No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Ultra-high heating rate densification of nanocrystalline magnesia at high pressure and investigation on densification mechanisms
Abstract
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.
... The result is the separation between densification and grain growth processes. Specifically, Joule heating, generates high heating rates that makes all processes to occur in regime of fast kinetics with too short times for the relaxation of the grain boundaries toward an equilibrium structure [2], a fact that inhibits the particle coarsening at early stages of the process [3]. A further grain growth, which might occur at the final stages, is limited by the short dwelling time [4]. ...
... Xu et al. prepared high density nano-sized Al 2 O 3 with a grain size of 290 nm at 1050 • C by spark plasma sintering method under 200 MPa with limited grain growth [34]. Liu reported that nearly fully dense nanocrystalline MgO could be rapidly fabricated at 1350 • C and 170 MPa by combustion reaction and quick pressing method (CR-QP) [35]. Ji and coworkers successfully prepared nanocrystalline 3YSZ ceramics at 900 • C under 1.5 GPa using a piston-cylinder apparatus [36]. ...
Tungsten heavy alloy (WHA) is an ideal material employed for kinetic energy penetrators due to its high density and excellent mechanical properties. However, it is difficult to obtain ultrafine-grained tungsten alloy with excellent properties by traditional powder metallurgy method because of severe grain growth at a high sintering temperature with a long soaking time. In this study, the sintering behavior of tungsten alloys was studied at 800 to 1300 °C, and highly dense 93W-5.6Ni-1.4Fe (wt.%) WHA was successfully fabricated at a low temperature of 950 °C with a high pressure of 150 MPa by spark plasma sintering. The as-sintered tungsten alloy possesses a high relative density (98.6%), ultrafine grain size (271 nm) and high dislocation density (2.6 × 1016 m−2), which results in excellent properties such as a high hardness (1079 HV1). The high sintering pressure is considered to support an additional driving force for the sintering and lead to a low-temperature densification, which effectively limits grain growth.
... This manufacturing method allows higher heating rates (up to 1000 • C/min) and shorter holding times to be reached compared with conventional sintering techniques, which is undeniably one of the greatest advantages of the FAST/SPS method . It was found that high heating rates can suppress the grain growth in the sample (Liu et al. 2014). For this reason, the SPS technique can be successfully used to produce nanostructured materials (Wei et al. 2014;Nygren 2007). ...
This paper presents the technology of powder sintering by the spark plasma sintering method, also known as the field assisted sintering technique. The mechanisms, compared to other sintering techniques, advantages of this system, applied modifications and the history of the development of this technique are presented. Spark Plasma Sintering (SPS) uses uniaxial pressing and pulses of electric current. The direct flow of current through the sintered material allows high heating rates to be reached. This has a positive effect on material compaction and prevents the grain growth of sintered compact. The SPS mechanism is based on high-energy spark discharges. A low-voltage current pulse increases the kinetics of diffusion processes. The SPS temperature is up to 500 °C lower than the sintering temperature using conventional methods. The phenomena that occur during sintering with the Field Assisted Sintering Technology (FAST)/SPS method give great results for consolidating all types of materials, including those which are nonconductive. This method is used, among others, in relation to metals, alloys and ceramics, including advanced and ultra-high-temperature ceramics. Due to the good results and universality of this method, in recent years it has been developed and often used in research institutions, but also in industry.
... The well-matched crystal morphologies resulted in perfect lattice coherence and promoted the densification process during sintering. Meanwhile, neither dislocations nor subgrains were observed in the lamellar structure; thus, the deformation of the grain boundaries played a key role in adjusting the internal stress and preventing intragranular deformation of the grains [17]. ...
SiC ceramics were sintered with Mg2Si and Ni3Al additives at 900–1300 °C. Densification was achieved at temperatures as low as 1300 °C, and the microhardness and fracture toughness were significantly improved from 4.95 GPa to 8.17 MPa m1/2 for the ceramic without Ni3Al to 13.59 GPa and 10.40 MPa m1/2 for the ceramic sintered with 10 wt% Ni3Al, representing improvements of 174.55% and 27.29%, respectively. Some of the Ni3Al was converted into Ni with a (110)Ni3Al || (002)Ni crystal morphology, and the SiC bonded to Ni3Al with a (103)SiC || (112)Ni3Al crystal morphology. These well-matched crystal morphologies led to perfect lattice coherence. The Ni-rich regions prevented the elongation of cracks, resulting in improved fracture toughness.
... Miranzo et al. designed a special experiment to demonstrate the intrinsic effects of an electric field on the densification behaviors of non-conductive materials [170]. Two types of Si 3 N 4 , including monolithic ceramic and J o u r n a l P r e -p r o o f 40 The abridged general view of EWOD is depicted in Fig. 22. The latest theory points out that the decrease in the contact angle (θ) depends on the square of the applied voltage (V 2 ): cos = 0 + 0 2 2 = 0 + …………………………………………….. (6) where the subscript "0" suggests a voltage value of zero, ε 0 is the vacuum permittivity, ε d is the dielectric constant of the liquid, d EDL is the thickness of the electric double layer, σ lv is the liquid/vapor surface energy per unit area, and η is the dimensionless electrowetting number. ...
Spark plasma sintering (SPS), also known as pulsed electric current sintering (PECS) or field-assisted sintering technique (FAST), belongs to a class of powder metallurgy techniques. In SPS, the sample is simultaneously subjected to a uniaxial pressure and electrical current in a vacuum or protective atmosphere. Although the fundamental principles of this procedure were first proposed over 50 years ago, SPS acquired major importance only within the last 20 years. Scholars come to realize that SPS technique enables control of the powder surface condition, atomic diffusion behavior, and phase stability and crystal growth behavior, as well as accelerating densification of hard-to-sinter materials. This review summarizes the latest research findings with respect to experimental procedures, densification behaviors, microstructural characteristics, and mechanical properties of various traditional and novel materials synthesized using SPS, mainly highlighting the heating mechanisms in SPS and the effects induced by multi-physical fields on materials. In addition, influences of operating parameters containing current, voltage, and uniaxial pressure on product characteristics are reviewed for a wide range of materials including hard-to-sinter materials, carbon-containing materials, nanocrystalline materials, non-equilibrium materials, gradient materials, interconnect materials, complex shape materials, and advanced functional materials.
