tantalum carbide and hafnium carbide
Thermal Analysis of Tantalum Carbide-Hafnium Carbide Solid
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.
Thermal Analysis of Tantalum Carbide-Hafnium Carbide Solid Solutions from Room Temperature to 1400°C
Plasma Forming Laboratory, Department of Mechanical and Materials Engineering, Florida International University, Miami, 33139 FL, USA
The thermogravimetric analysis on TaC, HfC, and their solid solutions has been carried out in air up to 1400 °C.
Three solid solution compositions have been chosen: 80TaC-20 vol % HfC (T80H20), 50TaC-50 vol % HfC (T50H50), and 20TaC-80 vol % 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;
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 [ 1 , 2, 3, 4 ].
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.
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 [ 5, 6, 7 ].
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 [ 8 , 9 ].
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 [ 10 ].
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 [ 11 , 12 , 13, 14 ].
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.
Recently, Cedillos-Barraza et al.
as well as our research group sintered TaC-HfC solid solutions without sintering additions by spark plasma sintering (SPS) [ 16, 17 ].
The compositions cover the full spectrum of TaC-HfC solid solutions, and both studies noticed that TaC0.5HfC0.5 has the highest hardness and elastic modulus among the solid solutions.
Our group conducted oxidation testing using a plasma jet by exposing these solid solutions and pure carbides to a temperature of ~3000 °C at a gas flow rate of sonic speed [ 18 ].
In general, the solid solutions showed better oxidation resistance than pure carbides.
The best oxidation resistance was found in the TaC0.5HfC0.5 composition.
After 300 s of exposure to such extreme oxidation conditions, the thickness of the oxide scale in TaC0.5HfC0.5 was only 28 μm, which is 1/6 and 1/10 of the oxide scale thickness in pure HfC and TaC, respectively [ 18 ].
The improved oxidation mechanism was explained by a newly formed Hf6Ta2O17 phase.
More importantly, we found a similar dense solid scaffold and liquid phase structure as reported in HfB2-SiC/HfB2-B4C systems that protect the underlying materials [ 18 ].
In the case of TaC-HfC solid solutions, the solid scaffold consists of HfO2 and Hf6Ta2O17, and the liquid phase is made of molten Ta2O5.
Compared to the B2O3 and borosilicate phase in the diboride system, molten Ta2O5 is a much more stable phase with a higher melting point of 1900 °C.
Hence, the carbide solid solutions exhibit exceptional oxidation resistance.
One question arises after the investigation on the plasma jet oxidation behavior of the carbide solid solutions: How would the carbide solid solutions behave below 1800 °C, where the temperature is not high enough to melt the resultant Ta2O5? To address this question, we sought to understand the oxidation behavior of the carbide solid solutions from room temperature to 1400 °C using thermogravimetric analysis (TGA).
Five samples, namely pure TaC, 80TaC-20 vol % HfC (T80H20), 50TaC-50 vol % HfC (T50H50), 20TaC-80 vol % HfC (T20H80), and pure HfC, were chosen.
A detailed analysis of the oxidation behavior is carried out in the present study using TGA followed by scanning electron microscopy (SEM).
2. Experimental Details
Commercial TaC powder (Inframat Advanced Materials LLC, Manchester, CT, USA) and hafnium carbide powder (Materion LLC, Cleveland, OH, USA) were used as starting powders.
Powders for solid solutions were mixed by a high-energy vibratory ball milling machine (Across International LLC, Livingston, NJ, USA) according to their stoichiometric ratio.
Pure powders were milled for one hour separately in a tungsten carbide (WC) jar to breakdown the agglomeration.
Subsequently, TaC and HfC powders were mixed for another hour.
The ball to powder ratio was 1:3, using a 6-mm diameter WC ball.
The mixed TaC-HfC powders were consolidated by a spark plasma sintering (SPS) machine (Model 10-4, Thermal Technologies, Santa Rosa, CA, USA).
The powders were sintered at 1850 °C with a heating rate of 100 °C/min and a maximum uniaxial pressure of 60 MPa.
The holding time was 10 min to ensure the densification.
The environment in the vacuum was set at a pressure of 4 Pa.
The details of the processing can be found in our previous study [ 17 ].
2.2. TGA Testing and Post-Oxidation Characterization
The TGA oxidation testing was conducted on a small portion (~30 mg) of the sintered pellets.
A thermogravimetric analysis (TGA) analyzer (SDT-Q600, TA Instruments, New Castle, DE, USA) was used to evaluate the oxidation performance of TaC, HfC, and TaC-HfC solid solution samples.
Samples were tested in the air at a heating rate of 5 °C/min.
The maximum temperature was 1400 °C for all samples.
The morphologies of the post-oxidation samples were examined by a field emission SEM (JSM-6330F, JEOL Ltd., Tokyo, Japan).
3. Results and Discussions
3.1. Microstructure and Phases in Sintered TaC, HfC, and TaC-HfC Solid Solutions
The detailed characterization results of the microstructures and phases formed in spark plasma-sintered TaC-HfC solid solutions were published in our previous paper [ 17 ].
For the reader’s convenience and the sake of completeness, a summary of the key results is listed in Table 1.
All five samples had high densities varying between 97% and 99%.
With the addition of HfC, the samples’ densification increases, and the highest densification is found in the T20H80 sample.
The average grain size also decreased with the HfC additions.
The lattice parameters of the formed solid solutions matched the theoretical values calculated according to Vegard’s Law.[ 17 ]
3.2. Macro State Morphology of Post-Oxidation TaC-HfC Solid Solutions
The overall qualitative oxidation resistance of TaC, HfC, and their solid solutions can be inferred by the morphology of the post-oxidation samples.
Appearances of the post-oxidation samples from the TGA testing are shown in Figure 2 .
The pure TaC sample showed the worst oxidation resistance, as it turned into a powdery form with no mechanical integrity ( Figure 2 a).
The oxidized pure HfC, on the other hand, displayed a much better oxidation resistance.
Structural integrity can still be seen even though the oxidized sample broke into several pieces ( Figure 2 b).
The post-oxidation samples’ appearances of T80H20 and T20H80 is the combination of oxidation morphology exhibited by pure TaC and HfC.
In the oxidized T80H20 sample ( Figure 2 c), a large amount of powder is noticed with a few solid broken pieces.
The appearance of the post-oxidation T20H80 ( Figure 2 d) is analogous to pure HfC with a small amount of powder.
T50H50 is the only sample that does not show significant spallation and delamination.
This suggests that the outer layer oxide scale has good mechanical integrity and can protect the underlying carbide solid solution.
3.3. Mass Change during Thermogravimetric Analysis of Carbide Solid Solutions
The weight change curves of TaC-HfC solid solutions are presented in Figure 3 as the degree of oxidation (α) with the onset of oxidation temperatures for five samples.
The degree of oxidation (α) is defined as the ratio of the measured weight change over the theoretical weight change at 100% conversion.
The degree of oxidation (α) is calculated by the following equation:∝
The oxidation resistance of tantalum carbide-hafnium carbide
The oxy-carbide layer can act as an oxygen diffusion barrier and protect the underlying carbides. The formed HfO2 has a high melting point around
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 300 m/s for 60 s, 180 s, and 300 s, respectively.
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.
Processing development of 4TaC-HfC and related carbides
Also among the carbides, tantalum carbide and hafnium carbide have outstanding hardness; high melting points (3880˚C and 3890˚C respectively);
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.
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.
The ability to process these materials at relatively low temperatures would save energy and reduce cost.
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.
% of metal diborides with 40 vol.
% of AlMgB14 were produced and characterized.
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