A simple route to fabricate strong boride hierarchical composites for use at ultra-high temperature

Abstract

A simple method to obtain a highly refractory HfB 2 -based ceramic nano-composite is presented. The boride was hot pressed with additions of SiC and WC particles and subsequently annealed at 2100°C for 2 hours.The annealing procedure was beneficial for high temperature strength, which increased by about 300 MPa in the 1500°C to 1800°C temperature range compared to the as-sintered material. Peak strengths of 850 MPa at 1500°C and 650 MPa at 1800°C were achieved due to two main microstructural changes. First, rounded SiC particles that were surrounded by a silica-based glass in the as-sintered ceramics evolved into platelets with mostly clean grain boundaries after heat treatment. In addition, the (Hf,W)B 2 solid solution that formed as shells around HfB 2 grain cores during densification reached an equilibrium state after annealing that revealed nano-texturing of the shell, which constituted a nano-composite with metallic W nano-particles embedded within HfB 2 grains. These two features combined to contribute to refractoriness and increased strength at elevated temperature. The unique findings reported in this study launch significant opportunities for ceramic development, manufacturing, and applications.

Crystal Structure and Physical Properties of the Cage Compound Hf2B2–2δIr5+δ

“… 26 , 27 It has found its way into nuclear applications as neutron absorber materials 28 and high-temperature ceramic composites. 29 , 30 …”

Solid‐state formation mechanisms of core–shell microstructures in (Zr,Ta)B2 ceramics

“…[4][5][6] For example, the average flexural strengths reported for a core-shell (Zr,Ta)B2 ceramic were 598 MPa at 1200 • C and 374 MPa at 1500 • C, 4 compared to those of a ZrB2-TaB2 composite without coreshell microstructure, which had strengths of 401 MPa at 1000 • C, 336 MPa at 1600 • C, and 256 MPa at 1800 • C. 7 Under certain processing conditions, transition metal (TM) diborides containing a second dissolved TM can form core-shell microstructures that consist of a core of nominally pure diboride with an epitaxial shell that is composed of a solid solution diboride. 3,[8][9][10][11][12] Several TMs including Hf, Ta, Nb, Mo, and W can substitute onto the metal sites in ZrB 2 to form substitutional solid solutions. 13 In core-shell microstructures, the solid solution shells can contain from 3 to 20 at% or more of the second TM.…”

“…Several TMs including Hf, Ta, Nb, Mo, and W can substitute onto the metal sites in ZrB 2 to form substitutional solid solutions 13 . In core–shell microstructures, the solid solution shells can contain from 3 to 20 at% or more of the second TM 3,8,9,14 . Typically, these materials have been produced by hot pressing or spark plasma sintering of mixtures of a primary TM compound (e.g., ZrB 2 ) with a compound containing a second TM such as Mo, Ta, or W. Most reported diborides with core–shell microstructures also contain a source of silicon, such as SiC or a TM disilicide 10,15–17 …”

“…13 In core-shell microstructures, the solid solution shells can contain from 3 to 20 at% or more of the second TM. 3,8,9,14 Typically, these materials have been produced by hot pressing or spark plasma sintering of mixtures of a primary TM compound (e.g., ZrB 2 ) with a compound containing a second TM such as Mo, Ta, or W. Most reported diborides with core-shell microstructures also contain a source of silicon, such as SiC or a TM disilicide. 10,[15][16][17] Formation of the core-shell microstructure has been attributed to a transient liquid phase that forms due to the reaction of Si with residual surface oxides.…”

“…Under certain processing conditions, transition metal (TM) diborides containing a second dissolved TM can form core–shell microstructures that consist of a core of nominally pure diboride with an epitaxial shell that is composed of a solid solution diboride 3,8–12 . Several TMs including Hf, Ta, Nb, Mo, and W can substitute onto the metal sites in ZrB 2 to form substitutional solid solutions 13 .…”