Interdiffusion of hafnium carbide and titanium carbide during hot-pressing

The interdiffusion of HfC and TiC during hot pressing was investigated using concentration profiles obtained via EDS in a SEM. The results were interpreted using the Boltzmann–Matano method and the integrals and derivatives were determined via numerical means. The microstructural evolution of two materials with different HfC/TiC is then explained in terms of the rate of material movement.     

Tungsten carbide platelet-containing cemented carbide with yttrium containing dispersed phase

A fine and platelet tungsten carbide patterned structure with fine yttrium containing dispersed phase was observed in liquid phase sintered WC-20% Co-1%Y2O3 cemented carbide with ultrafine tungsten carbide and nano yttrium oxide as starting materials. By comparing the microstructures of the alloy prepared by hot-press at the temperature below the eutectic melting temperature and by conventional liquid phase sintering, it is shown that hexagonal and truncated trigonal plate-like WC grains are formed through the mechanism of dissolution-precipitation （recrystallization） at the stage of liquid phase sintering. Yttrium in the addition form of oxide exhibits good ability in inhibiting the discontinuous or inhomogeneous WC grain growth in the alloy at the stage of solid phase sintering.     

Mechanochemical combustion synthesis of vanadium carbide (VC), niobium carbide (NbC) and tantalum carbide (TaC) nanoparticles

The nanoparticles of vanadium, niobium, and tantalum carbides were synthesized by a mechanically induced magnesiothermic combustion in the separate Mg/V₂O₅/C, Mg/Nb₂O₅/C, and Mg/Ta₂O₅/C systems. Initial materials in these systems ignited after short milling times of 10, 10, and 23 min, respectively. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM) and elemental mapping techniques were employed to characterize the combustion products. In this process, magnesium reduces initial oxides to generate elemental V/Nb/Ta to react with carbon, forming the carbide phases.     

Oxidation of hafnium carbide and hafnium carbide with additions of tantalum and praseodymium

The oxidation behavior of HfC, HfC-25 wt. % TaC, and HfC-7 wt.% PrC₂ has been studied between 1200–2200° C. Parabolic growth of the oxide layer has been observed for both HfC and HfC-TaC over the entire temperature range. A break in the temperature dependence of the oxidation kinetics occurs around 1600°C. At lower temperatures, the kinetics are limited by gaseous diffusion via pores in the oxide. Above 1800°C, gaseous diffusion through pores becomes less important as scale-growth kinetics are dominated by bulk (ambipolar) diffusion of oxygen and electrons through the oxide.     

Coatings enrichment by carbide dissolution

Surface welding of hard alloys allows for the protection of components exposed to severe wear. Nevertheless, deposition of these hard alloys requires special procedures in order to minimize cracks and other welding defects. This work evaluated an alternative procedure to reduce welding defects as hard surfaces were produced. For that purpose fine WC-Co were mixed with iron powders and with an atomized cobalt based alloy, respectively, and processed with Plasma Transferred Arc (PTA). Powder mixtures with 5 wt.% and 35 wt.% WC-Co were deposited with two current intensities 150 A and 170 A on carbon steel plates. It was aimed to melt and dissolve the carbides to produce defect free hard iron based and cobalt based coatings. Surfaces were characterized for their soundness, hardness and microstructure regarding the effect of powder mixture chemical composition and current intensity used during processing. Visual inspection, dilution measurements, X-ray diffraction of powder mixtures and coatings, Vickers microhardness, optical and Scanning Electron Microscopy and pin-on-disc tests were used for surface evaluation. Results showed that PTA deposition allowed for the enrichment of coatings as carbides were melted in the plasma arc and dissolve in the different matrix.     

Sintering of alumina-niobium carbide composite

Several studies have been focused on particulate-dispersed Al₂O₃ composites in order to improve both room and high temperature mechanical properties and wear resistance. In the present work Al₂O₃-NbC composites have been pressureless sintered and their microstructures analysed as a function of NbC and Y₂O₃ concentration, the latter added as sintering aid. The compositions used in this study were Al₂O₃-xNbC and (Al₂O₃ 3%Y₂O₃)-xNbC, (x = 10, 20 and 40 wt%) and the sintering was performed at 1650 °C/30 min and 1750 °C/15 min. A density greater than 96% of the theoretical density was reached even for those materials sintered at 1650 °C. The observed microstructure was more homogeneous for the samples with Y₂O₃ addition and the Y₃Al₅O₁₂ phase was detected. The Al₂O₃ grain growth restraining due to the NbC concentration was more pronouncedly in samples sintered at 1750 °C.     

Oxidation resistant tungsten carbide hardmetals

We present a new method for retarding the oxidation rate of hardmetals. By diffusion impregnating a WC-FeCr hardmetal with silicon, we manufacture two-layered silicide coatings consisting of an FeSi_x outer crust and WSi₂ beneath. The structure results from a preferential reaction between silicon and the metallic binder. The FeSi_x outer layer is crucial to providing oxidation resistance as when exposed to oxygen it passivates, forming a protective SiO₂ surface film – while simultaneously preventing exposure of the underlying WSi₂, which is known to oxidise in an active manner. Our analysis shows the coating method is applicable to various hardmetals structures.     

