Key Engineering Materials Vols. 602-603

Abstract: A new process was used to improve microstructure homogeneity of ZrO₂-BN composites. Boric acid was deposited on the surface of ZrO₂ particles by heterogeneous precipitation method from aqueous solution of boron oxide under the action of heat by evaporation. In order to improve the microstructure homogeneity, the mixture powder was stirred over-vigorously until transforming into the highly viscosity slurry. Subsequently, the obtained mixture powders were dried, nitrided at 900 °C in ammonia and crystallized at 1500 °C in nitrogen atmosphere to obtain in-situ synthesized BN coated on the surface of ZrO₂ composite particles. Finally, ZrO₂-BN composites incorporated with 30 vol. % BN were prepared by hot-pressing. The SEM microstructure and fracture surface of ZrO₂-BN composites revealed that in-situ synthesis BN particles were dispersed in the ZrO₂ matrix homogeneously.

Abstract: ZrO₂ wet gel was prepared using zirconium n-butoxide as precursor and 1,2-propylene oxide (PO) as a gelation agent in sol-gel process. Then, tetraethoxysilane (TEOS) was used to strengthen the wet gel in aging process. Finally, the monolithic silica/zirconia aerogel was obtained by supercritical fluid drying. N₂ adsorption-desorption method, scanning electron microscope (SEM), and Fourier transform infrared spectroscopy (FTIR) were used to characterize microstructure of the aerogel. The influence of pH of the aging solution on microstructure of silica/zirconia aerogel was studied. The results indicate that the particle size distribution of SiO₂/ZrO₂ aerogel was more uniform and less aggregate when aging in acid solution. However, the average pore diameter and particle size of SiO₂/ZrO₂ aerogel was increased by aging in alkaline solution. Aging in alkaline condition was useful for largening the pore size of SiO₂/ZrO₂ aerogel, and the maximum specific surface area and pore volume were obtained at pH=9. Meanwhile, different reactions were produced between the acid/alkaline solution and the hydroxyl of SiO₂/ZrO₂ aerogel, and the ester group/amino group was obtained relatively which can modify the network of aerogel and decrease the shrinkage of aerogel during the supercritical fluid drying.

Abstract: Tests were performed to investigate the microstructure of MgAl₂O₄ spinel layer between magnesia and alumina after high temperature thermal treatment (1500 °C and 1600°C). The first test involved fused magnesia and alumina sol. Hydration of magnesia powders was studied in a controlled condition that soaked in aqueous solution at 80 °C for up to 1h and 2h. Microstructures of the samples were investigated by SEM, EDS, which showed that the formation of Mg (OH)₂ made magnesia particles cracking, separated many punier particles. The punier particles combined with alumina sol, generated spinel at high temperature. A microstructurally distinct layer with granulate grains of up to 50μm width was observed after 3 h at 1600 °C. Spinel layer bordered alumina layer, and separated from magnesia layer. However, it was not found distinct spinel layer after 3 h at 1500 °C. The formation of small amount of spinel embedded in the layer of alumina, and penetrated the alumina layer. In the spinel layer , Spinel phase close to alumina layer contained 8.79% Mg by atomic, while spinel phase away from alumina layer contained 13.07% Mg by atomic.

Abstract: The traditional method preparing Si₃N₄/Si₂N₂O composite ceramic is to sinter α-Si₃N₄ powder with additives at relatively high temperatures. But the intergranular phase transformed from the sintering additives can degrade the high-temperature mechanical behavior. Amorphous nanoSi₃N₄ is used to fabricate Si₃N₄/Si₂N₂O composite ceramic by its oxidation and nitridation without sintering additives at low temperatures. Thus, it is essential to study the oxidation behavior and mechanism of amorphous nanoSi₃N₄. Amorphous nanoSi₃N₄ powders were oxidized in different atmospheres, at varying temperatures and for various different length of time. The oxidation process and products was analyzed by means of XRD, TGA and FTIR. The results showed that amorphous nanoSi₃N₄ could be oxidized into SiO₂ from 600°C to 1300°C. Below 1300°C, the oxidation became serious as the temperature increased. Besides, the longer the oxidation time and the lager the ratio of O₂/N₂ partial pressure was, the deeper the oxidation level was. When the temperature was above 1300°C, amorphous nanoSi₃N₄ was completely oxidized into SiO₂.

