This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB2) and hafnium diboride (HfB2) ceramics. The refractory diborides exhibit partial or complete solid solution with other transition metal diborides, which allows compositional tailoring of properties such as thermal expansion coefficient and hardness. Carbothermal reduction is the typical synthesis route, but reactive processes, solution methods, and pre-ceramic polymers can also be used. Typically, diborides are densified by hot pressing, but recently solid state and liquid phase sintering routes have been developed. Fine-grained ZrB2 and HfB2 have strengths of a few hundred MPa, which can increase to over 1 GPa with the addition of SiC. Pure diborides exhibit parabolic oxidation kinetics at temperatures below 1100°C, but B2O3 volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi2 improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB2 and HfB2 ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re-entry, and rocket propulsion.

Refractory Diborides of Zirconium and Hafnium

Field-assisted sintering technology/Spark plasma sintering is a low voltage, direct current (DC) pulsed current activated, pressure-assisted sintering, and synthesis technique, which has been widely applied for materials processing in the recent years. After a description of its working principles and historical background, mechanical, thermal, electrical effects in FAST/SPS are presented along with the role of atmosphere. A selection of successful materials development including refractory materials, nanocrystalline functional ceramics, graded, and non-equilibrium materials is then discussed. Finally, technological aspects (advanced tool concepts, temperature measurement, finite element simulations) are covered.

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Field-Assisted Sintering Technology/Spark Plasma Sintering: Mechanisms, Materials, and Technology Developments

and Perspectives Sophie Carenco,†,‡,§,∥,⊥ David Portehault,*,†,‡,§ Ced́ric Boissier̀e,†,‡,§ Nicolas Meźailles, and Cleḿent Sanchez*,†,‡,§ †Chimie de la Matier̀e Condenseé de Paris, UPMC Univ Paris 06, UMR 7574, Colleg̀e de France, 11 Place Marcelin Berthelot, 75231 Paris Cedex 05, France ‡Chimie de la Matier̀e Condenseé de Paris, CNRS, UMR 77574, Colleg̀e de France, 11 Place Marcellin Berthelot, 75231 Paris Cedex 05, France Chimie de la Matier̀e Condenseé de Paris, Colleg̀e de France, 11 Place Marcellin Berthelot, 75231 Paris Cedex 05, France Laboratory Heteroelements and Coordination, Chemistry Department, Ecole Polytechnique, CNRS-UMR 7653, Palaiseau, France

Nanoscaled metal borides and phosphides: recent developments and perspectives

Among the ultra-high temperature ceramics (UHTC) are a group of materials consisting of zirconium diboride or hafnium diboride plus silicon carbide, and in some instances, carbon. These materials offer a good combination of properties that make them candidates for airframe leading edges on sharp-bodied reentry vehicles. These UHTCperform well in the environment for such applications, i.e. air at low pressure. The purpose of this study was to examine three of these materials under conditions more representative of a propulsion environment, i.e. higher oxygen partial pressure and total pressure. Results of strength and fracture toughness measurements, furnace oxidation, and high velocity thermal shock exposures are presented for ZrB2 plus 20 vol.% SiC, ZrB2 plus 14 vol.% SiC plus 30 vol.% C, and SCS-9a SiC fiber reinforced ZrB2 plus 20 vol.% SiC. The poor oxidation resistance of UHTCs is the predominant factor limiting their applicability to propulsion applications. # 2002 Elsevier Science Ltd. All rights reserved.

Evaluation of ultra-high temperature ceramics foraeropropulsion use

Seven equimolar, five-component, metal diborides were fabricated via high-energy ball milling and spark plasma sintering. Six of them, including (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2, (Hf0.2Zr0.2Ta0.2Mo0.2Ti0.2)B2, (Hf0.2Zr0.2Mo0.2Nb0.2Ti0.2)B2, (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2, (Mo0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2, and (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2, possess virtually one solid-solution boride phase of the hexagonal AlB2 structure. Revised Hume-Rothery size-difference factors are used to rationalize the formation of high-entropy solid solutions in these metal diborides. Greater than 92% of the theoretical densities have been generally achieved with largely uniform compositions from nanoscale to microscale. Aberration-corrected scanning transmission electron microscopy (AC STEM), with high-angle annular dark-field and annular bright-field (HAADF and ABF) imaging and nanoscale compositional mapping, has been conducted to confirm the formation of 2-D high-entropy metal layers, separated by rigid 2-D boron nets, without any detectable layered segregation along the c-axis. These materials represent a new type of ultra-high temperature ceramics (UHTCs) as well as a new class of high-entropy materials, which not only exemplify the first high-entropy non-oxide ceramics (borides) fabricated but also possess a unique non-cubic (hexagonal) and layered (quasi-2D) high-entropy crystal structure that markedly differs from all those reported in prior studies. Initial property assessments show that both the hardness and the oxidation resistance of these high-entropy metal diborides are generally higher/better than the average performances of five individual metal diborides made by identical fabrication processing.

https://escholarship.org/content/qt99m7h5w8/qt99m7h5w8.pdf

High-Entropy Metal Diborides: A New Class of High-Entropy Materials and a New Type of Ultrahigh Temperature Ceramics.

