Thermal properties of ZrB2-TiB2 solid solutions

Zirconium diboride ceramics were prepared with additions of up to 50 vol.% TiB₂. The resulting (Zr,Ti)B₂ ceramics formed complete solid solutions based on x-ray diffraction. The addition of TiB₂ resulted in grain size decreasing from 22 μm for nominally pure ZrB₂ to 7 μm for ZrB₂–50 vol.% TiB₂. The thermal conductivity at 25°C ranged from 93 W/m⋅K for nominally pure ZrB₂ to 58 W/m⋅K for ZrB₂–50 vol.% TiB₂. Thermal conductivity was as high as 67 W/m⋅K for nominally pure ZrB₂ at 2000°C, but dropped to 59 W/m K with the addition of 50 vol.% TiB₂. Electrical resistivity measurements were used to calculate the electron contribution to thermal conductivity, which was 76 W/m⋅K for nominally pure ZrB₂ decreasing to 57 W/m⋅K when 50 vol.% TiB₂ was added. The phonon contribution to thermal conductivity did not change significantly for ≤10 vol.% TiB₂. Additions of ≥25 vol.% TiB₂ reduced the phonon contribution to nearly zero for all temperatures.     

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.     

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.     

Effects of Ti,Y, and Hf additions on the thermal properties of ZrB2

Reactive hot pressing was utilized to synthesize and densify four ZrB₂ ceramics with impurity contents low enough to avoid obscuring the effects of dopants on thermal properties. Nominally pure ZrB₂ had a thermal conductivity of 141 ± 3 W/m K at 25 °C. Additions of 3 at% of Ti, Y, or Hf decreased the thermal conductivity by 20 %, 30 %, and 40 %, respectively. The thermal conductivity of (Zr,Hf)B₂ was similar to ZrB₂ synthesized from commercial powders containing the natural abundance of Hf as an impurity. This is the first study to demonstrate that Ti and Y additions decrease the thermal conductivity of ZrB₂ ceramics and report intrinsic values for thermal conductivity and electrical resistivity of ZrB₂ containing transition metal additions. Previous studies were unable to detect these effects because the natural abundance of Hf present masked the effects of these additions.     

Potential-current characteristics in SiC/ZrB2 composite ceramics

ZrB₂/SiC composite ceramics were fabricated to improve the electrical conductive properties of SiC matrix. The debinding and sintering temperatures were determined by computation of Gibbs free energy. As a result, all the samples have the relative density above 99%, and have excellent mechanical and electrical properties. The effects of ZrB₂ content on the microstructure, mechanical and electrical properties were systematically studied. With increasing ZrB₂ content, as-prepared composites show great improvement in their mechanical properties. Importantly, the introduction of ZrB₂ weakened varistor nonlinear characteristic of composite and reduced its resistivity. The reason is the evolution of grain boundary in conductive paths. The sharp decrease of resistivity indicates the formation of percolation paths. The percolation threshold at 1?mA?cm^?2 obtained via percolation model is 10.7963?vol% (19.7098?wt%) ZrB₂. This value is much less than conventional composites, because the percolation path originates from grain boundary breakdown other than continuous conductor chains.     

Processing,microstructure, and mechanical properties of zirconium diboride-boron carbide ceramics

The processing, microstructure, and mechanical properties of zirconium diboride-boron carbide (ZrB₂-B₄C) ceramics were characterized. Ceramics containing nominally 5, 10, 20, 30, and 40 vol% B₄C were hot-pressed to full density at 1900 °C. The ZrB₂ grain size decreased from 4 to 2 µm and B₄C inclusion size increased from 3 to 5 µm for B₄C additions of 5 and 40 vol% B₄C, respectively. Elastic modulus decreased from 525 to 515 GPa and Vickers hardness increased from 15 to 21 GPa as the B₄C content increased from 5 to 40 vol%, respectively, following trends predicted using linear rules of mixtures. Flexure strength and fracture toughness both increased with increasing B₄C content. Fracture toughness increased from 4.1 MPa m^½ at 5 vol% B₄C to 5.3 MPa m^½ at 40 vol% B₄C additions. Flexure strength was 450 MPa with a 5 vol% B₄C addition, increasing to 590 MPa for a 40 vol% addition. The critical flaw size was calculated to be ~30 µm for all compositions, and analysis of the fracture surfaces indicated that strength was controlled by edge flaws generated by machining induced sub-surface damage. Increasing amounts of B₄C added to ZrB₂ led to increasing hardness due to the higher hardness of B₄C compared to ZrB₂ and increased crack deflection. Additions of B₄C also lead to increases in fracture toughness due to increased crack deflection and intergranular fracture.     

