ZrB2–SiC–G Composite Prepared by Spark Plasma Sintering of In‐Situ Synthesized ZrB2–SiC–C Composite Powders

To avoid introduction of milling media during ball‐milling process and ensure uniform distribution of SiC and graphite in ZrB₂ matrix, ultrafine ZrB₂–SiC–C composite powders were in‐situ synthesized using inorganic–organic hybrid precursors of Zr(OPr)₄, Si(OC₂H₅)₄, H₃BO₃, and excessive C₆H₁₄O₆ as source of zirconium, silicon, boron, and carbon, respectively. To inhabit grain growth, the ZrB₂–SiC–C composite powders were densified by spark plasma sintering (SPS) at 1950°C for 10 min with the heating rate of 100°C/min. The precursor powders were investigated by thermogravimetric analysis–differential scanning calorimetry and Fourier transform infrared spectroscopy. The ceramic powders were analyzed by X‐ray diffraction, X‐ray photoelectron spectroscopy, and scanning electron microscopy. The lamellar substance was found and determined as graphite nanosheet by scanning electron microscopy, Raman spectrum, and X‐ray diffraction. The SiC grains and graphite nanosheets distributed in ZrB₂ matrix uniformly and the grain sizes of ZrB₂ and SiC were about 5 μm and 2 μm, respectively. The carbon converted into graphite nanosheets under high temperature during the process of SPS. The presence of graphite nanosheets alters the load‐displacement curves in the fracture process of ZrB₂–SiC–G composite. A novel way was explored to prepare ZrB₂–SiC–G composite by SPS of in‐situ synthesized ZrB₂–SiC–C composite powders.     

Processing Low‐Oxide ZrB2 Ceramics with High Strength Using Boron Carbide and Spark Plasma Sintering

A phase diagram‐assisted powder processing approach is shown to produce low‐oxygen (0.06 wt%O) ZrB₂ ceramics using minimal B₄C additions (0.25 wt%) and spark plasma sintering. Scanning electron microscopy and scanning transmission electron microscopy with elemental spectroscopy are used to identify “trash collector” oxides. These “trash collector” oxides are composed of manufacturer metal powder impurities that form discreet oxide particles due to the absence of standard Zr–B oxides found in high oxygen samples. A preliminary Zr–B–C–O quaternary thermodynamic database developed as a part of this work was used to calculate the ZrO₂–B₄C pseudobinary phase diagram and ZrB₂–ZrO₂–B₄C pseudoternary phase diagrams. We use the calculated equilibrium phase diagrams to characterize the oxide impurities and show the direct reaction path that allows for the formation of ZrB₂ with an oxygen content of 0.06 wt%, fine grains (3.3 μm) and superior mechanical properties (flexural strength of 660 MPa).     

Evaluation of Rare‐Earth Modified ZrB2–SiC Ablation Resistance Using an Oxyacetylene Torch

Rare‐earth modified ZrB₂–SiC coatings were prepared via mechanical mixing Sm₂O₃ or Tm₂O₃ powders with spray‐dried ZrB₂, or by chemically doping samarium ions into spray‐dried ZrB₂. In either approach, SiC powders were also added and coatings were fabricated via shrouded air plasma spray. An oxyacetylene torch was utilized to evaluate the coatings under high heat flux conditions for hold times of 30 and 60 s. The resulting phases and microstructures were evaluated as a function of rare‐earth type, modification approach, and ablation time. A brittle m‐ZrO₂ scale was observed in the ZrB₂/SiC‐only coating after ablative tests; during cooling this scale detached from the unreacted coating. In contrast, rare‐earth modified coatings formed a protective oxide scale consisting primarily of either Sm_0.2Zr_0.8O_1.9 or Tm_0.2Zr_0.8O_1.9, along with small amount of m‐ZrO₂. These rare‐earth oxide scales displayed high thermal stability and remained adhered to the unreacted coating during heating and cooling, offering additional oxidation protection.     

Spark Plasma Sintering of Superhard B4C–ZrB2 Ceramics by Carbide Boronizing

A carbide boronizing method was first developed to produce dense boron carbide‐ zirconium diboride (“B₄C”–ZrB₂) composites from zirconium carbide (ZrC) and amorphous boron powders (B) by Spark Plasma Sintering at 1800°C–2000°C. The stoichiometry of “B₄C” could be tailored by changing initial boron content, which also has an influence on the processing. The self‐propagating high‐temperature synthesis could be ignited by 1 mol ZrC and 6 mol B at around 1240°C, whereas it was suppressed at a level of 10 mol B. B₈C–ZrB₂ ceramics sintered at 1800°C with 1 mole ZrC and 10 mole B exhibited super high hardness (40.36 GPa at 2.94 N and 33.4 GPa at 9.8 N). The primary reason for the unusual high hardness of B₈C–ZrB₂ ceramics was considered to be the formation of nano‐sized ZrB₂ grains.     

