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 Y2O3 on the physical properties and biocompatibility of β–SiAlON ceramics

To investigate the effects of Y₂O₃ on the physical properties and biocompatibility of β–SiAlON ceramics, β–SiAlON ceramics were prepared with Al, Si, and α–Al₂O₃ powders using a direct nitriding technique. As a sintering additive, Y₂O₃ helps lower the sintering temperature and forms β–SiAlON ceramics. In this study, the physical and biological properties of the prepared ceramics were investigated to evaluate their use as bone-repairing material. Experiments revealed that the main crystal composition of the sample was Si₄Al₂O₂N₆, containing small amount of additional phases Y₃Al₅O₁₂ with increasing content of Y₂O₃. The porosity and compressive strength initially decrease and then increase to their initial values, whereas the bulk density exhibits the opposite trend with an increased proportion of Y₂O₃. The proliferation of osteoblastic and angiogenic cells demonstrates that β–SiAlON and Y₃Al₅O₁₂ have good biocompatibility; however, the sample porosity has a slight effect on the cell proliferation rate. This implies that in human tissues, bone-repairing speed can be adjusted by modifying the sample porosity or material surface roughness. Therefore, Y₂O₃ can be added to β–SiAlON ceramics to regulate their microstructure, physical properties, and biological properties for tissue engineering applications.     

Influence of sintering additives on densification kinetics and high‐temperature strength in γ‐Y2Si2O7 ceramics

Pressureless sintering of pure γ‐Y₂Si₂O₇ powders that had been synthesized by a solid‐liquid reaction method using Y₂O₃ and SiO₂ powders with Li₂O, MgO, and Al₂O₃ additives was reported. The sintering kinetics of γ‐Y₂Si₂O₇ powders was analyzed to track details of densification evolution. Apparent activation energies of the densification of γ‐Y₂Si₂O₇ powders were reported for the first time, which was 57.1, 96.6, and 100.2 kJ/mol for the powders with Li₂O, MgO, and Al₂O₃ additives, respectively, indicating that Li₂O could promote the densification behavior effectively. The flexural strengths as a function of temperature for the γ‐Y₂Si₂O₇ ceramics with different additives were also investigated. The degradation of high‐temperature flexural strength was mainly ascribed to the softening of grain‐boundary glassy phase. γ‐Y₂Si₂O₇ specimens fabricated using the powders with MgO or Al₂O₃ additives exhibited better high‐temperature mechanical properties.     

Gelcasting and pressureless sintering of silicon carbide ceramics using Al2O3–Y2O3 as the sintering additives

In this paper, silicon carbide ceramics were prepared by aqueous gelcasting and pressureless sintering using Al₂O₃ and Y₂O₃ as the sintering additives. In order to develop well dispersed SiC slurries in the presence of sintering additives, the Al₂O₃ and Y₂O₃ powder was treated in the citric acid solution in advance. Zeta potential measurement showed that the isoelectric point (IEP) of Al₂O₃ and Y₂O₃ powder moved toward low pH region after treatment. Rheological measurement confirmed that the addition of as-treated powder showed very limited influence on the slurry properties as compared to that of untreated powder. SiC slurries with solid content of 54 vol% and enough fluidity can be developed. After gelcasting and pressureless sintering, SiC ceramics with nearly full density, fine grained and homogeneous microstructure can be obtained. Results showed that the surface treatment of Al₂O₃ and Y₂O₃ with citric acid is effective for the gelcasting process of SiC.     

Low-temperature joining of SiC ceramics with Y2O3-Al2O3 interlayer by SiO2-based liquid phase extrusion strategy

In order to reduce the joining temperature of SiC ceramics by glass-ceramic joining, some oxides were usually introduced into to Y₂O₃–Al₂O₃ for reducing the eutectic temperature. However, the joints might have poor high-temperature resistance due to the low melting point of the joining layer. In the present work, based on novel SiO₂-based liquid phase extrusion strategy, joining of SiC ceramics with Y₂O₃–Al₂O₃ interlayer was carried out by using Y₂O₃–Al₂O₃–SiO₂ as the filler through spark plasma sintering (SPS). The SiO₂-free interlayer of Y₂O₃–Al₂O₃ was used for comparison. It was found that SiC joints using Y₂O₃–Al₂O₃ could be only joined at a high temperature of 1800 °C, and the thickness of the interlayer was about 20 μm. The shear strength of the joint obtained at 1800 °C was 89.62 ± 4.67 MPa and the failure located in the SiC matrix. By contrast, reliable joining of SiC ceramics could be finished at as low as 1550 °C by extrusion of SiO₂-containing liquid phase when using Y₂O₃–Al₂O₃–SiO₂ as the interlayer, alongside the interlayer thickness of only several microns. The joint strengths after joining at 1550 °C was 84.90 ± 3.48 MPa and the failure located in matrix position. The joining mechanism was discussed by combining the detailed microstructure analysis and phase diagram.     

