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

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

In-situ synthesis and densification of HfB2 ceramics by the spark plasma sintering technique

Hafnium diboride (HfB₂) ceramics were in-situ synthesized and densified by the spark plasma sintering (SPS) method using HfO₂ and amorphous boron (B) as starting powders. Both synthesis and densification processes were succesfully accomplished in a single SPS cycle with one/two step heating schedules, which were designed by considering thermodynamic calculations made by Factsage software. In two step heating schedule, soaking at 1000 °C, which was supposed to be the synthesis temperature of HfB₂ particles, caused a creep like behaviour in final ceramic microstructures. A single step synthesis/densification schedule at 2050 °C with a 30 min hold time under 60 MPa uniaxial pressure leads to obtain monolithic HfB₂ ceramics up to 94% of it's theoretical density. Considering the literature, low hardness values (max. 12 GPa) were achieved, which were directly attributed to the low bonding between HfB₂ grains in terms of the residual stresses occurred during the synthesis and cooling steps. Samples produced by applying one step heating schedule showed transgranural fracture behaviour with a, fracture toughness of 3.12 MPa m^1/2. The fracture toughness of the samples produced by applying two step heating schedule was higher (5,06 MPa m^1/2) and the fracture mode changed from transgranular to mixed mode.     

Synthesis,microstructure and property characterization of Mo4Y2Al3B6 ceramic fabricated by spark plasma sintering

Polycrystalline Mo₄Y₂Al₃B₆ ceramic (92.84 wt% Mo₄Y₂Al₃B₆ and 7.16 wt% MoB) was prepared by spark plasma sintering at 1250 ℃ under 30 MPa using Mo, Y, Al, and B as starting materials. The dense sample obtained has a high relative density of 96.6 %. The average thermal expansion coefficient is 8.38 × 10^?6 K^?1 in the range of 25–1000 ℃. The thermal diffusivity decreases from 6.50 mm²/s at 25 °C to 4.33 mm²/s at 800 °C. The heat capacity, thermal conductivity, and electrical conductivity are 0.30 J·g^?1·K^?1, 11.73 W·m^?1·K^?1, and 0.66 × 10⁶ Ω^?1·m^?1 at 25 °C, respectively. Vickers hardness with increasing load in the range of 10–300 N at room temperature decreases from 10.82 to 9.49 GPa, and the fracture toughness, compressive strength, and flexural strength are 5.14 MPa·m^1/2, 1255.14 MPa, and 384.82 MPa, respectively, showing the promising applications as structural-functional ceramics.     

Densification,microstructure, and mechanical properties of V-substituted HfB2-based ceramics

This study firstly developed Hf_1-xV_xB₂ (x = 0, 0.01, 0.02, 0.05) powders, which were derived from borothermal reduction of HfO₂ and V₂O₅ with boron. The results revealed that significantly refined Hf_1-xV_xB₂ powders (0.51 μm) could be obtained by solid solution of VB₂, and x ≥ 0.05 was a premise. However, as the content of V-substitution for Hf increased, Hf_1-xV_xB₂ ceramics sintered by spark plasma sintering at 2000 °C only displayed a slight densification improvement, which was attributed to the grain coarsening effect induced by the solid solution of VB₂. By incorporating 20 vol% SiC, fully dense Hf_1-xV_xB₂-SiC ceramics were successfully fabricated using the same sintering parameters. Compared with HfB₂-SiC ceramics, Hf_0.95V_0.05B₂-20 vol% SiC ceramics exhibited an elevated and comparable value of Vickers hardness (23.64 GPa), but lower fracture toughness (4.09 MPa m^1/2).     

Microstructure and magnetic properties of nickel-zinc ferrite ceramics fabricated by spark plasma sintering

Dense nickel-zinc (NiZn) ferrite ceramics were successfully fabricated within tens of seconds via spark plasma sintering. The phase composition and microstructure of the sintered samples were characterized by X-ray diffraction and scanning electron microscopy, respectively. The static magnetic properties at room temperature and Curie temperature of the samples were investigated by vibrating sample magnetometry. The results indicated that the main phase of the sintered samples was Ni_0.75Zn_0.25Fe₂O₄ with spinal structure, and the sintering temperature and heating rate observably affected the microstructure and density, then the magnetic properties of the sample. The Joule heat generated by NiZn ferrite during spark plasma sintering was very important for the rapid preparation of the sample with high density and small grain size. The low sintering temperature and heating rate would be helpful to obtain samples with small grain size, high density, and then good magnetic properties. The samples sintered at 900 °C with the heating rate of 5–10 °C/s were characterized of the relative density above 95%, 4πM_s value beyond 4000 Gs and coercivity below 27.7 Oe.     

