ZrB2 SiC Ceramics Research Articles

Mechanical and ablation properties of laminated ZrB2–SiC/BN ceramics

Fully dense laminated ZrB2–SiC/BN ceramics were successfully prepared by tape casting and hot pressing, and the mechanical and ablation properties were investigated in detail. Compared with monolithic ZrB2–SiC ceramics, the flexural strength of laminated ZrB2–SiC/BN ceramics slightly decreased due to the introduction of the weak BN interlayer. However, the fracture toughness of laminated ZrB2–SiC/BN ceramics was significantly improved. The remarkable improvement of fracture toughness was mainly attributed to crack deflection and crack bifurcation. Under oxyacetylene torch environment, the laminated ZrB2–SiC/BN ceramics exhibited an excellent configurational stability compared with monolithic ZrB2–SiC ceramics due to the introduction of weak interlayer.

Changed oxidation behavior of ZrB2–SiC ceramics with the addition of ZrC

Oxidation behavior of ZrB2–SiC ceramics containing 0, 10 and 20vol% ZrC was studied at 1600°C in stagnant air. The structures of oxide scales depended on ZrC content. The thickness of the SiO2-rich glass layer on the outermost surface decreased steeply with the addition of ZrC. One “ZrC-corroded layer”, containing ZrCxOy, SiC and ZrB2, was observed in ZrB2–SiC–ZrC. The oxidation kinetics was converted from parabolic to linear for the ceramics with 20vol% ZrC.

Modeling of Thermal and Mechanical Behavior of ZrB2-SiC Ceramics after High Temperature Oxidation

The effects of oxidation on heat transfer and mechanical behavior of ZrB2-SiC ceramics at high temperature are modeled using a micromechanics based finite element model. The model recognizes that when exposed to high temperature in air ZrB2-SiC oxidizes into ZrO2, SiO2, and SiC-depleted ZrB2 layer. A steady-state heat transfer analysis was conducted at first and that is followed by a thermal stress analysis. A “global-local modeling” technique is used combining finite element with infinite element for thermal stress analysis. A theoretical formulation is developed for calculating the thermal conductivity of liquid phase SiO2. All other temperature dependent thermal and mechanical properties were obtained from published literature. Thermal stress concentrations occur near the pore due to the geometric discontinuity and material properties mismatch between the ceramic matrix and the new products. The predicted results indicate the development of thermal stresses in the SiO2 and ZrO2 layers and high residual stresses in the SiC-depleted ZrB2 layer.

Synthesis of ZrB2–SiC composite powders by sol–gel method using acetic acid as chemical modifier

ZrB2–SiC ceramics are known to have better performance than monolithic ZrB2. Therefore, the addition of SiC can improve the mechanical properties and sintering behavior of ZrB2. In the present work, ZrB2–SiC composite powders were synthesized by sol–gel method using zirconium n-propoxide (Zr(OPr)4), boric acid (H3BO3), sucrose (C12H22O11), TEOS, and acetic acid (AcOH). Clearly, it was a non-aqueous solution system at the very beginning of the reactions. Here, AcOH was used as both chemical modifier and solvent to control Zr(OPr)4 hydrolysis. Actually, AcOH could dominate the hydrolysis by self-produced water of the chemical propulsion, rather than the help of outer water. C12H22O11 was selected since it can be completely decomposed to carbon. Thus, carbon might be accounted precisely for the carbothermal reduction reaction. Moreover, the photomicrograph revealed a spherical morphology of ZrB2–SiC composite particles with an uniform size distribution.

Effects of SiC shape and oxidation on the infrared emissivity properties of ZrB2–SiC ceramics

ZrB2–SiC ceramics were prepared by hot-pressing sintering using SiC whiskers and SiC particles and the effects of SiC shape and oxidation on the infrared emissivity properties of ZrB2–SiC ceramics were investigated systematically. SiC whiskers and SiC particles affected the spectral emissivity of ceramics by changing the propagation direction of electromagnetic waves. Compared with SiC particles, SiC whiskers had smaller surface area at the same volume ratio and caused relatively higher infrared emissivity in ceramics. After oxidation at 1300°C for different time, the total emissivity of ZrB2–SiC ceramics varied in 0.70–1.00. The oxide layer on sample surface slowed down the increase rate of infrared emissivity and the substrate dominated the radiant behavior of ceramics. The investigation also illustrated that the exposed ZrO2 on the samples surface degenerated the infrared emissivity properties of ZrB2–SiC ceramics.

