3Y-TZP材料的液相烧结

通过在3Y-TZP(Tetragonal Zirconia Polycrystals stabilized with 3mol% Y2O3)中加入适当含量的硅酸盐玻璃添加剂,使其烧结温度明显降低,并且制备出具有细晶粒、高强度的四方相氧化锆增韧陶瓷材料.本文分析了添加剂对3Y-TZP材料烧结特性及显微结构的影响.发现液相烧结的3Y-TZP具有良好的抗弯强度,但韧性有待于提高.     

Reaction Sintering and Properties of Silicon Oxynitride Densified by Hot Isostatic Pressing

Silicon oxynitride ceramics were reaction sintered and fully densified by hot isostatic pressing in the temperature range 1700°C to 1950°C from an equimolar mixture of silicon nitride and silica powders without additives. Conversion to Si₂N₂O increases steeply from a level around 5% of the crystalline phases at 1700°C to 80% at 1800°C, and increases a few percent further at higher temperatures. α -Si₃N₄ is the major residual crystalline phase below 1900°C. The hardness level for materials containing 85% Si₂N₂O is approximately 19 GPa, comparable with the hardness of Si₃N₄ hot isostatically pressed with 2.5 wt% Y₂O₃, while the fracture toughness level is around 3.1 MPa. m^1/2, being approximately 0.8 MPa.m^1/2 lower. The three-point bending strength increased with HIP temperature from approximately 300 to 500 MPa.     

利用主烧结曲线预测TiO2陶瓷无压烧结显微组织演变

纳米金红石TiO2粉末在高温热膨胀仪中进行恒速无压烧结,升温速率为2℃/min和5℃/min,热膨胀仪自动记录烧结收缩量,根据全期烧结模型,建立TiO2主烧结曲线;利用阿基米德法测量烧结体的相对密度,利用扫描电镜观察烧结体在不同温度的显微组织演变.结果表明:利用主烧结曲线得到的相对密度和Archimedes法实测的密度吻合,证明了主烧结曲线对烧结路径不敏感,烧结体的相对密度仅是时间和温度的函数.烧结体的显微组织是烧结温度和时间的函数,烧结体的相对密度和显微组织有关联,主烧结曲线理论可以用来预测和控制烧结体的相对密度、显微组织和陶瓷性能.     

Reaction sintering of alumina/mullite nanocomposites from nano-sized starting powders

Alumina/mullite ceramic nanocomposites were prepared by the mixtures of nano-sized starting powders of alumina with silica and alumina with silicon carbide. Silica from deliberate addition and as the product of silicon carbide oxidation reacted completely with alumina to form mullite. Silica from direct addition segregated at the grain boundary and intergranular mullite was formed whereas silica from oxidation was surrounded by alumina matrix and intragranular mullite was formed after reaction sintering. The most significant difference was fracture behaviour where intragranular mullite nanoparticles promoted transgranular fracture in alumina matrix due to thermal mismatch around nanoparticles and intergranular mullite nanoparticles gave rise to intergranular fracture similar to pure alumina. Wear resistance of the nanocomposites was better than that of alumina. Pull-out formation in the nanocomposites was less and pull-out size was also smaller. Fracture toughness of the nanocomposites was significantly higher than that of alumina.     

Properties and microstructural aspects of TiO2‐doped sintered Alumina‐Zirconia composite ceramics

The effect of titania content on the densification, the phase transformation, the microstructures, and mechanical properties of 50 wt% Al₂O₃‐50 wt% ZrO₂ (12 mol% CeO₂) was evaluated. Ceramic composites with different TiO₂ content (0.27, 5, 10 wt%) were prepared by pressureless sintering at low temperature (1400°C) for 2 hours in air. Dense ceramic was obtained by adding 5 wt% of TiO₂ loading to improved mechanical properties. The microstructure analysis provided lots of information about solid‐state reactivity in alumina‐zirconia‐titania ternary system. The content of TiO₂ strongly affected the phases evolution and the grain growth during sintering. Furthermore, a significant effect on mechanical properties and fracture behavior was also observed.     

