ZrB2-SiC Ultra-high Temperature Ceramics Research Articles

Fabrication and characterisation of high-performance joints made of ZrB2-SiC ultra-high temperature ceramics

Joining is crucial for ultra-high temperature ceramics (UHTCs) to be used in demanding environments due to the difficulty in manufacturing large and complex ceramic components. In this study, ZrB2-SiC composite UHTCs parts were joined via Ni foil as filler, and the mechanical properties and oxidation behaviour of the fabricated ZrB2-SiC/Ni/ZrB2-SiC (ZS/Ni/ZS) joint were investigated. Firstly, dense ZrB2-SiC composites were prepared from nano-sized powders by spark plasma sintering (SPS). The ZrB2-SiC parts were then joined using SPS. Furthermore, the elastic modulus, hardness, shear strength and high temperature oxidation behaviour of the ZS/Ni/ZS joint were examined to evaluate its properties and performance. The experimental results showed that the ZrB2-SiC parts were effectively joined via Ni foil using SPS and the resultant microstructures were free from any marked defects or residual metallic layers in the joint. Although the elastic modulus and hardness in the joining zone were lower than those in the base ZrB2-SiC ceramics, the shear strength of the joint reached ∼161 MPa, demonstrating satisfactory mechanical properties. Oxidation tests revealed that the ZS/Ni/ZS joint possesses good oxidation resistance for a wide range of elevated temperatures (800–1600 oC), paving the way for its employment in extreme environments.

Role of hot-pressing temperature on densification and microstructure of ZrB2–SiC ultrahigh temperature ceramics

This examination is aimed to study the various densification mechanisms in the ZrB2-SiC system at different sintering temperatures. For such an objective, hot-pressing process was implemented to fabricate three ceramics at sintering temperatures of 1650, 1850, and 2050 °C under 10 MPa for 1 h. According to the results, particle rearrangement and fragmentation were the predominant densification mechanisms at the lowest sintering temperature. Additionally, it was found that the activation of the liquid phase sintering mechanism was advantageous in the particle rearrangement at low temperatures. However, rising the sintering temperature to 1850 °C changed the dominant mechanism to the plastic deformation. Such a phenomenon was accompanied by the creation of many dislocations in both ZrB2 and SiC grains. Implementing a higher sintering temperature (2050 °C) activated another consolidation mechanism called diffusion. This occurrence, together with the evaporation of the majority of the liquid phase at elevated temperatures, resulted in emerging transgranular fracture mode in the sample.

Cyclic thermal shock behaviours of ZrB2-SiC ultra-high temperature ceramics joints bonded with Ni interlayer

Joining technique of ultra-high temperature ceramics is of great importance for their applications. In this work, ZrB2-20 vol% SiC composites were joined with Ni powders as brazed fillers by spark plasma sintering (SPS) and hot pressing (HP) at 1200 οC. Joining mechanisms and microstructure evaluation of the joints were discussed. In addition, a quartz lamp thermal shock testing platform was established, based on which thermal shock tests (20, 30, 40 and 50 cycles) with a rapid heating rate of 50 οC/s were conducted to investigate cyclic thermal shock resistance of the ceramics joints. Results showed that the joints were composed of the residual Ni layer, transition layer and initial ceramics layer. The average grains size of ZrB2 in the joints using SPS was lower than that in the joints using HP. The microstructures analysis indicated that joining using SPS could obviously improve the diffusion ability of different elements. Experimental results showed that cracks were prone to occur near the residual Ni layer and the interface between transition layer and initial ceramic layer, due to the joining defects and high thermal mismatching stress. Compared with the joints using HP, joints using SPS had better cyclic thermal shock resistance.

A novel microstructural design to improve the oxidation resistance of ZrB2-SiC ultra-high temperature ceramics (UHTCs)

Directionally Solidified 72.8 mol% ZrB2 - 27.2 mol% SiC (equivalent to 20 vol%) composite was fabricated using a crucibleless-melting technique. Microstructural and XRD analyses confirmed the texturing of grains and lamellar structure. The Lotgering (00l) orientation factor was as high as 0.97. The thickness of the lamellae was about 5 μm. Bright-field TEM images confirmed that the eutectic composite has continuous ZrB2 and SiC phases. Electron diffraction patterns from both the ZrB2 and SiC phases were indexed as hexagonal and 3C-cubic structures, respectively. Line defects such as dislocations, planar defects such as stacking faults and twins were observed in the SiC. Oxidation studies performed at 1873K for 1 h demonstrated that the samples obtained by melting showed an improved oxidation resistance compared to ZrB2-SiC composites obtained via sintering due to the formation of a passivating continuous SiO2 layer. The oxidation mechanisms are discussed, the orientation and SiC lamellae acted as an effective barrier to the inward oxygen flux and played a key role in retarding oxidation.

