Kantesh Balani

Bactericidal effect of silver-reinforced carbon nanotube and hydroxyapatite composites

Bacterial infection remains an important risk factor after orthopedic surgery. The present paper reports the synthesis of hydroxyapatite-silver (HA-Ag) and carbon nanotube-silver (CNT-Ag) composites via spark plasma sintering (SPS) route. The retention of the initial phases after SPS was confirmed by phase analysis using X-ray diffraction and Raman spectroscopy. Energy dispersive spectrum analysis showed that Ag was distributed uniformly in the CNT/HA matrix. The breakage of CNTs into spheroid particles at higher temperatures (1700℃) is attributed to the Rayleigh instability criterion. Mechanical properties (hardness and elastic modulus) of the samples were evaluated using nanoindentation testing. Ag reinforcement resulted in the enhancement of hardness (by ∼15%) and elastic modulus (∼5%) of HA samples, whereas Ag reinforcement in CNT, Ag addition does not have much effect on hardness (0.3 GPa) and elastic modulus (5 GPa). The antibacterial tests performed using Escherichia coli and Staphylococcus epidermidis showed significant decrease (by ∼65–86%) in the number of adhered bacteria in HA/CNT composites reinforced with 5% Ag nanoparticles. Thus, Ag-reinforced HA/CNT can serve as potential antibacterial biocomposites.

Multi-scale hierarchy of Chelydra serpentina: microstructure and mechanical properties of turtle shell.

Carapace, the protective shell of a freshwater snapping turtle, Chelydra serpentina, shields them from ferocious attacks of their predators while maintaining light-weight and agility for a swim. The microstructure and mechanical properties of the turtle shell are very appealing to materials scientists and engineers for bio-mimicking, to obtain a multi-functional surface. In this study, we have elucidated the complex microstructure of a dry Chelydra serpentinas shell which is very similar to a multi-layered composite structure. The microstructure of a turtle shells carapace elicits a sandwich structure of waxy top surface with a harder sub-surface layer serving as a shielding structure, followed by a lamellar carbonaceous layer serving as shock absorber, and the inner porous matrix serves as a load-bearing scaffold while acting as reservoir of retaining water and nutrients. The mechanical properties (elastic modulus and hardness) of various layers obtained via nanoindentation corroborate well with the functionality of each layer. Elastic modulus ranged between 0.47 and 22.15 GPa whereas hardness varied between 53.7 and 522.2 MPa depending on the microstructure of the carapace layer. Consequently, the modulus of each layer was represented into object oriented finite element (OOF2) modeling towards extracting the overall effective modulus of elasticity (~4.75 GPa) of a turtles carapace. Stress distribution of complex layered structure was elicited with an applied strain of 1% in order to understand the load sharing of various composite layers in the turtles carapace.

Damping behavior of carbon nanotube reinforced aluminum oxide coatings by nanomechanical dynamic modulus mapping

Nanomechanical dynamic analysis has been utilized to evaluate damping behavior of plasma sprayed carbon nanotube (CNT) reinforced Al2O3 ceramic coatings. Addition and dispersion of CNTs in Al2O3 matrix elicited modulus enhancement from 200 to 400 GPa. Tan delta increases from 0.26 for Al2O3 to 0.39 with 8 wt % CNT coating. CNT bending and curling, Al2O3 coating on CNT, interparticle Al2O3 friction, and CNT/splat sliding serve as strong loss mechanisms in imparting enhanced damping to Al2O3 nanocomposites reinforced with CNTs. Damping and fracture toughness of CNT-Al2O3 coating is semiempirically related to the enhancement of storage modulus and tan delta with varying CNT content and degree of dispersion.

Analytical model to evaluate interface characteristics of carbon nanotube reinforced aluminum oxide nanocomposites

This research presents an analytical method to investigate the effect of volume fraction and the number of outer walls of multiwalled carbon nanotube (MWNT) reinforcement on load carrying capability in the aluminum oxide matrix. Interfacial shear stress transfer and energy dissipation have been estimated using the Cox model. Critical energy release rate for the debonding of MWNT from the matrix is also estimated based on the crack deflection. The computed results sufficiently manifest that MWNT pullout and crack deflection contributes greatly to improved fracture toughness of carbon nanotube reinforced aluminum oxide nanocomposites.

An experimental and numerical investigation of fracture resistance behaviour of a dissimilar metal welded joint

Abstract Dissimilar welds impose a challenge to the engineers concerned with the structural integrity assessment of these joints. This is because of the highly inhomogeneous nature of these joints in terms of their microstructure, mechanical, thermal, and fracture properties. Fracture mechanics-based concepts cannot be directly used because of the problems associated with the definition of a suitable crack-tip loading parameter such as J-integral crack tip opening displacement (CTOD), etc. Again, depending upon the location of initial crack (i.e. base, weld, buttering, different interfaces, etc.), further crack propagation can occur in any material. The objective of the current work is to use micro-mechanical models of ductile fracture for initiation and propagation of cracks in the bimetallic welds. The authors have developed a finite element formulation that incorporates the porous plasticity yield function due to Gurson—Tvergaard—Needleman and utilized it here for the analysis. Experiments have been conducted at MPA Stuttgart using single edge-notched bend (SEB) specimens with cracks at different locations of the joint. The micro-mechanical (Gurson) parameters of four different materials (i.e. ferrite, austenite, buttering, and weld) have been determined individually by simulation of fracture resistance behaviour of SEB specimens and comparing the simulated results with those of the experiment. In order to demonstrate the effectiveness of the damage model in predicting the crack growth in the actual bimetallic-welded specimen, simulation of two SEB specimens (with initial crack at ferrite—buttering and buttering—weld interface) has been carried out. The simulated fracture resistance behaviour compares well with those of the experiment.