... Recently, large amount of ceramic materials and ceramic matrices composites (CMC) are developed [1][2][3][4][5][6][7][8][9][10][11] and used in the industries because of their low density [12][13][14][15], hardness [17][18][19][20], wear resistance [21][22][23][24][25][26][27][28][29], toughness [30][31][32][33][34], mechanical strength [35][36][37][38][39][40][41][42][43][44], chemical [45][46][47][48][49], biological [50][51][52] and physical [53][54][55][56][57][58][59][60] properties. Many researches have been conducted to find low-cost solutions for the manufacture of porous ceramic composites, such as the combination of kaolin and alumina raw materials, to increase the toughness [61][62][63]. ...
Based on conventional kaolinite and IG-017 bio-original additives of IGREX Ltd. the authors have developed new ceramic composite materials for different industrial purposes. In this work, different powder mixtures of kaolinite and IG-017 bio-original additive were milled and uniaxially pressed at different compaction pressures into cylindrical discs, after compaction, the discs were sintered in electric kiln under oxidation atmosphere and in oxygen-free atmosphere. Using the oxygen-free sintering process, new high porosity ceramic materials were created. Through the examination of the microstructure of the produced specimens the authors have found that the ceramic structure is reinforced with carbon nanofibers.
... Thanking to the several years research and investigation in development and processing of high-tech ceramics and ceramic matrix composites the following scientists have achieved considerable new results: -S. Hampshire's group in study and development of Si3N4 and SiAlON [68][69][70], -S N Kulkov's and S P Buyakova's group in ZrO2, ZrWO8 and zirconia based CMCs [71][72][73][74][75][76][77], -E V Zharikov's group in development of carbon nanotubes (CNT) and their application for reinforce ceramic nanocomposites [78][79][80], -K Niihara's group in high-pressure synthesis and in development and application of boron nitride (BN) nanocomposites and nanotechnology [81][82][83][84]. The papers of S G Chuklina, A I Pylinina and I I Mikhalenko [85,86] also well represent the wide range application opportunities of ceramic materials and ceramic based composites especially the tetragonal ZrO2 ceramics. ...
This book has six chapters, they are focusing on applied material science based on my research works in technical ceramics and high tech materials, incuding the investigations of zircon-oxide, carbon, silicon-carbid, silicon nitride and heteromodulus ceramics, composites and their properties, application and processing.
Citation of articles in this volume should be cited as follows:
L.A. Gömze (2019) APPLIED MATERIALS SCIENCE III., Compilation of Selected Scientific Papers, pp. 1-227. Published by IGREX Ltd., Igrici (Hungary) or
(pp. …) (year) (DOI:…….)
... In addition, other fast-densification techniques were also adopted to fabricate MgO ceramics [25]. Liu et al. reported that pure MgO ceramics with a relative density of 99.1% and an average grain size of only 50 nm were obtained at 1620 K with the utilization of an ultra-high heating rate (above 1700 K/min) and high pressure (up to 170 MPa) by the combustion reaction plus quick pressing method [26]. Generally, the published SPS pressure for the fabrication of transparent MgO ceramics is higher than 150 MPa, which cannot be accessed easily in a typical experimental or production setup, especially for the large-size sample. ...
Transparent MgO ceramics were fabricated by spark plasma sintering (SPS) of the commercial MgO powder using LiF as the sintering additive. Effects of the additive amount and the SPS conditions (i.e., sintering temperature and heating rate) on the optical transparency and microstructure of the obtained MgO ceramics were investigated. The results showed that LiF facilitated rapid densification and grain growth. Thus, the MgO ceramics could be easily densified at a moderate temperature and under a low pressure. In addition, the transparency and microstructure of the MgO ceramics were found to be strongly dependent on the temperature and heating rate. For the MgO ceramics sintered at 900 °C for 5 min with the heating rate of 100 °C/min and the pressure of 30 MPa from the powders with 1 wt% LiF, the average in-line transmittance reached 85% in the range of 3 – 5 μm, and the average grain size is ∼0.7 μm.
... Thanking to the several years research and investigation in development and processing of high-tech ceramics and ceramic matrix composites the following scientists have achieved considerable new results: -S. Hampshire's group in study and development of Si 3 N 4 and SiAlON [68][69][70], -S N Kulkov's and S P Buyakova's group in ZrO 2 , ZrWO 8 and zirconia based CMCs [71][72][73][74][75][76][77], -E V Zharikov's group in development of carbon nanotubes (CNT) and their application for reinforce ceramic nanocomposites [78][79][80], -K Niihara's group in high-pressure synthesis and in development and application of boron nitride (BN) nanocomposites and nanotechnology [81][82][83][84]. The papers of S G Chuklina, A I Pylinina and I I Mikhalenko [85,86] also well represent the wide range application opportunities of ceramic materials and ceramic based composites especially the tetragonal ZrO 2 ceramics. ...
Materials with different crystalline and morphological compositions have different chemical, physical, mechanical and rheological properties, including wear protection, melting temperature, module of elasticity and viscosity. Examining the material structures and behaviors of differentceramic bodies and CMCs under high speed collisions in several years the authors have understood the advantages of hetero-modulus and hetero-viscous complex material systems to absorb and dissipate the kinetic energy of objects during high speed collisions. Applying the rheo-mechanical principles the authors successfully developed a new family of hetero-modulus and hetero-viscous alumina matrix composite materials with extreme mechanical properties including dynamic strength. These new corundum-matrix composite materials reinforced with Si2ON 2, Si3N4, SiAlON and AlN submicron and nanoparticles have excellent dynamic strength during collisions with high density metallic bodies with speeds about 1000 m/sec or more. At the same time in the alumina matrix composites can be observed a phase transformation of submicron and nanoparticles of alpha and beta silicone-nitride crystals into cubicc-Si3N4diamond-like particles can be observed, when the high speed collision processes are taken place in vacuum or oxygen-free atmosphere.
