Effect of ZrB2 powders on densification,microstructure, mechanical properties and thermal conductivity of ZrB2-SiC ceramics

With the view to improve the densification behaviour and mechanical properties of ZrB₂-SiC ceramics, three synthesis routes were investigated for the production of ZrB₂, prior to the fabrication of ZrB₂-20 vol. % SiC via spark plasma sintering (SPS). Two borothermal reduction routes, modified with a water-washing stage (BRW) and partial solid solution of Ti (BRS), were utilised, alongside a boro/carbothermal mechanism (BRCR) were utilised to synthesise ZrB₂, as a precursor material for the production of ZrB₂-SiC. It was determined that reduction in the primary ZrB₂ particle size, alongside a diminished oxygen content, was capable of improving densification. ZrB₂-SiC ceramics, with ZrB₂ derived from BRW synthesis, exhibited a favorable combination of high relative density (98.6%), promoting a marked increase in Vickers hardness (21.4 ± 1.7 GPa) and improved thermal conductivity (68.7 W·m^-1K^-1).     

Crystal-size dependence of the spark-plasma-sintering kinetics of ZrB2 ultra-high-temperature ceramics

The crystal-size dependence of the spark-plasma-sintering (SPS) kinetics of ZrB₂ ultra-high-temperature ceramics (UHTCs) was investigated. It was found that refining the starting powder enhances the SPS kinetics, reducing the onset temperatures of sintering and of the intermediate and final sintering regimes, as well as promoting a greater maximum shrinkage rate at lower temperatures. This enhancement was only relevant with reduction in crystal size to the nanoscale. Finally, the implications for low-temperature sintering of ZrB₂ UHTCs are discussed.     

Effect of sintering temperature on electrical properties of SiC/ZrB2 ceramics

SiC/20?wt% ZrB₂ composite ceramics were fabricated via pressureless solid phase sintering in argon atmosphere at different temperature. The effect of sintering temperature on microstructure, electrical properties and mechanical properties of SiC/ZrB₂ ceramics was investigated. Electrical resistivity exhibits twice significant decreases with increasing sintering temperature. The first decrease from 1900?°C to 2000?°C is attributed to the obvious decrease of continuous pore channels in as-sintered materials. The second decrease from 2100?°C to 2200?°C results from the improvement of carbon crystallization and the disappearance of amorphous layers enveloping ZrB₂ grains. Additionally, the increase of sintered density with increasing temperature caused greatly advance of flexural strength, elastic modulus and Vickers hardness. But excessive temperature is detrimental to flexural strength because of SiC grain growth.     

ZrB2–SiC laminated ceramic composites

The oxidation behavior for ZrB₂–20 vol% SiC (ZS20) and ZrB₂–30 vol% SiC (ZS30) ceramics at 1500 °C was evaluated by weight gain measurements and cross-sectional microstructure analysis. Based on the oxidation results, laminated ZrB₂–30 vol% SiC (ZS30)/ZrB₂–25 vol% SiC (ZS25)/ZrB₂–30 vol% SiC (ZS30) symmetric structure with ZS30 as the outer layer were prepared. The influence of thermal residual stress and the layer thickness ratio of outer and inner layer on the mechanical properties of ZS30/ZS25/ZS30 composites were studied. It was found that higher surface compressive stress resulted in higher flexural strength. The fracture toughness of ZS30/ZS25/ZS30 laminates was found to reach to 10.73 MPa m^1/2 at the layer thickness ratio of 0.5, which was almost 2 times that of ZS30 monolithic ceramics.     

High thermal-conductivity rGO/ZrB2-SiC ceramics consolidated from ZrB2-SiC particles decorated GO hybrid foam with enhanced thermal shock resistance

Graphene derivative materials exhibit excellent mechanical and thermal properties, which have been extensively used to toughen ceramics and improve thermal shock resistance. To overcome the thermal agglomeration of graphene oxide (GO) during heating and drying process, ZrB₂-SiC particles decorated GO hybrid foam with uniformly anchored ceramic particles was synthesized by electrostatic self-assembly and liquid nitrogen-assisted freeze-drying process. Densified rGO/ZrB₂-SiC ceramics with varying microstructure, thermal physical and mechanical properties were obtained by adjusting the content of decorated ceramic particles. Although the flexural strength of rGO/ZrB₂-SiC ceramics have an attenuation compared with that of ZrB₂-SiC ceramic, the thermal conductivity, work of fracture and thermal shock resistance are greatly improved. rGO/ZrB₂-SiC ceramics exhibit delayed fracture and increasing R-curve behavior during the crack propagation. The novel preparation technology allows for the well dispersion of rGO in ZrB₂-SiC ceramics and can be easily extended to other ceramic or metal materials systems.     

