Nanostructured Composite Powders Research Articles

Enhanced properties of hard metal WC-Ni processed from nanostructured powders

In this study, we investigated the effects of nanostructured composite powders (WC-Ni), nickel binder content, and sintering techniques on the morphology and mechanical properties of tungsten carbide. An alternative low-temperature gas-solid reaction route with a short reaction time was employed to produce nanostructured composite powders with 5 and 15 wt% Ni. The powders were consolidated using spark plasma sintering (SPS) and conventional vacuum furnace sintering at temperatures of 1350 °C and 1450 °C. The composite powders were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and particle size measurements. The expansion/contraction of the compacted nanocomposite powders was studied through dilatometric analysis. The composite powders exhibited a uniform distribution of Ni, in both compositions. For the WC-5 wt% Ni nanocomposite powders, the average crystallite sizes for WC and Ni were 41.6 nm and 46.2 nm, respectively. While, for the WC-15 wt% Ni nanocomposite, the crystallite sizes for WC and Ni were 45.6 and 38.4 nm. The sintered composites were evaluated through XRD, SEM, measurements of density and Vickers hardness. The enhanced sinterability of the nanostructured powders enabled their consolidation into (WC-Ni) hard metal with densities approaching the theoretical relative density of 96.07 %, even with low binder content (5 wt% Ni). This is attributed to the uniform distribution of Ni and an extensive Ni-WC interface present prior to sintering. The SPS process at 1350 °C produced sintered bodies (WC-5 wt% Ni and WC-15 wt% Ni) with higher Vickers hardness (2322 HV and 1512 HV, respectively) and superior microstructural quality compared to samples produced by conventional vacuum furnace techniques. These results demonstrated that the mechanical properties of the system WC-Ni processed by SPS are superior to those obtained by vacuum furnace. Therefore, WC-Ni hard metals processed by this approach is a promising alternative to conventional hard metals (WC-Co) for a wide range of applications.

Development of technology for obtaining nanostructured composite powders by high-speed mechanosynthesis

The results of the development of technology for obtaining nanostructured composite powders in two ways – surface reinforcement and cladding are presented. The possibility of manufacturing functional coatings based on the obtained powders by the method of supersonic cold gas-dynamic spraying is shown. The characteristics of the resulting coatings (adhesive strength, microhardness, wear resistance, low porosity) were studied. The real prospect of using the created functional protective coatings for precision engineering is shown.

Influence of Graphene Sheets on Compaction and Sintering Properties of Nano-Zirconia Ceramics.

The use of a nanostructured graphene-zirconia composite will allow the development of new materials with improved performance properties and a high functionality. This work covers a stepwise study related to the creation of a nanostructured composite based on ZrO2 and graphene. A composite was prepared using two suspensions: nano-zirconia obtained by sol-gel synthesis and oxygen-free graphene obtained sonochemically. The morphology of oxygen-free graphene sheets, phase composition and the morphology of a zirconia powder, and the morphology of the synthesized composite were studied. The effect of the graphene sheets on the rheological and sintering properties of a nanostructured zirconia-based composite powder has been studied. It has been found that graphene sheets in a hybrid nanostructure make it difficult to press at the elastic deformation stage, and the composite passes into the plastic region at a lower pressure than a single nano-zirconia. A sintering mechanism was proposed for a composite with a graphene content of 0.635 wt%, in which graphene is an important factor affecting the process mechanism. It has been determined that the activation energy of the composite sintering is more than two times higher than for a single nano-zirconia. Apparently, due to the van der Waals interaction, the graphene sheets partially stabilize the zirconia and prevent the disordering of the surface monolayers of its nanocrystals and premelting prior to the sintering. This leads to an increase in the activation energy of the composite sintering, and its sintering occurs, according to a mixed mechanism, in which the grain boundary diffusion predominates, in contrast to the single nano-zirconia sintering, which occurs through a viscous flow.

Nanostructured composite powders for producing protective coatings of machinebuilding parts and units

The paper studies a method for producing nanostructured composite powders of the metal – non-metal system. Powders preparation by universal disintegrator-activator technology, processing in planetary ball mills and bowl grinders is shown on the Al–Zn–Sn composition and titanium nitride nanoparticles. Judging by engineering-and-economical performance, the most promising method for obtaining nanostructured composite powders is the method of processing in a bowl grinder.

