Synthesis of chromium carbide (Cr3C2) nanopowders by the carbonization of the precursor

The solution-derived precursor method was used to synthesize chromium carbide (Cr₃C₂) nanopowders, ammonium dichromate ((NH₄)₂Cr₂O₇) and nanometer carbon black were used as raw materials. The products were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that the single phase Cr₃C₂ can be synthesized under the conditions of 21 wt.% C, 1100 °C and 30 min, and the average crystallite size is 27.2 nm. The powders show good dispersion and are mainly composed of spherical or near- spherical particles with a mean diameter of ~ 30 nm. The surface of the specimen mainly consists of Cr, C and O three species elements. The XPS spectrum of Cr2p consists of two peaks with the binding energies of 577.5 eV and 575.3 eV, which are assigned to the Cr2p_3/2 species of Cr₂O₃ and Cr₃C_2 − x (0 ≤ x ≤ 0.5), respectively. The XPS spectrum of O1s energy region for chromium carbide contains three peaks (Oa, Oh and Od), which are considered to be due to O⁻, OH⁻ and Cr₂O₃, respectively.     

Producing nanocrystalline bulk Mg-3Al-Zn alloy by powder metallurgy assisted hydriding-dehydriding

Nanocrystalline bulk Mg-3Al-Zn alloy with an average grain size of 48 nm has been prepared by powder metallurgy assisted hydriding-dehydriding. Evolutions of nanograined structure powders and bulk alloy have been investigated by TEM, SEM and XRD, respectively. The results showed that by milling in hydrogen for 60 h, as-hydriding powder possessed an average grain size of 5.9 nm. After a subsequent process of desorption-recombination treatment (at 350 °C) and consolidation process (extruded at 200 °C) resulted in bulk samples with an average crystallite size of 48 nm and MgH₂ was fully turned into Mg. The consolidated samples of 60 h milled powder had a final density of 1.77663 ± 0.006 g/cm³, which corresponded to 97.57 ± 0.3% of theoretical density. The highest microhardness of the nanocrystalline bulk alloy reached about 872.5 MPa, which is about three times higher than that of the coarse-grained AZ31.     

VC, Cr3C2 doped ultrafine WC-Co cemented carbides prepared by spark plasma sintering

The influence of the addition of 0.3, 0.5 and 0.7 wt.% VC on the density, microstructure and mechanical properties of WC-Cr₃C₂-11 wt.% Co with 0, 0.2, 0.4 and 0.6 wt.% Cr₃C₂ hard metals prepared by spark plasma sintering (SPS) at a temperature of 1200 °C (5 min, 40 MPa) was investigated. Microstructure analysis revealed that the WC grain size in the sintered hard metals was strongly influenced by the VC and Cr₃C₂ content. With the addition of inhibitors and the increased amount of Cr₃C₂, the density is reduced, and on the contrary, the addition of VC as an inhibitor contributes to promoting the densification. The combined addition of Cr₃C₂ and VC could strongly reduce the WC grain growth to about 350 nm. Observation suggests that the fracture of WC-Co cemented carbide is brittle and intergranular. The amount of added VC/Cr₃C₂ should be controlled in a certain range. Samples with an appropriate proportion of VC/Cr₃C₂ added exhibit higher hardness which can be up to 1938 HV₃₀. Toughness, too, can reach 16.34 MPa m^1/2.     

Influence of consolidation process and sintering temperature on microstructure and mechanical properties of near nano- and nano-structured WC-Co cemented carbides

In this paper the influence of the consolidation process and sintering temperature on the properties of near nano- and nano-structured cemented carbides was researched. Samples were consolidated from a WC 9-Co mixture by two different powder metallurgy processes; conventional sintering in hydrogen and the sinter-HIP process. Two WC powders with different grain growth inhibitors were selected for the research. Both WC powders used were near nanoscaled and had a grain size of 150 nm and a specific surface area of 2.5 m²/g. Special emphasis was placed on microstructure and mechanical properties; hardness and fracture toughness of sintered samples. Consolidated samples are characterised by different microstructural and mechanical properties with respect to the sintering temperature, the consolidation process used and grain growth inhibitors in starting powders. Increasing sintering temperature leads to microstructure irregularities and inferior hardness, especially for samples sintered in hydrogen. The addition of Cr₃C₂ in the starting powder reduced a carbide grain growth during sintering, improved microstructural characteristics, increased Vickers hardness and fracture toughness. The relationship between hardness and fracture toughness is not linear. Palmqvist toughness does not change with regard to sintering temperature or the change of Vickers hardness.     

