In this study, we used the Density functional theory (DFT) to obtain the relationship between mechanical property and electronic property of Boron nitride nanotubes (BNNTs) under the uni-axial strain. Moreover, we also investigated one CO molecule adsorbed on the BNNTs under the uni-axial strain. We also use the molecular dynamics to introduce the mechanical property and dynamic behavior of (8,8)BNNT under the uni-axial strain. There were three parts in this study: The first part: The effect of uni-axial strain on the electronic properties of (5,5) and (8,0)boron nitride nanotubes were obtained by DFT calculation. We used the HOMO-LUMO Gap¡Bbond angle¡Bbond length and radial buckling to analyze the electronic properties and mechanical properties. The stress-strain profiles indicated that different BNNTs types displayed very similar mechanical properties, but there were variations in HOMO-LUMO gaps at different strains, indicating that the electronic properties of BNNTs not only depend on uni-axial strain, but on BNNT type. In addition, the variations in nanotube geometries, partial density of states (PDOS) and charges of boron and nitride atoms were also discussed for (8,0) and (5,5) BNNTs at different strains. The second part: The DFT was used to investigate electronic properties of CO molecule adsorbed on BNNT under the uni-axial strain. The stress-strain profiles indicated that the CO molecule adsorption on BNNT leaded only to a local mechanical deformation. The strength of BNNT could not be affected when the CO molecule adsorbed on that. Moreover, we obtained that the charge of CO will slightly transfer to the adsorbed atom of BNNT when strain increased. Hence, the adsorption energy increased slightly under the uni-axial strain. The third part: The molecular dynamics simulations were performed to investigate deformation behaviors of (8,8)BN nanotubes under axial tensile strains at 300k. Variations with the tensile strain in the axial stress, bond lengths, bond angles, radial buckling, and slip vectors were all examined. The axial, radial, and tangential components of the slip vector were also employed to monitor, respectively, the local elongation, necking, and twisting deformation near the failure of the nanotube. The components of the slip vector grew rapidly and abruptly after the failure strain, especially for the axial component. This implies that the local elongation dominates the failure of the loaded BN nanotube and finally results in a chain-like tensile failure mode.

Hydroxyapatite (HA) has received wide attention in orthopedics, due to its biocompatibility and osseointegration ability. Despite these advantages, the brittle nature and low fracture toughness of HA often results in rapid wear and premature fracture of implant. Hence, there is a need to improve the fracture toughness and wear resistance of HA without compromising its biocompatibility. The aim of the current research is to explore the potential of nanotubes as reinforcement to HA for orthopedic implants. HA- 4 wt.% carbon nanotube (CNT) composites and coatings are synthesized by spark plasma sintering and plasma spraying respectively, and investigated for their mechanical, tribological and biological behavior. CNT reinforcement improves the fracture toughness (>90%) and wear resistance (>66%) of HA for coating and free standing composites. CNTs have demonstrated a positive influence on the proliferation, differentiation and matrix mineralization activities of osteoblasts, during in-vitro biocompatibility studies. In-vivo exposure of HA-CNT coated titanium implant in animal model (rat) shows excellent histocompatibility and neobone integration on the implant surface. The improved osseointegration due to presence of CNTs in HA is quantified by the adhesion strength measurement of single osteoblast using nano-scratch technique. Considering the ongoing debate about cytotoxicity of CNTs in the literature, the present study also suggests boron nitride nanotube (BNNT) as an alternative reinforcement. BNNT with the similar elastic modulus and strength as CNT, were added to HA. The resulting composite having 4 wt.% BNNTs improved the fracture toughness (~85%) and wear resistance (~75%) of HA in the similar range as HA-CNT composites. BNNTs were found to be non-cytotoxic for osteoblasts and macrophages. In-vitro evaluation shows positive role of BNNT in osteoblast proliferation and viability. Apatite formability of BNNT surface in ~4 days establishes its osseointegration ability.

