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Hafnium Tantalum Carbide

Tantalum 07/10/2020

Tantalum Carbide for Spacecraft

Hafnium Tantalum Carbide

Tantalum Carbide – new shielding material for spacecraft

This is a golden age of science fiction cinema.

If you wish to watch any science fiction cinema, all you need to do is, just wander into one of your local multiplexes and you’re faced with limitless sci-fi movies featuring aliens and superheroes, giant robots and fast-moving spacecraft/hypersonic vehicles with lightning speed.

This ability to travel at incredibly fast speeds is a hallmark of science fiction movies.

In the near future, the fast-moving spacecraft/hypersonic vehicles which we are watching in the cinemas is becoming reality.

Because this is the era with more space research is happening.

And even, the dream of traveling across the galaxy in a few short hours with lightning speed is getting a little closer to reality.

How it’s possible?

How can they withstand scorching temperatures in the space?

If possible, but how?

It is possible!, says scientists from Imperial College London.

In particular, the team from Imperial College London discovered that the melting point of hafnium carbide is the highest ever recorded for a material.

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Being able to withstand temperatures of nearly 4000°C could pave the way for hafnium carbide and tantalum carbide materials to be used in ever more extreme environments.

Tantalum carbide (TaC) and hafnium carbide (HfC) are refractory ceramics, meaning they are extraordinarily resistant to heat.

Their ability to withstand extremely harsh environments means that refractory ceramics could be used in thermal protection systems on high-speed vehicles and as fuel cladding in the super-heated environments of nuclear reactors.

What is Tantalum carbide?

Tantalum carbides form a family of binary chemical compounds of tantalum and carbon.

Tantalum carbides are commercially used in tool bits for cutting applications and are sometimes added to tungsten carbide alloys.

The melting points of a tantalum carbides peak at about 3880 °C depending on the purity and measurement conditions;

this value is among the highest for binary compounds.

Chemical Properties

The bonding between tantalum and carbon atoms in tantalum carbides is a complex mixture of ionic, metallic, and covalent contributions, and because of the strong covalent component, these carbides are very hard and brittle materials.

The hardness, yield stress, and shear stress increase with the carbon content of tantalum carbide.

hafnium tantalum carbide

Thermal Analysis of Tantalum Carbide-Hafnium Carbide Solid Solutions from Room Temperature to 1400°C

The thermogravimetric analysis on TaC, HfC, and their solid solutions has been carried out in air up to 1400°C.

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Three solid solution compositions have been chosen: 80TaC-20vol% HfC (T80H20), 50TaC-50vol% HfC(T50H50), and 20TaC-80vol% HfC(T20H80), in addition to pure TaC and HfC.

Solid solutions exhibit better oxidation resistance than pure carbides.

The onset of oxidation is delayed in solid solutions from 750°C for pure TaC, to 940 °C for the T50H50 sample.

Moreover, T50H50 samples display the highest resistance to oxidation with the retention of the initial carbides.

The oxide scale formed on the T50H50 sample displays mechanical integrity to prevent the oxidation of the underlying carbide solid solution.

The improved oxidation resistance of the solid solution is attributed to the reaction between Ta2O5 and HfC, which stabilizes the volume changes induced by the formation of Ta2O5 and diminishes the generation of gaseous products.

Also, the formation of solid solutions disturbs the atomic arrangement inside the lattice, which delays the reaction between Ta and O.

Both of these mechanisms lead to the improved oxidation resistances of TaC-HfC solid solutions.

Keywords: tantalum carbide; hafnium carbide; solid solutions; oxidation; thermogravimetric analysis tantalum carbide; hafnium carbide; solid solutions; oxidation; thermogravimetric analysis

1. Introduction

The interest in tantalum carbide (TaC) and hafnium carbide (HfC) has been growing in recent years due to their extremely high melting points, high hardness, and elastic moduli, and more importantly, their ability to form solid solutions.

The major applications of these two carbides are leading edges of reentry vehicles and lining materials for rocket thrusters.

In both cases, excellent oxidation resistance is required.

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However, gaseous products like CO and CO2 are inevitably formed during oxidation, which leads to porous oxide scales that delaminate and spall.

The major oxide of TaC is Ta2O5, which has a melting point of ~1900 °C, lower than the desired application temperature of 2000 °C or more.

As a result, the resultant oxide would melt and lose its structural integrity, and fail catastrophically.

To reduce the gaseous products as well as retain the integrity of oxide scales under extremely high temperatures, Hafnium diboride (HfB2) and its composites have been investigated as promising candidate materials for use on next-generation hypersonic vehicles.

During oxidation, HfB2 forms a solid scaffold-like structure that mainly consists of HfO2 and molten B2O3 infiltrated between the HfO2.

The resultant oxide scale is dense and crack-free, which provides exceptional oxidation resistance.

Unfortunately, the B2O3 starts to evaporate around 700°C, and therefore its protection of the underlying materials is lost.