Tribological properties and corrosion behavior of AC-HVAF sprayed nanostructured NiCrCoAlY-TiB2 coatings

In this paper, agglomerated and dense nanostructured NiCrCoAlY-TiB2 composite powders prepared by spray drying and plasma spheroidization were used for activated combustion high velocity air fuel (AC-HVAF) spray. The effect of sprayed powders on the microstructure, tribological properties and corrosion behavior of the AC-HVAF sprayed coatings was systematically investigated. The present results demonstrate that plasma spheroidized powders are beneficial for dense microstructure and can effectively enhance coating's wear and corrosion resistance in turn. The coating sprayed by plasma spheroidized powders exhibits good wear resistance with the coefficient of friction of 0.48 ± 0.01 and wear rate of 1.6 ± 0.2 × 10−5 mm3 N−1 m−1. The good wear resistance is mainly due to the uniformly distributed nanostructured TiB2, which can work as the hard-skeleton support. The electrochemical polarization tests and electrochemical impedance spectroscopy (EIS) analysis show that the good corrosion resistance can be attributed to the TiO2 and B2O3 oxide films formed on the surface and dense microstructure of as-prepared coatings.

Study of the formation of nanostructured composite powders in a plasma jet

The preparation of a titanium composite powder containing high-melting TiN nanoparticles using an electric-arc plasma torch is studied. Using an Evo MA15 electron microscope (Carl Zeiss) and a DRON-4.0 diffractometer, both the surface morphology and the internal structure of powder particles are investigated, an X-ray phase analysis of these particles is performed, and a distribution map of the elements both over the surface of the obtained composite particle and over its volume is constructed. It is shown that the nanoparticles are uniformly distributed over the entire volume of titanium particles and are completely clad with Ti. The nanostructured powders obtained in this way and used as nanomodifying additives increase the homogeneity of the distribution of the high-melting elements introduced into the melt, protect the nanoparticles from the direct interaction with ambient medium, increase their wettability, and prolong their shelf life. These factors improve the efficiency of metal and alloy nanomodification.

Structure and Deformation Behavior of Ti-SiC Composites Made by Mechanical Alloying and Spark Plasma Sintering.

Combining high energy ball milling and spark plasma sintering is one of the most promising technologies in materials science. The mechanical alloying process enables the production of nanostructured composite powders that can be successfully spark plasma sintered in a very short time, while preserving the nanostructure and enhancing the mechanical properties of the composite. Composites with MAX phases are among the most promising materials. In this study, Ti/SiC composite powder was produced by high energy ball milling and then consolidated by spark plasma sintering. During both processes, Ti3SiC2, TiC and Ti5Si3 phases were formed. Scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction study showed that the phase composition of the spark plasma sintered composites consists mainly of Ti3SiC2 and a mixture of TiC and Ti5Si3 phases which have a different indentation size effect. The influence of the sintering temperature on the Ti-SiC composite structure and properties is defined. The effect of the Ti3SiC2 MAX phase grain growth was found at a sintering temperature of 1400–1450 °C. The indentation size effect at the nanoscale for Ti3SiC2, TiC+Ti5Si3 and SiC-Ti phases is analyzed on the basis of the strain gradient plasticity theory and the equation constants were defined.

Understanding structural evolution of nanostructured Cu-Al2O3 composite powders during thermomechanical processing

A better understanding of structural coarsening mechanisms is of significance to effectively tackle structural instabilities in nanostructured materials. In this work, the microstructural evolution of nanostructured metallic powder particles during thermomechanical consolidation has been investigated to gain new insights into the evolution of reinforcing particles and matrix grains of a mechanically alloyed nanostructured Cu-5 vol.% Al2O3 composite powder extruded at either 300 or 900 °C, respectively. Unusual coarsening of Al2O3 particles and rapid growth of Cu grains occurred during extrusion at a low temperature of 300 °C; meanwhile the formation of elongated micron Cu grains was observed at a high extrusion temperature of 900 °C. Both the applied shear stress/strain and the sizes and distribution of second-phase nanoparticles were found to determine the grain structure of the Cu matrix after extrusion. Specifically, the shear stress-induced dynamic and particle-stimulated recrystallization and the movement of the Al2O3 particles are respectively responsible for the low-temperature rapid growth of the Cu grains and the coarsening of the Al2O3 particles. The development of the elongated micron Cu grains during extrusion at 900 °C is associated with anisotropic grain boundary migration and particle re-distribution.