As firearms continuously become more sophisticated, there have been commensurate efforts to optimize the ballistic performance of armours, with ceramic materials currently at the forefront of such studies. These efforts have focused on improving processing and microstructural design with reinforcements using dispersion particles, carbon nanotubes (CNT) and boron nitride nanotubes (BNNT). In most studies, ballistic testing has been used to identify parameters affecting the performance. The research documented here focuses on: (1) the investigation of two commercial ceramics, namely silicon carbide (SiC) and zirconia toughened alumina (ZTA). The primary material properties evaluated for the characterization included: hardness, fracture toughness, flexural strength and Young’s modulus. Other properties investigated included the microstructure, porosity/density, and mode of failure or fracture. (2) Ballistic depth of penetration (DOP) testing for six candidate ceramic armour systems including three monolithic ceramics (Al2O3, SiC and B4C) and three nanotube toughened ceramic composites (Al2O3-BNNT, Al2O3-single walled CNT and SiC-BNNT). SiC showed a hardness of 2413 HV, which is far beyond the requirements for armour ceramic. In contrast, ZTA barely met the hardness requirement of 1500 HV, but showed improved toughness of 4.90 MPa m1/2 beyond values reported for monolithic alumina. SiC and ZTA showed that microstructural design improves fracture toughness but processing introduces defects that can substantially reduce other armour related properties such as the strength. The results of the Charpy and drop tower impact tests are in agreement with indentation fracture toughness results suggesting a great degree of reliability of this cost efficient method. The addition of nanotubes produced an increase in toughness and a decrease in hardness in the ceramics, which resulted in an overall drop in performance during ballistic depth of penetration (DOP) tests. A microstructure-quasi-static mechanical properties-ballistic performance relationship was established which led to the development of a novel ballistic performance index and a new DOP model. The proposed ballistic performance index yielded a ranking, which agrees better with experimental observations than the currently published indices. The developed semi-empirical model suggests that the ballistic performance of ceramics is improved with increased fracture toughness, reduced flaw size and higher density.

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The discovery of carbon nanotubes (CNTs) in 1991 by Iijima revealed the possibility of ultra-strong materials exploiting the properties of materials at smaller length scales. The superior strength, stiffness, and ability to perform under extreme conditions motivated researchers to investigate further on CNTs and similar materials at nanoscale. This resulted in discovery of various nanostructures such boron nitride nanotubes (BNNTs), graphene, hexagonal boron nitride sheets etc. Many of such nanostructures exhibited superior strength and stiffness comparable to that of CNTs. Out of these nanotubes, BNNTs have recently attracted attention from researchers due to their excellent mechanical properties similar to that of CNTs along with better chemical and thermal stability. Thus, BNNTs can be used for varieties of applications such as protective shield for nanomaterials, optoelectronics, bio-medical, nano spintronics, field-emission tips in scanning tunneling and atomic force microscope, and as reinforcement in composites. BNNTs are also used in other applications such as water cleansing, hydrogen storage, and gas accumulators. To exploit these ultra-strong materials, the mechanics of materials under different conditions of loading and failure need to be studied and understood. Also, to make use of the material in a nanocomposite or other applications, the material properties should be evaluated. The present work is focused on the computational study of the mechanics of nanotubes with special reference to BNNTs and CNTs. Note that the attention is not given to the material but to the nanostructure and mechanics. Hence depending on the state-of-the-art, BNNTs and CNTs are used wherever it is appropriate along with justifications. The chapter-wise outline of the present work is given below. The first chapter is an introduction along with a state-of-the-art literature review. The second chapter introduces the molecular simulation methodology in brief. The chapters from the third to the seventh present the work in detail and describe the major contributions. The final chapter summarizes the work along with a few possible directions to extend the present work. Chapter 1 In this chapter, the importance of computational techniques to study the mechanics at the nanoscale is outlined. A brief introduction to various nanostructures and nanotubes are also given. A detailed literature review on the mechanics of nanotubes with special attention to elastic properties, buckling, tensile failure, and as reinforcement in nanocomposites is presented. Chapter 2 In this chapter, the molecular simulation technique is outlined. The molecular dynamics (MD) simulation is one of the most common simulation techniques used to study materials at the nanoscale. A few interatomic potentials that are used in an MD simulation are explained. Theories linking continuum mechanics with the molecular dynamics are also explained here. Chapter 3 In this chapter, the elastic behavior of single-walled BNNTs under axial and torsional loading is studied. Molecular dynamics (MD) simulation is carried out with a tersoff potential for modeling the interatomic interactions. Different chiral configurations with similar diameter are considered to study the effect of chirality on the elastic and shear moduli. Furthermore, the effects of tube length on elastic modulus are also studied by considering different aspects ratios. It is observed that both elastic and shear moduli depend on the chirality of