Type of refractory ceramics that can withstand extremely high temperatures without degrading
Ultra-high-temperature ceramics (UHTCs) are a type of refractory ceramics that can withstand extremely high temperatures without degrading, often above 2,000 °C. They also often have high thermal conductivities and are highly resistant to thermal shock, meaning they can withstand sudden and extreme changes in temperature without cracking or breaking. Chemically, they are usually borides, carbides, nitrides, and oxides of early transition metals.
UHTCs are used in various high-temperature applications, such as heat shields for spacecraft, furnace linings, hypersonic aircraft components and nuclear reactor components. They can be fabricated through various methods, including hot pressing, spark plasma sintering, and chemical vapor deposition. Despite their advantages, UHTCs also have some limitations, such as their brittleness and difficulty in machining. However, ongoing research is focused on improving the processing techniques and mechanical properties of UHTCs.
History.
Beginning in the early 1960s, demand for high-temperature materials by the nascent aerospace industry prompted the United States Air Force Materials Laboratory to begin funding the development of a new class of materials that could withstand the environment of proposed hypersonic vehicles such as the Boeing X-20 Dyna-Soar and the Space Shuttle at Manlabs Incorporated. Through a systematic investigation of the refractory properties of binary ceramics, they discovered that the early transition metal borides, carbides, and nitrides had surprisingly high thermal conductivity, resistance to oxidation, and reasonable mechanical strength when small grain sizes were used. Of these, ZrB2 and HfB2 in composites containing approximately 20% volume SiC were found to be the best performing.
UHTC research was largely abandoned after the pioneering mid-century Manlabs work due to the completion of the Space Shuttle missions and the elimination of the Air force spaceplane development. Three decades later, however, research interest was rekindled by a string of 1990s era NASA programs aimed at developing a fully reusable hypersonic spaceplane such as the National Aerospace Plane, Venturestar/X-33, Boeing X-37, and the Air Force's Blackstar program. New research in UHTCs was led by NASA Ames, with research at the center continuing to the present through funding from the NASA Fundamental Aeronautics Program. UHTCs also saw expanded use in varied environments, from nuclear engineering to aluminum production.
In order to test real world performance of UHTC materials in reentry environments, NASA Ames conducted two flight experiments in 1997 and 2000. The slender Hypersonic Aero-thermodynamic Research Probes (SHARP B1 and B2) briefly exposed the UHTC materials to actual reentry environments by mounting them on modified nuclear ordnance Mk12A reentry vehicles and launching them on Minuteman III ICBMs. Sharp B-1 had a HfB2/SiC nosecone with a tip radius of 3.5 mm which experienced temperatures well above 2,815 °C during reentry, ablating away at an airspeed of 6.9 km/s as predicted; however, it was not recovered and its axially-symmetric cone shape did not provide flexural strength data needed to evaluate the performance of UHTCs in linear leading edges. To improve the characterization of UHTC mechanical strength and better study their performance, SHARP-B2, was recovered and included four retractable, sharp wedge-like protrusions called "strakes" which each contained three different UHTC compositions which were extended into the reentry flow at different altitudes.
The SHARP-B2 test that followed permitted recovery of four segmented strakes which had three sections, each consisting of a different HfB2 or ZrB2 composite as shown in Figure 1. The vehicle was successfully recovered, despite the fact that it impacted the sea at three times the predicted velocity. The four rear strake segments (HfB2) fractured between 14 and 19 seconds into reentry, two mid segments (ZrB2/SiC) fractured, and no fore strake segments (ZrB2/SiC/C) failed. The actual heat flux was 60% less than expected, actual temperatures were much lower than expected, and heat flux on the rear strakes was much higher than expected. The material failures were found to result from very large grain sizes in the composites and pure ceramics, with cracks following macroscopic crystal grain boundaries. Since this test, NASA Ames has continued refining production techniques for UHTC synthesis and performing basic research on UHTCs.
Physical properties.