Fabrication of full density near-nanostructured cemented carbides by combination of VC/Cr3C2 addition and consolidation by SPS and HIP technologies

The aim of present work is to study the effect of VC and/or Cr₃C₂ in densification, microstructural development and mechanical behavior of nanocrystalline WC-12wt.%Co powders when they are sintered by spark plasma sintering (SPS) and hot isostatic pressing (HIP). The results were compared to those corresponding to conventional sintering in vacuum. The density, microstructure, X-ray diffraction, hardness and fracture toughness of the sintered materials were evaluated. Materials prepared by SPS exhibits full densification at lower temperature (1100 °C) and a shorter stay time (5 min), allowing the grain growth control. However, the effect of the inhibitors during SPS process is considerably lower than in conventional sintering. Materials prepared by HIP at 1100 °C and 30 min present full densification and a better control of microstructure in the presence of VC. The added amount of VC allows obtaining homogeneous microstructures with an average grain size of 120 nm. The hardness and fracture toughness values obtained were about 2100 HV₃₀ and close to 10 MPa m^1/2, respectively.     

Properties and fast low-temperature consolidation of nanocrystalline Ni-ZrO2 composites by high-frequency induction heated sintering

Nanopowders of Ni and ZrO₂ (11 nm and 90 nm, respectively) were synthesized from NiO and Zr by high energy ball milling. A highly dense nanostructured 2Ni-ZrO₂ composite was consolidated at low temperature by high-frequency induction heat sintering within 2 min of the mechanical synthesis of the powders (Ni-ZrO₂) with horizontal milled NiO + Zr powders under 500 MPa pressure. This process allows very quick densification to near theoretical density and prohibits grain growth in nano-structured materials. The grain sizes of Ni and ZrO₂ in the composite were calculated. Finally, the average hardness and fracture toughness values of nanostructured 2Ni-ZrO₂ composites were investigated.     

Properties of praseodymium-doped bismuth potassium titanate (Bi0.5K0.5TiO3) synthesised using the soft combustion technique

Bismuth potassium titanate (Bi_0.5K_0.5TiO₃; BKT) and praseodymium-doped BKT (Bi_0.5(1−x)Pr_xK_0.5TiO₃; BPKT) powders were synthesised using the soft combustion technique. Fine particles of 10-100 nm of BKT and BPKT were produced. A single phase BKT was obtained with a minimum of 0.5 mol of glycine. Various compounds of Bi_0.5(1−x)Pr_xK_0.5TiO₃ where x = 0.01, 0.03, 0.05, 0.10, 0.15 and 0.20 were prepared. Pure BKT and BPKT powders were obtained after calcination at 800 °C for 3 h. After sintering at 1050 °C for 5 h, pure BKT and BPKT pellets were obtained for x = 0 and 0.01. However, for BPKT with x = 0.03, 0.05, 0.10, 0.15 and 0.20, a minor amount of Bi₄Ti₃O₁₂ (BIT) secondary phase was present after sintering at 1050 °C for 5 h. The crystallite size and grain size of all the samples followed similar trends, first increasing from x = 0 (undoped BKT) to x = 0.05 and then decreasing above x = 0.05. Among the undoped and doped samples, BPKT with x = 0.05 had the highest dielectric properties (?_r = 713.87) due to its large crystallite size (68.66 nm), large grain size (∼435 nm) and high relative density (93.39%).     

Role of Cr ions on the microstructure development, and magnetic phase evolution of Ni0.7Zn0.3Fe2O4 ferrite nanoparticles

A series of ferrite samples with the chemical formula Ni_0.7Zn_0.3Cr_xFe_2−xO₄ (x = 0.0-0.5) were prepared by a sol-gel auto-combustion method and annealed at 600 °C for 4 h. The resultant powders were investigated by various techniques, including X-ray diffractometry (XRD), vibrating sample magnetometry (VSM), and permeability studies. The prepared samples have a cubic spinel structure with no impurity phase. As the Cr³⁺ content x increases, bulk density and crystallite size decrease, whereas porosity increases. The saturation magnetization decreases linearly from 58.31 to 42.90 emu/g with increasing Cr³⁺ content. However, coercivity increases with increasing Cr³⁺ substitution. The magnetic moments calculated from Neel's molecular-field model are in agreement in the experiment results. The initial permeability (μ_i) decreases with increasing Cr³⁺ substitution. The decrease in initial permeability (μ_i) is attributed to decrease in magnetization on addition of Cr³⁺. The real part of the permeability decreases gradually with increasing frequency in accordance with Snoek's law. The Curie temperature decreases linearly with increasing Cr³⁺ content.     

