First matrix cracking behavior of ceramic composites at elevated temperatures

The first matrix cracking behavior of a silicon carbide fiber (SCS-6™)-reinforced zircon matrix composite is studied as a function of flaw size and temperature. Flaws of controlled size are created in the monolithic zircon and silicon carbide fiber-reinforced zircon matrix composite by means of a Vicker's indentation technique. The first matrix cracking stress is measured at three different temperatures of 25, 500, and 1200°C as a function of the crack length. The results on ceramic composites demonstrated both steady state and non-steady state matrix cracking behaviors and an increase in the steady state matrix cracking stress with an increase in temperature as predicted by the theoretical models. These results are compared with the predictions of the theoretical models of matrix cracking based on fracture mechanics analysis.     

Mechanical behavior of glass and Blackglas ceramic matrix composites

Room temperature tensile tests are reported on two low-cost ceramic matrix composite materials, comprised of matrices of Blackglas^® and a proprietory glass composition each reinforced with Nicalon^® SiC-based fibers. The measured mechanical behaviors, supplemented by post-fracture analysis of fiber pullout and fiber fracture mirrors, are compared in detail to the performance predicted theoretically. This allows for an assessment of the roles of the matrix, fiber srength, residual stresses, fiber geometry, and the fiber/matrix interfacial properties in determining mechanical response. The Blackglas^® matrix cracks extensively during processing, and so the mechanical response is controlled by the deformation and fracture of the fiber bundle. The interfacial sliding resistance, τ, is determined to be ≈17 MPa and thein-situ (post-processed) fiber characteristic strength,σ_c is found to be ≈2.0 GPa, both similar to values reported in the literature for Nicalon^®/CAS-glass systems. For the glass matrix, the unidirectional and cross-ply materials show marked differences in mechanical behavior. In the cross-ply composites, τ ≈ 14 MPa andσ_c≈2.9 GPa; in the unidirectional variants, these values were 1.7 MPa and 1.6 GPa, respectively. With these data and other derived micromechanical parameters, the stress-strain and failure point of these materials was predicted using existing models, and excellent agreement with the experiments was obtained. These materials thus perform as expected given the in-situ fiber and interface properties. Notably, the cross-ply glass matrix composites exhibit high fiber strength retention and hence show tensile strengths that are better than other Nicalon^®-based materials tested to date.     

A mean-field micromechanical model of thermal-ratcheting behavior in short fiber-reinforced metal matrix composites

Thermal-ratcheting behavior in short fiber-reinforced metal matrix composites has been investigated. The internal stresses due to mismatch in thermal expansion coefficients between the fiber and matrix in the composites can introduce anomalous deformation under thermal-cycling conditions with or without external mechanical loads. This study has resulted in a more comprehensive understanding of such thermal-ratcheting behavior in short fiber-reinforced composites subject to a wide range of external uniaxial loads based on a mean-field micromechanical model considering the local mass transfer by diffusion along the fiber–matrix interface. The validity of the model has been verified through systematic experiments with directionally solidified Al–Al₃Ni eutectic composites.     

Tensile behavior and cyclic creep of continuous fiber-reinforced glass matrix composites at room and elevated temperatures

In this study we investigated the stress-strain behavior at room and elevated temperatures and the tensile creep and cyclic creep response of a unidirectional SiC fiber-reinforced aluminosilicate glass matrix composite. The interfacial condition of the as-received material was measured by a push-out indentation technique. The stress-strain behavior was that expected for this kind of composite, i.e. “pseudoductile” behavior with extensive fiber “pull-out” at room temperature and brittle failure at intermediate temperatures (750 °C) due to oxidation embrittlement. The stiffness of the composite at 750°C was analyzed for different loading rates, highlighing the influence of the loading rate on apparent composite stiffness, due to matrix softening. The creep studies were conducted at temperatures above and below the softening temperature of the glass (T _g, 745 °C) in air. The cyclic creep experiments showed the existence of extensive viscous strain recovery during the unloading period. The creep strain recovery was quantified using strain recovery ratios. These ratios showed a slight dependence on the temperatures investigated (700 and 750 °C). The crept composites retained their “graceful” fracture behavior only partially after testing, indicating that oxidation of the fiber/matrix interface due to oxygen diffusion through the matrix occurred in the peripheral area of the samples. Dr. Boccaccini is presently at the Institute for Mechanics and Materials, University of California, San Diego, CA 92093, USA.     

陶瓷基复合材料焊接技术研究进展

对陶瓷基复合材料的焊接技术作了总体概述.系统总结了陶瓷基复合材料的可用焊接方法、工艺、焊接过程中易出现的问题及对策,并对陶瓷基复合材料及其焊接技术的发展前景作了展望.     

