Key Engineering Materials Vols. 512-515

Abstract: Dense Y³⁺-dopted α-SiAlON/BN composition ceramics was fabricated by hot pressing at 1900 °C for 60 min, and flextural strength at room and 1000 °C was investigated through three-point bending test. The results show that h-BN contribute to the densification of powder body, but h-BN have little affect on the key temperature region of sintering densification of α-SiAlON ceramics. When content of BN is lower, which didn’t affect on the phase transformation of α-SiAlON, but when content of BN is higher, besides of major phase α-SiAlON, a little of β-SiAlON was also formed. For the α-SiAlON/BN composite ceramics content of 20 wt.% h-BN, the higer flexural strength( room-temperature and 1000 °C) were abtained, the formation of β-SiAlON should be an important resonon for the higher strentgh, more important is , whose flexural strength at 1000 °C is higher 50 MPa than at room temperature, the differnce of thermal properties between SiAlON and BN should contribute to the improve of high-temperature flextural strength.

Abstract: In this study, porous Si₃N₄ ceramics with interconnected pores were prepared by cold isostatic pressing using benzoic acid as a pore-foring agent. The pore-forming agent of green bodies was sublimation rapidly below 200°C without carbon remained. Mechanical Properties were tested by universal testing machine, and the microstructures of the specimen were observed by scanning electron microscopy (FEI Sirion 200). The resulting shows, many fibrous β-Si₃N₄ grains grown from the internal wall of the round pores, and the porous distribution uniform in the matrix. By changing the content of benzoic acid, the silicon nitride ceramics with different porosity was fabricated. As the acid content decreased from 15wt% to zero, the porosities decreased from 57.6% to 48.0%, flexural strength increased from 36.8MPa to 141.2MPa. Both the grains and suitable interfacial bonding strength contributes to the high fracture strength.

Abstract: A silicon nitride porous ceramics having excellent mechanical strength and dielectric properties can be employed as a wave-transparent material. The silicon nitride porous ceramic contains a plurality of silicon nitride crystal grains with pores formed in grain boundary which forms a three-dimensional network structure. The properties of the silicon nitride porous ceramics was studied , the porous ceramics was prepared by different process parameters, including the pressure of cold isostatic pressing, temperature of sintering and sintering atmosphere, etc.; A high porosity(>50%), high strength(>120MPa), low dielectric properties(ε<3.2) silicon nitride ceramic can be prepared by appropriate process parameters.

Abstract: Porous Si3N4/SiO2/BN composite ceramics with high strength and low dielectric constant were prepared by dry-pressing process and pressureless sintering at 1750°C for 1.5 h in flow nitrogen. The influences of BN content on microstructure, porosity, mechanical and dielectric properties of the porous Si3N4/SiO2/BN composite ceramics were discussed. The results showed that the porous Si3N4/SiO2/BN composite ceramics with porosity ranging from 29% to 48% were fabricated by adjusting the content of BN. The flexural strength of the porous Si3N4/SiO2/BN composite ceramics was 78215 MPa. The dielectric constant of the porous Si3N4/SiO2/BN composite ceramics was 3.9~5 at 1 MHz.

Abstract: Si₃N₄ based electric conductive ceramics were fabricated by pressureless liquid phase sintering with La₂O₃-AlN and TiN as sintering and conductive additives. The Si₃N₄ powder was synthesized by SHS (self-propagation high-temperature synthesis) method. The bulk density, mechanical properties and electrical conductivity of the sintered samples were studied. Microstructure and phase composition were analyzed by SEM and XRD. The results show that the density of the samples is above 96%. The sintered samples consist of β-Sialon and the conductive phase. β-Sialon grains shows elongated and the conductive phase granular which formed electric conductive network. The sintered samples have good mechanical properties and its electric resistivity is from 10¹² Ω•cm above drop to 1 Ω•cm below.