Enhanced mechanical properties and thermal shock resistance of Cf/ZrB2-SiC composite via an efficient slurry injection combined with vibration-assisted vacuum infiltration

An efficient slurry injection combined with vibration-assisted vacuum infiltration process has been developed to fabricate 3D continuous carbon fiber reinforced ZrB₂-SiC ceramics. Homogenous distribution between carbon fiber and ceramic was achieved successfully, leading to an enhancement in mechanical properties. The C_f-PyC/ZrB₂-SiC composite exhibited a typical non-brittle fracture mode with a superior fracture toughness of 6.72 ± 0.21 MPa·m^1/2 and an extraordinary work of fracture of 2270 J/m², respectively, increasing by nearly 14.8 % and 36 % as compared with those of a parent composite fabricated by only slurry injection and slurry infiltration. The enhancement in fracture toughness and work of fracture were attributed to multiple toughening mechanism including crack deflection, PyC coated fiber bundles pull-out and fiber bridging. Moreover, a critical thermal shock temperature difference of 814 °C was achieved, higher than that of traditional ZrB₂-based ceramics. This work presents an efficient approach to fabricate high-performance C_f/UHTCs with uniform architecture.     

Combustion synthesis of high-temperature ZrB2-SiC ceramics

The work is dedicated to researching into combustion kinetics and mechanism as well as the stages of the chemical transformations during self-propagating high-temperature synthesis of ZrB₂-SiC based ceramics. Dependences of the combustion temperature and rate on the initial temperature (T₀) have been studied. It has been shown that the stages of the chemical reactions of ZrB₂ diboride and SiC carbide formation do not change within the range of T_0?=?298–700?К. The effective activation energy of the combustion process amounted to 170–270?kJ/mol, from which it has been concluded that chemical interaction through the melt plays a leading role. The stages of the chemical transformations in the combustion wave have been studied by dynamic X-ray diffraction. First, ZrB₂ phase forms from Zr-Si melt saturated with boron, and SiC phase is registered later. The SHS method has successfully been used in order to obtain ZrB₂-SiC composite powders and compact ceramics with a silicon carbide content of 25–75%. The ceramics are characterized by a residual porosity of 1.5%, hardness up to 25?GPa, the elastic modulus of 318?±?21?GPa, elastic recovery of 36% and thermal conductivity of 54.9?W/(m?×?K) at T_room.     

Aqueous Gelcasting and Pressureless Sintering of Zirconium Diboride Ceramics

Dense (97.3%) zirconium diboride (ZrB₂) ceramics were obtained via gelcasting and pressureless sintering. Four wt% B₄C was used as sintering aid. ZrB₂, SiC, and B₄C can codisperse well in the alkaline region, using a polyacrylate dispersant. Compared with monolithic ZrB₂ (Z), the mechanical properties of ZrB₂‐SiC (ZS) were enhanced. The Vickers hardness and fracture toughness of ZS were (13.1 ± 0.6) GPa and (2.5 ± 0.4) MPa m^1/2, respectively.     

High temperature oxidation behavior of ZrB2-SiC added MoSi2 ceramics

In this paper, MoSi₂, MoSi₂-20?vol% (ZrB₂-20?vol% SiC) and MoSi₂-40?vol% (ZrB₂-20?vol% SiC) ceramics were prepared using pressureless sintering. The oxidation behaviors of these MoSi₂-(ZrB₂-SiC) ceramics were investigated at 1600?°C for different soaking time of 60, 180 and 300?min, respectively. The oxidation behaviors of the MoSi₂-(ZrB₂-SiC) ceramics were studied through weight change test, oxide layer thickness measurement, and microstructure analysis. Further investigation of the oxidation behaviors of the MoSi₂-(ZrB₂-SiC) ceramics was conducted at a higher temperature of 1800?°C for 10?min. The microstructure evolution of the ceramics was also analyzed. It was finally found that the oxidation resistance of MoSi₂ was improved by adding ZrB₂-SiC additives, and the MoSi₂-20?vol% (ZrB₂-20?vol% SiC) ceramic exhibited the optimal oxidation resistance behavior at elevated temperatures. From this study, it is believe that it can give some fundamental understanding and promote the engineering application of MoSi₂-based ceramics at high temperatures.     

