Sintering with Rigid Inclusions

Rigid inclusions retard the densification of a sintering body by creating a hydrostatic tensile stress in the matrix. Two models of this process are presented and compared with others from the literature. The composite can be represented by a composite sphere, with the core representing the inclusion. Alternatively, a self-consistent (s-c) calculation can be performed in which the sintering material is regarded as being surrounded by a composite matrix with a slower densification rate. The results differ only in that the shear modulus of the matrix is replaced by the shear modulus of the composite in the s-c calculation. The stress in the inclusion cannot exceed twice the sintering pressure, unless the Poisson's ratio of the matrix is negative. Predictions of higher stresses by previous authors carry the erroneous implication that Poisson's ratio is negative. Since the predicted stresses are not large, models of this type cannot account for the experimentally observed retardation of densification of polycrystalline matrices by inclusions.     

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.     

Effect of Green Density on Densification and Creep During Sintering

The effect of green density on both the densification rate and the creep rate was measured simultaneously during sintering by loading dilatometry. The experiments were performed on zinc oxide powder compacts with five different green densities covering a range of 0.39 to 0.73 of theoretical. The samples were heated at a constant rate of 4°C/min up to 1100°C in air. The densification rate at any temperature increases significantly with decreasing green density. The data for the densification rate and creep rate as a function of density show two quite distinct regimes of behavior; the rates were strongly dependent on density below 0.80, while above this value they were weakly dependent on density. The ratio of the densification rate to the creep rate was almost independent of temperature but increased almost linearly with increasing green density. The representation of the data in terms of models for sintering and creep is discussed.     

Factors Controlling the Sintering of Ceramic Particulate Composites: II, Coated Inclusion Particles

Composite powders were synthesized by coating coarse ZrO₂ inclusion particles with a cladding of fine-grained, crystalline ZnO powder using a chemical precipitation technique. Three different inclusion sizes (1, 3, and 14 μm) were used by selecting the size of the starting ZrO₂ powder, and the volume fraction of the inclusions was controlled by the amount of ZnO precipitated. The powders were compacted by uniaxial pressing in a die and then sintered at a constant heating rate of 4°C/min to 1500°C. The sintering kinetics were almost independent of the inclusion volume fraction, and of the inclusion size, for inclusion contents up to ∼40 vol%. Furthermore, composites containing up to ∼40 vol% inclusions were sintered to almost full density under the same conditions used for the unreinforced matrix. This is considerably better than the densities obtained for conventionally mixed powders, where a modest inclusion content (< ∼ 10 vol%) has been observed to cause a severe reduction in the sintered density of the composite matrix. The kinetic data and microstructural observations are a further indication that the main factors which oppose the free sintering of ceramic particulate composites are processing-related; these factors are (i) inclusion-inclusion interactions which constrain the matrix and (ii) the packing of the matrix phase in regions immediately surrounding the inclusions.     

Effect of Rigid Inclusions on the Densification and Constitutive Parameters of Liquid-Phase-Sintered YBa2Cu3O6+x Powder Compacts

The presence of rigid inclusions in a powder compact leads to a reduction in the densification rate of the compact and may also lead to processing defects. In this paper, the densification rate and the constitutive parameters of both homogeneous YBa₂Cu₃O_{6+ x} and composite powder compacts (YBa₂Cu₃O_{6+ x} powder with 10 vol% dense inclusions of YBa₂Cu₃O_{6+ x} ) are reported. A small amount of liquid phase, which formed during sintering, was present in the samples. However, even with the presence of a liquid phase, the addition of inclusions still reduces the densification rate of the composite and increases its viscosity. The results have been compared with a published analysis of the problem using measured values of the constitutive parameters. Both the viscosity and viscous Poisson's ratio of the porous body have been measured.     

Journal Effect of a Liquid Phase on the Sintering of Heterogeneous YBa2Cu3O6+x Compacts

Dense, polycrystalline YBa₂Cu₃O_6+x inclusions were incorporated into YBa₂Cu₃O_6+x: powder in order to investigate the effect of nondensifying inclusions on the sintering behavior of the matrix. The presence of these inclusions caused a significant reduction in the densification rate of the matrix, as well as some microstructural damage. However, when approximately 2.5 vol% of a liquid phase was present during sintering, there was some retardation of densification in the early stages, but this disappeared with time. Also, the final sintered microstructures were damage-free and essentially identical to those of samples containing no inclusions. Possible roles for the liquid phase in correcting this microstructural damage are briefly discussed.     

Pressureless Sintering of a Ceramic Matrix with Multiple Rigid Inclusions: Finite Element Model

Interaction among the dispersed second phase of rigid inclusions in a ceramic matrix undergoing densification is investigated by a viscoelastic finite element method with a two-dimensional multiple-inclusion model. The interaction is negligible when the volume fraction of inclusions is low (<20%) and/or when the inclusions are uniformly distributed. The interaction becomes significant when the volume fraction of inclusions is high or the inclusions are agglomerated. The densification rates in the region between the inclusions are enhanced to some degree when the inclusions are closely packed. This enhancement will, however, cease when the inclusions get much closer, eventually coming in contact with each other. In general, the agglomeration or close packing of inclusions results in retardation in the overall densification of the sintering matrix.     