Design and Processing of Novel Ceramic Composite Structures for Use in Medical Surgery

. In order to fulfill the clinical requirements for strong, tough and stable ceramics for dental applications, we have designed and developed innovative Ceria-stabilized zirconia (Ce-TZP)-based composites. In particular, we have added two kinds of second phases to the Ce-TZP matrix: equiaxed a-Al2O3 grains, for increasing the hardness and the fracture strength, and elongated hexa-aluminates (both SrAl12O19 and CeMgAl11O19), to provide an additional toughening effect by crack deflection/bridging mechanisms. In order to carefully control the composition and the microstructure in those complex composite systems, we have used a novel surface-coating approach for the preparation of the nanostructured composite powders, which allows a perfect tailoring of the microstructural, morphological and compositional features of the composites. Once optimized the sintering cycle for each composite material, both composites reached full densification. Mechanical properties (Vickers hardness, flexural strength and fracture toughness) were evaluated, while the zirconia transformability was followed by means of an optical microscope during load-unload bending tests. The sensitivity to ageing was estimated by autoclave treatments. In spite of a remarkable different behavior – mainly in terms of stress-induced tetragonal to monoclinic zirconia transformability - both materials showed excellent mechanical properties as well as a negligible sensitivity to ageing, thus demonstrating their high potential for new reliable and safe devices for structural biomedical applications.

Mechanically Synthesized Composite Powder Based on AMg2 Alloy with Graphite Additives: Particle Size Distribution and Structural-Phase Composition

Nanostructured composite powders based on AlMg2 alloy with graphite additives (1–9 wt %) have been obtained by mechanochemical synthesis in a planetary ball mill. Particle size distribution and the specific surface area of powders are measured. X-ray diffraction, Raman spectroscopy, and transmission electron microscopy are used to study the structural and phase composition of the powders. It is established that the hardening of composite powders is caused by the reduction of grain size, as well as by mechanisms of solid-solution and dispersion hardening. These nanostructured composite powders can be used in the manufacture of products by forming techniques and additive technologies.

Investigation of possibility to fabricate Si3N4-TiN ceramic nanocomposite powder by azide SHS method

The process of self-propagating high-temperature synthesis with use of a powder of sodium azide NaN3 as a nitriding agent (the SHS-Az method) was applied to fabricate a nanocomposite powder Si3N4-TiN. Combustion of the initial mixtures of NaN3 only with precursors that are halides of silicon and titanium: Na2SiF6, (NH4)2SiF6, Na2TiF6, (NH4)2TiF6 did not allow us to synthesize the composite powder of Si3N4-TiN, as the phases of silicon nitride Si3N4 were not formed. After water washing, the ultrafine powdered product of combustion consisted of one target phase of titanium nitride only and a large amount of impurity of side phases. Replacement of the halide salt of one of the elements (Si or Ti) by the powder of this element in the initial mixture of SHS-Az system resulted in formation of silicon nitride together with the titanium nitride and impurities. The least amount of impurities was obtained by burning the initial mixtures of xSi + y(NH4)2TiF6 + zNaN3 system. Only the initial mixture of 9Si + (NH4)2TiF6 + 6NaN3 allowed us to obtain as a result of the SHS-Az process the nanostructured composite powder of Si3N4-TiN without impurities.

Formation of Nanostructured Ni-TaC and Fe-TaC Composite Powders via Mechanical Alloying

In this study, mixtures of 25 wt.% of tantalum carbide (TaC) and iron (Fe) or nickel (Ni) powders were mechanical alloyed against time in order to investigate the interactions between the reinforcement (harder TaC particles) and surrounding ductile metallic matrixes (Fe and Ni). After mechanical alloying, apparent densities and particle size and distribution (PSD) of the powders were measured and morphological observations were realized via scanning electron microscopy (SEM). X-ray diffraction (XRD) was applied for phase analysis and with increasing mechanical alloying time certain shifts were observed for the two theta values of the samples. These data used for characterization of strain rates and crystallite sizes by fundamentals parameters approach applied with Lorentzian function and Williamson-Hall plot analysis.

