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Despite the ultrahigh intrinsic strength of multiwalled carbon nanotube (MWCNT), the strengthening effect on ceramic matrix composite remains far from expectation mainly due to the weak load transfer between the reinforcement and ceramic matrix. With the assistance of the in situ pullout test, it is revealed that the liquid‐phase sintering (LPS) can serve as a novel strategy to achieve effective load transfer in MWCNT reinforced ceramic matrix composites. The YAlO3 formed liquid phase during spark plasma sintering of SiC composite greatly facilitates radical elastic deformation of MWCNT, leading to highly increased interfacial shear strength (IFSS) as well as interlayer shear resistance (ISR) of nested walls. The liquid phase with superior wettability can even penetrate into the defects of MWCNT, which further increases the ISR of MWCNT. Moreover, the first‐principles calculation indicates that the oxygen terminated YAlO3 phase displays much stronger bonding compared with SiC matrix, which is also responsible for the large IFSS in the composite. As a result, as high as 30% improvement of bending strength is achieved in the composite with only 3 wt% MWCNT in comparison to the monolithic ceramic, manifesting the unprecedented strengthening effect of MWCNT assisted by LPS. [ABSTRACT FROM AUTHOR]

Multi-walled carbon nanotube (MWCNT) reinforced copper (Cu) matrix composites, which exhibit chromium (Cr) carbide nanostructures at the MWCNT/Cu interface, were prepared through a carbide formation using CuCr alloy powder. The fully densified and oriented MWCNTs dispersed throughout the composites were prepared using spark plasma sintering (SPS) followed by hot extrusion. The tensile strengths of the MWCNT/CuCr composites increased with increasing MWCNTs content, while the tensile strength of MWCNT/Cu composite decreased from that of monolithic Cu. The enhanced tensile strength of the MWCNT/CuCr composites is a result of possible load-transfer mechanisms of the interfacial Cr carbide nanostructures. The multi-wall failure of MWCNTs observed in the fracture surface of the MWCNT/CuCr composites indicates an improvement in the load-bearing capacity of the MWCNTs. This result shows that the Cr carbide nanostructures effectively transferred the tensile load to the MWCNTs during fracture through carbide nanostructure formation in the MWCNT/Cu composite. [ABSTRACT FROM AUTHOR]

The defect-type evolution and gradual increase in nanodefects in the outer walls of multiwalled carbon nanotubes (MWCNTs) in a chemically oxidizing environment were thoroughly investigated using a mixture of sulfuric acid (H2SO4) and nitric acid (HNO3). A fairly low temperature of 323K was employed for the acid treatment, and this limited the reaction rate to provide a mild acidic environment for gradual chemical oxidation compared to commonly used treatment conditions. High-resolution transmission electron microscopy (HRTEM) observations clearly demonstrated the formation of groove-type defects in the outer walls of MWCNTs in the early period around 0.5-3h, followed by a morphological change into circumference-type defects and further into unzipped graphene nanoribbons. A unique parabolic variation in the intensity ratio of D and G bands (ID/IG) was observed. This observation supports the gradual oxidation of graphitic walls with increasing acid treatment time and corresponds well with the formation and evolution of nanodefects. Herein, the appropriate acid treatment time that would result in an effective number of nanodefects via suitable carboxyl functionalization, providing preferential nanocarbide sites for interfacial improvement as well as uniform dispersibility of MWCNTs, has been discussed. [ABSTRACT FROM AUTHOR]

The thermal expansion response of multi-walled carbon nanotube (MWCNT) reinforced Al matrix composites was employed to discuss the improvement of the load transfer at the interface between the MWCNTs and the Al matrix. An aluminum carbide (Al 4 C 3 ) nanostructure at the end of the MWCNTs, incorporated in the Al matrix, was produced by appropriate heat-treatment. The stress contrast around the Al 4 C 3 observed in the high-resolution transmission electron microscopy (HRTEM) image revealed the evidence of a trace of friction, which would lead to the enhancement of the anchor effect from the Al matrix. This anchor effect of Al 4 C 3 may hinder the local interfacial slippage and constrain the deformation of the Al matrix. As a result, the thermal expansion behavior became linear and reversible under cyclic thermal load. It is concluded that the formation of Al 4 C 3 could effectively enhance the load transfer in MWCNT/Al composites. The yield strength of MWCNT/Al composites was substantially increased under the appropriate quantity of Al 4 C 3 produced at the MWCNT-Al interface by precisely controlled heat-treatment. [ABSTRACT FROM AUTHOR]

