Reaction bonded niobium carbide ceramics from polymer-filler mixtures

Manufacturing of novel reaction bonded Niobium Carbide (NbC) containing ceramic composites derived from polymer/filler mixtures was investigated. Poly(methylsiloxane) filled with 40 vol.% of a mixture of metallic Niobium (Nb) (reactive filler) and alumina powder (inert filler) was pyrolysed in inert atmosphere up to 1450°C. During pyrolysis metallic niobium reacted with carbon from the decomposition products of the preceramic polymer binder to form microcrystalline composites of NbC, Al₂O₃ and a silicon oxycarbide glass. Microstructure formation of specimens prepared with different niobium to alumina ratio in the starting mixture was experimentally examined and compared to thermodynamic phase equilibria calculations. Materials of high NbC content exhibit high hardness and wear resistance.     

Interfacial microstructure of Si3N4/Si3N4 brazing joint with Cu–Zn–Ti filler alloy

In this study, Si₃N₄ ceramic was jointed by a brazing technique with a Cu–Zn–Ti filler alloy. The interfacial microstructure between Si₃N₄ ceramic and filler alloy in the Si₃N₄/Si₃N₄ joint was observed and analyzed by using electron-probe microanalysis, X-ray diffraction and transmission electron microscopy. The results indicate that there are two reaction layers at the ceramic/filler interface in the joint, which was obtained by brazing at a temperature and holding time of 1223 K and 15 min, respectively. The layer nearby the Si₃N₄ ceramic is a TiN layer with an average grain size of 100 nm, and the layer nearby the filler alloy is a Ti₅Si₃N_x layer with an average grain size of 1–2 μm. Thickness of the TiN and Ti₅Si₃N_x layers is about 1 μm and 10 μm, respectively. The formation mechanism of the reaction layers was discussed. A model showing the microstructure from Si₃N₄ ceramic to filler alloy in the Si₃N₄/Si₃N₄ joint was provided as: Si₃N₄ ceramic/TiN reaction layer/Ti₅Si₃N_x reaction layer/Cu–Zn solution.     

Synthesis and growth mechanism of NbC–SiC micro/nanowire by carbothermal reduction

Abstract

NbC–SiC micro/nanowires (MNWs) with NbC content varying from 5 to 20 mol.-% were synthesised at 1600–1800°C via carbothermal reduction utilising silica sol, niobium pentoxide powder and carbon black as starting materials. The synthesis process and growth mechanism of NbC–SiC system were investigated. Results show that the morphology of the synthesised products mainly appears as curve shaped microwires or nanowires. The crystalline consists of both SiC and NbC phases which doped with each other by substitution and interstitial reactions in solid solution. NbC–SiC MNWs were developed by vapour–liquid–solid mechanism according to the existence of liquid droplet phase in the tip at reaction temperature. β-SiC twin crystal growing along [112] direction was formed in the stem, and NbC polycrystal was dissociated from Nb–Si liquid phase. The varied concentration of Nb and Si in the Nb–Si liquid phase could be a significant reason for the curved growth of NbC–SiC MNWs.     

Interlayer design for joining pressureless sintered sialon ceramic and 40Cr steel brazing with Ag57Cu38Ti5 filler metal

An interlayer design and test was made to enhance the joining strength of the pressureless sintered sialon ceramic and 40Cr steel. Joining was preformed by vacuum brazing using Ag₅₇Cu₃₈Ti₅ filler metal. The joint strength was evaluated by four-point bending. A strong interfacial bond of the Ag₅₇Cu₃₈Ti₅ filler metal on the sialon ceramic with formation of Ti₂AlN, Ti₅Si₄ and TiAg was obtained at brazing temperatures over 1123 K, which could be weakened by a brazed metal such as Kovar or Ni-15Cr-15Co alloy. The joint strength of sialon ceramic with 40Cr steel can be improved by using a layer of soft interlayer such as Cu with a suitable thickness, particularly by the composite interlayer such as Cu/Nb alloy, Cu/Ta, Cu/Mo etc. The maximum strength of the ceramic/steel joint, 280 MPa, was obtained by using Cu/Nb alloy as interlayer and brazing at 1153 K for 5 min. Finally, we discuss how to design an interlayer in ceramic/metal joining.     

