Equiaxed β–Si3N4 ceramics prepared by rapid reaction‐bonding and post‐sintering using TiO2–Y2O3–Al2O3 additives

Sintered reaction‐bonded Si₃N₄ ceramics with equiaxed microstructure were prepared with TiO₂–Y₂O₃–Al₂O₃ additions by rapid nitridation at 1400°C for 2 hours and subsequent post‐sintering at 1850°C for 2 hours under N₂ pressure of 3 MPa. It was found that α–Si₃N₄, β–Si₃N₄, Si₂N₂O, and TiN phases were formed by rapid nitridation of Si powders with single TiO₂ additives. However, the combination of TiO₂ and Y₂O₃–Al₂O₃ additives led to the formation of 100% β–Si₃N₄ phase from the nitridation of Si powders at such low temperature (1400°C), and the removal of Si₂N₂O phase. As a result, dense β–Si₃N₄ ceramics with equiaxed microstructure were obtained after post‐sintering at high temperature.     

Crystal structure of rare-earth silicon-oxynitride J-phases,Ln4Si2O7N2

Rare-earth silicon-oxynitride J-phases, Ln₄Si₂O₇N₂ (Ln=Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), were prepared by the N₂ gas-pressured sintering method at 1 MPa of N₂ and 1500–1700 °C. The Rietveld analysis was carried out for X-ray powder diffraction data measured at room temperature. The crystal structures of Ln₄Si₂O₇N₂ were refined with the structure model of La₄Si₂O₇N₂ for Ln=La, Pr, Nd, and Sm, and with that of Lu₄Si₂O₇N₂ for Ln=Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The refined monoclinic unit-cell parameters (lengths a, b, c, angles β, and volume V) increased linearly in their two series of Ln with increasing ionic radii of rare-earth atoms. Discontinuities of the unit-cell parameters were found between the two Ln series.     

Low-temperature densification by plasma activated sintering of Mg2Si-added Si3N4

In this study, highly dense Si₃N₄ ceramics with excellent mechanical properties were fabricated using Mg₂Si as a sintering additive by plasma-activated sintering at 1400–1500 °C. The effects of the sintering temperature and content of Mg₂Si on the densification, microstructures, and mechanical properties of the Si₃N₄ ceramics were investigated. The mechanism responsible for the effect of Mg₂Si in the promotion of the sinterability of Si₃N₄ is discussed. The results showed that the addition of Mg₂Si could effectively remove the oxide layers on the Si₃N₄ particles and form a liquid phase during the sintering, promoting the densification and phase transition of the Si₃N₄ ceramics. The Si₃N₄ ceramic sintered at 1450 °C with 6.0 wt% of Mg₂Si exhibited the maximum strength of 1050 MPa.     

Intergranular Nanostructure Effects on Strength and Toughness of Si3N4

Strength, toughness, microstructure, and atomic adsorption arrangement in silicon nitrides with MgO and RE₂O₃ additions (RE = La, Gd, Y, Lu) were examined. Mechanical properties were high for La, Gd, and equal La–Lu additions, but surprisingly were progressively lower for Y‐ and Lu‐doped samples. The lower strength and toughness were associated with fewer visible crack deflections and grain bridges. Detailed microstructural analysis of the Lu‐doped material revealed a complex intergranular nanostructure with variable Lu content and Si₃N₄ nanocrystals. Furthermore, the Lu‐rich areas showed an extra Lu‐adsorption site on the Si₃N₄ prismatic planes not previously observed in other studies. This inhomogeneous structure was attributed to grain growth impingement and higher viscosity of the Lu‐doped oxynitride glass that slows homogenization. The Y‐doped material with nearly identical glass viscosity demonstrates intermediate behavior. Finally, substituting half of the Lu₂O₃ with La₂O₃ resulted in a homogenous intergranular structure, attributed to a lower viscosity of the oxynitride glass phase, and high mechanical properties. Overall, care must be taken when adapting Si₃N₄ processing parameters for the smaller ionic radius rare earth dopants such as Lu and Y.     

