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This work addresses the challenges in brazing thermohydraulic sealings involving dissimilar materials, specifically 316Ti stainless steel and alumina, which have significantly different coefficients of thermal expansion (CTEs) and elastic constants. Traditional brazing methods require complex interlayers or extensive metallization steps, leading to issues such as dimensional changes, braze voids, and inadequate corrosion resistance due to prolonged high‐temperature exposure. A novel laser‐based brazing technique utilizing a diode laser is introduced to create high‐quality, localized joints while preserving the integrity of the parent materials. The study systematically optimizes laser process parameters using finite‐element modeling and the Taguchi method to achieve the desired bead geometry and thermal stress distribution with minimal heat input. Comparative analysis between laser active brazing (LAB) and conventional furnace brazing was conducted through metallography, shear testing, and autoclave testing. Results indicate that LAB parameters significantly affect bead thickness and shear strength, with laser‐brazed joints demonstrating superior quality and stability post‐autoclave testing compared with furnace‐brazed joints. The thermodynamic and kinetic aspects of the brazing process were also analyzed. In conclusion, the LAB method for brazing 316Ti to alumina proves to be a successful and efficient alternative, with joint properties meeting or exceeding those of traditional furnace brazing. [ABSTRACT FROM AUTHOR]

Spark plasma sintering (SPS) is one of the most promising technologies for producing polycrystalline transparent ceramics. However, it has limitations regarding the complexity of the geometries that can be produced. This study addresses these limitations by combining additive manufacturing techniques with spark plasma sintering. Additionally, a Multiphysics sintering model based on the continuum theory of sintering is employed to predict the outcomes of the SPS process, particularly the microstructure of the densified parts. As a result, the geometrically complex Al 2 O 3 transparent parts manufactured in this study exhibit a uniform microstructure, high density (∼99 %), and a linear transmittance of 16 % at 490 μm. [ABSTRACT FROM AUTHOR]

Vat photopolymerization (VPP) printing of ceramic parts offers advantages such as low cost, simple operation, and short fabrication cycles. However, drawbacks include low toughness and brittleness in the printed parts. This study explores enhancing the toughness and strength of alumina (Al2O3) ceramics by incorporating silicon carbide (SiC) particles as additives. The impact of varying SiC contents on the quality of VPP‐printed Al2O3 parts is examined, encompassing microstructure, physical properties, and mechanical properties. Results indicate that optimal SiC addition reduces Al2O3 ceramics' porosity, enhances crystalline quality, and boosts mechanical properties. Excessive SiC, however, diminishes these benefits. The most significant strengthening of Al2O3 parts occurred with a 1.5 wt.% SiC content, increasing bending strength and fracture toughness by 239.7% and 564.7%, respectively. This underscore SiC's positive role in enhancing the quality of VPP‐printed Al2O3 parts. [ABSTRACT FROM AUTHOR]

This study investigates the fabrication, microstructural characteristics and plasma resistance of Y–Al–Si–O (YAS) glass-ceramics coated on alumina ceramics. YAS frits were initially prepared using a melt-quenching method, then homogenously milled and coated onto alumina ceramics. The melt-coating process was conducted at 1650 °C for 1 h. The composition and microstructure of the glass frits and coatings were thoroughly characterized using X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. These analyses revealed a dense microstructure with a polycrystalline structure predominantly composed of Y3Al5O12 (YAG) phase and a minor phase of Y2Si2O7. The YAS coatings on alumina revealed a dense layer with strong adhesion to the substrate. Subsequently, the coatings underwent C4F6/Ar/O2 plasma treatment for 1 h. Plasma exposure tests demonstrated that the YAS-coated alumina exhibited significantly better etching resistance compared to uncoated alumina, with minimal surface damage observed on the YAS coating, confirming its protective properties against plasma. The superior plasma resistance of YAS coatings is attributed to the predominance of its YAG phase. This research offers a more stable and cost-efficient solution for protecting ceramics in demanding plasma environments. [ABSTRACT FROM AUTHOR]

The Bi 2 O 3 –B 2 O 3 –SiO 2 –ZnO–Al 2 O 3 (BBSZA) glass/Al 2 O 3 composites are prepared by quenching and quenching-free methods and conducted a comparative analysis of their microstructures and properties. The results reveal that the BBSZA/Al 2 O 3 composite obtained by quenching free method has smaller pores and superior performance. Specifically, at a BBSZA content of 50 wt%, the composite exhibits a thermal conductivity of 5.352 W/m·K and a flexural strength of 193.54 MPa, which are 1.38 times and 2.08 times those obtained using the quenching method, respectively. XRD and TEM analyses, along with parallel experiments using other glasses, suggest that presintering crystallization may be the prerequisite for achieving denser composites via the quenching-free method. BBSZA glass reacts with Al 2 O 3 at the presintering process to generate a great amount of ZnAl 2 O 4 crystals, which enhances grain boundary energy and increases sintering density. This work provides insights into the low-cost and high-performance development of low temperature co-fired ceramics (LTCC). [ABSTRACT FROM AUTHOR]

Binder jetting (BJ) additive manufacturing faces significant challenges in fabricating high performance alumina ceramics. In this study, a multi-phase infiltration method was successfully employed to prepare high-performance mullite strengthened alumina ceramics via the BJ process. The effects of printing, sintering, and infiltration processes on the microstructure and performance of the alumina-based ceramics were systematically investigated. The results indicate that the density and strength of the green body increased with increasing binder saturation and decreased with thicker powder layers. Upon sintering, the maximum bending strength of the alumina ceramics reached approximately 2.2 MPa, which increased with the sintering temperature. After further multi-phase infiltration, the maximum bending strength of the alumina-based ceramic composites reached 97.3 MPa, a 44-fold increase compared to before infiltration. Additionally, triple-periodic minimal surface structures from high-performance alumina-based ceramic composites were successfully fabricated. The compression behaviour and thermal properties of these TPMS structures were investigated for potential insulation applications.

