Microstructure and thermal conductivity of gas-pressure-sintered Si3N4 ceramic: the effects of Y2O3 additive content

Si₃N₄ ceramics were sintered at 1900 °C under a nitrogen pressure of 1 MPa using Y₂O₃-MgO additives. The effects of Y₂O₃ content (0.5-4 mol%) on microstructure and thermal conductivity were systematically investigated. The increasing Y₂O₃ content led to increases in amount and viscosity of liquid phase during sintering, which induced a “bimodal to normal” transition in distribution of grain size, decreased Si₃N₄/Si₃N₄ contiguity and enhanced devitrification degree of intergranular phase in sintered bulks. Moreover, the decreasing Y₂O₃ content was found to improve the elimination efficiency of SiO₂ impurity during sintering, resulting in lower lattice oxygen content in densified specimens. The microstructure had a strong effect on thermal conductivity. The samples sintered for 3 h gained an increase of thermal conductivity from 65 to 73 W·m^-1 K^-1 with increasing Y₂O₃ content, while the samples sintered for 12 h obtained a substantial increase of thermal conductivity from 87 to 132 W·m^-1 K^-1 with decreasing Y₂O₃ content.     

Effect of Y2O3 content on the microstructural characteristics and the mechanical and thermal properties of Yb-doped SiAlON ceramics

SiAlON ceramics are primarily employed in ceramic cutting tools, which exploit their stable physical properties in high-temperature cutting environments due to their excellent mechanical properties. Here, Yb/Y-co-doped SiAlON ceramics are prepared by adding Yb and Y rare-earth (RE) ions in the RE_xSi_12-(m+n)Al_m+nO_nN_16-n (m = 0.4, n = 1.0) composition. Yb₂O₃, the RE oxide, is the main sintering additive. For RE_x composition design with (Yb_1-y + Y_y)_x, Yb₂O₃ is replaced by Y₂O₃ (y = 0.00, 0.25, 0.50, 0.75, 1.00). While Yb₂O₃ has excellent high-temperature stability, it is limited by its microstructural characteristics, that is, the β-SiAlON morphology and limited fracture toughness due to the small cation sizes. Thus, the changes in the above properties are investigated for various Y₂O₃ additive contents substituting for Yb₂O₃. The average grain width decreases, and the equiaxed β-SiAlON grains are elongated with increasing Y₂O₃ content. Regarding the mechanical properties, the hardness and fracture toughness are evaluated using the indentation fracture method. The hardness decreases with increasing Y₂O₃ content; however, the fracture toughness exhibits a significant increase (～53.6%) from 4.6 to 7.0 MPa?m^1/2. Regarding crack propagation, intergranular fracture is mainly observed in the Yb/Y-co-doped SiAlON ceramics, whereas transgranular fracture is primarily observed in the Yb-single-doped SiAlON ceramic. Y₂O₃ substitution increases the α/β-SiAlON phase ratio, and the grain boundary phase exhibits increasing vitrification with increasing Y₂O₃ content. Moreover, the thermal properties of the Yb/Y-co-doped compositions are analyzed and discussed regarding intrinsic properties such as phonon scattering. The microstructural characteristics and improved fracture toughness derived from the Yb/Y-co-doped system designed in this study suggest considerable potential for the future composition design of ceramic cutting tools.     

Improvement on the phase stability,mechanical properties and thermal insulation of Y2O3-stabilized ZrO2 by Gd2O3 and Yb2O3 co-doping

Gd₂O₃ and Yb₂O₃ co-doped 3.5 mol% Y₂O₃–ZrO₂ and conventional 3.5 mol% Y₂O₃–ZrO₂ (YSZ) powders were synthesized by solid state reaction. The objective of this study was to improve the phase stability, mechanical properties and thermal insulation of YSZ. After heat treatment at 1500 °C for 10 h, 1 mol% Gd₂O₃–1 mol% Yb₂O₃ co-doped YSZ (1Gd1Yb-YSZ) had higher resistance to destabilization of metastable tetragonal phase than YSZ. The hardness of 5 mol% Gd₂O₃–1 mol% Yb₂O₃ co-doped YSZ (5Gd1Yb-YSZ) was higher than that of YSZ. Compared with YSZ, 1Gd1Yb-YSZ and 5Gd1Yb-YSZ exhibited lower thermal conductivity and shorter phonon mean free path. At 1300 °C, the thermal conductivity of 5Gd1Yb-YSZ was 1.23 W/m K, nearly 25% lower than that of YSZ (1.62 W/m K). Gd₂O₃ and Yb₂O₃ co-doped YSZ can be explored as a candidate material for thermal barrier coating applications.     

