Ceramics

High strength transparent Y₂O₃ ceramics were fabricated from commercial powders using spark plasma sintering (SPS) technique by optimizing the heating rate. The heating rate significantly influenced the microstructures and the optical/mechanical properties of the Y₂O₃ ceramics. Grain growth was limited accordingly with increasing the heating rate. The ball milling process of the commercial Y₂O₃ powders is likely to further enhance the sinterability during the SPS processing. The dense Y₂O₃ ceramics, which were sintered by SPS with 100 °C/min, showed good transmittance range from visible to near infrared (IR). For a high heating rate of 100 °C/min, the in-line transmittance at a visible wavelength of 700 nm was 66%, whereas for a slow heating rate of 10 °C/min, it reduced to 46%. The hardness H_v tends to increase with increasing the heating rate and rigorously followed the Hall–Petch relationship; that is, it is enhanced with a reduction of the grain size. The toughness K_IC, on the other hand, is less sensitive to both the heating rate and the grain size, and takes a similar value. This research highlighted that the high heating rate SPS processing can fabricate fully dense fine-grained Y₂O₃ ceramics with the excellent optical and mechanical properties. Full article

Al₂O₃ + 30 vol% ZrO₂ matrix composites with 20 and 30 vol% cBN have been prepared with the optimized processing route, using spark plasma sintering (SPS) at temperatures of 1400 °C and 1250 °C. The influence of cBN addition on the microstructure characteristics, micro/nanohardness, elastic modulus, and crack-extension resistance of the composites and their constitutions have been investigated using scanning electron microscopy (SEM), statistical analyses of the individual grain size and micro/nanoindentation methods. The matrix consists of alumina and zirconia grains with grain sizes/diameter of approximately 220 and 160 nm with approximately 1.9 μm cBN grains in the Al₂O₃ + ZrO₂ + cBN composites. The microhardness is slightly increasing with cBN addition from 16.2 to 17.1 GPa and the crack-extension resistance from 3.72 to 4.29 MPa.m^1/2. The toughening mechanisms are in the form of crack deflection, crack branching, and crack bridging. The nanohardness and indentation modulus of the matrix are approximately 30 and 420 GPa, and the cBN grains 70 and 777 GPa, respectively. Full article

Spark plasma sintering (SPS) has gained recognition in the last 20 years for its rapid densification of hard-to-sinter conventional and advanced materials, including metals, ceramics, polymers, and composites. Herein, we describe the unconventional usages of the SPS technique developed in the field. The potential of various new modifications in the SPS technique, from pressureless to the integration of a novel gas quenching system to extrusion, has led to SPS’ evolution into a completely new manufacturing tool. The SPS technique’s modifications have broadened its usability from merely a densification tool to the fabrication of complex-shaped components, advanced functional materials, functionally gradient materials, interconnected materials, and porous filter materials for real-life applications. The broader application achieved by modification of the SPS technique can provide an alternative to conventional powder metallurgy methods as a scalable manufacturing process. The future challenges and opportunities in this emerging research field have also been identified and presented. Full article

The present work merges two key strategies for the manufacturing of advanced ceramics, in particular, the development of functionally graded materials (FGMs) and the addition of graphene-based fillers into a ceramic matrix. A silicon nitride/reduced graphene oxide FGM composite is produced, in one step, from a single powder composition using the spark plasma sintering (SPS) technique with an asymmetric setting of the punches and die to create a continuous temperature gradient along the cross section of the powder compact. A deep microstructural and mechanical characterization has been done across the specimen thickness. The FGM composite exhibits bottom-top gradients in both the matrix grain size (150% increase) and α-phase content (89→1%). The FGM bottom surface is 10% harder than the top one and, on the other hand, the latter is 15% tougher. The presence of reduced graphene oxide sheets homogeneously distributed within the ceramic composite reduces the mechanical gradients compared to the monolithic silicon nitride FGM, although allows reaching a maximum long-crack toughness value of 9.4 MPa·m^1/2. In addition, these graphene-based fillers turn the insulating ceramics into an electrical conductor material. Full article

Material from the electric arc furnace smelting of municipal solid waste incineration (MSWI) bottom ash was easily converted into highly porous glass-ceramics by a combination of inorganic gel casting and sinter-crystallization at 1000 °C. In particular, the gelation of aqueous suspensions of fine glass powders, transformed into “green” foams by intensive mechanical stirring, occurred with a limited addition of alkali activator (1 M NaOH). The products coupled the stabilization of pollutants with good mechanical properties (e.g., compressive strength approaching 4 MPa). Interestingly, they could be used also as raw material for new glass-ceramic foams, obtained by the same gel casting and sintering method, with no degradation of chemical stability. Limitations in the crushing strength, derived from the limited viscous flow densification of semi-crystalline powders, were overcome by mixing powders from recycled foams with 30 wt% soda-lime glass. The new products finally featured an even higher strength-to-density ratio than the foams from the first cycle. Full article

