Sintering of Spherical Glass Powder under a Uniaxial Stress

The sintering of spherical borosilicate glass powder (particle size 5 to 10 μm) under a uniaxial stress was studied at 800°C. The experiments allowed the measurement of the kinetics of densification and creep, the viscosities for creep and bulk deformation, and the sintering stress which was found to increase with density. The data show excellent qualitative agreement with Scherer's theory of viscous sintering. In addition, the quantitative comparison between theory and experiment shows good agreement; the measured viscosity of the bulk glass was ∽1×10⁹ P (∽1×10⁸ Pa·s) compared to ∽3×10⁹ P (∽3 Pa·s) obtained by fitting the data with Scherer's theory.     

Stress Corrosion Cracking of Bioactive Glass Composites

Bioactive glass was reinforced with 20 vol% Ag particles and 20 vol% SiC whiskers. Fatigue parameters and lifetime data for the two composites were determined via the stressing rate dependence of the bend strength in a bio-simulating buffered solution (pH of 7.4). A lifetime of 10 yr was estimated for the SiC-reinforced composites under bend stresses of 34 MPa and for Ag composites under 23 MPa. Stresses for similar survival of pure bioactive glass are 17 MPa.     

Effects of Aspect Ratios of Ellipsoidal Inclusions on Elastic Stress Transfer of Ceramic Composites

The elastic stress transfer from a ceramic matrix to aligned elliposidal inclusions is analyzed for the case where the axis of the applied load is parallel to the axis of the ellipsoid. Perfect bonding between the matrix and inclusions is assumed. The calculation is based on Eshelby's model for loading of an infinite matrix containing an ellipsoidal inclusion, and the modification developed by Mori and Tanaka to account for the effect of finite volume fractions of inclusions. The results show that the transfer of the axial stress from the matrix to inclusions increases with the increase in the aspect ratio of the ellipsoid, and decreases with the increase in the volume fraction of inclusions when Young's modulus of the inclusion is higher than that of the matrix.     

Prediction of Crack Paths in Particulate Composites Using Electrical Analog

An electrical analog technique was used to simulate crack paths in the vicinity of elastic inhomogeneities in ceramic composites. The technique makes use of the analogy that exists between electrical potential differences between clamped ends of a conducting foil with electrical inhomogeneities (second phases of different electrical conductivity) and the load-point displacements of a mechanically loaded plate with elastic inhomogeneities (second phases of different elastic modulus). In crack problems, this analogy is exact for mode III loading. The specific problems investigated were the interaction between a crack and a well-bonded second phase of higher elastic modulus (simulating a fiber or a second-phase particle) and the interaction between a crack and a pore. Crack paths were incrementally determined by maximizing the end-point potential differences (corresponding to the maximum energy release rate criterion in the mechanical system) with respect to the direction of crack growth. Comparable mechanical tests were carried out using precracked Plexiglas * plates with either circular holes or elastic inclusions. Good agreements were obtained between the electrically simulated crack paths and the crack paths observed in the mechanical tests despite the fact that the mechanical tests were carried out in mode I loading and the analogy is not exact.     

Pressureless Sintering of Alumina-Titanium Carbide Composites

The densification of Al₂O₃-TiC composites is detrimentally affected by chemical reactions between Al₂O₃ and TiC. These reactions must be suppressed in order to promote sintering. In this study, the specific reactions occurring in Al₂O₃-TiC composites were modeled, using thermodynamic calculations, and verified by experiments. The reaction between Al₂O₃ and TiC was suppressed by the use of specially prepared embedding powders allowing pressureless sintering to closed porosity. The Al₂O₃-TiC composites were subsequently hot isostatically pressed to > 99% of theoretical density without encapsulation. Typical flexural strength and fracture toughness of Al₂O₃-30 wt% TiC composites were 690 MPa and 4.3 MPa · m^1/2, respectively.     

Reaction Sintering and Mechanical Properties of Hydroxyapatite–Zirconia Composites with Calcium Fluoride Additions

The effects of calcium fluoride (CaF₂) additions on the densification and mechanical properties of hydroxyapatite–zirconia composites (HA–ZrO₂) were investigated. When small amount of CaF₂ was added, the density of the composites was markedly enhanced. The reactions of HA with CaF₂, which led to the formation of fluorapatite (FA), were attributed to the observed improvements in densification. When HA–20-vol%-ZrO₂ composites were sintered, with the addition of 5 vol% of CaF₂, in air at 1300°C, the density of the specimen approached 98% of the theoretical value. The flexural strength and fracture toughness of the composites were also improved, as a result of the enhanced densification.     

