This study reviews densification behaviour, mechanical properties, thermal, and electrical conductivities of the ZrB2 ceramics and ZrB2-based composites. Hot-pressing is the most commonly used densification method for the ZrB2-based ceramics in historic studies. Recently, pressureless sintering, reactive hot pressing, and spark plasma sintering are being developed. Compositions with added carbides and disilicides displayed significant improvement of densification and made pressureless sintering possible at ≤2000 °C. Reactive hot-pressing allows in situ synthesizing and densifying of ZrB2-based composites. Spark plasma sintering displays a potential and attractive way to densify the ZrB2 ceramics and ZrB2-based composites without any additive. Young's modulus can be described by a mixture rule and it decreased with porosity. Fracture toughness displayed in the ZrB2-based composites is in the range of 2–6 MPa m1/2. Fine-grained ZrB2 ceramics had strengths of a few hundred MPa, which increased with the additions of SiC and MoSi2. The small second phase size and uniform distribution led to higher strengths. The addition of nano-sized SiC particles imparts a better oxidation resistance and improves the strength of post-oxidized ZrB2-based ceramics. In addition, the ZrB2-based composites showed high thermal and electrical conductivities, which decreased with temperature. These conductivities are sensitive to composition, microstructure and intergranular phase. The unique combinations of mechanical and physical properties make the ZrB2-based composites attractive candidates for high-temperature thermomechanical structural applications.

Densification of ZrB2-based composites and their mechanical and physical properties: A review

Zirconium diboride (ZrB 2 ) was densified by pressureless sintering using <4-wt% boron carbide and/or carbon as sintering aids. As-received ZrB 2 with an average particle size of ∼ 2 μm could be sintered to ∼ 100% density at 1900°C using a combination of boron carbide and carbon to react with and remove the surface oxide impurities. Even though particle size reduction increased the oxygen content of the powders from ∼ 0.9 wt% for the as-received powder to ∼ 2.0 wt%, the reduction in particle size enhanced the sinterability of the powder. Attrition-milled ZrB 2 with an average particle size of <0.5 μm was sintered to nearly full density at 1850°C using either boron carbide or a combination of boride carbide and carbon. Regardless of the starting particle size, densification of ZrB 2 was not possible without the removal of oxygen-based impurities on the particle surfaces by a chemical reaction.

Pressureless Sintering of Zirconium Diboride: Particle Size and Additive Effects

The densification behavior and grain growth of single-phase ZrB2 ceramics by a spark plasma sintering (SPS) process were systematically investigated. The sintering was conducted at temperatures ranging from 1800° and 1950°C in vacuum under a pressure of 50 MPa. The heating rate and the soaking time varied from 50° to 430°C/min and from 0 to 20 min, respectively. The microstructure was observed by field emission scanning electron microscopy. A near-fully dense ZrB2 ceramic can be obtained at a lower temperature, within a much shorter time, than those used in conventional hot-pressing process. The density of the sintered ceramics increases with increase in sintering temperature, heating rate, and holding time. On the other hand, microstructural examination shows a strong dependence of grain growth on sintering temperature, heating rate, and holding time. The fully dense ZrB2 ceramic with fine grain and homogeneous structure could be achieved by the processing conditions as follows: sintering temperature of 1900°C, holding time of 3 min, and heating rate of 200°–300°C/min.

Spark Plasma Sintering of Zirconium Diborides

Powder metallurgy can proceed in two fundamentally different ways for the fabrication of products from powders of refractory borides. One way — conventional sintering — includes molding or compacting of products from the powder (compression in steel die, isostatic compression, injection molding, rolling, slip casting etc.) with subsequent sintering of the compacts. The other way — hot pressing — combines the pressing and sintering processes and is discussed in Chapter C.XIII. For both ways, the raw boride material must previously be milled to a sufficiently fine particle size.

G.A. Meerson

Papers

Activated sintering of zirconium boride.