Abstract: The influences of hexagonal BN content on the porous Si₃N₄ material was investigated. SEM micrographs showed that the columnar crystal overlapping structure of the porous Si₃N₄ material with a small BN addition was almost invariant. BN uniformly dispersed in the spaces of crystal overlapping. The experiment showed that the apparent densities of the porous Si₃N₄ material with a small BN addition marginally increased, the porosity, the strength and the dielectric constant marginally decreased, but The thermal shock resistance was greatly improved.

Abstract: Porous silicon nitride (Si₃N₄) ceramics green were prepared by sol-gel and freeze drying processing. Al₂O₃ and MgO were selected as sintering additives. Porous Si₃N₄ ceramics were sintered at 1200~1300 °C. The porosity of porous Si₃N₄ ceramics reached 60~80%, the pore size of porous Si₃N₄ ceramics dried by freeze drying is less than 5μm. Two kinds of pores were formed, including open pores with pore size of 1~5μm and closed pores pore size with the nanometer level. The compression strength of porous Si₃N₄ ceramics was 15~25MPa. Thermal conductivity of porous Si₃N₄ ceramics was 0.08-0.1 W/m·K.

Abstract: SiAlON-cBN composites with different contents of cBN were consolidated by spark plasma sintering (SPS) at 1450°C using Y₂O₃, B₂O₃ and Al as additives. The effect of cBN content on the density, phase compositions, microstructures and mechanical properties of β-SiAlON-cBN composites was investigated. With increasing the cBN content, the density and hardness of β-SiAlON-cBN composites decreased. Fracture toughness could increase thanks to the crack deflection resulted from the cBN particles. For β-SiAlON-10 wt% cBN composites, the optimum hardness and highest relative density were 13 GPa and 96.4 %, respectively. For β-SiAlON-40 wt% cBN composites, the highest fracture toughness was K_IC = 5.3 MPa∙m^1/2.

Abstract: Macroporous polymer-derived SiCN ceramics are fabricated directly by mixing polysilazane precursors followed with crosslinking and pyrolysis. Two kinds of polysilazanes namely polyvinylsilazane and polyhydrosilazane are mixed, crosslinked by 2, 2-Azo-bis-isobutyronitrile to form resins before pyrolyzed to form ceramics in argon flow at 1000°C. The density of the SiCN ceramic is 1.65 g/cm³ with corresponding porosity of 30 % compared to dense SiCN ceramics. SEM images show that the ceramics possess high porosity and homogeneous honeycomb-like macropores of ~2 μm. The porous SiCN exhibits good mechanical property with Vicker hardness of 11-13 GPa under a load of 0.2 kg.

Abstract: The pyrolysis kinetic behaviors and reaction mechanism of polycarbosilane (PCS) were characterized by means of XRD, SEM and on-line TG-DSC-FTIR-MS coupling technique, which showed that the pyrolysis process accorded with the F2 integral model (300-600°C), and the values of apparent activation energy (E_a) and pre-exponential factor (A) were 56.10KJ/mol and 10.84, respectively. During the pyrolysis process, PCS was converted into amorphous inorganic solid from organic polymer with breakage and rearrangement of chemical bonds, then to well-defined crystal structure of good thermal stability. Furthermore, the weight loss occurred mainly among 300°C and 800°C with CH₄, (CH₃)₄Si,(CH₃)₃SiH,(CH₃)₂SiH₂ and other silane gases releasing from pyrolysis product leading to the conversion from organic polymers to inorganic ceramic. The pyrolysis product was converted into β-SiC crystal completely at 1400°C, and the crystallization of α-SiC phase occurred after 1550°C which might influence the stability of SiC matrix. Key words:polycarbosilane, organic precursor, pyrolysis kinetic behavior

Abstract: An oxidation protective coating of SiBCN(O) is prepared by slurry method, using polyborosilazane (PBSZ)-ethanol solution as the precursor. The dense and uniform coatings are obtained by pyrolysis and sintering at 1573K. The sintering temperature is significantly reduced and the densification is improved by introducing oxygen. The coating shows high oxidation protecting performance during the thermal shock between 1773K and room temperature for 10 cycles. The oxidation mechanism of the coating is studied and the results show that two stages are included: the relatively stable stage and the weight loss stage.