in BNNT-containing polymers is obtained, and such composites maintain good electrical insulation. The coefficient of thermal expansion (CTE) of the BNNT-loaded polymers is dramatically reduced because of interactions between the polymer chains and the nanotubes. Moreover, the composites possess good mechanical properties, as revealed by Vickers microhardness tests. This detailed study indicates that BNNTs are very promising nanofillers for polymeric composites, allowing the simultaneous achievement of high thermal conductivity, low CTE, and high electrical resistance, as required for novel and efficient heat-releasing materials.
Boron nitride nanotubes (BNNTs)/alumina composites were fabricated by hot pressing. The mechanical properties of the composites are greatly dependent upon the content of BNNTs. In comparison with monolithic alumina, the incorporation of BNNTs results in the improvement of bending strength and fracture toughness owing to the effective inhibition of grain growth. A routine toughening mechanism, especially the bridging of BNNTs at grain boundaries and the sufficient physical bonding between BNNTs and alumina matrix, is dominantly responsible for the increase in mechanical properties.Highlights► BNNTs are used as reinforcing phase to improve mechanical properties of Al2O3. ► The bending strength and fracture toughness are increased obviously. ► The improvement is ascribe to BNNTs bridging and sufficient physical bonding.
Ball milling was used to disperse MWCNTs of two different morphologies (stiff and straight vs. bent and entangled) and diameters (very large diameter and 3.5 times smaller diameter) in aluminium powders, which were subsequently hot consolidated by hot extrusion. Characterization of the produced composites revealed that the CNT morphology plays an important role in affecting dispersion. It was found that the smaller diameter bent and entangled CNTs were more difficult to disperse with increase in CNT content compared to the larger diameter stiff and straight ones; which in turn affected the tensile properties and hardness of the composites. Furthermore, cold welding of the milled powders as well as carbide formation in the final composite was found to depend on the CNT diameter. The smaller diameter CNTs – having a larger effective interfacial contact area with the aluminium matrix compared to the larger ones for a given CNT wt.% – were found to reduce particle welding during milling and to be more affected by carbide formation. Nano-sized particles of aluminium oxide as well as nano-rods of aluminium carbide, in addition to CNT damage were observed upon TEM analysis of the smaller diameter CNTs.
This review summarises the research work carried out in the field of carbon nanotube (CNT) metal matrix composites (MMCs). Much research has been undertaken in utilising CNTs as reinforcement for composite material. However, CNT-reinforced MMCs have received the least attention. These composites are being projected for use in structural applications for their high specific strength as well as functional materials for their exciting thermal and electrical characteristics. The present review focuses on the critical issues of CNT-reinforced MMCs that include processing techniques, nanotube dispersion, interface, strengthening mechanisms and mechanical properties. Processing techniques used for synthesis of the composites have been critically reviewed with an objective to achieve homogeneous distribution of carbon nanotubes in the matrix. The mechanical property improvements achieved by addition of CNTs in various metal matrix systems are summarised. The factors determining strengthening achieved by CNT reinforcement are elucidated as are the structural and chemical stability of CNTs in different metal matrixes and the importance of the CNT/metal interface has been reviewed. The importance of CNT dispersion and its quantification is highlighted. Carbon nanotube reinforced MMCs as functional materials are summarised. Future work that needs attention is addressed.
The current status of research on boron nitride nanotubes (BNNTs) - carbon nanotube structural analogues - is discussed. Latest achievements in BNNT synthesis, morphology, and atomic structure analysis as well as physical, chemical, and functional property evaluations are reviewed. Similarities and differences between structural parameters and properties of BNNTs in comparison with conventional carbon nanotubes are particularly highlighted. Recent breakthroughs in BNNT filling, doping and functionalization, morphology, and electronic structure engineering are examined. Finally, prospective BNNT applications for fabricating field-effect transistors, gas accumulators, and reinforcing polymer films are presented.
Due to the remarkable physical and mechanical properties of individual, perfect carbon nanotubes (CNTs), they are considered
to be one of the most promising new reinforcements for structural composites. Their impressive electrical and thermal properties
also suggest opportunities for multifunctional applications. In the context of inorganic matrix composites, researchers have
particularly focussed on CNTs as toughening elements to overcome the intrinsic brittleness of the ceramic or glass material.
