Properties and rapid low-temperature consolidation of nanocrystalline Fe-ZrO<Subscript>2</Subscript> composite by pulsed current activated sintering

Nanopowders of Fe and ZrO₂ were synthesized from Fe₂O₃ and Zr by high-energy ball milling. The powder sizes of Fe and ZrO₂ were 70 nm and 12 nm, respectively. Highly dense nanostructured 4/3Fe-ZrO₂ composite was consolidated by a pulsed current activated sintering method within 1 minute from the mechanically synthesized powders (Fe-ZrO₂) and horizontal milled Fe₂O₃+Zr powders under the 1 GPa pressure. The grain sizes of Fe and ZrO₂ in the composite were calculated. The average hardness and fracture toughness values of nanostuctured 4/3Fe-ZrO₂ composite were investigated.     

Synthesis and Characterization of Functionally Gradient Ni-ZrO<Subscript>2</Subscript> Composite Coating

For the first time, functionally ZrO₂ content graded Ni-ZrO₂ composite coating has been successfully co-electrodeposited from a bath with gradually increasing stirring rate. For this, different composite coatings were electroplated in the same bath with different stirring rates to find the optimum stirring rate in which the maximum content with uniform distribution of ZrO₂ particles in the coating can be achieved. To produce ZrO₂ content graded Ni-ZrO₂ composite coating, the stirring rate was continuously increased from 0 to optimum value. By increasing of ZrO₂ particles content, the microhardness increases from interface towards the surface of the coating. The results of wear resistance measurements, Electrochemical impedance spectroscopy and potentiodynamic polarization test revealed that wear and corrosion resistances of functionally graded Ni-ZrO₂ (FGNZ) is higher than that of ordinary Ni-ZrO₂ (ONZ) composite coating. This result has been attributed to lower mechanical mismatch between coating and substrate in the functionally graded composite coating with respect to the uniformly distributed one.     

Electrodeposition of sol-enhanced nanostructured Ni-TiO2 composite coatings

A novel electroplating method has been developed to produce nanocrystalline metal-matrix nano-structured composite coatings. A small amount of transparent TiO₂ sol was added into the traditional electroplating Ni solution, leading to the formation of nanocrystalline Ni-TiO₂ composite coatings. These coatings have a smooth surface. The Ni nodules changed from traditional pyramid-like shape to spherical shape. The grain size of Ni was also significantly reduced to the level of 50 nm. It was found that the amorphous anatase TiO₂ nano-particles (∼ 10 nm) were highly dispersed in the coating matrix. The microhardness was significantly increased from 320 HV₁₀₀ of the traditional Ni coating to 430 HV₁₀₀ of the novel composite coating with 3.26 wt.% TiO₂. Correspondingly, the wear resistance of the composite coating was improved by ∼ 50%.     

Densification and mechanical properties of fine-grained Al2O3-ZrO2 composites consolidated by spark plasma sintering

Alumina matrix composites containing 5 and 10 wt% of ZrO₂ were sintered under 100 MPa pressure by spark plasma sintering process. Alumina powder with an average particle size of 600 nm and yttria-stabilized zirconia with 16 at% of Y₂O₃ and with a particle size of 40 nm were used as starting materials. The influence of ZrO₂ content and sintering temperature on microstructures and mechanical properties of the composites were investigated. All samples could be fully densified at a temperature lower than 1400 °C. The microstructure analysis indicated that the alumina grains had no significant growth (alumina size controlled in submicron level 0.66-0.79 μm), indicating that the zirconia particles provided a hindering effect on the grain growth of alumina. Vickers hardness and fracture toughness of composites increased with increasing ZrO₂ content, and the samples containing 10 wt% of ZrO₂ had the highest Vickers hardness of 18 GPa (5 kg load) and fracture toughness of 5.1 MPa m^1/2.     

