Zirconium diboride (ZrB 2 ) and ZrB 2 ceramics containing 10, 20, and 30 vol% SiC particulates were prepared from commercially available powders by hot pressing. Four-point bend strength, fracture toughness, elastic modulus, and hardness were measured. Modulus and hardness did not vary significantly with SiC content. In contrast, strength and toughness increased as SiC content increased. Strength increased from 565 MPa for ZrB 2 to >1000 MPa for samples containing 20 or 30 vol% SiC. The increase in strength was attributed to a decrease in grain size and the presence of WC.

High-Strength Zirconium Diboride-Based Ceramics

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

Two zirconium diboride-base composites were produced and characterised The chosen starting compositions were: 55 wt% ZrB2+41 wt%TiB2+4 wt% Ni and 83 wt% ZrB2+13 wt% B4C+4 wt% Ni The microstructure and properties of these composites were compared to those of a monolithic ZrB2+4 wt% Ni material In all cases, metallic Ni as the sintering aid promoted the formation of the liquid phase which improved mass transfer mechanisms during sintering From the powder mixture ZrB2+TiB2, two solid solutions of Zr–Ti–B were obtained In the case of the other mixture, B4C particles were dispersed in the ZrB2 matrix The composite materials have better mechanical properties than those of the monolithic ZrB2 ceramic; in particular the fracture toughness and the flexural strength were almost doubled at room temperature Long term oxidation tests indicated that the ZrB2-based composites, particularly the composite containing B4C as the second phase, were more resistant to oxidation than the monolithic ZrB2 due to the formation of surface oxide products which were protective against the complete degradation by oxidation observed for the ZrB2 matrix material

Processing and properties of zirconium diboride-based composites

Analytical modeling of thermal and mechanical response is a fundamental step in the design process for ultra-high-temperature ceramic components, such as nose tips and wing leading edges for hypersonic applications. The purpose of the analyses is to understand the response of test articles to high-enthalpy flows in ground tests and to predict component performance in particular flight environments. Performing these analyses and evaluating the results require comprehensive and accurate physical, thermal, and mechanical properties. In this paper, we explain the nature of the analyses, highlight the essential material properties that are required and why they are important, and describe the impact of property accuracy and uncertainty on the design process.

Material property requirements for analysis and design of UHTC components in hypersonic applications

Zirconium diboride (ZrB 2 ) ceramics were sintered to a relative density of ∼ 98% without applied external pressure. Densification studies were performed in the temperature range of 1900 -2150 C. Examination of bulk density as a function of temperature revealed that shrinkage started at ∼2100°C, with significant densification occurring at only 2150°C. At 2150°C, isothermal holds were used to determine the effect of time on relative density and microstructure. For a hold time of 540 min at 2150 C, ZrB 2 pellets reached an average density of 6.02±0.04 g/cm 3 (98% of theoretical) with an average grain size of 9.0 ±5.6 μm. Four-point bend strength, elastic modulus, and Vickers' hardness were measured for sintered ZrB 2 and compared with values reported for hot-pressed materials. Vickers' hardness of sintered ZrB 2 was 14.5±2.6 GPa, which was significantly lower when compared with 23 GPa for hot-pressed ZrB 2 . Strength and elastic modulus of the ZrB 2 were 444±30 MPa and 454 GPa, which were comparable with values reported for hot-pressed ZrB 2 . The ability to densify ZrB 2 ceramics without hot pressing should enable near-net shape processing, which would significantly reduce the cost of fabricating ZrB 2 components compared with conventional hot pressing and machining.

Pressureless Sintering of Zirconium Diboride

Advanced ultrahigh temperature materials are critical to the development of next-generation rocket engines and hypersonic spacecrafts.

K Upadhya

Papers

Materials for ultrahigh temperature structural applications