Zirconia Transport by Liquid Convection during Oxidation of Zirconium Diboride–Silicon Carbide

During high-temperature oxidation of ZrB₂–SiC composites, a multi-layer oxide scale forms with a silica-rich borosilicate liquid as the surface oxide layer. Here, a recently proposed novel mechanism for the high-temperature oxidation of ZrB₂–SiC composites is further investigated and verified. This mechanism involves the formation of convection cells in the oxide surface layer during high-temperature oxidation of the composite. The formation of zirconia deposits found in the center of the convection cells is proposed here to be the consequence of liquid transport. The nature and deposition mechanism of the zirconia is reported in detail, using calculated phase equilibrium diagrams and microstructure observations of a ZrB₂-15 vol% SiC composite tested at 1550° and 1700°C in ambient air for various times. The calculated phase equilibrium diagrams for the binary ZrO₂–B₂O₃ system as well as the ternary B₂O₃–SiO₂–ZrO₂ system at 1500°C are reported here to interpret these results.     

Convection Patterns in Liquid Oxide Films on ZrB2–SiC Composites Oxidized at a High Temperature

During the high-temperature oxidation of ZrB₂–SiC composites, liquid boron oxide (B₂O₃) is formed at the zirconium diboride–zirconium oxide interface and transported through the overlying layer of silica liquid by convection, forming distinct convection cells arranged like the petals of a flower. The convection cells are localized by a viscous fingering phenomenon, as the fluid B₂O₃ rich liquid solution rises through the viscous silica layer. The upwelling B₂O₃ rich liquid contains dissolved zirconium dioxide, which deposits in the center of the flower-like structure as the B₂O₃ evaporates. The driving force for the B₂O₃ liquid flow is the volume increase upon oxidation of ZrB₂. Convective transport of B₂O₃ liquids suggests a novel mechanism for the high-temperature oxidation of these materials.     

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.     

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.     

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.     

Homogeneous Fabrication of Al2O3–ZrO2–SiC Whisker Composite by Surface-Induced Coating

Al₂O₃–ZrO₂–SiC whisker composites were prepared by surface-induced coating of the precursor for the ZrO₂ phase on the kinetically stable colloid particles of Al₂O₃ and SiC whisker. The fabricated composites were characterized by a uniform spatial distribution of ZrO₂ and SiC whisker phases throughout the Al₂O₃ matrix. The fracture toughness values of the Al₂O₃–15 vol% ZrO₂–20 vol% SiC whisker composites (∼12 MPa.m^1/2) are substantially greater than those of comparable Al₂O₃–SiC whisker composites, indicating that both the toughening resulting from the process zone mechanism and that caused by the reinforced SiC whiskers work simultaneously in hot-pressed composites.     

Preparation and Microstructure of a ZrB2–SiC Composite Fabricated by the Spark Plasma Sintering–Reactive Synthesis (SPS–RS) Method

A mixture of Zr, B₄C, and Si powders was adopted to synthesize a ZrB₂–SiC composite using the spark plasma sintering–reactive synthesis (SPS–RS) method. SPS treatments were carried out in the temperature range of 1350°–1500°C under a varying pressure of 20–65 MPa with a 3-min holding time. A dense (∼98.5%) ZrB₂–SiC composite was successfully fabricated at 1450°C for 3 min under 30 MPa. The microstructure of the composite was investigated. The in situ formed ZrB₂ and SiC phases dispersed homogeneously on the whole. The grain size of ZrB₂ and SiC was <5 and 1 μm, respectively. A number of in situ formed ultrafine SiC particles were observed entrapped in the ZrB₂ grains.     

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.     

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.     

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.     

Pressureless Densification of Zirconium Diboride with Boron Carbide Additions

Zirconium diboride (ZrB₂) was densified (>98% relative density) at temperatures as low as 1850°C by pressureless sintering. Sintering was activated by removing oxide impurities (B₂O₃ and ZrO₂) from particle surfaces. Boron oxide had a high vapor pressure and was removed during heating under a mild vacuum (∼150 mTorr). Zirconia was more persistent and had to be removed by chemical reaction. Both WC and B₄C were evaluated as additives to facilitate the removal of ZrO₂. Reactions were proposed based on thermodynamic analysis and then confirmed by X-ray diffraction analysis of reacted powder mixtures. After the preliminary powder studies, densification was studied using either as-received ZrB₂ (surface area ∼1 m²/g) or attrition-milled ZrB₂ (surface area ∼7.5 m²/g) with WC and/or B₄C as a sintering aid. ZrB₂ containing only WC could be sintered to ∼95% relative density in 4 h at 2050°C under vacuum. In contrast, the addition of B₄C allowed for sintering to >98% relative density in 1 h at 1850°C under vacuum.     

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.     

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.     

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.     

Resistance to Thermal Shock and to Oxidation of Metal Diborides–SiC Ceramics for Aerospace Application

Two SiC-containing metal diborides materials, classified in the ultra-high-temperature ceramics (UHTCs) group, were fabricated by hot-pressing. SiC, sinterability apart, promoted resistance to oxidation of the diboride matrices. Both the compositions, oxidized in air at 1450°C for 1200 min, had mass gains lower than 5 mg/cm². Slight deviations from parabolic oxidation kinetics were seen. The resistance to thermal shock (TSR) was studied through the method of the retained flexure strength after water quenching (20°C of bath temperature). Experimental data showed that the (ZrB₂+HfB₂)–SiC and the ZrB₂–SiC materials retained more than 70% of their initial mean flexure strength for thermal quenchs not exceeding 475° and 385°C, respectively. Certain key TSR properties (i.e., fracture strength and toughness, elastic modulus, and thermal expansion coefficient) are very similar for the two compositions. The observed superior critical thermal shock of the (ZrB₂+HfB₂)–SiC composite was explained in terms of more favorable heat transfer parameters conditions that induce less severe thermal gradients across the specimens of small dimensions (i.e., bars 25 mm × 2.5 mm × 2 mm) during the quench down in water. The experimental TSRs are expected to approach the calculated R values (196° and 218°C for ZrB₂+HfB₂–SiC and ZrB₂–SiC, respectively) as the specimen size increases.     

