Densification,microstructure and properties of ultra-high temperature HfB2 ceramics by the spark plasma sintering without any additives

Ultra-high temperature HfB₂ ceramic with nearly full densification is achieved by using gradient sintering process of SPS without any additives. The effect of the sintering temperature on the densification behavior, relative density, microstructure, mechanical and thermionic properties is systematically investigated. The results show that the fast densification of HfB₂ ceramic occurs at the heating stage, and the highest relative density of 96.75% is obtained at T =1950 °C, P = 60 MPa and t =10min. As the temperature is increased from 1800 to 1950 °C, the grain size of HfB₂ increases from 6.12 ±1.33 to 10.99 ± 2.25 μm, and refined microstructure gives the excellently mechanical properties. The highest hardness of 26.34 ±2.1GPa, fracture toughness of 7.12 ± 1.33 MPa m^1/2 and bending strength of 501 ±10MPa belong to the HfB₂ ceramic obtained at T =1950°C. Moreover, both the Vickers hardness and fracture toughness obey the normal indentation size effect. HfB₂ ceramic also exhibits the thermionic emission characterization with the highest current density of 6.12 A/cm² and the lowest work function of 2.92 eV.     

Microstructure and properties of Al2O3-TiC-Ti3SiC2 composites fabricated by spark plasma sintering

Abstract

Abstract

Highly densified Al₂O₃-TiC-Ti₃SiC₂ composites were fabricated by spark plasma sintering technique and subsequently characterised. From fracture surface observation, it is found that Al₂O₃ is 0·2-0·4?μm, TiC is 1-1·5?μm and Ti₃SiC₂ is 1·5-5?μm in grain size. With the increase in Ti₃SiC₂ volume contents, Vickers hardness of the composites decreases because of the low hardness of monolithic Ti₃SiC₂. The fracture toughness rises remarkably when the contents of Ti₃SiC₂ increase, which is attributed to the pullout and microplastic deformation of Ti₃SiC₂ grains. At the same time, the flexural strength of the composites shows a considerable improvement as well. The electrical conductivity rises significantly as the Ti₃SiC₂ contents increase because of the formation of Ti₃SiC₂ network and the increase in conductive phase contents.     

Synthesis,microstructure and properties of ternary layered i-MAX (Mo2/3M1/3)2AlC (M = Sc and Lu) ceramics fabricated by spark plasma sintering

Recently, a new i-MAX phase with in-plane chemical order has been discovered. In this study, the i-MAX phase (Mo_2/3Sc_1/3)₂AlC and (Mo_2/3Lu_1/3)₂AlC ceramics were synthesized by spark plasma sintering (SPS) with purity up to 96.45 wt% and 95.46 wt%, respectively. Relative densities were up to 98.29 % (M = Sc) and 98.23 % (M = Lu). Microstructure, physical and mechanical properties of (Mo_2/3M_1/3)₂AlC (M = Sc and Lu) ceramics were systematically investigated. Grain sizes of (Mo_2/3M_1/3)₂AlC (M = Sc and Lu) were also counted. The measured coefficients of thermal expansion (CTE) were 8.95 × 10⁻⁶ K⁻¹ (M = Sc) and 8.97 × 10⁻⁶ K⁻¹ (M = Lu), from 25 °C to 1200 °C. Regarding mechanical properties, the Vickers hardness of (Mo_2/3Lu_1/3)₂AlC was much higher than that of (Mo_2/3Sc_1/3)₂AlC, while the differences in flexural strength, fracture toughness and compressive strength were small. It indicates that both ceramics have good application prospects.     