Residual stress estimation in laminated ZrB2-SiC ultra-high temperature ceramics with strong interfaces using X-ray diffraction and indentation techniques

Abstract Laminated ZrB2–SiC ultra-high temperature ceramics (UHTCs) with different layer thickness ratios and strong interfaces were fabricated by hot-pressing. Residual stresses developed on the surfaces of laminated ZrB2-SiC ceramics were evaluated by X-ray diffraction (XRD) and indentation techniques. Results showed that the characteristic XRD peaks of the ZrB2 phase on the surface of the laminated ceramics presented a shift due to the residual surface stress existence. Both XRD and indentation tested results suggested that the surface residual stress increases with the increasing layer thickness ratio confirming the theoretical analysis and calculation reported by our previous work. The presence of a compressive residual stress in the external layer of the heterogeneous material is expected to improve the mechanical properties of laminated ZrB2-SiC ceramics.

Temperature Jump Phenomenon During Plasmatron Testing of ZrB2-SiC Ultrahigh-Temperature Ceramics

Temperature Jump Phenomenon During Plasmatron Testing of ZrB2-SiC Ultrahigh-Temperature Ceramics

Temperature Jump Phenomenon During Plasmatron Testing of ZrB2-SiC Ultrahigh-Temperature Ceramics

U LTRAHIGH temperature ceramic (UHTC) materials containing hafnium diboride (HfB2) and zirconium diboride (ZrB2) with a silica former, most commonly silicon carbide (SiC), have been studied extensively over the last decade as materials for leading-edge and control surface components on hypersonic vehicles [1–3]. Such components experience extreme aerothermal heating in chemically aggressive, partially dissociated air environments. Promising aspects of diboride-based UHTC materials include the very high melting points of HfB2 and ZrB2 and their refractory oxides hafnia (HfO2) and zirconia (ZrO2), as well as the high thermal conductivities of HfB2 and ZrB2, which enables efficient heat conduction away from stagnation point regions [4]. Zirconium diboride has some advantages over hafnium diboride as an aerospace material, because it is lighter and less expensive. The oxidation of ZrB2 produces both zirconia and boron oxide (B2O3). Significant oxidation of ZrB2 in atmospheric air begins at about 1050 K. The softening temperature for amorphous B2O3 is in the range of 830–900 K [5]; below about 1500 K, the oxide scale consists of a porousZrO2 network filled with liquidB2O3 that acts as an effective oxygen diffusion barrier [6,7]. However, the vapor pressure of B2O3 increases rapidly with temperature [8], resulting in rapid loss of B2O3 above 1500 K. The residual porous zirconia scale provides little resistance to inward oxygen transport and further oxidation [9,10], making the oxidation resistance of pure ZrB2 insufficient for high-temperature hypersonic vehicle applications. The addition of a silica former to ZrB2 improves its oxidation resistance [11–14]. Compositions containing from 10 to 30% (by volume) SiC have generally been found to be optimal in this regard. The virgin ZrB2-SiC surfaces oxidize through parallel reactions that generate ZrO2,B2O3, and SiO2. LiquidB2O3 mixes with amorphous SiO2 to form a borosilicate glass that seals the ZrO2 scale [15]. With increasing temperature, boron oxide evaporates preferentially from Received 10 August 2011; revision received 13 February 2012; accepted for publication 19 February 2012. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0887-8722/12 and $10.00 in correspondence with the CCC. ∗Senior Research Scientist, Molecular Physics Laboratory, 333 Ravenswood Avenue; jochen.marschall@sri.com. Senior Member AIAA (Corresponding Author). Research Physicist, Molecular Physics Laboratory, 333 Ravenswood Avenue. Member AIAA. Professor, Department ofMaterials Science andEngineering, 223McNutt Hall; billf@mst.edu. Professor, Department ofMaterials Science andEngineering, 223McNutt Hall; ghilmas@mst.edu. Ph.D. Candidate, Aeronautics and Aerospace Department. Chaussee De Waterloo 72; panerai@vki.ac.be. ∗∗Associate Professor, Aeronautics and Aerospace Department, Chaussee De Waterloo 72; olivier.chazot@vki.ac.be. JOURNAL OF THERMOPHYSICS AND HEAT TRANSFER Vol. 26, No. 4, October–December 2012

