Thermophysical Properties of ZrB2 and ZrB2–SiC Ceramics

Thermophysical properties were investigated for zirconium diboride (ZrB₂) and ZrB₂–30 vol% silicon carbide (SiC) ceramics. Thermal conductivities were calculated from measured thermal diffusivities, heat capacities, and densities. The thermal conductivity of ZrB₂ increased from 56 W (m K)⁻¹ at room temperature to 67 W (m K)⁻¹ at 1675 K, whereas the thermal conductivity of ZrB₂–SiC decreased from 62 to 56 W (m K)⁻¹ over the same temperature range. Electron and phonon contributions to thermal conductivity were determined using electrical resistivity measurements and were used, along with grain size models, to explain the observed trends. The results are compared with previously reported thermal conductivities for ZrB₂ and ZrB₂–SiC.     

Thermal and Electric Properties in Hot-Pressed ZrB2–MoSi2–SiC Composites

The thermal and electrical properties of MoSi₂ and/or SiC-containing ZrB₂-based composites and the effects of MoSi₂ and SiC contents were examined in hot-pressed ZrB₂–MoSi₂–SiC composites. The thermal conductivity and electrical conductivity of the ZrB₂–MoSi₂–SiC composites were measured at room temperature by a nanoflash technique and a current–voltage method, respectively. The results indicate that the thermal and electrical conductivities of ZrB₂–MoSi₂–SiC composites are dependent on the amount of MoSi₂ and SiC. The thermal conductivities observed for all of the compositions were more than 75 W·(m·K)⁻¹. A maximum conductivity of 97.55 W·(m·K)⁻¹ was measured for the 20 vol% MoSi₂-30 vol% SiC-containing ZrB₂ composite. On the other hand, the electrical conductivities observed for all of the compositions were in the range from 4.07 × 10–8.11 × 10 Ω⁻¹·cm⁻¹.     

Joining of ZrB2-Based Ultra-High-Temperature Ceramic Composites to Cu–Clad–Molybdenum for Advanced Aerospace Applications

Three hot-pressed ZrB₂-based ultra-high-temperature ceramic composites (UHTCC), ZrB₂–SiC_p (ZS), ZrB₂–SiC_p–C (ZSC), and ZrB₂–SCS9A (SiC fiber)–SiC_p (ZSS), were joined to Cu–clad–Mo using AgCuTi brazes ( T _L∼1073–1173K) and Pd-base brazes ( T _L∼1493–1513K). More extensive chemical interactions occurred in Pd-base joints than in AgCuTi-base joints. The Pd-braze region displayed higher hardness in joints made using ZS than ZSS and ZSC. Residual stress calculations point toward negative strain energy up to ∼23% clad layer thickness because α_{Cu–clad–Mo}<α_ZS (α=coefficients of thermal expansion). Above this thickness, α_{Cu–clad–Mo}>α_ZS, strain energy is positive, and it increases with increasing thickness. Projected reductions in the thermal resistance highlight the benefits of joining the UHTCC to Cu–clad–Mo.     

Fabrication and Characterization of ZrB2-Based Ceramic Using Synthesized ZrB2–LaB6 Powder

ZrB₂–LaB₆ powder was obtained by reactive synthesis using ZrO₂, La₂O₃, B₄C, and carbon powders. Then ZrB₂–20 vol% SiC–10 vol% LaB₆ (ZSL) ceramics were prepared from commercially available SiC and the synthesized ZrB₂–LaB₆ powder via hot pressing at 2000°C. The phase composition, microstructure, and mechanical properties were characterized. Results showed that both LaB₆ and SiC were uniformly distributed in the ZrB₂ matrix. The hardness and bending strength of ZSL were 17.06±0.52 GPa and 505.8±17.9 MPa, respectively. Fracture toughness was 5.7±0.39 MPa·m^1/2, which is significantly higher than that reported for ZrB₂–20 vol% SiC ceramics, due to enhanced crack deflection and crack bridging near SiC particles.     

Effects of Phase Change and Oxygen Permeability in Oxide Scales on Oxidation Kinetics of ZrB2 and HfB2

A wide range of experimental data on the oxidation of ZrB₂ and HfB₂ as a function of temperature (800°–2500°C) is interpreted using a mechanistic model that relaxes two significant assumptions made in prior work. First, inclusion of the effect of volume change associated with monoclinic to tetragonal phase change of the MeO₂ phases is found to rationalize the observations by several investigators of abrupt changes in weight gain, recession, and oxygen consumed, as the temperature is raised through the transformation temperatures for ZrO₂ and HfO₂. Second, the inclusion of oxygen permeability in ZrO₂ is found to rationalize the enhancement in oxidation behavior at very high temperatures (>1800°C) of ZrB₂, while the effect of oxygen permeability in HfO₂ is negligible. Based on these considerations, the significant advantage of HfB₂ over ZrB₂ is credited to the higher transformation temperature and lower oxygen permeability of HfO₂ compared with ZrO₂.     

