Interface characterization of fiber-reinforced Ni3Al matrix composites

The interfacial reaction characteristics of SCS-6, Sigma, and B₄C/B fibers with nickel aluminide (Ni₃Al) matrix have been investigated between 780°C to 980°C for times ranging from 1 to 100 hours. The microstructure and elemental compositions across the reaction zone have been analyzed quantitatively using microscopy and electron probe microanalyses, respectively. The results show that Ni₃Al reacts extensively with SCS-6, Sigma, and B₄C/B fibers to form complex reaction products, and Ni is the dominant diffusing species controlling the extent of reaction. In the SiC/Ni₃Al composite, the C-rich layer on the SiC surface can slow down but cannot stop the inward diffusion of Ni into SiC fiber. When the C-rich layer is depleted, a rapid increase in reaction zone thickness occurs. Diffusion barrier coating on the fibers is required to minimize the interfacial reactions.     

Fracture resistance of fiber-reinforced ceramic matrix composites

The influence of the properties of the fibers, the matrix and the interface on the mechanical properties of fiber reinforced ceramics is analyzed by a simplified method previously developed by the authors for cohesive materials. The method parts from the assumption that crack displacements are known a priori and furnishes, in a simple and easy way, the fracture resistance curves versus crack length. The numerical results from the model are compared with experimental data from the literature. Finally, the model is used to assess the influence of fiber strength, interface slipping shear stress, fiber radius and fiber defect distribution on the fracture resistance and ductility of fiber-reinforced ceramic composites.     

Interface characterization of duplex metal-coated SiC fiber-reinforced Ti-15-3 matrix composites

The interfacial reaction behavior of duplex metal (Cu/Mo and Cu/W)-coated SiC (SCS-6) fiber-reinforced Ti-15-3 composites, before and after thermal exposure, has been studied. The effect of thermal exposure on the shear sliding resistance of these composites was also obtained using a thin-specimen push-out test. The results are compared to those of an original SiC (SCS-6) fiber-reinforced Ti-15-3 composite. The interfacial reaction behavior is strongly affected by the existence of a coating layer. Both the Cu/Mo and Cu/W coating layers prevent the growth of a reaction layer. However, the coatings could not effectively prevent diffusion of alloying elements; only the W layer exists after the thermal exposure. On the other hand, the interface shear sliding stress minimally depends on the duplex metal coating layers prior to the thermal exposure, and this sliding stress in both the SiC/Cu/Mo/Ti-15-3 and SiC/Cu/W/Ti-15-3 composites decreases slightly relative to that in the SiC/Ti-15-3 composite. After thermal exposure, the interface shear sliding stress increases for the SiC/Ti-15-3 composite. In distinction, the interface shear sliding stress significantly decreases after thermal exposure in both the SiC/Cu/Mo/Ti-15-3 and SiC/Cu/W/Ti-15-3 composites. Theses behaviors are attributed to the decrease of radial clamping stress, which originates from a volume expansion associated with the β → α phase transformation.     

Effects of composite processing on the strength of sapphire fiber-reinforced composites

The current interest in tough, high-temperature materials has motivated fiber coating development for sapphire fiber-reinforced alumina composites. For this system, it has been demonstrated that the interfacial properties can be controlled with coatings which can be eliminated from the interface subsequent to composite consilidation. However, these fugitive coatings can contribute to the high temperature strength degradation of sapphire fibers. Such degredation,which compromises the composite strength and toughness, is the focus of the current investigation. It has been observed that, in some cases, by selecting appropriate composite processing conditions, such effects can be minimized. But overcoming fiber strength loss remains an important issue.     

Fabrication of fiber-reinforced metal-matrix composites by variable pressure infiltration

A model for variable pressure infiltration of fibrous preforms by molten metal has been developed. The mechanism of the infiltration and the effects of fiber distribution and wettability on infiltration resistance and composite microstructure have been studied. It is shown that the model is in good agreement with the experimental data on infiltration of carbon-fiber preforms by Al-Si eutectic. The solution of the resulting equation shows that the rate of infiltration is only a function of the rate of change of pressure Ф, by which the infiltration processing is controlled precisely. Two kinds of infiltration modes have been found. A critical fiber volume fractionV _c exists, which is the turning point of the infiltration modes as well as permeability. As fiber volume fraction exceedsV _c, the infiltration mode changes from nonuniform to uniform, resulting in a sharp decrease in permeability. The permeability and resistances of these two infiltration modes are well predicted by the variable pressure infiltration theory. If fibers are wetted by molten metal, the preforms can be completely infiltrated at low applied pressure. In the case of nonwetting, poor infiltration of the preforms up toV _cresults, though high pressure is applied, but quality composites are formed at a low applied pressure if the fibers in the bundles are fixed relative to each other. A novel process, variable pressure infiltration technique, has been generated, which offers the advantages of low applied pressure, easy control of the pro-cessing, and no requirement of wetting. Quality C/A356 composites have been fabricated by this technique with the investment precision casting molds at a pressure of 0.6 MPa. Also, the mechanical properties of the composites are studied. The composites have high strength with a special fracture mechanism.     

