Effects of various rare-earth additives on the sintering and transmittance of γ-AlON

Several rare earth elements (Sc, La, Pr, Sm, Gd, Dy, Er, and Yb) were evaluated to develop a sintering additive for transparent γ-AlON. After adding 0.2 wt. % of each of the rare earth additives to the α-Al₂O₃ and AlN starting material, hot pressing was performed at 1850 °C for 1 h under 20 MPa as a screening test, whereby the γ-AlON with Pr-nitrate showed the highest transmittance of 60.4 % at 632 nm. Two-step pressureless sintering was conducted as a separate test for the same starting material after adjusting the amount of Pr-nitrate to 0, 0.1, or 0.2 wt. % to enhance transmittance. The γ-AlON with 0.1 wt. % Pr-nitrate showed the highest transmittance of 80.36 % after 1st and 2nd sintering steps at 1610 and 1940 °C, respectively, indicating that Pr can be an alternative sintering additive to conventional Y₂O₃, MgO, and La₂O₃ for the fabrication of transparent γ-AlON.     

Aluminum oxynitride by SHS under high pressure of nitrogen gas

Aluminum oxynitride (γ-AlON) powders were prepared from low-caloric Al–Al₂O₃ mixtures by SHS under high pressure of nitrogen gas in the presence of heat-generating agent, Mg(ClO₄)₂, as a booster. The preparation procedure was characterized and optimized with respect to Al/Al₂O₃ ratio, amount of heatgenerating additive, and nitrogen gas pressure. The morphology of synthesized γ-AlON powders was found to depend on Al powder brand. Compared to γ-AlON powders obtained in chemical furnace, we managed to prepare less sintered arenaceous cakes that could be easily disintegrated into target γ-AlON powder. SHSproduced powders may find their application in production of optical ceramics.     

Phase composition,structural, and plasma erosion properties of ceramic coating prepared by suspension plasma spraying

Y₃Al₅O₁₂ (YAG), Y₂O₃, and Al₂O₃ ceramic coatings were manufactured to investigate the plasma erosion properties. The X‐Ray Diffraction (XRD) analysis confirmed that YAG coating was synthesized successfully by Y₂O₃ and Al₂O₃ mixture suspension using the plasma spraying method. Meanwhile, metastable phases were found in Y₂O₃ and Al₂O₃ coatings due to the quenching in cooling process of melted droplets. The coating surface morphology and microstructure of cross sections were characterized by SEM. The results reveal that coatings are composed by ultrafine splats and exhibit dense lamellar structure. The plasma erosion properties were evaluated at different etching test power under Ar/CF₄/O₂ plasma gas. The experimental results clarify that both of YAG and Y₂O₃ coatings show the better plasma erosion resistance than Al₂O₃ coatings. The formation of fluorination layer surface prevents the coatings from further erosion with plasma gas. Moreover, the etching rate of coatings depended on the fluorination and removing rate of fluoride layer.     

Uniform and fine Mg-γ-AlON powders prepared from MgAl2O4: A promising precursor material for highly-transparent Mg-γ-AlON ceramics

High-purity and sinterability Mg-γ-AlON (Mg_0.1Al_1.53O_1.89N_0.27) powders were synthesized by gas pressure sintering (GPS) of mixed powders of commercial Al₂O₃ and AlN, and lab-made MgAl₂O₄. The Mg-γ-AlON powders exhibited a uniform particle morphology and a small particle size of d₅₀ = 3.4 μm, owing to the use of MgAl₂O₄ as the Mg source. Highly-transparent Mg-γ-AlON ceramics were fabricated using the synthesized Mg-γ-AlON powders by spark plasma sintering (SPS) at 1800 °C for 5 min under an axial pressure of 80 MPa, followed by hot isostatic pressing (HIP) at 1800 °C for 2 h under a nitrogen gas pressure of 190 MPa. The ceramics showed a high in-line transmittance of ～ 80.5% at 450 nm, ascribed to the high sinterability of the MgAl₂O₄ raw powder that leads to a pore-free and fully densified microstructure. This indicates that MgAl₂O₄ as sintering additive is superior over MgO and MgF₂ in the fabrication of Mg-γ-AlON transparent ceramic.     