Hypersonic flight involves extremely high velocities and gas temperatures with the attendant necessity for thermal protection systems (TPS). New high temperature materials are needed for these TPS systems. A systematic investigation of the thermodynamics of potential materials revealed that low oxidation rate materials, which form pure scales of SiO2, Al2O3, Cr2O3, or BeO, cannot be utilized at temperatures of 1800°C (and above) due to disruptively high vapor pressures which arise at the interface of the base material and the scale. Vapor pressure considerations provide significant insight into the relatively good oxidation resistance of ZrB2- and HfB2-based materials at 2000°C and above. These materials form multi-oxide scales composed of a refractory crystalline oxide (skeleton) and a glass component, and this compositional approach is recommended for further development. The oxidation resistance of ZrB2-SiC and other non-oxide materials is improved, to at least 1600°C, by compositional modifications which promote immiscibility in the glass component of the scale. Other candidate materials forming high temperature oxides, such as rare earth compounds, are largely unexplored for high temperature applications and may be attractive candidates for hypersonic TPS materials.

Oxidation-based materials selection for 2000°C + hypersonic aerosurfaces: Theoretical considerations and historical experience

The thermal conductivity, thermal expansion, Youngs Modulus, flexural strength, and brittle–plastic deformation transition temperature were determined for HfB2, HfC0·98, HfC0·67, and HfN0·92 ceramics. The oxidation resistance of ceramics in the ZrB2–ZrC–SiC system was characterized as a function of composition and processing technique. The thermal conductivity of HfB2 exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2·5 at 820°C. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200°C for HfC0·98 to 1100°C for HfC0·67 ceramics. The transition temperature of HfB2 was 1100°C. The ZrB2/ZrC/SiC ceramics were prepared from mixtures of Zr (or ZrC), SiB4, and C using displacement reactions. The ceramics with ZrB2 as a predominant phase had high oxidation resistance up to 1500°C compared to pure ZrB2 and ZrC ceramics. The ceramics with ZrB2/SiC molar ratio of 2 (25 vol% SiC), containing little or no ZrC, were the most oxidation resistant.

Mechanical, Thermal, and Oxidation Properties of Refractory Hafnium and zirconium Compounds

3000°C. That's not just hot ... it's EXTREMELY hot. It is above the melting or decomposition temperatures for most of the materials known to man. But in the world of extreme environment engineering, it is just a baseline.

https://www.electrochem.org/dl/interface/wtr/wtr07/wtr07_p30-36.pdf

UHTCs: Ultra-High Temperature Ceramic Materials for Extreme Environment Applications

The effect of Si 3 N 4 , Ta 5 Si 3 , and TaSi 2 additions on the oxidation behavior of ZrB 2 was characterized at 1200°-1500°C and compared with both ZrB 2 and ZrB 2 /SiC. Significantly improved oxidation resistance of all Si-containing compositions relative to ZrB 2 was a result of the formation of a protective layer of borosilicate glass during exposure to the oxidizing environment. Oxidation resistance of the Si 3 N 4 -modified ceramics increased with increasing Si 3 N 4 content and was further improved by the addition of Cr and Ta diborides. Chromium and tantalum oxides induced phase separation in the borosilicate glass, which lead to an increase in liquidus temperature and viscosity and to a decrease in oxygen diffusivity and of boria evaporation from the glass. All tantalum silicide-containing compositions demonstrated phase separation in the borosilicate glass and higher oxidation resistance than pure ZrB 2 , with the effect increasing with temperature. The most oxidation-resistant ceramics contained 15 vol% Ta 5 Si 3 , 30 vol% TaSi 2 , 35 vol% Si 3 N 4 , or 20 vol% Si 3 N 4 with 10 mol% CrB 2 . These materials exceeded the oxidation resistance of the ZrB 2 /SiC ceramics below 1300°-1400°C. However, the ZrB 2 /SiC ceramics showed slightly superior oxidation resistance at 1500°C.

Inna G. Talmy

Papers

Refractory Diborides of Zirconium and Hafnium

Oxidation-based materials selection for 2000°C + hypersonic aerosurfaces: Theoretical considerations and historical experience

Mechanical, Thermal, and Oxidation Properties of Refractory Hafnium and zirconium Compounds

UHTCs: Ultra-High Temperature Ceramic Materials for Extreme Environment Applications

High‐Temperature Chemistry and Oxidation of ZrB2 Ceramics Containing SiC, Si3N4, Ta5Si3, and TaSi2