Sintering of Ceramic Particulate Composites: Effect of Matrix Density

Composites consisting of a fine-grained, polycrystalline zinc oxide matrix and <10 vol% coarse, rigid silicon carbide inclusions were prepared by the same mixing procedure and then compacted to produce samples with matrix densities of 0.45 and 0.68 of the theoretical. The samples were sintered under identical temperature profiles in separate experiments that employed either a constant rate of heating of 4°C/min or near isothermal heating at 735°C. The ratio of the densification rate of the composite matrix to the densification rate of the unreinforced zinc oxide was found to be independent of the initial matrix density. This ratio increased significantly with temperature in the constant-heating-rate experiments but was relatively constant in the isothermal experiments. The results indicate that microstructural coarsening may be an important mechanism for explaining the reduced sinterability of polycrystalline matrix composites.     

Mullite/Alumina Particulate Composites by Infiltration Processing: III, Mechanical Properties

The effect of incorporating mullite into alumina by an infiltration process on the mechanical properties was investigated. Data for Young's modulus, strength, and fracture toughness for various composite compositions were compared with those for the unreinforced matrix (alumina). Measurements of Young's modulus by a resonance technique showed that the addition of mullite decreased Young's modulus. Up to 14 vol%, these changes were close to those expected, but above this mullite content, the decrease was more dramatic and indicated specimen damage during processing. The addition of mullite led, in some cases, to increases of more than 60% in both the strength (biaxial flexure) and indentation fracture toughness. These increases have been attributed to the method of introducing mullite and the resulting residual compressive surface stresses. The strength of the indented composite bodies deviated from the ideal behavior, indicating the probability of R -curve behavior in these materials.     

MgO-ZrO_2复合材料的烧结和热震稳定性

以烧结镁砂和单斜氧化锆粉为原料,进行了Mg O-Zr O2复合材料的制备。通过XRD和SEM-EDS分析及显气孔率、体积密度、常温抗折强度、线膨胀系数和1100℃~室温水冷热震稳定性的测试,探讨了Zr O2的用量对Mg O-Zr O2复合材料的烧结和热震稳定性的影响。结果表明:随着Zr O2用量的增加,复合材料试样的致密度和常温抗折强度呈现出先增大后减小的趋势,复合材料试样的热膨胀系数逐步减小,复合材料试样的热震稳定性呈现出先变好后变差的趋势;当Zr O2用量为30%时,Mg O-Zr O2复合材料试样的烧结性能和热震稳定性最好。     

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。     

Fabrication of Mullite and Mullite-Matrix Composites by Transient Viscous Sintering of Composite Powders

Mullite was fabricated by a process referred to as transient viscous sintering (TVS). Composite particles which consisted of inner cores of α-alumina and outer coatings of amorphous silica were used. Powder compacts prepared with these particles were viscously sintered to almost full density at relatively low temperatures (∼1300°C). Compacts were subsequently converted to dense, fine-grained mullite at higher temperatures (∼1500°C) by reaction between the alumina and silica. The TVS process was also used to fabricate mullite/zirconia/alumina, mullite/silicon carbide particle, and mullite/silicon carbide whisker composites. Densification was enhanced compared with other recent studies of sintering of mullite-based composites. This was attributed to three factors: viscous flow of the amorphous silica coating on the particles, avoidance of mullite formation until higher temperatures, and increased threshold concentration for the development of percolation networks.     

烧结法制微晶玻璃的机理研究

微晶玻璃是近些年来在材料科学上的一项重大发现,在国防尖端技术和建筑业等多种领域起到了举足轻重的作用。微晶玻璃更具有高机械强度、绝缘性和良好的耐酸耐碱性等优点,本文通过烧结法就微晶玻璃的材料特征、微观结构和性能、降低烧结温度的方法等做了详细的介绍。     

Sintering and Microstructure Modification of Mullite/Zirconia Composites Derived from Silica-Coated Alumina Powders

This paper addresses the densification and microstructure development during firing of mullite/zirconia composites made from silica-coated-alumina (SCA) microcomposite powders. Densification occurs in two stages: in the presence of a silica–alumina mixture and after conversion to mullite. The first stage of densification occurs through transient viscous phase sintering (TVS). This is best promoted by rapid heating, which delays the crystallization of silica to higher temperatures. A further sintering stage is observed following mullitization. The introduction of seeds promotes solid-state sintering, most probably due to refinement of the mullite matrix. For seed concentrations up to about 1% the sintering kinetics depend on seed concentration. This suggests that nucleation still remains the rate-controlling mullitization step. Above this concentration the reaction becomes growth controlled. Introduction of seeds also promotes direct mullitization without transient zircon formation that was observed in a previous study of the same process without seeding. Seeding also promotes the development of elongated grains by way of a solid-state recrystallization process.     