Microstructure and sliding wear behavior of nanostructured Ni60-TiB2 composite coating sprayed by HVOF technique

A nanostructured Ni60-TiB₂ composite coating (Ni60 is a brand of Ni-based self-fluxing alloy with a hardness of HRC60) was sprayed on steel substrate by high velocity oxy-fuel (HVOF) process using high energy ball milled powders. Its sliding wear resistance at room-temperature was evaluated by ball-on-disc testing. For comparison, conventional Ni60-TiB₂ composite coating was prepared by HVOF using mechanically mixed Ni60 and TiB₂ powders and tested under the same conditions. The results show that the nanostructured composite coating has excellent mechanical properties and sliding wear resistance due to the microstructural homogenization and the well preserved nanostructure characteristic of the ball milled powders. Adhesive and abrasive wears are found to be responsible for the wear down mechanisms of the nanostructured Ni60-TiB₂ composite coating.     

Nanocarbon-dependent synthesis of ZrB2 in a binary ZrO2 and boron system

Thermodynamically, ZrO₂ may react with boron to form B₂O₃/B₂O₂ and ZrB₂ at room temperature. However, this reaction is incomplete at temperatures lower than 1550 °C, even with the use of metastable reactants, i.e., as-synthesized amorphous hydrous nano-ZrO₂ and amorphous boron powders. In this study, a complete disintegration of ZrO₂ was achieved by introducing nanocarbon to the binary system of ZrO₂ and boron at 1550 °C. The metastable reactants affected the temperature required for the solid-state reactions and also strongly affected the kinetics of the transformation. Single crystal and plate-like ZrB₂ particles with a uniform distribution and a size of ca. 1.0 μm in two-dimensions were obtained using 5 wt.% nanocarbon and a B/Zr molar ratio of 4.     

Effects of sintering additives on microstructure and mechanical properties of TiB2-WC ceramic-metal composite tool materials

TiB₂-WC ceramic-metal composite tool materials were fabricated using Co, Ni and (Ni, Mo) as sintering additives by vacuum hot-pressing technique. The microstructure and mechanical properties of the composite were investigated. The composite was analyzed by the observations of scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometry (EDS). The microstructure of TiB₂-WC ceramic-metal composites consisted of the fine WC grains and uniform TiB₂ grains. The brittle phase of Ni₃B₄ and a few pores were found in TiB₂-WC-Ni ceramic-metal composite. A lot of pores and brittle phases such as W₂CoB₂ and Co₂B were formed in TiB₂-WC-Co ceramic-metal composite. The liquid phase of Co was consumed by the reaction which led to the formation of the pores and the coarse grains of TiB₂. The pores, brittle phases and coarse grains of TiB₂ were harmful to the improvement of the mechanical properties of the composite. The sintering additive of (Ni, Mo) had a significant effect on the density and the mechanical properties of TiB₂-WC ceramic-metal composite. The formation of intermetallic compound of MoNi₄ inhibited the consumption of liquid phase of (Ni, Mo). The liquid phase of (Ni, Mo) not only inhibited the formation of the pores and the coarse grains of TiB₂ but also strengthened the interface energy between WC and TiB₂ grains. The grain size was fine and the average relative density of TiB₂-WC-(Ni, Mo) ceramic-metal composite reached 99.1%. The flexural strength, fracture toughness and Vickers hardness of TiB₂-WC-(Ni, Mo) ceramic-metal composite were 1307.0 ± 121.4 MPa, 8.19 ± 0.29 MPa m^1/2 and 22.71 ± 0.82 GPa, respectively.     

Synthesis and crystallization behavior of 3 mol% yttria stabilized tetragonal zirconia polycrystals (3Y-TZP) nanosized powders prepared using a simple co-precipitation process

The synthesis and crystallization behavior of 3 mol% yttria stabilized tetragonal zirconia polycrystals (3Y-TZP) nanopowders prepared using a simple co-precipitation process at 348 K and pH = 7 were investigated using differential scanning calorimetry/thermogravimetry (DSC/TG), an X-ray diffractometer (XRD), the Raman spectra, transmission electron microscopy (TEM), selected area electron diffraction (SAED), and an energy dispersive spectrometer (EDS). The activation energy of tetragonal ZrO₂ crystallization from 3Y-TZP freeze-dried precursor powders using a non-isothermal method, namely, 169.2 ± 21.9 kJ mol⁻¹, was obtained. The growth morphology parameter n was approximated as 2.0, which indicated that it had a plate-like morphology. The XRD, Raman spectra, and SAED patterns showed that the phase of the tetragonal ZrO₂ was maintained at 1273 K. The crystallite size of 3Y-TZP freeze-dried precursor powders calcined at 1273 K for 5 min was 21.3 nm.     