Thermal properties of La2O3-doped ZrB2- and HfB2-based ultra-high temperature ceramics

Thermal properties of La₂O₃-doped ZrB₂- and HfB₂-based ultra high temperature ceramics (UHTCs) have been measured at temperatures from room temperature to 2000 °C and compared with SiC-doped ZrB₂- and HfB₂-based UHTCs and monolithic ZrB₂ and HfB₂. Thermal conductivities of La₂O₃-doped UHTCs remain constant around 55–60 W/mK from 1500 °C to 1900 °C while SiC-doped UHTCs showed a trend to decreasing values over this range.     

Pursuing enhanced oxidation resistance of ZrB2 ceramics by SiC and WC co-doping

The oxidative degradation of ZrB₂ ceramics is the main challenge for its extensive application under high temperature condition. Here, we report an effective method for co-doping suitable compounds into ZrB₂ in order to significantly improve its anti-oxidation performance. The incorporation of SiC and WC into ZrB₂ matrix is achieved using spark plasma sintering (SPS) at 1800?°C. The oxidation behavior of ZrB₂-based ceramics is investigated in the temperature range of 1000?°C–1600?°C. The oxidation resistance of single SiC-doped ZrB₂ ceramics is improved due to the formation of silica layer on the surface of the ceramics. As for the WC-doped ZrB₂, a dense ZrO₂ layer is formed which enhances the oxidation resistance. Notably, the SiC and WC co-doped ZrB₂ ceramics with relative density of almost 100% exhibit the lowest oxidation weight gain in the process of oxidation treatment. Consequently, the co-doped ZrB₂ ceramics have the highest oxidation resistance among all the samples.     

Spark plasma sintering of ZrB2- and HfB2-based Ultra High Temperature Ceramics prepared by SHS

The combination of the SHS technique and the Spark Plasma Sintering (SPS) technology was adopted in this work for the fabrication of fully dense MB₂-SiC and MB₂-MC-SiC (M = Zr, Hf) Ultra High Temperature Ceramics (UHTCs). Specifically, Zr or Hf, B₄C, Si, and (for the cases of ternary systems) graphite powders were first reacted by SHS to successfully form the desired composites. The resulting powders were then subjected to consolidation by SPS. In particular, by setting a dwell temperature level of 1800°C, a mechanical pressure of 20 MPa, and a non-isothermal heating time of 10 min, products with relative densities ≥98.5% were obtained for the all systems investigated within 30 min of total processing time. The characteristics of the resulting dense UHTCs, i.e. hardness, fracture toughness, and oxidation resistance, are similar to, and in some cases superior than, those related to analogous products synthesized by alternative, less rapid, methods. The article is published in the original.     

Spark-plasma sintering of ZrB2 ultra-high-temperature ceramics at lower temperature via nanoscale crystal refinement

We have explored the feasibility of reducing the spark-plasma-sintering (SPS) temperature of additive-free ZrB₂ ultra-high-temperature ceramics (UHTCs) via crystal size refinement of the starting powder down to the low nanoscale. We found that under otherwise the same SPS conditions (75 MPa pressure, and 100 °C/min heating ramp) nanoscale ZrB₂ can be densified at temperatures about 450 °C lower than for the typical micrometre and submicrometre ZrB₂ powders, and at least 250 °C below the ultra-fine powder temperature. Furthermore, the nanoscale crystal refinement also promotes the production of fine-grained ZrB₂ UHTCs. We also found that elimination of the B₂O₃ impurities plays an important role in the complete densification. The unequalled sinterability of the nanoscale ZrB₂ powders highlights the need to use high-energy ball-milling for the comminution of the typical commercially available ZrB₂ powders.     

Ultra-high temperature ceramics: Aspiration to overcome challenges in thermal protection systems

