Effect of Sintering Atmosphere on Grain Shape and Grain Growth in Liquid-Phase-Sintered Silicon Carbide

We investigated the effects of the sintering atmosphere on the interface structure and grain-growth behavior in 10-vol%-YAG-added SiC. When α-SiC was liquid-phase-sintered in an Ar atmosphere, the grain/matrix interface was faceted, and abnormal grain growth occurred, regardless of the presence of α-seed grains. In contrast, when the same sample was sintered in N₂, the grain interface was defaceted (rough), and no abnormal grain growth occurred, even with an addition of α-seed grains. X-ray diffraction analysis of this sample showed the formation of a 3C (β-SiC) phase, together with a 6H (α-SiC) phase. These results suggest that the nitrogen dissolved in the liquid matrix made the grain interface rough and induced normal grain growth by an α→β reverse phase transformation. Apparently, the growth behavior of SiC grains in a liquid matrix depends on the structure of the grain interface: abnormal growth for a faceted interface and normal growth for a rough interface.     

Effect of Grain Shape on Abnormal Grain Growth in Liquid-Phase-Sintered Nb1−xTixC–Co Alloys

The effect of titanium substitution for niobium on the grain shape change, grain growth inhibition, and abnormal grain growth during liquid-phase sintering of Nb_{1− x} Ti_xC–Co alloy was studied. With increased titanium substitution, the shape of the Nb_{1− x} Ti _x C grains in the liquid cobalt matrix was changed from a cube with round corners to a cube with angular corners, which implied increased edge energy. As the grain corners became more angular, the grain growth became markedly inhibited, and abnormal grain growth occurred. The results could be best explained by the increased edge energy of the interface of the Nb_{1− x} Ti _x C grains, which increased the barrier for the growth by two-dimensional nucleation.     

Evolution of the Grain Size Distribution during the Sintering of Alumina at 1350°C

The objectives of this study are to provide some rare unfolded grain size distribution data for the sintering of alumina and to test for time invariance of the normalized grain size distribution as required by normal grain growth. The results show that ln ς doubled during the sintering times studied. The changes in the grain size distribution may be due to an increase in the number of relatively large grains combined with a reduction in the number of grain annihilation events compared with that required for time invariance of the normalized grain size distribution.     

Tape Casting, Pressureless Sintering, and Grain Growth in Ti3SiC2 Compacts

In this work we demonstrate that fine Ti₃SiC₂ powders can be tape-cast and/or cold-pressed and pressureless-sintered in Ar- or Si-rich atmospheres to produce fully dense, oriented microstructures in which the basal planes are parallel to the surfaces. Carbon- and/or Si-rich environments suppress grain growth. In the case of the tape casting, the C-residue from binder burnout results in small core grains relative to the surface grains that can grow significantly. When sintering in high Si activities, titanium silicide phases form at the grain boundaries that slow grain growth. Annealing the latter in Ar at 1600°C, for extended periods (30 h), rids the samples of these grain-boundary phases, which in turn results in grain growth. The advantage of the latter process is that the final grain size distribution is more uniform from surface to bulk.     

Effect of Sintering Atmosphere on Grain Boundary Segregation and Grain Growth in Niobium-Doped SrTiO3

To investigate the donor segregation in grain boundary regions and its effect on grain growth in SrTiO₃, SrTiO₃ powder compacts were doped with Nb₂O₅ and sintered in air or in hydrogen. The Nb-doped SrTiO₃ sintered in air did not show any detectable Nb segregation at the grain boundary region while an appreciable segregation was observed in the space-charge region of the sample sintered in H₂. The observed donor segregation in H₂ suggests a negative grain boundary charge and compensating positive space charge, which are the opposite to those in air. The negative grain boundary core was attributed to the segregation of inherently present acceptor impurities and the trapping of electrons at grain boundaries. In the H₂-sintered sample, where the added Nb ions were segregated in the space-charge region, the grain growth was suppressed. This result may indicate that the grain growth suppression in H₂ is due to the Nb solute drag of the boundary motion and the reduction in Ti vacancies.     