Here, we report the successful development of reduced graphene oxide (rGO) (0.1, 0.3 and 0.9 wt.%) reinforced alumina (Al2O3) novel nanocomposites by powder metallurgy route. The composite powder samples were prepared using a high-energy dry planetary ball mill for 2 h under an argon atmosphere. Then, the optimized compacted samples (at 170 MPa for 180 s) having high density were sintered at 1450 °C for 180 min. under an argon atmosphere. The characteristics of the materials have been evaluated to improve microstructure, microhardness, and Young's modulus of composites. rGO has 4-5 numbers of graphene sheets with an energy band gap of 2.28 eV and a specific surface area of 509.177 m2/g. The rGO retains structural quality before and after making composites with Al2O3 samples. The purity of rGO and Al2O3/rGO composite samples was confirmed by EDS analysis. While rGO shows peaks of C and O, Al2O3/rGO (0.1, 0.3, and 0.9 wt.%) composite samples show peaks of Al, O, and C. Micro Raman analysis establishes the successful formation of composite between Al2O3 and rGO. Al2O3/rGO (0.1, 0.3, and 0.9 wt.%) composite samples mostly show similar kinds of D, G, and 2D peaks as shown by pure rGO. The outstanding result achieved for the typical Al2O3/rGO (0.3 wt.%) sample showed significantly higher microhardness (2488 ± 12 VHN) and Young's modulus (520 ± 9 GPa) values compared to pure Al2O3. The microhardness and Young's modulus values of Al2O3/rGO (0.3 wt.%) composite are more than 79% and 31% compared to those observed for pure Al2O3. When the rGO amount further increased to 0.9 wt.% in Al2O3, agglomeration of graphene sheets developed, which caused degradation of the mechanical property of the composite. The excellent outcome of this work further advances critical applications of Al2O3 in various industrial fields. [ABSTRACT FROM AUTHOR]

Owing to the native character of the coefficient of thermal expansion (CTE) mismatch between Al2O3 and 304 stainless steel, obtaining high strength for Al2O3/304 stainless steel joints remains challenging. In this study, the residual stress of an Al2O3/304 stainless steel brazed joint was relieved using the Ag‐Cu‐Ti + boron nitride (BN) composite filler. The results indicated that the elements between the filler and base metals on both sides were mutually diffused. Moreover, reaction layers formed at the interface, connecting the Al2O3 ceramic and 304 stainless steel. The microstructure of the joint was as follows: Al2O3/Cu3Ti3O + TiCu/Ag (s, s) + Cu (s, s) + TiN + TiB2 + TiCu/Cu0.8Fe0.2Ti/TiFe2/Fe‐Cr/304 stainless steel. When the.1 wt% BN was added, the average strength of the joint reached 151 ± 10 MPa, 15% higher than the average strength of the joint only using Ag‐Cu‐Ti filler (131 ± 9 MPa). The improvement in the shear strength and reduction in the residual stress was attributed to the formation of TiN and TiB2, reducing the CTE difference and refining the microstructure. [ABSTRACT FROM AUTHOR]

In this work, hot-pressing of alumina in contact with hexagonal boron nitride or doped with boron carbide was conducted at 1500 °C for 30 min. After hot-pressing, abnormal grain growth induced by boron diffusion from these substances into alumina was detected, as clearly demonstrated with SEM, EDS, EBSD, and Raman spectroscopy. Grain boundary complexion transformations, solute drag, or another mechanism relating to interface-controlled grain boundary mobility are presumed to be the fundamental mechanism responsible for abnormal grain growth observed in this work.

Starch consolidation casting (SCC) technique was successfully employed to produce both porous alumina and graded porous alumina ceramics. The solid content in the alumina suspension was maintained at 40 vol.%, with potato starch varying from 5 to 15%. Structures of the porous alumina (monolithic) samples obtained by SCC and uniaxial pressing were compared. In addition, the influence of the SCC consolidation temperature and the starch content were evaluated in the monolithic samples, while the consolidation temperature and the number of layers numbers were evaluated in the graded samples. The lower SCC consolidation temperature resulted in lower linear shrinkage and a slight increase in total porosity due to the increased pore size. The compressive strength values for the monolithic samples ranged from 60 to 200MPa, which can be considered high when compared to previous works. The graded samples exhibited porosity variations across layers and interfaces were free of cracks and imperfections. Linear shrinkage was the same for the adopted consolidation temperature and the porosity was slightly higher for the 3-layer samples. They achieved strength of 60MPa with fracture mode parallel to the applied load.

The features of spark plasma sintering of submicron Al2O3 powders with different contents (0, 0.5, 1.5, 5 vol %) of β-SiC nanoparticles have been studied. The microstructure and hardness of Al2O3 + 5 vol % SiC ceramics obtained by sintering Al2O3 powders with β-SiC particles of various types (nanoparticles, submicron particles, fibers) have been studied. Sintering was carried out at heating rates (Vh) from 10 to 700°C/min. The sintering process of Al2O3 + SiC ceramics with low heating rates (Vh= 10–50°C/min) has a complex three-stage character, with a flat area in the temperature range of 1200–1300°C. At high heating rates (Vh> 250°C/min), the usual three-stage character of sintering is observed. The analysis of temperature dependences of compaction was carried out using the Young–Cutler model; it was found that the kinetics of powder sintering is limited by the intensity of grain boundary diffusion. It is shown that the dependence of the hardness of Al2O3 + SiC ceramics on Vh has a nonmonotonic character, with a maximum. In the case of pure alumina, an increase in Vh leads to a monotonic decrease in hardness. [ABSTRACT FROM AUTHOR]