Although there are now a number of studies published in the literature, these inorganic systems have received much less attention
than CNT/polymer matrix composites. This paper reviews the current status of the research and development of CNT-loaded ceramic
matrix composite (CMC) materials. It includes a summary of the key issues related to the optimisation of CNT-based composites,
with particular reference to brittle matrices and provides an overview of the processing techniques developed to optimise
dispersion quality, interfaces, and density. The properties of the various composite systems are discussed, with an emphasis
on toughness; a comprehensive comparative summary is provided, together with a discussion of the possible toughening mechanism
that may operate. Last, a range of potential applications are discussed, concluding with a discussion of the scope for future
developments in the field.
Aluminum (Al)/carbon nanotube (CNT) composites with nanoscale dispersion and regular orientation of the CNTs were fabricated by a combination of some advanced powder processes. The CNTs were well dispersed onto the Al particles by a nanoscale dispersion method. Moreover, the highly densified CNT composites were prepared by spark plasma sintering and subsequent hot extrusion. Microstructural observations by optical, field-emission scanning electron, and high-resolution transmission electron microscopies confirmed that the sintered Al/CNT compact and extruded bulk material had a good dispersion of oriented CNTs. Raman spectroscopy showed that the processing did little damage to the CNTs. As a result, the composites exhibited tensile strengths that were thrice larger than pure aluminum because of the CNT reinforcement.
Carbon nanotubes (CNTs) have recently emerged as materials with outstanding properties. Researchers have investigated their use as reinforcements in – mainly – polymer, and ceramic matrices. Due to the anticipated fabrication difficulties, a few research groups have explored their use to reinforce metal matrices. Recently, conventional powder metallurgy techniques (compaction and sintering) were used with some success. In this paper, a powder can rolling technique is used to fabricate carbon nanotube-reinforced aluminium strips. The Al–CNT mixtures are blended in either a mixer-shaker at a rotary speed of 46 rpm, or under argon in a planetary mill at a rotary speed of 300 rpm, prior to rolling. The dispersion of the nanotubes is shown to be better under the higher energy planetary action. The strength of the rolled strips is evaluated for various wt% CNT samples. The Al–0.5 wt% composite strips exhibited enhanced mechanical properties. The CNT-reinforced aluminium strips can have numerous attractive applications in the aerospace, automotive and electronics industries.
Powder metallurgy techniques have emerged as promising routes for the fabrication of carbon nanotube (CNT) reinforced metal matrix composites. In this work, planetary ball milling was used to disperse 2 wt% MWCNT in aluminum (Al) powder. Despite the success of ball milling in dispersing CNTs in Al powder, it is often accompanied with considerable strain hardening of the Al powder, which may have implications on the final properties of the composite. Both un-annealed and annealed Al–2 wt% CNT composites were investigated. It was found that, ball-milled and extruded (un-annealed) samples of Al–2 wt% CNT demonstrated high notch-sensitivity and consistently fractured outside the gauge length during tensile testing. In contrast, extruded samples annealed at 400 and at 500 °C for 10 h prior to testing, exhibited more ductile behavior and no notch sensitivity. Under the present investigated processing conditions, ball milling for 3 h followed by hot extrusion and annealing at 500 °C resulted in enhancements of around 21% in tensile strength compared with pure aluminum with the same process history. The ball-milling conditions used were found to result in the creation of a nanostructure in all samples produced, as shown by XRD and TEM analysis. Such nanostructure was retained after prolonged exposures to temperatures up to 500 °C. The tensile testing fracture surfaces showed uniform dispersion and alignment of the CNTs in the aluminum matrix but also showed CNTs acting as nucleation sites for void formation during tensile testing. This has contributed to the observation of CNT pull-out due to the poor bond between the CNTs and the matrix.
Aluminum composites reinforced with CNTs were fabricated by pressureless infiltration process and the tribological properties of the composites were investigated. Al has been infiltrated into CNTs–Mg–Al preform by pressureless infiltration in N2 atmosphere at 800 °C. By means of scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS), it was found that CNTs are well dispersed and embedded in the Al matrix. The friction and wear behaviors of the composite were investigated using a pin-on-disk wear tester under unlubricated condition. The tests were conducted at a sliding speed of 0.1571 m/s under an applied load of 30 N. The experimental results indicated that the friction coefficient of the composite decreased with increasing the volume fraction of CNTs due to the self-lubrication and unique topological structure of CNTs. Within the range of CNTs volume fraction from 0% to 20%, the wear rate of the composite decreased steadily with the increase of CNTs content in the composite. The favorable effects of CNTs on wear resistance are attributed to their excellent mechanical properties, being well dispersed in the composite and the efficiency of the reinforcement of CNTs.