Fabrication of Si3N4 ceramics by post-reaction sintering using Si–Y2O3–Al2O3 nanocomposite particles prepared by mechanical treatment

Post-reaction sintering of a powder compact of Si and sintering aids is a useful technique for fabricating silicon nitride (Si₃N₄) ceramics at low costs. In order to inhibit the inhomogeneous and uncontrollable exothermic nitridation of Si in the powder compact, Si–Y₂O₃–Al₂O₃ nanocomposite particles are designed as an aid for post-reaction sintering. These Si–Y₂O₃–Al₂O₃ nanocomposite particles are prepared via mechanical treatment applying high shear stress. Scanning electron microscopy (SEM) observations show that Y₂O₃ and Al₂O₃ particles are homogenously dispersed, and fixed to the Si particles. A green compact prepared using the Si–Y₂O₃–Al₂O₃ nanocomposite particles results in lower electrical resistivity than that prepared using a powder mixed by wet ball-milling, which suggests that Si particles in the green compact prepared using the nanocomposite particles are isolated by Y₂O₃ and Al₂O₃ particles. The isolation of Si particles by the sintering aids successfully prevents the Si particles from melting and agglomerating during the nitridation process, resulting in a higher nitridation ratio and higher α-Si₃N₄ phase content due to the inhibition of rapid heat transfer caused by the exothermic reaction. The nitridation ratio also increases with the applied power during mechanical treatment. As a result of firing the homogeneously nitrided powder compacts at high temperatures, Si₃N₄ ceramics with homogeneous microstructure and improved density are successfully fabricated in this manner.     

Reactive sintered highly transparent AlON ceramics with Y2O3-MgAl2O4-H3BO3 ternary additive

In this study, highly transparent aluminate oxynitride (AlON) ceramics were prepared via the reactive sintering of Al₂O₃ and AlN powders using a Y₂O₃-MgAl₂O₄-H₃BO₃ ternary sintering additive. The ternary doping process resulted in the efficient preparation of transparent AlON ceramics with small grains and high transmittance as compared to the binary doping (Y₂O₃-MgAl₂O₄) process. The addition of 0.1 wt.% Y₂O₃-0.4 wt.% MgAl₂O₄-0.12 wt.% H₃BO₃ resulted in the formation of a 4-mm-thick AlON ceramic with high transmittance (81% at 600 nm) and low haze (3.46%). This is the best performance in terms of the thickness and transmittance reported for AlON transparent ceramics prepared by the reactive sintering method.     

Effects of mechanically alloying Al2O3 and Y2O3 additives on the liquid phase sintering behavior and properties of SiC

In order to improve the sintering of SiC, mixtures of Al₂O₃ and Y₂O₃ powders are commonly included as sintering additives. The aim of this work was to use mechanically alloyed Al₂O₃–Y₂O₃ mixtures as sintering additives to promote liquid phase sintering of SiC using spark plasma sintering. The results showed that milling reduced the particle size of the powders and led to the formation of complex oxide phases (YAP, YAM, and YAG) at low temperatures. As the ball milling time increased, the mass loss of specimens sintered with mechanically alloyed Al₂O₃–Y₂O₃ mixtures decreased, and accordingly the relative density increased. However, the hardness and flexural strength of sintered SiC specimens first increased and then decreased. Because the specimens prepared with oxides milled for a long time contained too much YAG/YAP and accordingly too much liquid at sintering temperature. This negatively affected the mechanical properties of the SiC specimens because of the increased volume of the complex oxide phases, which have inferior mechanical properties to SiC, in the sintered specimens. When the ball milling time was 6 h, the hardness (24.02 GPa) and flexural strength (655.61 MPa) of the SiC specimens reached maximum values.     

