Densification mechanism and microstructure characteristics of nano- and micro- crystalline alumina by high-pressure and low temperature sintering

Fully dense ceramics with retarded grain growth can be attained effectively at relatively low temperatures using a high-pressure sintering method. However, there is a paucity of in-depth research on the densification mechanism, grain growth process, grain boundary characterization, and residual stress. Using a strong, reliable die made from a carbon-fiber-reinforced carbon (C_f/C) composite for spark plasma sintering, two kinds of commercially pure α-Al₂O₃ powders, with average particle sizes of 220 nm and 3 μm, were sintered at relatively low temperatures and under high pressures of up to 200 MPa. The sintering densification temperature and the starting threshold temperature of grain growth (T_sg) were determined by the applied pressure and the surface energy relative to grain size, as they were both observed to increase with grain size and to decrease with applied pressure. Densification with limited grain coarsening occurred under an applied pressure of 200 MPa at 1050 °C for the 220 nm Al₂O₃ powder and 1400 °C for the 3 μm Al₂O₃ powder. The grain boundary energy, residual stress, and dislocation density of the ceramics sintered under high pressure and low temperature were higher than those of the samples sintered without additional pressure. Plastic deformation occurring at the contact area of the adjacent particles was proved to be the dominant mechanism for sintering under high pressure, and a mathematical model based on the plasticity mechanics and close packing of equal spheres was established. Based on the mathematical model, the predicted relative density of an Al₂O₃ compact can reach ～80 % via the plastic deformation mechanism, which fits well with experimental observations. The densification kinetics were investigated from the sintering parameters, i.e., the holding temperature, dwell time, and applied pressure. Diffusion, grain boundary sliding, and dislocation motion were assistant mechanisms in the final stage of sintering, as indicated by the stress exponent and the microstructural evolution. During the sintering of the 220 nm alumina at 1125 °C and 100 MPa, the deformation tends to increase defects and vacancies generation, both of which accelerate lattice diffusion and thus enhance grain growth.     

Densification of RO2 type oxides during hot pressing

Conclusions The optimal conditions for obtaining high-density products by the hot pressing of pure oxides of the type RO₂ were established. In order to prepare high-density samples with simultaneous hot pressing and stabilization of the zirconium and hafnium dioxide the heating must be slowed down in the polymorphic interval (800–1200°C) to generate the necessary number of vacancies which ensure significant densification. The amount of stabilizer (calcium or magnesium oxide) should not be less than 7% (by weight). Prestabilized zirconium and hafnium dioxide can be sintered by hot pressing with heating without holding in the interval 800–1200°C to a relative density of 0.98–0.99 under a pressure of 160 kg/cm², and also with a holding time of 10 min and a temperature of 1700–1800°C (zirconium dioxide) and 2100–2200°C (hafnium dioxide).Fused quartz can be sintered by hot pressing to zero water absorption and a relative density of about 1.0 at 1400°C, a pressure of 200 kg/cm², and a holding time of 10 min.Deceased.Translated from Ogneupory, No. 2, pp. 36–40, February, 1969.     

烧结工艺对β-氮化硅陶瓷的影响

以β-Si3N4粉末为原料,以YAG（钇铝石榴石）为烧结助剂,通过气氛压力烧结（GPS）制备出致密的β-氮化硅陶瓷材料,形成大小均匀的柱状颗粒和小球状颗粒复合显微结构,研究了烧结助剂质量分数、烧结温度以及保温时间对β-氮化硅陶瓷致密化程度及力学性能的影响.     

Densification behavior and microstructure evolution of yttrium-doped ThO2 ceramics

Sintering of Th_1-xY_xO_2-x/2 ceramics (x = 0.01, 0.08, 0.15 and 0.22), planned to be used as solid electrolytes in oxygen sensors for sodium-cooled fast nuclear reactors, was investigated. High densification state (i.e. up to 98% TD) was reached after 4 h of heat treatment at 1600 °C and beyond. In addition, ESEM observations showed a major effect of yttrium on grain size due to solute drag effects. Sintering maps were plotted for all the samples and evidenced different stages driven by densification and grain growth. Grain growth was found to be strongly slowed down for x > 0.01, resulting in high values of relative density correlated to submicrometric grain size. Also, activation energies related to densification and grain growth were evaluated around 450 and 500–650 kJ mol⁻¹, respectively. These results led to deliver guidelines for the formulation and sintering of Th_1-xY_xO_2-x/2 ceramics in prospect of their use as a solid electrolyte.     

