Carbon nanotube reinforced aluminum based nanocomposite fabricated by thermal spray forming

The present research concentrates on the fabrication of bulk aluminum matrix nanocomposite structures with carbon nanotube reinforcement. The objective of the work was to fabricate and characterize multi-walled carbon nanotube (MWCNT) reinforced hypereutectic Al-Si (23 wt% Si, 2 wt% Ni, 1 wt% Cu, rest Al) nanocomposite bulk structure with nanocrystalline matrix through thermal spray forming techniques viz. plasma spray forming (PSF) and high velocity oxy-fuel (HVOF) spray forming. This is the first research study, which has shown that thermal spray forming can be successfully used to synthesize carbon nanotube reinforced nanocomposites. Microstructural characterization based on quantitative microscopy, scanning and transmission electron microscopy (SEM and TEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy and X ray photoelectron spectroscopy (XPS) confirms (i) retention and macro/sub-macro level homogenous distribution of multiwalled carbon nanotubes in the Al-Si matrix and (ii) evolution of nanostructured grains in the matrix. Formation of ultrathin β-SiC layer on MWCNT surface, due to chemical reaction of Si atoms diffusing from Al-Si alloy and C atoms from the outer walls of MWCNTs has been confirmed theoretically and experimentally. The presence of SiC layer at the interface improves the wettability and the interfacial adhesion between the MWCNT reinforcement and the Al-Si matrix. Sintering of the as-sprayed nanocomposites was carried out in an inert environment for further densification. As-sprayed PSF nanocomposite showed lower microhardness compared to HVOF, due to the higher porosity content and lower residual stress. The hardness of the nanocomposites increased with sintering time due to effective pore removal. Uniaxial tensile test on CNT-bulk nanocomposite was carried out, which is the first ever study of such nature. The tensile test results showed inconsistency in the data attributed to inhomogeneous microstructure and limitation of the test samples geometry. The elastic moduli of nanocomposites were computed using different micromechanics models and compared with experimentally measured values. The elastic moduli of nanocomposites measured by nanoindentation technique, increased gradually with sintering attributed to porosity removal. The experimentally measured values conformed better with theoretically predicted values, particularly in the case of Hashin-Shtrikman bound method

Effect of sintering temperature on phase evolution of Al86Ni6Y4.5Co2La1.5 bulk amorphous composites synthesized via mechanical alloying and spark plasma sintering

Al86Ni6Y4.5Co2La1.5 amorphous powders were synthesized by mechanical alloying for 200 h. Subsequent consolidation was performed via spark plasma sintering in the temperature range of 250 °C to 500 °C at the pressure of 500 MPa. The role of viscous flow on densification was investigated by studying the viscosity change of the amorphous phase at different consolidation temperatures. The decrease in viscosity at higher sintering temperatures resulted in better particle bonding and densification of consolidated samples. The formation of only FCC Al was observed in the consolidated samples at sintering temperatures ≤ 300 °C and the intermetallic phases formed at temperatures ≥ 400 °C. The mechanical properties of the bulk samples were measured by Vickers microhardness and nanoindentation tests. The testing results showed that the average values of microhardness, nanohardness and elastic modulus of the sample consolidated at 500 °C were 3.06 ± 0.14 GPa, 4.85 ± 1.14 GPa and 89.53 ± 9.25 GPa, respectively. The increase in hardness and elastic modulus of the higher temperature consolidated samples is attributed to the improvement in particle bonding, densification and distribution of various hard intermetallic phases in the amorphous matrix. Keywords: Al based glassy alloy, Mechanical alloying, Spark plasma sintering, Intermetallic phase, Viscosit

Al-MWCNT nanocomposite synthesized via spark plasma sintering: effect of powder milling and reinforcement addition on sintering kinetics and mechanical properties

