Preparation of ZrB2 based composites by reactive melt infiltration at relative low temperature

ZrB₂ based composites were prepared by a novel reactive melt infiltration process. The porous boron bars were used as preforms and infiltrated by the low melting Zr₂Cu intermetallic compound. Thermodynamics calculations revealed that B could react with liquid Zr₂Cu to form ZrB₂ as low as 1100 °C. Composites were prepared by heating the two materials to 1200 °C for 3 h in vacuum. The resultant composites were studied with XRD and SEM. ZrB₂ was identified to be the main constituent. The composites had a flexural strength of 414.3 MPa, a flexural modulus of 183.6 GPa, and a fracture toughness of 5.5 MPa m^1/2.     

Preparation of w–cu nano-composite powders with high copper content using a chemical co-deposition technique

A novel chemical co-precipitation was used to produce W-70%Cu nanocomposite powders with coating structure. The precursors consisting of CuC₂O₄·xH₂O and WO₃·2H₂O were first synthesized using copper nitrate, ammonium metatungstate(AMT) and oxalic acid as the raw materials at 80?°C for 1.5?h when the concentrations of the reactants were 0.8?mol/L and the hydrogen ion concentration was 1.2?mol/L. The precursors were calcined to produce the powders with different phase components and microstructure at various temperatures. The CuWO₄ and CuO nano-powders were obtained at 300?°C, which is colder than the traditional reaction temperature (1000?°C) of CuO?+?WO₃ = CuWO₄. However, the cubic Cu₂O and Cu₂WO₄ could be formed when the calcining temperature was 600?°C. The hydrogen reduction results show that the calcined powder is reduced to obtain W-Cu composite powder at 750?°C and 800?°C. In reduction process, volatile WO₂(OH)₂ through chemical vapor transport(CVT) continuously spreads to the copper surface and is reduced to form W and the coated particle is eventually formed. This particle is Cu particle coated by W phase and the interface between W and Cu phases is semi-coherent. It is found that the average particle size of the reduced powder is 30–50?nm observed by TEM images.     

Preparation of YAG gel coated ZrB2–SiC composite prepared by gelcasting and pressureless sintering

In this paper, gelcasting and pressureless sintering of YAG gel coated ZrB₂–SiC (YZS) composite were conducted. YAG gel coated ZrB₂–SiC (YZS) suspension was firstly prepared through sol–gel route. Poly (acrylic acid) was used as dispersant. YZS suspension had the lowest viscosity when using 0.6 wt.% PAA as dispersant. Gelcasting was conducted based on AM–MBAM system. The gelcast YZS sample was then pressureless sintered to about 97% density. During sintering, YAG promoted the densification process from solid state sintering to liquid phase sintering. The average grain sizes of ZrB₂ and SiC in the YZS composite were 3.8 and 1.3 μm, respectively. The flexural strength, fracture toughness and microhardness were 375 ± 37 MPa, 4.13 ± 0.45 MPa m^1/2 and 14.1 ± 0.5 GPa, respectively.     

Stable controlled growth of 3D CuO/Cu nanoflowers by surfactant-free method for non-enzymatic hydrogen peroxide detection

Sensitive, convenient and rapid detection of hydrogen peroxide(H_2 O_2) is highly desirable in fields like fundamental biological research, food industries, and clinical environmental analysis. Herein, a hierarchical porous CuO/Cu flower-like active electrode material for non-enzymatic H_2 O_2 sensor was synthesized via a low-cost and one-step chemical oxidation of Cu powder in water bath without surfactants. In order to discuss the growth mechanism of the product, products with different growth time length were fabricated. The electro-catalysis of all products were first exhibited by cyclic-voltammetry,and the product under 6 h reaction shows the best result. The detailed electro-catalytic behaviors of the best product(under 6 h reaction) are characterized by cyclic-voltammetry and amperometry under alkaline conditions. The materials have high sensitivity of 103 μA mM~(-1) cm~(-2)(R~2= 0.9979), low detection limit of 2 μmol/L and wide concentration range(from 2 μmol/L to 19.4 mmol/L). Large specific surface area and stabled nanostructure enabled good features, such as stability and sensitivity for the H_2 O_2 determination.     