Effects of Y2O3/MgO ratio on mechanical properties and thermal conductivity of silicon nitride ceramics

In this work, the effects of Y₂O₃/MgO ratio on the densification behavior, phase transformation, microstructure evolution, mechanical properties, and thermal conductivity of Si₃N₄ ceramics were investigated. Densified samples with bimodal microstructure could be obtained by adjusting the ratio of Y₂O₃/MgO. It was found that a low Y₂O₃/MgO ratio facilitated the densification of Si₃N₄ ceramics while a high Y₂O₃/MgO ratio benefited the phase transformation of Si₃N₄ ceramics. Best mechanical properties (flexural strength of 875 MPa, and fracture toughness of 8.25 MPa·m^1/2, respectively) and optimal thermal conductivity of 98.04W/(m·K) were achieved in the sample fabricated with Y₂O₃/MgO ratio of 3:4 by sintering at 1900°C for 4 h.     

Preparation of porous Al2O3 ceramics with enhanced properties by SLS using Al2O3 poly-hollow microspheres (PHMs) coated with CaSiO3 sintering additive

The mechanical properties of porous ceramics prepared by poly-hollow microspheres (PHMs) is usually low because of the weak bonding between different ceramic PHMs. In this paper, CaSiO₃ were coated to the surface of Al₂O₃ PHMs through co-precipitation method as sintering additive to improve the properties of Al₂O₃ poly-hollow microsphere ceramics (Al₂O₃ PHM ceramics). The influence of different amount of CaCl₂ solution on properties of the Al₂O₃ PHM ceramics such as phase composition, microstructure, porosity and mechanical properties were studied. The porosity of the Al₂O₃ PHM ceramics decreased from 77.03% to 68.16% with the increase of CaCl₂ solution amount, while compressive strength increased 29 times from 0.29 MPa to 8.39 MPa. The addition of the CaSiO₃ could decrease the sintering temperature of Al₂O₃ PHM ceramics and significantly improve the mechanical properties of Al₂O₃ PHM ceramics, which is beneficial for preparing highly porous Al₂O₃ PHM ceramics with high mechanical properties and complex shapes.     

Microwave sintering of IR-transparent Y2O3–MgO composite ceramics

A method for preparation of dense Y₂O₃–MgO composite ceramics by the microwave sintering was developed. The initial powders were obtained by glycine-nitrate self-propagating high-temperature synthesis (SHS) with different oxidant-to-fuel ratio. Density and IR-transmission of microwave sintered Y₂O₃–MgO ceramics increase with respect to dispersity of the SHS-powders and reach its maximum values for the powder prepared in a 20% fuel excess. The sintering behavior of Y₂O₃–MgO compacts was investigated by optical dilatometry and measuring an electric conductivity upon heating. Significant microwave radiation power surges at temperatures of 900–1000 °C, caused by the decomposition of magnesium carbonate, have been found. As a result of matching the conditions for the synthesis of powders and sintering modes, a transmission of composite ceramics of 78% at a wavelength of 6 μm was achieved at a maximum processing temperature of 1500 °C.     

Effects of Al2O3–Y2O3/Yb2O3 additives on microstructures and mechanical properties of silicon nitride ceramics prepared by hot-pressing sintering

Silicon nitride (Si₃N₄) ceramics doped with two different sintering additive systems (Al₂O₃–Y₂O₃ and Al₂O₃–Yb₂O₃) were prepared by hot-pressing sintering at 1800℃ for 2 h and 30 MPa. The microstructures, nano-indentation test, and mechanical properties of the as-prepared Si₃N₄ ceramics were systematically investigated. The X-ray diffraction analyses of the as-prepared Si₃N₄ ceramics doped with the two sintering additives showed a large number of phase transformations of α-Si₃N₄ to β-Si₃N₄. Grain size distributions and aspect ratios as well as their effects on mechanical properties are presented in this study. The specimen doped with the Al₂O₃–Yb₂O₃ sintering additive has a larger aspect ratio and higher fracture toughness, while the Vickers hardness is relatively lower. It can be seen from the nano-indentation tests that the stronger the elastic deformation ability of the specimens, the higher the fracture toughness. At the same time, the mechanical properties are greatly enhanced by specific interlocking microstructures formed by the high aspect ratio β-Si₃N₄ grains. In addition, the density, relative density, and flexural strength of the as-prepared Si₃N₄ ceramics doped with Al₂O₃–Y₂O₃ were 3.25 g/cm³, 99.9%, and 1053 ± 53 MPa, respectively. When Al₂O₃–Yb₂O₃ additives were introduced, the above properties reached 3.33 g/cm³, 99.9%, and 1150 ± 106 MPa, respectively. It reveals that microstructure control and mechanical property optimization for Si₃N₄ ceramics are feasible by tailoring sintering additives.     