Flash Spark Plasma Sintering (FSPS) of Pure ZrB2

Pure ZrB₂ powder was Flash sintered in an SPS furnace (FSPS). The samples were densified up to 95.0% in 35 s under an applied pressure of 16 MPa. Compared to Conventional SPS (CSPS), the newly developed FSPS technique resulted in an unprecedented energy and time savings of about 95% and 98% respectively. ZrB₂ monoliths obtained by CSPS and FSPS were compared with respect to microstructures, densification behavior, and grain growth. The developed methodology might find application to a wide range of highly conductive ceramics as such refractory borides and carbides.     

PMN–PT Ceramics Prepared By Spark Plasma Sintering

(1− x )Pb(Mg_1/2Nb_2/3)O₃− x PbTiO₃ (PMN–PT) ceramics of stoichiometric composition were fabricated by conventional pressureless sintering (CS) and spark plasma sintering (SPS). The CS ceramics exhibited a change from relaxor to normal ferroelectric behavior (FE) with increasing PT content. However, low dielectric constants, frequency dispersion, and diffuse phase transition behavior typical for relaxors were obtained for all SPS ceramics. FE and piezoelectric measurements further demonstrated low remanent polarization and strain, high coercive field, and low electromechanical response from SPS materials. Normal dielectric and enhanced FE performance appeared following high-temperature heat treatment after SPS. The effects of grain size, microstructure, and chemical heterogeneity formed during fast sintering are considered.     

Low‐Temperature Spark Plasma Sintering of Pure Nano WC Powder

For the first time we have demonstrated the densification of high‐purity nanostructured (d_avg ≈ 60 nm) tungsten carbide by High Pressure Spark Plasma Sintering (HPSPS) in the unusually low temperature range of 1200°C–1400°C. The high‐pressure sintering (i.e., 300 MPa) produced dense material at a temperature as low as 1400°C. In comparison with more conventional sintering techniques, such as SPS (80 MPa) or hot isostatic pressing, HPSPS lowered the temperature required for full densification by 400°C–500°C. High Pressure Spark Plasma Sintering, even in absence of any sintering aid or grain growth inhibitor, retained a very fine microstructure resulting in a significant improvement in both hardness (2721 HV₁₀) and fracture toughness (7.2 MPa m^1/2).     

Flash Spark Plasma Sintering (FSPS) of α and β SiC

A novel processing methodology that allows combined preheating and Flash‐SPS (FSPS) of silicon carbide‐based materials has been developed. Beta‐SiC (+10 wt% B₄C) powders were densified (Ф 20 mm) up to 96% of their theoretical density in 17 s under an applied pressure of 16 MPa (5 kN). The flash event was attributed to the sharp positive temperature dependence of the electrical conductivity (thermal runaway) of SiC, and a sudden increase in electric power absorption (Joule heating) of the samples after a sufficient preheating temperature (>600°C) was reached. The microstructural evolution was analyzed by examining materials densified by FSPS in the range of 82%–96% theoretical densities. FEM modeling results suggest that the FSPS heating rate was of the order of 8800°C/min. A comparative analysis was done between FSPS and reference samples (sintered using conventional SPS in the temperature range of 1800°C–2300°C). This allowed for a better understanding of the temperatures generated during FSPS, and in turn the sintering mechanisms. We also demonstrated the scalability of the FSPS process by consolidating a large α‐SiC disk (Ф 60 mm) in about 60 s inside a hybrid SPS furnace equipped with an induction heater, which allowed us to achieve sufficient preheating (1600°C) of the material to achieve FSPS.     

Mechanical Characterization of Annealed ZrB2–SiC Composites

Composites consisting of 70 vol% ZrB₂ and 30 vol% α‐SiC particles were hot pressed to near full density and subsequently annealed at temperatures ranging from 1000°C to 2000°C. Strength, elastic modulus, and hardness were measured for as‐processed and annealed composites. Raman spectroscopy was employed to measure the thermal residual stresses within the silicon carbide (SiC) phase of the composites. Elastic modulus and hardness were unaffected by annealing conditions. Strength was not affected by annealing at 1400°C or above; however, strength increased for samples annealed below 1400°C. Annealing under uniaxial pressure was found to be more effective than annealing without applied pressure. The average strength of materials annealed at 1400°C or above was ~700 MPa, whereas that of materials annealed at 1000°C, under a 100 MPa applied pressure, averaged ~910 MPa. Raman stress measurements revealed that the distribution of stresses in the composites was altered for samples annealed below 1400°C resulting in increased strength.     