Using the rheological principles and the energy engorgement by fractures, heating and melting of components the authors successfully developed several new hetero-modulus, hetero-viscous and hetero-plastic complex materials. These materials generally are based on ceramic matrixes and components having different melting temperatures and modules of elasticity from low values like carbon and light metals (Mg, Al, Ti, Si) up to very high values like boride, nitride and carbide ceramics. Analytical methods applied in this research were scanning electron microscopy, X-ray diffractions and energy dispersive spectrometry. Digital image analysis was applied to microscopy results to enhance the results of transformations.
... These changes could be due to the following factors. First, a significant amount of grain growth accompanied by pore entrapment is believed to happen at higher sintering temperatures (Liu et al., 2014). On the other hand, grain boundary diffusion as discussed above is believed to be relatively small at lower sintering temperatures. ...
The aim of this work is to study the behavior of cobalt ferrite nanoparticles during sintering. The size of prepared nanoparticles was in the range between 5 and 25 nm. The density of nanoparticles was measured using a Pycnometer and found to be 4.93 g cc-1. The nanoparticles were uniaxially pressed into disk samples, and sintered under a single, continuous, ramp rate. The maximum relative density of 96% was achieved when the sintering temperature was 1350 °C. This was considered 4% improvement in comparison with the density reported for cobalt ferrite prepared employing micro-particles. Pycnometer analysis on sintered disk samples determined the ratio of open and closed pores inside the sintered compacts.
... Sintering is a thermal process during which a powder compact or porous material is converted to a solid with mechanical stability and rigidity due to a decrease in free energy [1]. Various mass diffusion mechanisms play an important role in this process [2][3][4][5][6]. Simultaneously, heat diffusion is also involved and essential. ...
The modeling and a confirming experimental study of a sintering process by using intense thermal radiation (SITR) is demonstrated. Experiments were conducted in a modified spark plasma sintering setup for the purpose of rapid consolidation of porous SiC ceramics. A finite element method based software package, COMSOL Multiphysics, was employed to simulate the temperature distributions with two different die geometries. Experimental verifications were performed by sintering of SiC foams and measuring the temperature differences between two fixed points on the die and samples at 1000–1800 • C. SiC foams sintered under two geometries with 10 min' dwell resulted in the formation of grain necks. The compressive strengths are 19.2 ± 0.7 MPa (65.0 ± 0.1 vol% porosity) and 15.3 ± 1.9 MPa (69.0 ± 0.3 vol% porosity), respectively. The simulation and experimental results showed that the temperature distributions are strongly related to these geometries.
The poor mechanical and tribological properties limit the higher requirements of aluminum alloys in engineering and industrial applications, which leads to the rapid development of aluminum matrix composites (AMCs). Particulate reinforced AMCs have attracted extensive attention in automobile, electronics and military industries due to their low density, high strength, and excellent wear resistance. However, the interfacial reaction between reinforcements and the Al matrix tends to occur in conventional preparation processes owing to the higher reaction temperatures. The spark plasma sintering (SPS) technique is considered to be an efficient method for the fabrication of metal matrix composites, which can achieve rapid sintering, lower sintering temperatures, and higher densities than conventional fabrication processes. In addition, SPS can produce AMCs with excellent non-porous microstructure, fine grain size, and a strong bonding interface between reinforcement and Al matrix. Therefore, the interfacial reaction is effectively controlled and the structural integrity is maintained, resulting in enhanced strength and ductility. Based on the advantages of particulate reinforced AMCs and the SPS technique, the particulate reinforced AMCs fabricated by SPS have been extensively studied in recent decades, but have not been systematically evaluated. Therefore, this paper reviews the state-of-the-art particulate reinforced AMCs fabricated by SPS, focusing on the microstructure characterization, strengthening mechanisms, and mechanical and physical properties. Furthermore, the future research priorities and challenges of the high-performance particulate reinforced AMCs fabricated by SPS are also prospected.
Short-range order is a new strengthening effect that can significantly affect the mechanical properties of high-entropy materials. Furthermore, simulation results show that this strengthening effect at a quasi-atomic scale can suppress the grain size softening, leading to the disappearance of inverse Hall-Petch behavior in nanocrystalline high-entropy materials. In this work, the evident inverse Hall-Petch behavior is confirmed in the translucent nanocrystalline high-entropy ceramic (HEC) with an average grain size below 10 nm, fabricated by a high-pressure low-temperature sintering technique. Besides, the as-obtained nanocrystalline HEC also shows an improving fracture toughness compared with the corresponding coarse-crystalline HEC.
Ultra-high-strength M54 steel has been considered a promising candidate for structural applications in spacecraft and aircraft because of its excellent match of strength and toughness. Spark plasma sintering (SPS) technique characterized by simultaneous uniaxial pressure and pulsed current allows achieving fully dense bulk materials at low temperature with short heating, soaking, and cooling times. SPS has been widely used to produce metal materials due to its efficiency and advantages caused by its distinctive heating mode. The sintering behaviors, structural transformations, and mechanical properties of the ultra-high-strength M54 steel prepared by spark plasma sintering (SPS) were studied in this work. The actual density of the SPSed specimen was measured according to Archimedes’ principle, and X-ray diffraction (XRD) analysis was used to identify the phases of the raw powders and SPSed specimens. The microstructure of the SPSed specimens was characterized by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) coupled with an energy-dispersive spectrometer (EDS). After SPS consolidation, metallurgical bonding between powders was completed, and the ultra-high-strength M54 steel adjacent to nearly full densification was obtained directly. The quasistatic tensile tests indicated that the SPSed M54 steel exhibited better ultimate tensile strength (UTS) of 1933.88 MPa compared to the wrought M54 steel. According to TEM analysis, the primary strengthening mechanism was its lath martensite structure with high-density dislocations. The elongation was not significant due to the presence of micro-voids and inclusions.