Microstructure and room/elevated-temperature mechanical properties of hot-pressed h-BN composite ceramics with La2O3-Al2O3-SiO2 addition

Hexagonal boron nitride (h-BN) composite ceramics were fabricated by hot pressing using h-BN, La₂O₃, Al₂O₃ and amorphous SiO₂ as the raw materials. The effects of La₂O₃-Al₂O₃-SiO₂ addition on the bulk density, apparent porosity, microstructural evolution and room/elevated-temperature mechanical properties of h-BN composite ceramics were investigated. The results indicated that liquid phase generated by the reaction of La₂O₃-Al₂O₃ and SiO₂ exhibited a good wettability with h-BN grains, contributing to fill the pores and improve the densification. Therefore, h-BN composite ceramics with synchronous introduction of La₂O₃-Al₂O₃ and SiO₂ show the better mechanical properties compared with separate addition of La₂O₃-Al₂O₃ or SiO₂. h-BN composite ceramic incorporated with 10 vol.% La₂O₃-Al₂O₃ and 20 vol.% SiO₂ shows the highest room-temperature strength of 266.4 MPa and elevated-temperature strength of 389.0 MPa at 800 °C.     

IR-transparent MgO-Y2O3 ceramics by self-propagating high-temperature synthesis and spark plasma sintering

A glycine–nitrate self-propagating high temperature synthesis (SHS) was developed to produce composite Y₂O₃–MgO nanopowders. Based on the thermodynamic calculations a 0.25YMg₂(NO₃)₇-0.75NH₂CH₂COOH precursor composition was selected to prepare low agglomerated uniform composite yttria-magnesia powder. Near full dense composite ceramics were fabricated based on the prepared powders by the spark plasma sintering method. IR-transmittance and hardness of the Y₂O₃–MgO ceramics were studied in correlation with sintering conditions. The best transmittance of 80.9%@5 μm and H_v = 10.2 GPa were measured for the sample obtained at 1150 °C.     

Effects of Ho3+ concentration on the fabrication and properties of Ho: Y2O3-MgO nanocomposite for Mid-infrared laser applications

Infrared transparent Ho: Y₂O₃-MgO nanocomposite ceramics with a volume ratio of 50:50 (RE₂O₃: MgO) were prepared by combining sol-gel powder synthesis and hot-pressing sintering techniques. In order to obtain Ho: Y₂O₃-MgO nanocomposite ceramics with fine grain size, dense microstructure and homogeneous phase domains, the effect of sintering temperature and Ho³⁺ doping concentration were studied. Transmittance and SEM measurement revealed that the grain size of 3 at.% Ho: Y₂O₃-MgO ceramic sintered at 1250 °C is 141 nm, and the transmission is up to 85.2% at 5 μm. The detailed spectroscopic investigation of x at.% Ho: Y₂O₃-MgO (x = 1, 3, 5, 7, 9, 15) ceramics was performed. The nanocomposites exhibited photoluminescence properties similar to that of Ho: Y₂O₃ crystals and ceramics. In addition, the thermal conductivity of 3 at.% Ho: Y₂O₃-MgO ceramic is 13.04 W/m·K, which is superior to that of Ho:Y₂O₃ ceramics. The high transmission, excellent thermal conductivity, and outstanding optical characteristics indicated that Ho: Y₂O₃-MgO ceramics is a promising material for efficient infrared solid-state laser.     

Preparation of high-porosity Al2O3 ceramic foams via selective laser sintering of Al2O3 poly-hollow microspheres

In this paper, high-porosity Al₂O₃ ceramic foams called Al₂O₃ PHM ceramics were fabricated through selective laser sintering (SLS) via Al₂O₃ poly-hollow microspheres (Al₂O₃ PHMs). SLS parameters were optimized by an orthogonal experiment as to be laser power = 6 W, scanning speed = 1800 mm/s, and scanning space = 0.15 mm. The effect of sintering temperature on microstructure, shrinkage, porosity, phase composition, mechanical properties and pore size distribution of Al₂O₃ PHM ceramics were investigated. When sintering temperature increased, Al₂O₃ PHM ceramics contained only Al₂O₃ phase and were gradually densified. With the raise of sintering temperature, the porosity of Al₂O₃ PHM ceramics decreased gradually from 77.09% to 72.41%, but shrinkage in H direction and compressive strength of Al₂O₃ PHM ceramics increased from 6.63% and 0.18 MPa to 13.10% and 0.72 MPa, respectively. Sintering temperature had little effect on pore size distribution of Al₂O₃ PHM ceramics, which only declined from 24.2 to 21.4 μm with the increase of sintering temperature from 1600 to 1650 °C. This method can not only directly prepare ceramic foams with complex shapes, but also control properties of ceramic foams. It provides a simple preparation method for many kinds of ceramic foams with complex structure and high porosity by using PHMs with different composition.     