Deposition of Phase-pure Cr2AlC Coating by DC Magnetron Sputtering and Post Annealing Using Cr-Al-C Targets with Controlled Elemental Composition but Different Phase Compositions

Polycrystalline Cr_2AlC coatings were prepared on M38G superalloy using a two-step method consisting of magnetron sputtering from Cr-Al-C composite targets at room temperature and subsequent annealing at 620°C. Particularly, various targets synthesized by hot pressing mixture of Cr, Al, and C powders at 650–1000°C were used. It was found that regardless of the phase compositions and density of the composite targets, when the molar ratio of Cr:Al:C in the starting materials was 2:1:1, phase-pure crystalline Cr_2AlC coatings were prepared by magnetron sputtering and post crystallization. The Cr_2AlC coatings were dense and crack-free and had a duplex structure. The adhesion strength of the coating deposited on M38G superalloy from the 800°C hot-pressed target and then annealed at 620°C for 20 h in Ar exceeded82 ± 6 MPa, while its hardness was 12 ± 3 GPa.     

Ultra-high-temperature tensile properties and fracture behavior of ZrB2-based ceramics in air above 1500 °C

Nearly fully dense ZrB₂–SiC–graphite composites were fabricated from commercially available powder at 1900 °C by hot pressing. The tensile strength of ZrB₂-based ceramics was measured in air up to 1750 °C, which is the first reported tensile strength measurement in air above 1500 °C. A mechanical testing apparatus capable of testing material in ultra-high temperature under air atmosphere was built, evaluated, and used. Tensile strength was measured as a function of temperature up to 1750 °C in air. The respective average values of the tensile strength measured at 1550 °C, 1650 °C, and 1750 °C are 58.4, 44.8, and 21.8 MPa, which are 49.4%, 37.9%, and 18.4% of their room-temperature strength (118.2 MPa), respectively. Moreover, the tensile fracture behaviors and mechanism of ZrB₂-based ceramics at different testing temperatures were discussed based on microstructure characterization.     

A comparison study of hydrogen storage properties of as-milled Sm5Mg41 alloy catalyzed by CoS2 and MoS2 nano-particles

The influences of the catalysts of CoS₂ and MoS₂ nano-particles on microstructure and hydrogen storage behaviors of as-milled Sm₅Mg₄₁ alloy have been compared in this work. The Sm₅Mg₄₁ + 5 wt.% M (M = CoS₂, MoS₂) alloys were prepared by milling the mechanical ground as-cast Sm₅Mg₄₁ alloy powders (particle size ≤ 75 μm) with 5 wt.% CoS₂ or MoS₂ nano-particles (particle size ≤ 30 nm), respectively. The results demonstrate that the CoS₂ and MoS₂ nanoparticles are embedded into the alloy surface, which is nanostructure containing some crystal defects, such as dislocation, grain boundary and twin etc. Those microstructures play a beneficial role in reducing the total potential barrier that the hydrogen absorption or desorption reactions must overcome, hence improving the hydrogen storage kinetics of the alloys. The as-milled alloys are composed of Sm₅Mg₄₁ and SmMg₃ phases, and ball milling refines their crystal grains. The MgH₂ and Sm₃H₇ phases appear after hydrogenation, while Mg and Sm₃H₇ phases exist after dehydrogenation. The dehydriding activation energy of M = CoS₂ and MoS₂ alloys are 101.67 and 68.25 kJ/mol H₂ respectively. The initial hydrogen desorption of M = CoS₂ and MoS₂ alloys are 252.9 °C and 247.8 °C. The hydrogenation and dehydrogenation enthalpy changes of M = MoS₂ alloy are a little smaller than that of M = CoS₂ alloy. Therefore, the catalyst MoS₂ can improve the as-milled Sm₅Mg₄₁ alloy in hydrogen storage property more effectively than CoS₂.     

Low-temperature synthesis of ZrB2 nano-powders using a sorbitol modified sol–gel processing route

High-purity zirconium diboride nano-powders were synthesized by carbothermal reduction reaction at a relatively low temperature from a novel sorbitol modified sol–gel method. Phase composition and morphology of the ZrB₂ nano-powders were characterized by X-ray diffraction and transmission electron microscopy, respectively. The effect of sorbitol on sol–gel process and the formation mechanism of ZrB₂ powders were investigated by fourier transform infrared spectroscopy and thermo gravimetric/differential thermal analysis. Sorbitol, used as bridging and chelating ligand, led to the formation of chelate complex through its polyhydroxy opening reaction with H₃BO₃, and then induced the condensation of zirconia forming Zr–O–C–B network, which promoted the carbothermal reduction reaction to complete at 1450 °C for 1 h. The synthesized powders exhibited near-spherical morphology with a small average crystalline size of about 100 nm. With respect to the conventional solid state method, the sorbitol modified sol–gel method route guaranteed a faster, easier and energy-saving process for obtaining single-phase nano-powders.     