内晶结构复相陶瓷的裂纹扩展行为

以ZrO2-Alo2O3微-纳米复合陶瓷为研究对象,采用跟踪压痕裂纹法测试其裂纹扩展阻力曲线.结果表明,三种微-纳复合ZTA陶瓷的阻力曲线均为显著的上升曲线.成分相同、烧结温度高的试样和烧结温度相同、氧化锆含量少的试样,由于内晶颗粒数量多,阻力曲线的上升趋势更加明显.内晶颗粒附近区域产生位错网、亚晶界甚至微裂纹,使微-纳米复合ZTA陶瓷的断裂方式向穿晶断裂方向变化.裂纹穿晶扩展遇到的阻力大,所需要的裂纹扩展动力也大,形成开裂-止裂交替进行的裂纹扩展过程,故其阻力曲线上升明显.     

陶瓷基复合材料与金属连接的研究进展

陶瓷基复合材料是一种新兴的热结构材料，解决其自身及其与金属的连接工艺，是实现其推广应用的重要课题之一。首先分析了陶瓷基复合材料自身连接及其与金属连接的难点，在此基础上从解决被连接材料的化学相容性与物理匹配性两方面出发，综述了陶瓷基复合材料自身及其与金属连接的研究进展，并介绍了几种典型的连接实例——活性金属钎焊、部分瞬间液相扩散连接以及宏观结构梯度中间层设计。     

Thixoforming of SiC ceramic matrix composites in pseudo-semi-solid state

1INTRODUCTIONIt has great economic profit to exploit the uti-lization of SiC ceramic material for its strength,hardness,wear resistance,corrosion resistanceand high temperature resistance,abundant re-source and low price.But the brittle charactergreatly restricts its application in the field of engi-neering structure materials.Recently a series ofceramic matrix composites with exi mious perform-ance have been developed by reinforcing the matrixmaterial with other materials.Al/SiC is a la…     

Elastic properties of short glass fiber-reinforced ZA-27 alloy metal matrix composites

A well-consolidated composite of ZA-27 alloy reinforced with short glass fiber at volume fractions of 2, 7, 12, and 17% was prepared by liquid infiltration techniques and its elastic properties were determined by destructive testing. The results showed that the modulus of elasticity and ultimate tensile strength of the composite gradually increased with increasing volume fraction of the fiber, although the ductility decreased with an increase in volume fraction of the fibers. In addition, the data obtained from Young’s modulus measurements were compared with theoretical results predicted by the shear-lag model, Nielsen-Chen model, and computational model. The experimental results were shown to be in better agreement with those of the latter two models. The ultimate tensile strength test results were also compared with theoretical results predicted by the shear-lag and Miwa models. The Miwa model agreed favorably with the experimental results.     

The prospects for hybrid fiber-reinforced Ti-TiB-matrix composites

Significant cost reductions in continuously reinforced titanium-matrix composites have enabled their successful insertion into two aerospace applications. Additional applications are being pursued, but reduced anisotropy is required to achieve the broadest impact of this technology. Novel hybrid composites consisting of continuous SiC reinforcements and titanium alloys enhanced with TiB and/or TiC particles are being developed to reduce the anisotropy between longitudinal and transverse properties. For more information, contact W.M. Hanusiak, FMW Composite Systems, 1200 W. Benedum Industrial Drive, Bridgeport, WV; (304) 842-1864; fax (304) 842-1866; e-mail: bhanusiak@fmwcomposite.com.     

Damage behavior of tungsten fiber-reinforced copper matrix composite after high-speed impact

Wf/Cu82Al10Fe4Ni4 composite was fabricated by means of infiltration casting. By the microstructure observation of the composite with the aid of scanning electron microscopy(SEM) and transmission electron microscopy(TEM), it can be found that there are(Fe, Ni),AlFe, Al3 Ni, and Cu3 Al precipitated phases existing in the matrix alloy. By two-stage light gas gun, Wf/Cu82Al10-Fe4Ni4 composite hypervelocity projectile into concrete target test with the speed of 2.2 kmás-1is finished. By microstructure observation, it can be found that the failure mode of Wf/Cu82Al10Fe4Ni4 composite projectile during penetration is the rapid peeling of tungsten fibers from the projectile, which makes the projectile display good selfsharpening property. Meanwhile, it can be found that microstructure morphology change of Wf/Cu82Al10Fe4Ni4 composite occurs after hypervelocity impact. The density of dislocations around the large-dimensional(Fe, Ni),AlFe, Al3 Ni, and Cu3 Al precipitated phases in the matrix alloy rises sharply. At the same time, there are largedimensional deformed twins existing in local regions and stacking faults existing inside the twins.     

Recent progress in ceramic matrix composites reinforced with graphene nanoplatelets

Graphene nanoplatelets(GNPs) are considered to be one of the most promising new reinforcements due to their unique two-dimensional structure and remarkable mechanical properties. In addition, their impressive electrical and thermal properties make them attractive fillers for producing multifunctional ceramics with a wide range of applications. This paper reviews the current status of the research and development of graphene-reinforced ceramic matrix composite(CMC) materials. Firstly, we focused on the processing methods for effective dispersion of GNPs throughout ceramic matrices and the reduction of the porosity of CMC products. Then, the microstructure and mechanical properties are provided, together with an emphasis on the possible toughening mechanisms that may operate. Additionally, the unique functional properties endowed by GNPs, such as enhanced electrical/thermal conductivity, are discussed, with a comprehensive comparison in different ceramic matrices as oxide and nonoxide composites. Finally, the prospects and problems needed to be solved in GNPs-reinforced CMCs are discussed.     