The SiC addition was used to stabilize the B2O3 by forming a borosilicate glass phase, which increases the onset evaporation temperature to 1400 °C.

However, 1400 °C is still not high enough to withstand higher application temperatures of 2000 °C or more.

The studies on the solid solutions of TaC-HfC began with the discovery of a TaC0.8HfC0.2 phase that possesses the highest melting point (~4000 °C) of known substances.

Preliminary oxidation studies have been carried out on TaC0.8HfC0.2 and HfC-rich compositions, but no improvement in the oxidation behavior was observed compared to pure carbides.

Additionally, sintering aids were inevitable in those studies, which introduced secondary phases that clouded the understanding of oxidation behaviors.

Although TaC and HfC can form solid solutions above 887°C in all compositions, as shown in the phase diagram in Figure 1, oxidation studies on TaC-HfC solid solutions have barely been investigated.

as well as our research group sintered TaC-HfC solid solutions without sintering additions by spark plasma sintering (SPS).

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The compositions cover the full spectrum of TaC-HfC solid solutions, and both studies noticed that TaC0.5HfC

hafnium tantalum carbide

Advanced missile defense interceptors will provide our nation with the capability of defeating threats to our homeland and our deployed troops.

However, the fielding of these advanced interceptors is strongly dependent upon technologies that enable the production of interceptor boost nozzles capable of surviving extreme temperatures and corrosive environments with minimal erosion.

(PPI) has already produced high-quality vacuum plasma sprayed tantalum carbide (TaC) material.

Analytical models predict that PPI’s TaC will survive subscale hot fire testing.

In the proposed effort, the knowledge learned during the TaC studies will be applied to the development and characterization of materials superior to TaC, such as tantalum-hafnium-carbide (Ta4HfC5) and tantalum-hafnium-carbonitride (Ta4HfC3N2).

Ta4HfC5 has a melting temperature up to 265°C higher than TaC, and Ta4HfC3N2 is less susceptible to thermal shock due to its expected higher thermal conductivity than TaC.

Development, characterization, and testing of Ta4HfC5 and Ta4HfC3N2 is proposed to 1) increase the fundamental scientific knowledge of these ultra-high temperature materials;

2) produce material property data to be utilized in existing performance models;and 3) determine if these materials are superior to TaC for employment as boost nozzle components.

The proposed material development and low-cost fabrication techniques will ultimately lead to improved performance and reduced costs for advanced missile defense interceptors.

Carbides, nitrides, and borides ceramics are of interest for many applications because of their high melting temperatures and good mechanical properties.

Wear-resistant coatings are among the most important applications for these materials.

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Materials with high wear resistance and high melting temperatures have the potential to produce coatings that resist degradation when subjected to high temperatures and high contact stresses.

Among the carbides, Al4SiC4 is a low density (3.03 g/cm3), high melting temperature (>2000˚C) compound, characterized by superior oxidation resistance, and high compressive strength.

These desirable properties motivated this investigation to (1) obtain high-density Al4SiC4 at lower sintering temperatures by hot pressing, and (2) to enhance its mechanical properties by adding WC and TiC to the Al4SiC4.

Also among the carbides, tantalum carbide and hafnium carbide have outstanding hardness;

high melting points (3880˚C and 3890˚C respectively);

good resistance to chemical attack, thermal shock, and oxidation;

and excellent electronic conductivity.

Tantalum hafnium carbide (Ta4HfC5) is a 4-to-1 ratio of TaC to HfC with an extremely high melting point of 4215 K (3942˚C), which is the highest melting point of all currently known compounds.

Due to the properties of these carbides, they are considered candidates for extremely high-temperature applications such as rocket nozzles and scramjet components, where the operating temperatures can exceed 3000˚C.

Sintering bulk components comprised of these carbides is difficult, since sintering typically occurs above 50% of the melting point.

Thus, Ta4HfC5 is difficult to sinter in conventional furnaces or hot presses;

furnaces designed for very high temperatures are expensive to purchase and operate.

Our research attempted to sinter Ta4HfC5 in a hot press at relatively low temperatures by reducing powder particle size and optimizing the powder-handling atmosphere, milling conditions, sintering temperature, and hot-pressing pressure.

Also, WC additions to Ta4HfC5 were found to improve densification and increase microhardness.

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The ability to process these materials at relatively low temperatures would save energy and reduce costs.

Boron-based hard materials are used in numerous applications such as industrial machining, armor plating, and wear-resistant coatings.

It was often thought that in addition to strong bonding, super-hard materials must also possess simple crystallographic unit cells with high symmetry and a minimum number of crystal defects (e.g., diamond and cubic boron nitride (CBN)).

However, one ternary boride, AlMgB14, deviates from this paradigm;

AlMgB14 has a large, orthorhombic unit cell (oI64) with multiple icosahedral boron units.

TiB2 has been shown to be an effective reinforcing phase in AlMgB14, raising hardness, wear resistance, and corrosion resistance.