The processing and characterizing of bonding Ti6Al4V to Al2O3 brazed joints using interlayer thin films was investigated. The brazing was conducted in a tubular furnace with an argon flux at 980 °C for 30 min. The brazing fillers consisted of different combinations of thin Ag/Cu and Ti films with variable thicknesses. The joint interface analysis involved using digital microscopy (DM) and optical microscopy (OM). Microstructural characterization and chemical composition were performed via scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Mechanical properties were assessed through microhardness and shear strength tests. Brazing successfully produced interfaces with a combination of titanium films and Ag/Cu as brazing filler. The results revealed that the interface mainly comprises Ti2Cu, TiCu2Al, α-Ti, and Ti2(Cu,Ag). Some segregation of (Ag) was observed at the interfaces, but a decrease in its amount was observed when compared to joints produced using Ag/Cu fillers. The thickness of the titanium film in the brazing filler strongly influenced the integrity of the joints. The amount of (Ag) at the interface diminished as the Ti film's thickness decreased, leading to an improvement in the mechanical properties of the joints. Using a combination of Ag/Cu and Ti thin films revealed a potential approach to reduce the segregation of soft phases at interfaces, promoting a significant improvement in joining metal to ceramic materials. [ABSTRACT FROM AUTHOR]

We created AA2024–AA1050 and AA2024–AA1050/0.005 vol.% Al2O3 nanocomposites by six accumulative roll bonding (ARB) process cycles. We used AA2024 and AA1050 sheets with a thickness of 0.7 mm and plate-shaped alumina nanoparticles to create a composite. The two AA1050 and one AA2024 sheets (among the two AA1050 sheets) were ARB-ed up to six cycles with and without adding alumina nanoparticles. Also, a sample of the AA1050 without composite making was ARB-ed up to six cycles. We aged some composites after the ARB process in the furnace at 110, 150, and 190 °C. This project performed SEM, TEM, and EDS-MAP analyses, tensile strength, microhardness, and Pin-on-Disc tests to study the ARB-ed sheets. The results of the tensile tests showed that the tensile strength of AA2024–AA1050 created by the six cycles ARB process was two times more than primary AA1050. Also, the wear resistance of this composite was 74% more than six cycles ARB-ed the AA1050. Using 0.005 vol.% alumina nanoparticles in AA2024–AA1050 composite improved its wear resistance by 30%. In the following, the aging process caused an improvement in tensile strength and total elongation of AA2024–AA1050/Al2O3 nanocomposites. [ABSTRACT FROM AUTHOR]

Wire arc additive manufacturing is an emerging technology in advance fabrication. It has become a very promising alternative to high value large metal components in many manufacturing industries. Even though it has high deposition rate and high metal utilization rates, coarse grains and anisotropy are the major drawbacks of this process. In this work, a study was done to investigate how to overcome these challenges. In this study, during the deposition of SS308L, Al2O3 particles are added in the middle of deposition. Analysis of the microstructure, hardness and related radiography tests are performed on the WAAM in-situ specimens and WAAMed specimens. An improvement in the grain size and hardness was observed with in the in-situ specimens when compared with WAAMed specimens. [ABSTRACT FROM AUTHOR]

Wire arc additive manufacturing is an emerging technology in advance fabrication. It has become a very promising alternative to high value large metal components in many manufacturing industries. Even though it has high deposition rate and high metal utilization rates, coarse grains and anisotropy are the major drawbacks of this process. In this work, a study was done to investigate how to overcome these challenges. In this study, during the deposition of SS308L, Al2O3 particles are added in the middle of deposition. Analysis of the microstructure, hardness and related radiography tests are performed on the WAAM in-situ specimens and WAAMed specimens. An improvement in the grain size and hardness was observed with in the in-situ specimens when compared with WAAMed specimens. [ABSTRACT FROM AUTHOR]

Reports on freeze-cast ceramic materials frequently focus on the study of the organized macroporosity and the properties of the materials. This study aims to describe the microstructure evolution of freeze-cast alumina during the sintering process, analyzing grain growth, densification, pore elimination and crystal structure at different sintering temperatures (1300-1500 °C). Aqueous suspensions with 20 vol% alumina were freeze-cast in liquid N2 and sintered. The microstructure was analyzed by stereological analysis, N2 adsorption and X-ray diffraction. Grain sizes varied within 237-500 nm, and the intergranular porosity decreased from 8.8% at 1300 °C to 1.4% at 1500 °C. N2 isotherm analysis revealed pore shrinking from the region of macro and mesopores (20-80 nm), to smaller residual mesopores (3.7-15 nm) at temperatures above 1400 °C. Rietveld refinement of the XRD diffractograms confirmed increased crystallite size and decreased lattice strain at higher sintering temperatures. This comprehensive description of microstructural evolution of the freeze-cast alumina contributes to understanding the sintering of highly porous ceramics produced via freeze-casting.

Alumina and 304 stainless steel were brazed with silver‐copper‐titanium filler+copper foil. The effect of adding copper foil to the filler on the microstructure of the alumina/304 joint was studied, and the reasons for the effect of adding copper foil with different thicknesses on the joint performance were also analyzed. The standard microstructure of alumina/304 joint with silver‐copper‐titanium+copper foil filler is alumina ceramic/copper compound with titanium and oxygen (3 : 3 : 1) (Cu3Ti3O) continuous layer+copper titanide (TiCu)/silver solid solution+copper solid solution/titanium compound with iron (1 : 2) (TiFe2)+iron‐chromium compound/304 stainless steel. The added copper foil inhibits the diffusion of titanium elements, thereby reducing the formation of the brittle copper compound with titanium and oxygen (3 : 3 : 1) (Cu3Ti3O) and alleviating the residual stress. In addition, after adding copper foil, the copper solid solution is distributed in blocks in the joint, which also improves the plastic deformation ability of the joint. Under the synergistic effect of these two effects after adding copper foil, the joint strength is improved. When the copper foil was 200 μm, the shear strength is 198.10 MPa. [ABSTRACT FROM AUTHOR]

