Pressure-enhanced densification of TaC ceramics during flash spark plasma sintering

Flash spark plasma sintering (FSPS) offers extremely high heating rates to consolidate ceramics at a short time. However, significant grain growth sometimes occurs accompanied by rapid densification. In this work, a FSPS apparatus available for applying pressure was used to sinter TaC ceramics from powder compacts without preheating. It is found that the use of a higher pressure can efficiently promote densification and retard significant grain growth. Dense bulk TaC ceramics (95.18%) with average grain size of 4.09 μm were obtained in 90 seconds under 80 MPa. Such a process should facilitate the fast preparation of refractory ceramics with fine-grained microstructure.     

Grain growth stagnation in the dense nanocrystalline yttria prepared by combustion reaction and quick pressing with an ultra-high heating rate

Combustion reaction plus quick pressing was a developing technique that used the Joule heating effect of combustion reaction to sinter ceramics, and allows very high heating rate, short soaking duration and high pressure for densification of ceramics. By taking advantages of the particular conditions of this method, pure yttria ceramics with a relative density of 98.5% and an average grain size of 50 nm were obtained at 1620 K and 170 MPa. Moreover, the investigation on the grain growth of sintered yttria was carried out by analyzing the microstructure evolutions and responsible mechanisms. The combined effect of the ultra-high heating rate and the high pressure applied on compact at the peak temperature was effective in suppressing particle coarsening and enhancing densification. Besides, under the decreased sintering temperature and soaking duration, the retained nanostructure assisted to inhibit final-stage grain growth while without impeding the further densification of nanocrystalline ceramics.     

Sintering and densification mechanisms of tantalum carbide ceramics

Dense tantalum carbide (TaC) ceramics were prepared using TaC nanopowder via spark plasma sintering (SPS). The effects of the sintering temperature and applied pressure on the densification and grain growth behaviour of TaC ceramics were investigated. The results showed that high temperature and pressure promoted sintering densification, while their increase caused an increase in the grain size of TaC ceramics. A highly dense TaC ceramic (∼97.19%) with a fine grain size of 2.67 μm was obtained by sintering at 1800 °C for 10 min under 80 MPa. The Vickers hardness, Young's modulus and fracture toughness were 15.60 GPa, 512.66 GPa and 3.59 MPa·m^1/2, respectively. The densification kinetics were investigated using a creep deformation model. Diffusion and grain boundary sliding were proven to be the dominant densification mechanisms based on the stress and grain size exponents combined with the microstructural characteristics. The apparent activation energy of the mechanism controlling densification was 252.94 kJ/mol.     

Two-Stage Sintering of Alumina with Submicrometer Grain Size

This work verifies the applicability of two-stage sintering as a means of suppressing the final stage grain growth of submicrometer alumina. The first heating step should be short at a relatively high-temperature (1400°–1450°C) in order to close porosity without significant grain growth. The second step at temperatures around 1150°C facilitates further densification with limited grain growth. Fine-grained alumina with a relative density of 98.8% and a grain size of 0.9 μm was prepared by two-stage sintering. A standard sintering process resulted in ceramics with identical relative density and a grain size of 1.6 μm.     

Highly dense spinel ceramics with completely suppressed grain growth prepared via SPS with NaF as a sintering additive

For the preparation of transparent spinel ceramics it is common practice to use LiF as a sintering additive to achieve transparency. However, it is well known that in this case the grain size exhibits a significant increase compared to pure spinel ceramics, which can lead to a deterioration in mechanical properties. The findings of this paper indicate that when NaF is used as a sintering additive for the preparation of spinel ceramics via spark plasma sintering (SPS) translucent materials with a high level of densification can be obtained without observable grain growth. It is shown that the grain size after SPS at 1500 °C with 1 h dwell is essentially the same as the primary particle size of the spinel powder, whereas pure spinel ceramics prepared via SPS under the same conditions exhibit grain growth by approximately a factor 5.     