Mechanical properties and ablation behavior of ZrB2–SiC ceramics fabricated by spark plasma sintering

ZrB2–SiC ceramics were prepared by spark plasma sintering at 1375°C for 5min under a uniaxial load of 25MPa. Mechanical properties and ablation behavior of ZrB2–SiC ceramics were investigated. The results showed that the flexural strength of ZrB2–30vol.% SiC ceramic was significantly higher than that of ZrB2–15vol.% SiC ceramic due to the larger amount of SiC whereas the ablation resistance was reduced. The increase in flexural strength was attributed to the sintering of SiC particles which located in the boundary of ZrB2 and enhanced the binding force between ZrB2 particles. The decrease in the ablation resistance at 3000°C was due to the increase in weight loss and voids which were mainly caused by the rapid evaporation of SiO2. Ablation mechanism revealed that excessive SiC could supply much flaws and was hard to form continuous layer to seal the defects, which led to the dissatisfactory resistance to ablation for the ZrB2–30vol.% SiC. The different ablation behaviors of central and outer ablation regions were also discussed.

Single-cycle thermal shock resistance of ZrB2–SiCnp ceramic composites

SiC nanoparticles were introduced in a ZrB2 matrix prepared via vacuum hot pressing sintering to improve the thermal shock resistance of ultra-high temperature ZrB2–SiC ceramics, namely, ZrB2-20vol% nanoparticle SiC composites (ZrB2–20SiCnp). The thermal shock resistance of the material were determined at different temperatures via the water quenching-residual strength test method. The specific temperature of the water bath environment and the influence of different nanoparticle sizes on the thermal shock resistance of the material were also investigated. The results show that the critical thermal shock temperature difference (ΔTc) of the ZrB2–20SiCnp composite is 428°C, which is 11.2% higher than 385°C, ΔTc of a SiC microparticle-strengthened ZrB2 composite material. The results from the thermal shock analysis of the ZrB2–SiCnp composite indicate that ΔTc is strongly dependent on the formation of unusual intragranular nanostructures.

Improved Green Strength and Green Machinability of ZrB2 -SiC Through Gelcasting Based on a Double Gel Network

ZrB2–SiC green ceramics were fabricated by aqueous gelcasting based on single AM-MBAM, single Na-alginate, and double gel network system. ZrB2–SiC ceramics obtained by aqueous gelcasting based on AM-MBAM and Na-alginate double gel network had a dramatically highest green strength of 98.6 ± 5.1 MPa, which was 103% and 61% higher than that of ZrB2–SiC ceramics based on single AM-MBAM system and Na-alginate system, respectively. A “scratch test” was conducted to evaluate the green machinability of as-prepared ZrB2–SiC ceramics. The ZrB2–SiC ceramics based on this double gel network was found to have the best green machinability.

A top-down approach to densify ZrB2–SiC–BN composites with deeper homogeneity and improved reliability

A novel top down approach was developed to fabricate dense ZrB2–SiC–BN (ZSN) composites with a fine-grained (<1 μm) microstructure using coarse-grained (∼10 μm) ZrN and Si precursors that reacted with fine (∼0.5 μm) B4C powders at 1850 °C. The results show that the “reaction” and “densification” mechanisms acting during sintering could be separated or happen simultaneously by changing the pressure loading strategy. Loading cycles not only have a substantial influence on the microstructure homogeneity and the strength reliability of as sintered composites, but might alter the electric current path during the Spark Plasma Sintering process as well. A residual compressive stress of 361 MPa, generated upon the SiC grains in the ZSN composites, was measured by Raman spectroscopy. The fine SiC grains tended to form larger clusters in the dense microstructure if the loading cycle is inappropriate, which was further employed to interpret the observed scattering of strength values in one batch of ZSN. The 48ZrB2–24SiC–28BN (number in vol%) composite has an average strength of 473 MPa, which was 94% of that of in situ densified ZrB2–SiC ceramics (ZS). Considering the corresponding Younǵs modulus was only half of that for ZS, the in situ ZSN composite owns a better mechanical strain tolerance.

Electrical properties of ZrB2–SiC ceramics with potential for heating element applications

ZrB2–10SiC, ZrB2–20SiC and ZrB2–30SiC ceramics were fabricated by hot pressing. The typical effects of volume fraction of SiC on the electrical properties were investigated. The electrical resistivities of as-prepared ZrB2–SiC ceramics increased as the volume fraction of SiC increased. The room temperature electrical resistivity values for ZrB2–10SiC, ZrB2–20SiC and ZrB2–30SiC ceramics were 6.54, 8.65 and 11.29μΩcm, respectively. ZrB2 acted as an electrical conductor and SiC acted as an electrical insulator in the ZrB2–SiC ceramics. The microstructure of ZrB2–SiC ceramics was also analyzed, which further proved the electrical resistivity measurement results.