Evaluation of interfacial shear strength,residual clamping stress and coefficient of friction for fiber-reinforced ceramic composites

The shear strength, the residual clamping stress, the coefficient of friction and the frictional stress at the fiber/matrix interface are evaluated for fiber-reinforced ceramic composites by using the theoretical analysis for fiber push-out and the corresponding experimental results. The shear strength is evaluated from the load at which debonding initiates. Sliding occurs at the interface after complete debonding. For a fiber with a Poisson's ratio greater than zero, the characteristics of the nonlinear relationship between the load required to push out the fiber and the sample thickness enable the residual clamping stress, the coefficient of friction and the interfacial frictional stress to be evaluated in the present analysis.     

高韧性纤维增强水泥基复合材料的收缩变形

通过试验研究了砂灰比、水灰比、纤维种类和减缩剂对高韧性纤维增强水泥基复合材料(ECC)收缩变形的影响.结果表明:随着砂灰比的增大,ECC收缩应变值逐渐减小;随着水灰比的增大,ECC收缩应变值逐渐增大;国产PVA纤维对控制ECC早期收缩变形有较明显的效果,而日本产的高弹性模量PVA纤维对控制ECC后期收缩变形效果显著;水灰比为0.40时,混杂纤维对控制ECC收缩变形的效用比单独掺入国产PVA或日本产PVA好;水灰比为0.40时,掺入减缩剂可使ECC收缩应变约减少200×10^-6,可见减缩剂控制ECC收缩变形效果显著.     

Fatigue of ceramic matrix composites

Fatigue in ceramic matrix composites typically occurs when matrix cracks are present. It proceeds by cyclic degradation of the sliding resistance of the interface. The basic mechanisms are discussed and a methodology is developed that enables fatigue life predictions to be made, based on a minimum number of experimental measurements. The methodology relies on analysis of hysteresis loops. Changes in modulus upon cyclic loading as well as the permanent strains are predicted, as well as the fatigue threshold and the S-N curve.     

The mechanical properties of ceramic composites produced by melt oxidation

The strength and fracture toughness of a range of DMOX materials, with and without SiC particulates, have been measured. The strength has been shown to correlate with the SiC particulate size and to be consistent with a model based on the expansion misfit between the particulates and the matrix. The toughness has contributions for both the residual alloy and the SiC. The former has been shown to conform well with the expectations of models based on plastic dissipation in the alloy. The influence of the SiC is found to be strongly dependent on the particulate size and is qualitatively consistent with a contribution to toughness from frictional dissipation at debonded SiC/Al₂O₃ interfaces.     

低温模压法制备的C/C-SiC复合材料的摩擦磨损性能

为深入了解低成本法制备的C/C-SiC复合材料的摩擦磨损规律,以短炭纤维、Si粉、炭粉和粘结剂为原料,通过均匀混合、模压成形、1 600℃反应烧结制备了C/C-SiC复合材料,研究了孔隙度、SiC含量及环境湿度对该复合材料摩擦磨损性能的影响,并用光学显微镜及X射线衍射仪对磨屑进行观测分析,对不同状况下的摩擦磨损机理进行研究。结果表明:C/C-SiC复合材料的致密度决定其磨损方式;SiC在摩擦过程中作为硬质支撑点,其含量对摩擦系数及其稳定性具有关键性影响;湿态时的摩擦系数与线磨损均略有下降,但仍能保持其良好的摩擦磨损性能。     

反应熔渗法制备C/C-ZrC复合材料的微观结构及烧蚀性能

采用反应熔渗法(reactive melt infiltration,RMI)制备ZrC改性多孔C/C复合材料,研究不同孔隙度的C/C多孔体在熔渗过程中的增密行为和渗Zr后的相组成及微观形貌,探寻具有最佳熔渗效果的C/C多孔体,并研究所得C/C-ZrC复合材料在不同温度下的氧乙炔焰烧蚀行为。结果表明,随C/C多孔体密度增加,C/C-ZrC复合材料的密度降低;其中密度为1.40 g/cm3的多孔体熔渗效果最佳,开孔隙率由熔渗前的28.2%降低到6.6%。;熔渗的Zr液易与网胎层处的炭纤维和基体炭反应,生成的ZrC陶瓷相主要分布在原网胎层位置。择取原始密度为1.40 g/cm3的C/C多孔体熔渗后进行60 s的氧乙炔焰烧蚀实验,在3 000℃下的线烧蚀率和质量烧蚀率分别为0.003 3 mm/s和0.004 2 g/s,在2 500℃下的线烧蚀率和质量烧蚀率分别为0.008 0 mm/s和0.009 0 g/s,C/C-ZrC复合材料在3 000℃下的抗烧蚀性能明显优于2 500℃下的抗烧蚀性能。     