Effect of MgO particle size on the microstructure, mechanical properties and wear performance of ZTA-MgO ceramic cutting inserts

The mechanical properties, microstructure and wear performance of zirconia-toughened alumina (ZTA) cutting inserts with Magnesia (MgO) in different particle sizes as additives was investigated. The MgO particle sizes were varied from 80 nm to 7000 nm. The alumina (Al₂O₃), yittria stabilized zirconia (YSZ) and MgO powders were mixed, compacted and sintered at 1600 °C using the solid-state sintering method. The mechanical and physical properties of the samples such as wear resistance, Vickers hardness, fracture toughness, microstructure and density were analyzed. Commercially available stainless steel (316L) was used as the workpiece for the wear resistance study. It was observed that smaller MgO particle sizes induce better wear performance and mechanical properties for the cutting inserts. Wear resistance analysis showed that the cutting insert with nano-sized MgO (particle size 80 nm) had the lowest wear area of 0.019 mm². The same cutting insert also possessed the highest Vickers hardness value of 1740 Hv compared to the other samples. Furthermore, microstructural observations show that the Al₂O₃ grain size depends on the particle size of MgO, and is directly related to its hardness property.     

Crystallization and magnetic behavior of nanosized nickel ferrite prepared by citrate precursor method

NiFe₂O₄ nanoparticles have been synthesized by citrate precursor gel formation with subsequent heat treatment. Differential thermal and thermogravimetric (DTA/TG) analyses show that the metal citrates decomposed around 230 °C followed by crystallization of the ferrite. X-ray diffraction (XRD) patterns reveal the formation of the cubic spinel phase in the samples after sintering the gel at 350 °C, 500 °C and 700 °C. For the samples annealed at 350 °C and 500 °C a small amount of α-Fe₂O₃ was detected whereas single phase was obtained for the sample annealed at 700 °C. The lattice constant a for all the samples is comparable to the value of the bulk material. The mean crystallite size D_XRD of the samples determined from XRD line broadening is 26.2-28.5 nm. Transmission electron microscope (TEM) analysis shows that the single-phase particles form clusters with the particle size in the range of 21-82.5 nm and the most probable value D_TEM of 55.4 nm. Magnetic measurements show that its Curie temperature T_C is close to the bulk value while the spontaneous magnetization M_s at 5 K is lower than that of the bulk. The thermal variation of M_s in the temperature range from 5 to 300 K can be best fitted to a modified Bloch T^α law with the exponent value α ≈ 2. The magnetization data are explained with reference to the disordered surface spins and the finite size effects. In this investigated temperature range, the coercive force H_c decreases linearly with increasing temperature. The coercivity mechanism in the nanoparticle sample with broad particle size distribution is expected to be complex and different factors which affect the H_c value were proposed.     

Effect of Cr7C3 on the mechanical, thermal, and electrical properties of Cr2AlC

In this work, phase pure Cr₂AlC and impure Cr₂AlC with Cr₇C₃ have been fabricated to investigate the mechanical, thermal, and electrical properties. The thermal expansion coefficient is determined as 1.25 × 10⁻⁵ K⁻¹ in the temperature range of 25-1200 °C. The thermal conductivity of the Cr₂AlC is 15.73 W/m K when it is measured at 200 °C. With increasing temperature from 25 °C to 900 °C, the electrical conductivity of Cr₂AlC decreases from 1.8 × 10⁶ Ω⁻¹ m⁻¹ to 5.6 × 10⁵ Ω⁻¹ m⁻¹. For the impure phase of Cr₇C₃, it has a strengthening and embrittlement effect on the bulk Cr₂AlC. And the Cr₂AlC with Cr₇C₃ would result in a lower high-temperature thermal expansion coefficient, thermal conductivity, specific heat capacity and electrical conductivity.     

Synthesis of chromium carbide nanopowders via a microwave heating method

Chromium carbide nanopowders were firstly synthesized via a simple microwave heating technique using nanometer chromic oxide (Cr₂O₃) and nanometer carbon black as raw materials in argon gas atmosphere. The samples were characterized by X-ray diffractometry (XRD), thermogravimetric and differential scanning calorimetry (TG–DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The results show that chromium carbide nanopowders with an average crystallite size of 24 nm can be synthesized at 1000 °C for 1 h. The synthesis temperature required by the method is 400 °C lower than those required by the conventional approaches for preparing chromium carbide. SEM and TEM results show that the powders show good dispersion and are mainly composed of spherical or nearly spherical particles with a mean diameter of about 30 nm. The phase transformation sequence during the heat treatment is: Cr₂O₃ → CrO → Cr₇C₃ → Cr₂C → Cr₃C₂.     