This review is devoted to describing the main synthesis methods, the properties of MgO nanoparticles, and their areas of application, such as the creation of coatings for transformer and other types of steels, coatings on glass to control the energy of transmitted solar radiation, the creation of transparent and translucent one-component ceramics, the creation of multicomponent ceramic compositions, in medicine to make antibacterial agents, etc. The main methods of obtaining MgO nanoceramics are considered: sintering at low temperatures under high pressure, high-temperature sintering, the spark plasma sintering (SPS) method, and obtaining MgO melt in plasma in a cooled crucible followed by crystallization and the formation of dense ceramics. It is shown that variations in the conditions for the synthesis of nanosized MgO powders make it possible to obtain precursors that, upon sintering, form ceramics with a density close to the theoretical one.
Spark plasma sintering (SPS) is a type of pulsed electric current-assisted sintering technique. This method allows for rapid consolidation of powder materials into dense bulk when simultaneously applying uniaxial pressure and pulsed electrical current in a vacuum or protective atmosphere. Many scholars and researchers have realized the importance of the SPS due to its significant advantages in controlling the powder surface condition, atomic diffusion, phase stability, and crystal growth behavior. All these features inevitably influence the densification behavior and resulting physical and mechanical properties of the sintered materials. This review represents an extensive introduction of recent developments and fundamental principles in SPS techniques after a general description of the method and its outstanding advantages. A possible design for the SPS technique is proposed as well. Subsequently, the effects of each operating parameter, including current, voltage, and uniaxial pressure, on the densification behavior of advanced metals and alloys under various sintering conditions are reviewed. Finally, the successful applications of the SPS in preparing novel metal structural materials, such as high-quality special steels, new lightweight alloy materials with high performance, high entropy alloys, and other metal alloys, are described in detail.
The aim of this work is to study the behavior of cobalt ferrite nanoparticles during sintering. The size of prepared nanoparticles was in the range between 5 and 25 nm. The density of nanoparticles was measured using a Pycnometer and found to be 4.93 g cc⁻¹. The nanoparticles were uniaxially pressed into disk samples, and sintered under a single, continuous, ramp rate. The maximum relative density of 96% was achieved when the sintering temperature was 1350°C. This was considered 4% improvement in comparison with the density reported for cobalt ferrite prepared employing micro-particles. Pycnometer analysis on sintered disk samples determined the ratio of open and closed pores inside the sintered compacts.
MgO-based refractories have been regarded as ideal linings for various furnaces owing to high refractoriness and excellent corrosion resistance to basic slag. However, thermal shock damage resulting from poor thermal shock resistance (TSR) of magnesia is the common failing. The present work aimed at improving TSR of magnesia aggregates by introducing microscale monoclinic ZrO2. The results showed that the ZrO2 added could increase cation vacancy concentration and inhibit abnormal growth of MgO grains, which therefore enhanced densification of the specimens. However, when the ZrO2 amount exceeded 15wt%, densification decreased slightly due to agglomeration of ZrO2 and formation of more microcracks at the MgO grain boundaries. With increasing ZrO2 content, although the flexural strength degraded, the fracture toughness increased constantly because of the toughening effects of crack deflection and crack branching. Besides, although the addition of ZrO2 decreased the thermal conductivity, TSR of the specimens was significantly improved with increasing ZrO2 content due to the increase in toughness and decrease in strength, thermal expansion coefficient as well as Young’s modulus. After thermal shock tests, 15wt% ZrO2 containing specimen exhibited the highest TSR, whose residual strength ratio was about triple of that of the pure magnesia specimen. However, further increasing ZrO2 content to 20wt% reduced the TSR attributing to the spalling of intergranular ZrO2 agglomerations.
Lightweight microporous magnesia aggregates with closed porosity of 6.2%, apparent porosity of 2.1% and median pore size of 2.36 μm were fabricated by introducing nano-sized ZrO2 combined with magnesite decomposition and sintered at 1780 °C. The sintering properties and microstructure revealed that nano-sized ZrO2 promoted the sintering and formation of closed intragranular pores by increasing abundant crystal defects and producing cation vacancies (VMg'') raising the migration rate of grain boundaries, which induced a change in porosity type from large open pores to small closed pores. The nano-sized ZrO2 changed the bonding of magnesia grains from single CaO–SiO2–MgO phases bonding to composited CaO–SiO2–MgO phases-CaZrO3 phases bonding, significantly enhancing the slag penetration resistance. The closed porosity played a more important role in reducing the thermal conductivity compared with total porosities. The microporous magnesia aggregates and thermal conductivity of 8.994 W (m K)⁻¹ at 800 °C exhibit potential for application in wear lining refractories.
The conventional fabrication of bulk thermoelectric materials usually involves time and energy consuming steps including melting, annealing and sintering et al, which restricts the large-scale application. Herein, high thermoelectric performance ZrNiSn bulks were synthesized by one step self-propagating high-temperature synthesis combining with in-situ quick pressing (SHS-QP). The correlation among SHS process, QP process and the critical pressure, is systematically investigated. The reduced grain size due to non-equilibrium feature of SHS-QP leads to a significant reduction in the thermal conductivity and a high figure of merit ZT ~ 0.7.
Cu 2 Se is a promising material for high temperature thermoelectric energy conversion due to its unique combination of excellent electronic properties and low thermal conductivity owing to its ionic liquid characteristics at high temperature. In this paper, fully dense single-phase bulk Cu 2 Se material was prepared by the combination of self-propagating high-temperature synthesis (SHS) with in situ quick pressing (QP) for the first time. This new approach shortens the duration of the synthesis from days to hours compared to conventional preparation methods. SHS-QP technique is an ultra-fast preparation method, which utilizes the heat released by the SHS reaction and an external applied pressure to achieve the synthesis and densification of materials in one-step. The ultra-fast process of the SHS-QP technique enables the fabrication of single-phase Cu 2 Se bulk materials with relative density of over 98% and with precise control over the stoichiometry owing to the ability to suppress the Se vapor during the reaction. The SHS-QP prepared Cu 2 Se samples exhibit excellent thermoelectric figure of merit, ZT ∼ 1.5 at 900 K, which is comparable to those of Cu 2 Se materials prepared by conventional methods. This study opens a new avenue for the ultra-fast and low-cost fabrication of Cu 2 Se thermoelectric materials.