Densification mechanisms and microstructural evolution during spark plasma sintering of boron carbide powders

Micron-sized boron carbide (B₄C) powders were subjected to spark plasma sintering (SPS) under temperature ranging from 1700 °C to 2100 °C for a soaking time of 5, 10 and 20 min and their densification kinetics was determined using a creep deformation model. The densification mechanism was interpreted on the basis of the stress exponent n and the apparent activation energy Q_d from Harrenius plots. Results showed that within the temperature range 1700–2000 °C, creep deformation which was controlled by grain-boundary sliding or by interface reaction contributed to the densification mechanism at low effective stress regime (n = 2,Q_d = 459.36 kJ/mol). While at temperature higher than 2000 °C or at high stress regime, the dominant mechanism appears to be the dislocation climb (n = 6.11).     

Densification and mechanical properties of boron carbide prepared via spark plasma sintering with cubic boron nitride as an additive

Hexagonal boron nitride (h-BN) can reinforce boron carbide (B₄C) ceramics, but homogeneous dispersion of h-BN is difficult to achieve using conventional methods. Herein, B₄C/h-BN composites were manufactured via the transformation of cubic (c-) BN during spark plasma sintering at 1800 °C. The effects of the c-BN content on the microstructure, densification, and mechanical properties of B₄C/h-BN composites were evaluated. In situ synthesized h-BN platelets were homogeneously dispersed in the B₄C matrix and the growth of B₄C grains was effectively suppressed. Moreover, the c-BN to h-BN phase transformation improved the sinterability of B₄C. The sample with 5 vol.% c-BN exhibited excellent integrated mechanical properties (hardness of 30.5 GPa, bending strength of 470 MPa, and fracture toughness of 3.84 MPa⋅ m^1/2). Higher c-BN contents did not significantly affect the bending strength and fracture toughness but clearly decreased the hardness. The main toughening mechanisms were crack deflection, crack bridging, and pulling out of h-BN.     

Influence of processing parameters on the densification and the microstructure of pure zinc oxide ceramics prepared by spark plasma sintering

Due to the sensitivity of nanopowders and the challenges in controlling the grain size and the density during the sintering of ceramics, a systematic study was proposed to evaluate the densification and the microstructure of ZnO ceramics using spark plasma sintering technique. Commercially available ZnO powder was dried and sintered at various parameters (temperature (400–900?°C), pressure (250–850?MPa), atmosphere (Air/Vacuum) etc.). High pressure sintering is desirable for maintaining the nanostructure, though it brings a difficulty in obtaining a fully dense ceramic. Whereas, increasing the temperature from 600 to 900?°C results in fully densified ceramics of about 99% which shows to have big impact on the grain size. However, a high relative density of 92% is obtained at a temperature as low as 400?°C under a pressure of 850?MPa. The application of pressure during the holding time seems to lower the grain size as compared to ceramics pressed during initial stage (room temperature).     

Densification, microstructure and mechanical properties of SiO2-cBN composites by spark plasma sintering

SiO₂-cBN composites were consolidated by spark plasma sintering at 1473-1973 K. The effects of cBN content and sintering temperature on the relative density, phase transformation, microstructure and mechanical properties of the SiO₂-cBN composites were investigated. The relative density of the SiO₂-cBN composites increased with increasing SiO₂ content. The phase transformation of cBN to hBN in SiO₂-cBN composites was identified at 1973 K, showing the highest transformation temperature in cBN-containing composites. The SiO₂-20 vol% cBN composites sintered at 1673 K showed the highest hardness and fracture toughness of 12.5 GPa and 1.5 MPa m^1/2, respectively.     