Fast pressureless solid-state reaction sintering of highly infrared transparent MgAlON ceramics from MgAl2O4 and AlON powders by two-step heating

A two-step heating strategy was proposed to fabricate transparent MgAlON ceramics by solid-state reaction of MgAl₂O₄ and AlON powders via pressureless sintering. By dwelling 60 min at 1700 ℃ followed by 150 min at 1880 ℃, highly infrared transparent MgAlON ceramics with transmittance up to 80.4 % were successfully fast prepared. The phase transformation and microstructure evolution during heating from 1400 ℃ to 1800 ℃ and dwelling at 1700 ℃ for 0–90 min was thoroughly studied to reveal the solid-state reaction and densification mechanism of MgAlON by two-step heating. Surprisingly, it was found that the grown grains could break during dwelling at 1700 ℃. This secondary massive fragmentation of grown grains resulted in the minimized grain size and improved moveability of grains, which in turn prompted fast and high densification with pore free in the following sintering step. The grain breakage at 1700 ℃ could be attributed to the decomposition of AlON and formation of MgAlON.     

Two-step pulsed electric current sintering of transparent Al2O3 ceramics

Abstract

Fully densified Al₂O₃ ceramics with fine grain size were obtained by pulsed electric current sintering through a two-step heating profile (referred to as TS-PECS). Highly transparent Al₂O₃ polycrystals with fine grain size (400 nm) were successfully fabricated by the TS-PECS process, namely, sintering at 1000°C for 1 h and followed at 1200°C for 20 min under uniaxial pressure of 100 MPa. Effects of the first step temperature and heating rate were discussed for bulk density, grain size and transparency. The temperature in the first step strongly affects densification and grain growth of Al₂O₃. On the other hand, heating rate, even of 100 K min^?1, in TS-PECS does not give significant influences on densification and grain growth of Al₂O₃. Inline transmittance at 640 nm in wavelength normalised to 1 mm in thickness is increased by decreasing heating rate even in TS-PECS.     

Effect of heating rate on properties of transparent aluminum oxynitride sintered by spark plasma sintering

High heating rates ranging from 50 to 250°C/min are selected to rapidly sinter transparent aluminum oxynitride (AlON) ceramics by spark plasma sintering (SPS) at 1600°C under 60 MPa using a bimodal AlON powder synthesized by the carbothermal reduction and nitridation method. With 1 minute holding time before cooling, all the specimens show high density and high transparence. The maximum transmittance is up to 74.5%-80.6%, where the maximum transmittance is positively correlated with the heating rate. Further analysis reveals that faster heating rates enable the decrease in the amount of the AlON phase decomposed into the α-Al₂O₃ and AlN phases during heating. These α-Al₂O₃ and AlN phases have to be converted back to AlON at the final stage of sintering, which indicates that a decrease in the amount of the α-Al₂O₃ and AlN phases via the boosted heating leads to the higher transmittance of the AlON ceramics. The high heating rates and short holding duration of the SPS utilized in this study result in the fine grain size of the obtained ceramics (1-6 μm) compared to that of the AlON ceramics fabricated by the conventional sintering method. This effect of high heating rates is confirmed by the coupled densification-grain growth modeling. In turn, the obtained AlON specimens exhibit a Vickers hardness of 15.87-16.62 GPa.     

Fabrication of YAG transparent ceramics by two-step sintering

Yttrium aluminum garnet (YAG) nanopowders with mean particle size of about 50 nm synthesized by a modified co-precipitation method were used to sinter bulk YAG ceramic by two-step sintering method. Full densification was achieved by heating the sample up to 1800 °C followed by holding at 1550 °C for 10 h. Transparent YAG ceramics were obtained by suppressing grain-boundary migration while promoting grain-boundary diffusion during the two-step sintering process. The microstructure of the YAG ceramic is homogeneous without abnormal grain growth and the transmittance of the sintered sample is 43%.     

Effects of local Joule heating during the field assisted sintering of ionic ceramics

Recent investigations regarding the role of applied fields on the grain growth and densification behavior of ionic ceramics are providing strong insights into the efficacy of Field Assisted Sintering Technique (FAST), aka Spark Plasma Sintering (SPS). Explanations of the observed behaviors, such as grain growth suppression and densification enhancement, are based upon the conjectured presence of a Joule heating driven temperature differential between grain interfaces and grain cores. These differentials were thought to be responsible for providing increased densification rates and lower densification temperatures through grain growth suppression and/or increased local kinetics at the forming necks. In this paper, we analyze the energetic, thermal, and practical details of this process in the context of the commonly accepted stages of sintering.     