In this research the compacting and rheological properties of alumina and quartz powder mixtures were examined depending on forming pressures and times. On the basis of the achieved relative compaction curves as function of forming pressure and time the authors could successfully describe the changes of rheological behavior and rheological model of these powder mixtures. With increasing the compression pressure of the alumina and quartz powder mixtures considerable changes of rheological properties could be observed.
In this work the authors have shown how the compacting deformations of cylindrical specimens are depending on compaction pressures and times. On the basis of their research the authors could successfully illustrate also how the rheological behaviors and models of alumina-quartz ceramic powder mixtures are changing in the compacting die cavities at low values of forming pressures.
Ultrafine crystalline functionally graded cemented carbides (FGCCs) with a surface zone enriched in binder phase were prepared by a one-step Sinter-HIP method. The influence of sintering pressure and cubic carbide composition on the formation of gradient layer was examined. The results show that the ultrafine FGCC with surface zone enriched in binder phase can be formed by the one-step Sinter-HIP method. The process of the gradient layer formation is accelerated under higher sintering pressure; the gradient layer thickness increases with the sintering pressure increasing. The gradient layer thickness is controlled by diffusion distance of cubic carbide formers, such as Ti, Ta and Nb. The addition of (Ta,Nb)C leads to decrease the thickness of gradient layer.
Effect of different additives, namely Cr2O3, Fe2O3 and TiO2, up to 2 wt% was studied on the sintering and microstructural developments of the chemically pure magnesia using the pressureless sintering technique between 1500 and 1600°C. Sintering was evaluated by per cent densification and microstructural developments were studied by electron microscopy and elemental distribution of the additives in the sintered products was also investigated for their distribution in the matrix. Cr2O3 and TiO2 were found to deteriorate the densification associated with grain growth. Fe2O3 was found to improve the densification and well-compacted grain distribution was observed in the microstructure.
High-temperature creep experiments were performed on a Cu-2.8 ат.% Co solid solution. Cylindrical foils of 18 micrometers thickness were used for this purpose. Creep tests were performed in a hydrogen atmosphere in the temperature range of about from 1233 K to 1343 K and at stresses lower than 0.25 MPa. For comparison, a foil of pure copper and Cu-20 at.% Ni solid solution were investigated on high temperature creep. Measurements on the Cu foil showed classical diffusional creep behavior. The activation energy of creep was defined and turned out to be equal 203 kJ/mol, which is close to the activation energy of bulk self-diffusion of copper. There was a significant increase in activation energy for the Cu-20 at.% Ni solid solution. Its activation energy was about 273 kJ/mol. The creep behavior of Cu-Co solid solution was more complicated. There were two stages of diffusional creep at different temperatures. The extremely large activation energy (about 480 kJ/mol) was determined at relatively low temperature and a small activation energy (about 105 kJ/mol) was found at high temperatures. The creep rate of Cu-Co solid solution was lower than that of pure copper at all temperatures. In addition, the free surface tension of Cu-2.8 ат.% Co was measured at different temperatures from 1242 K to 1352 K. The surface tension increases in this temperature range from 1.6 N/m to 1.75 N/m. There were no features on the temperature dependence of the surface tension.
Etude de la liaison covalente de SiC et interaction sur ses proprietes physiques, la diffusion, le fluage et la resistance mecanique a haute temperature. Interet de ce materiau dans les applications a haute temperature
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.
The use of electric current to activate the consolidation and reaction-sintering of materials is reviewed with special emphasis of the spark plasma sintering method. The method has been used extensively over the past decade with results showing clear benefits over conventional methods. The review critically examines the important features of this method and their individual roles in the observed enhancement of the consolidation process and the properties of the resulting materials. (c) 2006 Springer Science + Business Media, Inc.
Some of the general background necessary for an understanding of the deformation of ceramics is presented. This includes explanations of the various deformation tests and descriptions of the various processes which give rise to the observed deformation. The more common models used to explain deformation behavior are presented. The paper concludes with a brief description of deformation maps.
An ultra-fast densification method based on high heating rate from the combustion reaction for producing carbon nanotubes (CNTs) reinforced alumina ceramics is reported. The heat generated by combustion reaction is adopted to act as a high temperature source to the sample, which results in a heating rate of 1660°C/min of the sample. Then a great mechanical pressure is applied to the sample when the sample gets the expected temperature. The densification process is finished in several minutes. The results indicate that the densification method is beneficial to protect the CNTs from destruction and creates good interfacial combination between the nanotubes and the matrix. With the addition of 1 wt% CNTs, the fracture toughness of the ceramics prepared increases about 50%.
Deformation mechanism diagrams show the fields of stress and temperature ; in which a given mechanism is dominant and the strain rate that it yields. ; Detailed maps are presented for five pure fcc metals (Ni, Cu, Ag, Al, Pb), six ; pure bcc metals (V, Cr, Nb, Mo, Ta, W) and a recrystallized Ni-1 vol% ThO ; alloy, which are based on direct comparison to available experimental data. The ; various deformation mechanisms are discussed, with refinements of their rate ; equations. Various applications of the diagrams are illustrated. They provide a ; convenient means for the normalized comparison of the behavior of different ; metals. They also demonstrate the effects of various changes in materials in a ; manner useful for qualitative engineering design. (auth)
Recently densification maps for spark plasma sintering (SPS) of nanocrystalline MgO were constructed using a HIP model and constant particle size during the process. Here, the effects of particle coarsening during the early stage and grain growth during the later stage of densification were integrated into the model. Densification diagrams were calculated between 500°C and 900°C, pressures range 30–500MPa and initial particle size 10–40nm. Plastic yield and diffusion process dominated during the densification in SPS. The power law creep exhibited negligible densification rates. Threshold pressure exists for full densification of nanocrystalline compact with minimal grain growth. Particle coarsening and grain growth resulted in densification patterns that follow the experimental SPS results. The calculated densities were lower than the experimental results and fall short in describing the experimental densification rates of nanocrystalline MgO. Therefore additional densification mechanisms with faster kinetics should be incorporated.