ZrB2-based solid solution ceramics and their mechanical and thermal properties

Zirconium diboride ceramics as one of the main members of ultrahigh-temperature ceramics are capable of being used as structural components at ultrahigh temperatures. Entropy adjusting is a newly developed approach to improving the properties of ceramics. In this work, a series of ZrB₂-based solid solution ceramics with different mixing entropies, formulated (Zr_xTi_yNb_yTa_y)B₂ (x = .25, .85, .925, .9625, 1; x + 3y = 1), were prepared by adjusting the content of other diborides. Diboride solid solution powders were synthesized by boro/carbothermal reduction process and then densified by spark plasma sintering. The results show that the formation of a single-phase solid solution is independent of the mixing entropy in (Zr_xTi_yNb_yTa_y)B₂ system. The addition of other diborides into ZrB₂ is beneficial to reduce the particle size of the synthesized powder and promote the densification process. The dense sintered samples with higher mixing entropy have finer grain size, higher hardness, and modulus. The (Zr_0.25Ti_0.25Nb_0.25Ta_0.25)B₂ ceramic has the highest hardness of 31 GPa and a modulus of 682 GPa. Severe lattice distortion in samples with higher mixing entropy will result in increased phonon scattering and lower thermal conductivity.     

High temperature erosion behavior of spark plasma sintered ZrB2-SiC composites

Damage of structural components of hypersonic vehicles by atmospheric particles demands thorough understanding on their wear behavior. In the present work, dense ZrB₂-SiC (10, 20, and 30 vol%) composites are prepared by spark plasma sintering at 55 MPa in two stages: 1400 °C for 6 min followed by 1600 °C for 2 min. With increase in SiC content, microstructures of sintered composites reveal strongly bonded ZrB₂ grains with SiC particles. A combination of maximum hardness of 23 GPa, elastic modulus of 398 GPa and fracture toughness of 5.4 MPa m^1/2 are obtained for the composite containing 30 vol% SiC particles. It is found that cracks are bridged or deflected by SiC particles in the composites. When the composites are subjected to SiC particle erosion at 800 °C, a 14% decrease in erosion rate is obtained with increase in SiC content from 10 to 30 vol%. The formation of large extent of boro-silicate rich viscous surface on eroded surfaces is attributed to reduced fracture or removal of ZrB₂ grains of the composites with increased SiC content.     

Investigation of the density and microstructure homogeneity of square-shaped B4C-ZrB2 composites produced by spark plasma sintering method

Square-shaped monolithic B₄C and B₄C-ZrB₂ composites were produced by spark plasma sintering (SPS) method to investigate the effect of 5, 10, 15 vol% ZrB₂ addition on the densification, mechanical and microstructural properties of boron carbide. The relative density of B₄C increased with the increasing volume fraction of ZrB₂ and density differences in different regions of the sample narrowed down. Homogeneous density distribution and microstructure were accomplished with the increasing holding time from 7 to 20 min for the B₄C-15 vol% ZrB₂ composites, and the highest overall relative density was achieved as 99.23%. The hardness and fracture toughness of composites were enhanced with the addition of ZrB₂ compared to monolithic B₄C. The enhancement in fracture toughness was observed due to the crack deflection, crack bridging and crack branching mechanisms. The B₄C-15 vol% ZrB₂ composite exhibited the combination of superior properties (hardness of 33.08 GPa, Vickers indentation fracture toughness of 3.82 MPa.m^1/2).     