Microstructure and mechanical properties of B4C based ceramics with Fe3Al as sintering aid by spark plasma sintering

B₄C based ceramics were fabricated with different Fe₃Al contents as sintering aids by spark plasma sintering at relatively low temperature (1700 °C) in vacuum by applying 50 MPa pressure and held at 1700 °C for 5 min. The effect of Fe₃Al additions (from 0 to 9 wt%) on the microstructure and mechanical properties of B₄C has been studied. The composition and microstructure of as-prepared samples were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalyzer (EPMA) equipped with WDS (wavelength dispersive spectrometry) and EDS. The mixtures of B₄C and Fe₃Al underwent a major reaction in which the metal borides and B₄C were encountered as major crystallographic phases. The sample with 7 wt% of Fe₃Al as a sintering aid was found to have 32.46 GPa Vickers hardness, 483.40 MPa flexural strength, and 4.1 MPa m^1/2 fracture toughness which is higher than that of pure B₄C.     

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.     

放电等离子烧结合成单相MgAlON材料

以氮化铝、富铝镁铝尖晶石和氧化铝为原料 ,用放电等离子烧结 (SPS)技术合成了单相MgAlON,研究了其显微结构 ,并与用传统的无压烧结 (PLS)技术制备的单相MgAlON材料在显微结构和断裂行为上做了比较。结果表明 :用SPS法在 170 0℃保温 1min的条件下合成出的单相MgAlON材料 ,显微结构比用PLS法合成的更加均匀致密 ,且晶粒细小 ;前者的断裂模式主要是穿晶断裂 ,后者的断裂模式则主要是沿晶断裂。     

Study on carbon contamination and carboxylate group formation in Y2O3-MgO nanocomposites fabricated by spark plasma sintering

For Y₂O₃-MgO nanocomposites fabricated by spark plasma sintering (SPS), the strong absorption peaks, which are assigned to the stretching vibrations of carboxylate group (OCO), are commonly observed in IR spectra. These absorption peaks can negatively affect military applications such as IR windows or domes. In order to suppress the formation of these absorption peaks, therefore, it is important to understand how the carboxylate group in the SPSed Y₂O₃-MgO nanocomposite originated. In this study, it is demonstrated that the formation of the carboxylate group is related to the carbon contamination during SPS processing. The carbon phase transforms into CO₂ gas by annealing and this leads to a reaction with metal oxide phases, resulting in the formation of carbonate phase. This carbonate phase contributes to the absorption peaks of the carboxylate group.     

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.     

Coarse-grain CeO2 doped ZrO2 ceramic prepared by spark plasma sintering

In the present work, coarse grain cerium stabilized zirconia bulk ceramic was prepared by spark plasma sintering technique. The relatively high temperature of 2000 °C used for sintering led to enormous grain growth up to approximately 100 μm. Sintering at high temperatures and in the vacuum caused oxygen depletion and thus transformation from tetragonal to cubic phase during the sintering process. The tetragonal phase was recovered by annealing at 1400 °C in air. This led to a change in fracture behavior. Mostly transgranular fracture of the cubic phase was changed to intergranular fracture after recovering the tetragonal phase. On the intergranular fracture surface, twinning-like structure and structures similar to antiphase domain were observed.Mechanical properties represented by indentation hardness of prepared samples were evaluated.     

Investigation of the density and microstructure homogeneity of square-shaped B4C-ZrB2 composites produced by spark plasma sintering method

Square-shaped monolithic B₄C and B₄C-ZrB₂ composites were produced by spark plasma sintering (SPS) method to investigate the effect of 5, 10, 15 vol% ZrB₂ addition on the densification, mechanical and microstructural properties of boron carbide. The relative density of B₄C increased with the increasing volume fraction of ZrB₂ and density differences in different regions of the sample narrowed down. Homogeneous density distribution and microstructure were accomplished with the increasing holding time from 7 to 20 min for the B₄C-15 vol% ZrB₂ composites, and the highest overall relative density was achieved as 99.23%. The hardness and fracture toughness of composites were enhanced with the addition of ZrB₂ compared to monolithic B₄C. The enhancement in fracture toughness was observed due to the crack deflection, crack bridging and crack branching mechanisms. The B₄C-15 vol% ZrB₂ composite exhibited the combination of superior properties (hardness of 33.08 GPa, Vickers indentation fracture toughness of 3.82 MPa.m^1/2).     

