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A dense β-Si 3 N 4 -based waterproof environmental barrier coating was fabricated using the liquid infiltration and filling method on porous Si 3 N 4 ceramic. Microstructure and thermal shock resistance of the coating were investigated. The results showed that the coating consisted of β-Si 3 N 4 , Y 10 Al 2 Si 3 O 18 N 4 and Y-Si-Al-O glass. As-prepared coating exhibited excellent thermal shock resistance. During thermal shock at ∆T=1200 °C, cracks were formed within the coating, but they did not penetrate through the whole coating due to the toughening mechanisms of the coating, including β-Si 3 N 4 particle bridging and crack branching. In addition, the coating had a good crack self-sealing ability during thermal cycling, which is mainly attributed to glass flow and oxidation of β-Si 3 N 4 particles within the crack. After thermal cycling for 25 times, the coating still had a good waterproof ability, and the water absorption of the coated porous Si 3 N 4 sample increased slightly from 4.5% to 9.3%. [ABSTRACT FROM AUTHOR]

The mechanism of interaction between crack-tips and pores in the porous ceramics was investigated in this paper. By using the J -integral numerical method, the effect of the pores on the stress-intensity factors of the crack tips is analyzed quantitatively. The presence of the pores may enhance or weaken the stress-intensity factors. The simulation results show that the pores in front of the crack will enhance the stress-intensity factors of the crack tip and promote the crack propagation and the pores at the side of the crack will weaken the stress-intensity factors of the crack tip and prevent the crack propagation. At the same time, the effects of different pores on the stress distribution around the crack-tip, with different pore-crack tip distance or positions, are presented. [ABSTRACT FROM AUTHOR]

This work presents computational solutions and experimental results on the most important parameters influencing thermal shock behavior. In this paper, the mesoscopic cracks were observed on the surface of the specimen which experimentally confirmed the decay in flexural strength after thermal shock. It has been shown that the crack width decreases with increasing porosity. For higher porosity, it is evident that the crack extended with obvious deflection and bifurcates. The numerical simulations within the framework of damage mechanics considering the coupling between the thermo and mechanical behavior of porous Si 3 N 4 on the mesoscopic level of observation can conveniently reproduce the evolution of thermal shock cracks, which is difficult to observe experimentally. The present theoretical-numerical-experimental study has led to an improved under -standing of the formation and evolution of thermal shock crack in porous ceramics. [ABSTRACT FROM AUTHOR]

Porous β- Si3 N4 ceramics are sintered at 1600°C in N2 and postheat treated at 1500°C under vacuum using Li2 O and Y2 O3 as the sintering additives. The partial sintering and phase transformation are promoted at low temperature by the addition of Li2 O. The addition of Y2 O3 is advantageous for the formation of high aspect ratio β- Si3 N4 grains. After postheat treatment, a large amount of intergranular glassy phase is removed, and the Li content in the samples is decreased. By this method, the β- Si3 N4 porous ceramic with a porosity of 54.1% and high flexural strength of 110 ± 8.1 MPa can be prepared with a small amount of sintering additives, 0.66 wt% Li2 O and 0.33 wt% Y2 O3, and it is suitable for high-temperature applications. [ABSTRACT FROM AUTHOR]

Si2 N2 O ceramics were prepared by plasma activated sintering using nanosized amorphous Si3 N4 powder without sintering additives within a temperature range of 1400°C-1600°C in vacuum. A mixed Si- N4− n- O n ( n = 0, 1...4) amorphous structure was formed in the process of sintering, and Si2 N2 O crystals were nucleated where the local structure was similar with Si2 N2 O. After sintering at 1600°C, the Si2 N2 O ceramic was composed of elongated plate-like Si2 N2 O grains and amorphous phase. The Si2 N2 O grains showed a width of less than 100 nm and a very high aspect ratio. [ABSTRACT FROM AUTHOR]

Abstract: Based on orthogonal experimental results of porous Si3N4 ceramics by gel casting preparation, a three-layer back propagation artificial neural network (BP ANN) was developed for predicting the performances of porous Si3N4 ceramics. The results indicated that BP ANN was a very useful and accurate tool for the prediction and optimization of porous Si3N4 ceramics performances. By using the developed ANN model, the influences of the compositions on performances of porous Si3N4 ceramics were investigated, and some important conclusions were drawn as follows: for the flexural strength of Si3N4 ceramics, solid loading has an optimum value where can achieve a maximum value, and the optimum solid loading decreases with the increase of monomer content; the porosity of sintering body monotonically decreases with the increase of solid loading, and it increases with monomer content; the porosity of sintering body monotonically increases with the increase of the ratio of crosslinking agent to monomer. [Copyright &y& Elsevier]

