Microstructure evolution and densification kinetics of Al2O3/Ti(C,N) ceramic tool material by microwave sintering

The microstructure evolution and densification kinetics of Al₂O₃/Ti(C,N) ceramic tool material during microwave sintering were studied. The density and grain growth significantly increases at the temperatures higher than 1400 °C. The calculated kinetics parameter n indicates that volume diffusion is the main densification mechanism when the sintering temperature is below 1300 °C, while grain boundary diffusion plays a leading role in the densification process when the sintering temperature is higher than 1300 °C. The grain growth activation energy of Al₂O₃/Ti(C,N) composite is 48.82 KJ/mol, which is much lower than those of monolithic Al₂O₃ in the microwave sintering and conventional sintering. The results suggested that the Al₂O₃/Ti(C,N) ceramic tool material with nearly full densification and fine grains can be prepared by two-step microwave sintering.     

Nano-sized zirconium carbide powder: Synthesis and densification using a spark plasma sintering apparatus

Nano-sized zirconium carbide powder was synthesized at 1600 °C by the carbothermal reduction of ZrO₂ using a modified spark plasma sintering (SPS) apparatus. The synthesized ZrC powder had a fine particle size of approximately 189 nm and a low oxygen content of 0.88 wt%. The metal basis purity of the synthesized powder was 99.87%. The low synthesis temperature, fast heating/cooling rate and the effect of current during the modified SPS process effectively suppressed the particle growth. Using the synthesized powder, monolithic ZrC ceramics with high relative density (97.14%) were obtained after the densification at 2100 °C for 30 min at a pressure of 80 MPa by SPS. The average grain size of the densified ZrC ceramics was approximately 9.12 μm.     

Effect of reinforcement NbC phase on the mechanical properties of Al2O3-NbC nanocomposites obtained by spark plasma sintering

The sintering behavior of Al₂O₃-NbC nanocomposites fabricated via conventional and spark plasma sintering (SPS) was investigated. The nanometric powders of NbC were prepared by reactive high-energy milling, deagglomerated, leached with acid, added to the Al₂O₃ matrix in the proportion of 5 vol% and dried under airflow. Then, the nanocomposite powders were densified at different temperatures, 1450–1600 °C. Effect of sintering temperature on the microstructure and mechanical properties such as hardness, toughness and bending strength were analyzed. The Al₂O₃-NbC nanocomposites obtained by SPS show full density and maximum hardness value > 25 GPa and bending strength of 532 MPa at 1500 °C. Microstructure observations indicate that NbC nanoparticles are dispersed homogeneously within Al₂O₃ matrix and limit their grain growth. Scanning electron microscopy examination of the fracture surfaces of dense samples obtained at 1600 °C by SPS revealed partial melting of the particle surfaces due to the discharge effect.     

Microstructure and optical properties of transparent alumina

The microstructure and optical properties are evaluated for alumina sintered by spark plasma sintering at temperatures between 1100 and 1550 °C. With increasing sintering temperature, grain growth and densification occur up to 1250 °C, and above 1300 °C, rapid grain growth and pore growth occur. Light transmission increases with the densification and decreases with the grain/pore growth. It is found that the total forward transmission and the reflection of light are related to the porosity and the pore growth, whereas the in-line transmission and the light absorption are related to the grain size and the defects, respectively. The relationships are explained by using the Mie scattering theory, model prediction and observed microstructural characteristics.     

Spark plasma sintering of a commercially available granulated zirconia powder: Comparison with hot-pressing

A commercially available granulated TZ3Y powder has been sintered by hot-pressing (HP). The “grain size/relative density” relationship, referred to here as the “sintering path”, has been established for a constant value of the heating rate (25 °C min^?1) and a constant value of the macroscopic applied pressure (100 MPa). It has then been compared to that obtained previously on the same powder but sintered by spark plasma sintering (SPS, heating rate of 50 °C min^?1, same applied macroscopic pressure). By coupling the analysis of a sintering law (derived from creep rate equations) and comparative observations of sintered samples using transmission electron microscopy, a hypothesis about the densification mechanism(s) involved in SPS and HP has been proposed. Slight differences in the densification mechanisms lead to scars in the microstructure that explain the higher total ionic conductivity measured, in the temperature range 300–550 °C, when SPS is used for sintering.     