Ultra-high heating rate densification of nanocrystalline magnesia at high pressure and investigation on densification mechanisms

The pressure-assisted densification method based on combustion reaction heating was applied to prepare dense nanocrystalline ceramics. The densification process of magnesia compact with a particle size of 50 nm was investigated, under the pressure range of 0–170 MPa, and the temperature range of 1620–1880 K with ultra-high heating rate (above 1600 K/min). The pressure was found to have an effect on enhancing densification while suppressing grain growth, and the higher sintering temperature lead to the larger grain size and lower density of the compact. Pure magnesia nanocrystalline ceramics with a relative density of 99.1% was obtained at 1620 K and 170 MPa, and the concurrent grain growth was almost completely restrained. Furthermore, the investigation on the pressure-dependent densification mechanisms including plastic flow, diffusion and power-law creep was also carried out. The result indicated the rate-controlling mechanism was the plastic flow accommodated by grain-boundary diffusion creep.     

Highly oriented α-Al2O3 transparent ceramics shaped by shear force

Alumina platelets were arranged horizontally in submicron alumina particles by shear force in the flow of slurries during casting. The obtained alumina green bodies with platelets were pressureless-sintered in vacuum, producing ceramics with thoroughly oriented grains and high transmittance. The effects of sintering parameters on the densification, microstructure evolution, and orientation degree of alumina ceramics were investigated and discussed. The results showed that the densification, grain size, orientation degree, and in-line transmittance were increased with increasing sintering temperature. The enhancement of orientation degree was mainly coherent with grain growth. The grain-oriented samples exhibited a much higher in-line transmittance (at 600 nm) of 61 % than that of the grain random sample (29 %). Moreover, the transmission remained a high level in the ultraviolet range (<300 nm).     

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 behavior and mechanical properties of nano-alumina ceramics prepared by Spark Plasma Sintering with pressure applied at different sintering stages

High densification and fine grain size are the key to achieve excellent mechanical properties of ceramic materials. Pressure-assisted sintering is an effective approach to achieve this goal. However, the pressure at different sintering stages has different effects on the densification behavior of nano-ceramics. In this work, it is found that adjusting the pressure applying regime during Spark Plasma Sintering of nano-alumina ceramics can effectively increase the densification rate and balance the relationship between the densification behaviors of particle coarsening, grain growth and vapor migration. When the pressure is applied at the beginning of the second sintering stage, the high densification and fine grain size microstructures can be both obtained at lower temperatures, leading to the best mechanical properties. This result is of great significance for the preparation of nano-ceramics with excellent mechanical properties.     

Influence of processing atmosphere on the microstructural evolution of submicron alumina powder during sintering

Investigations into the sintering of submicron oxide powders have revealed interesting behavior, particularly insofar as it concerns their microstructural evolution in the early, low temperature transformations during heating. In this work, experiments were conducted on a submicron alumina powder, whose microstructural evolution and densification were characterized after sintering from 900 °C to 1400 °C in air, dry air and high vacuum (10⁻⁸ atm). The results indicated that the processing atmosphere strongly influences the particle size distribution at low temperatures before shrinkage occurs. Shrinkage began concomitantly with grain growth and the sintering atmosphere influenced the sintering kinetics. This factor, which is associated with previous narrowing of the particle size distribution, may affect grain growth and densification during the final stage of sintering.     

Sintering highly dense ultra-high temperature ceramics with suppressed grain growth

Grain coarsening normally occurs at the final stage of sintering, resulting in trapped pores within grains, which deteriorates the density and the performance of ceramics, especially for ultra-high temperature ceramics (UHTCs). Here, we propose to sinter this class of ceramics in a specific temperature range and coupled with a relatively high pressure. The retarded grain boundary migration and pressure-enhanced diffusion ensure the proceeding of densification even at final stage. A highly dense TaC ceramic (98.6 %) with the average grain size of 1.48 μm was prepared under 250 MPa via high pressure spark plasma sintering using a C_f/C die at 1850 °C. It was suggested that the final-stage densification is mainly attributed to grain boundary plastic deformation-involved mechanisms. Compared to the usual sintering route using a high temperature (>2000 °C) and normal pressure (<100 MPa), this work provides a useful strategy to acquire highly dense and fine-grained UHTCs.     

