Key Engineering Materials Vols. 368-372

Abstract: The effect of different sintering additives on the high temperature oxidation and corrosion behaviour of Si3N4 ceramics is investigated. A corrosion furnace was set up which allows testing of ceramic discs at test temperatures upto 1500°C. It was possible to conduct tests under ambient oxygen conditions, or with increased water vapour as well as testing with corrosive gases such as HCl or SO2. Si3N4 was prepared with MgO, Al2O3, Y2O3 and Al2O3+Y2O3 sintering additives. These discs were subjected to different oxidation and corrosion test conditions at temperatures of 1200°C and 1500°C for upto 128 h. The effects that the different corrosion enviroments have on the corrosion resistance of the Si3N4 materials are presented.

Abstract: Rod-like α-sialon was synthesized successfully using pure SiO2 and AlN as the starting materials, carbon black as reductant, CaF2 and Y2O3 as addition agent by carbothermal reduction-nitridation. The effects of reaction temperature (1450°C, 1500°C, 1600°C and 1700°C) and additive (Li2CO3, CaF2, Y2O3 and Y2O3+CaF2) on phases and microstructure of the final products were studied by XRD and SEM. The results showed that α-sialon was synthesized by carbothermal reduction-nitridation at 1700°C for 3 hours. The morphology of the synthesized α-sialon was rod-like.

Abstract: Nd-doped α-SiAlON starting composition (Nd0.33Si9.38Al2.62O1.62N14.38) was prepared by gas pressure sintering at 1825°C for 3 hrs. In order to explore the effect of post heat treatment on the developments of elongated α-SiAlON grains, sample was heat treated at 1800°C for 4-12 hrs. It was found that post heat treatments promoted formation of the elongated α-SiAlON grains. The controlling mechanism of grain growth was determined via plotting on a graph the growth in width/length versus time graphics using Image Analysis method. Different growth rates were found between the length and width direction of the α-SiAlON crystals, resulting in anisotropic grain growth in the microstructural development.

Abstract: Slurry pumps are used to transport the liquid fluids with solid particles in the industries. The materials of flow passage in a slurry pump are subjected to severe damage due to abrasive erosion. This paper compares the abrasive erosion for an engineering ceramics i.e. hot pressed Si3N4 ceramics and a high chrome cast iron i.e. Cr26 by laboratory test and industrial operation. The results indicate that the erosion rate of Cr26 is much larger than that of hot pressed Si3N4 ceramics under the same test conditions. That indicates that the engineering ceramics is a promising alternation of the expensive material such as Cr26 in the application of slurry pumps. It is noted that the erosion pattern such as scale ripple has the similar features for both materials, even the hardness of hot pressed Si3N4 ceramics is much higher than Cr26. Further, the scale ripple is suspected to be originated from the relatively weak grain boundary and enhanced by cavitation.

Abstract: In this paper, highly porous Si3N4 ceramics with high strength, homogeneous microstructure were fabricated by introducing a proper amount of nanometer carbon in the Si3N4 slurry by gel-casting. Scanning electron microscopy, X-ray diffraction, Archimedes water-displacement method and three-point bending tests were employed to analyze the microstructures and mechanical properties of the sintered bodies. It was shown by the XRD analysis that SiC particles were formed in the sintered bodies. The pillar β- Si3N4 morphology, homogeneous microstructure and the SiC particles as a reinforcement phase are the contributing factors for high porosity and good mechanical behavior.

Abstract: Toray T300 PAN-based carbon fibers were surface oxidized in air at 300, 400 and 500 °C. The composition of surface was determined by X-ray photoelectron spectrometry (XPS), and the monofilaments of original carbon fiber and surface oxidized carbon fibers were tensile tested at room temperature. Three-dimensional carbon fiber reinforced BN-Si3N4 matrix composites were prepared by precursor infiltration and pyrolysis using a hybrid precursor mixed by borazine and perhydropolysilazane. With the increase of the oxidation temperature, the content of size on the surface of fiber reduces, and the tensile strength of carbon fiber declines. Carbon fiber oxidized at 400 °C has a 93% residual strength and the fiber oxidized at 500 °C is seriously decayed. The composite reinforced by original carbon fibers exhibits excellent mechanical properties, including high flexural strength (182.3 MPa) and good toughness; while the composite reinforced by 400 °C oxidized carbon fibers is weak (only 102.4 MPa) and brittle. The distinct difference of mechanical properties between the two composite is attributed to the change of the interfaces between carbon fibers and nitride matrices.

Abstract: In order to investigate the effect of sintering temperature on the microstructure and densification process of Y-SiAlON ceramics, by hot pressing at 1750°, 1800°, 1850° and 1900°C for 1 h, respectively, Y-α-SiAlON (m=1.2, n=1.2 according to formula Mm/3Si12-(m+n)Alm+nOnN16-n) was fabricated from α-Si3N4, AlN, Al2O3 and Y2O3 starting powers. The results showed that α-SiAlON phase is formed in all sintered specimens, and during Y-SiAlON sintering, all the samples begin to quickly shrink at 1550°C, and the shrinkage is finished at 1750°C, which indicate that 1550 ~ 1750 °C is very important for the densification process of Y-α-SiAlON ceramics, and 1750 °C is necessary for the Y-SiAlON densification. Higher sintering temperature contributes to α-SiAlON grain development and the SiAlON ceramics densification.

Abstract: Multi-step hot pressing sintering was adopted to prepare Y-SiAlON (Y0.4Si9.6Al2.4O1.2N14.8) at 1950°C for 60 min under 25 Mpa in nitrogen atmosphere, and the microstructure was studied. The phase assemblages, microstructures and densification of the sintered samples were discussed in detail. In order to contrast to multi-step sintering, one step sintering also was adopted. The results reveal that single-phase α-SiAlON with elongated grain pattern is formed in the multi-step sintering specimens, because of holding at low temperature in multi-step sintering is helpful to reduce the kinetics of crystal growth, the mount of elongated grain in multi-step sintering samples is more than that of in one step sintering ones.

Abstract: Single phase Sialon was synthesized successfully from fly ash by carbothermal reductionnitridation reaction in this paper. The effects of synthesizing temperature (1200°C, 1300°C, 1400°C and 1450°C) and carbon content (stoichiometric content, exceeding 10%, 50% and 100%) on the final production were studied by XRD and SEM. Synthesis mechanism of β-Sialon was also analysized. The results indicated that single phase β-Sialon with z=3 was obtained using fly ash as raw materials and carbon black of stoichiometric content as reducer by carbothermal reduction- nitridation reaction in flow nitrogen at 1450°C for 3h. Temperature and content of carbon had significant effect on the synthesis of β-Sialon.

Abstract: The Si-B-O-N microwave-transparent materials were prepared by gas pressure sintering (GPS).The effect of BN and nano-SiO2 contents on the mechanical and dielectric properties of the composites was studied. The microstructural characteristic and reinforced mechanism of the composites were also investigated. The results showed that a series of Si-B-O-N wave-transparent materials could be obtained by controlling the contents of raw materials and technological parameter. The bending strength of composites is from 74.7MPa to 174.83MPa, the dielectric constant is from 3.5 to 4.2 and the tangent of loss angle is from 0.5×10-3 to 4.5×10-3.