Creep and Densification During Sintering of Glass Powder Compacts

The simultaneous creep and densification of glass powder compacts was studied as a function of low applied uniaxial stress, temperature, and particle size. The creep rate can be expressed as the sum of the contribution from the applied stress that varies linearly with stress, and a contribution due to anisotropic densification that varies linearly with the densification rate. For a constant applied stress, the ratio of the creep rate to the densification rate is almost independent of both termperature and density. While these observations are consistent with the model of Scherer for the viscous sintering of glass, other observations show significant deviations from the model. Both the densification rate and the creep viscosity, which has an exponential dependence on porosity, show much stronger dependence on density compared with theoretical predictions.     

Effect of Shear Stress on Sintering

The effect of small uniaxial stresses on the sintering of CdO powder compacts was studied using a loading dilatometer. Compacts of two different green densities were sintered at 1123 K and subjected to stresses between 0 and 0.25 MPa. Densification and creep occur simultaneously, and the effects of these two processes can be separated. Between relative densities of 0.5 and 0.9, the dependence of the uniaxial creep rate on density can be described in terms of a stress intensification factor which depends exponentially on the porosity but is independent of the grain size. Comparison of the densification and creep rates permits definition of the sintering stress, which is found to decrease with increasing density, and verification of the Zener relation. The stress and grain size dependence of the creep rate, and the grain size dependence of the densification rate, support grain-boundary diffusion as the rate-controlling step in both processes.     

Liquid-Phase Sintering of MgO–Bi2O3

Liquid-phase sintering of MgO-5 wt% Bi₂O₃ was studied by loading dilatometry. The ratio of the creep viscosity to the densification viscosity (∼1.8) and the sintering stress remained nearly constant in a wide density interval. These results, together with results on several other systems, indicate that the constancy of the sintering stress during densification may be a general phenomenon, regardless of densification mechanism.     

Effect of Rigid Inclusions on the Sintering of Glass Powder Compacts

The effect of rigid inclusions on the sintering of glass powder compacts has been investigated at 600°C. The densification rates show good agreement with the rule of mixtures for inclusion volume fractions of °0.1 The transient stresses generated during sintering by the presence of the inclusions were evaluated from the sintering data. Below inclusion volume fractions of °0.12, the results are in excellent agreement with Scherer's theory for viscous sintering with rigid inclusions. At higher inclusion volume fractions, interactions between the inclusion particles lead to large deviations from theoretical predictions.     

Creep During Sintering of Porous Compacts

Creep during sintering of CdO powder compacts was studied in a loading dilatometer by applying a small, transient, uniaxial load to the compacts. After the load was removed, the axial shrinkage rate was lower, but the radial shrinkage rate was actually higher than that of a compact sintered under no load. This reduction in the axial shrinkage rate is more pronounced for longer transient loading times. The results provide further support for a mechanism of simultaneous creep and densification in which creep at constant volume occurs by diffusion-controlled grain-boundary sliding.     

Use of heating microscopy to assess sintering anisotropy in layered glass metal powder compacts

Abstract

The objective of the present study was to demonstrate that heating microscopy can be used to investigate the deformation of layered materials during sintering. Three composite systems with layered microstructure were prepared using borosilicate glass matrix and vanadium particles as the inclusion phase. The sintering shrinkage of cylindrical compacts was recorded in axial and radial directions. As expected, sintering was impaired with increasing concentration of vanadium particles. A shrinkage anisotropy factor was determined based on experimental measurements and its evolution during sintering for each sample was discussed. In samples containing three layers with different volume fractions of vanadium inclusions, similar densification was observed in the bottom layer with maximum concentration of inclusions (30 vol.-%) and in the top layer with minimum concentration (2 vol.-%). This indicates that sintering anisotropy of the samples is dependent not only on the composition, but also on the position arrangement of the layers in the sintering part. The results show that heating microscopy is a simple technique which can be used to support the design and fabrication of layered (and by extension functionally graded) materials. Smart choice of composition, dimension and position of the sample in the furnace during sintering should lead to adequate control or prediction of the final sintered shape.     

Sintering of TiO2–Al2O3 Composites: A Model Experimental Investigation

In recent theoretical work we have shown that the sintering of a composite is strongly affected by the shear deformation of the continuous phase. This phenomenon was studied experimentally in a model system consisting of a TiO₂ matrix dispersed with nondensifying agglomerates of Al₂O₃. The results of this study are reported here. Several interesting results were obtained: (a) The experimentally obtained sintering rate of the composite could be successfully predicted by measuring the concomitant shear and densification rate of the matrix phase in a sinter-forging experiment and using the developed analysis; (b) the densification rate of the composite changed with the volume fraction of the dispersed phase, but was unaffected by the size of the dispersed phase; and (c) processing flaws appeared to form only when the size of the dispersed phase was greater than about 10 μm. A technique for measuring the parameter β, which was used to correlate theory and experiments, is described. The procedures used for preparation of powders (from alkoxides) and the green compacts are also described in detail.     