Theoretical Study on the Relationship Between Crystal Chemistry and Properties of Quaternary Y–Si–O–N Oxynitrides

Y–Si–O–N quaternary oxynitrides (Y₅Si₃O₁₂N, Y₄Si₂O₇N₂, YSiO₂N, Y₂Si₃O₃N₄, and Y₃Si₅ON₉) are recognized as important secondary grain‐boundary phases in silicon nitride and believed to have important impacts on the high‐temperature mechanical properties and thermal conductivity of Si₃N₄ ceramic. In this work, equilibrium crystal structures, theoretical mechanical properties (second‐order elastic constants, polycrystalline bulk modulus, shear modulus, Young's modulus, and Vickers hardness) of the five quaternary phases are calculated using first‐principle total energy calculations. Meanwhile, temperature dependence of thermal conductivities of all five compounds is obtained based on Debye–Clarke model and Slack equation. On the basis of theoretical prediction, we establish the relationship between the componential (cation/anion or cation/cation ratios) and structural characteristics (bonding configurations) and mechanical/thermal properties. Our results are expected to provide helpful guidelines to improve the performances of Y–Si–O–N ceramics, and further guide the optimization of mechanical and thermal properties of Si₃N₄ by properly tailoring the secondary grain‐boundary phases.     

Experimental and first-principles simulation study on oxidation behavior at 1700 °C of Lu2O3–SiC-HfB2 ternary coating for SiC coated carbon/carbon composites

The oxidation behavior and microstructure evolution of Lu₂O₃–SiC-HfB₂ ceramic coating specimen at 1700 °C were investigated systematically by experimental study and first-principles simulation. The prepared ternary coating possesses a compact morphology, which effectively defends C/C substrate against oxidation at 1700 °C for 130 h, showing a good antioxidant property. The formed HfSiO₄, Lu₂Si₂O₇, and HfO₂ with high melting points play an active role in developing the thermal stability of the oxidized scale. Besides, Lu and Hf atoms incline to diffuse into SiO₂, which enhances its structural stability. The improved thermal property of the oxidized scale for the Lu₂O₃–SiC-HfB₂/SiC ceramic coating can delay the effective delivery of oxygen inwardly and thus prolong its oxidation protection time. The quick volatilization of SiO₂ at 1700 °C induces that some glass phase evaporates with being not completely stabilized, which causes the formation of holes and the consumption of the inner coating.     

Damage Tolerance and Extensive Plastic Deformation of β‐Yb2Si2O7 from Room to High Temperatures

β‐Yb₂Si₂O₇ is a promising environmental barrier coating (EBC) material and recently attracted attention for its damage tolerance. To investigate the mechanisms of its damage tolerance and possible plasticity, dense β‐Yb₂Si₂O₇ sample was synthesized by in situ reaction/hot‐pressing method, and its mechanical properties were measured from room to high temperatures. The low magnitudes of hardness to Young's modulus ratio H_V/E, shear modulus to bulk modulus ratio G/B, and high fracture toughness to strength ratio K_IC/σ provide evidences of damage tolerance of β‐Yb₂Si₂O₇. β‐Yb₂Si₂O₇ exhibits extensive plastic deformation in Hertzian contact tests at both room and high temperatures. Transmission electron microscopy (TEM) observations show that the deformation mechanisms are different at low and high temperatures. Deformation twinning and parallel dislocation arrangement occur in plastic deformation at room temperature. Above the brittle‐to‐ductile transition temperature (between 1200°C and 1300°C), plastic deformation brings out extensive slip and climb of dislocations, while twinning is seldom observed. Measurement of temperature‐dependent dynamic Young's modulus demonstrates excellent elastic stiffness retention up to 1300°C.     

Microstructure and EMW absorption properties of CVI Si3N4–SiCN ceramics with BN interface annealed in N2 atmosphere

A kind of chemical vapor infiltration (CVI) Si₃N₄–BN–SiCN composite ceramic with excellent electromagnetic wave (EMW) absorbing properties is obtained by CVI BN interface and SiCN matrix on porous Si₃N₄ ceramics, and then annealed at high temperatures (1200°C‐1500°C) in N₂ atmosphere. The crystallization behavior, EMW absorbing mechanism and mechanical properties of the composite ceramics have been investigated. Results showed CVI SiCN ceramics with BN interface were crystallized in the form of nanograins, and the crystallization temperature was lower. Moreover, both EMW absorbing properties and mechanical properties of CVI Si₃N₄–BN–SiCN composite ceramics firstly increased and then decreased with the increase in annealing temperature due to the influence of BN interface on the microstructure and phase composition of the composite ceramics. The minimum reflection coefficient (RC) and maximum effective absorption bandwidth (EAB) of the composite ceramics annealed at 1300°C were ?47.05 dB at the thickness of 4.05 mm and 3.70 GHz at the thickness of 3.65 mm, respectively. The flexural strength and fracture toughness of the composite ceramics annealed at 1300°C were 94 MPa and 1.78 MPa/m^1/2, respectively.     