Densification behavior of mullite-Al2TiO5 composites by reaction sintering of natural andalusite and TiO2

In this work, mullite-Al₂TiO₅ composites were fabricated by natural andalusite with TiO₂ as an additive. The densification characteristic, phase composition and mullitization process of andalusite with TiO₂ addition was investigated by the Archimedes’ principle, dilatometry, X-ray diffraction and scanning electron microscopy (SEM-EDS) techniques. The results showed that the incorporation of TiO₂ not only enhanced the thermal stability of in-situ Al₂TiO₅ in the silica liquid yielded from the mullitization of andalusite, but also accelerated andalusite decomposition and retarded mullite formation, which facilitated the sintering and densification of mullite-Al₂TiO₅ composites.     

Densification of “active” zirconia powders during hot pressing

Conclusions We studied the influence of temperature and heat process time for zirconium nitrate on the phase composition, dispersion of grains and crystalline lattice defects in the resulting zirconia, and also on its capacity for subsequent compaction during hot pressing. This capacity is connected with the formation, during a short heating period up to 400°C, of metastable tetragonal and cubic modifications having a highly defective crystalline lattice.We also studied the densification during hot pressing of active zirconia obtained by decomposing zirconium nitrate. During decomposition of zirconium nitrate at 400°C with a short soak we noted the formation of zirconium dioxide which was actively compacted at 1100°C and a pressure of 200 kg/cm². We studied the influence of additions of CaO and the methods of adding it on the densification of active zirconia. An increase in the content of CaO leads to a reduction in the capacity of ZrO₂ for compaction during hot pressing. It is necessary to introduce the additive to the nitrate before it decomposes. During hot pressing of active zirconia containing additions of CaO and Y₂O₃ at low temperatures (1100–1300°C) cubic solid solutions are not formed owing to the short process time, and the zirconia in the specimens consists of monoclinic form. Subsequent heating of the pressed specimens forms cubic solid solutions.It was found that the introduction of 10–20% additive of zirconia obtained by heat processing the nitrate to industrial ultrapure zirconium dioxide yields dense specimens with a reduction in the optimum pressing temperature of 200°C.Translated from Ogneupory, No.7, pp. 39–45, July, 1967.     

Hot pressing of nanocrystalline zinc oxide compacts: Densification and grain growth during sintering

Sintering behavior of nanocrystalline zinc oxide (ZnO) powder compacts using hot pressing method was investigated. The sintering conditions (temperature and total time) and results (density and grain size) of two-step sintering (TSS), conventional sintering (CS) and hot pressing (HP) methods were compared. The HP technique versus CS was shown to be a superior method to obtain higher final density (99%), lower sintering temperature, shorter total sintering time and rather fine grain size. The maximum density achieved via HP, TSS and CS methods were 99%, 98.3% and 97%, respectively. The final grain size of samples obtained by HP was greater than that of TSS method. However, the ultra-prolonged sintering total time and the lower final density (88 ks and 98.3%) are the drawbacks of TSS in comparison with the faster HP (17 ks and 99%) method.     

Reactive hot pressing of ZrB2-based composites with changes in ZrO2/SiC ratio and sintering conditions. Part I: Densification behavior

ZrB₂–SiC–ZrO₂ composites were hot pressed in order to investigate the effects of adding nano-sized ZrO₂ particles as well as the hot pressing parameters on the densification behavior of ZrB₂–SiC composites. An L9 orthogonal array of the Taguchi method was employed to study the significance of each parameter such as the sintering temperature, time, the applied external pressure, and ZrO₂/SiC volume ratio on the densification process. The statistical analyses revealed that among the mentioned parameters, the hot pressing temperature had a great influence over the densification. By being hot pressed at 1850 °C for 90 min under 16 MPa, fully dense ZrB₂-based composites were obtained. The relative density of the composites decreased at first and then enhanced as a function of ZrO₂/SiC ratio. Microstructural investigation of the fracture surfaces of the composites, which was carried out using the SEM analysis, showed the formation of new phases on the surfaces of SiC grains. The EDS and XRD analyses identified the ZrC as the newly formed interfacial phase due to the reaction between nano-ZrO₂ and SiC. The ZrC acted as an adhesive interphase between the ZrB₂/SiC grains, which could assist the sintering process.     