Role of Carbon in the Sintering of Boron-Doped Silicon Carbide

The effect of carbon on the sintering of boron-doped SiC was studied. The free carbon present in the green compact was found to react with the SiO₂ covering the surfaces of the SiC particles; however, even if no carbon was added, the surface SiO₂ reacted with the SiC itself at a slightly higher temperature. This latter reaction was associated with the onset of substantial pore growth in the shrinking green body, which, as the pores continued to grow at higher temperatures, prevented complete densification. Therefore, the reaction of the SiC with the SiO₂ may have led to the fracture of interparticle contacts, resulting in the onset of coarsening. Thus, the role of the carbon was to prevent reaction between the SiC and the surface SiO₂, by removing the SiO₂ at a temperature below that at which this reaction could occur.     

Pressureless Sintering of Boron Carbide

B₄C powder compacts were sintered using a graphite dilatometer in flowing He under constant heating rates. Densification started at 1800°C. The rate of densification increased rapidly in the range 1870°–2010°C, which was attributed to direct B₄C–B₄C contact between particles permitted via volatilization of B₂O₃ particle coatings. Limited particle coarsening, attributed to the presence or evolution of the oxide coatings, occurred in the range 1870°–1950°C. In the temperature range 2010°–2140°C, densification continued at a slower rate while particles simultaneously coarsened by evaporation–condensation of B₄C. Above 2140°C, rapid densification ensued, which was interpreted to be the result of the formation of a eutectic grain boundary liquid, or activated sintering facilitated by nonstoichiometric volatilization of B₄C, leaving carbon behind. Rapid heating through temperature ranges in which coarsening occurred fostered increased densities. Carbon doping (3 wt%) in the form of phenolic resin resulted in more dense sintered compacts. Carbon reacted with B₂O₃ to form B₄C and CO gas, thereby extracting the B₂O₃ coatings, permitting sintering to start at ∼1350°C.     

Development of WC–ZrO2 Nanocomposites by Spark Plasma Sintering

In the present work, we report the processing of ultrahard tungsten carbide (WC) nanocomposites with 6 wt% zirconia additions. The densification is conducted by the spark plasma sintering (SPS) technique in a vacuum. Fully dense materials are obtained after SPS at 1300°C for 5 min. The sinterability and mechanical properties of the WC–6 wt% ZrO₂ materials are compared with the conventional WC–6 wt% Co materials. Because of the high heating rate, lower sintering temperature, and short holding time involved in SPS, extremely fine zirconia particles (∼100 nm) and submicrometer WC grains are retained in the WC–ZrO₂ nanostructured composites. Independent of the processing route (SPS or pressureless sintering in a vacuum), superior hardness (21–24 GPa) is obtained with the newly developed WC–ZrO₂ materials compared with that of the WC–Co materials (15–17 GPa). This extremely high hardness of the novel WC–ZrO₂ composites is expected to lead to significantly higher abrasive-wear resistance.     

Electrode Effects on Microstructure Formation During FLASH Sintering of Yttrium‐Stabilized Zirconia

Systematic microstructural statistics for 3 mol% yttria‐stabilized zirconia synthesized by both conventional sintering and flash sintering with AC and DC current were obtained. Within the gage section, flash sintered microstructures were indistinguishable from those synthesized by conventional sintering procedures. With both techniques, full densification was obtained. However, from both AC and DC flash sintered specimens, heterogeneous grain size distributions and residual porosity were observed in the proximity of the electrodes. After DC sintering, an almost 400 times increased average grain size was observed near cathode compared to the gage section, unlike areas close to the anode. Concepts of Joule heating alone were not sufficient to explain the experimental observations. Instead, the activation energy for grain growth close to the cathode is lowered considerably during flash sintering, hence suggesting that electrode effects can cause significant heterogeneities in microstructure evolution during flash sintering. Microstructural characterization further indicated that microfracturing during green‐pressing and variations in contact resistance between the electrodes and the ceramic may also contribute to grain size gradients and hence local variations of physical properties.     