Magnesium diboride (MgB₂) ceramics, due to their impressive transition temperature of 39 K for superconductivity, have been widely investigated. The possibility to obtain highly dense MgB₂ ceramics with fine microstructure and grain boundaries acting as pinning sites by novel high-pressure-assisted spark plasma sintering (HP-SPS) is reported in this article. HP-SPS was employed to reach 100% density in MgB₂ ceramics, and high pressure was utilized in the consolidation of MgB₂. An increase in pressure helped in stabilizing the MgB₂ phase above thermal decomposition, thus avoiding the formation of non-superconducting phases such as MgO and MgB₄. Pressure allowed strengthening of the covalent bond (condensation effect) to increase the thermal stability of MgB₂. HP-SPS yielded high mechanical hardness in MgB₂ (1488 HV). For better electrical connectivity, which leads to large magnetic moments in high density samples were obtained with the beneficial effect of high applied pressure (1.7–5 GPa) at high temperature (>1000 °C). The combination of the SPS process and high pressure ensured retention of the homogeneous fine microstructure required to obtain high current density and high hardness. Full article

Sintering under pressure by means of the spark plasma sintering (SPS) technique is a common route to reduce the sintering temperature and to achieve ceramics with a fine-grained microstructure. In this work, high-density bulk TiO₂ was sintered by high pressure SPS. It is shown that by applying high pressure during the SPS process (76 to 400 MPa), densification and phase transition start at lower temperature and are accelerated. Thus, it is possible to dissociate the two densification steps (anatase then rutile) and the transition phase during the sintering cycle. Regardless of the applied pressure, grain growth occurs during the final stage of the sintering process. However, twinning of the grains induced by the phase transition is enhanced under high pressure resulting in a reduction in the crystallite size. Full article

Glass is a familiar material that requires abundant mineral sources, with devastating consequences for the environment. Rice husk ash (RHA) presents a very high silica content (>95%) and it can be a very promising alternative source for silica in silica-based glass. However, impurities like manganese and iron, which depend on the rice harvest, might limit RHA use, particularly in the production of optical transparent glasses. In this work, we discussed how Mn and Fe can affect the coloring of the produced glass, and how the effect of these impurities can be removed. First, the RHA was treated with acid solutions, leading to the production of a soda-lime glass with similar transparency to commercial glass (>70%). Secondly, another simpler approach was studied: a small amount of antimony oxide was added in the composition of the glass, obtaining a transparent glass (>80%, same thickness) with RHA. Full article

Summarizing the work of nearly a decade of research on spark plasma sintering (SPS), a review is given on the specificities and key factors to be considered in SPS of ceramic materials, based on the authors’ own research. Alumina is used primarily as a model material throughout the review. Intrinsic inhomogeneities linked to SPS and operational parameters, which depend on the generation of atomistic scale defects, are discussed in detail to explain regularly observed inhomogeneities reported in literature. Adopting an engineering approach to overcome these inherent issues, a successful processing path is laid out towards the mastering of SPS in a wide range of research and industrial settings. Full article

Piezoelectric thick films are of real interest for devices such as ceramic Micro-ElectroMechanical Systems (MEMS) because they bridge the gap between thin films and bulk ceramics. The basic design of MEMS includes electrodes, a functional material, and a substrate, and efforts are currently focused on simplified processes. In this respect, screen-printing combined with a sacrificial layer approach is attractive due to its low cost and the wide range of targeted materials. Both the role and the nature of the sacrificial layer, usually a carbon or mineral type, depend on the process and the final device. First, a sacrificial layer method dedicated to screen-printed thick-film ceramic and LTCC MEMS is presented. Second, the recent processing of piezoelectric thick-film ceramic MEMS using spark plasma sintering combined with a protective layer approach is introduced. Whatever the approach, the focus is on the interdependent effects of the microstructure, chemistry, and strain/stress, which need to be controlled to ensure reliable and performant properties of the multilayer electroceramics. Here the goal is to highlight the benefits and the large perspectives of using sacrificial/protective layers, with an emphasis on the pros and cons of such a strategy when targeting a complex piezoelectric MEMS design. Full article

Yttria stabilized zirconia (Y-TZP) is frequently used in dental and engineering applications due to its high strength and fracture resistance. In this study, 2Y-TZP samples were manufactured from commercially available powder produced by detonation synthesis. Tests of the mechanical properties exhibited an unusual combination of both very high strength and toughness. The materials show a very weak correlation between toughness and grain size. The transformability, measurable by XRD, cannot explain the high toughness. Fractographic analysis revealed a broad transformation affected zone with secondary cracks and shear bands on the tensile side of bending bars which can be made responsible for the high toughness and non-linear stress–strain curves. Full article

The technology of multi-wire sawing is well established in the production of silicon wafers but can also be applied in the production of ceramic substrates. In this study, the influence of the Al₂O₃-grain size of the alumina ceramic on the efficiency of the multi-wire slurry process was investigated. The grain size of HIPed alumina ceramics was changed by heat treatment processes at 1350 °C and 1400 °C. A B₄C slurry was used for the investigation of the cutting of high purity alumina ceramic. With increasing grain size of the ceramic, the efficiency of the sawing process increases. The analysis of the as-cut surface morphology of the substrates shows a change in material removal from trans- to intergranular micro-fracture with increasing grain size. Furthermore, grain coarsening leads to substrates with increased roughness values and reduced biaxial strength. Full article