The effect of polysiloxane on the properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-NbC composite material produced by pyrolysis process

Recently, studies have been developed in order to obtain Al₂O₃-NbC composite materials. The reinforced materials have shown good potential to be used as cutting tool materials at high-speed cutting and high temperature as a substitute to WC-Co material. The main disadvantage to produce these alumina-reinforced materials is the necessity to use pressure assisted sintering or high sintering temperatures to produce dense bodies. Manufacturing of composite ceramic materials derived from polymer reactive filler has been intensively investigated. Polymer pyrolysis is a relatively new and very promising method for obtaining ceramic material in complex shapes and lower sintering temperatures. This work investigated a ceramic composite matrix based in SiC_xO_y and Al₂O₃ and reinforced with NbC obtained by means of the active fillers pyrolysis process. The results obtained in this work demonstrate that using a mixture of polysiloxanes produces a composite material with better properties when compared to others polymer materials.     

Kinetic study on the coarsening behaviour of equilibrium phases in Nb alloyed ferritic stainless steels at 700 °C

Nb alloyed ferritic stainless steel is an attractive material to be used in automobile exhaust systems. Recently, in some published experimental work it was reported that coarsening rate of Laves phase (Fe₂Nb) can be higher than NbC in Nb alloyed ferritic stainless steels during aging at 700 °C. This observation was attributed to the fact that NbC has a more coherent interface with ferrite than has Laves phase. We explore this conclusion and find that the real reason for the smaller coarsening rate of NbC is the incredibly low solubility of carbon in ferrite.     

Precipitation and mechanical properties of Nb-modified ferritic stainless steel during isothermal aging

The influence of isothermal aging on precipitation behavior and mechanical properties of Nb-modified ferritic stainless steel was investigated using Thermo-calc software, scanning electron microscopy and transmission electron microscopy. It was observed that TiN, NbC and Fe₂Nb formed in the investigated steel and the experimental results agreed well with the results calculated by Thermo-calc software. During isothermal aging at 800 °C, the coarsening rate of Fe₂Nb is greater than that of NbC, and the calculated average sizes of NbC and Fe₂Nb of the aged specimen agreed with the experimental results. In addition, the tensile strength and micro-hardness of the ferritic stainless steel increased with increased aging time from 24 h to 48 h. But aging at 800 °C for 96 h caused the coarsening of the precipitation, which led to a decrease of tensile strength and micro-hardness.     

Chemical vapor deposition of niobium on graphite

Chemical vapor deposition of niobium on a graphite rod has been carried out at temperatures exceeding 1200 °C using NbBr₅ as a feed material. The properties of the deposits have been studied by X-ray diffraction analysis, surface hardness measurements, metallographic and scanning electron microscopy observations. It was found that coatings deposited at a low temperature (below 1300 °C) have a smooth surface and a fine grained structure with layers consisting of Nb + Nb₂C or Nb₂C + NbC depending on their deposition temperature. In contrast, the coatings deposited at a higher temperature were found to have decreased smoothness of the surface and a coarse grained structure with a single layer consisting of NbC. Annealing experiments on the coatings with Nb and Nb₂C layers showed that these layers readily transformed to a single layer of NbC at 900 °C within 10 h. It was found that the layer growth rates of carbides in the deposits are much faster than the layer growth rates estimated from ordinary diffusion data.     

Magnetic and processability studies on rubber ferrite composites based on natural rubber and mixed ferrite

Polycrystalline single phasic mixed ferrites belonging to the series Ni_1–x Zn _x Fe₂O₄ for various values of _x have been prepared by conventional ceramic techniques. Pre-characterized nickel zinc ferrites were then incorporated into a natural rubber matrix according to a specific recipe for various loadings. The processability and cure parameters were then determined. The magnetic properties of the ceramic filler as well as the ferrite loaded rubber ferrite composites (RFC) were evaluated and compared. A general equation for predicting the magnetic properties was also formulated. The validity of these equations were then checked and correlated with the experimental data. The coercivity of the RFCs almost resemble that of the ceramic component in the RFC. Percolation threshold is not reached for a maximum loading of 120 phr (parts per hundred rubber by weight) of the filler. These studies indicate that flexible magnets can be made with appropriate magnetic properties namely saturation magnetisation (M _s) and magnetic field strength (H _c) by a judicious choice of x and a corresponding loading. These studies also suggest that there is no possible interaction between the filler and the matrix at least at the macroscopic level. The formulated equation will aid in synthesizing RFCs with predetermined magnetic properties.     

Effect of different binders and secondary carbides on NbC cermets

Due to the rapidly increasing price of tungsten carbide and the significant health risks associated with the wear products of WC-Co (Co₃O₄ and Wo₃), an alternative is required. Niobium carbide (NbC) is well suited as a cutting tool due to its high melting point and low solubility in iron. Compared to pure NbC, a complete substitution of WC to NbC-Co resulted in an increased toughness and strength. As alternative binders, nickel and iron-based binders were subsequently investigated. Although iron-based cermets would be an economical, low-cost alternative to NbC-Ni cermets, they showed a higher coefficient of friction and wear rate. So far, NbC-Ni cermets best met the requirements of high hardness and toughness. Various secondary carbides such as VC, Mo₂C, TiC, but also WC were added to further improve the hardness. Elemental analyses of NbC-Ni-MeC cermets (Me = metal) showed that the binder is a face-centered cubic solid solution, while the NbC phase is a solid solution of the type (Nb, Me)C.