Effect of a weak fiber interface coating in ZrB2 reinforced with long SiC fibers

This study reports the microstructural analysis and mechanical properties of a ZrB₂ ceramic containing long BN-coated Hi-Nicalon SiC fibers. A composite was produced and thoroughly characterized by transmission electron microscopy to study the interfaces at the nanoscale level. Full densification was accomplished by hot pressing at 1450 °C. The fiber in the sintered material retained its pristine aspect, confirming that the coating was effective in preventing degradation due to interactions with the matrix. Pull-out was observed on fractured surfaces, but toughness values were about 4.5 MPa√m, which was comparable to those of ZrB₂ materials with SiC additions in the form of particles or short fibers. However, the composites exhibited a controlled fracture behavior, as confirmed by a notably higher work of fracture, 140 J/m², compared with 20–30 J/m² of unreinforced ZrB₂ or ZrB₂ containing chopped fibers.     

Morphology-controlled synthesis of octahedron and hexagonal plate of Co3O4

Hexagonal plate-like Co₃O₄ was prepared by the thermal oxidation of a Co(OH)₂ precursor with a hexagonal plate morphology at 600 °C in air. The Co(OH)₂ precursor was obtained by hydrothermal synthesis from CoCl₂, KCN and hydrazine at 180 °C. The hexagonal plates had a side length of approximately 3.5 μm and 1 μm in thickness. Octahedral Co₃O₄ was prepared by a hydrothermal method with CoCl₂, KCN, and H₂O₂ at 180 °C. H₂O₂ played an important role in the formation of Co₃O₄ with an octahedral morphology. A hierarchical hexagonal skeleton-like morphology assembled with small octahedral Co₃O₄ particles was also prepared using a hydrothermal method with CoCl₂ and KCN. The mechanism of the morphology-controlled synthesis of Co₃O₄ is discussed.     

Microstructure and brazing mechanism of porous Si3N4/Invar joint brazed with Ag-Cu-Ti/Cu/Ag-Cu multi-layered filler

Porous Si_3N_4 was brazed to Invar alloy in this study, and Ag-Cu-Ti/Cu/Ag-Cu multi-layered filler was designed to inhibit the formation of Fe_2Ti and Ni_3Ti intermetallic compounds. The effects of the brazing temperature and the thickness of Cu interlayer on the microstructure and mechanical properties of brazed joints were investigated. The typical microstructure of the joint brazed with multi-layered filler was porous Si_3N_4/TiN + Ti_5Si_3/Ag-Cu eutectic/Cu/Ag-Cu eutectic/Cu-rich layer + diffusion layer/Invar. When the brazing temperature increased, the reaction layer at the ceramic/filler interface grew thicker and the Cu interlayer turned thinner. As the thickness of Cu interlayer increased from 50 to 150μm, the joint strength first increased and then decreased. In this research, the maximum shear strength(73 MPa) was obtained when being brazed at 1173 K with a 100μm Cu interlayer applied in the filler, which was 55% higher than that brazed with single Ag-Cu-Ti brazing alloy and had reached 86% of the ceramic. The release of residual stress and the barrier effect of Cu interlayer to inhibit the formation of Fe_2Ti and Ni_3Ti intermetallics played the major role in the improvement of joint strength.     

Effect of environment atmosphere on thermal shock resistance of the ZrB2–SiC–graphite composite

The thermal shock resistance of the ZrB₂–SiC–graphite composite was evaluated by measuring the retention of the flexural strength after the electrical resistance heating to the temperature ranging from 1000 °C up to 2500 °C. The experiment was operated in two different environment atmospheres (pure oxygen and low oxygen partial pressure which mixed O₂ and Ar with 1:9) at total pressure 2000 Pa. The residual strength for the specimen decreased gradually as the temperature increased up to 2200 °C, and it was slightly higher when heated in low oxygen partial pressure environment than in pure oxygen. In contrast to the specimen heated in low oxygen partial pressure environment, the residual strength for the specimen in pure oxygen increased steeply as the temperature increased from 1600 °C up to 1800 °C. The analysis of the SEM observations combined with EDS confirmed that the surface oxidation played a positive role in the thermal shock resistance of the ZrB₂–SiC–graphite composite with different environment atmospheres. The results here pointed out a potential method for charactering the effect of environment atmosphere on thermal shock resistance of the ZrB₂–SiC–graphite composite.     

