Tungsten Carbide Ceramics Research Articles

Oscillatory pressure sintering of binderless tungsten carbide

Tungsten carbide ceramics were fabricated by a novel oscillatory pressure sintering process and the influence of sintering temperature on the microstructure and mechanical properties of tungsten carbide ceramics was investigated. It was found that tungsten carbide would decarbonize to form W2C under high temperature environment. As the sintering temperature increased, the density and hardness of tungsten carbide ceramics increased apparently, and the growth of tungsten carbide grains became more obvious. The samples fabricated by oscillatory pressure sintering exhibited superior mechanical properties in comparison with some published data obtained by other pressure-assisted sintering methods. The optimum sintering temperature for tungsten carbide ceramics under oscillatory pressure was considered to be 1900 °C. Flexural strength, hardness and fracture toughness of the corresponding sample prepared at this temperature were found to be 1014 MPa, 26.98 GPa and 5.91 MPa·m1/2, respectively.

The Influence of the Cobalt Content on the Strength Properties of Tungsten Carbide Ceramics under Dynamic Loads

Based on the registration and analysis of the full wave profiles, the Hugoniot elastic limit and spall strength of ceramics based on tungsten carbide with different cobalt content are measured. We also study the influence of the cobalt content on the mechanical characteristics of tungsten carbide such as hardness, fracture strength, Young’s modulus, shear modulus, and sound velocity. It is shown that in the process of spalling, the failure stresses grow and the dynamic elastic limit decreases almost linearly within the scatter of their values with growing cobalt content; moreover, the value of the Hugoniot elastic limit is abruptly practically halved as the cobalt content grows from 0 to 2 wt %.

Evaluation of microstructure and growth kinetics of tungsten carbide ceramics at the interface of iron and tungsten

The microstructure evolution and growth kinetics of tungsten carbide (WC or W2C) ceramics at the interface of iron and tungsten during isothermal annealing process from 1100 °C to 1150 °C were investigated by X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and scanning electron microscopy (SEM), respectively. The results show that carbide formation that involves W2C, WC and a small quantity of Fe2W2C, is strongly dependent on a diffusion-controlled reaction. During the evolution process of tungsten carbides, W2C emerges as an initial phase by the reaction of W and C, and WC is formed as the second phase, because the value of Gibbs free energy of formation of W2C is more negative than that of WC at the annealing temperature. The initial W2C transforms into WC until complete consumption of the W2C. Additionally, Fe2W2C between W2C and WC was formed at a relatively low temperature (1100 °C and 1125 °C) with a long annealing time (60 min). After the W2C is consumed completely, WC grows by C diffusion, where the growth kinetics of a dominant WC ceramic exhibits a parabolic growth law before metal tungsten is completely consumed. The growth activation energy of the WC layer is calculated to be achieved with a value of 208.68 kJ/mol. When the metal tungsten is completely consumed after annealing at 1150 °C for 60 min, the W2C and Fe2W2C disappear, and only the WC phase exists in the final product.

In situ reactive synthesis and plasma-activated sintering of binderless WC ceramics

ABSTRACTBinderless micrometre tungsten carbide ceramics (∼1.1 μm) were in situ synthesised and densified by plasma-activated sintering (PAS) from mixed powders of tungsten trioxide and carbon black. The influence of sintering process and powders’ composition ratio on the phase composition, microstructure and mechanical properties of as-prepared samples was clarified in detail. The phase evolution was ascertained by X-ray diffraction to be WO3→WO2.72→WO2→W2C→WC, with the formation of CO and CO2 gases. The sample with nearly single WC phase, dense structure and excellent mechanical properties was fabricated under the optimised process with the proper composition ratio. Owing to the micrograin size, the fracture toughness (8.88 MPa m1/2) was enhanced while high hardness (2159 HV10) was maintained, in the absence of any ceramic toughening phase.