A commercially available methylphenylvinylhydrogen polysiloxane preceramic polymer was cross-linked and pyrolyzed in argon in order to study if this compound could be used as an organic precursor for the manufacture of silicon carbide parts by photopolymerisation-based three-dimensional printing. X-Ray diffraction experiments and transmission electron microscopy observations showed that the pyrolyzed material was constituted by an interconnected polycrystalline network made of faulted/twinned β-SiC grains surrounded by an amorphous and porous second phase mainly constituted (at least 80 at%) of carbon. The free carbon residual content was estimated to be around 17.2 wt%. Additional efforts are then required to be able to use such a preceramic polymer to manufacture SiC parts by photopolymerisation-based three-dimensional printing. Full article

Fly ash or coal fly ash causes major global pollution in the form of solid waste and is classified as a “hazardous waste”, which is a by-product of thermal power plants produced during electricity production. Si, Al, Fe Ca, and Mg alone form more than 85% of the chemical compounds and glasses of most fly ashes. Fly ash has a chemical composition of 70–90%, as well as glasses of ferrous, alumina, silica, and CaO. Therefore, fly ash could act as a reliable and alternative source for ferrous, alumina, and silica. The ferrous fractions can be recovered by a simple magnetic separation method, while alumina and silica can be extracted by chemical or biological approaches. Alumina extraction is possible using both alkali- and acid-based methods, while silica is extracted by strong alkali, such as NaOH. Chemical extraction has a higher yield than the biological approaches, but the bio-based approaches are more environmentally friendly. Fly ash can also be used for the synthesis of zeolites by NaOH treatment of variable types, as fly ash is rich in alumino-silicates. The present review work deals with the recent advances in the field of the recovery and synthesis of ferrous, alumina, and silica micro and nanoparticles from fly ash. Full article

Refractory materials are subjected to thermomechanical loads during their working life, and consequent creep strain and stress relaxation are often observed. In this work, the asymmetric high temperature primary and secondary creep behavior of a material used in the working lining of steel ladles is characterized, using uniaxial tension and compression creep tests and an inverse identification procedure to calculate the parameters of a Norton-Bailey based law. The experimental creep curves are presented, as well as the curves resulting from the identified parameters, and a statistical analysis is made to evaluate the confidence of the results. Full article

Ultra-high temperature ceramic composites have been widely investigated due to their improved sinterability and superior mechanical properties compared to monolithic ceramics. In this work, high-entropy boron-carbide ceramic/SiC composites with different SiC content were synthesized from multicomponent carbides HfC, Mo₂C, TaC, TiC, B₄C, and SiC in spark plasma sintering (SPS) from 1600 °C to 2000 °C. It was found that the SiC addition tailors the phase formation and mechanical properties of the high-entropy ceramic (HEC) composites. The microhardness and fracture toughness of the HEC composites sintered at 2000 °C were improved from 20.3 GPa and 3.14 MPa·m^1/2 to 26.9 GPa and 5.95 MPa·m^1/2, with increasing SiC content from HEC-(SiC)0 (0 vol. %) to HEC-(SiC)3.0 (37 vol. %). The addition of SiC (37 vol. %) to the carbide precursors resulted in the formation of two high-entropy ceramic phases with two different crystal structures, face-centered cubic (FCC) structure, and hexagonal structure. The volume fraction ratio between the hexagonal and FCC high-entropy phases increased from 0.36 to 0.76 when SiC volume fraction was increased in the composites from HEC-(SiC)0 to HEC-(SiC)3.0, suggesting the stabilization of the hexagonal high-entropy phase over the FCC phase with SiC addition. Full article

In this study, 5Y-PSZ-based ceramics with 15 mol.% of manganese oxide were obtained from PSZ + MnO₂ powders mixtures by pressing and direct firing. The resulting materials show a stable cubic fluorite structure with only minor traces of segregated manganese oxides and relative density from 90% to 98%. The linear thermal expansion coefficient is in the order of 10⁻⁵ K⁻¹ at 500 K and increases gradually with temperature, due to the onset of a contribution of chemical expansion, reaching about 13 × 10⁻⁶ K⁻¹ at 1100 K. These results are suitable for prospective applicability as buffer layers to minimize degradation and delamination of electrolyte/oxygen electrode interfaces in solid electrolyte cells. The electrical conductivity remains close to 1 S/m at 973 K and close to 7 S/m at 1273 K, suggesting mixed conductivity with a prospective contribution to electrode processes occurring at electrode/electrolyte interfaces. Guidelines for further improvement were also established by a detailed analysis of the impact of heating/cooling rate, firing temperature, and time on those properties, based on Taguchi planning. Full article