添加Y2O3-Al2O3烧结助剂的氮化硅陶瓷的超高压烧结

以Y2O3-Al2O3体系为烧结助剂,在5.4～5.7 GPa,1 570～1 770K的高温高压条件下进行了氮化硅陶瓷的超高压烧结研究.用X射线衍射及扫描电镜对烧结样品进行了分析和观察,探讨了烧结温度及压力对烧结的陶瓷样品性能的影响.结果表明:得到的氮化硅由相互交错的长柱状β-Si3N4晶粒组成,微观结构均匀,α-Si3N4完全转变为β-Si3N4.经5.7GPa,1 770K且保温15min的超高压烧结,样品的相对密度达99.0%,Rockwell硬度HRA为99,Vickers硬度HV达23.3GPa.     

Influence of high energy ball milling on structural,microstructural and optical properties of TiO2 nanoparticles

The influence of high-energy ball milling on structural, microstructural, and optical properties of TiO₂ by modifying the nanoparticle size was studied. Five samples were extracted at different milling times (0, 2, 4, 8, and 13 h). The average particle sizes estimated by dynamic light scattering (DLS) were 205, 155.8, 116.8, 82.9, and 82.7 nm at 0, 2, 4, 8, and 13 h, respectively. X-ray diffraction analysis confirmed progressive broadening of the peaks as the milling time elapsed. Besides, a correlation was found between d spacing and the average crystal size. The UV–Vis diffuse reflectance spectra of TiO₂ revealed a decrease in reflectance due to particle size reduction. Similarly, an alteration of the bandgap transition energy was presented, whose values gradually decreased from 2.966 eV to 2.861 eV for the sample without and with the maximum duration milling performed (13 h), respectively. Likewise, the SEM analysis showed a distribution in nanoparticle size that became more homogeneous and smaller average grain size as the milling duration was longer.     

Interplay of Sintering Microstructures, Driving Forces, and Mass Transport Mechanisms

The equilibrium shapes of segments of a row of sintering particles are calculated for the case where interparticle distances are constrained and, in particular, for only vapor-phase transport or surface diffusion active. These equilibrium states are contrasted with those obtained by mechanisms which allow densification. The sintering potentials and surface curvatures are calculated for the equilibrium shapes as a function of dihedral angle. Expressions for the rate of shrinkage are presented and, for the case of equilibrium structures, calculated exactly.     

In situ reactive synthesis and plasma-activated sintering of binderless WC ceramics

Binderless micrometre tungsten carbide ceramics (～1.1?μm) were in situ synthesised and densified by plasma-activated sintering (PAS) from mixed powders of tungsten trioxide and carbon black. The influence of sintering process and powders’ composition ratio on the phase composition, microstructure and mechanical properties of as-prepared samples was clarified in detail. The phase evolution was ascertained by X-ray diffraction to be WO₃→WO_2.72→WO₂→W₂C→WC, with the formation of CO and CO₂ gases. The sample with nearly single WC phase, dense structure and excellent mechanical properties was fabricated under the optimised process with the proper composition ratio. Owing to the micrograin size, the fracture toughness (8.88?MPa?m^1/2) was enhanced while high hardness (2159 HV10) was maintained, in the absence of any ceramic toughening phase.     

Sintering behavior of Cr2AlC MAX phase synthesized by Spark plasma sintering

In the current study, the sintering and mechanical properties of the Cr₂AlC MAX phase synthesized by Spark plasma sintering at 1000, 1100, and 1200°C were investigated. The X-ray diffraction (XRD) patterns showed that the synthesis of the Cr₂AlC MAX phase was associated with the presence of impurities such as Cr₇C₃ and Al₂O₃. On the basis of the FESEM images equipped with energy dispersive spectrometer, the MAX phases had been formed successfully and the length of these layers increased by enhancing the sintering temperature. The results of the density showed that by increasing the temperature, the density increases from 5.10 to 5.33 g/cm³ and finally decreases to 5.25 g/cm³. Vickers hardness method applied to determine the hardness of the samples showed that the hardness decreases from 8.52 to 8.07 GPa for the prepared samples.     

