Nano-structured MgAl2O4 spinel consolidated by high pressure spark plasma sintering (HPSPS)

Nano-structured transparent polycrystalline magnesium aluminate spinel (PMAS) was fabricated using a high pressure (up to 1000 MPa) spark plasma sintering (HPSPS) apparatus and various properties of the spinel, such as transparency, micro-structure and mechanical properties (specifically, hardness and fracture toughness), were tested. Using a creep densification model, it was concluded that densification in the final stage of HPSPS is controlled by grain boundary sliding (GBS), rather than by oxygen diffusion. The average grain size of PMAS fabricated under 400 MPa pressure at 1200 °C was about 170 nm, while for samples fabricated under 1000 MPa at 1000 °C the average grain size was remarkably smaller (about 50 nm). HRTEM analysis clearly demonstrated clean grain boundaries and triple points with no evidence for the existence of amorphous regions. Fully dense specimens displayed in-line transmittance higher than 80%. It was moreover established that hardness and fracture toughness values did not depend on the indentation load applied. Finally, hardness values for grains sized between tens of microns and tens of nm strictly followed the Hall-Petch relationship.     

Preparation of dense SiHf(B)CN-based ceramic nanocomposites via rapid spark plasma sintering

Dense SiHf(B)CN-based ceramic nanocomposites were prepared by spark plasma sintering (SPS) using high heating rates (∼450 ^°C/min.) and high pressures (≥100 MPa). The obtained nanocomposites were investigated by X-ray diffraction, Raman spectroscopy and electron microscopy concerning their phase evolution and microstructure.The hardness and the elastic modulus of dense SiHfCN were found to be 26.8 and 367 GPa, respectively. Whereas the SiHfBCN samples exhibited a hardness of 24.6 GPa and an elastic modulus of 284 GPa. The investigation of the oxidation of the prepared dense ceramic nanocomposites at high temperature revealed that the parabolic oxidation rates of SiHfCN were comparable to those of ultra-high temperature ceramics (UHTCs, e.g. HfC-20 vol% SiC); whereas the parabolic oxidation rates of SiHfBCN were several orders of magnitude lower than those. The results obtained within this study indicate the feasibility of SPS for rapid preparation of dense though nano-scaled Hf-containing ceramic nanocomposites that are promising candidates for high-temperature applications in harsh environments.     

Fabrication of transparent Y2O3 ceramics by two-step spark plasma sintering

Transparent Y₂O₃ ceramics were successfully fabricated by spark plasma sintering applying a two-step pressure and heating profile. Through the shrinkage curve of the single-step SPS profile, it was confirmed that shrinkage occurred at 800°C–1250°C, and it was selected as the two-step pressure profile. After the first-step SPS stage at 1250°C, the second-step SPS stage, which had the highest real in-line transmittance, was completed at 1500°C. The two-step SPS profile improved the shrinkage behavior and was able to achieve sufficient densification without excessive coarsening. As a result, the normalized real in-line transmittance to 1 mm was 80.6% at 1100 nm, which is close to the theoretical transmittance of 81.6%. The two-step pressure and heating profile in the SPS process was a significant advantage in manufacturing ceramics that were transparent and had sufficient densification.     

Defects and microstructure of highly conducting Al-doped ZnO ceramics obtained via spark plasma sintering

Al-doped ZnO ceramics were sintered by conventional sintering method and spark plasma sintering (SPS) respectively. Electrical properties and microstructure have been investigated by various measurements. The samples sintered via SPS exhibit a huge electrical conductivity, up to 3.0 × 10⁵ S/m at room temperature, which was much higher than that of the sample sintered via the conventional sintering. Structural and morphorlogical characterizations pointed out that the further incorporation of Al ions and the absence of a secondary phase, contribute to the increase of the carrier concentration. Raman spectroscopy revealed the occurrence of structural distortions and a disorder induced by Al doping. Photoluminescence spectra were interpreted by different electronic active defects such as the defect complexes (Al_Zn-Zn_i) which play a key for the high electrical conductivity. Thus, SPS and Al doping modified the microstructure and the concentration of the electronic active defects to ensure high electrical conductivities in doped ZnO-based ceramics.     