Pressureless sintered silicon carbide matrix with a new quaternary additive for fully ceramic microencapsulated fuels

This study suggests a new additive composition based on AlN–Y₂O₃–Sc₂O₃–MgO to achieve successful densification of SiC without applied pressure at a temperature as low as 1850 °C. The typical sintered density, flexural strength, fracture toughness, and hardness of the SiC ceramics sintered at 1850 °C without applied pressure were determined as 98.3%, 510 MPa, 6.9 MPa·m^1/2, and 24.7 GPa, respectively.Fully ceramic microencapsulated (FCM) fuels containing 37 vol% tristructural isotropic (TRISO) particles could be successfully sintered at 1850 °C using the above matrix without applied pressure. The residual porosity of the SiC matrix in the FCM fuels was only 1.6%. TRISO particles were not damaged during processing, which included cold isostatic pressing under 204 MPa and sintering at 1850 °C for 2 h in an argon atmosphere. The thermal conductivity of the pressureless sintered FCM pellet with 37 vol% TRISO particles was 44.4 Wm^?1 K^?1 at room temperature.     

Improvement on thermophysical and mechanincal properties of LaMgAl11O19 magnetoplumbite by Nd2O3 and Sc2O3 co-doping

LaMgAl₁₁O₁₉-type magnetoplumbite holds great promise to be used above 1300 °C as thermal barrier coatings (TBCs), but its practical application has been restricted because of inferior thermophysical properties. Herein, we focus on optimizing the thermophysical properties of LaMgAl₁₁O₁₉ by simultaneously substituting La³⁺ and Al³⁺ ions with Nd³⁺ and Sc³⁺ ions, respectively. Results show that the effects of co-substitution on reducing thermal conductivity are pronounced. The thermal conductivities of La_1-xNd_xMgAl_11-xSc_xO₁₉ (x = 0, 0.1, 0.2, 0.3) ceramics decrease progressively with dopant concentration and a lowest thermal conductivity of 2.04 W/(m·K) is achieved with x = 0.3 at 1000 °C, which is a value superior to pure LMA and even lower than YSZ. The mechanisms behind the lowered thermal conductivity are investigated. Increase of the thermal expansion coefficient is also realized (8.53 × 10⁻⁶ K⁻¹ for pure LMA, 9.07 × 10⁻⁶ K⁻¹ for x = 0.3, 1300 °C). Most importantly, Nd³⁺ and Sc³⁺ combination doping indeed facilitates mechanical properties of La_1-xNd_xMgAl_11-xSc_xO₁₉ solid solutions as well. It should be noted that Sc³⁺ doping at Al³⁺ site plays more effective role in improving thermal properties than Nd³⁺ does at La³⁺ site. This work provides a path to simultaneously integrate low thermal conductivity, good phase stability, moderate thermal expansion behavior and excellent mechanical properties on LMA for the next generation TBCs.     

Microstructure and mechanical properties of CaAl12O19 reinforced Al2O3-Cr2O3 composites

Al₂O₃-Cr₂O₃ refractories have excellent slag corrosion resistance and can adapt to the oxidation/reduction atmosphere in the smelting reduction ironmaking furnace. However, Al₂O₃-Cr₂O₃ refractories have poor mechanical properties and sintering properties. In order to improve the mechanical properties of Al₂O₃-Cr₂O₃ materials, the CaAl₁₂O₁₉ reinforced Al₂O₃-Cr₂O₃ composites were prepared by pressureless sintering process, and the influences of CaO content on the sintering properties, mechanical properties, and microstructure evolution of the composites were studied. The results show that a small amount of CaO can significantly improve the compactness of the composites, which is mainly due to the formed sheet-like CA6 fill the gap between the solid solutions, and reduces the porosity of the composites. In addition, the sheet-like CA6 makes the connection between solid solutions closer, and the intergranular fracture gradually transforms into a mixed mode of intergranular and transgranular fracture. The best mechanical propertie is observed at S4 with the CaO content of 2 wt.%. Compared with sample S0 without CaO, the hardness, compressive strength and flexural strength of the S4 were increased by 35.19 %, 49.69 %, and 68.34 %, respectively. The addition of excessive CaO will deteriorate the mechanical properties of the composites, because the formation of a large number of layered CA6 increases the porosity of the composites. Furthermore, a small amount of CaO addition can significantly improve the thermal shock resistance of the composites. After 10 and 20 thermal shock cycles, the strength loss rates of S4 are only 5.83 % and 8.74 %, respectively.     