The present study investigated the microstructural characteristics and mechanical behavior of hybrid composites with an aluminum matrix reinforced with copper and alumina. To prepare these composites, copper and alumina were mixed in varied weight ratios and added to a 10 wt% hybrid reinforced Al–Mg–Si alloy using a two–step stir casting process. The resulting composites were characterized using hardness, tensile properties, and scanning electron microscopy. The findings reveal that composites with Al–90%/Cu–7.5%/Al2O3–2.5% and Al–90%/Cu–7.5%/Al2O3–2.5% exhibited the optimum mechanical properties, including hardness and tensile properties. The microstructure of all composites showed the presence of copper particles and Al2O3 reinforcement, which were distributed nearly uniformly throughout the metal matrix despite the presence of pores. The EDS profile of a representative composite showed peaks of aluminum (Al), copper (Cu), carbon (C), magnesium (Mg), silicon (Si), oxygen (O), manganese (Mn), and traces of calcium (Ca), sulfur (S), and potassium (K). Based on these results, further investigation could be conducted on the wear and corrosion properties of the aluminum alloy when reinforced with copper and alumina. [ABSTRACT FROM AUTHOR]

Porous mullite ceramics were prepared from mixtures of Egyptian kaolinite-clay and calcined α-alumina powder. The effects of two methods of compaction as well as two types of binder were investigated. The influence of sintering temperature on the physical properties, crystalline phase, compressive strength and microstructure was studied. Mullite bodies processed by uniaxial pressing utilizing 15wt. % water at 1500°C for 2h exhibited the highest compressive strength of 150 MPa and bulk density of 2.1 g/cm3, as well as, an open porosity of 35%. In the present study, the only stoichiometric mullite was achieved by the crystallization of metakaolin. The mullite crystallizing out from the liquid phase was rich in silica, as determined by EDS, while that resulting from alumina grains was rich in alumina. The microstructure showed that primary mullite appeared at 1200°C, beside the unreacted alumina particles. On the other hand, at 1400°C alumina particles started to react with the glassy phase to form parallel laminates of mullite. [ABSTRACT FROM AUTHOR]

The incorporation of high thermal neutron capture elements in ceramic materials is relevant for the design of traceable proppants for obtaining information in petrology, such as the location and height of fractures in unconventional gas and oil wells. Among these, GdAlO 3 has attracted interest because of its good thermo-mechanical properties. In this work, four Al 2 O 3 /GdAlO 3 reaction sintered composites were prepared using Gd 2 O 3 and Al 2 O 3 powders as raw materials and high-energy ball milling followed by sintering at 1600 °C. This combination of ball milling and reaction sintering is considered as a mechanism to produce large bulk ceramics with different Al 2 O 3 /GdAlO 3 compositions. The composite formulations range from pure alumina to eutectic ceramic. The obtained materials were characterized using X-ray diffraction for identifying the crystalline phases, Archimedes method for the textural properties, scanning electron microscopy for the microstructure, and Vickers indenter and three-point bending tests for the mechanical properties. In addition, the macroscopic capture cross-section Σ for thermal neutrons was estimated considering each composite formulation. It was found that the processing route is very suitable for producing low-porosity Al 2 O 3 /GdAlO 3 composites, which present similar mechanical properties to those of alumina. Both the material density and Σ increase with increasing the starting amount of Gd 2 O 3 , in a way that for the eutectic composite the density is 50% higher than that of pure alumina, and Σ is about 550000 c.u. The effect of the material porosity on Σ was also considered. The obtained results indicate that the proposed processing route is very attractive for producing these ceramic composites for technological applications. [ABSTRACT FROM AUTHOR]

Heating elements in the automotive industry are currently produced by several thermal spray processes and materials. However, simpler spraying technologies such as arc spraying are investigated regarding technological suitability as a cost-effective alternative to plasma spraying in the production process of these components. Thus, several mixtures and combinations of alternative pressurizing gases and further modifications of an arc spray process were examined in this study. Consequently, coatings based on NiCr were produced, since this alloy is typical for heating elements. Coating properties were investigated by SEM, EDS, and resistivity measurements. The results demonstrate reduced oxygen content and improved morphology compared with the industrially used plasma-sprayed coatings. Additionally, the improved microstructure affects the surface quality and specific resistivity of the coatings positively. This allowed for laser texturing the arc-sprayed coatings successfully. It must be considered a drawback, though, that cracks partially appeared in the underlying coatings. In contrast, the temperature coefficients of resistance and the resistivities of the heating elements were superior to the conventional coatings, which can lay the foundation for a future industrial application. To further investigate this and to minimize the influence of potential sample preparation issues on the analysis results, different methods of cross-sectioning, i.e., in detail hot mounting, cold mounting, and ion beam polishing, were evaluated. [ABSTRACT FROM AUTHOR]

Zirconia materials are commonly utilized in biomedical applications due to their esthetic appearance, mechanical properties, and biocompatibility. In this study, composites of 12 mol% Ce-TZP reinforced with a total of 25 vol% alumina and in situ formed manganese cerium hexaaluminate MnCeAl 11 O 19 were produced by a mixing and milling method, shaped by slip casting and pressureless sintered in air at 1500–1550 °C. Microstructure, mechanical properties, phase composition, and low temperature degradation resistance (LTD) were determined. Plain alumina toughened zirconia and a mixed composition containing 15% MnCeA6 combined the highest bending strength of > 650 MPa and highest fracture toughness of 11 MPa√ m. Balancing of contributions by different toughening effects assumes an underestimation of the transformation toughening effect. Evidence of a net synergetic effect between hexaaluminate reinforcement and transformation toughening was not observed. High amounts of manganese cerium hexaaluminate platelets lead to a substantial reduction in mechanical properties. [ABSTRACT FROM AUTHOR]