Computer modeling of the mechanical behavior of composites -Interfacial cracks in fiber-reinforced materials-

An interface crack between a fiber and a matrix has been analyzed and results of SIFs for both, mode I and mode II contributions, have been presented graphically. Trends in these SIFs have been identified and rationalized. It was found that the strong variation in SIFs is mainly a function of crack length, elastic mismatch and loading directions.     

Microstructure evolution of rare earth Pr modified alumina-silicate short fiber-reinforced Al-Si metal matrix composites

Al-Si metal matrix composites (MMCs) reinforced with 20 vol.% alumina-silicate shot fibers (Al2O3-SiO2(sf)) were fabricated by an infiltration squeeze method. Pure Pr metal was added into these composites. The effect of Pr addition on the microstructure evolution of Al-Si MMCs was investigated by SEM,TEM,and EDS. Pr addition is favorable to make uniform microstructures with the modified eutectic Si crystal. PrAlSi phase with high contents of Pr and Si is observed on the interface between the fiber and the m...     

Continuous fiber-reinforced titanium aluminide composites

Recent work on a lightweight, elevated-temperature intermetallic-matrix composite (SCS-6 SiC/α₂ Ti-14Al-21Nb), including investigations of fabrication techniques, microstructural characteristics and mechanical behavior, indicates that the material appears promising for a number of demanding applications. If successfully implemented, this material, or a derivative, will provide substantial weight savings in aerospace systems. To realize this potential, major challenges must be conquered—low-temperature ductility and environmental resistance must be improved, and the cost must be brought to competitive levels.     

The failure of fiber-reinforced ceramic-matrix composites under dynamic loading

High-strain-rate compressive failure mechanisms in fiber-reinforced ceramic-matrix composite materials have been characterized. These are contrasted with composite damage development at low strain rates and with the dynamic failure of monolithic ceramics. It is possible to derive significant strain-rate strengthening benefits if a major fraction of the fiber reinforcement is aligned with the load axis. This effect considerably exceeds the inertial microfracture strengthening observed in monolithic ceramics and nonaligned composites. Its basis is shown to be the transspecimen propagation time period for heterogeneously-nucleated, high- strain kink bands. For high-strain-rate tensile loading conditions, it is found that behavior is not correctly described by the current matrix fracture/fiber pullout models. This is a consequence of the rapid and extreme frictional heating produced at the fiber-matrix interface by sliding velocities on the order of 100 m/s. At rapid loading rates, the near-interface matrix appears to virtually melt, and the frictional interface shear resistance is reduced to the point that the fibers debond throughout the specimen, and pull out without failing. This suggests that for sufficiently rapid loading, the stress to fail the composite will approach that merely to create the initial matrix crack (i.e., a stress level well below the ultimate strength normally attainable under quasistatic conditions).     

Mechanical behavior of diamond matrix composites with ceramic Ti3(Si,Ge)C2 bonding phase

Polycrystalline diamond, PCD, compacts are usually produced by high pressure–high temperature (HP–HT) sintering. This technique always introduces strong internal stresses into the compacts, which may result in self-fragmentation or graphitization of diamond. This may be prevented by a bonding phase and Ti₃(Si,Ge)C₂ was so investigated. This layered ceramic was produced by Self Propagating High Temperature Synthesis and the product milled. The Ti₃(Si,Ge)C₂ milled powder was mechanically mixed, in the range 10 to 30 wt.%, with 3–6 μm diamond powder (MDA, De Beers) and compacted into disks 15 mm in diameter and 5 mm high. These were sintered at a pressure of 8.0 GPa and temperature of 2235 K in a Bridgman-type high pressure apparatus. The amount of the bonding phase affected the mechanical properties: Vickers hardness from 20.0 to 60.0 GPa and Young's modulus from 200 to 500 GPa, with their highest values recorded for 10 wt.% Ti₃(Si,Ge)C₂. For this composite fracture toughness was 7.0 MPa m^1/2, tensile strength 402 MPa and friction coefficient 0.08. Scanning and transmission electron microscopy, X-ray and electron diffraction phase analysis were used to examine the composites.     

Delamination failure in ceramic matrix composites: Numerical predictions and experiments

The cohesive-zone finite element approach is used to predict initiation and propagation of delamination in relatively complex ceramic matrix composite sub-elements. Two different generic attachment sub-elements are analyzed and tested under applied uniaxial load. Pre-test analyses predict that delamination initiation and growth are the predominant failure mechanism for both of the sub-elements. Experimental results confirm the finite element predictions, and a good qualitative and quantitative agreement is found between the two.