The remarkable mechanical properties exhibited by carbon nanotubes have stimulated much interest in their use to reinforce advanced composites. Their elastic modulus is over 1 TPa and tensile strength is over 150 GPa, which makes them many times stiffer and stronger than steel while being three to five times lighter. In promoting their products, several manufacturers of sports equipment have advertised carbon nanotubes as reinforcements of some of their top of the line products.This paper evaluates the technical and economic feasibility of using carbon nanotubes in reinforcing polymer composites. It is concluded that carbon nanotubes can be used in conjunction with carbon fibers in a hybrid composite in order to achieve elastic modulus values in the range 170–450 GPa. As the sole reinforcing phase, carbon nanotubes present a viable option if elastic modulus values on the order of 600 GPa are desired. These conclusions are confirmed by a case study to select the optimum material for a tennis racket using the analytic hierarchy process.The discussion also shows that carbon nanotubes face several challenges, which need to be overcome before they can be widely used. They need to be produced in larger quantities at a lower cost, they need to be synthesized in longer lengths, and improved techniques are required to align and evenly distribute them in the matrix.
Amino functionalized boron nitride nanotubes were used as the reinforcement material for the fabrication of Al-matrix composites using powder metallurgy process. It was found that the mechanical properties of these composites were improved significantly as compared to pure Al composites fabricated under similar conditions. The microhardness of these composites was found to improve by five times and compressive strength by 300% as compared to pure Al composites under similar processing conditions. The enhanced mechanical properties of these composites can be attributed to the proper dispersion of boron nitride nanotubes (BNNTs) in Al matrix and the formation of a strong interfacial bonding between BNNTs and Al matrix under the processing conditions. High-resolution transmission electron microscopy studies revealed the formation of transition layer of AlB2 which might lead to a better load transfer from Al matrix to the BNNTs. Further, these composites are believed to withstand high temperatures as compared to Al matrix composites reinforced with carbon nanotubes and, therefore, can be used for applications where lightweight and high strength materials are desired with stability at elevated temperatures.
The Young modulus of individual single-walled boron nitride nanotubes (SW-BNNTs) was
determined using a high-resolution transmission-electron microscope (HRTEM)–atomic force
microscope (AFM) set-up. The Young modulus and maximum stress for these NTs were
deduced from the analysis of the stress–strain curves, and discussed as a function of the
considered value for the shell thickness of an SW-BNNT. The elastic properties of bundles
of SW-BNNTs were also investigated. All these experiments revealed that SW-BNNTs are
very flexible. Furthermore, the electrical behavior of these SW-BNNTs was also analyzed
employing a scanning tunneling microscope (STM) holder integrated with the same HRTEM.
I/V
curves were measured on individual tubes as well as on bundles of SW-BNNTs.
This study proposes boron nitride nanotube (BNNT) reinforced hydroxyapatite (HA) as a novel composite material for orthopedic implant applications. The spark plasma sintered (SPS) composite structure shows higher density compared to HA. Minimal lattice mismatch between HA and BNNT leads to coherent bonding and strong interface. HA-4 wt% BNNT composite offers excellent mechanical properties-120% increment in elastic modulus, 129% higher hardness and 86% more fracture toughness, as compared to HA. Improvements in the hardness and fracture toughness are related to grain refinement and crack bridging by BNNTs. HA-BNNT composite also shows 75% improvement in the wear resistance. The wear morphology suggests localized plastic deformation supported by the sliding of outer walls of BNNT. Osteoblast proliferation and cell viability show no adverse effect of BNNT addition. HA-BNNT composite is, thus, envisioned as a potential material for stronger orthopedic implants.
Biodegradable polylactide-polycaprolactone copolymer (PLC) has been reinforced with 0, 2 and 5wt.% boron nitride nanotubes (BNNTs) for orthopedic scaffold application. Elastic modulus of the PLC-5wt.% BNNT composite, evaluated through nanoindentation technique, shows a 1370% increase. The same amount of BNNT addition to PLC enhances the tensile strength by 109%, without any adverse effect on the ductility up to 240% elongation. Interactions of the osteoblasts and macrophages with bare BNNTs prove them to be non-cytotoxic. PLC-BNNT composites displayed increased osteoblast cell viability as compared to the PLC matrix. The addition of BNNTs also resulted in an increase in the expression levels of the Runx2 gene, the main regulator of osteoblast differentiation. These results indicate that BNNT is a potential reinforcement for composites for orthopedic applications.