High-performance ceramic parts with complex shape prepared by selective laser sintering: a review

ABSTRACT

Selective laser sintering (SLS) is an additive manufacturing technology which has shown great advantages in direct formation of the polymer, metal and their composites. However, ceramic parts prepared by the SLS still exhibit some fatal defects, including low density and poor mechanical properties. In this respect, recent advances for preparing ceramics have improved the final density and performance by adopting post-processing methods. In this review, three commonly used powder preparation approaches (i.e. mechanical mixing, solvent evaporation and dissolution-precipitation process) and two powder sintering mechanisms for the SLS are introduced. Porous ceramic parts are prepared directly through the SLS by virtue of their high porosity. And dense, high-performance Al₂O₃, ZrO₂, kaolin and SiC ceramic parts with complex shape are prepared by introducing CIP technology into the SLS, indicating that the hybrid technology could be the promising route for preparing high-performance ceramic parts used in various fields.     

Effect of ZrB2 content on phase assemblage and mechanical properties of Si3N4–ZrB2 ceramics prepared at low temperature

Based on the previous work on Si₃N₄–ZrB₂ [Wu et al. J Eur Ceram Soc;2017,37:4217], the influence of ZrB₂ addition on the phase and microstructure evolution of Si₃N₄–ZrB₂ composites was emphatically investigated, and the mechanical properties were compared with pure Si₃N₄ ceramics. It was revealed that the ratio of β‐ to (α+β)‐Si₃N₄ significantly increased from 14.3% in pure Si₃N₄ ceramics to 39.8% in Si₃N₄ with 15 vol% ZrB₂ addition, indicating that the introduction of ZrB₂ promoted α‐ to β‐Si₃N₄ phase transformation. As a consequence, the microstructure of the composite showed the bimodal distribution, containing both elongated and equiaxed Si₃N₄ grains. For the pure Si₃N₄, Vickers hardness, fracture toughness and flexural strength was 22.8 GPa, 7.6 MPa m^1/2, and 334.5 MPa, respectively. In contrast, the composite of Si₃N₄–30 vol% ZrB₂ simultaneously possessed an excellent combination of mechanical properties: 19.5 GPa in hardness, 9.8 MPa m^1/2 in toughness and 702.0 MPa in strength. Present study suggested that Si₃N₄‐based ceramics with high hardness, high toughness, and high strength could be obtained by the combination of appropriate ZrB₂ content and low hot‐pressing temperature.     

Densification,solid solution formation,and microstructural investigation of reactive pressureless sintered HfB2-TiB2-SiC-MoSi2 quadruplet composite

Dense HfB₂-TiB₂-SiC-MoSi₂ quadruplet composite was produced by a reactive pressureless sintering method at 2050 °C for 5 h. The relative density was improved and reached 98% by in situ formation of SiC and MoSi₂ phases. Microstructural studies proved that SiC and MoSi₂ second phases were mostly formed during the sintering process. Moreover, the Sintering mechanism of the composite was investigated by HSC software. TiB₂ co-matrix was improved the sinterability of the composite by the formation of (Hf,Ti,Mo)–B and (Hf,Ti,Mo)–C solid solutions.Mechanical properties such as Vickers hardness (23.2 GPa), fracture toughness (5.4 MPa m^1/2), and elastic modulus (430 GPa) were effectively enhanced by tailoring the composite.     

Spark Plasma Sintering of double perovskite Ba2MgWO6 doped with Ce3+: Part I - Structural and microstructural characterizations

Ba2MgWO6 (BMW) translucent ceramics undoped or doped with Ce³⁺ were prepared by using Spark Plasma Sintering (SPS) method. Powders with submicrometric particles diameter and composed of pure BMW cubic phase were fabricated by simple solid-state reaction from primary powders. This work highlights that thermomechanical schedule applied during sintering must be carefully chosen as BMW tends to decompose for temperatures above 1400 °C. As a result, dense and translucent undoped BMW and BMW:Ce³⁺ ceramics were obtained at only 1350 °C for 5 min under 50 MPa of uniaxial pressure. Such lanthanide-free ceramics with high density (i.e. 6.8–7.2 g/cm⁻³) could be envisaged as new and more environmentally-sustainable scintillators for medical imaging.     

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.     