In-situ synthesis of 15R-Sialon from Al-Si3N4-Al2O3 composite at 1500°C via liquid-phase sintering

In this study, alumina-based composite with 12 wt% Al and 16 wt% Si₃N₄ was designed to achieve the synthesis of 15R-Sialon reinforced alumina composite. To investigate the reaction mechanism, two-step sintered Al-Si₃N₄-Al₂O₃ samples at different temperatures ranging from 600°C to 1500°C were prepared and characterized via X-ray diffraction and scanning electron microscope (SEM). The results revealed that 15R-Sialon was synthesized at 1500°C through a novel liquid Si phase sintering and Si₃N₄ played as a precursor and a reactant. First, Si₃N₄ precursor reacted with Al to form intermediate phases AlN and Si, which were not further transformed below 1400°C. When the sintering temperature was 1500°C, the formed Si presented as a liquid phase, under the influence of which plate-like15R-Sialon was generated from Al₂O₃, residual Si₃N₄, and derived AlN. The obtained Si was also involved in the synthesis of 15R-Sialon and completely transformed. In addition to the AlN from Si₃N₄, the AlN deriving from the nitridation of Al may not react with liquid Si. Compared to 15R-Sialon from liquid Si, plate-like 15R-Sialon with smaller size was generated from AlN, SiO, and O₂.     

Phase transformation and interface segregation behavior in Si3N4 ceramics sintered with La2O3–Lu2O3 mixed additive

Microstructure and mechanical property of silicon nitride (Si₃N₄) ceramic are strongly dependent on the selection of sintering additives. When rare‐earth (RE) oxide is used as the sintering additive, segregation of RE ions at interface between Si₃N₄ grain and intergranular glassy film (IGF) is believed to play a critical role. Although the ionic radius of RE ion is known to be an empirical parameter to modify the mechanical property, the correlation between the segregated ions and their ionic radii is still under controversy. In order to address this issue, (i) rate of α‐β phase transformation and (ii) segregation behavior at the interface were studied for Si₃N₄ ceramics sintered using mixture of La₂O₃ and Lu₂O₃ as additives in this study. Specimens of Lu content 30% and higher exhibited lower activation energies for the α‐β phase transformation as compared with those of Lu content 20% and lower. In terms of the segregation behavior, La was preferably segregated at one site and Lu at the other site along β‐Si₃N₄/IGF interface in the specimens of Lu content 30% and higher. It is understood from these results that Lu segregation site should be more closely related with grain growth.     

Silicon Oxynitride Ceramics Prepared by Plasma Activated Sintering of Nanosized Amorphous Silicon Nitride Powder without Additives

Si₂N₂O ceramics were prepared by plasma activated sintering using nanosized amorphous Si₃N₄ powder without sintering additives within a temperature range of 1400°C–1600°C in vacuum. A mixed Si–N_4?n–O_n (n = 0, 1…4) amorphous structure was formed in the process of sintering, and Si₂N₂O crystals were nucleated where the local structure was similar with Si₂N₂O. After sintering at 1600°C, the Si₂N₂O ceramic was composed of elongated plate‐like Si₂N₂O grains and amorphous phase. The Si₂N₂O grains showed a width of less than 100 nm and a very high aspect ratio.     

High‐Temperature Phase Relations in the Lu2O3–Ta2O5 System

The compounds formed from the Lu₂O₃–Ta₂O₅ system in the composition range 25–60 mol% Ta₂O₅ were prepared by solid‐state reaction from 1350°C to 2058°C, and the phase transitions were investigated by X‐ray diffraction (XRD). Cubic Lu₃TaO₇, M′‐LuTaO₄, M‐LuTaO₄, O‐Ta₂O₅, and T‐Ta₂O₅ are observed. With the temperature increase, there is an irreversible phase transition from M′ to M‐LuTaO₄ near 1770°C in the composition of 30–52 mol% Ta₂O₅, and another phase transition from T‐Ta₂O₅ to O‐Ta₂O₅ at about 1685°C when the ratio of Ta₂O₅ is >52 and ≤60 mol%. A phase diagram of the Lu₂O₃–Ta₂O₅ system in the range 0–100 mol% Ta₂O₅ was constructed. These results are helpful to explain the phase transition of Lu₂O₃–Ta₂O₅ system and design the preparation technique of LuTaO₄ single crystal or ceramic scintillator, which may be applied in the fields of nuclear medicine and high‐energy physics.     