Fabrication of transparent γ-AlON by direct 2-step pressureless sintering of Al2O3 and AlN using an AlN-deficient composition

Transparent polycrystalline aluminum oxynitride (γ-AlON) was fabricated by the pressureless two-step sintering of α-Al₂O₃ and AlN after adding a small amount of MgO and Y₂O₃. The process was based on two assumptions. The first was the utilization of AlN-deficient non-stoichiometric composition to increase the cationic vacancies and the second was the selection of the 1st step sintering temperature that suppresses the formation of γ-AlON phase to achieve a high density after the 2nd sintering step. The 1st and 2nd sintering steps were performed at 1610–1650 and 1940–1990 °C, respectively, for 10 h in a 2.5 atmospheric nitrogen pressure, and the optimal sintering conditions were determined. The fabricated γ-AlON showed a mean grain size of 164–248 μm without the presence of significant scattering centers, where the sample prepared using an optimal condition revealed a very high transmittance of 84.7% along with comparable mechanical properties.     

The role of fluorination during the physicochemical erosion of yttria in fluorine-based etching plasmas

A physicochemical mechanism acting between the reactive plasma and the material surface controls the erosion of polycrystalline ceramics in fluorine containing etching plasmas. In this study, a Y₂O₃/YOF composite was exposed to a fluorine etching plasma. Relocalization enables the direct correlation of crystalline orientation with material response. Our study reveals an orientation dependent surface fluorination of Y₂O₃, which controls the etching resistance and morphology formation. Orientations near the low index planes (001), (010) and (100) exhibit the lowest stability due to a homogeneous surface reaction. The presented results help to extend the mechanistic understanding of the plasma-material interaction of Y₂O₃.     

Highly transparent cubic γ-Al2O3 ceramic prepared by high-pressure sintering of home-made nanopowders

Highly transparent gamma-aluminum oxide (γ-Al₂O₃) ceramics were fabricated for the first time, by combining homogeneous precipitation and high-pressure sintering in the absence of exogenous dopants. The resulting cubic γ-Al₂O₃ transparent ceramic material exhibits a promising replacement for single-crystal sapphire. The optimum optical properties are achieved in response to sintering at 5 GPa and a temperature of 300 °C and include maximum transmittance of 86% in the range of 0.6–1.2 µm which are properties that are comparable to those of single-crystal sapphire (∼86%). Vickers hardness (16 GPa) and compressive strength (350 MPa) in response to high-pressure sintering are also similar to those of a conventional sapphire single crystal. Meanwhile, the dielectric constant (9.46) is comparable to that of sapphire in the C-axis direction. These findings will facilitate further development of transparent Al₂O₃ ceramics for use in a wider range of optical applications.     

Passivation effect on the surface characteristics and corrosion properties of yttrium oxide films undergoing SF6 plasma treatment

This study investigates the structures, compositions and fluorocarbon-plasma etching behaviors of yttrium oxyfluoride (YOF) passivation films fabricated on sputter-deposited yttrium oxide (Y₂O₃) by high-density SF₆ plasma irradiation. High-resolution transmission electron microscopy and nano-beam electron diffraction confirmed a YOF passivation film containing multiple phases of (104) and (006) crystal planes was formed on the fluorinated Y₂O₃ surface. X-ray photoelectron spectroscopy revealed few changes in the chemical compositions and surface roughness of the YOF passivation film after fluorocarbon plasma etching, confirming the chemical stability of the SF₆ plasma-treated Y₂O₃ sample. The etching depth was ～20% lower on the SF₆ plasma-treated Y₂O₃ film than on the commercial Y₂O₃ coating. These results showed that the SF₆ plasma-treated Y₂O₃ films have an excellent erosion resistance properties compared to the commercial Y₂O₃ coatings.     