Microstructure and magnetic properties of HVOF thermally sprayed Fe75Si15B10 coatings

Partially amorphous Fe₇₅Si₁₅B₁₀ coatings were prepared from nanostructured feedstock powders by using high velocity oxy-fuel spraying. Scanning electron microscopy, X-ray diffraction, Vickers indenter and magnetic measurements were used to investigate microstructural, structural, microhardness and magnetic properties of the coatings. The Rietveld refinement of the X-ray diffraction patterns reveals the presence of an amorphous phase, nanocrystalline α-Fe(Si,B) structure having a lattice parameter close to 0.2841 nm and an average crystallite size of about 78-83 nm in addition to small amounts of Fe₃O₄ oxide (104 nm) and Fe₂B boride (151 nm), which disappear completely with increasing coating thickness. Coercivity and microhardness values are 15.5 Oe and 478 Hv, respectively, for 84 μm thickness.     

Spark plasma sintering consolidation of nanostructured TiC prepared by mechanical alloying

Spark plasma sintering technique was used for the consolidation of nanostructured titanium carbide synthesized by mechanical alloying in order to avoid any important grain growth of the compact materials. The TiC phase was obtained after about 2 h of mechanical alloying. Towards the end of the milling process (20 h), the nanocrystalline powders reached a critical size value of less than 5 nm. Some physical and mechanical properties of the consolidated carbide were reported as a function of the starting grain size powders obtained after different mechanical alloying durations. The crystalline grain size of the bulk samples was found to be increased to a maximum of 120 nm and 91 nm for carbides mechanically alloyed for 2 h and 20 h respectively. The Vickers hardness showed to be improved to about 2700 Hv for a maximum density of 95.1% of the bulk material.     

Synthesis, vacuum sintering and dielectric characterization of zirconia (t-ZrO2) nanopowder

Phase pure zirconium oxide powders have been synthesized using the single step auto-ignition combustion method, the particles were nanometer sized (20 nm) and the size distribution was very narrow (3.4 nm). Systematic structural characterization revealed the t-ZrO₂ and indexed for its tetragonal structure (a = 3.5975 Å and c = 5.1649 Å). Calculated microstrain in most of the plane indicated the presence of compressive stress (65-288 MPa) along various planes of the particles. Observed space group (P4₂/nmc) revealed the presence of cations in the 8e positions (0.75, 0.25, 0.75) and the anions in the 16 h positions (0.25, 0.25, 0.4534). The metal-oxide (Zr-O) band observed at the low wavenumber region further confirmed the phase purity of the as-prepared ZrO₂ nanopowders. Peaks at the binding energy positions 2.042 and 0.525 keV in the energy dispersive X-ray spectrum revealed oxygen deficient zirconia. The particle size estimated by TEM was in good agreement with the results obtained through X-ray line broadening (20.81 nm) measurements. The nanopowders were sintered to above 98% of the theoretical density by using vacuum sintering technique at a relatively low temperature of 1300 °C. Stable tetragonal ZrO₂ experimentally yield the permittivity value of about 28 at 10 MHz.     

Nanostructured MgFe2O4 as anode materials for lithium-ion batteries

Transition metal oxides in the nano size region are enormous attention as a new generation of anode materials for high energy density Li-ion batteries. MgFe₂O₄ is used for the first time as active electrode vs. lithium metal in test cells. The research has been focused on the effect of grain size of MgFe₂O₄ and their electrochemical performance studied. In this studies, nanostructured milled MgFe₂O₄ (grain size 19 nm) sample have been compared with relatively large-sized as-prepared sample (grain size 72 nm). From the result, the 19 nm grain size sample delivered an improved discharge capacity of around 850 mAh/g, whereas it is only 630 mAh/g for as-prepared sample (72 nm). These values are two times higher than that of a carbon anode (372 mAh/g). The anomalous capacity may be associated with the formation of oxygen rich MgFe₂O₄ samples.     