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.     

Enhanced electromagnetic shielding property of cf/mullite composites fabricated by spark plasma sintering

Aiming to prepare high-performance electromagnetic interference (EMI) shielding materials, chopped carbon fibers were incorporated into mullite ceramic matrix via rapid prototyping process of spark plasma sintering (SPS). Results indicate that C_f/mullite composites with only 1 wt% of carbon fibers exhibit highest shielding effectiveness (SE_T) over 40 dB at a small thickness of 2.0 mm, showing great advantages both in terms of performance and thickness compared with many mature carbon/ceramic composites. The high EMI shielding properties mainly depend on two mechanisms of absorption and reflection in this present work. The enhanced absorption and reflection of electromagnetic wave are ascribed to the promotional electrical conductivity arising from the formation of conductive network by introduction of carbon fibers. Regarding enhanced electrical conductivity, notable intensified interfacial polarization on a large number of interfaces between mullite matrix and carbon fibers is also the key factor to the improved absorption, which makes absorption play a dominant role in the significant improvement of EMI SE_T. The C_f/mullite composites with excellent EMI shielding properties and thin thickness show great potential application as EMI materials.     

An optimized method for synthesizing phase-pure Ti3AlC2 MAX-phase through spark plasma sintering

So far many attempts have been made to synthesize phase-pure Ti₃AlC₂ MAX-phase. But still the challenge posed by the presence of TiC and Ti-Al based intermetallic transient impurity phases in the final product is a persisting problem. Spark plasma sintering (SPS) technique has been the most successful method to decrease the impurity content of the final product. Even so, synthesis of phase-pure Ti₃AlC₂ MAX-phase, without any TiC and Ti-Al based intermetallic impurities, has not been achieved and reported in literature with substantial evidences. Further, high purity Ti₃AlC₂ MAX-phase synthesized using SPS technique has shown lack of phase and microstructural stability above 1350°C temperature. In this work, we have reported an optimized method for producing phase-pure Ti₃AlC₂ MAX-phase (having more than 99 % purity) using commercial grade Ti, Al and C elemental powders through SPS technique. The final product also showed very good high temperature stability up to 1500°C under flowing Argon inert atmosphere.     

Preceramic paper-derived Ti3Al(Si)C2-based composites obtained by spark plasma sintering

The paper describes the structure and properties of preceramic paper-derived Ti₃Al(Si)C₂-based composites fabricated by spark plasma sintering. The effect of sintering temperature and pressure on microstructure and mechanical properties of the composites was studied. The microstructure and phase composition were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. It was found that at 1150 °C the sintering of materials with the MAX-phase content above 84 vol% leads to nearly dense composites. The partial decomposition of the Ti₃Al(Si)C₂ phase becomes stronger with the temperature increase from 1150 to 1350 °C. In this case, composite materials with more than 20 vol% of TiC were obtained. The paper-derived Ti₃Al(Si)C₂-based composites with the flexural strength > 900 MPa and fracture toughness of >5 MPa m^1/2 were sintered at 1150 °C. The high values of flexural strength were attributed to fine microstructure and strengthening effect by secondary TiC and Al₂O₃ phases. The flexural strength and fracture toughness decrease with increase of the sintering temperature that is caused by phase composition and porosity of the composites. The hardness of composites increases from ~9.7 GPa (at 1150 °C) to ~11.2 GPa (at 1350 °C) due to higher content of TiC and Al₂O₃ phases.     

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.     