Densification,mechanical, and tribological properties of ZrB2-ZrCx composites produced by reactive hot pressing

ZrB₂-ZrC_x composites were produced using Zr:B₄C powder mixtures in the molar ratios of 3:1, 3.5:1, 4:1, and 5:1 by reactive hot pressing (RHP) at 4-7 MPa, 1200°C for 60 minutes. X-ray diffraction analyses confirmed the formation of nonstoichiometric zirconium carbide (ZrC_x) with different lattice parameters and enhanced carbide formation by increasing the Zr mole fraction. An increase in applied pressure from 4 to 7 MPa was responsible for the improved relative density (RD) of 4Zr:B₄C composition from 86% to 99%. Microstructural studies on Zr-rich composites showed a reduction in unreacted B₄C particles and enriched elongated ZrB₂ platelets. Reaction and densification mechanism in 4Zr:B₄C composition were studied as a function of temperature increased from 600 to 1200°C at an applied constant pressure of 7 MPa. After 1000°C, <40 vol.% of unreacted Zr was observed during the densification process. Concurrently, low energies of carbon diffusion and carbon vacancy formation were found to enhance nonstoichiometric ZrC_x formation, which was found to be responsible for the completion of the reaction. The plastic deformation of unreacted Zr was responsible for the densification of the ZrB₂-ZrC_x composite. The results clearly showed that the applied pressure is five times lower than the reported values. Moreover, a temperature of 1200°C was sufficient to produce dense ZrB₂-ZrC_x composites. The improved microhardness, flexural strength, fracture toughness, and specific wear rate were 8.2-15 GPa, 265-590 MPa, 2.82-6.33 MPa.m^1/2, and 1.43-0.376 × 10⁻² mm²/N, respectively.     

Fabrication and mechanisms of densification of ZrB2-based ultra high temperature ceramics by reactive hot pressing

Dense ZrB₂–SiC (25–30 vol%) composites have been produced by reactive hot pressing using stoichiometric Zr, B₄C, C and Si powder mixtures with and without Ni addition at 40 MPa, 1600 °C for 60 min. Nickel, a common additive to promote densification, is shown not to be essential; the presence of an ultra-fine microstructure containing a transient plastic ZrC phase is suggested to play a key role at low temperatures, while a transient liquid phase may be responsible at temperatures above 1350 °C. Hot Pressing of non-stoichiometric mixture of Zr, B₄C and Si at 40 MPa, 1600 °C for 30 min resulted in ZrB₂–ZrC_x–SiC (15 vol%) composites of 98% RD.     

Fabrication of ZrB2 ceramics by reactive hot pressing of ZrB and B

Zirconium diboride (ZrB₂) ceramics were prepared by reactive hot pressing of ZrB+B powder mixture. Formation of a transient liquid due to eutectic reaction of ZrB₂+Zr→L_eu(ZrB2+Zr) at 1661°C following peritectic decomposition of 2ZrB=ZrB₂+Zr at 1250°C during heating up of the ZrB+B mixture facilitated densification. The liquid phase was subsequently eliminated via reaction of B with Zr in the eutectic liquid L_eu(ZrB2+Zr) to result in a dense ZrB₂ ceramic. Full density was reached after reactive hot pressing at 1900°C under 30 MPa for 1 h. The ZrB₂ ceramic had a refined microstructure consisting of grains of <1.5 μm in size and relatively good Vickers hardness (21 ± 2 GPa) and flexural strength (595 ± 63 MPa).     

Densification and grain growth kinetics of Ce0.9Gd0.1O1.95 in tape cast layers: The influence of porosity

The sintering behavior of Ce_0.9Gd_0.1O_1.95 (CGO) tape cast layers with different porosity was investigated by an extensive characterization of densification, microstructural evolution, and applying the constitutive laws of sintering. The densification of CGO tapes associates with grain coarsening process at the initial sintering stage at T < 1150 °C, which is mainly influenced by small pores and intrinsic characteristics of the starting powders. At the intermediate sintering stage, densification is remarkably influenced by large porosity. Moreover, the sintering constitutive laws indicate that increasing the initial porosity from 0.38 to 0.60, the densification at the late stage is thermally activated with typical activation energy values increasing from 367 to 578 kJ mol⁻¹. Similar effect of the porosity is observed for the thermally activated phenomena leading to grain growth in the CGO tapes. The analysis of sintering mechanisms reveals that the grain growth behavior at different porosity can be described using an identical master curve.     