Mechanism of improving the mechanical properties of Si3N4/TiC ceramic tool materials prepared by spark plasma sintering

Spark plasma sintering (SPS) is a new sintering method having shorter sintering time and higher densification speed than the traditional sintering methods. In this paper, the Si₃N₄/TiC ceramic tool material is sintered by SPS. The microstructure and mechanical properties of the material under different sintering parameters are compared. The sintering process of the material is then analyzed, and the best sintering parameters are obtained. Heat the material to 1600°C and keep the temperature for 15 min, then continue to heat to 1700°C and keep the temperature for 10 min, Si₃N₄/TiC ceramic tool material has high mechanical properties, its bending strength, fracture toughness, and Vickers hardness are 959 MPa, 8.61 MPa·m^1/2, and 15.21 GPa, respectively. The scanning electron microscope (SEM) analysis shows that under this condition, the sintering additives and Si₃N₄/TiC material form the liquid phase, which makes the Si₃N₄ particles rearrange, dissolve, precipitate, and transform into rod shape β-Si₃N₄. In addition, under the action of pulse current and external pressure, electric sparks are generated between TiC particles, which allows the material transfer and particle refinement. Therefore, the β-Si₃N₄ has uniform grain size, and it is vertically and horizontally arranged in the structure, which makes the material have excellent mechanical properties.     

Synthesis and characterization of Al2O3-TiC-ZrO2 ceramics with intragranular nanostructure by spark plasma sintering

In this paper, Al₂O₃-TiC ceramic composites with intragranular nano-ZrO₂ were prepared in vacuum by spark plasma sintering (SPS). The effect of ZrO₂ particles with different nano-sizes on the microstructure and mechanical properties of ceramics was studied. The results show that SPS can achieve relative densification of materials without generating new impurity phases. At the same time, the sintering densification temperature of ceramic materials can be reduced by adding ZrO₂ (20 nm) particles. Under the action of SPS strong electric field, the nano-ZrO₂ adsorbed on the surface of the matrix particles can enter the interior of matrix grains, and form intragranular nanostructures when the grain boundaries move and the particles merge. The microstructure and mechanical properties of ceramic materials can be improved through the intragranular structure formed by nanoparticles. The main reasons for the increased strength and toughness of ceramic materials are crack deflection, crack bridging and transgranular fracture.     

Microstructure and thermal conductivity of fully ceramic microencapsulated fuel fabricated by spark plasma sintering

Fully ceramic microencapsulated pellet (FCM), consisting of tristructural isotropic (TRISO) particles embedded in silicon carbide (SiC) matrix, was fabricated using spark plasma sintering. The parameters affecting the densification of SiC matrix were first investigated, and then FCM pellets were prepared using TRISO particles with/without outer pyrolytic carbon (OPyC) layer. Effects of thermal exposure on the TRISO particles during SPS were evaluated. In addition, the thermal condcutvitities of FCM pellet, as well as the SiC matrix, were measured using laser flash. It was revealed that the TRISO particles with OPyC layers significantly lower the thermal conductivity of FCM pellet. Based on Maxwell‐Eucken model, the predicted effective thermal conductivities of TRISO particles with/without OPyC layers were 14.4 W/m K and 25.2 W/m K, respectively. Finite elements simulation indicated that the SiC layer in TRISO particle plays a dominant role on the thermal conductivity of FCM. The presence of OPyC layers would generate gaps/porous SiC near the interface and resist the heat flows, leading to a lower thermal conductivity of FCM.     

Effect of porosity and hexagonality on the infrared transmission of spark plasma sintered ZnS ceramics

Tailoring phase transition and microstructural evolution during sintering is crucial for the fabrication of ZnS ceramics transparent to infrared (IR) radiation. Herein, we have described the phase transition, microstructure, and related IR transmission of spark-plasma-sintered ZnS ceramics in terms of sintering temperature and pressure. The pore characteristics of spark-plasma-sintered ZnS ceramics were evaluated using Mie scattering theory. Changes in hexagonality and residual pore characteristics of the microstructure affected IR transmission of the sintered specimens. High temperature and pressure condition of SPS were found to increase excessive hexagonal phase (>20%), mainly contributing to a transmittance decay in the range 2–4 μm.     

