Ultrasonic examination of reaction bonded silicon nitride

An ultrasonic examination has been made of a series of partially nitrided reaction-bonded silicon nitride (RBSN) ceramics whose weight gains varied from 22% to nearly 64% representing full nitridation. A pulse echo overlap technique was used which enabled both the longitudinal and shear velocities of propagation to be measured at 15 MHz; from these measurements values of Young's modulus (E) and the bulk modulus (K) at room temperature were derived. For fully nitrided RBSN the values obtained wereE=160 GN m^–2,K=90 GN m^–2 in good agreement with published values obtained by ultrasonic methods. Both Young's modulus and the bulk modulus were found to be markedly sensitive to the changes in fractional porosity due to changes in weight gain (and green density) each decreasing by over 50% as the fractional porosity increased from 0.16 for a fully nitrided ceramic to 0.26 for 60% weight gain material.     

Nitridation of whisker-reinforced reaction bonded silicon nitride ceramics

Ceramic matrix composites were fabricated from silicon carbide whisker-reinforced reaction bonded silicon nitride. Optimal dispersion of the SiC whiskers in the silicon powder slip was achieved by milling and pH control; a pH range of 4–5 giving the best results. Only a slight drop in green density was observed for a 30 wt% addition of SiC whiskers. The effects of the whisker additions on the nitridation kinetics of reaction bonding were investigated and the additions were found to increase the induction period before nitridation and to slightly decrease the nitridation rate but green density and temperature were still found to be the main factors controlling nitridation. Modulus of Rupture measurements for the composites showed a decrease in strength compared to the monolithic material.     

Properties of reaction bonded silicon nitride obtained from slip cast preforms

Fabrication of silicon preforms of high green density (>1·2 g/cm³) by slip casting of silicon (in aqueous medium) has been studied. The nitridation product consists of 59–85% α-Si₃N₄, 7–22%β-Si₃N₄ and 7–23% Si₂N₂O phase. The amounts of un-nitrided silicon were negligible. The microstructure is either granular or consists of needle-like grains (α-Si₃N₄) and whiskers deposited in the large pores. MOR values of the specimens are almost constant up to 1000°C or 1400°C or show slight increase up to 1000°C or 1200°C. In some cases a little dip around 1200°C, then a sharp increase in MOR up to 1400°C was observed.K _ic values are almost constant up to 1000°C, and thereafter increase sharply. Pore size distribution, existence of Si₂N₂O phase and oxidation of RBSN at high temperatures have been considered for the explanation of the observed behaviour.     

Low pressure injection moulding of SiC platelet reinforced reaction bonded silicon nitride

Reaction-bonded Si3N4 toughened by oriented SiC platelets was fabricated via low pressure injection moulding (LPIM). Initially, the rheology of ceramic suspensions was optimized with respect to solid content, SiC platelet loading, particle surface properties and binder composition. Surface active additives were used to modify the particle–polymeric binder interphase in order to prevent particle reagglomeration, to reduce the viscosity and/or to increase the solid content. The relationship between LPIM processing variables and platelet orientation in injection moulded reaction bonded silicon nitride ceramics was studied and the resultant mechanical properties were compared to composites containing randomly dispersed platelets.     

The temperature and frequency dependencies of permittivity and dielectric loss in reaction bonded silicon nitride

The temperature dependencies of the permittivity and dielectric loss of reaction bonded silicon nitride (RBSN) have been measured between 20 and 900° C at frequencies covering the range from 3 Hz to 300 kHz. Above about 300° C both parameters have a large effect. Analysis of the permittivity data in terms of the theory discussed by Jonscher and by Dissado and Hill predicts loss peaks at 48 Hz and 8 Hz at temperatures of 900 and 800° C, respectively. These values are in close agreement with those found independently from direct observation of the temperature and frequency variations of dielectric loss. On the assumption that a thermal activation process is responsible for the temperature dependence of the loss peak frequency, the associate activation energy is found to be about 1.94 eV.     

Microstructures and mechanical properties of silicon nitride bonded silicon carbide ceramic foams

Silicon nitride bonded silicon carbide foams have been produced by nitridation of the foamed compacts containing silicon carbide and silicon powders. When no nitridation additive was used the ceramic foams nitrided at all temperatures studied contained a significant amount of whisker phase α-Si₃N₄ formed both inside and outside the cell walls leading to a loose microstructure and a low mechanical strength. When the Al₂O₃ and Y₂O₃ were used as nitridation additives, the ceramic foams nitrided at temperatures of 1360 and 1395 °C containing certain amount of Si₂N₂O and whisker α-Si₃N₄ phases that are bonded by a glassy phase and behave as reinforcements for the ceramic foams exhibited a much higher mechanical strength. At nitridation temperature of 1430 °C, the ceramic foam showed the locally formed β-Si₃N₄ as the main nitrided phase that caused no increase in bonding area between the nitrided phase and the silicon carbide particles. Thus, a relatively lower mechanical strength was observed for the ceramic foam.     