Microstructure and mechanical properties of Al2O3/Y3Al5O12/ZrO2 hypereutectic directionally solidified ceramic prepared by laser floating zone

Al₂O₃/Y₃Al₅O₁₂/ZrO₂ directionally solidified ceramic has been considered as a promising candidate for ultrahigh temperature structural materials due to its excellent performance even close to its melting point. In this work, laser floating zone (LFZ) solidification experiments were performed on Al₂O₃/Y₃Al₅O₁₂/ZrO₂ hypereutectic with the solidification rates between 2 μm/s and 30 μm/s. The full eutectic lamellar microstructure is obtained with hypereutectic composition. The solid/liquid interface morphology is investigated. The microstructure characteristic is discussed based on the solid/liquid interface. The variation of lamellar spacing with different compositions and solidification rates was reported and discussed by considering an irregular eutectic growth model. The maximum hardness and fracture toughness are 19.06 GPa and 3.8 MPa m^1/2, respectively. The toughening mechanism of ZrO₂ is discussed based on the scenario of the crack propagation pattern.     

Comparison of the homemade and commercial graphene in heightening mechanical properties of Al2O3 ceramic

Graphene has been successfully fabricated by a novel method, using graphite powder and NMP (N-Methyl Pyrrolidone) as the raw materials based on the principles of liquidoid exfoliation and mechanical milling. SEM, TEM and Raman spectrum were utilized to characterize the morphology of the homemade graphene, illustrating the few defects and rare layers were endowed in this study. Afterwards, the homemade and commercial graphene were doped into Al₂O₃ powder with the mass ratio of 0%, 1%, 2%, and 3% to reinforce the mechanical properties of the matrix. The composites were processed at 1600 °C, pressure of 30 MPa and soaking time of 1 h by vacuum hot pressing. The test results illustrated the bending strength and fracture toughness tended to be intensive at first and subdued afterwards, achieving the optimal performance of 625.4±18.2 MPa and 6.07±0.22 MPa m^1/2 at 2 wt% prepared graphene additive, and the commercial grapheme owned the best heighten effect in 3 wt% graphene/Al₂O₃ composites. Compared to the blank Al₂O₃ sintered samples, the graphene/Al₂O₃ specimens (both prepared and commercial additive) behaved evident increase in mechanical properties, even upon 30% enhanced in fracture toughness and bending strength generally by the prepared grapheme. Moreover, the prepared graphene had better improvement effect than commercial graphene in enhancing mechanical properties of Al₂O₃ ceramic.     

Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations

Transparent Y₂O₃ ceramics were fabricated by the solid-state reaction and vacuum sintering method using La₂O₃, ZrO₂ and Al₂O₃ as sintering aids. The microstructure of the Y₂O₃ ceramics sintered from 1550 °C to 1800 °C for 8 h were analyzed by SEM. The sintering process of the Y₂O₃ transparent ceramics was optimized. The results showed that when the samples were sintered at 1800 °C for 8 h under vacuum, the average grain sizes of the ceramics were about 3.5 µm. Furthermore, the transmittance of Y₂O₃ ceramic sintered at 1800 °C for 8 h was 82.1% at the wavelength around the 1100 nm (1 mm thickness), which was close to its theoretical value. Moreover, the refractive index of the Y₂O₃ transparent ceramic in the temperature range from 30 °C to 400 °C were measured by the spectroscopic ellipsometry method.     