Polymorphism in the Lu2 − xYxSi2O7 system at high temperatures

Samples in the system Lu_2 − xY_xSi₂O₇ (1.25 ≤ × ≤ 2) have been synthesised following a sol–gel method and calcined to high temperatures (≥1400 °C). X-ray diffraction (XRD) has shown that all compositions crystallize as β-Lu_2 − xY_xSi₂O₇ at the low temperatures, while increasing calcination temperature produces the formation of the γ- and δ-polymorphs, the temperatures of formation of each polymorph depending on the Y/Lu ratio. Unit cell parameters of the samples crystallizing as γ-Lu_2 − xY_xSi₂O₇ have been calculated and plotted as a function of composition. They show a linear change with increasing Y content, indicating a degree of solid solubility of Lu₂Si₂O₇ in γ-Y₂Si₂O₇. Based on these data and on those reported in our previous studies [Becerro, A.I. and Escudero, A., XRD and ²⁹Si MAS NMR spectroscopy across the β-Lu₂Si₂O₇–β-Y₂Si₂O₇ solid solution. J. Solid State Chem., 2005, 178; Becerro, A.I. and Escudero, A., Phase transitions in Lu-doped Y₂Si₂O₇ at high temperatures. Chem. Mater., 2005, 17, 112] a temperature–composition diagram of the Lu₂Si₂O₇–Y₂Si₂O₇ system is given. Finally, the influence of Lu on the reversibility of the γ-Y₂Si₂O₇ → β-Y₂Si₂O₇ transition is studied by means of XRD and ²⁹Si MAS NMR spectroscopy.     

Experiments and transient finite element simulation of γ-Y2Si2O7/B2O3-Al2O3-SiO2 glass coating on porous Si3N4 substrate under thermal shock

A dense γ-Y₂Si₂O₇/B₂O₃-Al₂O₃-SiO₂ glass coating was fabricated by slurry spraying method on porous Si₃N₄ ceramic for water resistance. Thermal shock failure was recognized as one of the key failure modes for porous Si₃N₄ radome materials. In this paper, thermal shock resistance of the coated porous Si₃N₄ ceramics were investigated through rapid quenching thermal shock experiments and transient finite element analysis. Thermal shock resistance of the coating was tested at 700 °C, 800 °C, 900 °C and 1000 °C. Results showed that the cracks initiated within the coating after thermal shock from 800 °C to room temperature, thus leading to the reduction of the water resistance. Based on the finite element simulation results, thermal shock failure tended to occur in the coating layer with increasing temperature gradient, and the critical thermal shock failure temperature was measured as 872.24 °C. The results obtained from finite element analysis agree well with that from the thermal shock tests, indicating accuracy and feasibility of this numerical simulation method. Effects of thermo-physical properties for the coating material on its thermal shock resistance were also discussed. Thermal expansion coefficient of the coating material played a more decisive role in decreasing the tangent tensile stress.     

Effects of different types of sintering additives and post-heat treatment (PHT) on the mechanical properties of SHS-fabricated Si3N4 ceramics

Porous silicon nitride (Si₃N₄) ceramics were fabricated by self-propagating high temperature synthesis (SHS) using Si, Si₃N₄ and sintering additive as raw materials. Effects of different types of sintering additives with varied ionic radius (La₂O₃, Sm₂O₃, Y₂O₃, and Lu₂O₃) on the phase compositions, development of Si₃N₄ grains and flexural strength (especially high-temperature flexural strength) were researched. Si₃N₄ ceramics doped with sintering additive of higher ionic radius had higher average aspect ratio, improved room-temperature flexural strength but degraded high-temperature flexural strength. Besides, post-heat treatment (PHT) was conducted to crystallize amorphous grain boundary phase thus improving the creep resistance and high-temperature flexural strength of SHS-fabricated Si₃N₄ ceramics. Excellent high-temperature flexural strength of 140 MPa~159 MPa and improved strength retention were achieved after PHT at 1400 °C.     

A high-temperature structural and wave-absorbing SiC fiber reinforced Si3N4 matrix composites

The SiC_f/Si₃N₄ composite with low–high–low permittivity sandwich structure was designed for high-temperature electromagnetic (EM) wave absorption and mechanical stability. The SiC_f/Si₃N₄ possessed the remarkable mechanical properties at room temperature (the flexural strength is 357 ± 16 MPa and the fracture toughness is 10.8 ± 1.7 MPa m^1/2) for the strong fiber strength, moderate interface bonding strength and uniform matrix. Furthermore, the retention rate is as high as 80% at 800 °C. The A/B/C nanostructure and the sandwich meta-structure endowed the SiC_f/Si₃N₄ with an excellent EM absorbing property at room temperature. The SiC_f/Si₃N₄ still absorbed 75% of the incident EM waves energy in X and Ku bands when the temperature increases up to 600 °C, which is only 6% lower than that at room temperature, for the partial compensation of the decreased interfacial polarization loss for the increased conductivity loss and dipole polarization loss.     