Surface characteristics and electrochemical corrosion behavior of NiTi alloy coated with IrO2

The aim of this work is to investigate the surface characteristics and corrosion behavior of NiTi (50.6 at.% Ni) shape memory alloy coated by a ceramic-like and highly biocompatible material, iridium oxide (IrO₂). IrO₂ coatings were prepared by thermal decomposition of H₂IrCl₆ · 6H₂O precursor solution at the temperature of 300 °C, 400 °C and 500 °C, respectively. The surface morphology and microstructure of the coatings were investigated by scanning electron microscope (SEM) and glancing angle X-ray diffraction (GAXRD). X-ray photoelectron spectroscopy (XPS) was employed to determine the surface elemental composition. Corrosion resistance property of the coated samples was studied in a simulated body fluid at 37 ± 1 °C by electrochemical method. It was found that the morphology and microstructure of the coatings were closely related to the oxidizing temperatures. A relatively smooth, intact and amorphous coating was obtained when the H₂IrCl₆·6H₂O precursor solution (0.03 mol/L) was thermally decomposed at 300 °C for 0.5 h. Compared with the bare NiTi alloy, IrO₂ coated samples exhibited better corrosion resistance behavior to some extent.     

Characterization and properties of CuZrAlTiNi high entropy alloy coating obtained by mechanical alloying and vacuum hot pressing sintering

CuZrAlTiNi High entropy alloy (HEA) coating was synthesized on T10 substrate using mechanical alloying (MA) and vacuum hot pressing sintering (VHPS) technique. The MA results show that the final product of as-milled powders is amorphous phase. The obtained coating sintered at 950 °C is compact and about 0.9 mm in thickness. It is composed of a couple of face-centered cubic (FCC), one body-centered cubic (BCC) solid solutions and AlNi₂Zr phase. The interface strength between coating and substrate is 355.5 MPa measured by three point bending test. Compared with T10 substrate, the corrosion resistance of CuZrAlTiNi HEA coating is enhanced greatly in the seawater solution, which is indicated by the higher corrosion potential, wider passivation region, and secondary passivation. The average microhardness of the coating reaches 943 HV_0.2, and is about 3.5 times higher than the substrate, which is mainly ascribed to the uniformly dispersed nano-size precipitates, phase boundary strengthening and solid solution strengthening. Moreover, the wear resistance of the coating is slightly improved in comparison with the substrate.     

Preparation of nanocrystalline nickel oxide thin films by sol–gel process for hydrogen sensor applications

Preparation of nanocrystalline NiO thin films by sol–gel method and their hydrogen (H₂) sensing properties were investigated. The thin films of NiO were successfully deposited on the glass and SiO₂/Si substrate by a sol–gel coating method. The films were characterized for crystallinity, electrical properties, surface topography and optical properties as a function of calcination temperature and substrate material. It was found that the films produced by this method were polycrystalline and phase pure NiO. The H₂ gas sensitivity of these films was studied as a function of H₂ concentration and calcination temperature. The results indicated that the sol–gel derived NiO films could be used for the fabrication of H₂ gas sensors to monitor low concentration of H₂ in air quantitatively at low temperature range (< 200 °C).     

Study on tribological behavior of electrodeposited Cu–Si3N4 composite coatings

Cu–Si₃N₄ composite coatings were prepared by electrolysis from a copper sulphate solution containing dispersed Si₃N₄ particles of 0.4 or 1 μm mean size. Wear behavior of Cu–Si₃N₄ composite and pure copper coatings were evaluated using a pin-on-disc test machine under dry condition sliding. Effects of current density and particle concentration on the incorporation percentage of Si₃N₄, the preferred orientation of copper crystallites, the microstructure, the microhardness and the wear resistance of the coatings were determined. Si₃N₄ particles in the copper matrix resulted in the production of composite deposits with smaller grain sizes and led to change the preferred orientation growth from [1 0 0] to [1 1 0]. It was proved that the presence of Si₃N₄ particles decreases the wear loss and the friction coefficient of the coating. According to the results, the friction coefficient decreased dramatically from 0.52 to 0.26 for pure copper coatings to 0.16–0.24 for Cu–Si₃N₄ composite coatings. In addition, fluctuation of friction coefficient values for Cu–Si₃N₄ composite coating was lower compared with the pure copper coating. The wear properties of Cu–Si₃N₄ composite coatings were shown to depend on the weight fraction, the size and the distribution of co-deposited particles.     