Viscous Sintering under a Uniaxial Load

An analysis is presented for viscous sintering under a uniaxial load; numerical results are presented for compressive loads. The load increases the axial contraction rate, hut counteracts the radial contraction rate. When the load is similar in magnitude to the capillary stress driving sintering, the radius remains essentially constant as the body densifies. Following a suggestion by Bordia, the viscosity and sintering kinetics can be determined in one experiment by measuring the axial and radial shrinkages simultaneously.     

Characterization of Fracture in Fiber-Reinforced Ceramic Composites under Shear loading

This study developed a test technique for mode II fracture testing of a fiber-reinforced ceramic composite. This method employed a small, straight-notched, precracked, end-notched flexure specimen subjected to three-point bending. This method was demonstrated by measuring the mode II critical strain energy release rate, G _{II c} of a fiber-reinforced glass-ceramic composite at room temperature.     

Mechanochemical Synthesis and Pressureless Sintering of TiB2–AlN Composites

TiB₂-AlN composites have been fabricated by the pressureless sintering of a mechanochemically processed Ti, Al, and BN powder mixture. TiB₂-AlN powder was obtained from the mixture of Ti, Al, and BN, which had a composition corresponding to 45.7 wt% TiB₂-54.3 wt% AlN, after mechanochemical processing for longer than 24 h. X-ray diffraction and transmission electron microscopy analysis showed that the powder subjected to mechanochemical processing for 60 h consisted of crystallites less than 300 nm in size with a disordered crystal structure. TiB₂-AlN composites with 95% relative density, a flexural strength of 172 MPa, a fracture toughness of 4.6 MPa·m^1/2, a hardness of 12.0 GPa, and an electrical resistivity of 1488 μΩ·cm were obtained by pressureless sintering at 1700°C for 2 h of the powder subjected to mechanochemical processing for 60 h.     

Sintering Stress of Nonlinear Viscous Materials

An analytical model for the sintering stress of materials characterized by a nonlinear viscous behavior during densification is proposed. The growing applications in the field of nanosized powders processing (in particular, consolidation of high surface area components used in supercapacitors, rechargeable batteries, gas absorbers) have renewed the interest in this fundamental parameter of sintering science, because of the sintering stress’ characteristic inverse proportionality with respect to the powder particles radius. This increase in the magnitude of the sintering stress is also responsible for power‐law creep being the mechanism that underlies densification even without the application of any additional external load, and therefore for a nonlinear viscous behavior of the solid material. The analytical treatment of problems involving nonlinear viscous materials has traditionally involved complex self‐consistent methods and approximations, unless the local case of an isolated pore embedded in a fully dense skeleton was considered. The paper proposes a simple first‐order iterative method that allows the derivation of both bulk modulus and sintering stress of a material containing an arbitrary amount of pores, as functions of porosity and of the material's nonlinearity parameter, namely strain rate sensitivity. An expression for densification kinetics is also obtained and compared with experimental data.     

B6O–c-BN Composites Prepared by High-Pressure Sintering

High-pressure sintering behavior in the B₆O– c -BN system was investigated using in-laboratory-synthesized B₆O and commercially available c -BN powders (with an average grain size of 0.5, 3, or 6 μm). No reaction occurred between the two components under the high-pressure (4–6 GPa) and high-temperature (1500°–1800°C) conditions that have been investigated. Well-dispersed, sintered B₆O– x ( c -BN) composites (where x = 0–60 vol%) of almost-full density were prepared by sintering at a pressure of 6 GPa and temperature of 1800°C for 20 min. The maximum Vickers microhardness (46 GPa) of these composites was attained by adding 40 vol% c -BN with an average grain size of 0.5 μm. The fracture toughness of these composites increased as the c -BN content increased; the maximum fracture toughness (1.5–1.8 MPa^.m^1/2) was observed for x = 40–60 vol%. Crack deflection along the B₆O– c -BN grain boundary contributed to increasing the fracture toughness.     

Sintering with Rigid Inclusions: Pair Interactions

The interaction between two spherical, rigid inclusions in an infinite, linearly viscous, densifying medium has been studied. The stresses in the vicinity of the two spheres are highly anisotropic, with significantly enhanced rates of densification in the gap between these particles. A pairwise-additive approximation for the densification rate of the composite is presented and compared with the composite-sphere and self-consistent models described by Scherer.