Processing of ZrB2- and HfB2-Based Ultra-High Temperature Ceramic Materials: A Review

Abstract

ZrB2 and HfB2 are leading materials in the category of ultra-high temperature ceramics (UHTCs). UHTCs are a group of ceramic materials that can withstand ultra-high temperature (>2,000°C) in oxidizing conditions. Such a high temperature will be encountered by future hypersonic reentry vehicles. Future reentry vehicles will have sharp edges to improve aerodynamic performance. The sharp leading edges will cause higher surface temperature than that of the actual blunt-edged vehicles. The sharp edges have less surface area to dissipate heat and thus the temperature gets increased. To withstand the intense heat generated when these vehicles reenter the earth’s upper atmosphere, UHTC materials are needed. UHTC materials are composed of borides of early transition metals. 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 that of other borides because of the stability of the ZrO2 and HfO2 scales that form on these materials at elevated temperatures in oxidizing environments. Processing of these materials is very difficult because these materials are very refractory in nature. Powder synthesis usually involves high-temperature solid state reactions. Dense bodies are generally produced by hot pressing and spark plasma sintering. For coating preparation, plasma spray, chemical vapor deposition, and physical vapor deposition is used. All these processing methods for ZrB2 and HfB2 are difficult because of the involvement of very high temperatures. In this review, the present state of the knowledge on processes employed for powder synthesis, fabrication of dense bodies, and coating processes is discussed. The advantages, limitations, and challenges in the different processing methods are addressed.

Early events in the mechanism of single-source chemical vapor deposition of zirconium and hafnium diboride: a computational investigation

“…Borides of the group 4 transition metals (TiB 2 , ZrB 2 , HfB 2 ) are hard, infusible, inert solids with good thermal and electrical conductivity. [1][2][3][4] Their uncommonly high melting points (3245 1C ZrB 2 , 3380 1C HfB 2 ) qualify them as ultra-high temperature ceramics (UHTCs), 5 and this property in conjunction with their resistance to oxidation at high temperatures makes them attractive materials for thermal coatings on hypersonic vehicles. 5,6 TiB 2 and ZrB 2 have found applications in protective coatings for cutting tools, for similar reasons.…”

“…[9][10][11] Chemical vapor deposition (CVD) has proved to be a powerful technique for generating such films. [12][13][14] Two processes dominate the chemical vapor deposition of group 4 transition metal diboride ceramics: independentprecursor systems centered around metal chlorides, 4,8,[15][16][17][18][19] and single-source systems involving metal borohydrides. [20][21][22][23][24][25][26][27][28][29] The single-source technique has significant advantages over the independent-precursor system.…”

“…Films may be deposited at temperatures as low as 200-250 1C, [20][21][22][23][24][25][26]29 whereas the independent-precursor systems require temperatures in excess of 700-900 1C. 4,[16][17][18][19]30 The films deposited from a single source are cleaner, free of halogen impurities. 8 While the kinetics of deposition have been studied extensively and this knowledge leveraged to tune the process, 14,22,29 the overall mechanism by which the ceramic is generated has not been elucidated.…”

“…Two processes dominate the chemical vapor deposition of group 4 transition metal diboride ceramics: independent-precursor systems centered around metal chlorides, 4,8,15–19 and single-source systems involving metal borohydrides. 20–29 The single-source technique has significant advantages over the independent-precursor system.…”

Oxidation of Ceramic Materials Based on HfB2-SiC under the Influence of Supersonic CO2 Jets and Additional Laser Heating

“…Ultra-high-temperature ceramic materials (UHTC) based on ZrB 2 -SiC and HfB 2 -SiC are currently recognized as very promising for operation under extreme conditions, such as at temperatures above 2000 • C in an oxygen-containing atmosphere with simultaneous high ablative loading [1][2][3][4][5][6][7][8]. These materials are positioned as interesting from the practical point of view for manufacture of the most thermally loaded parts (e.g., sharp edges of wings and nose parts) of high-speed aircraft, which are subjected to aerodynamic heating up to temperatures of 2000-2500 • C; the smaller the radius of curvature, the greater the thermochemical effect on the material [9][10][11].…”