Investigation of Ti coatings on cubic boron nitride (cBN) grits by discharge treatment in spark plasma sintering system

The Ti coatings on cubic boron nitride (cBN) grits were prepared by discharge treatment on a mixture of Ti powders and cBN grits in spark plasma sintering system. The uniform and full coatings with a thickness of ～1.2 μm were prepared at 850 °C for 60 min, which were constituted with TiB₂, TiN, and Ti phases. The compressive fracture strength and toughness impact of the Ti-coated cBN grits were 11.6% and 7.4% higher than the cases of the pristine ones, respectively. With the aid of Ti coatings, the interface bonding strength between cBN grits and Fe-based matrix was improved by 335 MPa in the Fe-based matrix/cBN composites.     

Development and characterization of laser clad high temperature self-lubricating wear resistant composite coatings on Ti–6Al–4V alloy

To enhance the wear resistance and friction-reducing capability of titanium alloy, a process of laser cladding γ-NiCrAlTi/TiC + TiWC₂/CrS + Ti₂CS coatings on Ti–6Al–4V alloy substrate with preplaced NiCr/Cr₃C₂–WS₂ mixed powders was studied. A novel coating without cracks and few pores was obtained in a proper laser processing. The composition and microstructure of the fabricated coating were examined by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) techniques, and tribological properties were evaluated using a ball-on-disc tribometer under dry sliding wear test conditions at 20 °C (room-temperature), 300 °C, 600 °C, respectively. The results show that the coating has unique microstructure consisting of α-Ti, TiC, TiWC₂, γ-NiCrAlTi, Ti₂CS and CrS phases. Average microhardness of the composite coating is 1005 HV_0.2, which is about 3-factor higher than that of Ti–6Al–4V substrate (360 HV_0.2). The friction coefficient and wear rate of the coating are greatly decreased due to the combined effects of the dominating anti-wear capabilities of reinforced TiC and TiWC₂ carbides and the CrS and Ti₂CS sulfides which have excellent self-lubricating property.     

Building metallurgical bonding interfaces in an immiscible Mo/Cu system by irradiation damage alloying (IDA)

For the immiscible Mo/Cu system with a positive heat of mixing(△H_m 0), building metallurgical bonding interfaces directly between immiscible Mo and Cu and preparing Mo/Cu laminar metal matrix composites(LMMCs) are very difficult. To solve the problem, a new alloying method for immiscible systems, which is named as irradiation damage alloying(IDA), is presented in this paper. The IDA primarily consists of three steps. Firstly, Mo is damaged by irradiation with multi-energy(186, 62 keV) Cu ion beams at a dose of 2 × 10~(17) ions/cm~2. Secondly, Cu layers are superimposed on the surfaces of the irradiation-damaged Mo to obtain Mo/Cu laminated specimens. Thirdly, the irradiation damage induces the diffusion alloying between Mo and Cu when the laminated specimens are annealed at 950℃ in a protective atmosphere.Through IDA, Mo/Cu LMMCs are prepared in this paper. The tensile tests carried out for the Mo/Cu LMMCs specimens show that the Mo/Cu interfaces constructed via IDA have high normal and shear strengths.Additionally, the microstructure of the Mo/Cu interface is characterized by High Resolution Transmission Electron Microscopy(HRTEM), X-ray diffraction(XRD) and Energy Dispersive X-ray(EDX) attached in HRTEM. The microscopic characterization results show that the expectant diffusion between Mo and Cu occurs through the irradiation damage during the process of IDA. Thus a Mo/Cu metallurgical bonding interface successfully forms. Moreover, the microscopic test results show that the Mo/Cu metallurgical interface is mainly constituted of crystalline phases with twisted and tangled lattices, and amorphous phase is not observed. Finally, based on the positron annihilation spectroscopy(PAS) and HRTEM results,the diffusion mechanism of IDA is discussed and determined to be vacancy assisted diffusion.