An Introduction to Ultra-High Temperature Ceramics

Ultra-high temperature ceramics (UHTCs) are a class of materials that can be used in environments that exhibit extremes in temperature, chemical reactivity, erosive attack, etc.¹ Extreme environments could be considered as being encountered in applications including handling of molten metals and electrodes for electric arc furnaces, but this article will focus on materials being examined for aerospace applications such as hypersonic flight, scramjet propulsion, rocket propulsion, and atmospheric re-entry.

Figure 1. Conceptual design for the X43-A, a reusable hypersonic aerospace vehicle that would utilize UHTC leading edges and control surfaces. Image courtesy of NASA.

A variety of criteria can be used to define UHTCs including ultimate use temperature, environmental resistance, and strength at elevated temperature. For this article, UHTCs are defined as compounds with melting temperatures in excess of 3000°C.² Using the melting temperature criterion, only a few materials can be classified as UHTCs, most of which are borides, carbides, or nitrides of early transition metals.

Table 1 lists some compounds that meet the melting temperature criterion. From the 1950s through about 1970, many of these compounds were studied extensively in the U.S. and U.S.S.R. for potential aerospace applications. After a period of relative inactivity, research on UHTCs has experienced a resurgence in recent years with significant efforts in countries including China, Japan, Italy, Ukraine, and the United States, among others.

Table 1. Some ceramics with reported melting temperatures of 3000°C and higher. Melting temperatures estimated from phase diagrams.³

The remainder of this article discusses the two main classes of UHTCs that are being considered for aerospace applications, the borides and the carbides.

Borides

Boride ceramics offer an unusual combination of ceramic-like properties including high melting temperature (>3000°C), elastic modulus (~500 GPa), and hardness (>20 GPa) with metallic characteristics such as high electrical conductivity (~10⁷ S/m) and thermal conductivity (60-120 W/m•K).⁴ This combination of properties makes UHTCs attractive for applications such as the leading edges of hypersonic aerospace vehicles and atmospheric re-entry vehicles, which require materials to retain their shape at temperatures in excess of 2000°C.

Further, these applications require high thermal conductivity as the primary method for removing heat from the sharp leading edge via conduction through the ceramic. From the larger list of borides, ZrB₂ and HfB₂ have received the most attention as potential candidates for leading edge materials because their oxidation resistance is superior to the other borides due to the stability of the ZrO₂ and HfO₂ scales that form on these materials at elevated temperatures in oxidizing environments.²

Often, the borides are combined with other refractory phases such as SiC or MoSi₂ to improve the strength and oxidation resistance.⁵ Traditionally, borides and boride-based particulate composites have been densified by hot pressing at temperatures of 2000°C or higher. More recently, additives such as C, B₄C, and MoSi₂ have been used to devise pressureless sintering methods that allow near-net shape forming of diboride ceramics.^6,7 The keys to pressureless densification appear to be the use of starting particles with high purity and an average size of 2 µm or less combined with additives to react with and remove oxide impurities present on the surfaces of powder particles.

Figure 2>. Image showing rectangular and circular plates prepared by pressureless sintering or ZrB2 powders.

The room temperature flexure strengths for ZrB₂ and HfB₂ ceramics are typically in the range of 300 to 500 MPa. When second phases such as SiC or MoSi₂ are present in volume fractions of 10% or higher, room strengths in the range of 800 MPa to 1000 MPa or higher have been reported.^8,9 Because the proposed applications will expose these ceramics to temperatures of 2000°C or above, one of the key issues with boride ceramics is the retention of strength at elevated temperature.

As temperature increases to ~1000°C, the strength of fine-grained boride ceramics (including those containing particulate reinforcements such as SiC or MoSi₂) tend to increase slightly. However, as temperatures reach the range of 1000°C to 1200°C, strength typically decreases dramatically, often falling by 50% or more. One of the few reports that has shown strength retention to temperatures as high as 1500°C involved spark plasma sintering of a HfB₂-based ceramic.¹⁰ Further research is needed to understand strength and other elevated temperature properties of boride ceramics.

Carbides

Compared to borides, carbide ceramics tend to have higher melting temperatures (typically 200°C or more higher than the corresponding boride) and lower values of thermal and electrical conductivities (electrical conductivity for ZrC is 10⁶ S/m compared to 10⁷ S/m for ZrB₂).¹¹ In particular, TaC is thought to have the highest melting temperature of any material at 3997°C. Carbides tend to have lower oxidation resistance at intermediate temperatures due to the formation of CO gas as one of the oxidation products.¹² However, carbide ceramics such as TaC have shown promise for use in environments that include a combination of ultra-high temperature, reactive phases, and erosion such as throats for solid rocket nozzles.