Diffusional flow of a polycrystal is classically treated as a continuum diffusion problem: its rate is calculated by solving for the diffusional flux of matter through or around each grain in a polycrystal, driven by the stress acting on it. Many, but not all, experimental observations are adequately explained by this model. Progress in understanding the discrepancies can be made by considering the microscopic processes involved in diffusional flow: the detailed nature and number, of sinks and sources in the grain boundaries; and their mobility. This paper treats these problems and derives expressions for the rate of diffusional flow when the density of sinks and sources becomes small and when their mobility is limited by impurities, solutes, or precipitates. The results become identical with those of the classical treatment in the appropriate limits.
Extensive high temperature creep data are available for a superplastic yttria-stabilized tetragonal zirconia containing 3mol% of Y2O3 (termed 3Y-TZP). These data are analyzed and it is shown that, contrary to superplastic metals, intragranular slip cannot occur to accommodate grain boundary sliding at the temperatures and stresses examined experimentally. It is proposed instead that flow occurs by Coble diffusion creep controlled by movement of the Zr4+ ions and, because of the small grain sizes in the 3Y-TZP materials, the creep rates are reduced through interface-controlled diffusion creep. A detailed analysis demonstrates this approach predicts the experimental creep rates to within one order of magnitude over the range of grain sizes and temperatures examined experimentally and it accounts also for the reported values for the stress exponent and the exponent of the inverse grain size at high and low stress levels.
Densification of pure nanocrystalline MgO powder with 10nm particle size by hot-pressing was investigated in the temperature range 700–800°C, applied pressure range 100–200MPa, and for durations of up to 240min. It was shown that significant densification under the pressure begins above 440°C. Densities higher than 99.5% with grain size of 73nm were achieved at 790°C and 150MPa for a 30min duration. Remarkable densification from 90 to 99.5% was observed by temperature change from 700 to 790°C, for which the grain size was doubled only. The final grain size decreased with increasing the applied pressure. Higher shrinkage rates and cumulative shrinkages were recorded by the application of pressure at 550°C rather than from room temperature. The temperature at which the pressure was applied is crucial in determining the maximum shrinkage rate in the nanocrystalline compacts. This effect was related to the morphological changes of the particles caused by plastic deformation at lower temperatures. Analysis of the densification rate and its comparison to the literature data was in agreement with Coble creep, where self-diffusion of Mg2+ cations along the grain boundaries acts as a main densification mechanism.
Polycrystalline matter can deform to large strains by grain-boundary sliding with diffusional accommodation. A new mechanism for this sort of deformation is described and modelled. It differs fundamentally from Nabarro-Herring and Coble creep in a topological sense: grains switch their neighbors and do not elongate significantly. A constitutive equation describing the mechanism is derived from the model. The strain-rate may be diffusion controlled, in which case the constitutive equation resembles the Nabarro-Herring-Coble equation but predicts strain-rates which are roughly an order of magnitude faster. Or it may be controlled by an interface reaction-roughly speaking, by the restricted ability of a boundary to act as a sink or source for point defects, or by its restricted ability to slide. The flow behavior of superplastic alloys can be explained as the superposition of this mechanism and ordinary power-law creep ("dislocation creep"). The combined mechanisms appear to be capable of explaining not only the observed relation between strain-rate and stress, but most of the microstructural and topological features of superplastic flow as well.
A hydrothermally processed bulky powder composed of loosely aggregated nano-sized rods was consolidated by spark plasma sintering. The use of a high pressure cell allows the application of pressure up to 500MPa. It was found that applying of high pressure is beneficial for widening up the kinetic window for attaining dense HAp nanoceramics. The high transparency of HAp nanoceramics obtained in this study is ascribed to the high density and homogeneous nano-grained structure achieved besides the unique intrinsic optical properties of the HAp crystal itself, i.e. its low refractive index and very small birefringence. Achieving full densification at the minimized sintering temperature allows for the first time the preparation of transparent HAp nanoceramics with stoichiometric composition, i.e. avoiding the loss of structural water that commonly takes place during the conventional ways of sintering.
Densification maps for spark plasma sintering (SPS) of nanocrystalline MgO powders were constructed using hot-isostatic pressing (HIP) model. Effects of the grain size, applied pressure, vacuum level, SPS temperature, and duration on densification were determined. Plastic flow and diffusion were the dominating mechanisms at the first and the final stages of densification, respectively, whereas power-law creep did not contribute to densification. Most impressive was the effect of the grain size on the SPS duration needed for full densification. At 800 °C and 150 MPa, the 20 nm particle size powder compact may be fully densified within 1 min. Significant densification is expected by the plastic yield if a threshold pressure is applied. Small temperature decrease of 50 °C may increase the densification duration from tens of seconds to tens of minutes. Relative densities as high as 75% may be reached within 1 min, irrespective of the initial particle size. However, threshold particle size exists (i.e. 50 nm at 800 °C and 100 MPa) for full densification at workable SPS durations. The densification maps highlight the presence of a temperature window between 750 °C and 850 °C within which fast densification of nanocrystalline MgO may be achieved by SPS. Reasonable agreement between the calculated and the experimental SPS results confirms the validity of the HIP model for description of the SPS process.
We investigated superfast densification of nanocrystalline MgO powders by spark plasma sintering (SPS) between 700 °C and 825 °C under applied pressures of 100and 150 MPa. Fully-dense transparent nanocrystalline MgO with a 52-nm average grain size was fabricated at 800 °C and 150 MPa for 5 min. In-line transmissionsof 40% and 60% were measured compared to MgO single crystal, for the yellowand red wavelengths, respectively. Densification occurs by particles sliding over each other; the nanometric grain size and pores lead to the optical transparency. The light brownish color of the nanocrystalline MgO is due to the oxygen vacancy color centers, originating from the reducing atmosphere of the SPS process.