Translucent Mg-α-Sialon Ceramics Prepared by Spark Plasma Sintering

The translucent Mg-α-sialon ceramics have been prepared by spark plasma sintering (SPS) α-Si₃N₄ powder with AlN and MgO as the additives at 1850°C for 5 min. The sample possesses a uniform, dense microstructure under the rapid densification of SPS process. The translucent Mg-α-sialon ceramics achieve the maximum transmittance of 66.4% for the sample of 0.5 mm in thickness in the medium infrared region, which could be attributed to the equiaxed microstructure and few glassy phase confirmed by the observation of transmission electron microscopy. The material also exhibits good mechanical properties of high hardness (21.4±0.3 GPa) and fracture toughness (6.1±0.1 MPa·m^1/2).     

放电等离子烧结TiBCN粉体材料的研究

采用放电等离子烧结在1700℃,1800℃及1900℃制备TiBCN陶瓷,并用扫描电镜、X射线衍射仪、万能材料试验机、热重分析仪及激光导热仪等分析了材料的微观结构、组分、力学、抗氧化及热性能。结果表明,在1800℃热压时,材料的压缩强度最高,达到951±206MPa。烧结后样品的氧化测试表明材料具有良好的抗氧化性能,在1200℃以内该材料的质量变化率均小于2.2 %。TiBCN的结构和性能与TiC,TiB₂及TiN有一定的相似性,具有重要的研究价值和潜在的应用前景。     

SiC Depletion in ZrB2–30 vol% SiC at Ultrahigh Temperatures

The formation of a porous SiC‐depleted region in ZrB₂–SiC due to active oxidation at ultrahigh temperatures was characterized. The presence/absence of SiC depletion was determined at a series of temperatures (1300°C–1800°C) and times (5 min–100 h). At T < 1627°C, SiC depletion was not observed. Instead, the formation of a ZrO₂ + C/borosilicate oxidation product layer sequence was observed above the ZrB₂–SiC base material. At T ≥ 1627°C, SiC was depleted in the ZrB₂ matrix below the ZrO₂ and borosilicate oxidation products. The SiC depletion was attributed to active oxidation of SiC to form SiO(g). The transition between C formation in ZrO₂ (T < 1627°C) and SiC depletion in ZrB₂ (T ≥ 1627°C) is attributed to variation in the temperature dependence of thermodynamically favored product assemblage influenced by the local microstructural phase distribution. The growth kinetics of the SiC depletion region is consistent with a gas‐phase diffusion‐controlled process.     

Oxidation Behavior of ZrB2–SiC Composites at Low Pressures

ZrB₂‐60 mol%SiC composite with a eutectic microstructure was oxidized at 1573 to 1873 K with reduced total pressures (P_tot) and low oxygen partial pressures (). The mass change was continuously measured by a thermobalance, and then fit with a multiple paralinear model. Oxidation scale of SiO₂/ZrO₂+SiO₂/ZrO₂/ZrB₂ was formed at  > 0.13 kPa, whereas only porous ZrO₂ remained at  < 0.13 kPa, P_tot < 1.33 kPa and higher than 1773 K. With increasing , the parabolic oxidation constant decreased, whereas the linear oxidation constant increased.     

无压烧结硼化锆基ZrB2-SiC复相陶瓷的结构与性能

以钇铝石榴石-YAG为烧结助剂,通过无压烧结制备了ZrB2-SiC复相陶瓷。研究了烧结助剂含量对烧结材料力学性能和显微结构的影响,材料的显微结构由扫描电镜SEM及其能谱分析EDS测定。研究结果表明,烧结助剂(YAG)和原料中的杂质形成玻璃相填充在晶界上,显著促进了硼化锆基ZrB2-SiC复相陶瓷的致密化。     

Ferrite‐Based Exchange‐Coupled Hard–Soft Magnets Fabricated by Spark Plasma Sintering