Effect of Y2O3-stabilized ZrO2 whiskers on the microstructure,mechanical and wear resistance properties of Al2O3 based ceramic composites

The aim of this study was to improve the mechanical properties of Al₂O₃ ceramics by the addition of Y₂O₃-stabilized ZrO₂ whiskers (designated as Al₂O₃/YSZ_W composite) through the flux method and hot-pressing technology. The effect of YSZ_W content on their microstructure, phase composition and transformability, mechanical properties, and wear resistance was systematically investigated. The Al₂O₃/YSZ_W composites containing 10 wt% YSZ_W exhibited the best mechanical performance, including the highest content of YSZ_W tetragonal phase and transformability as well as the largest values in their relative density (99.5%), hardness (1969 HV), fracture toughness (9.57 MPa m^1/2) and flexural strength (855 MPa). The strengthening and toughening of the Al₂O₃/YSZ_W composites were attributed to the YSZ_W tetragonal-monoclinic phase transformation as well as the whiskers reinforcing effect. Furthermore, the Al₂O₃/YSZ_W composites also showed the highest friction and wearing properties.     

Effect of Ba concentration on phase stability and mechanical and thermal properties of La2Mo2O9

Ba-substituted La₂Mo₂O₉ ((La_1−xBa_x)₂Mo₂O_9−δ, x = 0–0.12) was prepared and the thermal and mechanical properties were evaluated. The thermal expansion coefficients (TECs) were determined from high-temperature X-ray diffraction (XRD) analysis. Phase transition in La₂Mo₂O₉ was suppressed via substitution of Ba for La, as demonstrated by differential scanning calorimetry (DSC) analysis. The mechanical properties, such as the bulk modulus, shear modulus, Young’s modulus, compressibility, and Debye temperature were evaluated from the measured sound velocities. The thermal conductivity was evaluated from the thermal diffusivity, heat capacity, and density in the temperature range from room temperature to 1073 K. The thermal conductivity decreased with increasing Ba content. Theoretical calculations based on the Klemens–Callaway model were performed to analyze the thermal conductivity, and the results suggest that the reduction of the thermal conductivity was mainly attributed to oxygen defects in the anion sublattice of La₂Mo₂O₉.     

Characterization of B4C-SiC ceramic composites prepared by ultra-high pressure sintering

Additive-free boron carbide (B₄C) – silicon carbide (SiC) ceramic composites with different B₄C and β-SiC powders ratio were densified using the high-pressure “anvil-type with hollows” apparatus at 1500 °C under a pressure of 4 GPa for 60 s in air. The effect of starting powders ratio on the composites sintering behavior, relative density, microstructural development, and thermomechanical properties was studied. The sintered samples hardness was found to be in the range from 24 to 31 GPa. The thermal conductivity measurements, conducted in the temperature range from room temperature to 1000 °C, showed that the thermal diffusivity of sintered samples was between 6 and 9.5 mm²/s whereas the thermal conductivity was in the range from 16 to 28 W/(m K). The results of this study show that the high-pressure sintering can be a very effective low-temperature densification method for the obtainment of additive-free B₄C - β-SiC ceramic composites.     

Fabrication of high thermal conductivity silicon nitride ceramics by pressureless sintering with MgO and Y2O3 as sintering additives

The fabrication of silicon nitride (Si₃N₄) ceramics with a high thermal conductivity was investigated by pressureless sintering at 1800 °C for 4 h in a nitrogen atmosphere with MgO and Y₂O₃ as sintering additives. The phase compositions, relative densities, microstructures, and thermal conductivities of the obtained Si₃N₄ ceramics were investigated systemically. It was found that at the optimal MgO/Y₂O₃ ratio of 3/6, the relative density and thermal conductivity of the obtained Si₃N₄ ceramic doped with 9 wt% sintering aids reached 98.2% and 71.51 W/(m·K), respectively. EDS element mapping showed the distributions of yttrium, magnesium and oxygen elements. The Si₃N₄ ceramics containing rod-like grains and grain boundaries were fabricated by focused ion beam technique. TEM observations revealed that magnesium existed as an amorphous phase and that yttrium produced a new secondary phase.     

