Hot Pressed HfB₂ and HfB₂–20 vol%SiC Ceramics Based on HfB₂ Powder Synthesized by Borothermal Reduction of HfO₂^*

Abstract

Phase pure hafnium diboride (HfB2) powder was synthesized by borothermal reduction of hafnium dioxide using amorphous boron at relatively low temperature (1600°C) in vacuum. The synthesized HfB2 powder had an average particle size of 1.37 μm with an equiaxed shape, and a low oxygen content of 0.79 wt%. Using the as‐synthesized HfB2 powder and a commercial SiC, HfB2 monolithic, and HfB2–20 vol%SiC composite were hot pressed at 2000°C to relative densities of 95.7% and 99.2%, respectively. With the addition of SiC, the grain size decreased and the fracture behavior changed from intergranular to a mixed mode, which resulted in a high flexural strength of 993±90 MPa for the composite. Fracture toughness of the composite was 6.29±0.65 MPa m1/2, which was significantly higher than that of the HfB2 monolithic and the reported values in literature.

Densification, microstructure evolution and mechanical properties of WC doped HfB2–SiC ceramics

“…In addition, microcracks existed in HS ceramic (see Figs. 2a and 4a), possibly due to thermal expansion mismatch between HfB 2 (6.3 × 10 −6 K −1 at 298 K) and SiC (3.3 × 10 −6 K −1 at 298 K) grains [7,24].…”

“…In order to enhance the densification, hot pressing or processing aids for pressureless sintering started with fine source powders are common routines of practice. HfB 2 -SiC ceramic with relative density of 99.2% has been hot pressed at 2000 • C using fine HfB 2 powder [7]. Several silicides and carbides, such as HfSi 2 , MoSi 2 , TaSi 2 , B 4 C and WC, were selected as sintering additives to enhance densification of HfB 2 via either liquid phase sintering or oxygen removal mechanisms [6,[8][9][10][11][12][13].…”

Effects of SiC content on the densification, microstructure, and mechanical properties of HfB2–SiC composites

“…[28][29][30][31] This phenomenon can be witnessed by crack attraction to the second phase in case of tensile stress field present in the matrix (Figure 9). [28][29][30][31] This phenomenon can be witnessed by crack attraction to the second phase in case of tensile stress field present in the matrix (Figure 9).…”

Selection principle of the synthetic route for fabrication of HfB2 and HfB2‐SiC ceramics

“…[5][6][7] As a result of the strong covalent bonding and low self-diffusion rate, high sintering temperature (2000-2200°C) and external pressure (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) were generally required for the densification of HfB 2 , [1][2][3][4]8 and SiC was added targetting low-temperature densification and property enhancement. [5][6][7] As a result of the strong covalent bonding and low self-diffusion rate, high sintering temperature (2000-2200°C) and external pressure (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) were generally required for the densification of HfB 2 , [1][2][3][4]8 and SiC was added targetting low-temperature densification and property enhancement.…”

“…13 Borothermal and boro/ carbothermal reduction were considered to be advantageous owing to the feasibility of scale-up and the economical cost of the raw materials. 4 Guo et al developed a modified borothermal reduction route, including two-step reduction plus intermediate water-washing, and HfB 2 product was featured with the fine particle size (0.8 μm). 4 Guo et al developed a modified borothermal reduction route, including two-step reduction plus intermediate water-washing, and HfB 2 product was featured with the fine particle size (0.8 μm).…”