A novel method for producing full-dense nano-grained alumina was reported. The heat generated by the combustion reaction (or self-propagating high-temperature synthesis (SHS)) was applied to act as a heating source. An alumina compact was loaded inside the combustion reactants. After reaction, the temperature of the alumina compact was increased at a heating rate of 1600°C/min. A large mechanical pressure was applied, when the temperature reached the maximum. Two kinds of α-Al2O3 powders (200 and 600 nm) were used as the raw materials. The rapid densification process was performed within 2 min. Microstructure analysis indicated that the grains had no significant growth.
It was found that plastic deformation takes place in periclase above 1100°C., in rutile above 600°C., and in sapphire above 900°C. The mechanism is slip; in sapphire (0001) is the slip plane and [1120] is the slip directiog. All creep curves for sapphire in tension show the same qualitative features. Each consists of three stages: a stage of increasing creep rate (sometimes called an incubation period), a stage of large but decreasing creep rate (sometimes called first-stage creep), and a stage of small and nearly constant creep rate (sometimes called second-stage creep). The so-called third-stage creep, characteristic of metal behavior, has not been noted. Plastic deformation increases the electrical resistivity of sapphire at constant temperature.
This report introduces the idea of deformations-mechanism maps: maps which display the fields of stress and temperature in which a particular mechanism of plastic flow is dominant. Most materials have the capacity to deform by several alternative and independent mechanisms: dislocation glide, diffusional flow and dislocation creep are examples. Each appears on the map as a field. A point on a map then identifies the dominant mechanism and indicates the resulting strain-rate. Three applications of the maps are discussed. First, they permit a study of the effect of crystal structure and atomic bonding on plastic flow. Second, they help in the design of experiments to study a given flow mechanism and in locating, identifying and characterizing missing mechanisms. And, third, they are useful in a qualitative way for choosing a material for engineering applications, for predicting the mechanism by which it deforms and hence in selecting, or predicting the effects, of strengthening mechanisms.
Crack-tip bridging by particles is considered to be one of the primary strengthening mechanisms of ceramic nanocomposites. Small, brittle particulate inclusions have been shown to cause crack-tip bridging at short distances behind the crack tip. This mechanism leads to modest toughness but a very steep R-curve, and it is the latter that produces the very high fracture strength of the ceramic nanocomposite. Localized high residual stress around the particles (particularly in the case of silicon carbide-alumina material) causes the strengthening mechanism to operate effectively, even at a small volume fraction of 5%. The present study predicts the magnitude of the toughness increase and the extent of R-curve behavior for the nanocomposite.
Isothermal and constant-grain-size sintering have been carried out to full density in Y2O3 with and without dopants, at as low as 40% of the homologous temperature. The normalized densification rate follows Herring's scaling law with a universal geometric factor that depends only on density. The frozen grain structure, however, prevents pore relocation commonly assumed in the conventional sintering models, which fail to describe our data. Suppression of grain growth but not densification is consistent with a grain boundary network pinned by triple-point junctions, which have a higher activation energy for migration than grain boundaries. Long transients in sintering and grain growth have provided further evidence of relaxation and threshold processes at the grain boundary/triple point.
The densification rate and grain growth rate of pure MgO powder compacts are measured between 1450 and 1650 C in air. Densification rate in a semilog plot appears to be linear up to about 94% of the theoretical, followed by marked nonlinearity. The time dependence of grain growth is 1/2 at the beginning and then decreases considerably with further sintering. Application of lattice diffusion model to the densification and grain growth data gives calculated diffusion coefficients in fair agreement with the directly measured diffusion coefficients for magnesium; they are also in fair agreement with the literature value of the diffusion coefficients calculated from various other kinetic processes in MgO.
In several recent experiments on the Zn-22% Al eutectoid and the Pb-62% Sn eutectic, a sigmoidal relationship between stress and strain rate is noted and the mechanical behaviour has been divided into three regions: low-stress region (region I), intermediatestress region (the superplastic region or region II), and high-stress region (region III). In region II, the stress exponent,n, is 2 and the apparent activation energy,Q, is close to grain-boundary diffusion,Q
gb, but in both regions I and III the stress exponent and the activation energy increase (n > 2 andQ >Q
gb). Analysis of the experimental data of the two superplastic alloys suggests that the transition in behaviour between region II and region I may not necessarily reflect a change in deformation process but can arise from the presence of a threshold stress which decreases strongly with increasing temperature. Based on consideration of various possible threshold stress processes during superplastic flow, it seems most likely that a threshold stress which depends strongly on temperature may result from impurity atom segregation at boundaries and their interaction with boundary dislocations.
The creep rate (e˙) predicted by the boundary diffusion (D<sub>b</sub>) model is e˙≃150σD<sub>b</sub>WΩ/(GS)<sup>3</sup>kT , where σ is the stress, W is the boundary width, (GS) is the average grain size, and Ω is vacancy volume. The stress dependence is the same as the lattice diffusion model, given by C. Herring, while the grain size dependence and the numerical constant are greater for boundary diffusion. Discussion of the mechanism of creep in polycrystalline alumina is based on the differences between the lattice and boundary diffusion models.
Models for initial‐, intermediate‐, and final‐stage densification under pressure have been developed, which explicitly include both the surface energy and applied pressure as driving forces. For the initial stage, the time dependences and size effects given by the integrated equations are identical to those reported earlier for surface energy (alone) as the driving force. The only modification is that the surface energy (γ) is expanded into (γ+P a R/π), where P a is the applied pressure and R is the particle radius. For the intermediate stage of the process, the Nabarro‐Herring and Coble creep models may be adapted to give approximate (∼4×) densification rates for lattice and boundary diffusion models, respectively. In these cases the complex driving force is written as: (P a /D+γk), where D is the relative density, and k is the pore surface curvature. At the final stage of the process those models are invalid; an alternate model is developed based on diffusive transport between concentric spherical shells which will give a better assessment of the time dependence of densification high density (≫95%); the driving force is (P a /D+γk) in this case also. Because of the fact that the pore size is some unknown function of density, the rate equations cannot be integrated without further information. It is shown that of the various relations which have been assumed in development of models for hot pressing, for the effective stress in relationship to the applied stress and the porosity, (P a /D) is the only form which satifies the criteria demanded by self‐consistency in generation of steady‐state diffusion models.