In situ synthesis of nanocrystalline intermetallic layer during surface plastic deformation of zirconium

By means of a surface plastic deformation method a nanocrystalline (NC) intermetallic compound was in situ synthesized on the surface layer of bulk zirconium (Zr). Hardened steel shots (composition: 1.0C, 1.5Cr, base Fe in wt.%) were used to conduct repetitive and multidirectional peening on the surface layer of Zr. The microstructure evolution of the surface layer was investigated by X-ray diffraction and scanning and transmission electron microscopy observations. The NC intermetallic layer of about 25 μm thick was observed and confirmed by concentration profiles of Zr, Fe and Cr, and was found to consist of the Fe_100 − xCr_x compound with an average grain size of 22 nm. The NC surface layer exhibited an extremely high average hardness of 10.2 GPa. The Zr base immediately next to the compound/Zr interface has a grain size of ∼ 250 nm, and a hardness of ∼ 3.4 GPa. The Fe_100 − xCr_x layer was found to securely adhere to the Zr base.     

Solid-state synthesis and spark plasma sintering of SrZrO3 ceramics

SrZrO₃ powders are obtained by solid state reaction from SrCO₃ and ZrO₂ precursors, without involving intermediate calcination and grinding steps. The resulted powders are essentially within a single phase, with sub-micron average crystallite size. Pellets of these powders show a relatively poor sintering behavior, when fired up to 1600 °C. Alternatively, spark plasma sintering technique is used in order to obtain nearly 100% dense samples at the expense of excessive grain coarsening (i.e., up to 5 μm in diameter). Crystalline structure, composition and morphology of the specimens obtained in this work are investigated by X-ray diffraction, scanning and transmission electron microscopy together with energy dispersive X-ray spectroscopy.     

Development of multimaterial coatings by cold spray and gas detonation spraying

The basic objective is the development of multifunctional multimaterial protective coatings using cold spraying (CS) and computer controlled detonation spraying (CCDS).As far as CS is concerned, the separate injection of each powder into different zones of the carrier gas stream is applied. Cu-Al, Cu-SiC, Al-Al₂O₃, Cu-Al₂O₃, Al-SiC, Al-Ti and Ti-SiC coatings are successfully sprayed. As to CCDS, powders are sprayed with a recently developed apparatus that is characterized by a high-precision gas supply system and a fine-dosed twin powder feeding system. Computer control provides a flexible programmed readjustment of the detonation gases energy impact on powder thus allowing selecting the optimal for each component spraying parameters to form composite and multilayered coatings. Several powders are sprayed to obtain composite coatings, specifically, among others, WC-Co-Cr + Al₂O₃, Cu + Al₂O₃, and Al₂O₃ + ZrO₂.     

Characteristics of BaO-B2O3-SiO2 nano glass powders prepared by flame spray pyrolysis as the sintering agent of BaTiO3 ceramics

Nanosized BaO-B₂O₃-SiO₂ glass powders are directly prepared by flame spray pyrolysis. The mean size of the BaO-B₂O₃-SiO₂ glass powders with amorphous phase and spherical shape is 30 nm. The effects of glass powders on the sintering characteristics of the BaTiO₃ pellet formed from the nanosized BaTiO₃ powders are investigated. The mean size and BET surface area of the BaTiO₃ powders prepared by spray pyrolysis are 110 nm and 9.1 m²/g. The BaTiO₃ pellet with glass additive has large grain size with several microns, dense structure and pure tetragonal crystal structure at a sintering temperature of 1000 °C. The XRD pattern of the pellet has distinct split of (2 0 0) and (0 0 2) peaks at 2θ ≈ 44.95°. The dielectric constant of the pellet without glass additive is 2180. However, the dielectric constants of the pellets with 1, 3, 5 and 7 wt% glass additive with respect to BaTiO₃ are 2496, 2514, 2700 and 2225, respectively.     

Preparation of Mo(Si, Al)2–ZrO2 nanocomposite powders by mechanical alloying

Phase transformations and the final formation of Mo(Si, Al)₂–ZrO₂ nanocomposite during high-energy ball milling of a series of Mo–Si–Al–ZrO₂ powders were investigated. Mechanical alloying led to phase transformations from the initial Mo–Si–Al powders mixture to Mo_ss (2 h) → C40 Mo(Si, Al)₂ (4, 8 h) → Mo_ss (12 h) phases. The phase transformations studied by XRD are discussed considering the alloying and second phase effects. Finally, the Mo_ss matrix reinforced with ZrO₂ particles nanocomposite structure was studied by means of TEM. The Mo_ss matrix phase formed was revealed to be strongly inhomogeneous even after 12 h of mechanical alloying and Mo-, Si- and Al-enriched regions were observed. The ZrO₂ nanostructured phase, evenly distributed in the Mo_ss matrix, had grain size of about 5–20 nm.     