Sintering of β.q.ss. and gahnite glass-ceramic/silicon carbide composites

The sintering of β.quartz solid solution, β.q._ss., and gahnite glass-ceramic/particulate SiC composites has been studied by two different sintering procedures. In one procedure, the composites were fired above the melting point of the crystalline phase, identified by DTA, at a very high heating rate (800 °C min⁻¹) in air, nitrogen and argon atmospheres, for 1–6 min. It was found that the reduction of ZnO constituent of the glass by SiC particles gives rise to Zn, CO, and SiO gaseous products preventing complete densification of composites. In the other procedure, sintering was done at about crystallization peak temperature of the glass phase, employing a low heating rate (40 °C min⁻¹) in air for 60 min. In this case, the circumferential tensile stress in the glass-ceramic matrix phase, caused by the presence of incompressible SiC particles, retards the densification of the composites. The maximum amount of SiC particles yielding a reasonably dense composite was found to be 9 vol.%.     

Processing and mechanical properties of new hierarchical metal-graphene flakes reinforced ceramic matrix composites

Hierarchical tantalum-graphene flakes reinforced zirconia (3Y-TZP) ceramic matrix composites were fabricated by wet processing route and freeze drying followed by spark plasma sintering (SPS). The microstructures and mechanical properties were investigated. The results show that graphene and Ta particles are homogeneously dispersed in the ceramic matrix and the optimum sintering temperature for complete densification of composites and thermal reduction of the graphene oxide is 1500 °C. The addition of dual reinforcements of tantalum microflakes and graphene nanoflakes results in significant improvement in the mechanical properties of the ZrO₂ matrix. Approximately a 30% increase in flexural strength vs the zirconia-Ta composite and a 175% increase in fracture toughness vs the monolithic zirconia have been achieved by introducing 0.5 vol% GO and 20 vol% Ta particles.     

Sintering behavior and mechanical properties of nano-sized Cr3C2/Al2O3 composites prepared by MOCVI process

A process using metal-organic chemical vapor infiltration (MOCVI) conducted in fluidized bed was employed for the preparation of nano-sized ceramic composites. The Cr-species was infiltrated into Al₂O₃ granules by the pyrolysis of chromium carbonyl (Cr(CO)₆) at 300–450 °C. The granulated powder was pressureless sintered or hot-pressed to achieve high density. The results showed that the dominant factors influencing the Cr-carbide phases formation, either Cr₃C₂ or Cr₇C₃, in the composite powders during the sintering process were the temperature and oxygen partial pressure in the furnace. The coated Cr-phase either in agglomerated or dispersive condition was controlled by the use of colloidal dispersion. The microstructures showed that fine (20 –600 nm) Cr_xC_y grains (≤8 vol.%) located at Al₂O₃ grain boundaries hardly retarded the densification of Al₂O₃ matrix in sintering process. The tests on hardness, strength and toughness appeared that the composites with the inclusions (Cr₃C₂) had gained the advantages over those by the rule of mixture. Even 8 vol.% ultrafine inclusions have greatly improved the mechanical properties. The strengthening and toughening mechanisms of the composites were due to grain-size reduction, homogenous dispersion of hard inclusions, and crack deflection.     

Dense Al2O3/ZrO2 Particulate Composites by Free Sintering of Coated Powders

Composite powders, prepared by coating coarse ZrO₂ particles with fine Al₂O₃ powder using a chemical precipitation technique, were compacted and sintered freely at a constant heating rate of 4°C/min to ∼1600°C. Composites containing up to ∼30 vol% inclusions were sintered to nearly full density under the same conditions used for the unreinforced matrix. Furthermore, the sintering kinetics were not influenced significantly by the inclusion volume fraction. The sinterability of the composites formed from the coated powders was significantly better than that for similar composites formed from mechanically mixed powders. The present data provide a further demonstration that the use of coated powders may have widespread applicability for the fabrication, by free sintering, of dense ceramic particulate composites.     

Sintering, Creep, and Electrical Conductivity of Model Glass-Matrix Composites

The sintering, creep, and electrical conductivity of model glass-matrix composites were investigated as functions of inclusion content and size. The composites consisted of spherical soda–lime glass particles and spherical nickel inclusions. At inclusion volume fractions below 10 vol% there was no inclusion size effect on the creep viscosity. Above 10 vol% there was an inclusion size effect, with the viscosity increasing significantly with decreasing inclusion size. The increase in electrical conductivity commenced at lower inclusion volume fractions as the inclusion size decreased. The data indicate that interactions between the inclusions might be responsible for the observed inclusion size effect. The sintering rates of the composites were compared with the predictions of Scherer's self-consistent model. Good agreement was obtained when the measured creep viscosity was used in the model equations.