Mimicking the damage tolerance of biological materials such as nacre has been realised in textured layered alumina ceramics, showing improved reliability as well as fracture resistance at room temperature. In this work, the fracture behaviour of alumina ceramics with textured microstructure and laminates with embedded textured layers are investigated under uniaxial bending tests at elevated temperatures (up to 1200 °C). At temperatures higher than 800 °C monolithic textured alumina favours crack deflection along the basal grain boundaries, corresponding to the transition from brittle to more ductile behaviour. In the case of laminates, the loss of compressive residual stresses is counterbalanced by the textured microstructure, effective up to 1200 °C. This study demonstrates the potential of tailoring microstructure and architecture in ceramics to enhance damage tolerance within a wide range of temperatures. [ABSTRACT FROM AUTHOR]

In this paper, we report that the hardness of alumina ceramics can be enhanced by flash treatment. The effects of treatment conditions on the hardness of dense alumina ceramics were studied. The results show that as the current density increases, the hardness first increases and then decreases; and the highest hardness is more than 20% higher than that of the original sample. The microstructure analysis reveals that flash-treatment has two effects on alumina: (1) generation of dislocations, increasing hardness; and (2) generation of oxygen vacancies/pores, decreasing hardness. The variation of hardness with current density and treatment time is the combined result of these two effects. The current research provides a simple and feasible approach to strengthen alumina ceramics, which could be extended to other ceramic systems. [ABSTRACT FROM AUTHOR]

Composition and structure are the two most essential factors for insulator materials performance. In this paper, the differences in composition, structure and mechanical properties of three typical alumina insulator for ultra high voltage direct current (UHVDC) were systematically studied, and the reasons were analyzed. The results show that compared with NGK, two domestic manufacturers have added more calcined industrial alumina, less potash feldspar and quartz into the formula, and have not properly raised the sintering temperature according to the formula, the porcelains of two domestic manufacturers show the following characteristic: 1) the content of Al2O3 are 47.22% and 45.24%, respectively, 7.31% and 5.33% higher than that of NGK, the content of SiO2 are 46.32% and 48.44%, respectively, 7.09% and 4.97% lower than that of NGK, and the total content of K2O + Na2O are 3.21% and 3.14%, respectively, 0.41% and 0.48% lower than that of NGK; 2) the true porosity are 5.24% and 4.18%, respectively, 3.46% and 4.52% lower than that of NGK. The internal pores are basically less than 5 µm, and the pore size is generally smaller. There are many flat, slender, crack like pores, as well as annular pores and cracks surrounding the particles; 3) the content of internal crystalline phase is 48.10% and 49.04%, respectively, which is nearly 10% higher than NGK, with corundum phase being 10% higher, quartz phase being 4% higher, mullite phase being 4% less. The mullite phase is thinner and shorter, and the degree of crosslinking is lower; 4) the average value of the three-point bending strength of the porcelain head test strip are 155 and 158 MPa, respectively, which are higher than NGK, but the dispersion strength of the two domestic manufacturers is higher than NGK. [ABSTRACT FROM AUTHOR]

In this work, silica‐based ceramic cores with alumina as a mineralizer were prepared via an injection molding method, and the effects of alumina on the microstructural evolution and properties at 1450°C (simulating the process of equiaxed castings) and 1550°C (simulating the process of columnar/single crystal castings) were investigated. It was found that alumina promoted the cristobalite crystallization of fused silica refractory during sintering but inhibited the devitrification rate in the subsequent heating. The flexural strength of silica‐based ceramic cores at an ambient temperature and 1450°C improved with an increasing alumina content, whereas the opposite trend appeared at 1550°C. The creep resistances of silica‐based cores were improved significantly and then slightly deteriorated with an increasing alumina content from 5% to 20%, depending on the competition effects of alumina hindering the viscous flow of liquid silica (favorable), but suppressing the devitrification rate (unfavorable). The results of this work show that silica‐based cores need to follow different compositional design principles for equiaxed and columnar/single‐crystal turbine blade castings. [ABSTRACT FROM AUTHOR]

The effects of dopants with different valences on the flash sintering behavior of α-Al 2 O 3 are investigated. The results indicate that regardless of their valence and ionic radius, all the tested dopants reduce the onset temperature more effectively compared to undoped Al 2 O 3. Interestingly, the relative density and grain size of the flash-sintered samples exhibit an inverse linear relationship. The data for the samples doped with non-trivalent cations fall on a different line than those for the samples doped with trivalent cations and the undoped samples. Dopants have an effect on the flash sintering behavior of Al 2 O 3 because flash sintering increases the solubility of the dopants in alumina, creating more point defects, and thereby increasing the electrical conductivity of the material. The mechanisms for point defect generation in trivalent and non-trivalent dopants are different. [ABSTRACT FROM AUTHOR]

Thin double-layer coatings comprising an alumina top coat and mullite bond coat were deposited on SiC substrates by laser chemical vapor deposition (CVD). The effect of the presence of mullite bond coat and the phases of alumina top coat on the structural integrity against the thermal residual stress loaded by the high-temperature CVD process was examined by microstructural characterization and simulative consideration. A laminated layered structure having a dense γ-alumina top surface with a cone-like morphology are grown at a deposition temperature of 1323 K, whereas an α-alumina top layer comprising densely packed faceted grains was grown at 1473 K. The interfaces between the SiC and mullite layers were coherent owing to the formation of a thin transition layer. The γ-alumina layer formed an adhesive interface bordering the mullite layer, whereas small residual defects were formed in the α-alumina layer bordering the mullite layer. Spacings of surface cracking induced by the high-temperature deposition process in the double-layer coatings were approximately half of those in the coatings without mullite layers. As simulative results by finite element method suggested, the double-layer coatings were experimentally verified to be more tolerant to the formation of surface cracks and interfacial delamination, compared to single-layer coatings without the mullite bond layers. [ABSTRACT FROM AUTHOR]

Ceramic foam filters are applied to remove non-metallic inclusions during steel melt filtration. The flame-spray technology can contribute to improving such filters by increasing the deposition capability of inclusions and decreasing the impairment of the filter substrate by the steel melt. This makes them suitable for application to the continuous steel casting process and foundry casting. The ball-on-three-balls test was utilized at elevated temperatures to determine the high-temperature behaviour of thermally sprayed alumina. Additionally, the flame-spray technology was used to produce carbon-free filters composed of flame-sprayed alumina by coating and burning out the Al2O3–C filter substrate during a sintering step. The B3B revealed a ductile behaviour with increasing temperature and a bending strength of 23.28 MPa at 1400 °C. After sintering at 1400 °C, the filters showed a sufficient CCS of 0.16 MPa, which allows the performance of casting tests (immersion and pouring) with molten steel at 1580 °C.