Microstructure and mechanical properties of ZrB2–SiC joints fabricated by a contact-reactive brazing technique with Ti and Ni interlayers

A contact-reaction brazing technique with Ti and Ni interlayers was used to join ZrB2–SiC ceramics. The influence of Ti–Ni stack methods and Ti content to the microstructure of the joint was investigated. It was found that Ni could infiltrate into ZrB2–SiC during the heating process and led to excessive interfacial reactions. The TiB whiskers could be in situ synthesized in all range of Ti concentration (55–79at% Ti) with Ti/Ni/Ti stack foils. Due to the diffusion behavior of Ti, B atoms and low interfacial energy of TiB in TiNi phase, a cross woven structure of TiB whisker was produced in the brazing seam. The microhardness tests demonstrated tight bonding between TiB and the matrix. The highest shear strength of 134MPa was obtained from 65at% Ti.

서브마이크론/나노 크기의 SiC 비율변화에 따른 ZrB2-SiC 세라믹스의 열적, 기계적 특성

ZrB₂-SiC ceramics are fabricated via hot pressing with different ratios of submicron or nano-sized SiC in a ZrB₂-20 vol%SiC system, in order to examine the effect of the SiC size ratio on the microstructures and physical properties, such as thermal conductivity, hardness, and flexural strength, of ZrB₂-SiC ceramics. Five different ZrB₂-SiC ceramics (ZrB₂-20 vol%[(1-x)SiC + xnanoSiC] where x = 0.0, 0.2, 0.5, 0.8, 1.0) are prepared in this study. The mean SiC particle sizes in the sintered bodies are highly dependent on the ratio of nano-sized SiC. The thermal conductivities of the ZrB₂-SiC ceramics increase with the ratio of nano-sized SiC, which is consistent with the percolation behavior. In addition, the ZrB₂-SiC ceramics with smaller mean SiC particle sizes exhibit enhanced mechanical properties, such as hardness and flexural strength, which can be explained using the Hall-Petch relation.

Change in microstructures and physical properties of ZrB2–SiC ceramics hot-pressed with a variety of SiC sources

ZrB2–SiC ceramics were fabricated by hot pressing with a variety of SiC sources in order to examine the effect of the SiC size on the microstructures and physical properties, such as hardness and thermal conductivities, of ZrB2–SiC composite ceramics. Three different ZrB2–SiC ceramics, ZPS (ZrB2–20vol% polycarbosilane), ZFS (ZrB2–20vol% fine-grained SiC), and ZNS (ZrB2–20vol% nano-sized SiC), were prepared for this study. PCS is effectively transformed into β-SiC after hot pressing. By using PCS as a precursor for SiC, ZrB2 particles are surrounded by fine particles of SiC, which results in the grain-growth inhibition of ZrB2. The effects of the SiC size on the microstructures and the physical properties of ZrB2–SiC ceramics were also investigated. ZrB2–SiC ceramics were produced by using various SiC sources in order to investigate the grain-growth inhibition and the mechanical/thermal properties of ZrB2–SiC. The sizes of ZrB2 or SiC particles in the sintered bodies highly depend on the initial size of SiC. ZrB2–SiC ceramics with smaller SiC show enhanced mechanical properties, consistently with the Hall–Petch relation. The thermal conductivities of ZrB2–SiC ceramics with nano-SiC or PCS-derived SiC are higher than that of ceramics with conventional SiC, which can be explained by the percolation theory.

Thermal shock behavior of porous ZrB2–SiC ceramics

Porous ZrB2–SiC (ZS) ceramics with different microstructures were fabricated by partial hot-pressing or pressureless-sintering, in order to obtain high thermal shock damage resistance. Their thermal shock behavior was investigated by water-quenching. The influences of various microstructural characteristics: porosity, pore size and sintering degree, on mechanical properties were investigated in detail, and their effects on critical crack size, critical thermal shock temperature (ΔTc) and residual strength retention rate after thermal shock were also discussed. The pressureless-sintered ZS with 11% porosity had an optimum configuration of porosity and pore size, and exhibited the best combination of mechanical properties and thermal shock damage resistance. No cracks were visible under the scanning electron microscope on the surfaces of this specimen after thermal shock, showing its excellent structural stability.