Influence of TiO2 on the densification behaviour of Yb2O3

The effect of temperature and heating rate on the densification of ytterbia (Yb₂O₃), with and without titania (TiO₂) doping was investigated. It is shown that up to a certain doping level, titania doping enhances the densification behaviour of ytterbia. The effect of titania doping on crystal structure confirms that titania is substitutionally incorporated in ytterbia up to the solubility limit, which corresponds well with the densification results. The increased densification rate of titania-doped ytterbia is attributed to the formation of cation vacancy and lattice distortion. Using constant heating rate experiments, the activation energy for densification has been calculated and it is shown that in the intermediate density range (60 % to 85 %), the activation energy is independent of the density. Titania doping increases the activation energy for densification.     

Preparation of B4C composites toughened by MoB2/Mo2B5-SiC interlocking structure via reactive hot pressing

B₄C composites toughened by MoB₂/Mo₂B₅-SiC interlocking structure were prepared via reactive hot pressing with B₄C and MoSi₂ as raw materials. The phase composition, microstructure, and mechanical properties of the fabricated B₄C composites were studied. The crack propagation and fracture surface were observed, and the toughening mechanism was analyzed. The results indicate that the interlocking structure of MoB₂/Mo₂B₅-SiC is formed in the obtained B₄C composites. The relative density, flexural strength, and fracture toughness of the B₄C composites reach 99.3%, 480 MPa, and 5.2 MPa·m^1/2, respectively, when the MoSi₂ content is 30 wt%. The hardness is 33 GPa when the MoSi₂ content is 20 wt%. The special laminar fracture of the interlocking structure of MoB₂/Mo₂B₅-SiC elongates the crack extending path and thus consumes more energy of crack extension. The phenomena of crack bridging and branching and the special laminar fracture of the interlocking structure have a synergistic effect on promoting the overall fracture toughness.     

Densification as an exothermic process revealed by rapid high temperature consolidation of BaTiO3 nanopowder

Abstract

Densification is an exothermic process according to the classical sintering theories; however, it has never been explored experimentally. In the present work, such heat release was successfully detected from nanosized BaTiO₃ nanopowder compact, which was rapidly consolidated by spark plasma sintering. A reduction of total power consumption was observed immediately when rapid densification occurred. The effects of the deviation of overall electric resistance on total power consumption were analysed. The temperature at which a falling inflection point of the power supply was observed can be used as an indicator of the minimum temperature required for densification. This would be of help for defining the ‘kinetic window’ for processing of nanoceramics in sintering practice.     

Densification and grain growth of Gd2Zr2O7 nanoceramics during pressureless sintering

Gd₂Zr₂O₇ nanoceramics were fabricated using pressureless sintering method, in which the nanopowders were synthesized via solvothermal approach. The effects of starting powders on grain growth and densification during sintering of ceramics were revealed. Two distinct pressureless sintering methods were investigated, including conventional and two-step sintering. The sample grain size increases abruptly as sintering temperature increases during conventional sintering. In contrast, in two-step sintering, abnormal or discontinuous grain growth was suppressed in the second step, leading to Gd₂Zr₂O₇ nanoceramics formation (average grain size 83 nm, relative density ∼93%). Such distinct behaviors may originate from the interplay between kinetic factors such as grain boundary migration and diffusion. Moreover, suppression of grain growth and promotion of densification in the two-step sintering are mainly due to dominant role of grain boundary diffusion during the second-step sintering process.     

Effect of Al2O3 addition on texturing in a rotating strong magnetic field and densification of B4C

The properties of ceramics can be improved by controlling the microstructure through texturing ceramics in a strong magnetic field. Fabricating dense boron carbide (B₄C) requires high temperature sintering, therefore sintering additives are often used in order to densify B₄C ceramics at lower temperatures. However, combined effect of texturing and sintering additives on densification of B₄C has not been made clear yet. Here we report the effect of alumina (Al₂O₃) sintering additive on texturing in a strong magnetic field and densification of B₄C. Texturing was performed by rotating superconducting magnet at 12 T during slip casting process. Electron backscatter diffraction (EBSD) was used to observed the texturing projection. {0001} plane is clearly oriented in the plane parallel to rotating magnetic field. In addition, Lotgering factor was also calculated as quantitatively evaluation of texturing degree. Results on densification showed that addition of Al₂O₃ successfully increased density of B₄C sintered by spark plasma sintering (SPS) at 1800^oC to 97.8%. Formation of aluminum borate (Al₅BO₉) as secondary phase was detected by X-Ray diffraction (XRD). It is considered that the generation of Al₅BO₉ assisted finer densification of B₄C ceramic. Textured B₄C sintered at 1700^oC by SPS without alumina addition exhibited the highest orientation of c-axis. Addition of alumina caused decrease in degree of orientation of c-axis.     

