Of late, nanotechnology seems to be rapidly thrusting its applications in all aspects of life including engineering and medicine. Materials science and engineering has experienced a tremendous growth in the field of nanocomposite development with enhanced chemical, mechanical, and physical properties. A wide array of research has been conducted in the processing of nanocomposites. Consolidation of these systems from loose particles to bulk free form entities has always been a challenge. To name a few, traditional consolidation techniques such as cold pressing and sintering at high temperatures, hot pressing, and hot isostatic pressing have strong limitations of not being able to retain the nanoscale grain size due to the excessive grain growth during processing. This article reviews in detail the results from numerous studies on various methods for manufacturing nanocomposites with improved properties and retained nanostructures. Both challenges and recent advances are discussed in detail in this review.

Challenges and advances in nanocomposite processing techniques

Powder-bed fusion is a class of Additive Manufacturing (AM) processes that bond successive layers of powder to facilitate the creation of parts with complex geometries. As AM technology transitions from the fabrication of prototypes to end-use parts, the understanding of the powder properties needed to reliably produce parts of acceptable quality becomes critical. Consequently, this has led to the use of powder characterisation techniques such as scanning electron microscopy, laser light diffraction, X-ray photoelectron spectroscopy, and differential thermal analysis to study the effect of powder characteristics on part properties. Utilisation of these powder characterisation methods to study particle morphology, chemistry, and microstructure has resulted in significant strides being made towards the optimisation of powder properties. This paper reviews methods commonly used in characterising AM powders, and the effects of powder characteristics on the part properties in powder-bed fusion processes.

Powder characterisation techniques and effects of powder characteristics on part properties in powder-bed fusion processes

Interfacial phenomena in thermally sprayed multiwalled carbon nanotube reinforced aluminum nanocomposite

There are two challenges in nanoceramic sintering: fully densifying the sintered body and maintaining the sintered grains at <100 nm size. This review examines the fundamental factors underlying nanoceramic sintering and the approaches to effectively utilise the sintering factors to advantage. Nanoceramic sintering techniques are divided into four categories: pressureless sintering, pressure sintering, electrically assisted sintering, and other sintering related techniques. Pressureless sintering has mainly evolved around modifying sintering schedules, improving nanoparticle packing characteristics, and using additives to tailor the diffusion rates. Pressure sintering, which includes hot pressing, hot isostatic pressing, and sinter forging, can effectively achieve full densification for nanostructured ceramics but microstructural inhomogeneity and sintered shape limitation are difficult to overcome. For electrically assisted sintering, many nanoceramics have been sintered to full density with spar...

Sintering of nanoceramics

A study on sliding and erosive wear behaviour of atmospheric plasma sprayed conventional and nanostructured alumina coatings

Vacuum plasma spray (VPS) forming of tungsten-based metal matrix nanocomposites (MMCs) has shown to be a cost effective and time saving method for the formation of bulk monolithic nanostructured thermo-mechanical components. Spray drying of powder feedstock appears to have a significant effect on the improved mechanical properties of the bulk nanocomposite. The reported elastic modulus of the nanocomposite nearly doubles due to the presence of HfC nano particulates in the W matrix. High resolution transmission electron microscopy (HRTEM) revealed the retention of nanostructures at the select process conditions and is correlated with the enhanced mechanical properties of the nanocomposite.

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Structure and property evaluation of a vacuum plasma sprayed nanostructured tungsten-hafnium carbide bulk composite

Nanostructured coatings undergo many processing challenges during atmospheric plasma spray if the feedstock is not well prepared. The effects of using spray drying on achieving a suitable feedstock for an even flow of nanoparticles through the gun, the selection of plasma gases for optimum heat transfer, and the velocity of the agglomerates during flight are highlighted in this communication. Analytical heat transfer calculations are used for coating property optimization. The physical characteristics of the nanoagglomerates are studied using diagnostics sensor and scanning electron microscopy. Transmission electron microscopy revealed the retention of nanostructures.

Role of Spray Drying of Nanoagglomerates in Morphology Evolution in Nanostructured APS Coatings

This article, for the first time, presents the challenges toward the successful consolidation of near-net-shape bulk MoSi 2 –Si 3 N 4 –SiC nanocomposite using plasma spray forming. A detailed characterization of the spray formed bulk nanocomponent has been performed using optical microscopy (OM), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and Vickers hardness testing. Vickers hardness (900 Hv) and fracture toughness (∼>5 MPa m 1/2 ) of the nanocomposite showed a little deviation from the expected, which might be due to the difference in the particle (Si 3 N 4 ) size and their distribution in the MoSi 2 matrix as a function of component thickness. Relatively higher hardness is attributed to the retention of the nanostructure in the composite. In addition, the as fabricated bulk nanocomposite showed enhanced oxidation resistance.

Plasma spray formed near-net-shape MoSi2–Si3N4 bulk nanocomposites—structure property evaluation

A creative technique of in-situ focused ion beam (FIB) extraction was introduced to prepare a gas atomized rapidly solidified hypereutectic Al-Si single particle's cross-section for High Resolution Transmission Electron Microscopy (HRTEM) analysis. This preparation technique may be employed to characterize very inimitable samples that are abnormally wrought or intricate to prepare through traditional techniques. TEM results revealed that a gas-atomization/rapid solidification process leads to a homogeneous dispersion of 50-100-nm Si phase in the Al matrix. Stacking faults and dislocations are observed in the microstructure and will ultimately lead to the increased strength in a resultant bulk material manufactured from this powder to be further examined.

Keith E. Rea

Papers

Structure and property evaluation of a vacuum plasma sprayed nanostructured tungsten-hafnium carbide bulk composite

Role of Spray Drying of Nanoagglomerates in Morphology Evolution in Nanostructured APS Coatings

Plasma spray formed near-net-shape MoSi2–Si3N4 bulk nanocomposites—structure property evaluation

FIB cross-sectioning of a single rapidly solidified hypereutectic Al-Si powder particle for HRTEM.