Chemical vapor deposition of C on SiO2 and subsequent carbothermal reduction for the synthesis of nanocrystalline SiC particles/whiskers

This study investigates chemical vapor deposition of C from CH₄ on particulate SiO₂ and subsequent carbothermal conversion of the resultant composite particles to SiC powders. Mass measurements, HR-TEM, SEM and XRD were used to characterize the products at various stages of the processes. It was found that oxide particles gained mass rapidly at 1300 K under CH₄ atmosphere owing to enhanced C uptake. Pyrolytic carbon layers 5-8 nm thick were deposited on SiO₂ particles. The coated powders with high C loadings (40-42.6 wt.% C) were converted to SiC under Ar flow in a temperature range of 1700-1800 K. Almost pure SiC powders containing a mixture of particles and whiskers of ~ 100 nm were synthesized at 1750 K for 45 min and at 1800 K for 30 min using the starting powder with 40 wt.% C. Whisker diameter increased with the C content of the coated powder. It was proposed that SiC whisker was grown by a vapor-solid mechanism. Equilibrium thermodynamic analysis by the method of minimization of Gibbs’ free energy predicted the reaction pathways to SiC and to the product species in the Si-O-C-Ar system. This study demonstrated that either C shell-SiO₂ core powders or SiC powders could be synthesized rapidly in the same reactor.     

Fabrication of Nd:YAG transparent ceramics with both TEOS and MgO additives

Neodymium doped YAG transparent ceramics were fabricated by vacuum reactive sintering method using commercial α-Al₂O₃, Y₂O₃ and Nd₂O₃ powders as the starting materials with both tetraethyl orthosilicate (TEOS) and MgO as sintering aids. The morphologies and microstructure of the powders and Nd:YAG transparent ceramics were investigated. Fully dense Nd:YAG ceramics with average grain size of ∼10 μm were obtained by vacuum sintering at 1780 °C for 8 h. No pores and grain-boundary phases were observed. The in-line transmittance of the ceramic was 83.8% at 1064 nm.     

Microstructure, kinetic analysis and hardness of Sn-Ag-Cu-1 wt% nano-ZrO2 composite solder on OSP-Cu pads

Nano-sized, nonreacting, noncoarsening ZrO₂ particle-reinforced Sn-Ag-Cu composite solders were prepared by mechanically dispersing ZrO₂ nano-particles into Sn-Ag-Cu solder and the interfacial morphology between the solder and organic solderability preservative (OSP)-Cu pads were characterized metallographically. At their interfaces, island-shaped Cu₆Sn₅ and Cu₃Sn intermetallic compound (IMC) layers were found in solder joints with and without the ZrO₂ particles and the IMC layer thickness was substantially increased with reaction time and temperature. In the solder ball region, needle-shaped Ag₃Sn and spherically-shaped Cu₆Sn₅ IMC particles were found to be uniformly distributed in the β-Sn matrix. However, after the addition of ZrO₂ nano-particles, Ag₃Sn and Cu₆Sn₅ IMC particles appeared with a fine microstructure and retarded the growth rate of the IMC layers at their interfaces. From a kinetic analysis, the calculated activation energies for the total (Cu₆Sn₅ + Cu₃Sn) IMC layers for Sn-Ag-Cu and Sn-Ag-Cu-1 wt% ZrO₂ composite solder joints on OSP-Cu pads were about 53.2 and 59.5 kJ/mol, respectively. In addition, solder joints containing ZrO₂ nano-particles displayed higher hardness due to the uniform distribution of ZrO₂ nano-particles as well as the refined IMC particles. The hardness values of the plain Sn-Ag-Cu solder joint and solder joints containing 1 wt% of ZrO₂ nano-particles after 5 min reaction at 250 °C were about 15.0 Hv and 17.1 Hv, respectively. On the other hand, their hardness values after 30 min reaction were about 13.7 Hv and 15.5 Hv, respectively.