Microstructure and Properties of Spark Plasma-Sintered ZrO2–ZrB2 Nanoceramic Composites

In a recent work, ¹ we have reported the optimization of the spark plasma sintering (SPS) parameters to obtain dense nanostructured 3Y-TZP ceramics. Following this, the present work attempts to answer some specific issues: (a) whether ZrO₂-based composites with ZrB₂ reinforcements can be densified under the optimal SPS conditions for TZP matrix densification (b) whether improved hardness can be obtained in the composites, when 30 vol% ZrB₂ is incorporated and (c) whether the toughness can be tailored by varying the ZrO₂–matrix stabilization as well as retaining finer ZrO₂ grains. In the present contribution, the SPS experiments are carried out at 1200°C for 5 min under vacuum at a heating rate of 600 K/min. The SPS processing route enables retaining of the finer t -ZrO₂ grains (100–300 nm) and the ZrO₂–ZrB₂ composite developed exhibits optimum hardness up to 14 GPa. Careful analysis of the indentation data provides a range of toughness values in the composites (up to 11 MPa·m^1/2), based on Y₂O₃ stabilization in the ZrO₂ matrix. The influence of varying yttria content, t -ZrO₂ transformability, and microstructure on the properties obtained is discussed. In addition to active contribution from the transformation-toughening mechanism, crack deflection by hard second phase brings about appreciable increment in the toughness of the nanocomposites.     

Microstructural Evolution and Grain Growth Kinetics in ZrB2–SiC Composites During Heat Treatment

This work reported the microstructural evolution and grain growth kinetics of ZrB₂–SiC composites containing 10, 20, and 30 vol% SiC during heat treatment at 2000°C. The coarsening of ZrB₂ occurred in the three systems, whereas the obvious coarsening for SiC appeared only in the composite with 30 vol% SiC. The kinetics analysis showed the ZrB₂ grain growth rate in the ZrB₂–30 vol% SiC was 25 times lower than that for ZrB₂–10 vol% SiC during heat treatment. Furthermore, the grain growth controlling mechanisms of ZrB₂ and SiC were discussed. In addition, it was found that the heat treatment had little effect on Vickers hardness and fracture toughness of ZrB₂–SiC.     

Oxidation of ZrB2–SiC: Influence of SiC Content on Solid and Liquid Oxide Phase Formation

The effect of SiC concentration on the liquid and solid oxide phases formed during oxidation of ZrB₂–SiC composites is investigated. Oxide-scale features called convection cells are formed from liquid and solid oxide reaction products upon oxidation of the ZrB₂–SiC composites. These convection cells form in the outermost borosilicate oxide film of the oxide scale formed on the ZrB₂–SiC during oxidation at high temperatures (≥1500°C). In this study, three ZrB₂–SiC composites with different amounts of SiC were tested at 1550°C for various durations of time to study the effect of the SiC concentration particularly on the formation of the convection cell features. A calculated ternary phase diagram of a ZrO₂–SiO₂–B₂O₃ (BSZ) system was used for interpretation of the results. The convection cells formed during oxidation were fewer and less uniformly distributed for composites with a higher SiC concentration. This is because the convection cells are formed from ZrO₂ precipitates from a BSZ oxide liquid that forms upon oxidation of the composite at 1550°C. Higher SiC-containing composites will have less dissolved ZrO₂ because they have less B₂O₃, which results in a smaller amount of precipitated ZrO₂ and consequently fewer convection cells.     

The ZrB2 Volatility Diagram

A volatility diagram was calculated for temperatures of 1000, 1800, and 2500 K to understand the oxidation of ZrB₂. Applying the diagram, it can be seen that exposure of ZrB₂ to air produces ZrO₂ (cr) and B₂O₃ (l) over the temperature range considered. The pressure of the predominant vapor species was predicted to increase from ∼10⁻⁶ Pa at 1000 K, to 344 Pa at 1800 K, and to ∼10⁵ Pa at 2500 K. Predictions were consistent with experimental observations that ZrB₂ exhibits passive oxidation below 1200 K, but undergoes active oxidation at higher temperatures due to B₂O₃ (l) evaporation.     