The processing and characterizing of bonding Ti6Al4V to Al2O3 brazed joints using interlayer thin films was investigated. The brazing was conducted in a tubular furnace with an argon flux at 980 °C for 30 min. The brazing fillers consisted of different combinations of thin Ag/Cu and Ti films with variable thicknesses. The joint interface analysis involved using digital microscopy (DM) and optical microscopy (OM). Microstructural characterization and chemical composition were performed via scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Mechanical properties were assessed through microhardness and shear strength tests. Brazing successfully produced interfaces with a combination of titanium films and Ag/Cu as brazing filler. The results revealed that the interface mainly comprises Ti2Cu, TiCu2Al, α-Ti, and Ti2(Cu,Ag). Some segregation of (Ag) was observed at the interfaces, but a decrease in its amount was observed when compared to joints produced using Ag/Cu fillers. The thickness of the titanium film in the brazing filler strongly influenced the integrity of the joints. The amount of (Ag) at the interface diminished as the Ti film’s thickness decreased, leading to an improvement in the mechanical properties of the joints. Using a combination of Ag/Cu and Ti thin films revealed a potential approach to reduce the segregation of soft phases at interfaces, promoting a significant improvement in joining metal to ceramic materials.

In the present work, porous composites were fabricated from pure Al2O3 and mixed Ti3AlC2/Al2O3 powder by slip casting and sintering. The effect of sintering temperature and different composition ratio on microstructure, phase composition, porosity and gas permeation flux of the fabricated materials was investigated. The microstructure and phase composition of the samples were analyzed by scanning electron microscopy and X-ray diffraction, respectively. The gas permeation experiments were performed using pure hydrogen at 0.1–0.9 MPa pressure. It is shown that a decrease in sintering temperature from 1500 to 1350 °C results in an increase in hydrogen permeation flux of the alumina from 5 to 25 mol/(m2 × s), which is due to higher pore size and overall porosity of the samples. Sintering of Ti3AlC2/Al2O3 powder mixtures leads to the formation of Al2O3, Al2TiO5 and TiO2 phases as a result of oxidation of the Ti3AlC2 phase, resulting in an increased pore size in the composites compared with pure alumina. The open porosity of composites increases from 3.4 to 40% with an increasing Ti3AlC2/Al2O3 ratio from 1/10 to 1/2, respectively. The composites with the highest porosity (40%) had a maximum permeation flux of 200 mol/(m2 × s). The changes in the bending strength of the alumina and composite samples, depending on the microstructure and porosity, were also discussed. The investigated composites are considered promising materials for hydrogen separation membrane supports. [ABSTRACT FROM AUTHOR]

Copper electrodeposited composite coatings containing two types of aluminum-oxide powders with different dispersities (alumina Al2O3-1 and electroexplosive aluminum-oxide nanopowder Al2O3-2) are obtained during the work. The studies show that introducing the powders leads to a change in the microstructure of the composites and a change in the grain growth principles during the formation of the coatings. Refinement and ordering of the grain structure of the coatings occurs and twinning defects and texture are formed. The change in the formation of the microstructure of the composites leads to a change in some operational characteristics: an increase in the microhardness (by 10% in the composites with the addition of alumina and by more than 30% in the coatings with electroexplosive aluminum oxide) and ultimate tensile strength (by 20% in the composites with Al2O3-1 and almost 1.5-fold in the samples with Al2O3-2). [ABSTRACT FROM AUTHOR]

Pressure assisted casting of advanced ceramics is an emerging preparation technique as it permits using a colloidal slips in shaping under pressure. Application of pressure provides flexibility in forming of complex shapes in combination with high homogeneity, superior green density, strength and high productivity. Current study compares conventional slip casting (CSC), pressure slip casting (PSC) and cold isostatic pressing (CIP) used for preparation of alumina green bodies. Since particle size is a limitation in PSC process due to the defined pore size of the mould, the same starting alumina powder obtained by mixing of powders with two different average particle size (7 μm and 1.43 μm) in the proportion of 65:35 was employed in all three processes. Green densities of 65%TD at 35 bar in case of PSC and 66%TD at 1200 bar after CIP were obtained; however, density of the CSC samples was only 50%TD. Flexural strength and fractographic studies were carried out and correlated with the respective processes. The samples were also sintered at 1600 °C to evaluate their sinterability.

Ta–Al2O3 composite samples with different compositions are prepared using Field Assisted Sintering Technique (FAST). Two different alumina powders are used to investigate the influence of the starting powders particle size on the microstructural features and the resulting electrical conductivity of the prepared composite materials. Percolation threshold of the two material systems is influenced by the metal fraction, as well as the alumina particle size of the starting powder. The percolation threshold for the fine- and the coarse-grained alumina is found to be at 15 vol.-% Ta and 7.5 vol.-% Ta, respectively. Microstructural investigations show significant differences in terms of particle shape of both, Ta and Al2O3 after sintering, most likely being the reason for the different percolation thresholds of the investigated materials. Anisotropy effects resulting from the processing using FAST and the influence on electrical properties are also shown.

