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. Material Formula Crystal structure Lattice parameters (Å) Density (g/cm3) Melting point a b c (°C) (°F)Hafnium carbonitrideHfCNCubic–––12.654,1107,430 Hafnium carbide HfC FCC 4.638 4.638 4.638 12.76 3,958 7,156 Tantalum carbide TaC Cubic 4.455 4.455 4.455 14.50 3,768 6,814 Niobium carbide NbC Cubic - - - 7.820 3,490 - Zirconium carbide ZrC Cubic 4.693 4.693 4.693 6.56 3,400 6,152 Hafnium nitride HfN FCC 4.525 4.525 4.525 13.9 3,385 6,125 Hafnium boride HfB2 Hexagonal 3.142 – 3.476 11.19 3,380 6,116Thorium dioxideThO2Fluorite - - -10.03,3506,060 Zirconium boride ZrB2 Hexagonal 3.169 – 3.530 6.10 3,245 5,873 Titanium boride TiB2 Hexagonal 3.030 – 3.230 4.52 3,225 5,837 Titanium carbide TiC Cubic 4.327 4.327 4.327 4.94 3,100 5,612 Niobium boride NbB2 Hexagonal 3.085 - 3.311 6.97 3,050 Tantalum boride TaB2 Hexagonal 3.098 – 3.227 12.54 3,040 5,504 Titanium nitride TiN FCC 4.242 4.242 4.242 5.39 2,950 5,342 Zirconium nitride ZrN FCC 4.578 4.578 4.578 7.29 2,950 5,342Magnesium oxideMgOCubic - - -3.62,8525,166 Vanadium carbide VC Cubic - - - 5.77 2,810 unstable -Zirconium dioxideZrO2Monoclinic - - -5.682,7154,919 Tantalum nitride TaN Cubic 4.330 4.330 4.330 14.30 2,700 4,892Calcium oxideCaOCubic - - -3.342,6134,735 Niobium nitride NbN Cubic - - - 8.470 2,573 - Silicon carbide SiC Polymorphic – Various – 3.21 2,545 4,613Aluminum oxideAl2O3HCP4.7504.7504.7503.9872,0723,762 Vanadium nitride VN Cubic - - - 6.13 2,050 unstable ? -
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 × 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. Material Thermal expansion (10−6/K) Temp. range (°C) Thermal cond. (W/mK) Temperature (°C) HfB2–20%SiC 62 1,000 ZrB2–20%SiC 5–7.8 400–1,600 78 1,000 HfN 6.5 20–1,000 22 800 HfC 6.6 20–1,500 30 800 HfB2 7.6 20–2,205 70 800 TiB2 8.6 20–2,205 ZrB2 8.3 20–2,205 TaB2 8.4 1,027–2,027 36.2 2,027 ZrC 5.2 1,027–2,027 TiC 7.7 20–1,500 TaC 6.3 20–1,500 SiC 1.1–5.5 20–1,500 26.3 1,500
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.