Ultra-high temperature ceramics (UHTCs) have played a significant role in fulfilling demands for the thermal protection system (TPS) in the aerospace sector, however, a promising candidate has not emerged yet. This critical review provides typical inconsistencies and new perspectives related to UHTCs in terms of: (i) material and processing: i.e., sinterability, reinforcements, microstructural evolution, (ii) properties and performance correlation with the processing conditions and resulting microstructure, and (iii) outlook on the most promising ZrB₂-HfB₂-SiC-based composites as potential candidates for hypersonic leading edge and re-entry structures. An optimal selection of the content, size and reinforcing phase (such as silicides, refractory carbides, and carbon-based, etc.) is mandated in upgrading the thermo-mechanical performance of UHTCs to sustain elevated temperature (1700 °C), exhibiting flexural/fracture strength of >300 MPa, high thermal conductivity >14.5 Wm^?1K^?1, and high oxidation resistance (<80 gm^?2 over 2 h at 1400 °C). From emphasis on the powder purity, and sintering additives on affecting the densification, mechanical properties and high temperature oxidation, improvements in the functional performance of UHTCs are carried forward with emphasis mainly on borides and carbides. Emergence of SiC as most promising sintering additive with optimal content of ～20 vol%, and with supplemented HfB₂ addition in ZrB₂-HfB₂-SiC based UHTCs have exhibited higher oxidation resistance and may serve as conceivable entrants for hypersonic vehicles. Further, the review leads the reader to developing new materials (including silicides, MAX phases, and high entropy UHTCs), incorporating novel strategies like designing layered structures or functionally graded materials (FGM), and effective joining to allow the integration of smaller components into scaled up structures. On one hand, where plasma arc-jet exposure mimics high heat-flux exposures, the utilization of multi-length-scale computational modeling (such as finite element methods, density functional theory, ab initio etc.) allows assessing the material performance under dynamic changes (of variable partial pressure, temperatures, gradation, etc.) towards perceiving new insights into the structural stability and thermo-mechanical properties of UHTCs. This review critically underlines the present state of the art and guides the reader towards the futuristic development of new class of high-temperature materials for TPSs.     

Study of tribological properties of polymer derived ZrB2-SiC ceramics

ZrB₂–SiC composite ceramics with different compositions (20 and 60?vol% ZrB₂–SiC, 20ZS and 60 ZS, respectively) were prepared. Wear tests were conducted on the obtained ceramics in multiple distances using ball-on-flat tribotester. Volume loss and cross-sectional profiles of samples were measured by three–dimensional (3D) profilometer to study the onset of track wear damages. Pressure–depth curves and hardness were measured by indentation to investigate defects produced in the tribo-film by the debris. The debris of 20ZS was found to be joined to the tribo-film and accumulated with distance, shifting from microcrack (＜10,000 cycles) to abrasive wear (50,000 cycles). Compared to 20ZS, lower debris accumulation of 60ZS resulted in better wear resistance, leading to thinner and more stable non-substrate regions for this sample. These differences between both samples basically resulted from different particle sizes. Fine grains were easily pulled out in the experiment, resulting in abrasive wear of the specimen. While transgranular fracture of grains and the pinning led to larger grains with less debris, the damage mode remained transgranular fracture.     

Preparation of dense SiHf(B)CN-based ceramic nanocomposites via rapid spark plasma sintering

Dense SiHf(B)CN-based ceramic nanocomposites were prepared by spark plasma sintering (SPS) using high heating rates (∼450 ^°C/min.) and high pressures (≥100 MPa). The obtained nanocomposites were investigated by X-ray diffraction, Raman spectroscopy and electron microscopy concerning their phase evolution and microstructure.The hardness and the elastic modulus of dense SiHfCN were found to be 26.8 and 367 GPa, respectively. Whereas the SiHfBCN samples exhibited a hardness of 24.6 GPa and an elastic modulus of 284 GPa. The investigation of the oxidation of the prepared dense ceramic nanocomposites at high temperature revealed that the parabolic oxidation rates of SiHfCN were comparable to those of ultra-high temperature ceramics (UHTCs, e.g. HfC-20 vol% SiC); whereas the parabolic oxidation rates of SiHfBCN were several orders of magnitude lower than those. The results obtained within this study indicate the feasibility of SPS for rapid preparation of dense though nano-scaled Hf-containing ceramic nanocomposites that are promising candidates for high-temperature applications in harsh environments.     

Si-SiC-ZrB2 ceramics by silicon reactive infiltration

Silicon carbide ceramics obtained by silicon reactive infiltration are nowadays employed within industry in several high temperature applications. Although these ceramics show good thermo-mechanical properties and oxidation resistance, they suffer temperature limitations (1400 °C). At higher temperatures another type of ceramics, commonly known as ultra high temperature ceramics (UHTCs), is under study. These include the transition metal diborides of group IV; one in particular, zirconium diboride, is interesting in certain applications (e.g. aerospace) because of its low relative density. ZrB₂-SiC ceramics show good thermo-mechanical properties and maintain the “protective” passive oxidation regime of their scales over a wide range of temperatures.This paper presents a feasibility study on a manufacturing methodology to produce Si-SiC-ZrB₂ bulk ceramics taking advantage of the reactive infiltration technique. This technique allows lower processing temperatures and near to net shape capability due to low shrinking of the green compacted bodies. C-SiC-ZrB₂ preforms were successfully infiltrated with molten silicon. The resulting Si-SiC-ZrB₂ composites showed promising oxidation behavior, similarly to that reported in other works. Bulk material optimization was performed with a view to manufacturing Si-SiC-ZrB₂ ceramic matrix composites by silicon reactive infiltration in future.     