Densification,microstructure and mechanical properties of hot pressed tantalum carbide

Monolithic tantalum carbide (TaC) ceramics were prepared by hot pressing in order to investigate the effect of hot pressing temperature on the densification behavior, microstructure and mechanical properties of TaC. Monolithic TaC sample hot pressed at 2000 °C for 45 min under 40 MPa, with relative density value above 97%, Vickers hardness of 15.7 GPa and fracture toughness of 4.1 MPa m^1/2 was obtained. Fracture surfaces investigations of the samples, which were carried out using the SEM analysis, showed a significant grain growth by increasing the hot pressing temperature from 1700 to 2000 °C. Also, based on the X-ray diffraction pattern, a decrease in the lattice parameter of hot pressed TaC sample was observed.     

Densification mechanism,microstructure and mechanical properties of ZrC ceramics prepared by high-pressure spark plasma sintering

The traditional way of densifying high-melting-point ceramics at high temperatures with long soaking time leads to severe grain coarsening, which degrades the mechanical properties of ceramics. Here, highly dense (∼98%) zirconium carbide (ZrC) ceramics with limited grain growth were obtained by spark plasma sintering (SPS) at relatively low temperatures, 1900 ℃, with a high pressure up to 200 MPa in a reliable carbon-fiber-reinforced carbon composite (C_f/C) mold. Subgrains and high-density dislocations formed in the high-pressure sintered ceramics. The hardness and fracture toughness of the prepared highly dense ZrC ceramics reached 20.53 GPa and 2.70 MPa·m^1/2, respectively. The densification mechanism was mainly plastic deformation under high pressure. In addition, ZrC ceramics sintered at high pressure possessed a high dislocation density of 7.30 × 10¹² m⁻², which was suggested to contribute to the high hardness.     

Mechanical,tribological and electrical properties of ZrB2 reinforced Cu processed via milling and high-pressure hot pressing

The present work investigates the effect of (0–10 wt%) ZrB₂ reinforcement on densification, mechanical, tribological and electrical properties of Cu. The consolidation of Cu–ZrB₂ samples was carried out using a hot press (temperature: 500 °C, pressure: 500 MPa, time: 30 min, vacuum pressure: 1.3 × 10^-2 mbar). The bulk density of the hot-pressed Cu composites decreased from 8.84 g/cc to 8.16 g/cc and the relative density of samples lowered from 98.6% to 92.1% with the addition of ZrB₂. The incorporation of hard ZrB₂ (up to 10 wt%) improved the hardness of Cu (1.32–2.55 GPa). However, the yield strength and compressive strength of Cu composites increased up to 5 wt% ZrB₂, and further addition of ZrB₂ lowered its strength. The yield strength of Cu samples varied from 602 to 672 MPa and the compressive strength between ~834 and 971 MPa. On the other hand, the coefficient of friction (COF) (from 0.49 to 0.18) and wear rate (from 49.3 × 10^-3 mm³/Nm to 9.1 × 10^-3 mm³/Nm) of Cu–ZrB₂ samples considerably decreased with the addition of ZrB₂. Significantly low wear was observed with Cu-10 wt% ZrB₂ (Cu-10Z) samples, which is 5.41 times less than pure Cu. As far as the wear mechanisms are concerned, in pure Cu, continuous chips (wear debris) were formed during sliding wear by plowing. Whereas the major amount of material loss was occurred due to the plowing mechanism with discontinuous and short chip formation for Cu–ZrB₂ composites. The electrical conductivity of Cu–ZrB₂ samples decreased from 75.7% IACS to 44.1% IACS. In particular, Cu with ZrB₂ (up to 3 wt%) could retain the conductivity of 66.8% IACS. This study reveals that the addition of ZrB₂ (up to 3 wt%) is advantageous to have a good combination of properties for Cu.