Oxidation of ZrB2–SiC ultra-high temperature composites over a wide range of SiC content

The oxidation performance of ZrB2–SiC ultra-high temperature ceramics with SiC content ranging from 20 to 80 vol% has been evaluated at 1773 K for 50 h and at 2073 K for 20 min. Oxidation reaction pathways were interpreted using volatility diagrams of the ZrB2–SiC system. At 1773 K for 50 h, all ZrB2–SiC composites from 20 to 80 vol% SiC formed a protective SiO2 surface coating. Samples with ≤50 vol% SiC developed a distinguishable SiC-depleted layer at 1773 K and 2073 K. High temperature torch testing for 20 min at approximately 2073 K revealed that samples with ≥65 vol% SiC exhibit a depression under the torch flame. Samples rich in ZrB2 were dominated by a ZrO2 layer after a similar exposure. The overall weight density of ultra-high temperature ceramics can be reduced with improved oxidation performance at 1773 K by adding at least 65 vol% SiC.

Aqueous gelcasting of ZrB2–SiC ultra high temperature ceramics

ZrB2–SiC ultra high temperature ceramics containing B4C and C as sintering additives were successfully prepared by aqueous gelcasting and pressureless sintering. Polyacrylic acid (PAA) was used as the dispersant throughout this research. The various effects of zeta potential, pH value, dispersant concentration, solid loading and ball-milling time on the rheology and fluidity behavior of ZrB2–SiC suspension were investigated in detail. A well-dispersed suspension with 50vol.% solid loading was prepared at pH 10 with 0.4wt.% PAA after ball milling at 240rpm for 24h. Then crack-free green ZrB2–SiC ceramics were obtained by gelcasting process and then pressureless sintered at 2100°C to about 98% relative density. The microstructure and mechanical properties were examined, and the flexural strength and fracture toughness were 405±27MPa and 4.3±0.3MPam1/2, respectively.

Preparation and characterization of ZrB2-SiC ultra-high temperature ceramics by microwave sintering

ZrB2-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB2-SiC composites and almost fully dense ZrB2-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB2-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m1/2, respectively.

Ablation Resistant of Pressureless Sintered ZrB<sub>2</sub>-Based Ceramics

ZrB2-SiC and ZrB2-SiC-C ultra-high temperature ceramics (UHTCs) were fabricated by pressureless sintering under an argon atmosphere. The mass and linear ablation rates were tested in an oxyacetylene flame with high velocity. The microstructure and phase transformation of the ZrB2-based UHTCs were characterized by scanning electron microscopy along with energy dispersive spectrometry. Results show that the UHTCs have excellent properties of ablation resistance at ultra-high temperature. The values of mass and linear ablation rates were lower in the ZrB2-SiC UHTCs than those measured for ZrB2-SiC-C. The effect of C addition on the ablation resistant was not obvious but it influenced the microstructure of the ZrB2-SiC UHTCs. And the ablation resistant mechanisms of ZrB2-based UHTCs were discussed according to microstructure analysis.

Effects of Carbon Black on Microstructure and Mechanical Properties of Hot Pressed ZrB<sub>2</sub>-SiC Ceramics

Abstract. ZrB2-SiC ultra-high temperature ceramics (UHTCs) was hot-pressed at a temperature of 1900°C with the addition of carbon black as a reinforcing phase. Microstructure and mechanical properties were investigated. Analysis revealed that the amount of carbon black had a significant influence on the sinterability and mechanical properties of ZrB2-SiC ceramics. When a small amount ( < 10 vol.%) of carbon black was introduced, it may react with oxide impurities (i.e. ZrO2, B2O3 and SiO2) present on the surface of the starting powder, thus promote the densification and grain refining of ZrB2-SiC ceramics. As a result, the mechanical properties including flexural strength and fracture toughness were improved. However, with the further adoption of carbon black, mechanical properties were not improved much, which could be attributed to the redundant phase at grain boundaries. The results presented here point to a potential method for improving densification, microstructure and mechanical properties of ZrB2-based ceramic composites.