Strengthening and toughening of CNTs-reinforced B4C-SiC ceramic composite material via spark plasma sintering

For addressing the issue of low relative density, poor fracture toughness of boron carbide ceramics, carbon nanotubes (CNTs)-reinforced B₄C-SiC ceramic composite material was prepared via spark plasma sintering (SPS), and the impact of CNTs on the strengthening and toughening of the composite was studied. The evidence revealed that an appropriate amount of CNTs can enhance the discharge effect and improve the compactness. As the CNTs content increased, the flexural strength and fracture toughness took on a tendency to first rise and then drop. After mixing .5 wt.% CNTs, the flexural strength and fracture toughness were 499 MPa and 5.38 MPa·m^1/2, which increased by 59.4% and 28.4%, respectively. The transformation of fracture mode, grain refinement, bridging and pulling-out of CNTs efficiently enhance the mechanical properties.     

Effect of annealing process on IR transmission and mechanical properties of spark plasma sintered Yttria

In this study, fully dense Yttria ceramics were successfully fabricated by spark plasma sintering (SPS) at temperatures of 1300 and 1350 $℃$. The effects of post-annealing on IR transmission were investigated by Fourier transform infrared spectroscopy (FTIR) at various temperatures ranging from 1050 to 1250 $℃$. It was found that the optimum annealing temperature depends strongly on the sintering temperature. Annealed samples showed white opaqueness mainly due to the increase and coalescence of pores after annealing and showed an absorption band around 6.6 $\mu m$ which limits usage of yttria in IR applications. Sintering at 1350 $℃$ and annealing at 1250 $℃$ led to the maximum IR transmittance above 80% at wavelength of 5 $\mu m$ for a 3.5-mm-thick sample. The hardness and the fracture toughness of the samples were analyzed in detail and hardness of 9.2 GPa and fracture toughness of 1.65 MPa m^1/2 were obtained for the above sample.     

Microstructure and properties of a graphene platelets toughened boron carbide composite ceramic by spark plasma sintering

A kind of B₄C/SiC composite ceramic toughened by graphene platelets and Al was fabricated by spark plasma sintering. The effects of graphene platelets and Al on densification, microstructure and mechanical properties were studied. The sintering temperature was decreased about 125–300?°C with the addition of 3–10?wt% Al. Al can also improve fracture toughness but decrease hardness. The B₄C/SiC composite ceramic with 3?wt%Al and 1.5?wt% graphene platelets sintered at 1825?°C for 5?min had the optimal performances. It was fully densified, and the Vickers hardness and fracture toughness were 30.09?±?0.39?GPa and 5.88?±?0.49?MPa?m^1/2, respectively. The fracture toughness was 25.6% higher than that of the composite without graphene platelets. The toughening mechanism of graphene platelets was also studied. Pulling-out of graphene platelets, crack deflection, bridging and branching contributed to the toughness enhancement of the B₄C-based ceramic.     

Evolution of microstructure,mechanical, and optical properties of Y2O3-MgO nanocomposites fabricated by high pressure spark plasma sintering

A high-pressure spark plasma sintering (SPS) process was applied for consolidating Y₂O₃–MgO nanocomposites. This approach enabled to fabricate a fully dense infrared (IR) transparent nanocomposites, which possess an average grain size of ∼70 nm and high hardness, at a relatively low sintering temperature of 1130 °C under a high pressure of 300 MPa. The light transmittance was improved with increasing pressure and reached to the maximum transmittance of 64.5% at a wavelength of 0.2–1.6 μm owing to the fine-grained microstructure. The Vickers hardness exhibited 16.6 ± 0.7 GPa for the grain size of 74 nm, which is significantly higher than that of the sub-micro grains obtained at a conventional sintering pressure of 70 MPa (11.9 ± 0.8 GPa). The hardness rigorously followed the Hall–Petch relationship, that is, it is enhanced with a reduction of the grain size. Successful fabrication of the high-performance Y₂O₃–MgO nanocomposites indicates that the nanopowder processing followed by the high-pressure sintering process can be applied for fabricating fully dense fine-grained nanocomposites with excellent optical and mechanical properties.     