Rapid Rate Sintering of Nanocrystalline ZrO2−3 mol% Y2O3

Conventional ramp-and-hold sintering with a wide range of heating rates was conducted on submicrometer and nanocrystalline ZrO₂–3 mol% Y₂O₃ powder compacts. Although rapid heating rates have been reported to produce high density/fine grain size products for many submicrometer and smaller starting powders, the application of this technique to ZrO₂–3 mol% Y₂O₃ produced mixed results. In the case of submicrometer ZrO₂–3 mol% Y₂O₃, neither densification nor grain growth was affected by the heating rate used. In the case of nanocrystalline ZrO₂–3 mol% Y₂O₃, fast heating rates severely retarded densiflcation and had a minimal effect on grain growth. The large adverse effect of fast heating rates on the densification of the nanocrystalline powder was traced to a thermal gradient/differential densification effect. Microstructural evidence suggests that the rate of densification greatly exceeded the rate of heat transfer in this material; consequently, the sample interior was not able to densify before being geometrically constrained by a fully dense shell which formed at the sample exterior. This finding implies that rapid rate sintering will meet severe practical constraints in the manufacture of bulk nanocrystalline ZrO₂–3 mol% Y₂O₃ specimens.     

Sintering and densification of nanocrystalline ceramic oxide powders: a review

Abstract

Observation of the unconventional properties and material behaviour expected in the nanometre grain size range necessitates the fabrication of fully dense bulk nanostructured ceramics. This is achieved by the application of ceramic nanoparticles and suitable densification conditions, both for the green and sintered compacts. Various sintering and densification strategies were adopted, including pressureless sintering, hot pressing, hot isostatic pressing, microwave sintering, sinter forging, and spark plasma sintering. The theoretical aspects and characteristics of these processing techniques, in conjunction with densification mechanisms in the nanocrystalline oxides, were discussed. Spherical nanoparticles with narrow size distribution are crucial to obtain homogeneous density and low pore-to-particle-size ratio in the green compacts, and to preserve the nanograin size at full densification. High applied pressure is beneficial via the densification mechanisms of nanoparticle rearrangement and sliding, plastic deformation, and pore shrinkage. Low temperature mass transport by surface diffusion during the spark plasma sintering of nanoparticles can lead to rapid densification kinetics with negligible grain growth.     

Single CaO accelerated densification and microstructure control of highly transparent YAG ceramic

In this work, a small amount of CaO single dopant was adopted to realize the densification and microstructure control of fine grained YAG ceramic with excellent optical quality, by a simple solid‐state reaction and one‐step vacuum sintering method. Then, highly transparent YAG ceramics (T = 84.4% at 1064 nm) were obtained just after vacuum sintering at 1820°C for 8 hours. The average grain size was only 2.7 μm, when the total amount of CaO was as low as 0.045 wt%. The effect of CaO on the microstructural evolution and optical property of the as‐fabricated YAG ceramics was systematically investigated in detail. It was found that CaO dopant promoted both densification and grain growth of YAG ceramics when the sintering temperature was lower than 1660°C, however, it dramatically inhibited grain growth when the sintering temperature was further increased.     

Processing of dense high-entropy boride ceramics

Dense (Hf_0.2,Zr_0.2,Ti_0.2,Ta_0.2,Nb_0.2)B₂ high-entropy ceramics with high phase purity were produced by two-step spark plasma sintering of precursor powders synthesized by boro/carbothermal reduction of oxides. The reacted powders had low oxygen (0.404 wt%) and carbon (0.034 wt%) contents and a sub-micron average particle size (∼0.3 μm). Powders were synthesized by optimizing the excess B₄C content of the reaction mixture and densified by a two-step spark plasma sintering process. The relative density increased from 98.9% to 99.9% as the final sintering temperature increased from 2000 °C to 2200 °C. The resulting ceramics were nominally single-phase (Hf,Zr,Ti,Ta,Nb)B₂ with oxygen contents as low as 0.004 wt% and carbon as low as 0.018 wt%. The average grain size increased from 2.3 ± 1.2 μm after densification at 2000 °C to 4.7 ± 1.8 μm after densification at 2100 °C, while significant grain growth occurred during sintering at 2200 °C. The high relative densities, low oxygen and carbon contents, and fine grain sizes achieved in the present study were attributed to the use of synthesized precursor powders with high purity and fine particle size, and the two-step synthesis-densification process. These are the first reported results for dense high-entropy boride ceramics with high purity and fine grain size.