In this research, a novel aluminium metal matrix composite (AMMC) was developed using recycled aluminium alloy as a matrix with 5% alumina as reinforcement. The machining experiments were conducted by varying the input parameters such as voltage (Vs), wire feed rate (Fw), current (Ip), pulse on time (ONT) and pulse off time (OFFT), on wire breakage. The effect of voltage level and wire breakage frequency was analysed. The parameter combinations for machining the slot of size 5 mm width and 10 mm height with high machining rate (MR) and less surface roughness (Ra) were analysed using the CRiteria Importance Through Intercriteria Correlation (CRITIC) and simple additive weighting (SAW) methods. The wire breakage frequency is lesser at minimum peak current. The optimal parameter combination for higher MR and lower Ra is found to be at 30 V, 7 mm/min, 30 A, 120 μs (ONT) and 70 μs (OFFT). Analysis of variance (ANOVA) is performed to understand the significant factors affecting the WEDM process. ANOVA results predict that wire feed rate and voltage contribute 47.82% and 21.23%, respectively, to MR; and pulse on time shows a 23.06% influence on surface roughness. Scanning electron microscopy (SEM) was used to ascertain the pattern of wire breakage in WEDM, and based on the results obtained from employing this technique, it is inferred that the erosion and breakage of the wire are not instantaneous and that a cone shape is formed on the either portion of the wire. [ABSTRACT FROM AUTHOR]

The sintering temperature and holding time have a significant impact on the densification of a ceramic compact. In this study, alumina inserts were made with varying sintering temperatures and holding times. The procedure began with the compacting of alumina powders inside a mould with trapezium and round shapes. These inserts were then sintered between 1200 °C and 1400 °C for 5 to 9 h holding time. The sintered samples underwent analysis based on the shrinkage size, density, and microstructure. The sample with the highest density was chosen for additional machining tests. The results showed that the alumina shrinkage ranged from 3 to 6%, with a maximum relative density of 91.3% recorded when the sintering parameter was applied at 1400 °C and a 9 h holding time. The transition of the grain growth was observed depending on the sintering temperature and heating duration. When machined with AISI 1045 carbon steel, the sintered alumina inserts achieved a maximum tool life of 35 s at a cutting speed of 350 m/min. The sintered inserts exhibited brittle characteristics with a dominant notch wear and abrasive mechanisms. [ABSTRACT FROM AUTHOR]

The need for new solutions for electrical insulation is growing due to the increased electrification in numerous industrial sectors, opening the door for innovation. Plasma spraying is a fast and efficient way to deposit various ceramics as electrical insulators, which are used in conditions where polymers are not suitable. Alumina (Al 2 O 3 ) is among the most employed ceramics in the coating industry since it exhibits good dielectric properties, high hardness, and high melting point, while still being cost-effective. Various parameters (e.g., feedstock type, spray distance, plasma power) significantly influence the resulting coating in terms of microstructure, porosity, and metastable phase formation. Consequently, these parameters need to be investigated to estimate the impact on the dielectric properties of plasma-sprayed alumina coatings. In this work, alumina coatings with different spray distances have been prepared via atmospheric plasma spray (APS) on copper substrates. The microstructure, porosity, and corresponding phase formation have been analyzed with optical microscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Moreover, we present an in-depth analysis of the fundamental dielectric properties e.g., direct current (DC) resistance, breakdown strength, dielectric loss tangent, and permittivity. Our results show that decreasing spray distance reduces the resistivity from 6.31 × 10 9 Ω m (130 mm) to 6.33 × 10 8 Ω m (70 mm), while at the same time enhances the formation of the metastable δ -Al 2 O 3 phase. Furthermore, space charge polarization is determined as the main polarization mechanism at low frequencies. [ABSTRACT FROM AUTHOR]

In this work, Sedlecky ml kaolinite, Nabalox No. 315 alumina and corn starch were used. The powder mixtures were milled and uniaxially pressed into pellets. Some properties of the raw materials and mixtures were investigated like TG, DTA, and the gas composition formed during the sintering process. The pellet samples were pre-sintered in the electric kiln under an oxygenfree reduction atmosphere at 1250 °C and then sintered in nitrogen gas at 1450 °C. The composition and microstructure of the produced specimens were tested. The pyrolysis results show that in reducing pre-sintering 25 wt% of the mass of the corn additive remove from the ceramic body as gas. The remained part of the corn can incorporate into the material structure, and it may affect the further heat treatment processes. The mullite content of the ceramic samples increased from 43 wt% to 70 wt% because of high-temperature sintering in nitrogen gas. The mullite crystal structure has also changed. The increase in the amount of mullite phase is a consequence of the sintering process since the free SiO2 and Al2O3 formed mullite crystals. [ABSTRACT FROM AUTHOR]

Alumina ceramics used in microwave systems are susceptible to the multiplication of secondary electron emission on the surface due to the influence of resonance between electrons and the radiofrequency electric field, and a detrimental multipactor effect may therefore be triggered. For the alumina-loaded microwave components, it is essential to achieve low secondary electron yield (SEY) on the inserted alumina surfaces to mitigate multipactor. In this work, to achieve an ultralow SEY surface of alumina, two recognized low-SEY treatments were combined. For the primary SEY suppression, a series of microstructures were fabricated on the alumina surfaces with varied porosity and aspect ratio at the hundred-micrometer scale by infrared laser etching. The microstructure with 52.14% porosity and 1.78 aspect ratio showed an excellent low-SEY property, which could suppress the SEY peak value ( δ m) of alumina from 2.46 to 1.00. For the secondary SEY suppression, the SEY dependence of TiN coating on sputtering parameters was studied, and the lowest δ m of 1.19 was achieved when the gas flow ratio of Ar:N2 was 15:7.5. Thereafter, by depositing TiN ceramic coating onto the laser-etched porous samples, an ultralow SEY, with δ m of 0.69, was achieved on the alumina surfaces. The simulation work revealed the impact of dielectric surface charge on electron multiplication and revealed a mechanism of using low-SEY surfaces to inhibit multipactor. Some coaxial filters filled with alumina were fabricated for verification; the results revealed that the multipactor threshold increased from 125 W to 425 W after applying the TiN-coated porous alumina, and to 650 W after treating another multipactor-sensitive area with the same low-SEY process. This work developed an advisable method to sharply reduce SEY, which is of great significance for the multipactor mitigation of alumina-loaded microwave components. [ABSTRACT FROM AUTHOR]