Thus, it was thought that adding other, similar phases (i.e., ZrB2 and HfB2) to AlMgB14 could lead to useful improvements in properties vis-à-vis pure AlMgB14.

Group IV metal diborides (XB2, where X = Ti, Zr, or Hf) are hard, ultra-high temperature ceramics.

These compounds have a primitive hexagonal crystal structure (hP3) with planes of graphite-like boride rings above and below planes of metal atoms.

Unlike graphite, there is a strong bonding between the planes, resulting in high hardness.

For this study two-phase composites of 60 vol.% metal diborides with 40 vol.

Oxidation resistance of tantalum carbide-hafnium carbide solid

The oxidation behaviors of tantalum carbide (TaC) - hafnium carbide (HfC) solid solutions with five different compositions, pure HfC, HfC-20 vol% TaC (T20H80), HfC- 50 vol% TaC (T50H50), HfC- 80 vol% TaC (T80H20), and pure TaC have been investigated by exposing to a plasma torch which has a temperature of approximately 2800 °C with a gas flow speed greater than 300m/s for the 60s, 180s, and 300s, respectively.

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The solid solution samples showed significantly improved oxidation resistance compared to the pure carbide samples, and the T50H50 samples exhibited the best oxidation resistance of all samples.

The thickness of the oxide scales in T50H50 was reduced by more than 90% compared to the pure TaC samples, and more than 85% compared to the pure HfC samples after 300 s oxidation tests.

A new Ta2Hf6O17 phase was found to be responsible for the improved oxidation performance exhibited by solid solutions.

The oxide scale constitutes a scaffold-like structure consisting of HfO2 and Ta2Hf6O17 filled with Ta2O5 which was beneficial to the oxidation resistance by limiting the availability of oxygen.

Tantalum carbide (TaC) and hafnium carbide (HfC) possess extremely high melting points, around 3900 °C, which is the highest among the known materials.

TaC and HfC exhibit superior oxidation resistance under oxygen-deficient and rich environments, respectively.

A versatile material can be expected by forming solid solutions of TaC and HfC.

However, the synthesis of fully dense solid solution carbide is a challenge due to their intrinsic covalent bonding which makes sintering challenging.

The aim of the present work is to synthesize full dense TaC-HfC solid solutions by spark plasma sintering with five compositions: pure HfC, HfC-20 vol.% TaC (T20H80), HfC- 50 vol.% TaC (T50H50), HfC - 80 vol.% TaC (T80H20), and pure TaC.

To evaluate the oxidation behavior of the solid solutions carbides in an environment that simulate the various applications, an oxygen-rich, plasma-assisted flow experiment was developed.

While exposed to the plasma flow, samples were exposed to a temperature of approximately 2800 °C with a gas flow speed greater than 300 m/s.

Density measurements confirm near full density was achieved for all compositions, with the highest density measured in the HfC-contained samples, all consolidated without sintering aids.

Confirmation of the solid solution was completed using x-ray diffraction, which had an excellent match with the theoretical values computed using Vegard’s Law, which confirmed the formation of solid solutions.

The solid solution samples showed much-improved oxidation resistance compared to the pure carbide samples, and the T50H50 samples exhibited the best oxidation resistance of all samples.

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The thickness of the oxide scales in T50H50 was reduced by more than 90% compared to the pure TaC samples, and more than 85% compared to the pure HfC samples after 5 min oxidation tests.

A new Ta2Hf6O 17 phase was found to be responsible for improved oxidation performance.

Additionally, the structure of the HfO2 scaffold filled with molten Ta2O 5 was also beneficial to the oxidation resistance by limiting the availability of oxygen.

The addition of tantalum carbide. (TaC) in the HfC matrix was studied to improve the microstructure. The microstructure of. HfC, TaC, and co-deposited hafnium ...

Hafnium carbide is proposed as a structural material for aerospace applications at ultra-high temperatures.

The chemical vapor deposition technique was used as a method to produce monolithic hafnium carbide (HfC) and tantalum carbide (TaC).

The microstructure of HfC and TaC was studied using analytical techniques.

The addition of tantalum carbide (TaC) in the HfC matrix was studied to improve the microstructure.

The microstructure of HfC, TaC, and co-deposited hafnium carbide-tantalum carbide (HfC/TaC) were comparable and consisted of large columnar grains.

Two major problems associated with HfC, TaC, and HfC/TaC as a monolithic are lack of damage tolerance (toughness) and insufficient strength at very high temperatures.

A carbon fiber reinforced HfC matrix composite has been developed to promote graceful failure using a pyrolytic graphite interface between the reinforcement and the matrix.

The advantages of using carbon fiber reinforcement with a pyrolytic graphite interface are reflected in superior strain capability reaching up to 2%.

The tensile strength of the composite was 26 MPa and needs further improvement.

Heat treatment of the composite showed that HfC did not undergo any phase transformations and that the phases comprising composite were are thermochemically compatible.

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