Fabrication of SiC whisker reinforced Al2O3 composites

It is reported that SiC whiskers and alumina powder can be mixed in water medium when the whiskers are flocculated and the powder deflocculated, and that the composite slip can be cast to high green density. However, sintering of the composite to fully dense material was found to be difficult. The reasons were the formation of whisker networks and the development of the differential sintering stresses in the matrix around the whiskers. Hot pressing was subsequently employed, and the use of the computerized dilatometer allowed the establishment of a quantitative relationship of the densification rate with pressure, temperature and SiC whisker content. The present results help provide a useful basis for the effective design, development and fabrication of the whisker-reinforced ceramic composite.     

Interfacial structure of Si3N4 whisker reinforced 7064 aluminium composite

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

Influence of whisker toughening and microstructure on the wear behavior of Si3N4- and Al2O3-matrix composites reinforced with SiC

A comparative study of the influence of randomly-oriented SiC whiskers on the abrasive wear behavior of several commercially-produced Si₃N₄- and Al₂O₃-based ceramics suggested that the residual stress states present within the materials can be important in predicting their wear resistance. The addition of SiC whiskers to the Si₃N₄ matrix created residual tensile stresses at the whisker-matrix interfaces which led to enhanced bulk fracture toughness, but which degraded the fracture toughness at the microstructural level, and thus the abrasive wear resistance, by promoting easier whisker debonding and removal by the abrasive particles. The addition of SiC whiskers to an alumina matrix, on the other hand, led to the creation of residual compressive stresses at whisker-matrix interfaces, producing a locally tougher interface that was more able to withstand the rigors of the abrasive wear environment. These results indicate that in brittle materials, improved bulk mechanical properties do not always translate directly to improved performance in a tribological environment.     

Microstructure and properties of SiC whisker reinforced ceramic composites

Microstructures of SiC whisker reinforced alumina and tetragonal zirconia polycrystals (TZP) were investigated using analytical electron microscopy. In the Al₂O₃-SiC system, amorphous phases between the whisker and matrix were observed; these amorphous phases were virtually eliminated when the whiskers were leached with HF acid before being incorporated into the matrix. In the TZP/SiC system, reaction between the whisker and matrix had taken place during fabrication and resulted in the formation of a glassy phase. This reaction appeared to be associated with the presence of SiO₂ impurity present in the TZP matrix.Mechanical properties of the composites were measured both at ambient and elevated temperatures and fracture surfaces were examined. The results indicated that fracture of the composites was sensitively influenced by the whisker-matrix interface. The presence of amorphous interfacial phases was detrimental to the properties of the composites and caused a reduction in fracture energy. High temperature tests showed that the TZP composite had a structural transition with extensive cracking occurring at 1000 °C, whilst the alumina composites retained their properties up to 1200 °C, whereupon they deteriorated rapidly.     

Quantitative determination of the stability of SiC whisker in reaction-bonded/hot-pressed Si3N4 composites by X-ray diffraction

A method was developed for the quantitative determination of weight fractions of the phases in SiC whisker-reinforced reaction-bonded Si₃N₄ composites using X-ray diffraction. Composites with different amounts of sintering additives and whiskers were fabricated using reaction bonding followed by hot pressing. The amount of whiskers remaining in each composite after each processing stage was determined. In order to study the degradation mechanism, the microstructural development after each processing step was examined using scanning and transmission electron microscopy. Finally, the effect of sintering additives on the microstructural development and whisker stability was also investigated.     

Mechanical properties Of Si3N4 cerarnics reinforced with SiC whiskers and SiC platelets

Silicon nitride ceramics reinforced with SiC whiskers and SiC platelets were fabricated by hot pressing and their mechanical properties were studied. They showed higher fracture energy than conventional composites, particularly when they were consolidated by gas-pressure hot pressing at high temperature. A high fracture toughness (10.7 MPa m^1/2) which was measured by the single-edge pre-cracked beam method was achieved. Furthermore, a unique method to observe the crack propagation behaviours directly in a scanning electron microscope with loading devices was developed. As a result, much bridging and pull-out of the whiskers and the elongated Si₃N₄ grains, and crack deflection along the platelets, were observed behind the crack tip. This means that these grains are effective in enhancing the fracture resistance during crack propagation.     

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%.     

Creep properties of (Si–Al–O–N)SiC whisker composites

(Si–Al–O–N) (sialon)–SiC whisker (SiC_w) composites containing up to 10 mass% SiC_w were prepared by hot isostatic pressing. The strengths and the fracture toughness of composites remained relatively unchanged with the amount of SiC_w. The addition of SiC_w enabled us to improve the creep properties of sialon ceramics. The total creep strain and steady-state creep rate at 1473 K under a stress of 400–500 MPa decreased with increasing the amount of SiC_w. The experimental creep exponent values of monolithic sialon and sialon–SiC_w composites were nearly 1. It is supposed that the creep of both monolithic sialon and sialon–SiC_w composites are dominated by the viscous flow of the interglanular glassy phase.     