Spark plasma sintering of TiC particle-reinforced molybdenum composites

In order to improve the recrystallization resistance and the mechanical properties of molybdenum, TiC particle-reinforcement composites were sintered by SPS. Powders with TiC contents between 6 and 25 vol.% were prepared by high energy ball milling. All powders were sintered both at 1600 and 1800 °C, some of sintered composites were annealed in hydrogen for 10 h at 1100 up to 1500 °C. The powders and the composites were investigated by scanning electron microscopy and XRD. The microhardness and the density of composites were measured, and the densification behavior was investigated. It turns out that SPS produces Mo–TiC composites, with relative densities higher than 97%.The densification behavior and the microhardness of all bulk specimens depend on both the ball milling conditions of powder preparation and the TiC content. The highest microhardness was obtained in composites containing 25 vol.% TiC sintered from the strongest milled powders. The TiC particles prevent recrystallization and grain growth of molybdenum during sintering and also during annealing up to 10 h at 1300 °C. Interdiffusion between molybdenum and carbide particles leads to a solid solution transition zone consisting of (Ti_1 −x Mo_x)C_y carbide. This diffusion zone improves the bonding between molybdenum matrix and TiC particles. A new phase, the hexagonal Mo₂C carbide, was detected by XRD measurements after sintering. Obviously, this phase precipitates during cooling from sintering temperature, if (Ti_1 −x Mo_x)C_y or molybdenum, are supersaturated with carbon.     

Spark plasma sintering of W-10Ti high-purity sputtering target: Densification mechanism and microstructure evolution

The densification mechanism and microstructure evolution of W-10Ti sputtering target prepared by spark plasma sintering (SPS) method at a temperature ranges from 900 to 1600 °C, with dwelling time of 6 min and fixed pressure of 30 MPa were investigated. Densification occurs mainly at low temperatures (900 to 1300 °C), while grain growth occurs at high temperatures (1400 to 1600 °C). The creep model has been used to reveal the densification process. The effective stress exponent n is calculated systematically, which indicates that the densification process is mainly due to the particle rearrangement (n < 1), grain boundary diffusion (n = 1–2), and dislocation climbing (n = 3.77 or 4.14). In addition, the apparent activation energy Q_d is calculated to be 119.30 and 271.79 kJ/mol when the effective stress exponent n is equal to 1 and 2, respectively. It is also found that the microstructure of W-10Ti alloys is greatly affected by the sintering temperatures. The solution between W and Ti significantly improves with the increase of the sintering temperature. The solubility of W in βTi(W) exceeded the eutectoid point (28.97 wt% W) and the eutectoid structure (βW(Ti) + αTi) forms in cooling process when the temperature is up to 1300 °C. With the temperature increasing to 1500 °C, the composition of the βTi(W) phase is located in the miscibility gap of the (βTi(W), βW(Ti)) system, which tends to decompose in to βTi(W) and βW(Ti) phases.     

Spark plasma sintering of a commercially available granulated zirconia powder: I. Sintering path and hypotheses about the mechanism(s) controlling densification

Spark plasma sintering (SPS) of a commercially available granulated zirconia powder has been investigated. The “relative density/grain size” trajectory, or “sintering path”, has been established for a constant heating rate (50 °C/min) and a constant applied pressure (100 MPa). In addition, an attempt has been made to identify the mechanism(s) that could be invoked for the control of densification during the SPS experiments.     

Two-step hot pressing of bimodal micron/nano-ZrB2 ceramic with improved mechanical properties and thermal shock resistance

The densification and grain growth behaviors for micron- and nano-sized ZrB₂ particles were investigated. The densification on-set temperature (T_d-micron) and grain growth on-set temperature (T_g-micron) for micron-sized ZrB₂ particles were about 1500 °C and 1800 °C, respectively. And the densification on-set temperature (T_d-nano) and grain growth on-set temperature (T_g-nano) for nano-sized ZrB₂ particles were about 1300 °C and 1500 °C, respectively. A bimodal micron/nano-ZrB₂ ceramic was therefore prepared using a novel two-step hot pressing. A high relative density of 99.2%, an improved flexural strength of 580.2 ± 35.8 MPa and an improved fracture toughness of 7.2 ± 0.4 MPa·m^1/2 were obtained. The measured critical thermal shock temperature difference (ΔT_c) for this bimodal micron/nano-ZrB₂ ceramic was as high as 433 °C.     