The creep of pure polycrystalline aluminum was studied at low stresses, and temperatures near the melting point. It was found that at stresses between 3 and 13 lb/in² the secondary creep rate varied linearly with the stress, whereas at slightly higher stresses, the creep rate increased with the fourth power of the stress. The activation energy for creep was determined to be about 35,500 calories per mole over the entire stress range. The low stress creep results in the range of "viscous" creep were analysed from the viewpoint of the Nabarro-Herring model for stress directed self-diffusion of vacancies, and it was found that the experimental creep rates were about one thousandfold greater than those predicted theoretically. This factor plus additional experimental observations on the displacements of markers at grain boundaries, and on creep recovery upon removal of the stress led to the conclusion that the Nabarro-Herring model does not apply to the case of aluminum, and that creep occurs by a dislocation climb mechanism at all stresses considered.
Molecular-dynamics simulations are used to elucidate the effects of grain growth on grain-boundary diffusion creep and grain-boundary sliding during high-temperature deformation of a nanocrystalline Pd model microstructure. The initial microstructure consists of a 25-grain polycrystal with an average grain size of about 15 nm and a columnar grain shape. Prior to the onset of significant grain growth, the deformation proceeds via the mechanism of Coble creep accompanied by grain-boundary sliding. While grain growth is generally known to decrease the creep rate due to the increase of the average grain size, the results obtained in this study reveal an enhanced creep rate at the onset of the grain growth, when rapid grain-boundary migration occurs. The enhanced creep rate is shown to arise from topological changes during the initial growth phases, which enhance both the stress-induced grain-boundary diffusive fluxes and grain-boundary sliding. Dislocations generated as a result of grain-rotation-induced grain coalescence and grain-boundary decomposition in the vicinity of certain triple junctions also contribute to the deformation.
Effect of the particle size on the possible electric discharge during the SPS was examined. Nanoparticle compacts enable accumulation of high electric charge, and discharge under conventional voltages used for the SPS. The critical particle size for the electric discharge is both morphological and material dependent. The early stages of densification of the nanocrystalline powder compact proceed either by the plastic deformation or grain-rotation coalescence and sliding, aided by softening of the particle surfaces. The active densification mechanism depends on the changes both in the mechanical and electrical properties with temperature. Densification of 11 nm nc-MgO particles with low yield stress proceeds by plastic deformation already at 700 °C. However, densification of 34 nm nc-YAG particles with high yield stress proceeds by nano-grain rotation aided by particle surface softening. Densification at the final stages of SPS is associated with diffusional processes, where curvature driven grain growth predominates.
Dense zirconia and ceria ceramics with grain sizes approaching 10 nm have been obtained using a high-pressure modification of the field activated sintering (spark plasma sintering). Pressures up to 1 GPa and sintering times of 5 min have been used. The densification has been performed at temperatures as low as 930 °C for fully-stabilized zirconia, 815 °C for samaria-doped ceria, and 675 °C for pure ceria. Relative densities greater than 98% were achieved for all materials. The method overcomes the deleterious effect of powder agglomeration, producing fine grain size for these ceramics in bulk form.
Nanocrystalline (nc) Y2O3 powders with 18nmcrystallite size were sintered using spark plasma sintering (SPS) at 1100 ◦C and 100MPa for different durations. Specimens with 98% density and 106±33nm mean grain size were formed after 20 min. The grain size at the final stage of sintering first increased and then tended to stagnation with the SPS duration. The nanostructure consisted of convex tetrahedron shaped nano-pores at part of the grain boundary junctions. Theoretical calculations were made for grain growth stagnation imposed by either drag from nano-pores at grain junctions or from dense triple junctions; the experimental results were in agreement with grain growth stagnation due to nano-pore drag in nc-Y2O3. The conditions for the stabilization of the nanostructure in Y2O3 were determined. Extended SPS duration up to 40 min led to sudden grain coarsening and loss of the nanocrystalline character.
Isothermal and constant-grain-size sintering have been carried out to full density in Y2O3 with and without dopants, at as low as 40% of the homologous temperature. The normalized densification rate follows Herring’s scaling law with a universal geometric factor that depends only on density. The frozen grain structure, however, prevents pore relocation commonly assumed in the conventional sintering models, which fail to describe our data. Suppression of grain growth but not densification is consistent with a grain boundary network pinned by triple-point junctions, which have a higher activation energy for migration than grain boundaries. Long transients in sintering and grain growth have provided further evidence of relaxation and threshold processes at the grain boundary/triple point.
Sintering is the process whereby interparticle pores in a granular material are eliminated by atomic diffusion driven by capillary forces. It is the preferred manufacturing method for industrial ceramics. The observation of Burke and Coble that certain crystalline granular solids could gain full density and translucency by solid-state sintering was an important milestone for modern technical ceramics. But these final-stage sintering processes are always accompanied by rapid grain growth, because the capillary driving forces for sintering (involving surfaces) and grain growth (involving grain boundaries) are comparable in magnitude, both being proportional to the reciprocal grain size. This has greatly hampered efforts to produce dense materials with nanometre-scale structure (grain size less than 100 nm), leading many researchers to resort to the 'brute force' approach of high-pressure consolidation at elevated temperatures. Here we show that fully dense cubic Y2O3 (melting point, 2,439 degrees C) with a grain size of 60 nm can be prepared by a simple two-step sintering method, at temperatures of about 1,000 degrees C without applied pressure. The suppression of the final-stage grain growth is achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. Such a process should facilitate the cost-effective preparation of other nanocrystalline materials for practical applications.
Self-propagating high-temperature synthesis: twenty years of search and findings
- A G Merzhanov
Merzhanov AG. Self-propagating high-temperature synthesis: twenty years
of search and findings. In: Munir ZA, Holt JB, editors. Combustion and
plasma synthesis of high-temperature materialsPL New York. VCH Publishing Inc.; 1990. p. 1-53.