Hot‐Pressed ZrB2–SiC Ceramics with VC Addition: Chemical Reactions, Microstructures, and Mechanical Properties

Alterable roles of VC addition on the densification, microstructure development, and mechanical properties of the ZrB2–SiC ultrahigh temperature ceramics (UHTCs) were comprehensively investigated based on utilizing the thermodynamic analysis and experimental results. Chemical reactions of VC with the inevitably existed oxide (ZrO2) impurity on the particle surfaces of the ZrB2 starting powder, or with the ZrB2 matrix phase could be controlled by adjusting the atmospheres during hot pressing. Different chemical reactions during hot pressing also resulted in much different material microstructures and then different material properties. Benefiting the VC addition and proper sintering atmosphere, refined grain size of ZrB2 and improved mechanical properties were developed in ZrB2–SiC systems. By properly controlling these reactions, the microstructures and mechanical properties of ZrB2–SiC based UHTCs could be tailored.

Enhanced densification and mechanical properties of ZrB 2–SiC processed by a preceramic polymer coating route

Dense ZrB2–SiC ultra-high temperature ceramics were prepared by a novel pressureless sintering process. The relative density of sintered ceramics increased from 62.5% for ZrB2 to 96.7% by coating the starting ZrB2 powder with polycarbosilane, which was converted to C and SiC by pyrolysis. The mechanism of enhanced densification and the mechanical properties of the sintered ZrB2–SiC ceramics were investigated.

Pressureless Sintering of Ultra-High Temperature ZrB<sub>2</sub>-SiC Ceramics

ZrB2-SiC ultra-high temperature ceramics (UHTCs) were pressureless sintered with Y2O3-Al2O3 as the sintering additives. The effects of sintering additive and crystallization annealing on the microstructure and properties of ZrB2-SiC UHTCs were investigated. Sintering was activated by producing liquid phase of Y2O3 and Al2O3. The relative density of sintered ZrB2-20wt%SiC ceramic could reach 96% when the content of sintering additive was 6% and the sintering temperature was 1750°C and its bending strength, Vickers hardness, and fracture toughness were 412 MPa, 13 GPa, and 6.0 MPa•m1/2, respectively. The crystallization annealing can result in YAG phase from grain boundary and enhance the high temperature properties of the UHTCs. The UHTCs have excellent ablation resistance at ultra-high temperatures, and a very low ablation rate of 0.0006 mm/s after ablation for 900s at 2800°C.

Energy and Deformation during Explosive Compaction of ZrB2-SiC Ultrahigh Temperature Ceramics

To introduce a new technique and to choose the process parameters, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>ZrB</mml:mtext><mml:mn>2</mml:mn></mml:msub></mml:math>-SiC ultrahigh temperature ceramics (UHTCs) were prepared by mixing and explosive compaction. The explosive kinds or explosive mass was variable so as to change the explosive impact energy. We have studied the relationships of the explosive impact energy, the tube deformation energy, the powder compaction energy and the ratio of the explosive mass to the tube mass (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>R</mml:mi></mml:math>), the relationships of the tube deformation energy, the tube equivalent strain and the mass ratio <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>R</mml:mi></mml:math>, and the relationships of the densities of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>ZrB</mml:mtext><mml:mn>2</mml:mn></mml:msub></mml:math> composites and the powder compact energy. The results show that the densities of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>ZrB</mml:mtext><mml:mn>2</mml:mn></mml:msub></mml:math> composites reach 93.37% of theory density. For any kind of explosive, the reduction of the outer diameter and the equivalent strain of the steel tubes raises gradually with the rise of the mass ratio <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>R</mml:mi></mml:math>. Generally speaking, the higher the explosion speed of the explosive is, the larger the deformation degree and the equivalent strain of the steel tubes are. The explosive impact energy can be divided into two parts: the tube deformation energy and the powder compact energy; with the rise of the mass ratio <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>R</mml:mi></mml:math>, the tube deformation energy hardly changes, while the explosive impact energy and powder compact energy increase synchronously, and the densities of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>ZrB</mml:mtext><mml:mn>2</mml:mn></mml:msub></mml:math> composites also increase gradually. The density of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>ZrB</mml:mtext><mml:mn>2</mml:mn></mml:msub></mml:math> composites produced by different explosives orders from big to small as RDX, Ammonium Nitrate, TNT, and Urea Nitrate.