Effect of sintering additives on the behaviour of SiC whisker-reinforced Si3N4 composites

The effects of sintering additives on the microstructural development, whisker stability, oxidation resistance and room-temperature mechanical properties of the SiC whisker-reinforced Si₃N₄ matrix composites were investigated. Seven different combinations of Y₂O₃ and Al₂O₃ were used as sintering additives. The composites containing 20 vol % SiC whiskers were densified by hot pressing. The microstructure of the resulting composites was characterized using X-ray diffraction, scanning and transmission electron microscopy. Oxidation testing of the composite at 1400 °C was conducted to investigate the relationship between matrix compositions and oxidation resistance. The flexural strength, fracture toughness and crack propagation patterns were also characterized and correlated with the microstructural features.     

Effect of whisker aspect ratio on the density and fracture toughness of SiC whisker-reinforced Si3N4

Controlling the suspension properties prior to slip casting optimizes the homogeneity, density and fracture toughness of silicon carbide whisker reinforced silicon nitride (SiC_w/Si₃N₄). Further improvements in the mechanical properties are realized by combining ball milling with ultrasonic dispersion of the composite suspension. Ball milling reduces the SiCw aspect ratio from 25 to 15 which in turn increases the dispersion of the whiskers within the suspension, resulting in increases in the green and sintered density, along with the fracture toughness. In a binderless process, 20 volume% reduced aspect ratio (r = 15) SiC_w/Si₃N₄ can be densified to 95% theoretical density by pressureless sintering using 8% Y₂O₃ and 2% Al₂O₃ by weight as sintering aids. These composites had measured values of fracture toughness from 9–10.5 MPa · m^1/2, representing an average increase of approximately 30% over the fracture toughness for monolithic Si₃N₄ processed under identical conditions.     

Slurry processing for fabrication of SiC whisker-reinforced Si3N4 composites

In order to solve the major problems of processing whisker-reinforced ceramic composites, such as agglomeration of whiskers, correlation between pH and viscosity has been carefully investigated in a mixed slurry of whiskers and matrix powder. SiC whiskers and Si₃N₄ powder were dispersed homogeneously by controlling pH in aqueous suspension, and the state was successfully fixed by a sudden change of pH to make the slurry more viscous. The slurry was then filtrated rapidly and dried. The strength of hot pressed composites obtained by this procedure was scarcely lowered, with increased whisker loading in the range 0–30 wt% and fracture toughness increased more than 75%.     

Preparation and characterization of nano-structured monolithic SiC and Si3N4/SiC composite by hot isostatic pressing

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Si3N4／SiC纳米复合陶瓷的微观结构

利用ＪＥＭ２０００ＥＸⅡ高分辨电镜和ＨＦ２０００冷场发射枪透射电镜对Ｓｉ３Ｎ４ＳｉＣ纳米笔合陶瓷材料的微观组织，结构和成分进行了研究。结果表明，ＳｉＣ颗粒弥散分布基体相β－Ｓｉ３Ｎ４晶内和晶界，晶内ＳｉＣ颗粒与基体相的界面结构有三种类型；１）直接结合的的界面；２）完全非晶态的界面；３）混合型的界面，晶间ＳｉＣ颗粒与基体相的界面大部分是直接结合的。     

Fracture toughness of Si3N4 and its Si3N4 whisker composite without sintering aids

Si₃N₄ without sintering aids is studied with special interest to the fracture behaviour and its relation to microstructure. Cracks propagated almost transgranularly and no rising R-curve behaviour was found, because crack-wake region gave no contribution on toughening due to very high grain-boundary bonding strength. Microstructure with highly elongated grains was obtained by addition of 20%Si₃N₄ whisker, but fracture toughness was found to be similar to that of the monolithic Si₃IM₄ with equiaxed grains. It is recognized that fracture toughness is not determined simply by apparent microstructural parameters such as mean aspect ratio of grains when grain-boundary bonding is sufficiently strong. Detailed examination of microfracture behaviour is, therefore, necessary for the analysis of toughening in this kind of composites.     

Si3N4/TiC纳米复合陶瓷材料R曲线行为

在微米Si3N4基体中加入亚微米Si3N4及纳米TiC颗粒,热压烧结制备出力学性能良好的Si3N4/TiC纳米复合陶瓷材料。采用压痕-弯曲强度法测定了复合材料的裂纹扩展阻力曲线(R曲线)。结果表明:材料呈现出上升的阻力曲线特性,显示出增强的抗裂纹扩展能力。其中,加入质量分数为10%亚微米Si3N4颗粒和15%纳米TiC颗粒的复合材料显示出较为优越的抗裂纹扩展能力,其阻力曲线上升最陡,上升幅度最大。分析表明:弥散的TiC粒子同基体之间弹性模量和热膨胀失配以及Si3N4类晶须拔出与桥联补强协同增韧,有助于纳米复合材料抑制主裂纹失稳扩展,导致复合材料的阻力曲线行为。     

Analysis of flexural creep for an SiC fibre-reinforced Si3N4 composite

A numerical analysis for extracting the tensile and compressive creep power-law parameters from flexural test data is applied to an SiC fibre-reinforced hot-pressed Si₃N₄ composite. The evolution and steady-state conditions of the stress distribution during creep and the effect of neutral-plane migration on the creep behaviour are also analysed. Finally, the kinetics and mechanisms of high-temperature creep in the composite are discussed with respect to the results of the numerical analysis.     

Si3N4/BN纤维独石陶瓷阻力曲线的研究

通过压痕小裂纹直接测量法获得Si3N4/BN纤维独石陶瓷的阻力曲线,采用指数经验公式拟合了实验数据,对纤维独石陶瓷阻力曲线具有的独特的阶梯状进行分析,并对纤维独石陶瓷具有高韧性的机理作了研究。     

The microstructural characterization of in situ grown Si3N4 whisker-reinforced barium aluminum silicate ceramic matrix composite

The microstructure of Barium Aluminum Silicate (BAS)/Silicon Nitride in situ whisker reinforced ceramic matrix composite was examined by X-ray diffraction, transmission electron microscopy, electron diffraction and energy-dispersive X-ray microanalysis. Although we cannot conclusively exclude the presence of orthorhombic BAS, hexagonal BAS and both -Si₃N₄ and -Si₃N₄ were identified in this material. The crystallization process of the glass phase can be taken almost to completion but a small proportion of residual glass phase is present. Both whiskerlike and equiaxed -Si₃N₄ exist in this material.     

Strength degradation in SiC and Si3N4 ceramics by exposure to coal slags at high temperatures

Several commercial SiC and Si₃N₄ ceramics were exposed to the slag-like combustion products of coal—oil mixtures at approximately 1200° C to assess the effects of the ceramics composition on its corrosion behaviour and related changes in flexure strength including its temperature dependence. In the case of exposure to a slag from combustion of a basic coal—oil mixture, very accelerated corrosive attack occurs in both the SiC and Si₃N₄ ceramics such that the general rate of surface recession compared with localized corrosive attack limits the long-term load bearing capacity of the materials. Both the high-temperature corrosion for two different acidic slags and the resultant flexure strengths are dominated by either local enhanced surface corrosion (i.e. pitting) or slag penetration into the various materials which are strongly influenced by processing defects, impurities and/or second phases initially present in the ceramics. The SiC ceramics generally exhibited greater resistance to corrosive attack than did the Si₃N₄ ceramics.