Experimental Design Applied to Silicon Carbide Sintering

Silicon carbide is a promising structural ceramic used as abrasives and applied in metallurgical components, due to its low density, high hardness, and excellent mechanical properties. The composition and content of the additive can control liquid-phase sintering of SiC. Compositions based on the SiO₂–Al₂O₃–RE₂O₃ system (RE = rare earth) have been largely used to promote silicon carbide densification, but most studies are not systematically presented. The aim of this work is to study the effect of several oxide additives in the SiO₂–Al₂O₃–Y₂O₃ system on the densification of silicon carbide using experimental design. This technique seems to be effective in optimizing the values of maximum density with minimum weight loss.     

Grain Growth During Gas-Pressure Sintering of β-Silicon Nitride

The microstructures of gas-pressure-sintered materials from β-Si₃N₄ powder were characterized in terms of the diameter and aspect ratio of the grains. The size distributions of diameters in materials fabricated by heating for 1 h at 1850° to 2000°C were nearly constant when they were normalized by average diameters because of normal grain growth. The rate-determining step in the densification and grain growth was expected to be the diffusion of materials through the liquid phase. The activation energy for grain growth was 372 kJ/mol. The average aspect ratio of the grains was 3 to 4, whereas that of large grains was smaller because of shape accommodation. The fracture toughness was about the same as that of material from α-Si₃N₄ powder despite the smaller aspect ratio of the grains     

Fabrication of Dense Nanostructured Silicon Carbide Ceramics through Two-Step Sintering

SiC powder compacts were prepared with Al₂O₃, Y₂O₃, and CaO powders. By two-step sintering, fully dense nanostructured SiC ceramics with a grain sizes of ∼40 nm were obtained. The grain size–density trajectories are compared with those of conventional sintering processes.     

Effect of Li2O and PbO Additions on Abnormal Grain Growth in the Pb(Mg1/3Nb2/3)O3–35 mol% PbTiO3 System

Abnormal grain growth in Pb(Mg_1/3Nb_2/3)O₃–35 mol% PbTiO₃ (PMN-35PT) ceramics doped with Li₂O and PbO has been investigated. Replacing the PbO dopant with up to 2 mol% Li₂O caused an increase in the number of abnormal grains. For the composition containing 2 mol% Li₂O and 6 mol% PbO, the amount of abnormal grain growth decreased with increasing sintering temperature. Single crystals of ∼6 mm × 6 mm × 2 mm thickness were grown from the 2 mol% Li₂O, 6 mol% PbO-containing composition via the templated grain growth method. Grain growth behavior with temperature is explained in terms of the effect of Li₂O on interface-reaction-controlled grain growth and the critical driving force.     

Grain Growth of Zinc Oxide During the Sintering of Zinc Oxide—Antimony Oxide Ceramics

Grain growth in a high-purity ZnO with systematic additions of Sb₂O₃ from 0.29 to 2.38 wt% was studied for sintering in air from 1106° to 1400°C. The results are discussed and compared with previous studies of pure ZnO and ZnO with Bi₂O₃ additions in terms of the kinetic grain growth expression: Gⁿ – Gⁿ ₀= K ₀ t exp(— Q/RT ). Additions of Sb₂O₃ inhibited the grain growth of ZnO and increased the grain growth exponent ( n -value) to 6 from 3 for pure ZnO and 5 for the ZnO—Bi₂O₃ ceramic. The apparent activation energy for the grain growth of ZnO also increased to about 600 kJ/mol from 220 kJ/mol for pure ZnO and 150 kJ/mol for the ZnO—Bi₂O₃ ceramics. Both the grain growth exponent and the activation energy were independent of the Sb₂O₃ content. Particles of the Zn₇Sb₂O₁₂ spinel were observed on the grain boundaries and at the grain triple point junctions. It was also observed that the Sb₂O₃ additions caused twin formation in each ZnO grain. It is concluded that both the Zn₇Sb₂O₁₂ particles and the twins are responsible for the ZnO grain growth inhibition by Sb₂O₃.     