Polycrystalline infrared-transparent MgO fabricated by spark plasma sintering

In the present research, polycrystalline magnesium oxide (MgO) bodies were fabricated using spark plasma sintering (SPS) at different temperatures and times from MgO nanopowder. Microstructural development, densification, and optical properties were investigated during SPS. The critical pressure of plastic deformation of the MgO compacts during sintering was also analyzed. The results showed that the plastic deformation phenomenon had a profound effect on the grain size and optical properties. In addition, the optical properties and microstructure of MgO bodies were strongly dependent on sintering temperature and time. Full-dense infrared-transparent magnesium oxide with a relative density of 99.99% was prepared at 1200 °C for 5 min under the pressure of 80 MPa. The spark plasma sintered MgO demonstrated the highest infrared transmittance of 72% in the 3–7 μm wavelength range, which was comparable with the values reported for MgO single crystal.     

Influence of TiN dopant on microstructure of TiB2 ceramic sintered by spark plasma

In this study, the impact of TiN as a sintering aid on the relative density and microstructure of TiB₂ ceramic was investigated. Monolithic TiB₂ and TiB₂ doped with 5?wt% TiN were sintered at 1900?°C for 7?min dwell time under the pressure of 40?MPa by spark plasma. The addition of TiN affected the microstructure of TiB₂-based sample considerably depicting the finer grains in the as-sintered ceramic. X-ray diffraction evaluation indicated that no interaction occurred between the initial materials. However, detail investigation by the map analysis and energy dispersive spectroscopy results revealed the formation of in-situ nano-sized hBN secondary phase in the TiN-doped TiB₂. In addition, TiN played a remarkable role on increasing the relative density of TiN-doped TiB₂ ceramic producing a nearly fully dense ceramic with relative density of 99.9% in comparison with the monolithic ceramic having 96.7% relative density.     

Processing and mechanical properties of B4C-SiCw ceramics densified by spark plasma sintering

Homogenous distribution of whiskers in the ceramic matrix is difficult to be achieved. To solve this problem, B₄C-SiC_w powder mixtures were freeze dried from a slurry dispersed by cellulose nanofibrils (CellNF) in this work. Dense B₄C ceramics reinforced with various amounts of SiC_w up to 12 wt% were consolidated by spark plasma sintering (SPS) at 1800 °C for 10 min under 50 MPa. During this process, CellNF was converted into carbon nanostructures. As iron impurities exist in the starting B₄C and SiC_w powders, both thermodynamic calculations and microstructure observations suggest the dissolution and precipitation of SiC_w in the liquids composed of Fe-Si-B-C occurred during sintering. Although not all the SiC_w grains were kept in the final ceramics, B₄C-9 wt% SiC_w ceramics sintered at 1800 °C still exhibit excellent Vickers hardness (35.5 ± 0.8 GPa), flexural strength (560 ± 9 MPa) and fracture toughness (5.1 ± 0.2 MPa·m^1/2), possibly contributed by the high-density stacking faults and twins in their SiC grains, no matter in whisker or particulate forms.     

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

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

Sintering behavior and mechanical properties of spark plasma sintering SiO2-MgO ceramics

SiO₂-MgO ceramics containing different weight fractions (0, 0.5, 1, 2, and 4 wt%) of SiO₂ powder were prepared by mixing nano MgO powder, and the powder mixtures were densified by spark plasma sintering (SPS). The effect of SiO₂ addition and SPS method on the sintering behavior, microstructure and mechanical properties were investigated. Results were compared to specimens obtained by conventional hot pressing (HP) under a similar sintering schedule. The highest relative density, flexural strength and hardness of 2 wt% SiO₂-MgO ceramics reached 99.98%, 253.99 ± 7.47 MPa and 7.56 ± 0.21 GPa when sintered at 1400 °C by SPS, respectively. The observed improvement in the sintering behavior and mechanical properties are mainly attributed to grain boundary "strengthening" and intragranular "weakening" of the MgO matrix. Furthermore, the spark plasma sintering temperature could be decreased by more than 100 °C as compared with the HP method, SPS favouring enhanced grain boundary sliding, plastic deformation and diffusion in the sintering process.     