网状结构Si3N4陶瓷增强金属基复合材料的研究

利用有机前驱体浸渍法制备了Si3N4网络陶瓷预制体,利用液态金属浸渗法制备出Al基、Mg基复合材料：分析了在浸渗过程中浸渗温度、润湿角、浸渗时间、浸渗高度的相互关系。在压力下金属液克服浸渗阻力．使浸渗得以完成。网络陶瓷骨架孔筋表面覆盖一层氧化膜有利于自发娄渗的进行。合金中适量镁元素的存在使平面上发生轻微化学放热反应．对浸渗有利。指出压力浸渗制备铝基复合材料与无压浸渗制备镁基合材料的特点。探计了这种复合材料抗压强度和摩擦磨损性能特点。指出Si3N4／Al复合材料，Si3N4/Mg复合材料抗磨擦性能明显优于基休．抗拉强度大大高于基体。     

网状结构Si3N4陶瓷增强金属基复合材料的研究

利用有机前驱体浸渍法制备了Si3N4网络陶瓷预制体,利用液态金属浸渗法制备出Al基、Mg基复合材料.分析了在浸渗过程中浸渗温度、润湿角、浸渗时间、浸渗高度的相互关系.在压力下金属液克服浸渗阻力,使浸渗得以完成.网络陶瓷骨架孔筋表面覆盖一层氧化膜有利于自发浸渗的进行.合金中适量镁元素的存在使界面上发生轻微化学放热反应,对浸渗有利.指出压力浸渗制备铝基复合材料与无压浸渗制备镁基复合材料的特点.探讨了这种复合材料抗压强度和摩擦磨损性能特点.指出Si3N4/Al复合材料,Si3N4/Mg复合材料抗磨擦性能明显优于基体,抗拉强度大大高于基体.     

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.     

Thermomechanically induced embrittlement in hot isostatically pressed Si3N4/SiC composites

The macroscopic fracture properties of an Si₃N₄/SiC-platelet composite fabricated by hot isostatic pressing (HIP) without sintering aids were measured by the chevron-notch technique in bending and related to micromechanisms of fracture by means of a quantitative profilometric analysis of the fracture surfaces. Compositional and processing parameters were varied systematically in order to maximize both the fracture toughness and the work of fracture of the composite. Data were compared with those of monolithic Si₃N₄ fabricated by the same process. Cooling-rate from the HI Ping temperature was indicated as a critical parameter especially when cooling was performed under high pressure. A marked embrittlement of the composite body was found by cooling at around 650 °C h^–1 and it could not be completely recovered by successive annealing even up to temperatures above 1700 °C. The highest fracture toughness and work of fracture in the composite (obtained at a cooling rate of about 100 °C h^–1), were measured as 4.6 MPa m^1/2 and 58.6 J m^–2, respectively. In agreement with fractal analysis results, they were estimated to be about 60%–70% of the maximum values, respectively, obtainable in the present composite system, provided that a complete debonding at the platelet/matrix interface can occur.     

氮化硅晶须的研究现状

介绍了晶须的生长机理、氮化硅晶须的制备方法以及影响其生长的因素。阐述了氮化硅晶须增强复合材料的现状,指出加强晶须的基础研究,改进制备技术和降低成本是今后的发展方向。     

SiC-nanoparticle-reinforced Si3N4 matrix composites

Hot-pressed nanosized SiC-particle (SiC_p-reinforced Si₃N₄ composites have been studied with respect to their microstructures, room temperature mechanical properties and thermal shock resistance. The experimental results indicate that the flexural strength, fracture toughness and thermal shock resistance are all increased by the addition of 5 vol% SiC_p. Further additions of SiC_p, however, have a detrimental effect on these properties. These changes are closely related to the effects of SiC_p on the matrix grain morphology. The mechanisms of nucleation and growth of -Si₃N₄ grains in the presence of SiC nano-particles are discussed.     

Production of superplastic aluminium composites reinforced with Si3N4 by powder metallurgy

Superplastic aluminium composites were processed from these fine aluminium powders of less than 20 m and reinforcements of either Si₃N₄ whiskers or Si₃N₄ particulates by hot extrusion at temperatures between 733 and 793 K with a reduction ratio of 1001. The dispersion of the reinforcements was homogeneous, and the size of the grains of this matrix alloy after extrusion was fine, at less than 3 m for the composites reinforced with either Si₃N_4W or Si₃N_4P. All composites showed large superplastic elongations of more than 300% in a relatively high strain-rate range from 4×10^–22 s^–1 at testing temperatures between 788 and 833 K. These superplastic composites also exhibited excellent mechanical properties at room temperature, which are supposedly attributable to both the homogeneous dispersion in reinforcements and the fine-grained structures.     

Chemical reactions in mullite matrix SiC whisker reinforced composites in RF plasma

Decomposition of mullite by volatilization of SiO occurs during elevated temperature exposure to reducing gases. RF induction coupled argon plasma power sources have been used for rapid sintering of a number of ceramics and ceramic matrix composites. During plasma sintering of SiC whisker reinforced mullite matrix composites, whiskers were destroyed and the matrix converted to alumina by accelerated volatilization of silicon containing species. The results of these experiments are interpreted through the use of thermodynamic calculations of SiO pressures in the plasma environment.