Effect of nanostructured composite powders on the structure and strength properties of the high-temperature inconel 718 alloy

The experimental results of the effect of powder nanomodifiers of refractory compounds on the strength properties, the macro- and microstructure of the high-temperature Inconel 718 alloy have been presented. It has been shown that the introduction of powder modifiers into the melt leads to a decrease in the average grain size by a factor of 1.5–2 in the alloy. The long-term tensile strength of the alloy at 650°C increases 1.5–2 times, and the number of cycles at 482°C before fracture grows by more than three times. The effect of nanoparticles on the grain structure and strength properties of the alloy is due to an increase in the number of generated crystallization centers and the formation of nanoparticle clusters of refractory compounds at boundaries and junctions in the formed grain structure, which hinder the development of recrystallization processes in the alloy.

Effect of reinforcement phase on the mechanical property of tungsten nanocomposite synthesized by spark plasma sintering

Nanostructured tungsten composites were fabricated by spark plasma sintering of nanostructured composite powders. The composite powders, which were synthesized by mechanical milling of tungsten and Ni-based alloy powders, are comprised of alternating layers of tungsten and metallic glass several hundred nanometers in size. The mechanical behavior of the nanostructured W composite is similar to pure tungsten, however, in contrast to monolithic pure tungsten, some macroscopic compressive plasticity accompanies the enhanced maximum strength up to 2.4GPa by introducing reinforcement. We have found that the mechanical properties of the composites strongly depend on the uniformity of the nano-grained tungsten matrix and reinforcement phase distribution.

Microstructure, Mechanical Properties, and Two-Body Abrasive Wear Behavior of Cold-Sprayed 20 vol.% Cubic BN-NiCrAl Nanocomposite Coating

20 vol.% cubic boron nitride (cBN) dispersoid reinforced NiCrAl matrix nanocomposite coating was prepared by cold spray using mechanically alloyed nanostructured composite powders. The as-sprayed nanocomposite coating was annealed at a temperature of 750 °C to enhance the inter-particle bonding. Microstructure of spray powders and coatings was characterized. Vickers microhardness of the coatings was measured. Two-body abrasive wear behavior of the coatings was examined on a pin-on-disk test. It was found that, in mechanically alloyed composite powders, nano-sized and submicro-sized cBN particles are uniformly distributed in nanocrystalline NiCrAl matrix. Dense coating was deposited by cold spray at a gas temperature of 650 °C with the same phases and grain size as those of the starting powder. Vickers hardness test yielded a hardness of 1063 HV for the as-sprayed 20 vol.% cBN-NiCrAl coating. After annealed at 750 °C for 5 h, unbonded inter-particle boundaries were partially healed and evident grain growth of nanocrystalline NiCrAl was avoided. Wear resistance of the as-sprayed 20 vol.% cBN-NiCrAl nanocomposite coating was comparable to the HVOF-sprayed WC-12Co coating. Annealing of the nanocomposite coating resulted in the improvement of wear resistance by a factor of ~33% owing to the enhanced inter-particle bonding. Main material removal mechanisms during the abrasive wear are also discussed.

Thermochemical synthesis of nanostructured Cu-Al2O3 composite powder

Synthesis of Cu-Al2O3 nanocomposite powder through a thermochemical method from the water solution of copper nitrate (Cu (NO3)2.3H2O) and aluminum nitrate (Al (NO3)6.9H2O) is studied in this research. X-ray diffraction (XRD) technique, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to characterize the synthesized powder. XRD results show that γ-Al2O3 phase begins to form at the temperature ≈800°C during the heat treatment process. Studying SEM micrographs proves that the nano sized Al2O3 particles are homogenously dispersed in the copper matrix. XRD results also show that disappearing the reflects of CuO peaks after performing a reduction chemical reaction at the temperatures above 800°C in hydrogen atmosphere indicates that such chemical reaction at the temperatures above 800°C is required in order to achieve Cu-Al2O3 nanocomposite powder.