Oxidation Resistance of Fully Dense ZrB2 with SiC, TaB2, and TaSi2 Additives

Specimens of ZrB₂ containing various concentrations of B₄C, SiC, TaB₂, and TaSi₂ were pressureless-sintered and post-hot isostatic pressed to their theoretical densities. Oxidation resistances were studied by scanning thermogravimetry over the range 1150°–1550°C. SiC additions improved oxidation resistance over a broadening range of temperatures with increasing SiC content. Tantalum additions to ZrB₂–B₄C–SiC in the form of TaB₂ and/or TaSi₂ increased oxidation resistance over the entire evaluated spectrum of temperatures. TaSi₂ proved to be a more effective additive than TaB₂. Silicon-containing compositions formed a glassy surface layer, covering an interior oxide layer. This interior layer was less porous in tantalum-containing compositions.     

Solution-Based Synthesis of Submicrometer ZrB2 and ZrB2–TaB2

Zirconium diboride and a zirconium diboride/tantalum diboride mixture were synthesized by solution-based processing. Zirconium n -propoxide was refluxed with 2,4-pentanedione to form zirconium diketonate. This compound hydrolyzed in a controllable fashion to form a zirconia precursor. Boria and carbon precursors were formed via solution additions of phenol–formaldehyde and boric acid, respectively. Tantalum oxide precursors were formed similarly as zirconia precursors, in which tantalum ethoxide was used. Solutions were concentrated, dried, pyrolyzed (800°–1100°C, 2 h, flowing argon), and exposed to carbothermal reduction heat treatments (1150°–1800°C, 2 h, flowing argon). Spherical particles of 200–600 nm for pure ZrB₂ and ZrB₂–TaB₂ mixtures were formed.     

Microstructure and Mechanical Properties of ZrB2-Based Composites Reinforced and Toughened by Zirconia

The present work reported the effect of addition of ZrO₂ on the microstructure and mechanical properties of ZrB₂-based ceramic composites by means of hot-pressed sintering. Observation of microstructure and systematic testing results of mechanical properties were carried out. Through X-ray diffraction analysis and calculation of the volume fraction of ZrO₂ phase transformability, the toughening mechanism of the present composites was explored. The phase transformation toughening by ZrO₂ additive played an important role in improving the fracture toughness of ZrB₂-based composites.     

Properties of a Pressureless-Sintered ZrB2–MoSi2 Ceramic Composite

A ZrB₂-based composite was fully densified by pressureless sintering at 1850°C with addition of 20 vol% MoSi₂. The microstructure was very fine, with mean dimensions of ZrB₂ grains around 2.5 μm. The four-point flexural strength in air was in excess of 500 MPa up to 1500°C.     

Properties of ZrB2–SiC Ceramics by Pressureless Sintering

Pressureless sintering was used to densify ZrB₂–SiC ultra-high temperature ceramics. The physical, mechanical, thermal, electrical, and high temperature properties were investigated. This comprehensive set of properties was measured for ZrB₂ containing 20 vol% SiC in which B₄C and C were used as the sintering aids. The three-point flexural strength was 361±44 MPa and the elastic modulus was 374±25 GPa. The Vickers hardness and fracture toughness were 14.7±0.2 GPa and 4.0±0.5 MPa·m^1/2 respectively. Scanning electron microscopy studies of the microstructure of ZrB₂–SiC showed that SiC particles were distributed homogenously in the ZrB₂ matrix with little residual porosity.     

Thermal Shock Resistance and Fracture Behavior of ZrB2–Based Fibrous Monolith Ceramics

The thermal shock resistance and fracture behavior of zirconium diboride (ZrB₂)-based fibrous monoliths (FM) were studied. FMs containing cells of ZrB₂–30 vol% SiC with cell boundaries composed of graphite–15 vol% ZrB₂ were hot pressed at 1900°C. The average flexure strength of the FMs was 375 MPa, less than half of the strength of hot-pressed ZrB₂–30 vol% SiC. Flexure specimens failed noncatastrophically and retained 50%–85% of their original strength after the first fracture event. A critical thermal shock temperature (Δ T _c) of 1400°C was measured by water quench thermal shock testing, a 250% improvement over the previously reported Δ T _c values for ZrB₂ and ZrB₂–30 vol% SiC of similar dimensions (4 mm × 3 mm × 45 mm). The flexure strength was maintained with Δ T _c values of 1350°C and below. As Δ T _c increased, the stiffness of the flexure specimen decreased linearly. The lower stiffness and improvement in thermal shock resistance is attributed to crack propagation in the cell boundary and crack deflection around the load-bearing cells. The critical thermal shock was attributed to the fracture of the ZrB₂–30% SiC cell material.     