Combined role of SiC particles and SiC whiskers on the characteristics of spark plasma sintered ZrB2 ceramics

In this research work, the effects of silicon carbide (SiC) as the most important reinforcement phase on the densification percentage and mechanical characteristics of zirconium diboride (ZrB₂)-matrix composites were studied. In this way, a monolithic ZrB₂ ceramic (as the baseline) and three ZrB₂ matrix specimens each of which contains 25 vol% SiC as reinforcement in various morphologies (SiC particulates, SiC whiskers, and a mixture of SiC particulates/SiC whiskers), have been processed through spark plasma sintering (SPS) technology. The sintering parameters were 1900 °C as sintering temperature, 7 min as the dwell time, and 40 MPa as external pressure in vacuum conditions. After spark plasma sintering, a relative density of ~96% was obtained (using the Archimedes principles and mixture rule for evaluation of relative density) for the unreinforced ZrB₂ specimen, but the porosity of composites containing SiC approached zero. Also, the assessment of sintered materials mechanical properties has shown that the existence of silicon carbide in ZrB₂ matrix ceramics results in fracture toughness and microhardness improvement, compared to those measured for the monolithic one. The simultaneous addition of silicon carbide particulates (SiC_p) and whiskers (SiC_w) showed a synergistic effect on the enhancement of mechanical performance of ZrB₂-based composites.     

Mechanical properties of sintered ZrB2-SiC ceramics

ZrB₂ ceramics containing 10-30 vol% SiC were pressurelessly sintered to near full density (relative density >97%). The effects of carbon content, SiC volume fraction and SiC starting particle size on the mechanical properties were evaluated. Microstructure analysis indicated that higher levels of carbon additions (10 wt% based on SiC content) resulted in excess carbon at the grain boundaries, which decreased flexure strength. Elastic modulus, hardness, flexure strength and fracture toughness values all increased with increasing SiC content for compositions with 5 wt% carbon. Reducing the size of the starting SiC particles decreased the ZrB₂ grain size and changed the morphology of the final SiC grains from equiaxed to whisker-like, also affecting the flexure strength. The ceramics prepared from middle starting powder with an equiaxed SiC grain morphology had the highest flexure strength (600 MPa) compared with ceramics prepared from finer or coarser SiC powders.     

Reactive hot-pressing of ZrB2-ZrC-SiC ceramics via direct addition of SiC

A series of ZrB₂-ZrC-SiC composites with various SiC content from 0 to 20 vol% were prepared by reactive hot-pressing using Zr, B₄C and SiC as raw materials. Self-propagating high-temperature synthesis (SHS) occurred, and ZrC grains connected each other to form a layered structure when the SiC content is below 20 vol%. The evolution of microstructure has been discussed via reaction processes. The composite with 10 vol% SiC presents the most excellent mechanical properties (four-point bending strength: 828.6±49.9 MPa, Vickers hardness: 19.9±0.2 GPa) and finest grain size (ZrB₂: 1.52 µm, ZrC: 1.07 µm, SiC: 0.79 µm) among ZrB₂-ZrC-SiC composites with various SiC content from 0 to 20 vol%.     

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.     

Effect of SiC addition on the formation of ZrB2 through reaction of ZrO2, boric acid (H3BO3) and carbon black

ZrB₂-SiC powders with different amounts of SiC (10–30 wt%) were in-situ synthesized at 1600 °C for 90 min in Ar atmosphere. Effects of SiC addition on the formation of ZrB₂ via carbothermal reduction of ZrO₂, H₃BO₃ and carbon black were investigated. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and transmission electron microscope (TEM). The grain size of ZrB₂ in final powders decreased with adding SiC. Columnar ZrB₂ and granular SiC were combined interactively when the SiC content was 25 wt%. Layer-like hexagonal SiC was obtained in the product containing 30 wt% SiC, whereas the ZrB₂ grain growth was strongly inhibited. Furthermore, the growth mechanisms of ZrB₂ and SiC were studied.     

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).     