SiO2–PbO–CaO–ZrO2–TiO2–(B2O3–K2O), A New Zirconolite Glass–Ceramic System: Crystallization Behavior and Microstructure Evaluation

Zirconolite (CaZrTi₂O₇) is a mineral that has a high containment capacity for actinides and lanthanides and is considered to be a good candidate for the immobilization of radioactive wastes. The glass–ceramic technique seems to be a very suitable and convenient method to produce zirconolite crystals by precipitating them in a specific glass matrix. In this study, development of a new zirconolite-based glass–ceramic belonging to SiO₂–PbO–CaO–ZrO₂–TiO₂–(B₂O₃–K₂O) system was investigated. The presence of PbO, together with B₂O₃ and K₂O, allowed the preparation of a X-ray diffraction (XRD) amorphous glass with a relatively high concentration of ZrO₂ and TiO₂, which was successfully converted to a glass–ceramic containing 34 wt% of zirconolite after heating at 770°C for 4 h. Differential thermal analysis, XRD, scanning electron microscope, and energy dispersive X-ray spectroscopy were used to determine the crystallization conditions, identify the crystallized phases, determine their compositions and quantities and observe and analyze the microstructures. The zirconolite crystals showed a platelet morphology with a monoclinic structure characterized by a =1.246 nm, b =0.7193 nm, c =1.128 nm, and β=100.508°.     

New Route to Synthesize Ultra-Fine Zirconium Diboride Powders Using Inorganic–Organic Hybrid Precursors

Ultra-fine zirconium diboride (ZrB₂) powders have been synthesized using inorganic–organic hybrid precursors of zirconium oxychloride (ZrOCl₂·8H₂O), boric acid, and phenolic resin as sources of zirconia, boron oxide, and carbon, respectively. The reactions were substantially completed at a relatively low temperature (∼1500°C). The synthesized powders had a smaller average crystallite size (<200 nm), a larger specific surface area (∼32 m²/g), and a lower oxygen content (<1.0 wt%), which were superior to some commercially available ZrB₂ powders. The thermodynamic change in the ZrO₂–B₂O₃–C system was mainly studied by thermogravimetric and differential thermal analysis. The crystallite size and morphology of the synthesized powders were characterized by transmission electron microscopy and scanning electron microscopy.     

Oxidation Behavior of Zirconium Diboride Silicon Carbide Produced by the Spark Plasma Sintering Method

Dense samples of ZrB₂–20 vol% SiC were successfully fabricated by spark plasma sintering without the use of sintering aids. Oxidation behavior of these samples was characterized by exposing them to 1400°, 1500°, and 1600°C in an ambient atmosphere for 150 min, and by measuring the weight gains of the sample and crucible, as well as the thickness of the oxide scale and the glassy outer layer. The effects of gravity on the viscous outer layer are shown to result in significant heterogeneity within a sample. The oxidation scales were characterized by scanning electron microscopy and transmission electron microscopy with energy dispersive spectroscopy analysis. The oxide scale was found to be composed of three layers: (1) a SiO₂-rich glassy outer layer, (2) an intermediate layer of a ZrO₂ matrix with interpenetrating SiO₂, and (3) a layer containing a ZrO₂ matrix enclosing partially oxidized ZrB₂ with Si–C–B–O glass inclusions.     

Pressureless Sintering of Zirconium Diboride Using Boron Carbide and Carbon Additions

The synergistic roles of boron carbide and carbon additions in the enhanced densification of zirconium diboride (ZrB₂) by pressureless sintering have been studied. ZrB₂ was sintered to >99% relative density at 1900°C. The combination of 2 wt% boron carbide and 1 wt% carbon promoted densification by removing surface oxide impurities (ZrO₂ and B₂O₃) and inhibiting grain growth. Four-point bending strength (473±43 MPa), Vickers' microhardness (19.6±0.4 GPa), fracture toughness (3.5±0.6 MPa·m^1/2), and Young's modulus (507 GPa) were measured. Thermal gravimetry showed that the combination of additives did not have an adverse effect on the oxidation behavior.     

Strength and Phase Stability of Yttria-Ceria-Doped Tetragonal Zirconia/Alumina Composites Sintered and Hot Isostatically Pressed in Argon–Oxygen Gas Atmosphere

Yttria-ceria-doped tetragonal zirconia (Y,Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing at 1400° to 1450°C and 196 MPa in an Ar–O₂ atmosphere using the fine powders prepared by hydrolysis of ZrOCl₂ solution. The composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5–4 mol% CeO₂–ZrO₂ and 2.5 mol% YO_1.5–5.5 mol% CeO₂–ZrO₂ exhibited mean fracture strength as high as 2000 MPa and were resistant to phase transformation under saturated water vapor pressure at 180°C (1 MPa). Postsintering hot isostatic pressing of (4Y, 4Ce)-TZP/Al₂O₃ and (2.5Y, 5.5Ce)-TZP/Al₂O₃ composites was useful to enhance the phase stability under hydrothermal conditions and strength.