Ferrite‐based, hard‐soft magnetic nanocomposites with the composition (100%?x)SrFe₁₂O₁₉–xCoFe₂O₄, where x = 5, 10, and 15 wt%, were prepared by mixing the constituent powders, followed by spark plasma sintering. In order to control the particle size of the constituent materials, the SrFe₁₂O₁₉ and CoFe₂O₄ powders were synthesized using the hydrothermal method, mixed and then consolidated with spark plasma sintering. The conditions during the spark plasma sintering process (sintering temperature, time, and applied pressure) were varied in order to prepare composites with a high density and exchange‐coupled hard and soft magnetic phases, leading to an increase in the maximum energy product, when compared with pure SrFe₁₂O₁₉. The microstructural analysis revealed that the relative density of the sintered composite exceeded 90% of the theoretical value and that the CoFe₂O₄ was uniformly distributed in the SrFe₁₂O₁₉ matrix. Magnetic measurements of the sintered composites showed a single‐phase magnetic behavior. When compared with the single‐phase SrFe₁₂O₁₉ used in this study, the SPS composites exhibited a 22% increase in the maximum energy product (26.1 kJ/m³).     

High Hardness and Ductile Mosaic SiC/SiO2 Composite by Spark Plasma Sintering

SiC powder was coated with SiO₂ layer by chemical vapor deposition, and the SiC(core)/SiO₂(shell) composite powder was consolidated to a SiC/SiO₂ composite with a mosaic microstructure by spark plasma sintering (SPS) at 1923 K for 1.8 ks. The SiC(core)/SiO₂(shell) powder with a 80–100 nm thick SiO₂ layer resulted in a SiC/SiO₂ composite with a relative density of 97% and hardness and fracture toughness of 17.1 GPa and 8.4 MPa m^1/2, respectively.     

Photoluminescence Properties of Nanocrystalline ZnO Ceramics Prepared by Pressureless Sintering and Spark Plasma Sintering

Nanocrystalline ZnO ceramics with grain sizes of ∼100 nm were prepared by pressureless sintering at 800°C for 2 h and spark plasma sintering (SPS) at 550°C for 2 min, respectively. Excellent green emission properties were obtained in the ZnO ceramic prepared by the SPS process and in the pressureless-sintered ZnO ceramic prepared at 1000°C for 2 h, which are attributed to the vacuum ambience of the SPS process and the sublimeness of the interstitial Zn at >900°C in air, respectively.     

Low‐Temperature Sintering of HfC/SiC Nanocomposites Using HfSi2‐C Additives

HfC/SiC nanocomposites were fabricated via the reactive spark plasma sintering (R‐SPS) of a nano‐HfC powder and HfSi₂‐C sintering additives. The densification temperature decreased to 1750°C as the additive content increased. XRD analysis indicated the formation of pure HfC–(19.3–33.8 vol%) SiC within the sintered composites without residual silicide or oxide phases or secondary nonoxide phases. Ultrafine and homogeneously distributed HfC (470 nm) and SiC (300 nm) grains were obtained in the dense composites using nano‐HfC powder through the high‐energy ball‐milling of the raw powders and R‐SPS. Grain growth was further suppressed by the low‐temperature sintering using R‐SPS. No amorphous phase was identified at the grain boundary. The maximum Vickers hardness, Young's modulus, and fracture toughness values of the HfC/SiC nanocomposites were 22 GPa, 292 GPa, and 2.44 MPa·m^1/2, respectively.     

TEM Study of the High‐Temperature Oxidation Behavior of Hot‐Pressed ZrB2–SiC Composites

The oxidation behaviors of ZrB₂‐ 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 10⁴ and 10 ^{? 8} Pa, respectively. The oxidation of ZrB₂ and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica‐rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB₂ was oxidized to ZrO₂ accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO₂ with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB₂ – SiC composites tested under air and low oxygen partial pressure, the ZrB₂ begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.     

Oxygen diffusion mechanisms during high‐temperature oxidation of ZrB2‐SiC

Oxygen diffusion mechanisms during oxidation of ZrB₂‐30 vol% SiC were explored at temperatures of 1500°C and 1650°C using an ¹⁸O tracer technique. Double oxidation experiments in ¹⁶O₂ and ¹⁸O₂ were performed using a modified resistive heating system. A combination of scanning electron microscopy, energy‐dispersive spectroscopy, and time‐of‐flight secondary ion mass spectrometry was used to characterize the borosilicate and ZrO₂ oxidation products. Oxygen exchange with the borosilicate network was observed to occur quickly at the oxygen‐borosilicate surface at both 1500°C and 1650°C, while evidence of oxygen permeation was only observed at 1650°C for short time (<1 min) exposures. At longer times, >5‐9 min, complete oxygen exchange throughout both the borosilicate glass and ZrO₂ was observed at both temperatures preventing identification of the oxygen transport mechanisms, but demonstrating that oxygen transport is rapid in both oxide phases.