Abstract: A high molecular weight block copolymer dispersant with pigment affinic groups — DISPERBYK163 (BYK163) was chosen to disperse submicron-sized Si3N4 in tert-butyl alcohol (TBA) solvent. The stability of Si3N4 powders in TBA with dispersant BYK163 was studied by sedimentation. To achieve good casting behavior, rheological measurement was employed to find a reasonable dispersant content in the slurry. The adsorption isotherm of BYK163 on Si₃N₄ particles surface was obtained by thermogravimetric analysis (TGA). Based on the surface chemical properties of Si₃N₄ particles, the dispersing phenomena and mechanism of BYK163 in TBA were summarized. BYK163 could form a steric layer on the surface of Si₃N₄ particles, and effectively prevent particles agglomeration. Therefore, porous Si₃N₄ samples with porosity of 40% to 80% could be prepared from Si₃N₄ slurries with low and high solid loadings due to well dispersion with BYK163 addition.

Abstract: The big gap between the thermal conductivity of Si3N4 ceramics and the theoretical value of beta-Si3N4 single crystal implies that the low thermal conductive oxide grain-boundary phases in the ceramics possibly have a great influence on the thermal conductivity of the ceramics. In this work, yttrium chloride (YOCl) is introduced as sintering additive instead of the commonly-used Y2O3, in order to decrease the amount of the grain-boundary oxide; and the influence of YOCl on the sintering and the properties of Si3N4 ceramics was studied. Results show that YOCl additive can react with Si3N4 powders to form similar Y-Si-O-N grain-boundary phases just like Y2O3 while the Cl disappears during sintering. The thermal conductivity of the ceramics sintered with YOCl additive (85 W/m•K) increases about 15% as comparing with the ceramics sintered with Y2O3 additive, even though the relative density of the former reaches merely 97%. The microstructure of the sample sintered at different temperatures was investigated in the work, for elucidating the sintering process and the relationship between the properties and the microstructure of the YOCl added ceramics.

Abstract: The influences of the mullite hollow balls content on selected properties of Si₃N₄-BN matrix ceramics were investigated.The experiment showed that the apparent densities and the porosity of the nitride ceramic composites decreased as the mullite hollow balls contents increased.The strength of the porous nitride ceramic composite was greatly improved, and the dielectric constant decreased with a small addition of the mullite hollow balls. SEM micrographs showed that the mullite hollow balls became one flesh with the nitride ceramic composites.Many large pores appeared in the whole materials. The properties of the nitride ceramic composites were effectively improved.

Abstract: Recently, ceramic composites with low dielectric constant, low loss tangent, high flexural strength and high thermal shock resistance have received a considerable attention as candidate materials for certain high speed radome. In this paper, Si₃N₄-BN ceramic composites were fabricated by dry-press processing and cold isostatic pressing, with α-Si₃N₄ and BN as starting powder, Al₂O₃ and Yb₂O₃ as sintering additives, PMMA as pore-forming agen_{Subscript text}t. After sintering for 2 h at 1750°C, porous Si₃N₄-BN ceramic composites with a three-point bending strength of 50~120MPa and a dielectric constant of 3.2~4.4 at 7~18 GHz frequency were obtained. The sintered body was mainly β-Si₃N₄ grains; BN was dispersed in the grains. The formation of β-Si₃N₄ grains was demonstrated by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) respectively. Furthermore, the influence of the BN content on the dielectric and mechanical properties was also studied. The porous Si₃N₄-BN ceramic composites showed a lower dielectric constant and shrinkage. For above excellent properties, Si₃N₄-BN ceramic composites have been became one of the most hopeful candidate materials for high speed radome.

Abstract: H-BN coatings can be achieved by the Polymer Derived Ceramics (PDCs) route on metallic substrate, i.e. titanium. The ceramization process is led via an optical treatment by IR irradiation to overcome the problem of the metal stability at high temperature (needed in classical process). Structural and chemical characterisations by XRD, IR, RAMAN and EDS show that the obtained crackless coatings have a micro composite structure composed by a µ-layer of TiBx/TiN at the interphase, the h-BN coating and a layer of BN(O) at the surface. Hereafter, tribological and mechanical analyses investigated through nano- and micro-indentation give a weak friction coefficient and show a good adherence between the substrate and the coating. This good adherence can be explained by the formation of Ti-B-N bond at the interface.