Sintering behavior and non-linear properties of ZnO varistors processed in microwave electric and magnetic fields at 2.45 GHz

A study of the densification behavior and grain growth mechanisms of ZnO-based varistors composed of 98 mol.% ZnO–2 mol.% (Bi₂O₃, Sb₂O₃, Co₃O₄, MnO₂) has been carried out. The pressed samples were sintered in microwave electric (E) and magnetic (H) fields using a single-mode cavity of 2.45 GHz. The effect of the sintering temperature (900–1200 °C), holding time (5–120 min) and sintering mode (E, H) on the microstructure and electrical properties of the sintered varistor samples were investigated. The grain growth kinetics was studied using the simplified phenomenological equation Gⁿ = kte^(?Q/RT). The grain growth exponent (n) and apparent activation energy (Q) values were estimated for both electric and magnetic heating modes and were found to be n = 3.06–3.27, Q = 206–214 kJ mol^?1, respectively. The lower value of n estimated in the E field was attributed to a volume diffusion mechanism, whereas the higher n value in the H field sintering was correlated mainly to a combined effect of volume and surface diffusion processes. Samples sintered in the H and E fields showed high final densities. Moreover, the ones sintered in the H field presented slightly higher density values and bigger grains for all sintering temperatures than E field heated ones. The optimal sintering conditions were achieved at 1100 °C for a 5 min soaking time for both H and E field processed samples, where respectively densities of 99.2 ± 0.5% theoretical density (TD) and 98.3 ± 0.5% TD along with grain size values of G = 7.2 ± 0.36 μm and G = 6.6 ± 0.33 μm were obtained. Regarding the electrical properties, breakdown voltage values as high as 500–570 V mm^?1 were obtained, together with high non-linear coefficients α = 29–39 and low leakage currents (J_l ≈ 5 × 10^?3 mA cm^?2), respectively, for E and H field sintered varistor samples. Moreover, samples sintered in an H field systematically exhibited higher breakdown voltage values compared to the ones sintered in the E field. This was attributed to an improved coupling between the H field and the present dopants within the ZnO matrix, this latter being mostly semiconductive, thus leading to an enhanced reactivity and improved properties of the electrostatic barrier.     

Flexural strength behavior of a ZrB2–TaB2 composite consolidated by non-reactive spark plasma sintering at 2300 °C

A bulk zirconium–tantalum diboride ceramic composite was consolidated by non-reactive spark plasma sintering (SPS) at 2300 °C. In order to consolidate the ZrB₂–44 wt% TaB₂ composite and restrict grain-growth, a special loading procedure was used. Pressure was applied and released at 2150 °C and 1250 °C, respectively. These SPS conditions allowed us to obtain a crack-free bulk composite with a grain size of 4–8 μm. Flexural strength at temperatures up to 1800 °C was measured for the ZrB₂–TaB₂ composite. Importantly, at 1600 °C, the strength was 336 ± 23 MPa, which is superior to that of monolithic ZrB₂. Moreover, the ZrB₂–TaB₂ composite only showed plastic behavior at 1800 °C, a finding that is atypical for ZrB₂-based ceramics.     

Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-CuO powder mixtures

Understanding the microstructure of W–Cu nanocomposite powder is essential for elucidating its sintering mechanism. In this study, the effect of milling time on the structural characteristics and densification behavior of W-Cu composite powders synthesized from WO₃-CuO powder mixtures was investigated. The mixture of WO₃ and CuO powders was ball-milled in a bead mill for 1 h and 10 h followed by reduction by heat-treating the mixture at 800 °C in H₂ atmosphere with a heating rate of 2 °C/min to produce W-Cu composite powder. The microstructure analysis of the reduced powder obtained by milling for 1 h revealed the formation of W–Cu powder consisting of W nanoparticle-attached Cu microparticles. However, Cu-coated W nanocomposite powder consisting of W nanoparticles coated with a Cu layer was formed when the mixture was milled for 10 h. Cu-coated W nanopowder exhibited an excellent sinterability not only in the solid-phase sintering stage (SPS) but also in the liquid-phase sintering stage (LPS). A high relative sintered density of 96.0% was obtained at 1050 °C with a full densification occurring on sintering the sample at 1100 °C. The 1 h-milled W-Cu powder exhibited a high sinterability only in the LPS stage to achieve a nearly full densification at 1200 °C.     