Compositional tailoring effect on crystal structure,mechanical and thermal properties of γ-AlON transparent ceramics

In view of the practical application of γ-AlON as a promising transparent structural ceramic, in-depth insight into its mechanical and thermal properties is essential. The solid-state MAS NMR technique was combined with XRD Rietveld refinement to confirm the crystal structure of Al_(8+x)/3O_4-xN_x (x = 0.299–0.575). These structural parameters were further applied to predict hardness and elastic properties based on theoretical exploration, which are in good agreement with the experimental values. A slight enhancement of mechanical properties with increasing nitrogen concentration is attributed to the stronger chemical bond in octahedra. The experimental thermal conductivity of γ-AlON transparent ceramics was improved slightly with the rise of x in the temperature range from 298 K to 1074 K. The intrinsic lattice thermal conductivity was determined by eliminating the extrinsic phonon scattering as well as the thermal radiation. The reason for the discrepancy between experimental and intrinsic thermal conductivity was revealed. The present methods provided powerful and accessible guidelines in optimizing the mechanical and thermal properties of oxynitride materials.     

Reaction sintering of AlN–AlON composites

Sintering behavior of three different compositions in the AlN–Al₂O₃ system using Y₂O₃ as a sintering aid was investigated. Samples with various ratios of AlN/Al₂O₃ were sintered in nitrogen atmosphere using a gas pressure furnace in the temperature range 1750–1950 °C. The densification of the samples was studied by shrinkage and relative density measurements. Results showed that samples containing 1 and 70 wt.% alumina were sintered to near theoretical density at 1800 °C; whereas the sample with 20 wt.% alumina never reached densities higher than 93% in the temperature range considered. It was found that the AlN/Al₂O₃ ratio and the sintering temperature had a great influence on the microstructure and crystalline phases present in the samples, namely, AlN, γ-AlON, 27R, and YAG. In the sample with 20 wt.% alumina, porosity formation prevented further densification. These porosities were probably due to the release of oxygen during sintering.     

Theoretical study on composition- and pressure-dependent mechanical properties of AlON solid solution

In view of the application of γ-AlON as transparent structural materials, it is significant to have a deep understanding of mechanical properties of γ-AlON as well as their response to external pressure. First-principles calculation combined with bond valence models was used to reconstruct Fd3m structure, and explore the composition and applied pressure dependence of mechanical properties of Al_24−8/3xO_24+8xN_8−8x. The reconstructed Fd3m structure of Al_24−8/3xO_24+8xN_8−8x was proved to be reasonable. The investigation demonstrated that the hardness and bulk modulus of bonds can be enhanced by increasing the ratio of bond valence to bond length and the bonds in octahedra contribute more to hardness and bulk modulus of Al_24−8/3xO_24+8xN_8−8x. The applicability of the inferences was assessed by MgAl₂O₄ and c-Si₃N₄. It can be inferred that the inferences cannot be applied to other spinel-type compounds but also provide guidance to designing new spinel-type compounds with desirable hardness and bulk modulus.     

Comparison of directional solidification of γ-TiAl alloys in conventional Al2O3 and novel Y2O3-coated Al2O3 crucibles

The effects of novel Y₂O₃-coated Al₂O₃ (Y₂O₃/Al₂O₃) crucibles on the microstructure and composition of directionally solidified TiAl alloys were investigated and compared with those of single layered Al₂O₃ and Y₂O₃ crucibles, based on which the corresponding alloy–crucible interaction mechanisms were discussed. The DS alloys exhibited a fully lamellar γ/α₂ structure interspersed with some Al₂O₃ or Y₂O₃ particles. Differently from that in the case of using Al₂O₃ crucibles, no interfacial interaction layer was found in the ingots prepared using Y₂O₃/Al₂O₃ crucibles. Dissolution and erosion were the main mechanisms responsible for the alloy–crucible interactions which increased with the heating temperature and interaction time. Nevertheless, the interaction extents when using Y₂O₃/Al₂O₃ crucibles were much lower than using Al₂O₃ crucibles, making the former promising candidate crucibles for the high quality DS of highly reactive TiAl alloys.     