Sol-gel assisted synthesis of TiB2-B4C composite powder

Composite powders containing titanium diboride and boron carbide have been prepared by sol-gel method at 1450°C using titanium isopropoxide (titanium precursor), boric acid (Boron precursor), sucrose (carbon source), and acetic acid (AcOH) as a solvent. The effect of boron source (trimethyl borate and boric acid) and B₂O₃/TiO₂, C/B₂O₃ mole ratios of starting materials on the final phases has been studied. The progress of reactions was determined using thermal analysis (TGA-DTA). The resultant powders were characterized by X-ray diffraction (XRD) analysis and field-emission scanning electron microscope (FESEM). XRD patterns confirmed the formation of TiB₂, B₄C, and TiC phases after heat treatment at 1450°C at mole ratio of B₂O₃/TiO₂ = 4.5, C/B₂O₃ = 2.4. With increasing the content of boron oxide, the unwanted phases such as TiC and C were reduced. TiB₂ and B₄C composite powders (~5 µm diameter) containing residual carbon (<4 wt%) were synthesized using the mole ratio of B₂O₃/TiO₂ = 10 and C/B₂O₃ = 1.9 at low temperature of 1450°C.     

ZrB2-MoSi2 ceramics: A comprehensive overview of microstructure and properties relationships. Part II: Mechanical properties

The mechanical behavior of ZrB₂-MoSi₂ ceramics made of ZrB₂ powder with three different particle sizes and MoSi₂ additions from 5 to 70 vol% was characterized up to 1500 °C. Microhardness (12–17 GPa), Young’s modulus (450–540 GPa) and shear modulus (190–240 GPa) decreased with both increasing MoSi₂ content and with decreasing ZrB₂ grain size. Room temperature fracture toughness was unaffected by grain size or silicide content, whilst at 1500 °C in air it increased with MoSi₂ and ZrB₂ grain size, from 4.1 to 8.7 MPa m^½. Room temperature strength did not trend with MoSi₂ content, but increased with decreasing ZrB₂ grain size from 440 to 590 MPa for the largest starting particle size to 700–800 MPa for the finest due to the decreasing size of surface grain pullout. At 1500 °C, flexure strength for ZrB₂ with MoSi₂ contents above 25 vol% were roughly constant, 400–450 MPa, whilst for lower content strength was controlled by oxidation damages. Strength for compositions made using fine and medium ZrB₂ powders increased with increasing MoSi₂ content, 250–450 MPa. Ceramics made with coarse ZrB₂ displayed the highest strengths, which decreased with increasing MoSi₂ content from 600 to 450 MPa.     

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.     

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.     

ZrB2-MoSi2 ceramics: A comprehensive overview of microstructure and properties relationships. Part I: Processing and microstructure

This study systematically correlates processing with quantitative microstructural information over an extended compositional range for ZrB₂-MoSi₂ ceramics, with MoSi₂ contents ranging from 5 to 70 vol% and diboride starting particle sizes ranging from 3 to 12 μm. Fifteen different ceramics were hot pressed between 1750 and 1925 °C. Plastic deformation of MoSi₂ contributed to initial densification, but some of the MoSi₂ decomposed during the later stages of hot pressing. Finer diboride particles required lower temperatures to densify (1750 to 1850 °C) compared to coarser diboride particles (1900 °C). Increasing MoSi₂ content led to a decrease in sintering temperature. As MoSi₂ content increased, ZrB₂ grain size decreased and MoSi₂ cluster size increased. Starting powders with lower impurity contents and isothermal vacuum holds contributed to lower oxide impurity contents in the final ceramics. A diboride core-shell microstructure involving (Zr_1-x,Mo_x)B₂ solid solutions formed in all compositions with Mo contents in the solid solution shells ranging from 3 to 6 at%.This work identified specific relationships between starting composition, processing conditions and final microstructure, showing how microstructure and properties could be tailored by processing. The outcomes of this extensive study will serve as guidelines for the design of other structural ceramics that have to attain determinate thermo-mechanical properties for targeted applications.