In this work, the effect of mechanical milling of aluminium (Al) powder and subsequent addition of multiwalled carbon nanotubes (MWCNTs) as reinforcement followed by powder mixture consolidation via spark plasma sintering has been investigated. Grain growth during sintering, densification behaviour and mechanical properties (microhardness and compressive properties) were analyzed to complement the study of sintering kinetics. The results showed that for milled Al powder compact, densification started at a lower temperature as compared to the as-received powder compact. This is attributed to large specific surface area of milled Al powders and high amount of diffusivity path created during the milling. The addition of MWCNTs, reinforcement of high hardness and compressive strength, constricted Al particle deformation, sliding and rearrangement during compaction, thereby hindering densification. This behaviour was confirmed by observed increase in Tstart and Tend, the temperature at which densification started and ended, respectively, for nanocomposites containing MWCNTs. Mechanical properties were significantly improved as a result of milling. Microhardness and compressive strength of the milled powder compact increased by 97% and 53%, respectively. MWCNTs pinned grain boundaries and caused dislocation generation and accumulation, which led to further improvement in mechanical properties. However, Al-1.0 wt% CNT showed reduction in microhardness due to inefficient dispersion of MWCNTs leading to large amounts of porosity. Keywords: Al nanocomposite, Multiwalled carbon nanotube, Mechanical milling, Spark plasma sintering, Sintering behaviour, Compression tes

In situ processing of Fe-based bulk metallic glass nanocomposites in supercooled liquid region by spark plasma sintering

Current study reports fabrication of in-situ Fe-based bulk metallic glass (BMG) nanocomposites by carrying out spark plasma sintering of Fe57Cr9Mo5B16P7C6 amorphous alloy powder. In situ BMG composites were synthesized with varying amount of crystalline phases by controlled partial devitrification in supercooled liquid region. Application of high consolidation pressure (400 MPa) during sintering contributed to enhanced viscous flow in the amorphous powder resulted in high densification (> 97 %) in the sintered compacts. Microstructural and phase evolution in the consolidated BMGs and BMG nanocomposites were systematically investigated with respect to sintering conditions. Microscale heterogeneity with respect to phase evolution in the sintered samples was evaluated by nanoindentation studies. Hardness of the sintered BMG nanocomposites was found to be significantly higher than the sintered BMGs ascribed to the evolution of hard intermetallic phases during devitrification

A novel coating strategy towards improving interfacial adhesion strength of Cu-Sn alloy coated steel with vulcanized rubber

A comparative assessment in terms of uniformity, coating coverage and coating deposition mechanism has been carried out for two different types of Cu-Sn coatings on steel substrate with varying Sn composition (2-6.5 wt%) deposited via immersion technique, viz. (i) single layer Cu-Sn coating and (ii) double layer coating consisting of a thin Cu strike layer followed by a Cu-Sn layer. Coating morphology, surface coverage, coating-substrate interface, and coating composition at surface and along the depth were studied using laser confocal microscope (OLS), scanning electron microscope (SEM) coupled with energy dispersive spectroscope (EDS), glow discharge optical emission spectroscopy (GDOES), X-ray photoelectron spectroscopy (XPS) and cross-sectional transmission electron microscopy (TEM). Quantitative depth profiling using GDOES and surface compositional analysis via XPS suggested improvement in surface coverage in the case of double layer coatings. SEM-EDS and TEM analysis confirmed that the coating deposition was more uniform with sufficient coating penetration inside the deep roughness troughs resulting in compact and micro-porosity free interface for this type of coatings. Better adhesion strength with less variation in peel force and cohesive mode of fracture within the rubber was observed for the double layer coated samples during the peel test carried out on coated steel samples vulcanized with rubber. On the other hand, the single layer coated samples showed large variation in peel force with adhesive-cohesive or mixed mode of fracture at interface. (C) 2014 Elsevier B.V. All rights reserved

