Niobium carbide, chromium carbide and niobium–chromium carbide coatings were deposited using the thermo-reactive diffusion (TRD) deposition technique on AISI D2 steel substrates, and their wear and corrosion resistance was studied. The morphology of the coatings was characterized through optical and scanning electron microscopy (SEM), and the crystalline structure was studied through X-ray diffraction (XRD). Chemical composition was evaluated via energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The hardness of the coatings was measured through nanoindentation, and their wear was studied using the ball on disk test. The electrochemical behavior was assessed with potentiodynamic polarization and electrochemical impedance (EIS) tests. The XRD results show the formation of the NbC for the niobium carbide coating, Cr 23 C 6 and Cr 7 C 3 for the chromium carbide coating, and NbC, Cr 23 C 6 and Cr 7 C 3 for the niobium–chromium coating. Hardness value for the niobium–chromium carbide coating was 27.62 ± 2.56 GPa, which was higher in comparison to 21.66 ± 0.5 GPa for niobium carbide, 14.7 ± 1.1 GPa for chromium carbide and 6.70 ± 0.28 GPa for the uncoated steel. The wear resistance obtained was higher for the niobium–chromium carbide coating. However, its corrosion resistance was lower than the corrosion resistance for binary coatings.

Wear and corrosion resistance of niobium–chromium carbide coatings on AISI D2 produced through TRD

Hierarchical porous structures are highly desired for various applications. However, it is still challenging to obtain such materials with tunable architectures. Here, this paper reports hierarchical nanomaterials with oriented 2D pores by taking advantages of thermally instable bonds in vanadium-based metal-organic frameworks (MOFs). High-temperature calcination of these MOFs accompanied by the loss of coordinated water molecules and other components enables the formation of orderly slit-like 2D pores in vanadium oxide/porous carbon nanorods (VOx /PCs). This unique combination leads to an increase of the reactive surface area. In addition, optimized VOx /PCs demonstrate high-rate capability and ultralong cycling life for sodium storage. The assembled full cells also show high capacity and cycling stability. This report provides an effective strategy for producing MOFs-derived composites with hierarchical porous architectures for energy storage.

Thermal Instability Induced Oriented 2D Pores for Enhanced Sodium Storage.

A combination of plasma-enhanced chemical vapor deposition and magnetron sputtering techniques has been employed to deposit chromium-doped diamond-like carbon (DLC) coatings on stainless steel, silicon and glass substrates. The concentrations of Cr in the coatings are varied by changing the parameters of the bipolar pulsed power supply and the argon/acetylene gas composition. The coatings have been studied for composition, morphology, surface nature, nanohardness, corrosion resistance and wear resistance properties. The changes in I
 D
 /I
 G
 ratio with Cr concentrations have been obtained from Raman spectroscopy studies. Ratio decreases with an increase in Cr concentration, and it has been found to increase at higher Cr concentration, indicating the disorder in the coating. Carbide is formed in Cr-doped DLC coatings as observed from XPS studies. There is a decrease in sp
 3/sp
 2 ratios with an increase in Cr concentration, and it increases again at higher Cr concentration. Nanohardness studies show no clear dependence of hardness on Cr concentration. DLC coatings with lower Cr contents have demonstrated better corrosion resistance with better passive behavior in 3.5% NaCl solution, and corrosion potential is observed to move toward nobler (more positive) values. A low coefficient of friction (0.15) at different loads is observed from reciprocating wear studies. Lower wear volume is found at all loads on the Cr-doped DLC coatings. Wear mechanism changes from abrasive wear on the substrate to adhesive wear on the coating.