Formation of Oxide Films on ZrB2–15 vol% SiC Composites During Oxidation: Evolution with Time and Temperature

The evolution of convection cells features that form in oxide films on ZrB₂–SiC composites are quantified by the number population, size, and distribution in oxide films on ZrB₂–15 vol% SiC oxidized at temperatures between 1500° and 1600°C for various oxidation times. The number of convection cells per unit area increases after short exposure times of the oxidation at 1550°C, but then decreases slowly with increasing time. The results indicate that these convection cells are transient, they form and transport boria–silica–zirconia liquid to the surface but with increasing exposure time the increasing amount of flowing viscous SiO₂-rich liquid submerges them and they become extinct.     

In Situ Synthesis of Ultrafine ZrB2–SiC Composite Powders and the Pressureless Sintering Behaviors

Ultrafine ZrB₂–SiC composite powders have been synthesized in situ using carbothermal reduction reactions via the sol–gel method at 1500°C for 1 h. The powders synthesized had a relatively smaller average crystallite size (<200 nm), a larger specific surface area (∼20 m²/g), and a lower oxygen content (∼1.0 wt %). Composites of ZrB₂+20 wt% SiC were pressureless sintered to ∼96.6% theoretical density at 2250°C for 2 h under an argon atmosphere using B₄C and Mo as sintering aids. Vickers hardness and flexural strength of the sintered ceramic composites were 13.9±0.3 GPa and 294±14 MPa, respectively. The microstructure of the composites revealed that elongated SiC grain dispersed uniformly in the ZrB₂ matrix. Oxidation from 1100° to 1600°C for 30 min showed no decrease in strength below 1400°C but considerable decrease in strength with a rapid weight increment was observed above 1500°C. The formation of a protective borosilicate glassy coating appeared at 1400°C and was gradually destroyed in the form of bubble at higher temperatures.     

Fabrication and Characterization of an Ultra-High-Temperature Carbon Fiber-Reinforced ZrB2–SiC Matrix Composite

A novel carbon fiber-reinforced ZrB₂–SiC matrix composite was fabricated by heaterless chemical vapor infiltration through infiltration of SiC matrix into a carbon fiber-ZrB₂ powder preform. The C/ZrB₂–SiC composite presented a flexural strength of 148 MPa, a fracture toughness of 5.6 MPa·m^1/2, and a good oxidation and ablation resistance.     

Transmission Electron Microscopy Characterization of Hot-Pressed ZrB2 with MoSi2 Additive

Microstructure of the hot-pressed ZrB₂ with MoSi₂ additive was investigated by transmission electron microscopy (TEM). The effect of MoSi₂ addition on the microstructure of the ceramic was assessed. For the pure ZrB₂, the microstructure consisted of the equiaxed ZrB₂ grains and a few elongated ZrB₂ grains. For the ZrB₂ with MoSi₂ additive, the microstructure consisted almost entirely of equiaxed ZrB₂ grains. A few dislocations were present in the ZrB₂ grains. In addition, high-resolution TEM observations showed that the intergranular amorphous phase was absent at two ZrB₂ grain boundaries in the ZrB₂ with MoSi₂ additive.     

Densification and Mechanical Behavior of HfC and HfB2 Fabricated by Spark Plasma Sintering

Hafnium diboride (HfB₂)- and hafnium carbide (HfC)-based materials containing MoSi₂ as sintering aid in the volumetric range 1%–9% were densified by spark plasma sintering at temperatures between 1750° and 1950°C. Fully dense samples were obtained with an initial MoSi₂ content of 3 and 9 vol% at 1750°–1800°C. When the doping level was reduced, it was necessary to raise the sintering temperature in order to obtain samples with densities higher than 97%. Undoped powders had to be sintered at 2100°–2200°C. For doped materials, fine microstructures were obtained when the thermal treatment was lower than 1850°C. Silicon carbide formation was observed in both carbide- and boride-based materials. Nanoindentation hardness values were in the range of 25–28 GPa and were independent of the starting composition. The nanoindentation Young's modulus and the fracture toughness of the HfB₂-based materials were higher than those of the HfC-based materials. The flexural strength of the HfB₂-based material with 9 vol% of MoSi₂ was higher at 1500°C than at room temperature.