Preparation of zirconium pyrophosphate bonded silicon nitride porous ceramics

Abstract

A new method for preparing high bending strength porous silicon nitride ceramics with controlled porosity was developed using a pressureless sintering technique, using zirconium pyrophosphate as a binder. The fabrication process was described in detail and the sintering mechanism of porous ceramics was analysed by an X-ray diffraction method. The microstructure and mechanical properties of the porous Si₃N₄ ceramics were investigated, as a function of the content of ZrP₂O₇. The resultant porous silicon nitride ceramics sintered at low temperature (1000 and 1100°C) showed fine micropore structure and a high bending strength. Porous silicon nitride ceramics with porosity of 34–47%, a bending strength of 40–114 MPa and a Young's modulus of 20–50 GPa were obtained.     

Characterization of silicon nitride single crystals and polycrystalline reaction sintered silicon nitride by microhardness measurements

Identification of - and -phases of Si₃N₄ single crystals grown from Si melt could be made with the help of Vickers microhardness measurements. The effect of chemical additives, e.g. metallic Fe and BaF₂, on the microhardness of Si₃N₄ was also determined. Different constants involved in the empirical Meyer relationship between load and indentation diameters could be correlated with the porosity and microhardness of Si₃N₄ single crystals and polycrystalline, reaction sintered Si₃N₄.     

The development of strength in reaction sintered silicon nitride

The development of strength in reaction sintered silicon nitride has been investigated by determining the elastic moduli, fracture mechanics parameters, strengths and critical defect sizes of silicon compacts reacted to various degrees of conversion using static or flowing nitrogen. The relationship between each property and the nitrided density is shown to be independent of the green silicon compact density but is influenced by the nitriding conditions employed. Young's moduli, rigidity moduli and strengths vary linearly with the nitrided density. After an initial period when increases may occur, the critical defect sizes in both static and flow materials decrease continuously with increasing nitrided density, although at any particular density they are larger in material produced under flow conditions. A model is suggested for the development of the structure of reaction sintered silicon nitride involving the development of a continuous silicon nitride network within the pore space of the original silicon compact. The experimental data are discussed in terms of the proportion of silicon nitride which contributes effectively to the continuous network.     

Effect of microstructure on the high temperature strength of nitride bonded silicon carbide composite

Four compositions of nitride bonded SiC were fabricated with varying particle size of SiC of ∼ 9.67, ∼ 13.79, ∼ 60 μ and their mixture with Si of ∼ 4.83 μ particle size. The green density and hence the open porosity of the shapes were varied between 1.83 to 2.09 g/cc and 33.3 to 26.8 vol.%, respectively. The effect of these parameters on room temperature and high temperature strength of the composite up to 1300°C in ambient condition were studied. The high temperature flexural strength of the composite of all compositions increased at 1200 and 1300°C because of oxidation of Si₃N₄ phase and blunting crack front. Formation of Si₃N₄ whisker was also observed. The strength of the mixture composition was maximum.     

The influence of the nitriding parameters on the microstructure and strength of the open-cell reaction bonded silicon nitride foams fabricated via wet processing

In this study, the parameters which influence strength of the open-cell reaction bonded silicon nitride foams were investigated. These parameters include the monomer content in the suspension, the porosity level of the foam, the nitriding atmosphere including N₂ and N₂–4 %H₂, and the nitriding temperature ranging from 1350 to 1425 °C. The nitriding mechanisms dominating under different nitriding conditions were also studied based on the phase and microstructural analysis. It was observed that there is a minimum monomer concentration of 25 wt% required in the premix solution to obtain a defect-free and homogeneous RBSN foam. Increasing the monomer content only from 15 to 20 wt% resulted in a threefold increase in the foam strength. The high porosity level of the foam which is above 70 vol% significantly affects the nitriding mechanisms and microstructures compared to those of dense RBSN ceramics. The maximum strength was obtained for the foams nitrided under N₂–H₂ atmospheres, and the nitriding temperature had a negligible effect on the foam strength when H₂ is present in the atmosphere. α-Si₃N₄ is also the dominant phase in the microstructure in the presence of H₂ regardless of the nitriding temperature. It was observed that β-Si₃N₄ can also be present in high quantities when N₂ atmospheres are used. β-Si₃N₄ is present in the microstructures in two different morphologies including interlocking rods and angular grains. Each morphology forms based on a specific nitriding mechanism.     