Laminated ZrB2–SiC/graphite ceramics with simultaneously improved flexural strength and fracture toughness

Layered graphite was introduced into ZrB2–SiC ceramics to prepare laminated composite by tape casting and hot pressing. Both fracture toughness and flexural strength were improved in the laminates, reaching the peak values of 12.4MPam1/2 and 580MPa, respectively. Cracks propagated in a step-like mode resulting from the layered, soft interface layer, so crack branch and interface delamination were considered as the main toughening mechanisms. The compressive stress in the ZrB2–20vol% SiC layer was the main strengthening mechanism.

Spark plasma sintering of ZrB2–SiC composites with in-situ reaction bonded silicon carbide

This work presents a new hybrid manufacturing process to produce ZrB2–SiC ceramics with the SiC phase synthesized in situ by reaction bonding. Since ZrB2–SiC composites properties strongly depend on their grain size, finer and thus more expensive powders are usually employed. In the first part of the process, silicon carbide with a fine microstructure was produced by the reaction bonding between silicon powders and carbon derived from powders and pyrolysed phenolic resin The latter, being fluid before polymerization, filled the interstices between the powders thanks to the high pressure applied. Because of this behavior, after spark plasma sintering, the resulting microstructure presents the SiC phase dispersed between the ZrB2 grains in a different ways when compared with the standard ceramic powders consolidation methods.

Thermal shock behavior of ZrB2-SiC ceramics with different quenching media

The thermal shock behavior of ZrB2-SiC ceramics was studied with water, air and methyl silicone oil as quenching media, respectively. The temperature of all coolants was room temperature (25°C) and the residual strength of the ceramics after quenching was tested. The strength of the ceramics after water quenching had an obvious drop when the temperature difference, ΔT, was about 275°C, while the residual strength of the specimens quenched by air and silicone oil only varied a little and even increased slightly when the temperature difference was higher than 800°C. The different thermal conductive coefficient of the coolants and surface heat transfer coefficient resulted in the differences in the thermal shock behavior. The formation of oxidation layer was beneficial for improving the residual strength of the ceramics after quenching.

Preparation and Microstructure of Laminated ZrB2-SiC Ceramics with Porous ZrB2 Interlayers

Laminated ZrB2-SiC ceramics with porous ZrB2 interlayers were successfully prepared by Spark plasma Sintering (SPS)/tape casting, laminating and Reactive Spark Plasma Sintering (RSPS). ZrB2-SiC plates were sintered by SPS. And ZrO2-B4C sheets were prepared by tape casting, which will undergo reducing reaction that produces porous ZrB2 interlayers during SSPS processing. The ZrB2-SiC plates and ZrO2-B4C sheets were laminated alternately and reactive sintered to laminated ceramics with porous ZrB2 interlayers. The porosity and thickness of the porous layers were controlled by the tape casting processing and sintering pressure. The microstructure of the laminated ZrB2-SiC ceramics was analysed and discussed in details.

Diffusion bonding of ZrB2–SiC and Nb using dynamic compressed Ni foam interlayer

The ZrB2–SiC ceramics were diffusion bonded to Nb using different thicknesses of Ni foam interlayer. The bonding experiments were performed at 750°C, with a pressure of 20MPa and for a bonding time of 60min. A solid state intergranular penetration of Ni was found in ZrB2–SiC bonding region. The microstructures and chemical compositions of the joint interfaces were investigated by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. It was found that the Ni foam compression involving extensive ligaments merging and discontinuous interfacial bonding was beneficial for stiffness improvement and stress release. The asynchronous cracking along bonding interface resulted in a ductile failure mode. The maximum shear strength of the joints was 155.6MPa when using 4mm thickness Ni foam as interlayer.

Oxidation Behavior of Hot Pressed ZrB2-ZrC-SiC Ceramic Composites

Dense ZrB2-SiC ceramics containing 40 vol% ZrC particles are fabricated via hot pressing method. Then the sintered ceramics are oxidized in air up to 1500°C, and the oxidation kinetics of the ceramic composites is deduced in combination with the reacted fraction curves. As indicated by the experimental results, the oxidation kinetics changes from reaction-controlled process to diffusion-controlled one with increasing of oxidation temperature. In addition, the oxidation kinetics parameters are obtained, which indicates that the oxidation resistance decays at elevated temperatures. Furthermore, the evolution of surface morphology and oxide scale during oxidation process is clarified.