Influence of the thermo-chemical conditions on the carbo-chromization process of α-Fe matrices obtained by powder sintering technique

We report on the superficial layer formation resulting from the carburization followed by chromization of α-Fe samples obtained by powder sintering technique. The carburization and chromization were carried out by thermal diffusion between 880-980 °C and 950-1050 °C in a solid powder mixture of charcoal/BaCO₃ and ferrochromium/alumina/NH₄Cl, respectively. The obtained layers were investigated using X-ray diffraction, optical microscopy, Vickers micro-hardness technique and scanning electron microscopy. The results show that the layers are of micrometric size and consist mostly of chromium carbides of different phases. These phases as well as the thickness of the layers are closely related to the treatment temperature used for carburization and to the temperature and Cr initial concentration in the mixture used for chromization. For highly reactive carbo-chromization conditions (high concentration of Cr, and high carburization and chromization temperatures) the superficial layer is constituted of two chromium carbide sub-layers (Cr₃C₂/Cr₇C₃) separated by a sharp interface. The thickness and hardness of the coating layer reached 45 μm and 2300 HV, respectively. Such coating could be used for tools that have to be abrasion and oxygen resistant at high temperatures.     

Effects of mechanical activation of MoO3/C powder mixture in the processing of nano-crystalline molybdenum

Nano-crystalline molybdenum with a mean crystallite size of 45 nm was synthesized through reduction of MoO₃ by carbon using high energy ball milling and subsequent heat treatment. XRD, DTA/TG and SEM techniques were employed to evaluate the powder particle characteristics. It was found that initial mechanical activation at an ambient temperature did not result in the reduction of MoO₃ to molybdenum. DTA/TG results showed that in a sample milled for 25 h, MoO₂ and Mo₂C were formed at 485 and 925 °C, respectively during carbo-thermal reduction of MoO₃. At the final stage, Mo₂C reacts with the remaining MoO₂ at 1030 °C to produce nano-crystalline molybdenum.     

Synthesis of (Bi0.5Na0.5)TiO3 (BNT) and Pr doped BNT using the soft combustion technique and its properties

In this work, bismuth sodium titanate (Bi_0.5Na_0.5)TiO₃ (BNT) and praseodymium (Pr)-doped BNT were successfully produced using the soft combustion technique. The effects of Pr doping on stoichiometry, microstructure, density and dielectric properties were studied. Pure Pr-doped BNT was obtained in all samples containing 5, 10 and 20 mol% Pr after calcination at 800 °C for 3 h. The produced powders were then pressed into pellets and sintered at 1100 °C for 3 h. The very similar ionic radii of Pr³⁺ with Bi³⁺ and Na⁺ made it possible to substitute both Bi and Na. The crystallite size and grain size decreased with increasing Pr amount because Pr acted as grain growth inhibitor, both for calcined powders and for sintered pellets. Maximum density was obtained in 5 mol% Pr-doped BNT, beyond which density decreased. The maximum dielectric constant of 756 was obtained in 5 mol% Pr-doped BNT and decreased at higher levels of Pr doping. Pr doped into BNT also caused a decrease in dielectric loss.     

Fabrication and mechanical properties of ultrafine grained WC-10Co-0.45Cr3C2-0.25VC alloys

The ultrafine grained WC-10Co-0.45Cr₃C₂-0.25VC alloys were fabricated through planetary ball milling and low pressure sintering. The effects of the cobalt particle size, milling speed and sintering temperature on the microstructure, hardness and fracture toughness of the ultrafine grained alloys were investigated using optical microscopy, scanning electron microscopy and mechanical testing. The results showed that the mechanical properties of the low pressure-sintered alloys substantially depend on the milling speed and sintering temperature. At the same time, the hardness and fracture toughness of the samples can be increased from 1703 MPa and 8.90 MN m^−3/2 to 1789 MPa and 11.21 MN m^−3/2, respectively, when the cobalt particle size is reduced from 17 μm to 1.4 μm.     