Fabrication and evaluation of a pulse laser-induced Ca–P coating on a Ti alloy for bioapplication

In the present paper, we demonstrate the feasibility of depositing a tailored calcium phosphate (Ca-P) coating on a Ti-6Al-4V substrate by using a pulsed Nd:YAG laser system. Different textures were obtained by varying the laser spot overlap with change in laser traverse speed. Surface roughness measurements using laser confocal microscopy indicated a decrease in roughness with increasing laser scan speed. X-ray diffraction studies revealed the formation of alpha-TCP, TiO2, Ti and Al as the major phases. An instrumented nanoindenation technique used to study the mechanical properties of the coatings, revealed a very high hardness and Youngs modulus of the coating surface as compared to the substrate. This further proved the retainment of the ceramic phase on the surface. Wear studies in a simulated biofluid (SBF) environment demonstrated an increased wear resistance of the coated samples as compared to the bare Ti-6Al-4V. Formation of an apatite-like layer after immersion in SBF for different time periods further demonstrated the bioactivity of the coated samples.

Modified Eshelby tensor modeling for elastic property prediction of carbon nanotube reinforced ceramic nanocomposites

A modified model using the Eshelby equivalent tensor is developed to evaluate overall elastic properties of carbon nanotube (CNT) reinforced aluminum oxide nanocomposites. This model accounts for the effect of carbon nanotube geometry and porosity on the effective elastic modulus. Experimental results are compared with the computed predictions. Computed results show higher elastic modulus values, which are attributed to the perfect bonding between the reinforcement and the matrix. It is concluded that the modified Eshelby model can predict the elastic property of CNT reinforced ceramic nanocomposites.

The hydrophobicity of a lotus leaf: a nanomechanical and computational approach

The multi-scale microstructure of a lotus leaf is rendered non-wetting by micro-protrusions and nano-hairs present on its surface. The mechanical properties of the surface become important since the water droplet has to be supported on the micro-protrusions without wetting the surface. Current work correlates the non-wetting behavior of the lotus leaf with its mechanical properties (Youngs modulus and critical flexing stress) and areal spread of micro-protrusions on the leaf surface. Quasistatic nanoindentation of nano-hairs on the lotus leaf surface has shown a variation of elastic modulus between 359 and 870 MPa, which in turn dictates the critical flexing strength and consequent non-wetting. Computational fluid dynamics modeling is utilized to correlate wetting phenomena with the areal spread of micro-protrusions. A qualitative model is proposed for the way nature has chosen to render the lotus leaf surface non-wetting.

Superhydrophobic self-floating carbon nanofiber coating for efficient gravity-directed oil/water separation

The fabrication of a superhydrophobic carbon nanofiber (CNF) on various substrates (activated carbon fiber and glass) via a two-step process (plasma sputtering followed by chemical vapor deposition at a lower operating temperature of 300 °C) is reported, eliminating the need for multiple pre- and post-treatments with toxic chemicals (fluorine/Si-based chemicals, metal salts, and organic solvents). Entangled CNFs grown on the coated activated carbon fiber (ACF) and glass substrates showed superhydrophobicity with water contact angles of 146° and 156°, respectively. The superhydrophobicity of the coated substrate is attributed to its lower surface energy, more graphitic structure (lower ID/IG ratio) as observed by Raman spectroscopy, lower carbonization state ratio (sp3/sp2 ratio) and restricted carbonyl (CO) as analyzed by X-ray photo-electron spectroscopy, and hydroxyl functional groups (–OH) as shown by Fourier transform infrared spectroscopy. Coated substrates were found to be stable in both acidic and basic chemicals. The load carrying capacity of the deposited CNF was measured to be up to 153 times that of its own deposited weight. The coated ACF was used for oil/water separation and showed a separation efficiency of >99%. Thus, the as processed durable CNF coated substrates can potentially be used for many applications such as anti-wetting, corrosion resistance, support for floating aquatic micro-devices and oil–water separation applications.

Dielectric and Pyroelectric Properties of HAp-BaTiO3 Composites

In order to mimic the electrical properties of natural bone, the present work investigated the dielectric, AC conductivity, pyroelectric and piezoelectric properties of HA-40 wt% BaTiO3 (HA-26 vol% BaTiO3) and HA-60 wt% BaTiO3 (HA-44 vol% BaTiO3) composites. Multistage spark plasma sintering was used to achieve the desired combination of properties. The electrical parameters were measured as a function of temperature and frequency. The values of dielectric constant and loss for both the developed composites, measured at room temperature and at 1 KHz frequency was 21, 38 and 0.01 and 0.02, respectively. The AC conductivity for both the composites is found to be of the order of 10−10 and 10−9 (Ωcm)−1, measured under similar conditions. Activation energy calculated from σac vs. temperature plot for HA-40 wt% BaTiO3 is 0.50 eV. The room temperature pyroelectric coefficients for both the compositions are 2.35 and 21 μC/m2K, respectively. The piezoelectric coefficient values (d33) for both the compositions are 0.9 and 1 pC/N, respectively. The observed values of electrical parameters closely resemble with that of the natural human bone.