Influence of SPS parameters on the density and mechanical properties of sintered Ti3SiC2 powders

The densification of Ti₃SiC₂ MAX phase was performed by the Spark Plasma Sintering (SPS) technique. The SPS parameters, such as sintering temperature, pressure and soaking time, were optimized to obtain fully densified samples which were characterized to obtain the best mechanical properties. The sintering temperature was varied from 1070 to 1300 °C, the soaking time from 1 to 10 min and the applied pressure from 60 to 180 MPa. The best full densified samples were sintered at 1300 °C applying 60 MPa for 7 min. Ti_xC_y and TiSi₂ secondary phases were found in samples densified at 1200, 1250 and 1300 °C, due to decomposition of Ti₃SiC₂. These secondary phases, detected by XRD patterns, were confirmed by microhardness testing, FESEM observations and EDAX analyses.     

Temperature dependence of microstructure evolution during hot pressing of ZrB2–30 vol.% SiC composites

Microstructure evolutions of ZrB₂–30 vol.% SiC composites, prepared by hot pressing at different processing temperatures (1700, 1850 and 2000 °C) for 30 min under 10 MPa, were investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy (TEM). The microstructures of the fabricated composites were compared with and the effects of the processing temperature on the sintering process and densification behavior during the hot pressing were found. The amount and the orientation of dislocations which were indicated by TEM analysis in the sample hot pressed at 1700 °C showed that no plastic deformation and atomic diffusion occurred. But the presence of amorphous phases and rearrangement of particles are signs of the fact that liquid phase sintering and particle fragmentation/rearrangement is the main densification mechanism. On the other hand, in the sample hot pressed at 1850 °C, aggregation of dislocations behind the grain boundaries and the presence of twinnings addressed wide plastic deformations which were introduced as the main densification mechanism at 1850 °C. Finally in the sample hot pressed at 2000 °C, lower amounts of un-oriented dislocations and also some annealing twinnings were observed in TEM micrographs together with fractographical SEM analysis and showed that the atomic diffusion is the dominant densification mechanism of hot pressed ZrB₂–30 vol.% SiC composite.     

Sinterability studies of monolithic chromium diboride (CrB2) by spark plasma sintering

Spark plasma sintering (SPS) experiments were conducted to investigate the effect of the processing parameters such as temperature, mechanical pressure and dwell time on densification behavior of monolithic chromium diboride. The sintering experiments were performed at different temperatures ranging from 1100 °C to 1900 °C under the mechanical pressure of 30 MPa–70 MPa for 1 min–15 min duration. The onset temperature for the densification of CrB₂ is observed to be 1300 °C at 50 MPa. High dense chromium diboride (98.4%ρ_th) compact was obtained when processed at 1900 °C under a mechanical pressure of 70 MPa for 15 min duration. Hardness and fracture toughness of high density monolithic CrB₂ (98.4%ρ_th) sample were measured to be 15.89 ± 1.3 GPa and 1.8 ± 0.14 MPa·m^1/2 respectively.     

Pressure effects and grain growth kinetics in the consolidation of nanostructured fully stabilized zirconia by pulsed electric current sintering

The effect of applied uniaxial pressure on the densification and grain size of nanocrystalline cubic zirconia (c-YSZ) was investigated during sintering by the pulsed electric current sintering (PECS) method. The role of the pressure depended on temperature, being highly significant at lower temperatures and of little significance at higher temperatures. The kinetics of grain growth were determined under PECS conditions. Analysis of the results indicated a grain growth process that is retarded, probably due to the effect of the current on grain boundary energy or dopant segregation. The activation energy for grain growth of c-YSZ was determined as 252 ± 34 kJ mol^?1, a value that is slightly smaller than reported values for microcrystalline samples.     

Microstructure characterization of a cobalt-oxide-doped cerium-gadolinium-oxide by analytical and high-resolution TEM

The microstructure and chemistry of 2 mol.% and 5 mol.% cobalt-oxide-doped Ce_0.8Gd_0.2O_1.9 sintered at different temperatures were examined by a combination of electron energy-loss spectroscopy and energy-filtering and high-resolution transmission electron microscopy. Co grain boundary excess was evaluated. It is found that Co solubility in Ce_0.8Gd_0.2O_1.9 is low at temperatures between 800 and 1150 °C, resulting in a large number of Co precipitates at grain boundaries. With increasing sintering temperature, precipitates grow, influencing the Co redistribution and further altering the segregation amount in the grain boundary. The Co grain boundary concentration is shown to increase with the increase of sintering temperature from 890 to 1050 °C, which is suggested to be due to grain growth. It is found that Co grain boundary segregation induces a detectable variation in the ELNES of Ce-M_4,5 and O-K absorption edges, indicating a reduction of Ce atoms in the grain boundary region. The phase of the precipitates was identified as CoO at temperatures between 890 °C and 1150 °C. HRTEM reveals that grain boundaries are less disordered after prolonged sintering time at higher temperature. At a dopant level of 5 mol.% Co oxide in Ce_0.8Gd_0.2O_1.9, the grain boundaries become more disordered, and exhibit a high amount of Co segregation.     