Influence of alloy chemistry on microstructure and properties in NiCrBSi overlay coatings deposited by plasma transferred arc welding (PTAW)

The microstructures and performance of two NiCrBSi alloy overlays deposited by plasma transferred arc welding are studied. The coatings consist of a γ-Ni primary dendritic phase with harder Ni + Ni₃B or Ni + Ni₃Si eutectics and Cr-based particles (CrB, Cr₃C₂, and Cr₇C₃) situated at the interdendritic regions. It was found that the volume fraction of the soft primary dendritic phase drastically decreased and the proportion of chromium borides and carbides increased with an increase of C, B, Si, and Cr content. Microhardness testing revealed that the primary Ni dendrite, interdendritic, and Cr-particle phases had average hardness values of 405, 860, and 1200 HV respectively. An increase in the volume fraction of hard eutectics and Cr-particles lead to a substantial increase in hardness and wear resistance.     

Characterization and properties of quaternary Mo-Si-C-N coatings synthesized by magnetron sputtering technique

In this paper the direct current unbalanced reactive magnetron sputtering and composite target techniques were exploited to deposit quaternary Mo-Si-C-N coatings on Si wafer and stainless steel (1Cr18Ni9) in an Ar/N₂ gaseous mixture. The chemical composition, microstructure, morphology, hardness, and friction coefficient of these films were characterized by means of X-ray diffraction, XPS, field emission scanning electron microscopy, TEM and nanoindentation. With the increase of C content in the range of C/(C + Mo + Si + N) = 0-12 at.%, the crystallite size decreases from 32 nm to 5 nm and the average friction coefficient of Mo-Si-C-N coatings decreases from 0.24 to 0.17, while the hardness increases at first and then decreases after passing a maximum value of about 27 GPa at 9 at.% C. It was suggested that in the Mo-Si-C-N coatings C atoms substitute for the N atoms in the nano-sized crystalline Mo₂N to form Mo₂N(C) solid solution phase and the microstructure of the Mo-Si-C-N coatings may be nano-sized crystalline Mo₂N(C) embedded in the amorphous SiNx and CNx phases.     

Effects of Al and Al4C3 contents on combustion synthesis of Cr2AlC from Cr2O3-Al-Al4C3 powder compacts

Preparation of the ternary carbide Cr₂AlC was conducted by combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) from the Cr₂O₃-Al-Al₄C₃ powder compact. Effects of the contents of Al and Al₄C₃ on the product composition and combustion behavior were studied by formulating the reactant mixture with a stoichiometric proportion of Cr₂O₃:Al:Al₄C₃ = 3:5x:y, where x and y varied from 1.0 to 1.5. When compared to those of the powder compact with Cr₂O₃:Al:Al₄C₃ = 3:5:1 (i.e., x = y = 1.0), the combustion temperature and reaction front velocity increased with content of Al, but decreased with that of Al₄C₃. Besides Cr₂AlC and Al₂O₃, the final products always contained a secondary phase Cr₇C₃ that was substantially reduced by adopting additional Al and Al₄C₃ in the reactant compacts. For the sample with Cr₂O₃:Al:Al₄C₃ = 3:7.5:1 (x = 1.5), solid state combustion reached a peak temperature of 1245 °C and yielded Cr₂AlC with a trivial amount of Cr₇C₃. Although Cr₇C₃ was lessened by introducing extra Al₄C₃, the increase of Al₄C₃ from y = 1.1 to 1.5 produced almost no further reduction of Cr₇C₃ in the final product. This is partly attributed to the low combustion temperature in the range of 1065-1095 °C for the samples with additional Al₄C₃, and in part, due to the role of Al₄C₃ which might react with Cr to form Cr₇C₃, Cr₂Al, and Cr₂AlC.     

Mechanical synthesis and rapid consolidation of a nanocrystalline 3.3Fe0.6Cr0.3Al0.1-Al2O3 composite by high frequency induction heating

Nanopowders of 3.3Fe_0.6Cr_0.3Al_0.1 and Al₂O₃ were synthesized from Fe₂O₃, Cr, and Al powders by high-energy ball milling. A high density nanocrystalline 3.3Fe_0.6Cr_0.3Al_0.1-Al₂O₃ composite was consolidated by a high frequency induction heated sintering (HFIHS) method within 3 min from mechanically synthesized powders of Al₂O₃ and 3.3Fe_0.6Cr_0.3Al_0.1. The average grain sizes of Al₂O₃ and 3.3Fe_0.6Cr_0.3Al_0.1 were 84 and 32 nm, respectively.