反应烧结Si3N4/SiC复相陶瓷合成工艺优化的研究

利用反应烧结制备Si3N4结合SiC复合材料.设计了L9(34)正交试验方案,研究了原料中Si、添加剂Al2O3、Y2O3的含量对复合材料力学性能的影响,采用X射线衍射(XRD)和扫描电镜(SEM)对复合材料的相组成、断口形貌进行分析.结果表明,反应烧结后试样生成了颗粒状的α-Si3N4、针状或棒状的β-Si3N4和少量的Sialon,其中针状或棒状的β-Si3N4和SiC形成三维网络结构,提高了材料的力学性能.优化实验得到的试样力学性能显著提高,其中维氏硬度2205、抗弯强度410MPa、断裂韧性为8MPa·m1/2.     

Electrophoretic deposition [EPD] applied to reaction joining of silicon carbide and silicon nitride ceramics

Electrophoretic Deposition (EPD) was used to deposit a mixture of SiC or Si₃N₄ filler and reactive carbon (graphite and carbon black) particles onto various SiC or Si₃N₄ parts in preparation for reaction bonding. The particles had gained a surface charge when mixed into an organic liquid consisting of 90 w % acetone + 10 w % n-butyl amine to form a slurry. The charged particles then moved when placed under the influence of an electric field to form a green deposit on the ceramic parts. The green parts were then dried and subsequently joined using a reaction bonding method. In this reaction bonding, molten Si moves into the joint via capillary action and then dissolves carbon and precipitates additional SiC. An optimum mixture of SiC filler to C powder ratio of 0.64 was identified. Residual un-reacted or free Si was minimized as a result of selecting powders with well-characterized particle size distributions and mixing them in batch formulas generated as part of the research. Image analysis of resulting microstructures indicated residual free Si content as low as 7.0 vol % could be realized. Seven volume percent compares favorably with the lowest free Si levels available in experimental samples of bulk siliconized (reaction-bonded) SiC manufactured using conventional reaction-bonding techniques. The joints retained the residual silicon over a large number of high-temperature thermal cycles (cycling from below to above the melting point of silicon). Comparisons to commercial reaction-bonded SiC indicated the majority of residual silicon of the joint was retained in closed porosity. This infers that parts made with these joints might be successfully utilized at very high temperatures. It was demonstrated that the EPD technique could be applied to butt, lap, and scarf type joints, including the capability to fill large gaps or undercut sections between parts to be joined. The overall results indicate that EPD, combined with reaction bonding, should allow for the fabrication of large complex structures manufactured from smaller components consisting of silicon carbide or silicon nitride.     

The wetting,reaction and bonding of silicon nitride by Cu-Ti alloys

The wettability and reactivity of pressureless sintered Si₃N₄ by powdered Cu-Ti alloy were investigated using sessile drop tests conducted in a vacuum. Bonding of Si₃N₄ to itself was also carried out and joint strength was evaluated by compressive shear testing. The correlation of wetting behaviour with reaction and bond strength was interpreted. The wettability of Cu-Ti alloys on Si₃N₄ was improved greatly by addition of titanium up to 50 wt%. However, the reaction-layer thickness was increased up to 10 wt% and thereafter decreased up to 50 wt%. We propose the dovetail model which describes the reaction-layer growth behaviour with titanium. As the titanium content was increased, it tended to form a continuous thin reaction layer which greatly improved the wettability. From metallographic and XRD analyses, TiN and Ti suicide were found in the reaction layer. The thermodynamic reaction for TiN formation was suggested to be Si₃N₄(s) + 4Ti (1 ? sol) = 4TiN(s) + 3Si(s). Ti-silicide might be formed during cooling by the reaction with Ti and Si which had been decomposed from Si₃N₄, diffused to and dissolved in the liquid Cu-rich alloy. The reaction layer growth was controlled by diffusion of nitrogen or titanium in the reaction layer according to the titanium concentration. The shear strength of Si₃N₄ to Si₃N₄ was affected by the morphology and thickness of the reaction layer rather than the wettability. As the titanium content increased, shear strength also increased rapidly up to 5 wt% and then slowly up to 50 wt%. As the reaction temperature and time were increased, shear strength was lowered due to the greater thickness of the reaction layer despite improved wettability.     

Creep of silicon nitride

General features of silicon nitride based ceramics, which may well influence their creep behavior are presented. Then, the most commonly invoked models for the microscopic mechanisms assumed to take place during creep (viscous flow, solution-precipitation, cavitation and shear thickening) are analyzed. Finally, the very numerous macroscopic and microscopic experimental findings about the plastic deformation of silicon nitride based ceramics at high temperatures, such as the fundamental role played by the secondary phases, the essential compressive-tensile asymmetry, and the microstructural evolution accompanying creep are summarized and discussed in terms of those models.