Matrix cracking of cross-ply ceramic composites

A micromechanics study is presented of the matrix cracking behavior of laminated, fiber-reinforced ceramic cross-ply composites when subject to tensile stressing parallel to fibers in the 0° plies. Cracks extending across the 90° plies are assumed to exist, having developed at relatively low tensile stresses by the tunnel cracking mechanism. The problem addressed in this study is the subsequent extension of these initial cracks into and across the 0° plies. Of special interest is the relation between the stress level at which matrix cracks are able to extend all the way through the 0° plies and the well known matrix cracking stress for steady-state crack extension through a uni-directional fiber-reinforced composite. Depending on the initial crack distribution in the 90° plies, this stress level can be as large as the uni-directional matrix cracking stress or it can be as low as about one half that value. The cracking process involves a competition between crack bridging by the fibers in the 0° plies and interaction among multiple cracks. Crack bridging is modeled by a line-spring formulation where the nonlinear springs characterize the sliding resistance between fibers and matrix. Crack interaction is modeled by two representative doubly periodic crack patterns, one with collinear arrays and the other with staggered arrays. Material heterogeneity and anisotropy are addressed, and it is shown that a homogeneous, isotropic average approximation can be employed. In addition to conditions for matrix cracking, the study provides results which enable the tensile stress-strain behavior of the cross-ply to be predicted, and it provides estimates of the maximum stress concentration in the bridging fibers. Residual stress effects are included.     

The mechanics of failure of silicon carbide fiber-reinforced glass-matrix composites

Tensile and bending tests have been performed on a high-strength silicon carbide fiber-reinforced glass-matrix (LAS) composite. The experimental results indicate that the strength of the composite component is strongly dependent on the geometry and loading. A model of failure is developed which allows a uniform interpretation of the experiments when due account is taken of the strength of the matrix, the statistical nature of the properties of the fibers, and the dimension of the fiber pull-out length.     

Tailoring interfacial shear strength of SiC fiber-reinforced brittle matrix composites

The interfacial shear strength of Nicalon SiC fiber-reinforced glass-ceramic matrix composites was aimed to be tailored via two methods: (1) varying of the thickness of the carbon-rich interfacial layer between the fiber and the matrix by controlling hot pressing period and (2) formation of the secondary interfacial layer, TaC, at the carbon/matrix boundary by doping the Ta₂O₅ matrix addition. In the series of composites with varying carbon-rich layer thickness, fiber/matrix debonding mostly occurred at the carbon/matrix boundary and hence the increase in the carbon-rich layer thickness did not cause any apparent changes in the interfacial shear strength. In the TaC formed series of composites, the interfacial shear strength was affected considerably by the presence of the TaC phase at carbon/matrix boundary. The Ta₂O₅ addition to control the quantity of the TaC phase has shown to be a useful method to tailor the interfacial shear strength of SiC fiber/glass-ceramic composites.     

Fatigue and fracture behavior of Nb fiber-reinforced MoSi2 composites

Fatigue and fracture mechanisms in Nb fiber-reinforced MoSi₂ composites are elucidated in this article. The effects of fiber diameter on fracture and crack-tip shielding mechanisms are discussed after a review of micromechanical models which are applied to the prediction of residual stress levels, toughening, and microcracking phenomena. Toughening is shown to occur by a combination of crack bridging and crack-tip blunting under monotonic and cyclic loading. However, the observed failure mechanisms are different under monotonic and cyclic loading. Composites with smaller (250-μm) fiber diameters are shown to have better fatigue resistance and lower fracture toughness than composites with larger (750-μm) fiber diameters. The occurrence of slower fatigue crack growth rates in the composites reinforced with smaller diameter Nb fibers is rationalized by assessing the combined effects of fiber spacing and interfacial crack growth on the average crack growth rates within the composites.     

Creep rupture life prediction of short fiber-reinforced metal matrix composites

The creep rupture life of an Al/Al₂O₃ composite and its creep behavior were studied. The metal matrix composite was produced by using a squeeze casting technique. High-temperature tensile tests and creep experiments were conducted on a 15 vol pct alumina fiber-reinforced AC2B Al alloy metal matrix composite (MMC). The high-temperature tensile strength of Al/Al₂O₃ composite is 14 pct higher than that of an AC2B Al alloy. The steady-state creep rate and the creep life were measured. The stress exponent in Norton’s equation and the activation energy were computed. The stress exponents of the AC2B and Al/Al₂O₃ composites were found to be 4 and 12.3, respectively. The activation energy of the AC2B and Al/Al₂O₃ composites was found to be 242.74 and 465.35 kJ/mol, respectively. A new equation for predicting creep life was established, which was based on the conservation of the creep strain energy. The theoretical predictions were compared with those of the experiment results, and a good agreement was obtained. It was found that the creep life is inversely proportional to the (n + 1)th power of the applied stress and strain failure energy of creep is conserved. The creep fracture surface, examined by scanning electron microscopy (SEM), showed that the MMC specimen failed in a brittle manner.