Most research conducted in the last two decades has focused on improving the performance of the two most promising compounds developed by Manlabs, ZrB2 and HfB2, though significant work has continued in characterizing the nitrides, oxides, and carbides of the group four and five elements. In comparison to carbides and nitrides, the diborides tend to have higher thermal conductivity but lower melting points, a tradeoff which gives them good thermal shock resistance and makes them ideal for many high-temperature thermal applications. The melting points of many UHTCs are shown in Table 1. Despite the high melting points of pure UHTCs, they are unsuitable for many refractory applications because of their high susceptibility to oxidation at elevated temperatures.
Table 1. Crystal structures, densities, and melting points of selected UHTCs.
Structure.
UHTCs all exhibit strong covalent bonding which gives them structural stability at high temperatures. Metal carbides are brittle due to the strong bonds that exist between carbon atoms. The largest class of carbides, including Hf, Zr, Ti and Ta carbides have high melting points due to covalent carbon networks although carbon vacancies often exist in these materials; indeed, HfC has one of the highest melting points of any material. Nitrides such as ZrN and HfN have similarly strong covalent bonds but their refractory nature makes them especially difficult to synthesize and process. The stoichiometric nitrogen content can be varied in these complexes based on the synthetic technique utilized; different nitrogen content will give different properties to the material, such as how if x exceeds 1.2 in ZrNx, a new optically transparent and electrically insulating phase appears to form. Ceramic borides such as HfB2 and ZrB2 benefit from very strong bonding between boron atoms as well as strong metal to boron bonds; the hexagonal close-packed structure with alternating two-dimensional boron and metal sheets give these materials high but anisotropic strength as single crystals. Borides exhibit high thermal conductivity (on the order of 75–105 W/mK) and low coefficients of thermal expansion (5–7.8 x 10−6 K−1) and improved oxidation resistance in comparison to other classes of UHTCs. Thermal expansion, thermal conductivity and other data are shown in Table 2. The crystal structures, lattice parameters, densities, and melting points of different UHTCs are shown in Table 1.
Table 2. Thermal expansion coefficients across selected temperature ranges and thermal conductivity at a fixed temperature for selected UHTCs.
Thermodynamic properties.
In comparison with carbide and nitride-based ceramics, diboride-based UHTCs exhibit higher thermal conductivity (refer to Table 2, where we can see that hafnium diboride has thermal conductivity of 105, 75, 70 W/m*K at different temperature while hafnium carbide and nitride have values only around 20W/m*K). Thermal shock resistance of HfB2 and ZrB2 was investigated by ManLabs and it was found that these materials did not fail at thermal gradients sufficient for the failure of SiC; indeed, it was found that hollow cylinders could not be cracked by an applied radial thermal gradient without first being notched on the inner surface. UHTCs generally exhibit thermal expansion coefficients in the range of 5.9–8.3 × 10−6 K−1.The structural and thermal stability of ZrB2 and HfB2 UHTCs results from the occupancy of bonding and antibonding levels in hexagonal MB2 structures with alternating hexagonal sheets of metal and boride atoms. In such structures, the principal frontier electronic states are bonding and antibonding orbitals resulting from bonding between boron 2p orbitals and metal d orbitals; before group (IV), the number of available electrons in a unit cell is insufficient to fill all bonding orbitals, and beyond it they begin to fill the antibonding orbitals. Both effects reduce the overall bonding strength in the unit cell and therefore the enthalpy of formation and melting point. Experimental evidence shows that as one moves across the transition metal series in a given period, the enthalpy of formation of MB2 ceramics increases and peaks at Ti, Zr, and Hf before decaying as the metal gets heavier. As a result, the enthalpies of formation of several important UHTCs are as follows: HfB2 ; TiB2 ; ZrB2 ; TaB2 ; NbB2 ; VB2.
Mechanical properties.
Table 3 lists UHTC carbides and borides mechanical properties. It is extremely important that UHTCs are able to retain high bending strength and hardness at high temperatures (above 2000 °C). UHTCs generally exhibit hardness above 20 GPa due to the strong covalent bonds present in these materials. However, the different methods of processing UHTCs can lead to great variation in hardness values. UHTCs exhibit high flexural strengths of ; 200 MPa at 1,800 °C, and UHTCs with fine-grained particles exhibit higher flexural strengths than UHTCs with coarse grains. It has been shown that diboride ceramics synthesized as a composite with silicon carbide (SiC) exhibit increased fracture toughness (increase of 20% to 4.33 MPam1/2) relative to the pure diborides. This is due to material densification and a reduction in grain size upon processing.