Role of Sn on the adhesion in Cu-Sn alloy-coated steel-rubber interface

Cu-Sn coatings with varying Sn content were deposited on steel substrate by immersion route and the effect of variation of Sn content and the substrate roughness on the interfacial adhesion strength of Cu-Sn-coated steel substrates vulcanized with styrene butadiene rubber were investigated. The surface roughness of the coatings did not vary compared to pristine steel substrate with change in Sn weight% in the coatings. The coated surfaces exhibited bare spots or deep trough as micro-discontinuities in the coatings, where formation of Fe2O3 was evident from SEM-EDS, AES, and XPS analysis. Microstructural study of the coating cross-section and coating-substrate interface by transmission electron microscopy of cross-sectioned samples revealed inadequate penetration of coating inside these troughs. Peel test carried out on the Cu-Sn-coated steel-rubber joints showed mixed mode i.e. adhesive and cohesive mode of interfacial fracture irrespective of the coating composition. The peel test further indicated higher interfacial adhesion strength for Cu-Sn-coated samples than pure Cu-coated samples, with an optimum adhesion strength for the coatings containing 3-4 wt.% Sn

Structural Transformations In Carbon Nanotubes During Thermal Spray Processing

This study compares the interaction of carbon nanotubes (CNTs) with the flame/energy sources during different thermal spray processes viz. plasma spraying (PS), high-velocity oxy fuel spraying (HVOF), cold spraying (CS), and plasma spraying of liquid precursor (PSLP). CNTs were successfully retained as reinforcement in metal and ceramic composite coatings in all thermal spray processes except PSLP. The retention of CNT structure is attributed to micron size metal/ceramic powder which acts as a carrier and thermal shield against high heat in plasma spraying (PS) and high-velocity oxy fuel spraying (HVOF). However, vaporization of CNTs occurred in PSLP under the intense heat of the plasma which is attributed to phase transformation in unshielded CNTs. © 2009 Elsevier B.V. All rights reserved

Microstructural evolution in Cu-Sn coatings deposited on steel substrate and its effect on interfacial adhesion

The objective of the present work is to understand the microstructural evolution in Cu-Sn coatings with varying Sn content (3-6.5 wt%) deposited on steel substrate via immersion coating method and its effect on interfacial adhesion with styrene butadiene (SBR) based bead rubber. The phase formation prediction in different Cu-Sn alloy systems from Pourbaix diagrams constructed using FactSage revealed the formation of higher amount of SnO2 with an increase in Sn content in the coatings. Quantitative depth profiling by glow discharge optical emission spectroscopy, microstructural characterization by transmission electron microscopy and phase analysis by grazing incidence X-ray diffraction confirmed the presence of Cu3Sn precipitates with increasing volume fraction as Sn content in coatings increases. Adhesion strength measured by performing pull-out test on the SBR rubber vulcanized Cu-Sn coated steel wire samples exhibited maximum value for Cu-5 wt.% Sn coating. The formation of Cu3Sn and SnO2 played crucial role in controlling the Cu activity at the coating-rubber interface to form optimally thick Cu-sulfide layer in Cu-5 wt.% Sn coating and thus provided the maximum adhesion strength. (C) 2014 Elsevier B.V. All rights reserved

Evaluation of room temperature creep deformation of in situ Fe-based bulk metallic glass nanocomposites by instrumented indentation

In the present work, Fe57Cr9Mo5B16P7C6 (at. %) in situ bulk metallic glass nanocomposites of varying crystal-linity, i.e. 0 to 80 vol. %, were synthesized by spark plasma sintering. Room temperature indentation creep behaviour of these composites was evaluated with nanoindentation technique. Effect of crystalline phase content on overall creep behaviour of the BMG composites was evaluated. Creep resistance of the BMG composites was improved with increase in the amount of crystalline phases up to 40 vol. %. Improved creep resistance is attributed to the presence of nanocrystalline phases in amorphous matrix and lower defect concentration in the BMG composite samples. However, excess crystallinity imparted brittleness and resulted in poor creep perfor-mance of the BMG composites. Derived stress exponent values are significantly lesser for the BMG composite sample in comparison to the amorphous compact indicating the transition in the mode of plastic deformation from inhomogeneous to nearly homogeneous. Loading rate has more pronounced influence on final creep displacement of fully amorphous sample in comparison to that of BMG composite