Single-step,high pressure,and two-step spark plasma sintering of UO2 nanopowders

Three different spark plasma sintering (SPS) treatments were applied to highly sinteractive, near-stoichiometric UO_2.04 nanocrystalline (5 nm) powders produced by U(IV) oxalate hydrothermal decomposition at 170 °C. The sintering conditions for reaching 95 % theoretical density (TD) in regular SPS, high pressure SPS (HP-SPS), and, for the first time, two-step SPS (2S-SPS), were determined. Densification to 95 % TD was achieved at 1000 °C in regular SPS (70 MPa applied pressure), 660 °C in HP-SPS (500 MPa), and 650?550 °C in 2S-SPS (70 MPa). With the goal of minimising the grain growth during densification, the sintering treatments were optimised to favour densification over coarsening, and the final microstructures thus obtained are compared. Equally dense UO₂ samples of different grain sizes, ranging from 3.08 μm to 163 nm, were produced. Room-temperature oxidation of the powders could not be avoided due to their nanometric dimensions, and a final annealing treatment was designed to reduce hyperstoichiometric samples to UO_2.00.     

Structural properties and mechanical behavior of SWCNTs and MWCNTs reinforced Al2O3 fabricated by spark plasma sintering

Carbon nanotubes due to their structural and mechanical properties are good candidates as the second phase to improve the mechanical properties of alumina-based ceramics. In the present study, the effects of single wall and multi-wall carbon nanotubes on structural and mechanical properties of alumina were investigated. SWCNTs and MWCNTs were dispersed in alumina powder via a conventional method using 1 wt % PVA water solution as media. Sintering process for two different composite powders, alumina-2 wt. % SWCNTs and alumina 2 wt % MWCNTs was performed by spark plasma sintering technique at 1500 °C and 20 MPa for 10 min. Results showed that the presence of CNTs in alumina caused a considerable amount of porosity in final bodies. SEM images of fracture surfaces revealed the agglomeration of SWCNTs which played a dominant role in the deterioration of mechanical properties. MWCNTs reinforced alumina obtained higher Vickers hardness and bending strength values (12.91 GPa and 291 MPa, respectively) compared to that of SWCNTs (9.18 GPa and 276 MPa, respectively), due to sever agglomerate of SWCNTs throughout sintered composites. Typical load-displacement (P/h) curves were obtained from bending strength test and discussed. It was concluded that the addition of MWCNTs to alumina represented better densification and mechanical properties compared to SWCNTs.     

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.     

Synthesis and characterization of ternary layered i-MAX (Mo2/3Y1/3)2AlC ceramics fabricated by spark plasma sintering

In this paper, the i-MAX phase (Mo_2/3Y_1/3)₂AlC ceramic with high purity of 98.29 wt% (1.13 wt% Y₂O₃ and 0.58 wt% Mo₂C) and high relative density of 98.59% was successfully synthesized by spark plasma sintering (SPS) at 1500°C with the molar ratio of n(Mo):n(Y):n(Al):n(C) = 4:2:3.3:2.7. The positions of C atoms in the crystal of (Mo_2/3Y_1/3)₂AlC were determined. Microstructure and physical and mechanical properties of (Mo_2/3Y_1/3)₂AlC ceramic were systematically investigated. It was found that the obtained (Mo_2/3Y_1/3)₂AlC ceramic had an average grain size of 32.1 ± 3.1 μm in length and 14.2 ± 1.7 μm in width. In terms of physical properties, the measured thermal expansion coefficient (TEC) of (Mo_2/3Y_1/3)₂AlC was 8.99 × 10⁻⁶ K⁻¹, and the thermal capacity and thermal conductivity at room temperature were 0.43 J·g⁻¹·K⁻¹ and 13.75 W·m⁻¹·K⁻¹, respectively. The room temperature electrical conductivity of (Mo_2/3Y_1/3)₂AlC ceramic was measured to be 1.25 × 10⁶ Ω⁻¹·m⁻¹. In terms of mechanical properties, Vickers hardness under 10 N load was measured as 10.54 ± 0.29 GPa, while flexural strength, fracture toughness, and compressive strength were determined as 260.08 ± 14.18 MPa, 4.51 ± 0.70 MPa·m^1/2, and 855 ± 62 MPa, respectively, indicating the promising structural applications.