Influence of Silica and Aluminum Contents on Sintering of and Grain Growth in 6H-SiC Powders

To clarify the influence of impurities on the sintering of SiC powder, three 6H-SiC powder samples—with different levels of SiO₂ and aluminum impurities—were sintered with additions of boron and carbon. The densification, grain growth, and transformation of 6H-SiC during sintering were studied qualitatively. The powder that contained the most SiO₂ required the greatest amount of boron additive for complete densification. SiO₂ apparently reacted with the boron additive and was consumed during sintering. The powder with the greater aluminum impurity level exhibited partial transformation of 6H-SiC to 4H-SiC, and the sintered SiC from this powder had elongated grains. The partial transformation in the SiC crystal accelerated non-equiaxial grain growth.     

Structure of Sapphire Bicrystal Boundaries Produced by Liquid-Phase Sintering

The structure and composition of sapphire bicrystal boundaries produced by liquid-phase sintering depended on the crystallographic misorientation of the crystals across the boundary and on the orientation of the boundary. Basal twist boundaries of 15° or 30° were not wetted by glass, but contained significant amounts of Ca and Si at the boundary. For tilt boundaries of 7° or 12°, the glass wetted segments of boundaries that contained the basal plane of either crystal. Boundary segments with orientations of 40° or more from the basal plane, however, were dewetted (i.e., "dry"). Boundary segments oriented less than ∼40° from the basal orientation were partially wetted, consisting of segments of wetted and dry grain boundaries. For the 12° tilt boundary, Ca and Si could be detected on portions of the boundary that contained no glass. For bicrystal boundaries having tilts of ≤4°, dewetting occurred for all observed boundary orientations. Basal-oriented segments in these small angle tilt boundaries contained noticeable concentrations of adsorbed Ca and Si, while nonbasal segments were apparently free of Ca and Si. Most results could be explained based on a combined Wulff plot construction, which predicts partially wetted grain boundaries and "missing" angles for unwetted grain boundaries. Results that could not be explained by the construction included growth step ledges bounded by nonequilibrium facet planes.     

Thermodynamic Benefit of Abnormal Grain Growth in Pore Elimination During Sintering

Free-energy analyses performed on closed particle-pore arrays show that the presence of an abnormal grain thermodynamically favors the shrinkage of large pores to which is is adjacent. This gives an explanation to the experimentally observed phenomenon of abnormal-grain-growth-promoted densification in barium titanate.     

Quantification of Grain Boundary-Pore Contact During Sintering

A stereological method has been used to determine the degree of grain boundary-pore contact during sintering of Al₂O₃. Al₂O₃ doped with 200 ppm MgO exhibits a degree of contact of 5.7 times that expected from random intersections with pores, while pure Al₂O₃ shows a pore contact factor of 4.8. These data are larger than the values of 2.8 for sintered or hot-pressed UO₂, computed from published data, and values of 1.7 and 1.8 for sintered W and Cu powders, respectively. The degree of grain boundary-pore contact for each material remains constant throughout densification from pressed powder to near full density.     

Sintering in Nitrogen Atmosphere of Iron-Rich Glass-Ceramics

The sintering and the crystallization of two iron-rich glass compositions (45–75-μm powder fractions) were studied in air and nitrogen atmospheres. The phase formation was evaluated by differential thermal analysis, while the densification, by dilatometry; the crystalline phases were identified by X-ray diffraction and the structure observed by scanning electron microscopy. It was highlighted that, due to the absence of Fe²⁺ oxidation and lower viscosity of the parent and residual glasses, the sintering in nitrogen atmospheres occurs at 100°–200°C lower temperature. In the same time the higher amount of crystal phase, formed during sinter–crystallization in inert atmosphere, improves the mechanical properties. A value of 120 MPa for the bending strength was obtained after 1-h sintering at 960°C in N₂.     

Transparent Alumina with Submicrometer Grains by Float Packing and Sintering

Commercially available α-alumina powder with high-purity, submicrometer particle size and narrow particle-size distribution is used as starting material to prepare dense ceramic parts with transparent properties. The powder is dispersed and stabilized in a water-based suspension. Controlled consolidation and drying by float packing leads to homogeneous green compacts, which can be densified without additives by sintering in air at 1275°C to transparency, while the mean grain size remains at 0.4 μm. The in-line transmittance for wavelengths of 300–450 nm is comparable to commercial polycrystalline alumina tubes for lighting technologies, whose grain sizes are larger by a factor of 40.