Sintering and annealing effects on ZnO microstructure and thermoelectric properties

The influence of different thermal treatments on zinc oxide has been investigated regarding the thermal diffusivity and structural properties of doped and undoped samples. ZnO powders having various grain sizes and morphologies, with or without aluminum doping, have been prepared under different temperatures by spark plasma sintering (SPS). The microstructural properties and thermal diffusivities of the prepared samples have been measured before and after annealing treatments in air at 800 °C. In undoped samples, the crystallite sizes increased after the annealing treatments, while it was retained in the Al-doped samples. The thermal diffusivities, microstrain and degree of preferred orientation were affected by the SPS temperature and the annealing; however, the general trends were retained after the annealing treatments. Lower maximum temperature yielded a lower degree of preferred orientation, less microstrain, higher density of grain boundaries, lower thermal diffusivities and, for Al-doped samples, lower electrical conductivity and a difference in zT-values from 0.2 to 0.3 at 800 °C. Calculations of the wavelengths and mean free paths of the phonons that contribute to the main part of the thermal conductivity have been conducted and reveal that nanostructures <12 nm are required to lower the thermal conductivity by quantum confinement.     

Densification and grain growth during early-stage sintering of Ce0.9Gd0.1O1.95−δ in a reducing atmosphere

The present work investigates the processes of densification and grain growth of Ce_0.9Gd_0.1O_1.95?δ (CGO10) during sintering under reduced oxygen partial pressure. Sintering variables were experimentally characterized and analyzed using defect chemistry and sintering constitutive laws. Based on the results achieved, the grain size–relative density relationship, the densification rate and the grain-growth rate were determined. The activation energies for densification and grain growth were evaluated, and the dominant densification mechanism was indicated. For comparison, the densification behavior of CGO10 sintered in air was also studied. Accelerated densification was observed in early-stage sintering of CGO10 in a reducing atmosphere. This might be attributed to the oxygen vacancies generated by the reduction of Ce⁴⁺ to Ce³⁺ in the reducing atmosphere, which facilitate the diffusion of ions through the lattice. The densification activation energy of CGO10 in the reducing atmosphere was evaluated to be 290 ± 20 kJ mol^?1 in the relative density range of 0.64–0.82, which was much smaller than that of CGO10 sintered in air (770 ± 40 kJ mol^?1). The grain-growth activation energy of CGO10 sintered in the reducing atmosphere was evaluated to be 280 ± 20 kJ mol^?1 in the grain size range of 0.34–0.70 μm. The present work describes a systematic investigation of sintering behavior of CGO10 under reduced oxygen partial pressure, which contributes to the first known determination of the fundamental parameters associated with densification and grain growth during early-stage sintering of CGO10 in a reducing atmosphere.     

Effects of sintering temperature on fine-grained tungsten heavy alloy produced by high-energy ball milling assisted spark plasma sintering

Fine-grained tungsten heavy alloys (WHAs) were successfully produced using the high-energy ball milling assisted spark plasma sintering (SPS) method. The effects of increasing sintering temperatures on the microstructure and mechanical properties of the alloy were studied in detail. The hardness of the alloy was found to continuously decrease from 79.3 to 63.8 HRA. In contrast, the bending strength continuously increased from 353.6 to 954.5 MPa. W grain size increased with increased sintering temperature. The temperature ranges from 1000 to 1100 °C and 1150 to 1200 °C were a period of rapid growth of W grain. According to the color change in the scanning electron microscope (SEM) image, the W alloy microstructure were classified into white W grains, off-white W-rich particles, dark grey matrix γ-(Ni, Fe, W), as well as pitch-black W- and O-rich particles. The bending fracture of the alloy mainly displays the features of intergranular fracture. The microporosity of different sizes was distributed on the bending fracture, and grew with increased sintering temperature.