Influence of CeO2 addition on the microstructure and mechanical properties of Zirconia-toughened alumina (ZTA) composite prepared by spark plasma sintering

In this study, zirconia-toughened alumina (ZTA) samples with different amounts of CeO₂ were prepared by the spark plasma sintering method. The phase composition and microstructure of the samples were examined by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The addition of CeO₂ results in grain refinement and density increase; moreover, CeO₂ stabilises the high-temperature metastable phase. As the amount of CeO₂ reaches 7 wt%, a new CeAl₁₁O₁₈ phase appears. The Vickers hardness, modulus, and fracture toughness of the samples depend to a large extent on the grain size, relative density, and existence of the second phase. Among the composites, that with 5 wt% CeO₂ shows the best performance with the highest values of relative density, Vickers hardness, and fracture toughness: 96.51%, 1688 HV, and 9.91 MPa.√m, respectively.     

Fabrication of sub-micrometer MgO transparent ceramics by spark plasma sintering

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.     

Influence of spark plasma sintering conditions on microstructure,carbon contamination,and transmittance of CaF2 ceramics

Commercial CaF₂ powder was applied to fabricate transparent CaF₂ ceramics by spark plasma sintering under various sintering conditions. The low sintering temperatures and high pressures caused serious carbon contamination, while the soak time had less influence on the carbon concentration in the ceramics. The highest carbon contamination occurred to the CaF₂ ceramics sintered at 800 °C. A low sintering pressure suppressed carbon contamination but led to high porosity and large pore size. A high pre-loading pressure led to relatively high porosity and carbon concentration. Furthermore, the relatively fast densification in the edge region of the plates may cause the non-uniform distribution of porosity, thereby affecting the distribution of carbon concentration. The low pre-loading pressure and the high sintering pressure reduced porosity and carbon concentration to obtain dense transparent ceramic with uniform microstructure and high transmittance.     

Ultra-high elevated temperature strength of TiB2-based ceramics consolidated by spark plasma sintering

Spark plasma sintering of TiB₂–boron ceramics using commercially available raw powders is reported. The B₄C phase developed during reaction-driven consolidation at 1900 °C. The newly formed grains were located at the grain junctions and the triple point of TiB₂ grains, forming a covalent and stiff skeleton of B₄C. The flexural strength of the TiB₂–10 wt.% boron ceramic composites reached 910 MPa at room temperature and 1105 MPa at 1600 °С. Which is the highest strength reported for non-oxide ceramics at 1600 °C. This was followed by a rapid decrease at 1800 °C to 480–620 MPa, which was confirmed by increased number of cavitated titanium diboride grains observed after flexural strength tests.     

Monoclinic phase-free,low temperature spark plasma sintering of CeO2-doped ZrO2 ceramics and its associated benefits on mechanical properties

Consolidating a CeO₂-doped ZrO₂ ceramics, free from monoclinic phase using spark plasma sintering (SPS) is a major challenge faced by previous researchers; Ce⁺⁴ → Ce⁺³ conversion under reducing environments was assigned as the prime factor. We report dense (> 95 % of theoretical density) 20 mol. % CeO₂-doped ZrO₂ ceramics, free from monoclinic phase and any of micro/ macro-cracks via SPS. The sintering temperature (1175 ℃) used for the present work was the lowest compared to previous reports on the same system. Phase analysis revealed a mixture of tetragonal (major phase) and cubic phase (minor). No depletion of cerium (Ce) from the ZrO₂ matrix and no additional/impurity phases were noted after SPS; a common issue that has been observed in most of the previous works. Sintered ceramics showed appreciably high hardness (>11 GPa); the obtained toughness was in-between of tetragonal and cubic CeO₂-ZrO₂ ceramics.     

Electrical properties of reduced 3YTZP ceramics consolidated by spark plasma sintering

3 mol% Yttria doped zirconia ceramics were consolidated by spark plasma sintering (SPS) at two sintering temperatures with the aim of achieving two different reduction levels. Microstructural characterization of the ceramics was performed by scanning electron microscopy (SEM). Electrical properties were investigated by means of impedance spectroscopy from room temperature up to 500 °C. The two ceramics presented a remarkably different electrical behavior. The effect of the extra electrons introduced by reduction during SPS on both the bulk and the grain boundary conductivity was analyzed and discussed.