Densification of Yttria Transparent Ceramics: The Utilization of Activated Sintering

Highly transparent 0.5 at.% Tm:Y₂O₃ ceramics were prepared by using solid‐state reaction combined with vacuum sintering method, with ZrO₂ and Al₂O₃ as sintering aids. Doping amount of ZrO₂ was fixed at 1 at.%, while the effect of Al₂O₃ on densification, microstructure evolution, and transmittance of the Y₂O₃ ceramics was carefully studied. It was found that the addition of Al₂O₃ was very effective in improving densification of Y₂O₃, due to the formation of an Al‐rich eutectic phase Y₄Al₂O₉ (YAM) during the sintering process. As the content of Al₂O₃ was increased from 0 to 81.8 wt ppm, porosity of the ceramics was decreased and transmittance was increased. However, when the content of Al₂O₃ was increased to 137 wt ppm, a secondary phase began to segregate at grain junctions. Further increase in the amount of Al₂O₃ led to an increase in both amount and size of the secondary phase. At the optimized content of Al₂O₃ with 81.8 wt ppm, the Tm:Y₂O₃ ceramics sintered at 1860°C for 13 h exhibited an in‐line transmittance of 83.0% at 2000 nm and 76.5% at 600 nm. It is expected that this finding can be readily applied to other transparent ceramics.     

Air plasma-sprayed Y2O3 coatings for Al2O3/Al2O3 ceramic matrix composites

Al₂O₃/Al₂O₃ ceramic matrix composites (CMC) are candidate materials for hot-gas leading components of gas turbines. Since Al₂O₃/Al₂O₃ CMC are prone to hot-corrosion in combustion environments, the development of environmental barrier coatings (EBC) is mandatory. Owing to its favorable chemical stability and thermal properties, Y₂O₃ is considered a candidate EBC material for Al₂O₃/Al₂O₃ CMC. Up to 1 mm thick Y₂O₃ coatings were deposited by means of air plasma spraying (APS) on Al₂O₃/Al₂O₃ CMC with a reaction-bonded Al₂O₃ bond-coat (RBAO). APS Y₂O₃ coatings exhibit a good adherence in the as-deposited state as well as upon isothermal annealing up to 1400 °C. Moreover, furnace cyclic testing performed at 1200 °C revealed an excellent durability. This is explained by the formation of a continuous, approximately 1 μm thick reaction zone at the APS Y₂O₃/RBAO interface. The reaction zone between Y₂O₃ and Al₂O₃ comprises three layers of thermodynamically stable yttrium-aluminates exhibiting strong bonding, respectively.     

Comparison of hydrothermal corrosion behavior of SiC with Al2O3 and Al2O3 + Y2O3 sintering additives

Fully dense SiC bulks with Al₂O₃ and Al₂O₃ + Y₂O₃ sintering additives were prepared by spark plasma sintering and the effect of sintering additives on the hydrothermal corrosion behavior of SiC bulks was investigated in the static autoclave at 400°C/10.3 MPa. The SiC specimen with Al₂O₃ sintering additive exhibited a higher weight loss and followed a linear law. However, the SiC specimen with Al₂O₃ + Y₂O₃ additive exhibited a lower weight loss and followed a parabolic law, indicating that the corrosion kinetic and mechanism were different for these two SiC bulks. Further examination revealed that, a deposited layer was formed on the surface of SiC specimen with Al₂O₃ + Y₂O₃ sintering additive after corrosion, which can effectively protect the SiC specimen from further corrosion, and thereby improved the corrosion resistance of the SiC specimen with Al₂O₃ + Y₂O₃ sintering additive.     

Processing map to control the erosion of Y2O3 in fluorine based etching plasmas

Due to the increasing number of applications for ceramic components in reactive etching processes, the interest in the specific erosion behavior of highly etch-resistant materials like yttrium oxide (Y₂O₃) has increased in the past years. Despite the large number of investigations already existing in this field, a more general understanding of the erosion mechanisms still lacks due to the limited comparability of these investigations. The huge difference in the kind of etching setups, processing parameters (bias voltage and plasma gas composition), and sample microstructures prevented consistent conclusions so far. To achieve a more general understanding, this study investigates the erosion behavior Y₂O₃ under a broad spectrum of plasma etching parameters. Therefore, the bias voltage is increased from 50 to 300 V and the plasma gas composition is gradually changed from Ar-rich to CF₄-rich compositions. This systematic approach allows to directly correlate the morphology changes caused by plasma erosion with the related plasma etching parameters and enables to better understand their influence on the depth of physical and chemical interactions, surface damage, and etching rate. We discovered three distinct erosion regimes, which exhibit specific erosion characteristics. Using these observations, a schematic processing map for Y₂O₃ was developed, which could help to estimate the severity of the erosion attack dependent on the processing parameters.     