Quantitative characterization of interfacial shear strength (IFSS), in multi-walled carbon nanotube (MWCNT)-reinforced Al matrix composites, has been carried out using an in-situ pull-out technique. The end of an MWCNT protruding out of the tensile fracture surface of the composite was bonded to the tip of an atomic force microscopy (AFM) cantilever using an electron-beam-induced deposition method. The MWCNT was pulled out from the Al matrix using a nanomanipulator system installed inside a scanning electron microscope (SEM). The effective embedded length of the MWCNT incorporated in the Al matrix was evaluated by observing the pulled out MWCNT under high-resolution transmission electron microscopy (HRTEM). Valid results were obtained for six MWCNTs with straight, visibly damage-free embedded parts nearly perpendicular to the fracture surface. The IFSS between the MWCNT and the Al matrix was 24.8 ± 3.2 MPa. The ultimate tensile strength (UTS) of the MWCNT/Al composites is in good agreement with that estimated by the shear lag model using these IFSS values, resulting in the realization of 60% load transfer efficiency at the directly contacted MWCNT-Al interface in the MWCNT/Al composites. No MWCNT failure was observed during the pullout in this study. [ABSTRACT FROM AUTHOR]

The interface and interfacial reactions in Al-matrix composites reinforced with multi-walled carbon nanotubes (MWCNTs) were thoroughly investigated by high-resolution transmission electron microscopy combined with a precisely controlled heat treatment in the solid state. It was shown that MWCNT (002) formed a coherent interface with the low-index Al planes of Al (111), Al (220), and Al (002), realizing a stable interface. Aluminum carbides (Al 4 C 3 ) were preferentially formed at the active prism plane edges sited at the open ends and acid treatment-induced surface nanodefects of MWCNTs. The Al 4 C 3 maintained the shape of the pristine MWCNT and showed a typical orientation relationship with the Al matrix, that is, Al (111)//Al 4 C 3 (001). It was suggested that the diffusion of Al atoms through Al 4 C 3 dominated the growth of Al 4 C 3 ; the Al 4 C 3 originated at the open MWCNT tips may quickly grow in the <100> direction of the MWCNT, while the nanodefect-originated Al 4 C 3 may grow simultaneously in the <110> and <100> directions of the MWCNT. The activation energy of Al 4 C 3 formation, the appearance of twinning in single-crystal Al 4 C 3 , and the possible influence of the Al 4 C 3 formation on the enhancement of load transfer at the MWCNT/Al interface were also studied. [ABSTRACT FROM AUTHOR]

Abstract: We have, for the first time, fabricated chromium (Cr) carbide nanostructures on multiwalled carbon nanotubes (MWCNTs) in a MWCNT–copper (Cu) composite by adding small amounts of Cr. A single nanocrystal of carbide was epitaxially grown on the sidewall of chemically treated MWCNTs through the diffusion of solute Cr atoms to defects in the MWCNTs. Carbide has an island-shaped morphology with a maximum size of several hundred nanometers. The phase of the generated Cr carbide nanostructures was mostly Cr7C3, as determined by electron beam diffraction using the nanobeam diffraction mode of a transmission electron microscope. In particular, the Cr carbide nanostructures formed at radially unzipped sites were connected to the outermost wall and some inner walls of the MWCNTs. This is expected to increase the load-bearing capacity of the MWNCTs because the inner walls contribute to the load transfer, which is very effective for improving the mechanical and thermomechanical properties of the composites. [Copyright &y& Elsevier]

Abstract: Multi-walled carbon nanotube (MWCNT)-reinforced copper (Cu) nanocomposite coatings were successfully deposited on aluminum (Al) substrate by a cold spraying process at a low pressure. The microstructure and the Raman spectrum of the low-pressure-cold-sprayed MWCNT–Cu nanocomposite coating showed that the MWCNTs maintained their tube structure in the Cu matrix, even though structural damage to the MWCNTs increased slightly. MWCNT–Cu nanocomposite-coated Al exhibits higher thermal diffusivity than pure-Cu-coated Al with a comparable hardness. The higher thermal diffusivity of the MWCNT–Cu coating could be explained by the dispersion of MWCNTs within the clean and closed CNT/Cu interfaces, which were achieved with the aid of compressive stress during the cold spraying. [Copyright &y& Elsevier]

• The structural evolution of MWCNTs during composite consolidation was investigated. • The MWCNT was not damaged during spark plasma sintering. • The hot-extruded MWCNT showed a clear break and a sword-in-sheath failure. • No MWCNT failure was induced during tensile test. The crystalline integrity of multi-walled carbon nanotubes (MWCNTs) plays a critical role in determining the performance of MWCNT/Al composites. In this work, the structural evolution of MWCNTs during composite consolidation was studied via high-resolution transmission electron microscopy and Raman spectroscopy. The MWCNTs were not structurally damaged during the spark plasma sintering of MWCNT/Al mixed powders. However, two kinds of fractures, i.e., clear break and sword-in-sheath failure, were generated under severe plastic deformation of Al during hot extrusion. We concluded that the MWCNT failure, seen on the fractured composites, was caused by the consolidation process rather than the tensile tests. This work provides a better understanding of the strengthening behavior of MWCNTs, and the design of high-performance MWCNT/Al composites using suitable consolidation techniques. [ABSTRACT FROM AUTHOR]