硬质合金组合轧辊的结构分析

针对棒材轧机应用硬质合金轧辊时，辊环结构及辊环与轧辊轴连接方式存在的问题，介绍了国外3种获成功应用的硬质合金组合轧辊的结构、使用材质、安装和设计特点。     

Microstructure of vacuum plasma-sprayed boron carbide

In the present study, boron carbide was deposited on Ti-6%Al-4%V alloy by vacuum plasma spraying. Chemical and phase compositions of the initial starting powder and the as-sprayed deposit were characterized using hot gas extraction, x-ray photoelectron spectroscopy, Raman spectroscopy, x-ray diffraction, and transmission electron microscopy. Mechanical properties of the deposition were assessed by microhardness and nanohardness indentation. The microstructure consisted of equiaxed boron carbide grains, microcrystalline particles, and amorphous carbon regions. The amount of boron oxide and amorphous carbon in the coating was increased compared with the initial powder. The measured microhardness was slightly higher than values reported previously (1033±200 HV). There was significant variation in measured nanohardness (−100 + 39 GPa) from point to point caused by multiple phases, splat boundaries, and porosity in the deposited structure. Carbon segregation to grain boundaries and/or splat boundaries in boron carbide was observed directly using spatially resolved electron energy loss spectroscopy method.     

Electrochemical corrosion of silicon carbide ceramics

Recently, SiC‐based ceramics have been found to exhibit corrosion damage patterns, which can only be explained by electrochemical processes. Therefore, the current work focusses on the electrochemical test procedures to determine the corrosion behaviour of solid state sintered silicon carbide (SSiC) ceramics in acidic and alkaline media. The corrosion current densities have been determined from linear voltammetric scans. At anodic polarization potentials, electrochemically induced etching patterns were observed in alkaline solution. The formation of pores and crevices during electrochemical oxidation in acidic solution could be monitored by field emission scanning electron microscopy (FESEM) in addition to transmission electron microscopy (TEM). Impedance spectra measured after anodic polarization could be described by the assumption of a pore model equivalent circuit.     

Damage-free machining of monocrystalline silicon carbide

Cutting tests of monocrystalline SiC, on the surface of which an amorphous layer was preformed by ion implantation, were performed. Ductile-mode machining was observed at a depth of cut smaller than 60 nm. At a depth of cut larger than 60 nm, cracks were observed on the work surface. However, transmission electron micrographs show that crack propagation was obstructed at the interface between the amorphous and crystalline layers even under brittle-mode machining, and no subsurface damage extended into the crystalline layer. The results suggest that the damage-free machining of monocrystalline SiC is possible by surface modification to an amorphous structure.     

Aluminum-silicon carbide coatings by plasma spraying

An aluminum base composite (Al-SiC) powder has been developed for producing plasma sprayed coatings on Al and other metallic substrates. The composite powders were prepared by mechanical alloying of 6061 Al alloy with SiC particles. The concentration of SiC was varied between 20 and 75 vol%, and the size of the reinforcement was varied from 8 to 37 μm in the Al-50 vol% SiC composites. The 44 to 140 μm composite powders were sprayed using an axial feed plasma torch. Adhesion strength of the coatings to their substrates were found to decrease with increasing SiC content and with decreasing SiC particle sizes. The increase in the SiC content and decrease in particle size improved the erosive wear resistance of the coatings. The abrasive wear resistance was found to improve with the increase in SiC particle size and with the SiC content in the composite coatings.     

具有梯度结构的涂层硬质合金刀片

研究了Co和Ti(CN)含量对梯度硬质合金的力学性能、梯度结构的影响.测试了梯度硬质合金刀片的切削性能.结果表明:随着合金钴含量的增多,合金梯度层厚度增厚,梯度结构越明显;合金的强度提高,磁饱和提高,磁力、硬度和密度减小.随着合金Ti(CN)含量的增多,合金梯度层厚度有变薄的趋势;随合金的硬度提高,合金的强度和密度减小.切削试验表明:具有梯度结构涂层硬质合金刀片的切削性能比无梯度结构涂层硬质合金刀片的切削性能优良.达到同一磨损高度hB=0.15 mm时,前者的切削寿命较后者提高了近一倍.同时随着合金钴含量的增多,硬质合金刀片的切削性能提高.     

Elastic modulus of nanocrystalline cemented carbide

The purposes of this work were to obtain the accurate elastic modulus of the nanocrystalline WC–Co cemented carbides, and to propose the mechanism for the difference of elastic modulus between the nanocrystalline and conventional polycrystalline cemented carbides. The nanocrystalline cemented carbide was prepared by spark plasma sintering (SPS) technique. The conventional polycrystalline cemented carbides were prepared by SPS and sinter-HIP techniques as references, respectively. The sintered cemented carbides were characterized by X-ray diffractometry, scanning electron microscopy and the transmission electron microscopy with precession electron diffraction technology. The elastic modulus was obtained by averaging the values measured with the continuous stiffness measurement method of the nanoindentation technology. The results show that the nanocrystalline cemented carbide has a relatively low modulus, which could be attributed to the more interface area and higher fraction ratio of the hcp cobalt phase caused by the rapid heating and cooling process during SPS.