Effect of ZrB2 content on the densification,microstructure, and mechanical properties of ZrC-SiC ceramics

ZrC ceramics containing 30 vol% SiC-ZrB₂ were produced by high-energy ball milling and reactive hot pressing. The effects of ZrB₂ content on the densification, microstructure, and mechanical properties of ceramics were investigated. Fully dense ceramics were achieved as ZrB₂ content increased to 10 and 15 vol%. The addition of ZrB₂ suppressed grain growth and promoted dispersion of the SiC particles, resulting in fine and homogeneous microstructures. Vickers hardness increased from 23.0 ± 0.5 GPa to 23.9 ± 0.5 GPa and Young’s modulus increased from 430 ± 3 GPa to 455 ± 3 GPa as ZrB₂ content increased from 0 to 15 vol%. The increases were attributed to a combination of the higher relative density of ceramics with higher ZrB₂ content and the higher Young’s modulus and hardness of ZrB₂ compared to ZrC. Indentation fracture toughness increased from 2.6 ± 0.2 MPa⋅m^1/2 to 3.3 ± 0.1 MPa⋅m^1/2 as ZrB₂ content increased from 0 to 15 vol% due to the increase in crack deflection by the uniformly dispersed SiC particles. Compared to binary ZrC-SiC ceramics, ternary ZrC-SiC-ZrB₂ ceramics with finer microstructure and higher relative densities were achieved by the addition of ZrB₂ particles.     

Mechanical and high temperature oxidation behavior of HfB2-ZrB2-SiC-B4C-CNT composites joined with and without Ni interlayer

Difficulties associated with the manufacturing of diboride based large and complex shapes mandate them to be joined for extreme applications. In the present work, spark plasma sintering is utilized to join HfB₂-ZrB₂-SiC-B₄C-CNT (HZSBC) based composites with Ni-interlayer at 1100 °C and without interlayer at 1800 °C. Microstructural analysis has elicited the formation of Ni diffused reaction zone (150–200 μm), and unaffected HZSBC composite for HZSBC-Ni-HZSBC joined composite (at 1100 °C), which gets merged into a homogeneous microstructure (without any distinguishable interface) in the HZSBC-HZSBC joint (without Ni-interlayer). An overall reduction of 18% and 14% is elicited in hardness and elastic modulus, respectively, in the HZSBC-Ni-HZSBC joint, whereas in HZSBC-HZSBC joined composite, uniform hardness and elastic modulus of ～22 GPa and ~398 GPa, respectively, is observed. Further, the bending and shear strength of the joined composites was obtained to be ～209 MPa and ～41 MPa, respectively, for HZSBC-Ni-HZSBC, which increased to ~342 MPa and ～81 MPa, respectively, for HZSBC-HZSBC. In conjunction, though superior high-temperature (1500 °C) oxidation protection is witnessed in HZSBC-HZSBC joint, with a thinner oxide layer, both display SiO₂-rich protective glassy layer, making these composites potential candidates for use in atmospheric re-entry conditions.     

Multiscale toughening of ZrB2-SiC-Graphene@ZrB2-SiC dual composite ceramics

The design of bioinspired architectures is effective for increasing the toughness of ceramic materials. Particularly, a dual composite equiaxial architecture is ideal for fabricating weak interface-toughened ZrB₂-SiC ceramics with isotropic performance. In this paper, ZrB₂-SiC-Graphene@ZrB₂-SiC dual composite ceramics were synthesized via an innovative processing technique of granulating-coating method. ZrB₂-20 vol.% SiC containing 30 vol.% Graphene was selected as weak interface to realize multiscale toughening and improve the thermal shock resistance of ZrB₂-SiC ceramic materials. The incorporation of ZrB₂-SiC-Graphene weak interface into the ZrB₂-SiC matrix improved the damage tolerance and critical thermal shock temperature difference. The design of equiaxial structures moderated the anisotropy of performance in different planes. The graphene sheets incorporated in the ZrB₂-SiC-Graphene interface phase played a key role in multiscale toughening, including macroscopic toughening of crack deflection and microcracks, and microscopic toughening of graphene bridging and pull-out.