Abstract: The gelcasting technique was employed to prepare Si3N4 green body. The monomers used in the research were acrylamide (AM) and N,N′-methylenebisacrylamide (MBAM). The influences of the monomer content (AM and MBAM) and the ratio of monomers (AM/MBAM) on the warpage rate, shrinkage rate, and the flexural strength of Si3N4 ceramics green body were investigated. Both warpage rate and shrinkage rate of green body were found to decrease with the increase of monomer content, and monotonically increase with the ratio of monomers after drying. The variation of warpage rate with the ratio of monomers is evident when monomer content is 20wt.%, but the variations are not evident when monomer contents are 40 and 55wt.%. The flexural strength of the green body is highest at an optimum value of the monomers ratio, and increases with increasing monomer content, reaching 50–90MPa when monomer contents are 40 and 55wt.%. [Copyright &y& Elsevier]

Abstract: Porous silicon carbide/silicon nitride (SiC/Si3N4) composite ceramics with flexural strength of 78 MPa, fracture toughness of 1.68 MPa m1/2, and porosity up to 63% were obtained by gel casting and two-step sintering using commercial phenolic resin as carbon source. Thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and three-point bending methods were employed to characterize and analyze the microstructure and basic mechanical performance of the porous ceramics. The polymer was proved to be an effective source that could provide free carbon for fabrication of the SiC nanoparticles by in situ reaction with Si3N4 and SiO2. Sizes of the formed SiC particles were in the range of 50–300 nm. Low green density, significant weight loss, and little shrinkage led to high porosity. High aspect ratio of β-Si3N4, uniform porous distribution, and formation of proper amount of SiC particulates were the main reasons for good mechanical performance. However, only a limited amount of polymer additions had these beneficial effects. Excessive additions could cause agglomeration for residual carbon and inborn SiC, hence becoming fatal flaws for the materials. [Copyright &y& Elsevier]

The mechanical properties and failure mechanisms of the β-Si3N4 thin layers in basal plane under uniaxial tension are investigated by using molecular dynamics simulations. It is found that the thin layers display a nonlinear stress-strain relationship first at [variant_greek_epsilon] < 0.06, and then a linear response at 0.06 < [variant_greek_epsilon] < 0.09, and finally the stresses increase nonlinearly with the strains until fracture occurs. The fracture stresses and strains increase with increasing the side lengths of the thin layers, and the trend is same for Young's moduli accompanying little anisotropy. The deterioration in mechanical properties derives from the N6h-Si bonds where the fracture is initiated. [ABSTRACT FROM AUTHOR]

Porous β- Si3 N4 ceramics are sintered at 1600°C in N2 and postheat treated at 1500°C under vacuum using Li2 O and Y2 O3 as the sintering additives. The partial sintering and phase transformation are promoted at low temperature by the addition of Li2 O. The addition of Y2 O3 is advantageous for the formation of high aspect ratio β- Si3 N4 grains. After postheat treatment, a large amount of intergranular glassy phase is removed, and the Li content in the samples is decreased. By this method, the β- Si3 N4 porous ceramic with a porosity of 54.1% and high flexural strength of 110 ± 8.1 MPa can be prepared with a small amount of sintering additives, 0.66 wt% Li2 O and 0.33 wt% Y2 O3, and it is suitable for high-temperature applications. [ABSTRACT FROM AUTHOR]

Abstract: Al2O3 coatings were prepared using supersonic plasma spraying on the porous Si3N4 substrate at the spraying power of 52, 54, 58 and 60kW, respectively. The effect of spraying power on the microstructures and properties of Al2O3 coating was investigated. The feedstock particles were sufficiently melted and Al2O3 coatings become more and more compact with the increasing spraying power. However, the defects, including the surface cracks, interlamellar cracks and needle-like particles, were produced within the coating when the spraying power was 60kW due to the excessive power. As-received Al2O3 coatings sealed the surface pores of porous Si3N4, and decreased the water absorption obviously, but only with a slight decrease of the total porosity. Vickers hardness and flexural strength of porous Si3N4 with Al2O3 coating increased gradually as the spraying power increased from 52 to 58kW, but it began to decrease when the spraying power was up to 60kW due to the appearance of defects within the coating. [Copyright &y& Elsevier]