Excellent mechanical property of fast hot pressure sintering CaZr4(PO4)6 low thermal expansion ceramics

The low mechanical property of the CaZr₄(PO₄)₆ (CZP) ceramics restrict its application prospects in area of high anti-thermal shock applications. Herein, the fast hot pressure sintering (FHP) was adopted to modify the microstructure and optimize the mechanical property of CZP ceramics for the first time. The as-fabricated CZP ceramics exhibited smaller grain size and lower stomatal rates than that of the normal-pressure sintering ceramics owing to the lower processing temperature and shorter insulation time. Consequently, the CZP ceramics prepared with FHP method demonstrated an unprecedented bending strength of 94.5 MPa, with the thermal expansion coefficient of the CZP ceramics remaining at a low level. This study is important for promoting the application of CZP ceramics in flood anti-thermal shock applications, the fast hot pressure sintering would trigger a new direction of improvement methods of sodium zirconium phosphate ceramics.     

Preparation and characterizations of V2AlC-Al2O3 composite produced via spark plasma sintering

A 2:1:1 MAX phase, V₂AlC, composited with Al₂O₃ is prepared using spark plasma sintering (SPS). To obtain a sample with suitable physical and mechanical properties, an especial current rate regime is carried out for SPS. Different final temperatures of the synthesis (1000, 1200, 1400, and 1500°C) are considered (named S1000, S1200, S1400, and S1500, respectively). Phase analysis, physical and mechanical properties characterizations, and morphology of the samples are performed to determine the quality of the samples. The best characterizations belong to the samples of S1400 and S1500. Bending strengths about 325 and 316 MPa are obtained for the samples of S1400 and S1500, respectively. Regarding the total MAX phases formation and strength, the optimum sample is S1400. However, regarding bulk density, wear resistance, and hardness, S1500 has an advantage over the S1400. The advantages of S1500, as well as its larger grain size, are attributed to the better sintering of Al₂O₃.     

Thermoelectric properties of Cu2Sex prepared by solution phase methods and spark plasma sintering

Cu-Se compounds have attracted more attentions as a low-cost, low toxicity thermoelectric materials. In this work, p-type Cu₂Se_x compound powders were prepared by solution syntheses, including the wet chemistry method and hydrothermal synthesis method. The reaction mechanisms were investigated, showing that the NaOH contents affected the phase structure of the final product. Polycrystalline Cu₂Se_x bulk materials were obtained by densifying the powders using spark plasma sintering (SPS) and obtained the maximum ZT value of 0.75 at 623 K, which is currently the highest reported value for the Cu-Se system at this temperature. We also found that the introduced oxides could suppress the long-range migration of Cu ions in Cu-Se system to avoid the decomposition of the materials.     

Effects of powder preparation and sintering temperature on consolidation of ultrafine WC-8Co tool material produced by spark plasma sintering

In this study, ultrafine tool materials were produced by spark plasma sintering using three sets of WC-8Co nanopowders mixed by different methods. Effects of powder preparation method and sintering temperature on the consolidation of WC-8Co cemented carbides were investigated. At sintering temperature of 1250 °C, cemented carbide sintered from the powder mixed by ultrasonic vibration method exhibited homogeneous microstructure, high relative density (99.1%), small average grain size (280 nm), and excellent mechanical properties (H_V: 18.8 GPa, K_IC: 11.4 MPa⋅m^1/2). However, cemented carbide sintered from heavily ball-milled powder (ball milling for 24 h) showed increased grain coalescence and microdefects as well as lower relative density of 94.6%. Moreover, its hardness decreased to 17.7 GPa due to the decrease in relative density. Furthermore, straight cracks along grain boundary became dominant, causing fracture toughness to decrease to 10.5 MPa⋅m^1/2. Additionally, high sintering temperature caused grain coarsening, which was detrimental to mechanical properties of cemented carbides.     