Sintering of Al2O3-SiC composite from sol-gel method with MgO, TiO2 and Y2O3 addition

Al₂O₃-SiC composite ceramics were prepared by pressureless sintering with and without the addition of MgO, TiO₂ and Y₂O₃ as sintering aids. The effects of these compositional variables on final density and hardness were investigated. In the present article at first α-Al₂O₃ and β-SiC nano powders have been synthesized by sol-gel method separately by using AlCl₃, TEOS and saccharose as precursors. Pressureless sintering was carried out in nitrogen atmosphere at 1600 °C and 1630 °C. The addition of 5 vol.% SiC to Al₂O₃ hindered densification. In contrast, the addition of nano MgO and nano TiO₂ to Al₂O₃-5 vol.% SiC composites improved densification but Y₂O₃ did not have positive effect on sintering. Maximum density (97%) was achieved at 1630 °C. Vickers hardness was 17.7 GPa after sintering at 1630 °C. SEM revealed that the SiC particles were well distributed throughout the composite microstructures. The precursors and the resultant powders were characterized by XRD, STA and SEM.     

Effect of Y2O3 on the corrosion resistance of two-step sintered Al5Y3O12-MgAl2O4 sidewalls in the aluminum electrolyte

Magnesium–aluminum spinel (MAS) precursor powder was synthesized through a microwave hydrothermal method. The synergistic effects of sintering process and sintering aids on the densification, hardness and corrosion resistance of MAS were revealed. X-ray diffraction analysis (XRD), Archimedes’ drainage method, fully automatic micro-Vickers hardness test and scanning electron microscopy (SEM) were performed to analyze the phase composition, bulk density, hardness microstructure and corrosion depth of the samples, respectively. Results revealed that the best two-step sintering condition is 1650 °C/3 min/1550 °C/20 h. The MAS products obtained under the best condition have clear grain boundaries, uniform particle size distribution, and few pores. When the amount of Y₂O₃ added is 4 wt.%, Y₂O₃ and Al₂O₃ form the second-phase solid solution Al₅Y₃O₁₂, which activates the crystal lattice and benefits the sintering densification of MAS. Under these conditions, the relative density of the MAS composite ceramics prepared is relatively large (95.94 %), the grain size is relatively uniform, the hardness is relatively large (1264 HV), and the corrosion depth is relatively small (94.58 μm).     

Preparation of O′-Sialon-based ceramics by two-stage liquid phase sintering and study on the toughening mechanism of ultrafine-grained sintered clusters

Ultrafine-grained O′-Sialon-based ceramics were prepared by two-stage sintering at 1250 °C, with large particle GH4169 superalloy powder and nano Al₂O₃–Y₂O₃ as composite sintering aids. The effects of these aids on the densification, microstructure, and mechanical properties of O′-Sialon-based ceramics during two-stage sintering were also studied. Studies have shown that the densification process of O′-Sialon-based ceramics promoted by composite sintering additives, presents with the characteristics of two-stage liquid-phase sintering. In the first stage, GH4169 formed ultrafine-grained sintered clusters in the sintered material through liquid phase diffusion. In the second stage, the uniformly dispersed nano Al₂O₃–Y₂O₃ realized the uniform sintering of the material. In the fracture process, the ultrafine-grained sintered clusters hindered the crack propagation and promoted multiple deflections of the crack around the edge of the clusters, achieving the effect of crack deflection toughening. This effect, dominated by ultrafine-grained sintered clusters, significantly improved the fracture toughness of O′-Sialon-based ceramics up to 8.52 MPa m^1/2.     

Low temperature pressureless sintering of α-SiC with Al2O3 and CeO2 as additives

The combination of Al₂O₃ and CeO₂ was testified as suitable sintering additive for liquid phase sintering of SiC ceramics, which has lower sintering temperature than that sintered with Al₂O₃ and Y₂O₃ as sintering aids. However, the mechanical properties including flexural strength, Vickers’ hardness and fracture toughness of this system were similar to those of the samples sintered with Al₂O₃ and Y₂O₃ as sintering aids. The good wettability of the eutectic liquid phase on SiC plate, the high solubility of SiC particles into the liquid phase and the penetration of the liquid phase along the SiC–SiC grain boundaries all confirmed the suitability of the combination of Al₂O₃ and CeO₂ as liquid phase sintering additive for SiC.