Effect of milling type on the microstructural and mechanical properties of W-Ni-ZrC-Y2O3 composites

This study reports the effect of milling type on the microstructural, physical and mechanical properties of the W-Ni-ZrC-Y₂O₃ composites. Powder blends having the composition of W-1 wt% Ni-2 wt% ZrC-1 wt% Y₂O₃ were milled at room temperature for 12 h using a Spex™ 8000D Mixer/Mill or cryomilled in the presence of externally circulated liquid nitrogen for 10 min using a Spex™ 6870 Freezer/Mill or sequentially milled at room temperature and cryogenic condition. Then, powders were compacted in a hydraulic press under a uniaxial pressure of 400 MPa and green bodies were sintered at 1400 °C for 1 h under Ar/H₂ atmosphere. Phase and microstructural characterization of the milled powders and sintered samples were performed using X-ray diffractometer (XRD), TOPAS software, scanning electron microscope/energy dispersive spectrometer (SEM/EDS), X-ray fluorescence (XRF) spectrometer and particle size analyzer (PSA). Archimedes density and Vickers microhardness measurements, and sliding wear tests were also conducted on the sintered samples. The results showed that sequential milling enables the lowest average particle size (214.90 nm) and it is effective in inhibiting W grain coarsening during sintering. The cryomilled and sintered composite yielded a lower hardness value (5.80±0.23 GPa) and higher wear volume loss value (149.42 µm³) than that of the sintered sample after room temperature milling (6.66±0.39 GPa; 102.50 µm³). However, the sequentially milled and sintered sample had the highest relative density and microhardness values of 95.09% and 7.16±0.59 GPa and the lowest wear volume loss value of 66.0 µm³.     

Pressureless sintered Al4SiC4 ceramics with Y2O3 addition

Highly densified Al₄SiC₄ ceramics with a relative density of 96.1% were prepared by pressureless sintering using 2 wt% Y₂O₃ as additives. The densification mechanism, phase composition, microstructures and mechanical properties of Al₄SiC₄ ceramics were investigated. Y₂O₃ in-situ reacted with the oxygen impurities in Al₄SiC₄ powder to form a yttrium aluminate liquid phase during sintering, which promoted the densification and anisotropic grain growth. The final Al₄SiC₄ ceramics were composed of equiaxed grains and columnar grains, and presented a bimodal grain distribution. The mechanical properties of the pressureless sintered Al₄SiC₄ ceramics were better than those reported for hot pressed Al₄SiC₄, including a flexural strength of 369 ± 24 MPa, fracture toughness of 4.8 ± 0.1 MPa m^1/2 and Vickers hardness of 11.3 ± 0.2 GPa. Pressureless sintering of Al₄SiC₄ ceramics is of great significance for the development and practical application of Al₄SiC₄ ceramic parts, especially with big size and complex shape.     

Effect of nano-AlN addition on thermal diffusivity and mechanical strength of porous AL2O3 ceramic composites

In a properly made porous abrasive composite, the vitrified bond should ideally cover the grains and form a continuous network of bridges, and thus part of the heat energy from the grinding process is also transferred to the vitrified bond. Until recently, most studies on the design of composite properties have focused mainly on improving their mechanical strength and wear resistance, but increasingly the very important aspect of their thermal properties is noticed. The vitrified Al₂O₃ composites were made from Al₂O₃ grains, vitrified bond of Na₂OK₂OAl₂OB₂O₃SiO₂ and AlN nanopowder. The increase in porosity in the tested composites is the effect of the AlN decomposition reaction. Crystalline phases were identified in both composites - α-Al₂O₃ and NaAl₁₁O₁₇, but with a different percentage share in individual composites. In composites doped with AlN nanopowder, the proportion of NaAl₁₁O₁₇ crystalline phase decreases, due to its high susceptibility to reduction by Al, obtained from the AlN decomposition reaction. The product of the redox reaction is also Na⁺ ions, which may participate in the formation of the glass phase and thus increasing the fraction of the residual glass phase. As a result of the partial reduction of NaAl₁₁O₁₇ phase, an increase in α-Al₂O₃ content is observed. A higher proportion of α-Al₂O₃ phase with high thermal conductivity can be a factor that increases the rate of heat removal from the work zone.     