Optical characterizations of GaN nanorods fabricated by natural lithography

We fabricated GaN/Sapphire nanorods by nanosphere lithography (NSL) using SiO₂ nanospheres. Arrays of SiO₂ nanospheres were packed on GaN, followed by dry-etching via inductively coupled plasma (ICP) etching. SiO₂ nanospheres served as the etching mask under our etching conditions. Finally, a sapphire substrate under GaN was exposed by dry-etching. A significant blue shift was observed in the room temperature photoluminescence (PL) spectrum from GaN/Sapphire nanorods when the underlying Al₂O₃ was exposed. GaN nanorods were fabricated by simple and reproducible methods, where SiO₂ nanospheres were successfully used as the etching mask. In addition, a blueshift in PL by the band-filling effect was observed due to the GaN nanostructures.     

Diffusion bonding of transparent ceramics by spark plasma sintering (SPS) complemented by hot isostatic pressing (HIP)

Diffusion bonding is a viable technique for fabricating larger parts or composite transparent ceramics. Spark plasma sintering (SPS) and hot isostatic pressing (HIP) allow for simultaneous application of high temperature and pressure, making them suitable for bonding various materials, including transparent ceramics. In this study, we demonstrated the combined use of SPS followed by HIP for successful diffusion bonding of transparent MgAl₂O₄ (spinel) and Y₃Al₅O₁₂ (YAG) ceramics. The YAG samples had superior surface quality and were adequately bonded using SPS alone, while rougher spinel samples showed inadequate bonding initially due to insufficient surface contact. However, subsequent HIP treatment effectively closed the voids at the interface and promoted grain growth across the joint, resulting in a strong seamless bond. The combination of SPS and HIP, as a two-step process, holds promise for diffusion bonding of ceramics and offers a potential solution for compensating surface quality issues in the bonded parts.     

Heterogeneous material joining of sapphire and TC4 alloy with a functionally graded interface

The natural high brittleness of sapphire leads to its poor machinability, which limits its application shape and size. The connection between sapphire and metal provides a new method to prepare large-size sapphire components and complex structural parts. In this study, sapphire and TC4 alloy were joined by spark plasma sintering (SPS) diffusion technology with Al₂O₃-Ti as interlayer fillers. The effects of filler composition, sintering temperature, microstructure, and phase distribution on the strength of the joint were explored. The results showed that sapphire and TC4 could be well connected by SPS with Al₂O₃-Ti fillers. The microstructure of the joint showed no obvious defect or element enrichment. The optimized shear strength of sapphire/interlayer(Al₂O₃-Ti)/TC4 joint reached 121 MPa, and the fracture path mode was sapphire-interlayer-TC4, indicating the strong bonding between the heterogeneous materials. The sapphire/interlayer(Al₂O₃-Ti)/TC4 structure could relieve the residual stress caused by the difference in the thermal expansion coefficient in direct connection between sapphire and TC4.     

A study of phase evolution and crystal growth for nano-sized monoclinic Y4Al2O9 as a novel thermal barrier coatings

Nano-sized monoclinic Y₄Al₂O₉ was produced by sol-gel process as a novel potential candidate material for thermal barrier coatings. The thermal behavior, structural evolution of the products and the morphological characteristics of the compacted bodies were investigated by Thermogravimetric analysis and differential scanning calorimeter (TG-DSC), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and Field emission scanning electron microscopy (FESEM). Qualitative analyses indicate that monoclinic Y₄Al₂O₉ was formed at about 1000 °C, and exhibited good phase stability throughout the annealing temperature ranging from 1000 °C to 1400 °C. The thermophysical properties of Y₄Al₂O₉ ceramics were also evaluated compared with 8YSZ and La₂Zr₂O₇. The determined activation energy of crystal growth is about 72.71 ± 0.31 kJ mol⁻¹. Meanwhile, Y₄Al₂O₉ represents low thermal conductivity (1.71 W m⁻¹ K⁻¹), moderate thermal expansion coefficient (8.73 × 10⁻⁶ K⁻¹), and high sintering-resistance ability. Such results reveal that nano-sized Y₄Al₂O₉ is favorable for the application of TBCs.