Mechanical and tribological properties of SiC whisker-reinforced SiC composites via oscillatory pressure sintering

SiC whisker (SiCw)-reinforced SiC composites were prepared by an oscillatory pressure sintering (OPS) process, and the effects of SiCw content on the microstructure and mechanical and tribological properties of such composites were investigated. The addition of SiCw could promote the formation of long columnar α-SiC, and the aspect ratio of α-SiC grains first increased and then decreased with the increase of SiCw content. When the SiCw content was 5.42 wt%, the relative density of the SiC–SiCw composite reached up to 99.45%. The SiC–5.42 wt% SiCw composite possessed the highest Vickers hardness, fracture toughness, and flexural strength of 30.68 GPa, 6.66 MPa·m^1/2, and 733 MPa, respectively. In addition, the SiC–5.42 wt% SiCw composite exhibited the excellent wear resistance when rubbed with GCr15 steel balls, with a friction coefficient of .76 and a wear rate of 4.12 × 10⁻⁷ mm³·N⁻¹·m⁻¹. This could be ascribed to the improved mechanical properties of SiC–SiCw composites, which enhanced the ability to resist peeling and micro-cutting, thereby enhancing the tribological properties of the composites.     

Polycrystalline transparent magnesium aluminate spinel processed by a combination of spark plasma sintering (SPS) and hot isostatic pressing (HIP)

A two-stage processing approach combining spark plasma sintering (SPS) and hot isostatic pressing (HIP) was employed for the fabrication of relatively large (30?mm diameter) and thick (up to 8?mm) samples of transparent polycrystalline magnesium aluminate. The effects of sample thickness, heating rate during SPS, and the temperature and duration of HIP treatments were investigated. It was established that the heating rate during SPS had a major influence on discoloration due to carbon contamination, which increased with sample thickness. HIP treatment allowed for the elimination of cloudiness due to samples porosity, although carbon contamination present after the SPS step could not be reduced by HIP treatment, regardless of the temperature and duration applied. Highly transparent specimens with thicknesses of 4 and 8?mm exhibiting an in-line transmittance of 85.2 and 83.2% at 600?nm, respectively, were fabricated.     

Transparent LiOH-doped magnesium aluminate spinel produced by spark plasma sintering: Effects of heating rate and dopant concentration

The effects of LiOH doping of magnesium aluminate spinel powders and various Spark Plasma Sintering (SPS) schedules on densification behavior and final transparency of polycrystalline magnesium aluminate spinel were studied. Two commercial magnesium aluminate spinel powders, with different specific surface areas, were doped with up to 0.6 wt% of LiOH and consolidated using SPS with slow (2.75 °C/min) and fast (100 °C/min) heating rates. The slow heating rate was optimal for undoped magnesium aluminate spinel (LiOH-free) with the best real in-line transmittance (RIT) of 84.8% (measured at 633 nm on a disc 0.8 mm thick). For the magnesium aluminate spinel doped with 0.3 wt% of LiOH, the fast heating rate was beneficial, and an RIT of 76.5% was achieved. μ-Raman analysis confirmed that the addition of LiOH suppressed carbon contamination.     

Microstructure development and optical properties of Fe:ZnSe transparent ceramics sintered by spark plasma sintering

Fe:ZnSe transparent ceramics were prepared by spark plasma sintering. Fe:ZnSe powders synthesized via co-precipitation yielded well-dispersed particles with an average particle size of 550 nm. These powders were in the cubic phase Fe:ZnSe, indicating the successful substitution of Fe²⁺ for Zn²⁺. The highest relative density, 99.4%, was obtained by increasing the pressure and sintering time. The effects of sintering temperature, pressure, and time on the microstructure of SPS prepared ceramics were presented by micrographs. With increasing sintering temperature, from 600°C to 900°C, the average grain size increased from < 1 to 10 μm. The intergranular fracture indicated no neck formation in the sintering process. High pressure was essential for the densification process. The average grain size deceased from approximately 10 to 5 μm when the pressure was increased. Increasing the sintering time from 10 to 120 minutes lead to a change in the microstructure, from inter- to transgranular fracture, and eliminated the micropores. The as-prepared Fe:ZnSe ceramics were composed of single-phased cubic ZnSe. The sample sintered at 900°C under a pressure of 90 MPa for 120 minutes yielded a transmittance of approximately 60% at 1.4 μm and 68% at 7.5 μm and had residual micropores as its main scattering source. There was a strong characteristic absorption peak of Fe²⁺ ions at around 3 μm, which was red-shifted compared to Fe:ZnS transparent ceramics. Fe:ZnSe transparent ceramics have a reddish-brown color and it could be a promising mid-infrared laser material.