The Influence of Temperature on Frictional Behavior of Plasma-Sprayed NiAl-Cr2O3 Based Self-Adaptive Nanocomposite Coatings

Frictional behavior of nano and hybrid-structured NiAl-Cr2O3-Ag-CNT-WS2 adaptive self-lubricant coatings was evaluated at a range of temperatures, from room temperature to 700 °C. For this purpose, hybrid structured (HS) and nanostructured (NS) composite powders with the same nominal compositions were prepared by spray drying and heat treatment techniques. A series of HS and NS coating samples were deposited on steel substrate by an atmospheric plasma spraying process. The tribological behavior of both coatings was studied from room temperature to 700 °C at 100° intervals using a custom designed high temperature wear test machine. Scanning electron microscopy was employed for the evaluation of the composite coatings and worn surfaces. Experimental results indicated that the hybrid coating had inferior tribological properties when compared to the nanostructured coating, showing the attractive frictional behavior on the basis of low friction and high wear resistance; the NS coating possessed a more stable friction coefficient in the temperature range of 25-700 °C against alumina counterface. Microstructural examinations revealed more uniformity in NS plasma-sprayed coatings.

Preparing of nanostructured Al2O3–TiO2–ZrO2 composite powders and plasma spraying nanostructured composite coating

Nanostructured Al2O3–TiO2–ZrO2 composite powders for plasma spraying were prepared by spray drying granulation technology. The effects of processing parameters on the microstructure and properties of composite powders were investigated. The results show that with increasing the slurry solid content, the particle size of powders increases, and the bulk density of powders increases, and the flowability of powders increases firstly and then decreases. With increasing the binder content, the particle size of powders increases, and the bulk density of powders increases, and the flowability of powders increases firstly and then decreases. With increasing the spray drying temperature, the particle size of powders increases, and the bulk density and flowability of powders increases firstly and then decreases. The most appropriate spray drying parameters are the slurry solid content of 40 wt.%, the binder content of 2.0 wt.% and the spray drying temperature of 250 °C. The nanostructured composite coating was successfully prepared by using the as-prepared nanostructured Al2O3–TiO2–ZrO2 composite powders as feedstocks. The nanostructured coating possessed higher hardness and toughness compared with the conventional microstructured one, which was attributed to the use of the nanostructured composite powders feedstocks.

Effect of Microstructure of Composite Powders on Microstructure and Properties of Microwave Sintered Alumina Matrix Ceramics

Two kinds of different structured alumina–titania composite powders were used to prepare alumina matrix ceramics by microwave sintering. One was powder mixture of alumina and titania at a micron–submicron level, in which fused-and-crushed alumina particles (micrometers) was clad with submicron-sized titania. The other was powder mixture of alumina and titania at nanometer–nanometer level, in which nano-sized alumina and nano-sized titania particles were homogeneously mixed by ball-milling and spray dried to prepare spherical alumina–titania composite powders. The effect of the microstructure of composite powders on microstructure and properties of microwave sintered alumina matrix ceramics were investigated. Nano-sized composite (NC) powder showed enhanced sintering behavior compared with micro-sized composite (MC) powders. The as-prepared NC ceramic had much denser, finer and more homogenous microstructure than MC ceramic. The mechanical properties of NC ceramic were significantly higher than that of MC ceramic, e.g. the flexural strength, Vickers hardness and fracture toughness of NC ceramic were 85.3%, 130.3% and 25.7% higher than that of MC ceramic, respectively. The improved mechanical properties of NC ceramic compared with that of MC ceramic were attributed to the enhanced densification and the finer and more homogeneous microstructure through the use of the nanostructured composite powders.

Preparation of cBNp/NiCrAl nanostructured composite powders by a step-fashion mechanical alloying process

When mechanical alloying process is employed to synthesize composite powders strengthened by particle dispersion, the powders tend to fracture into small segments, especially when high content of ceramic particles is added. In the present work, a step-fashion mechanical alloying (MA) method was developed to synthesize 40vol.% cBNp/NiCrAl composite powders with both cBNp uniform dispersion in the metal alloy matrix and desirable particle size distribution. The effect of MA time on the particle size, microstructure, grain size and microhardness of the composite powder was investigated. The morphology and cross-sectional microstructure evolution of composite powders during milling were characterized by scanning electron microscopy (SEM). The change of the grain size of the alloy matrix phase with milling time was estimated based on X-ray diffraction analysis (XRD). It was found that after 40h of milling the nanostructured composite powder with a mean particle size of ~22μm and a narrow distribution was obtained. Moreover, the SEM examination showed that cBN particles were uniformly distributed in the nanostructured NiCrAl matrix. Finally, the milling mechanism of the proposed step-fashion MA was discussed.