Fundamentals of sintering nanoscaled binderless hardmetals

By reducing the metallic binder in cemented carbides, so-called binderless hardmetals or pure tungsten carbide ceramics can ultimately be achieved. Nanoscaled tungsten carbide has a very high hardness of above 3000HV1 and still offers quite a good fracture toughness of up to 7.5MPa∗m1/2. Since only solid state sintering can be used, sintering temperatures as high as 2000°C or higher are normally needed to completely densify these materials. At these temperatures abnormal grain growth which leads to a decrease in hardness is an often reported problem. However, using pressure assisted techniques such as SPS, ROC or HIP, WC ceramics can be produced at lower temperatures, although productivity and freedom in size and form are limited.The aim of this work was to investigate the drivers for densification, grain growth as well as the microstructure and chemical phases occurring during solid state sintering in a conventional SinterHIP furnace. A WC starting powder with DBET=115nm was milled to nanoscaled size (50 to 60nm), sintered at different temperatures (interrupted sintering experiments) and then analysed by using FESEM, XRD, Rietveld analysis, dilatometry and other methods.It could be shown that three main processes during sintering of slightly under-stoichiometric binderless WC hardmetals occur: first, the reduction of surface oxides at 800°C after which densification starts due to cleaned surfaces and the existence of W in a nascent state; second, the secondary carburisation of WC and W at around 1400°C; and, third, grain growth and pore annealing at around 1600°C when a closed porosity exists. By adding Cr3C2 as grain growth inhibitor the secondary carburisation step was lowered to around 1000°C and a solid solution of (W,Cr)2 instead of pure W2C was formed.

Grain growth during sintering of tungsten carbide ceramics

Grain growth and abnormal grain growth in tungsten carbide cobalt composites (cemented carbides, hardmetals) are usually discussed with respect to liquid phase sintering (Ostwald ripening). Densification and grain growth during solid state sintering are not as thoroughly studied but do play an important role in sintering hardmetals and, particularly tungsten carbide ceramics (binderless hardmetals). In this work the influences of sintering temperature, carbon content, additions of grain growth inhibitors, defects and dislocations (microstrain) introduced by milling on the densification and microstructure of WC ceramics were studied including density, micro structural, thermal and X-ray analysis. Microstrain promotes densification and results in lowering the sintering temperature, whereas free carbon seems to hinder densification at low temperatures and to promote it slightly at higher temperatures. Depending on sintering regime, free carbon and microstrain may drastically boost abnormal grain growth. By adding grain growth inhibitors, densification is shifted to higher temperatures. However, the addition prevents abnormal grain growth regardless of C-content and microstrain. Like in hardmetals grain growth inhibitors also inhibit normal grain growth. The findings are relevant for sintering of WC ceramics and hardmetals alike.

Fabrication of Nanosized Tungsten Carbide Ceramics by Reactive Spark Plasma Sintering

Binderless tungsten carbide ceramics were produced by reactive spark plasma sintering, using tungsten nitride and carbon black as starting materials. The phase and microstructure evolution during the sintering process was investigated. It was found that the tungsten nitride decomposed into nano tungsten with a particle size of about 25 nm firstly, and then reacted with carbon black to form carbides. Due to the high activity of the in situ formed carbides, samples could be densified at a temperature as low as 1350°C. The bulk carbides possessed an average grain size of 270 nm and Vickers’ hardness of 25.4 GPa.

Ballistic development of U.S. high density tungsten carbide ceramics

The United States and France, under a cooperative research project agreement are developing a new class of high density ceramics which inherently provide high space efficiency and reduced susceptibility to damage accumulation effects in thick sections. While many ceramics were considered, this research has focused on tungsten carbide based ceramics. The U.S. Army Research Laboratory, in cooperation with Cercom Inc. has developed a hot-pressed tungsten carbide ceramic for ballistic applications. This paper will present a survey of high density ceramics, document the mechanical and elastic properties of the U.S. WC ceramic and baseline the ballistic performance.

Metal Free Tungsten Carbide Ceramics

Over the past few decades, cemented carbides have been used in applications requiring high hardness, toughness, and strength. However, the metallic binder phase present in cemented ceramics can result in poor mechanical properties at high temperatures, and has proved susceptible to corrosion and erosion in aggressive environments. Metal free tungsten carbide ceramics developed by VM Advanced Carbides provide improved mechanical properties and chemical stability, and can be used in a variety of high temperature applications.

4828584 Dense, fine-grained tungsten carbide ceramics and a method for making the same : Raymond A Cutler assigned to Ceramatec Inc

4828584 Dense, fine-grained tungsten carbide ceramics and a method for making the same : Raymond A Cutler assigned to Ceramatec Inc