Al2TiO5添加量对MgO-Al2TiO5复合陶瓷烧结及抗热震性的影响

以纳米MgO粉为原料,选用以纳米Al2O3粉和纳米TiO2粉经1500℃保温3 h烧结制备的Al2TiO5为添加剂,采用固相烧结法经1500℃保温3 h制备了Al2TiO5质量分数分别为0、5%、10%和20%的MgO-Al2TiO5复合陶瓷,并采用XRD、SEM和EDS等研究了Al2TiO5添加量对MgO-Al2TiO5复合陶瓷烧结性能及抗热震性能的影响。结果表明:添加Al2TiO5有利于促进复合陶瓷的烧结,其体积密度和线收缩率随Al2TiO5添加量的增加而增大,当Al2TiO5添加量为20%(w)时,其体积密度和线收缩率分别为3.68 g·cm-3和22.07%;当Al2TiO5添加量为10%(w)时,其抗热震性能最佳。Al2TiO5位于方镁石晶粒交界处,抑制方镁石晶粒生长,阻碍裂纹扩展,使MgO-Al2TiO5复合陶瓷的抗热震性能得到改善。     

Structure-property relations of co-doped bismuth layer-structured Bi3.25La0.75(Ti1-xMox)3O12 ceramics

In this work, the fabrication and investigation of substituting higher-valence Mo⁶⁺ for Ti⁴⁺ ion on the B-site of La³⁺-doped Bi₄Ti₃O₁₂ [BLT] structure to form Bi_3.25La_0.75(Ti_1-x Mo _x )₃O₁₂ [BLTM] (when x = 0, 0.01, 0.03, 0.05 0.07, 0.09, and 0.10) ceramics were carried out. X-ray diffraction patterns of BLTM ceramics indicated an orthorhombic structure with lattice distortion, especially with a higher concentration of a MoO₃ dopant. Microstructural investigation showed that all ceramics composed mainly of plate-like grains. An increase in MoO₃ doping content increased the length and thickness of the grain but reduced the density of the ceramics. Electrical conductivity was found to decrease, while the dielectric constant increased with Mo⁶⁺ doping concentration. Ferroelectric properties were found to be improved with increasing MoO₃ content and were optimized at x = 0.1.     

Effect of TiO2 on the mullite formation and mechanical properties of alumina porcelain

The effect of adding TiO₂ to standard alumina porcelain on its microstructure and flexural strength was investigated. A series of alumina porcelain bodies containing increasing amounts of TiO₂ were prepared by extruding mixtures of raw materials and TiO₂. Porcelain rods were fired under industrial scheduling in a manufacturing kiln. The overall degree of crystalline and amorphous phase content within the porcelain bodies was quantitatively determined using a Rietveld analysis. Results indicated a higher amount of mullite formation in porcelain bodies containing TiO₂. Examination of the product materials using field emission scanning electron microscopy showed a high density of secondary mullite crystals present in the earlier feldspar grain areas of specimens with TiO₂. Energy dispersive X-ray analysis of secondary mullite crystals revealed that Ti⁴⁺ enters into the secondary mullite structure forming a Ti⁴⁺-mullite solid solution. Assessment of the mechanical properties of the TiO₂ containing bodies indicated that small addition raises the flexural strength of the standard porcelain. The improvement in mechanical properties could be associated with an increase of both specimen density and relative content of types II and III secondary mullites. Both observations may be attributed to a decrease in the viscosity of melted feldspar grains, which in turn favours the nucleation and growth of secondary mullite crystals and thus increases the final density of porcelain bodies.     

氧化钛对氧化铝瓷的莫来石形成和力学性能的影响

研究了添加氧化钛对标准氧化铝瓷的显微结构和抗折强度的影响。通过挤压原料和TiO2的混合料,制备了含有不同量氧化钛的系列氧化铝瓷,在生产窑内按照工业化制度烧成瓷棒。定量分析了瓷件内的晶化程度和非晶相含量。结果表明,含有TiO2的瓷件内形成了大量莫来石。用场发射扫描电镜分析材料,表明含有TiO2的试样在早期长石颗粒区域出现了密度较高的二次莫来石晶体。测量含TiO2瓷件的力学性能,表明加入少量能提高标准瓷件的抗折强度。