     

Carbon/ceramics composites — Preparation and properties

Fabrication methods for carbon/ceramics composites were established by using two different processes of hot-pressing and pressureless sintering without any binder phase. In the hot pressing method, some boron compounds were found to be an effective aid for sintering and graphitization of coke powder above 2000°C under some pressure. When the content of boron compound such as B₄C was high, graphite/B₄C composites could be fabricated. If some other ceramic powder such as NbC, TiC or TaC was mixed in addition to the B₄C, three component composites with graphite matrix could be obtained. In pressureless sintering method, raw coke carbon powder was ground for a long time to be transformed in to a sinterable and non-graphitizing-type carbon powder. From a mix of ceramic powders such as SiC or B₄C with the ground coke powder, the composites of carbon/SiC or carbon/SiC/B₄C systems could be fabricated by heat-treatment under normal pressure.Some properties of the graphite samples and carbon/ceramic composites were investigated. It was found that their mechanical properties were much better than those of conventional graphite samples and the resistance to oxidation and corrosion was also excellent. It is suggested that the composites could be applied as bearing or mechanical seals both for use in high temperature environments and as machine parts in contact with some molten metals.     

Dielectric characterization of BaTiO3-BaM1/3N2/3 O3 systems (with M=Co,Mg and N=Nb or Ta)

The dielectric behaviour of (1–x)BaTiO₃–x BaM_1/3N_2/3O₃ systems, in which M=Co, Mg and N=Nb, Ta is described whenx varies from 0 to 0.1. The evolution of the Curie temperature and of the dielectric constant, either in the ferroelectric state or in the paraelectric state are correlated to the structure and the ceramic microstructure. An observed diffuse phase transition is discussed.     

In situ fabricated metal-carbide with core–shell structure for high impact-toughness iron-matrix composite

A series of metal-carbide (Ta–TaC, Nb–NbC and W–WC) with core–shell structure for iron-matrix composites are fabricated by in situ solid-phase diffusion. Results show that the formation of metal-carbide with a rod-shaped core–shell structure, in which the metal-rod surface was covered with a carbide shell layer, in the iron- matrix after in situ solid-phase diffusion. The TaC, NbC, and WC shell layers are in situ synthesised by the diffusion of carbon atoms from the iron-matrix onto the surface of the Ta, Nb, and W rods, respectively. Metallurgical integration occurs between metal-carbide and iron-matrix. The metal-carbide-reinforced iron-matrix composites show excellent impact resistance, and the shell-layer hardness is extremely high.     

Optimization of NbC Concentration in (Bi,Pb)2Sr2Ca2Cu3O10 + x Superconducting Ceramics Using Multifractal Formalism

The microstructure effect on the superconducting properties of (Bi,Pb)₂Sr₂Ca₂Cu₃O_{10 + x} ceramics containing different additions of nanocrystalline NbC powder was analyzed using information interpretation of multifractal formalism. By introducing NbC additions, the midpoint transition temperature T _c of the ceramics can be raised by more than 10 K. The optimal NbC content evaluated with the use of the multifractal parametrization methodology is 0.35 wt %. The corresponding increase in T _c may attain 12–16 K. The crystallite and pore substructures in the ceramics are found to be closely related. The information approaches are shown to have great potential for structural characterization of superconducting materials and optimization analysis of property–structure–composition relationships.     

ZrC Segregation to the Surface of Dilute Solid Solutions of Zirconium Carbide in Niobium Carbide

Precipitation of crystalline ZrC in the surface region of dilute ZrC–NbC solid solutions (<1.0 mol % ZrC) was revealed by electron microscopy, x-ray microanalysis, laser mass spectrometry, and x-ray diffraction. The ZrC precipitation was shown to be a consequence of the diffusional decomposition of the dilute Zr_{1 – x} Nb _x C solid solutions. The boundaries of the solid-miscibility gap in the ZrC _y –NbC _y"system were calculated. The segregation energy of zirconium carbide was estimated at –33.3 kJ/mol.     