High-temperature properties and interface evolution of C/SiBCN composites prepared by precursor infiltration and pyrolysis

C/SiBCN composites with a density of 1.64 g/cm³ were prepared via precursor infiltration and pyrolysis and the bending strength and modulus at room temperature was 305 MPa and 53.5 GPa. The precursor derived SiBCN ceramics showed good thermal stability at 1600 °C and the SiC and Si₃N₄ crystals appeared above 1700 °C. The bending strength of the composites was 180 MPa after heat treatment at 1500 °C, and maintained at 40 MPa-50 MPa after heat treatment for 2 h at 1600 °C–1900 °C. In C/SiBCN composites, SiBCN matrix could retain amorphous up to 1500 °C and SiC grains appeared at 1600 °C but without Si₃N₄. The reason for no detection of Si₃N₄ was that the carbon fiber reacted with Si₃N₄ to form an interface layer (composed of SiC and unreacted C) and a polycrystalline transition layer (composed of B and C elements), leading to the degradation of the mechanical properties.     

Synthesis mechanism of AlN–SiC solid solution reinforced Al2O3 composite by two-step nitriding of Al–Si3N4–Al2O3 compact at 1500 °C

The in-situ controllable synthesis of AlN–SiC solid solution reinforcement in large-sized Al–Si₃N₄–Al₂O₃ composite refractory by two-steps nitriding sintering was examined. In the first step, a dynamic Al@AlN structure was constructed in the composite by pre-nitriding at 580 °C. During the subsequent sintering process, it cracked above ∼900 °C, and micronized Al cluster (mixture of droplets and vapor) was extracted out gradually. As a result, multiple AlN mesophases were formed through different reaction paths, including i) initial AlN shell formed by solid Al with N₂, ii) reaction of Al cluster with N₂, and iii) reaction of Al cluster with Si₃N₄ from 900 °C to 1500 °C. The Si₃N₄ precursor serves as both a solid nitrogen source and an active Si source, and the controllable reaction between Al and Si₃N₄ leading to uniformly distributed AlN and Si mesophases. AlN–SiC solid solution is significantly formed when liquid Si appears. The shell, granule and whisker SiC–AlN solid solution were observed mainly depending on the dynamic AlN mesophase. The SiC–AlN solid solution reinforced Al₂O₃ materials is a novel promising refractory for large-scale blast furnace lining.     

Nitridation of Silicon Powders Catalyzed by Cobalt Nanoparticles

The effect of Co nanoparticles (NPs) on the nitridation of silicon (Si) was studied. Co NPs were deposited homogeneously on the surfaces of Si powders using an in situ reduction method using NaBH₄ as a reducing reagent. Si powders impregnated with 0.5–2.0 wt% Co NPs were nitrided in 1200°C–1400°C for 2 h. The resultant silicon nitride powders were characterized by XRD, FE‐SEM, TEM, and EDS. The results showed that: (1) Co NPs significantly decreased the Si nitridation temperature, and the nitridation could be completed at 1300°C upon using 2 wt% Co NPs as catalysts. For comparison, the Si conversion could not be completed even at a temperature as high as 1400°C in the case without using a catalyst; (2) many Si₃N₄ whiskers with 80–320 nm in diameter and tens micrometers in length were generated and uniformly distributed in the final products. They were single‐crystalline α‐Si₃N₄ grown along the [101] direction. The enhanced nitridation in the case of using Co NPs as a catalyst was attributed two following factors, the increased bond length and weakened bond strength in N₂ caused by the electron donation from the Co atoms to the N atoms.     

Effect of in situ synthesis of Si2N2O on microstructure and the mechanical properties of fused quartz ceramics

Si/SiO₂ composite billets were prepared using a low-toxicity gel system, and the resulting billets were sintered at high temperature in nitrogen to synthesize Si₂N₂O in the central position of the fused silica ceramic matrix. The influences of in situ synthesized Si₂N₂O on the microstructure and mechanical properties of fused silica ceramics were studied. The results show that Si/SiO₂ composite billets can be used to synthesize spike-like and fibrous Si₂N₂O in situ in nitrogen at 1450 °C. Si₂N₂O synthesized in situ can improve the mechanical properties and microstructure of quartz ceramics. When the Si/SiO₂ composite billet is sintered in nitrogen at 1450 °C for 2 h, the volume density and bending strength of the quartz ceramics can reach 2.36 g/cm³ and 114.37 MPa, respectively.