Corrosion and Wear Behaviors of Cr-Doped Diamond-Like Carbon Coatings

Chromium carbide nanopowder (Cr 3 C 2 ) has been synthesized from chromium oxide (Cr 2 O 3 ) by chemical-reduction route under autogenic pressure of hydrogen and CO gases at 87 MPa. The reduction of Cr 2 O 3 to Cr 3 C 2 has taken place at a relatively low temperature (700 °C) in the presence of Mg in an autoclave. The increased pressure of hydrogen and CO gases facilitates the reduction and carburization simultaneously which helps in reducing the reaction temperature. The nanopowder shows faceted morphology as observed in TEM. High resolution transmission electron micrographs reveal that the size of Cr 3 C 2 powders varies in the range of 30–40 nm and is coated by 12–13 layers of carbon with average thickness of 3.5–4 nm. Thermal stability of the as-obtained product was investigated by TGA/DTA analysis. Based on the experimental results, possible reaction mechanism for the transformation and formation of nanopowder is discussed in the light of the obtained structure.

In-situ synthesis of chromium carbide (Cr3C2) nanopowders by chemical-reduction route

Electrospun CoCr7C3-supported C nanofibers: Effective, durable, and chemically stable catalyst for H2 gas generation from ammonia borane

The solution-derived precursor method was used to synthesize chromium carbide (Cr 3 C 2 ) nanopowders, ammonium dichromate ((NH 4 ) 2 Cr 2 O 7 ) 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 Cr 3 C 2 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 Cr 2 O 3 and Cr 3 C 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 Cr 2 O 3 , respectively.

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

Chromium carbide (Cr 3 C 2 ) nanopowders can be synthesized by carbon thermal reduction of nanometer chromic oxide (Cr 2 O 3 ) and with nanometer carbon black. Effect of reaction time on phase composition and microstructure of chromium carbide nanopowders was researched under the conditions of 28 wt.% C and 1100 °C. The products were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric and differential scanning calorimetry (TG–DSC) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that the product is mainly composed of Cr 2 O 3 when the reaction time is 0.5 h, and the diffraction peaks of carbon are not observable. Some powders show a spherical morphology, others appear as fused mass. The powders with the single phase of Cr 3 C 2 can be synthesized at 1 h, and an average crystallite size of 26 nm. The powders show good dispersion and are mainly composed of spherical or nearly spherical particles with a mean diameter of about 30 nm. Compared with those of 1 h, the diffraction lines of Cr 3 C 2 synthesized at 1.5 h and 2 h shift toward smaller angles, and the powders exhibit a lot of agglomerated particles.

Effect of reaction time on phase composition and microstructure of chromium carbide nanopowders

Chromium carbide (Cr3C2) nanopowders have been synthesized by a novel solution-derived precursor method, and the raw materials are ammonium dichromate ((NH4)2Cr2O7) and glucose (C6H12O6). The products were characterized by X-ray diffractometry (XRD), thermogravimetric and differential scanning calorimetry (TG-DSC), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that Cr3C2 nanopowders with an average crystallite size of 32 nm can be synthesized at 900 °C in 1 h. The powders synthesized by this approach show good dispersion and are mainly composed of spherical or nearly spherical particles with a mean diameter of about 30 nm. The weight loss ratio of the precursor during the reaction is about 55 wt.%, and it reduces rapidly before 400 °C (about 45 wt.%). The surface of the specimen mainly consists of three elements, Cr, C and O. The XPS spectrum of Cr2p consists of two peaks with the binding energies of 575.8 eV and 577.4 eV, which are assigned to the Cr2p3/2 species of Cr3C2 -x (0 ≤ x ≤ 0.5) and Cr2O3, respectively.

Low temperature synthesis of chromium carbide (Cr3C2) nanopowders by a novel precursor method

Chromium carbide nanopowders were firstly synthesized via a simple microwave heating technique using nanometer chromic oxide (Cr 2 O 3 ) 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: Cr 2 O 3  → CrO → Cr 7 C 3  → Cr 2 C → Cr 3 C 2 .

Hongjuan Zheng

Papers

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

Effect of reaction time on phase composition and microstructure of chromium carbide nanopowders

Low temperature synthesis of chromium carbide (Cr3C2) nanopowders by a novel precursor method

Synthesis of chromium carbide nanopowders via a microwave heating method

Effect of V8C7-Cr3C2 nanocomposite on microstructure and properties of vitrified bond cBN grinding tools prepared by microwave heating method