A new composition of ceramic varistor: SnO2 doped with (Co,Nb, Al)

A new ceramic voltage sensor based on SnO₂ doped with Co, Nb and Al has been successfully synthesized by ceramic route. The ceramic compact exhibited excellent non-linear current-voltage (I-V) characteristics withα ≈ 24 andE _1mA ≈ 700 V/mm. NCL Communication no. 6345     

Solidification microstructures and mechanical properties of vertical centrifugal cast high speed steel

Abstract

This study was undertaken to investigate the influence of Nb and V alloying elements and manufacturing conditions on the microstructural behaviour and mechanical characteristics of HSS (high speed steel) roll manufactured by a VCC (vertical centrifugal casting) process. In the Fe - 2C - 6Cr-1.5W - 3Mo - 4V alloy, the amount of MC carbide was increased and the the amount of M₇C₃ carbide decreased with an increase in V and Nb content. In steel containing 3%Nb, primary NbC carbide was formed within the cell in the matrix. The hardness of steel containing 6.5%V but no Nb was increased a little but when 9%V was added, the hardness decreased in the specimen owing to the soft ferritic matrix. The hardness of the matrix in steel containing 1.5%Nb increased, but decreased for 3%Nb addition. In wear tests, wear loss decreased with increasing rotational wear speed.     

Preparation and microwave characterization of BaWO<Subscript>4</Subscript> filled polytetrafluoroethylene laminates for microwave substrate applications

Phase pure BaWO₄ ceramic filler has been prepared through solid state ceramic route. Planar BaWO₄ filled Polytetrafluoroethylene (PTFE) composite substrates were fabricated through Sigma Mixing (SM), Extrusion (E), Calendering (C) followed by Hot pressing (H) (SMECH) processes. Morphology and filler distribution of the composites were analyzed using particle size analysis and scanning electron microscopy. The effect of BaWO₄ ceramic filler content on the dielectric properties of the composites was measured at microwave frequency using X-band waveguide cavity perturbation technique. Optimum BaWO₄ filler loading in the PTFE matrix has been found out as 74 wt%. The moisture absorption characteristics of the composite samples were ascertained as per IPC-TM-650 2.6.2.1 method. PTFE/BaWO₄ composite exhibits a dielectric constant of 4.3 and a loss tangent of 0.004 at optimum filler loading.     

Synthesis,characterization and structural refinement of polycrystalline uranium substituted zirconolite

Ceramic precursors of Zirconolite (CaZrTi₂O₇) family have a remarkable property of substitution on Zr^{4 +} cationic sites. This makes them potential material for nuclear waste management in ‘synroc’ technology. In order to simulate the mechanism of partial substitution of zirconium by tetravalent actinides, a solid phase of composition CaZr_0.95U_0.05Ti₂O₇ has been synthesized through ceramic route by taking calculated quantities of oxides of Ca, Ti and nitrates of uranium and zirconium respectively. Solid state synthesis has been carried out by repeated pelletizing and sintering the finely powdered oxide mixture in a muffle furnace at 1050°C. The polycrystalline solid phase has been characterized by its typical powder diffraction pattern. Step analysis data has been used for ab initio calculation of structural parameters. The SEM and EDAX analysis also confirm that zirconolite acts as a host material for uranium. The powder diffraction data of 3500 points between 2θ = 10–80° has been analysed by GSAS (general structure analysis system) software to obtain the best fit of the observed data points. The uranium substituted zirconolite crystallizes in monoclinic symmetry with space group C2/c (#15). The following unit cell parameters have been calculated: a = 12.4883(15), b = 7.2448(5), c = 11.3973(10) and β = 100.615(9)°. The calculated and observed values of the intensities, lattice parameters and density measurement shows good agreement. The Rietveld analysis and GSAS based calculations for bond distance Ti—O, Ca—O, Zr—O, and O—M—O bond angles have been made. The structure was refined to satisfactory completion.The and Rp and Rwp are found to be 7.48 and 9.74 % respectively.     

Synthesis of ferroelectric perovskites through aqueous-solution techniques

The hydrolysis of niobates in aqueous solutions has been applied to the coprecipitation of PbNb_2/3Mg_1/3O₃ leading to a low temperature synthesis of this ferroelectric relaxor ceramic. The effect of hydrolysis conditions — such as the concentration of bases and acids used, their rates of addition and the nature of the precursors salts — is described. The perovskite phase appears after heating at 350C and is obtained as a pure phase at 800C after heat treatment for 1 h. Relaxor ferroelectric ceramics with a high dielectric constant can be obtained by sintering at 1000C. The process has been also applied to the synthesis of other relaxor ceramic compositions such as PZN (PbNb_2/3Zn_1/3O₃) and PFN (PbNb_1/2Fe_1/2O₃) compounds.