a nanotube. For aspect ratios less than 15, the elastic modulus reduces monotonically with an increase in the chiral angle. For chiral nanotubes the torsional response shows a dependence on the direction of loading. The difference between the shear moduli against and along the chiral twist directions is maximum for a chiral angle of 15◦, and zero for zigzag (0◦) and armchair (30◦) configurations. Chapter 4 Buckling of nanotubes have been studied using many methods such as MD, molecular mechanics, and continuum based shell theories. In MD, motion of the individual atoms are tracked under an applied temperature and pressure, ensuring a reliable estimate of the material response. The response thus simulated varies for individual nanotubes and is only as accurate as the force field used to model the atomic interactions. On the other hand, there exists a rich literature on the understanding of continuum mechanics based shell theories. Based on the observations on the behavior of nanotubes, there have been a number of shell-theory-based approaches to study the buckling of nanotubes. Although some of these methods yield a reasonable estimate of the buckling stress, investigation and comparison of buckled mode shapes obtained from continuum analysis and MD are sparse. Previous studies show that a direct application of shell theories to study nanotube buckling often leads to erroneous results. In this chapter, the nonlocal effect on the mechanics of nanostructures is studied using Eringen’s nonlocal elasticity. The buckling of carbon nanotubes is considered as an example to demonstrate and understand the nonlocal effect in the nanotubes. Single-walled armchair nanotubes with the radius varying from 3.4nm to 17.7nm are considered and their critical buckling stresses are predicted based on multiscale modeling techniques including classical and nonlocal continuum mechanics theories and MD simulation. Fitting nonlocal mechanics models to MD simulation yields a radius-dependent length-scale parameter, which increases approximately linearly with the radius of carbon nanotube. In addition, the nonlocal shell model is found to be a better continuum model than the nonlocal beam model due to its ability to include the circumferential nonlocal effect. Chapter 5 In this chapter, the effects of geometrical imperfections on the buckling of nanotubes are studied. The present study reveals that a major source of the error in continuum shell theories in calculating the buckling stress can be attributed to the geometrical imperfections. Here, geometrical imperfections refer to the departure of the shape of the nanotube from a perfect cylindrical shell. Analogous to the shell buckling in the macro-scale, in this work the nanotube is modeled as a thin-shell with initial imperfection. Then a nonlinear buckling analysis is carried out using the Riks method. It is observed that this proposed approach yields significantly improved estimate of the buckling stress and mode shapes. It is also shown that the present method can account for the variation of buckling stress as a function of the temperature considered. Hence, this turn out to be a robust method for a continuum analysis of nanotubes taking in the effect of variation of temperature as well. Chapter 6 In this chapter, the effects of Stone-Wales (SW) and vacancy defects on the failure behavior of BNNTs under tension are investigated using MD simulations. The Tersoff-Brenner potential is used to model the atomic interaction and the temperature is maintained close to 300 K. The effect of a SW defect is studied by determining the failure strength and failure mechanism of nanotubes with different radii. In the case of a vacancy defect, the effect of an N-vacancy and a B-vacancy is studied separately. Nanotubes with different chirality but similar diameter are considered first to evaluate the chirality dependence. The variation of failure strength with the radius is then studied by considering nanotubes of different diameter but same chirality. It is observed that the armchair BNNTs are extremely sensitive to defects, whereas the zigzag configurations are the least sensitive. In the case of pristine BNNTs, both armchair and zigzag nanotubes undergo brittle failure, whereas in the case of defective BNNTs only the zigzag ones undergo brittle failure. An interesting defect-induced plastic behavior is observed in defective armchair BNNTs. For this nanotube, the presence of a defect triggers mechanical relaxation by bond breaking along the closest zigzag helical path, with the defect as the nucleus. This mechanism results in a plastic failure. Chapter 7 In this chapter, the utility of BNNTs as reinforcement for nanocomposites with metal matrix is studied using MD simulation. Due to the light weight, aluminium is used as the matrix. The influence of number of walls on the strength and stiffness of the nanocomposite is studied using single-and double-walled BNNTs. The three body tersoff potential is used to model the atomic interactions in BNNTs, while the embedded atom method (EAM) potential is used to model the aluminium matrix. The van der Waals interaction between different groups — the aluminium matrix with the nanotube or the between the concentric tubes in double walled BNNT — is modeled using a Lennard Jones potential. A representative volume element approach is used to model the nanocomposite. The constitutive relations for the nanocomposite is also proposed wherein the elastic constants are obtained using the MD simulation. The nanocomposite with reinforcement shows improved axial stiffness and strength. The double-walled BNNT provides more strength to the nanocomposite than the single-walled BNNT. The BNNT reinforcement can be used to design nanocomposites with varying strength depending on the direction of the applied stress. Chapter 8 The summary of the work with a broad outlook is presented in this chapter. The major conclusions of the work are reiterated and possible directions for taking the work further ahead are mentioned.