We have studied the effect of monoclinic ZrO2 additions (1.5, 5, and 10 vol %) on the shrinkage kinetics of submicron α-Al2O3 powder. Ceramic samples were prepared by two-step spark plasma sintering (SPS). It has been shown that the two-step SPS process (heating to the temperature corresponding to 90% relative density and isothermal holding at this temperature) allows one to obtain ceramics with ultrafine-grained microstructure and high, near-theoretical density. Large ZrO2 additions (10%) lead to an increase in the activation energy for SPS and a decrease in grain-boundary deformation rate in the isothermal holding step. [ABSTRACT FROM AUTHOR]

The work was aimed at the investigation of kinetics of Spark Plasma Sintering (SPS) of the α-Al 2 O 3 particles with amorphous surface layers and investigation of the effect of the amorphous layers on the grain growth and on the mechanical properties of alumina. The objects of investigations comprised:(i) submicron α-Al 2 O 3 powder, (ii) submicron α-Al 2 O 3 powder with the amorphous layers on the particles' surfaces, and (iii) the fine-grained α-Al 2 O 3 powder. The submicron powders (i) and (ii) were used to analyze the effect of the amorphous layers on the sintering kinetics. Powders (i) and (iii) were used to analyze the effect of the initial particle sizes on the shrinkage kinetics. The effect of the temperature regime and of the rate (V h) on the shrinkage kinetics of the submicron and fine alumina powders has been studied. The shrinkage curves were analyzed using the Young–Cutler and Coble models. The sintering kinetics was shown to be determined by the intensity of grain boundary diffusion for the submicron powders and by simultaneous lattice diffusion and grain boundary one for the fine powders. The amorphous layers on the surfaces of the submicron α-Al 2 O 3 particles were found to affect the grain boundary migration rate and the Coble equation parameters at the final stages of SPS. The abnormal characteristics of the alumina ceramics sintered from the submicron powder with the amorphous layers on the particles' surfaces were suggested to originate from the increased concentration of the defects and of the excess free volume at the grain boundaries formed during crystallization of the amorphous layers. [ABSTRACT FROM AUTHOR]

The nearly fully dense ultra‐high pure (>99.99%) α‐Al2O3 ceramics were prepared by flash sintering at the furnace temperature of 1300°C. Compared with the samples sintered at 1300 and 1650°C without electrical field, the flash‐sintered samples exhibited remarkably improved density and finer grains. The flash‐sintered samples also exhibited high hardness, which is even higher than that of the hot‐pressed sample. Therefore, it is believed that flash sintering could be an effective technique for the preparation of ultra‐high pure alumina ceramics. [ABSTRACT FROM AUTHOR]

This study describes the porosity and particle size effects of rice husk pore former on the diametral tensile strength of porous alumina (Al2O3) ceramics. Porous Al2O3 ceramics with high porosity and sufficient diametral tensile strength were successfully prepared by the pore-forming agent method using rice husk (RH) as the pore former according to the sample formulation Al2O3-xRHy (where 'x' denotes the particle size range in µm and 'y' denotes the percent weight content (wt%) of RH). The thermogravimetric analysis (TGA) and X-ray diffractometer (XRD) results revealed that silica was retained as rice husk ash in the developed porous Al2O3 after the decomposition of the starting rice husk pore former. Microstructures of the as-prepared porous Al2O3 ceramics having different RH additions exhibited hierarchical pore structures with increased particle size of the pore-forming agent. Porosity increased with larger particle size range of rice husk where the Al2O3-63RH5 demonstrated the least porosity (44.2 vol%), while the highest porosity (70.9 vol%) was demonstrated by the Al2O3-125–250RH20. The diametral tensile strength of the RH-shaped porous alumina ceramics declined from 16.97 to 0.65 MPa with increased particle size of the rice husk. [ABSTRACT FROM AUTHOR]

Highly transparent Dy3+ and Dy3+/Cr3+ polycrystalline alumina ceramics were prepared with the real in-line transmittance (RIT) up to 55% (at λ = 632 nm), one of the highest values reported for luminescent rare-earth elements doped alumina. The RIT of doped alumina decreased more sharply with increasing mean grain size than predicted by the models for pure alumina. Co-doping with Dy3+ and Cr3+ resulted in a moderate increase of the RIT, which was independent on grain size. EDX analysis revealed that dysprosium segregated at grain boundaries and chromium was dispersed throughout the alumina matrix. A thermodynamic model for diffusion of multiple species in systems with multiple sorts of traps was proposed. The photoluminescence excitation and emission spectra of Dy3+-doped alumina showed peaks characteristic for Dy3+-doped materials. In co-doped Dy3+/Cr3+ alumina, Dy3+ emitted in the blue and yellow regions and Cr3+ in the red region, creating a suitable combination for obtaining white light. [ABSTRACT FROM AUTHOR]

Magnesium- and calcium-aluminosilicate (MAS and CAS) glass–ceramics were used to join alumina with six different compositions. The fillers were applied onto the alumina by screen-printing, and then joining was performed slightly below and above the filler melting temperature (T m). The evolution of various intermediate compounds upon heat treatment between the filler itself and at the joining interface was compared. MgAl 2 O 4 and CaO·6Al 2 O 3 was the main crystalline phase presented at the joining interface for the MAS and CAS system, respectively, while more intermediate compounds were observed when only filler was heat-treated. The formation of MgAl 2 O 4 and CaO·6Al 2 O 3 was attributed to the diffusion of Al ions from the alumina base, which is desirable for obtaining a sound joint due to the similar coefficient of thermal expansion to the base alumina. The maximum joint strength of 250 ± 41 and 301 ± 48 MPa was obtained for MAS and CAS filler system, respectively, after joining at T ≥ T m due to complete interfacial wetting. [ABSTRACT FROM AUTHOR]