Gel-casting of transparent magnesium aluminate spinel ceramics fabricated by spark plasma sintering (SPS)

In this paper, a transparent magnesium aluminate spinel ceramic was fabricated through the newest colloidal gel casting method, using a synthetic powder with the average particle size of 90 nm and Isobutylene-Maleic Anhydride (ISOBAM) additive. ISOBAM served as both a dispersant and a gelation agent to achieve a dense body. Also, the suspension rheological behavior was optimized by the solid loading of 85 wt%, the additive content of 0.7 wt%, and the gelation time of 350 s. This led to a green body with a density equal to 65% of theoretical density and the green strength of 14.48 MPa. The results revealed that the reduction of porosity and the uniform distribution of pores in the green body (smaller than half of the initial powder particle size, 35 nm), as accompanied by spark plasma sintering (SPS), resulted in the final body density of 99.97%, as well as the high in-line transmittance of 86.7% at the wavelength of 1100 nm.     

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.     

Effect of sintering temperature on the structural and magnetic properties of MgFe2O4 ceramics prepared by spark plasma sintering

Spark plasma sintering (SPS) is a powerful technique to produce fine grain dense ferrite at low temperature. This work was undertaken to study the effect of sintering temperature on the densification, microstructures and magnetic properties of magnesium ferrite (MgFe₂O₄). MgFe₂O₄ nanoparticles were synthesized via sol–gel self-combustion method. The powders were pressed into pellets which were sintered by spark plasma sintering at 700–900 °C for 5 min under 40 MPa. A densification of 95% of the theoretical density of Mg ferrite was achieved in the spark plasma sintered (SPSed) ceramics. The density, grain size and saturation magnetization of SPSed ceramics were found to increase with an increase in sintering temperature. Infrared (IR) spectra exhibit two important vibration bands of tetrahedral and octahedral metal-oxygen sites. The investigations of microstructures and magnetic properties reveal that the unique sintering mechanism in the SPS process is responsible for the enhancement of magnetic properties of SPSed compacts.     

Mechanical properties and microstructures of TiCN/nano-TiB2/TiN cermets prepared by spark plasma sintering

The effects of TiN and nano-TiB₂ additions to titanium carbonitride (TiCN-WC-Cr₃C₂-Co)-based cermets processed by spark plasma sintering (SPS) are identified. The TiN and nano-TiB₂ additions were varied from 0 to 15?wt% to ascertain their combined effects on the mechanical properties. Scanning electron microscopy (SEM) revealed the combined chemical composition of the new phases formed during sintering. The hardness and fracture toughness values were recorded. Increase in the fracture toughness value with TiN addition was more compared with the nano-TiB₂ addition. In contrast, the hardness values were higher for the cermets formed with the nano-TiB₂ addition. Sintered bodies were made as tool inserts that meet SNGN120408 standard tool configuration. Using these tools, EN24 work-piece was turned at different cutting speeds of 11.87, 29.68, 71.46, 163.88?m/min under conditions of dry cutting. The performance was evaluated. Cutting force as well as surface roughness of the work-piece after machining was measured. For all cutting tools, initially the cutting force was high but it tended to decrease at higher cutting speeds. In addition, for all the tools, at higher cutting tools the surface roughness values were uniformly minimal. The cermet with a composition 55TiCN-15WC-10Co-5Cr₃C₂–15nanoTiB₂ (all in wt%), in particular, showed a balanced enhancement in both fracture toughness (6.8?MPa?m^1/2) and Vickers hardness (18?GPa) values. The surface finish of the work-piece was also the best after machining when a tool of the above composition was used.     

Chemical and microstructural characterisation of HfO2-Y2O3 ceramics with high amount of Y2O3 (33, 40 and 50 mol. %) manufactured using spark plasma sintering

Hafnia based ceramics are potential promising candidates to be used as thermal barrier coatings (TBC) for applications in the field of propulsion. In this study, Spark Plasma Sintering (SPS) of fully stabilised hafnia with yttrium oxide (yttria) was investigated to provide a better understanding of the effect of manufacturing parameters, on the crystallography, chemistry and microstructure of the material. Several hafnia powders, containing different amounts of yttria (33 mol. %, 40 mol. % or 50 mol. %), were sintered by SPS at different temperature levels ranging from 1600 °C to 1850 °C. On these materials, X-ray diffraction patterns associated with scanning electron micrographs have highlighted the influence of both the sintering temperature and the amount of yttria on the final composition, the lattice parameter and the microstructure of hafnia-based materials. In the end, it is established that, for all quantities of yttrium employed, the main phase is Y₂Hf₂O₇ with very high densification levels.     

The effect of slip casting and spark plasma sintering (SPS) temperature on the transparency of MgAl2O4 spinel

MgAl₂O₄ bulk samples were fabricated by two different approaches to investigate the effect of slip casting and sintering temperature on their transparency. Three MgAl₂O₄ samples containing 1 wt% LiF, as the sintering aid, were prepared by the spark plasma sintering process (SPS) at 1400 °C and 1500 °C, under 100 MPa, for 15 min. Also, another MgAl₂O₄ sample was prepared by slip casting followed by SPS under similar conditions. It was observed that utilizing slip casting led to more transparency (10% in the visible region and 20% in the IR region) due to the more homogeneous structure. It was also observed that by reducing the SPS temperature from 1500 °C to 1400 °C, the transparency increased (20% in the IR region) because of the lower grain growth rate at the lower temperature.     

High-temperature oxidation behavior of monolithic Zr3[Al(Si)]4C6 and its composite incorporating 30vol% ZrB2-SiC: Providing a basis for broadening the service temperature

To provide a basis for the high-temperature oxidation of ultra-high temperature ceramics (UHTCs), the oxidation behavior of Zr₃[Al(Si)]₄C₆ and a novel Zr₃[Al(Si)]₄C₆-ZrB₂-SiC composite at 1500 °C were investigated for the first time. From the calculation results, the oxidation kinetics of the two specimens follow the oxidation dynamic parabolic law. Zr₃[Al(Si)]₄C₆ exhibited a thinner oxide scale and lower oxidation rate than those of the composite under the same conditions. The oxide scale of Zr₃[Al(Si)]₄C₆ exhibited a two-layer structure, while that of the composite exhibited a three-layer structure. Owing to the volatilization of B₂O₃ and the active oxidation of SiC, a porous oxide layer formed in the oxide scale of the composite, resulting in the degradation of its oxidation performance. Furthermore, the cracks and defects in the oxide scale of the composite indicate that the reliability of the oxide scale was poor. The results support the service temperature of the obtained ceramics.