Microstructure control and mechanical properties of porous silicon nitride ceramics

Porous silicon nitride ceramics with a fibrous interlocking microstructure were synthesized by carbothermal nitridation of silicon dioxide. The influences of different starting powders on microstructure and mechanical properties of the samples were studied. The results showed that the microstructure and mechanical properties of porous silicon nitride ceramics depended mostly on the size of starting powders. The formation of single-phase β-Si₃N₄ and the microstructure of the samples were demonstrated by XRD and SEM, respectively. The resultant porous Si₃N₄ ceramics with a porosity of 71% showed a relative higher flexural strength of 24 MPa.     

A novel aerogels/porous Si3N4 ceramics composite with high strength and improved thermal insulation property

A novel ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite with high strength, low density, good dielectric properties and low thermal conductivity was synthesized by filling ZrO₂-SiO₂ aerogels into the porous Si₃N₄ ceramics through vacuum sol-impregnating. The effects of aerogels on the microstructure and properties of composite were discussed. The results show that aerogels could form a mesoporous structure and significantly decrease the thermal conductivity from 9.8 to 7.3 W m^?1 K^?1. Meanwhile, the density, mechanical and dielectric properties of the porous Si₃N₄ ceramics could not be affected after introducing ZrO₂-SiO₂ aerogels. The composite exhibits high porosity (62.6%), high flexural strength (53.86 MPa) and low dielectric constant (2.86). The ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite shows great potential as radome materials applied in the fields of aerospace.     

Effects of SmF3 addition on aluminum nitride ceramics via pressureless sintering

Aluminum nitride (AlN) ceramics with dense structure, high thermal conductivity, and exceptional mechanical properties were fabricated by pressureless sintering with a novel non-oxide sintering additive, samarium fluoride (SmF₃). The results showed that the use of a moderate amount of SmF₃ promoted significant densification of AlN and removed the oxygen impurity. This led to the formation of fine and isolated secondary phase that cleaned the grain boundaries and increased the contact between AlN grains, remarkably enhancing thermal conductivity. Furthermore, SmF₃ also exhibited grain refinement and grain boundary strengthening effects similar to traditional sintering additive, samarium oxide (Sm₂O₃), leading to high mechanical properties in SmF₃-doped AlN samples. The most optimal characteristics (thermal conductivity of 190.67 W·m⁻¹·K⁻¹, flexural strength of 403.86 ± 18.27 MPa, and fracture toughness of 3.71 ± 0.19 MPa·m^1/2) were achieved in the AlN ceramic with 5 wt% SmF₃.     

Mechanical properties and microstructure of Al2O3-SiCw ceramic tool material toughened by Si3N4 particles

Al₂O₃-SiC_w toughened ceramic tools play vital role in high-speed machining of nickel-based superalloys due to their superior mechanical properties. Herein, owing to synergistic toughening mechanism, α-Si₃N₄ particles are employed as reinforcement phase into Al₂O₃-SiC_w ceramic composite to optimize mechanical properties of Al₂O₃-SiC_w ceramic tools. Moreover, the influence of Si₃N₄ content and sintering parameters on microstructure and mechanical properties of Al₂O₃-20 vol%SiC_w ceramic tool material is systematically investigated. Results reveal that appropriate amount of Si₃N₄ particles is required to effectively increase the density of Al₂O₃-SiC_w ceramic composites. The presence of Si₃N₄ particles leads to formation of novel β-sialon phase during hot-press sintering, which effectively enhances fracture toughness and flexural strength of Al₂O₃-SiC_w ceramic composites. It is observed that grain size of newly formed β-sialon phase is extremely sensitive to hot-pressing sintering conditions. The degree of chemical transformation of α-Si₃N₄ into Si_6-zAl_zO_zN_8-z (β-sialon) and z-value of Si_6-zAl_zO_zN_8-z are significantly influenced by sintering temperature. Overall, Al₂O₃-20 vol%SiC_w-15 vol%Si₃N₄ ceramic tool material, with 1.5 vol%Y₂O₃-0.5 vol%La₂O₃-0.5 vol%CeO₂ (YLC) sintering additive, rendered optimal mechanical properties after sintering at 1600 °C under 32 MPa for 30 min. Improved mechanical performance can be ascribed to synergistic toughening and strengthening influence of whiskers and particles.