Table. 3 Flexural strength, hardness, and Young's Modulus at given temperatures for selected UHTCs.
The UHTC composites show higher mechanical properties like Tensile strength, Young's modulus, hardness, flexural strength, and fracture toughness at high temperatures as compared to monolithic UHTCs. The high sintering temperature and pressure result in high residual stress in the composites, which can be released at high temperatures. Therefore, the mechanical properties increase with the increase in temperature.
At 1,200 °C, the flexural strength of SiC is 170 MPa vs SiC-ZrC (10 wt%) is 350 MPa. At 2,000 °C, Titanium Carbide's flexural strength is 410 MPa vs TiC-WC (5% vol) is 491 MPa vs TiC-SiC (40% vol) is 543 MPa. Similarly the flexural strength for TaC-SiC (20% vol) is 715 MPa at 1,900 °C which is about 40% higher than TaC (500 MPa) at the same temperature.
The Young's modulus for TiC-WC (3.5 wt%) - CNT(2 wt%) at 1,600 °C is 428 GPa vs 300 GPa for TiC and the flexural toughness of TiC-WC (3.5 wt%) - CNT (2 wt%) at the same temperature is 8.1 MPa m1/2 as compared to TiC which is 3.7 MPa m1/2. For ZrC the fracture toughness at 1,900 °C is 4 MPa m1/2 which increases to 5.8 MPa m1/2 for ZrC-ZrO2(40 wt.%).
The high strength of the materials is obtained due to the high homogeneities of the microstructures and the solute dispersion in the microstructures. 
A significant enhancement in hardness (~30%) of (Hf-Ta-Zr-Nb)C material compared to the monolithic UHTCs (HfC, TaC, ZrC, NbC) and in comparison to the hardest monocarbide (HfC) and the binary (Hf-Ta)C was recorded. The mechanism behind this enhancement in hardness maybe because of bonding behavior or some solid solution hardening effects arising from localized lattice strains.
For applications based on combustion harsh environments and aerospace, Monolithic UHTCs are of concern because of their low fracture toughness and brittle behavior. UHTC composites are a potential approach to overcome these deficiencies.
Chemical properties.
While UHTCs have desirable thermal and mechanical properties, they are susceptible to oxidation at their elevated operating temperatures. The metal component oxidizes to a gas such as CO2 or NO2, which is rapidly lost at the elevated temperatures UHTCs are most useful at; boron, for example, readily oxidizes to B2O3 which becomes a liquid at 490 °C and vaporizes very rapidly above 1,100 °C; in addition, their brittleness makes them poor engineering materials. Current research targets increasing their toughness and oxidation resistance by exploring composites with silicon carbide, the incorporation of fibers, and the addition of rare-earth hexaborides such as lanthanum hexaboride (LaB6). It has been found that the oxidative resistance of HfB2 and ZrB2 are greatly enhanced through the inclusion of 30% weight silicon carbide due to the formation of a protective glassy surface layer upon the application of temperatures in excess of 1,000 °C composed of SiO2. To determine the effect of SiC content on diboride oxidation, ManLabs conducted a series of furnace oxidation experiments, in which the oxidation scale thickness as a function of temperature for pure HfB2, SiC and HfB2 20 v% SiC were compared. At temperatures greater than 2,100 K the oxide scale thickness on pure HfB2 is thinner than that on pure SiC, and HfB2/20% SiC has the best oxidation resistance. Extreme heat treatment leads to greater oxidation resistance as well as improved mechanical properties such as fracture resistance.
Synthesis of diboride (Zr, Hf, Ti) UHTCs.
UHTCs possess simple empirical formulas and thus can be prepared by a wide variety of synthetic methods. UHTCs such as ZrB2 can be synthesized by stoichiometric reaction between constituent elements, in this case Zr and B. This