Carbides are typically used as nominally single phase ceramics to maximize the melting temperatures by avoiding reactions, solution formation, or the formation of eutectics. The high melting temperatures combined with low self-diffusion coefficients makes densification of carbides difficult, or in some cases impossible, using conventional hot pressing of commercially available powders.

A further impediment to densification is the apparent overlap of the temperature regimes in which densification and grain coarsening occur, which can lead to the formation of porosity entrapped within individual grains in polycrystalline ceramics.¹³ The result is that some carbides reach what appears to be a limiting density where further increases in temperature can, in some cases, actually lead to a decrease in the relative density of the resulting ceramic.

Outlook

The development of ultra-high temperature ceramics for aerospace applications continues around the globe. While significant progress has been made in recent years in understanding fundamental microstructure-processing-property relationships in these materials, further work is needed to develop UHTCs for applications such as sharp leading edges for hypersonic aerospace vehicles and propulsion components for rocket motors. Development is likely to be driven by "market pull" based on applications where performance requirements necessitate the use of ceramics due to some combination of temperature requirements, weight savings compared to heavier refractory metals, or use of simpler passive designs as opposed to more complex actively cooled components.

References

1. E. Wuchina, E. Opila, M. Opeka, W. Fahrenholtz, and I. Talmy, "UHTCs: Ultra-High Temperature Ceramic Materials for Extreme Environment Applications," Interface, 16(4) 30-36 (2007).
2. W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, and J.A. Zaykoski, "Refractory Diborides of Zirconium and Hafnium," Journal of the American Ceramic Society, 90(5) 1347-1364 (2007).
3. Data collected from Phase Diagrams for Ceramists, Vol. X, ed. By A.E. McHale, American Ceramic Society, Westerville, OH (1994) and Phase Diagrams for Ceramists, Volume 1, ed. By E.M. Levins, C.R. Robbins, and H.F. McMurdie, The American Ceramic Society, Columbus, OH, 1964.
4. R.A. Cutler, "Engineering Properties of Borides," pp. 787-803 in Ceramics and Glasses: Engineered Materials Handbook Volume 4, ed. by S.J. Schneider, Jr., ASM International, Materials Park, OH (1991).
5. W.C. Tripp, H.H. Davis, and H.C. Graham, "Effect of an SiC Addition of the Oxidation of ZrB2," Ceramic Bulletin 52(8) 612-616 (1973).
6. S.C. Zhang, G.E. Hilmas, and W.G. Fahrenholtz, "Pressureless Densification of Zirconium Diboride with Boron Carbide Additions," Journal of the American Ceramic Society, 89(5) 1544-1550 (2006).
7. D. Sciti, S. Guicciardi, A. Bellosi, and G. Pezzotti, "Properties of a Pressureless Sintered ZrB2-MoSi2 Ceramic Composite,'' Journal of the American Ceramic Society, 89 [7] 2320-2322 (2006).
8. A.L Chamberlain, W.G. Fahrenholtz, G.E. Hilmas, and D.T. Ellerby, "High Strength ZrB2-Based Ceramics," Journal of the American Ceramic Society, 87(6) 1170-1172 (2004).
9. A. Balbo and D. Sciti, "Spark Plasma Sintering and Hot Pressing of ZrB2-MoSi2 Ultra-High Temperature Ceramics," Materials Science and Engineering A, 475 108-112 (2008).
10 . D. Sciti, S. Guicciardi, and M. Nygren, "Densification and Mechanical Behavior of HfC and HfB2 Fabricated by Spark Plasma Sintering," Journal of the American Ceramic Society, 91(5) 1433-1440 (2008).
11. P.T.B. Shaffer, "Engineering Properties of Carbides," pp. 804-811
in Ceramics and Glasses: Engineered Materials Handbook Volume 4, ed. by S.J. Schneider, Jr., ASM International, Materials Park, OH (1991).
12. A.K. Kuriakose and J.L. Margrave, "The Oxidation Kinetics of Zirconium Diboride and Zirconium Carbide at High Temperatures," Journal of the Electrochemical Society, 111(7) 827-8331 (1964).
13. X. Zhang, G.E. Hilmas, W.G. Fahrenholtz, and D.M. Deason "Hot Pressing of Tantalum Carbide with and without Sintering Aids," Journal of the American Ceramic Society, 90(2) 393-401 (2007).