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.     

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.     

Transverse creep of SiC/Ti-6Al-4V fiber-reinforced metal matrix composites

The transverse creep response of an 8-ply SiC (SCS-6)/Ti-6Al-4V composite was measured at 482 °C from 69 to 276 MPa. Creep samples with fibers exposed at the edges as well as specimens with fully embedded fibers were tested under stepped loading conditions with increasing load. The response of each sample geometry was compared with creep data from the unreinforced matrix (‘neat’ material). The samples with exposed fiber ends exhibited minimum creep rates that were higher than those of the neat material at all stresses, and the stress exponent was slightly large than the neat material. The embedded fiber samples possessed minimum creep rates that were smaller than the neat material at low stresses (<115 MPa), but became equivalent to the exposed fiber data at the highest stress (276 MPa). The apparent stress exponent for the embedded fiber composite was significantly larger than the neat material. The exposed fiber test data were well represented by a standard Crossman analysis, where the fibers were considered to have completely debonded. A stress singularity in the interfacial region at the sample edge is responsible for this behavior. The Crossman model was modified to incorporate the effect of a finite interface strength (120 MPa), and this was used to describe the response of the samples with embedded fibers. A reasonable fit to this representation was obtained. However, the measured minimum creep rate at the lowest stress was significantly lower than that predicted by the Crossman analysis for fully bonded fibers. This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee.     

Structure and properties of hot-pressed boron carbide base ceramics

Translated from Poroshkovaya Metallurgiya, No. 7(331), pp. 16–20, July, 1990.     

Tensile properties of short fiber-reinforced SiC/Ai composites: Part I. effects of matrix precipitates

The tensile behavior of aluminum matrix composites reinforced with 8 and 20 pet SiC whiskers or paniculate was characterized. Two matrix alloys were employed, a solution-hardened Al-Mg alloy (5456) and a precipitation-hardened Al-Cu-Mg alloy (2124). The precipitation-hardened alloy was aged to develop a variety of precipitate microstructures. It was found that additions of SiC caused monotonie increases in the elastic modulus, 0.2 pct offset yield stress, work-hardening rate, and ultimate tensile stress. The proportional limit, however, was found to first decrease and then increase with SiC content. Whiskers caused a greater increase in the longitudinal elastic modulus than particles. For the 2124 alloy, it was found that the proportional limit could be varied between 60 and 650 MPa by changing the precipitate microstructure, while changes in the SiC content had much smaller effects. These observations are discussed in relation to current theories of the strengthening of short fiber composites, with primary emphasis being placed on the effects of SiC additions on the elastic modulus and the work-hardening rate.     

Sintering and strength of hot-pressed ceramics based on titanium diboride

The paper examines how the grain-size composition of starting titan diboride powders and calcium silicon and calcium hexaboride additives influence the consolidation of titanium diboride in hot pressing and the structure and properties of hot-pressed materials. It is shown that 3 and 5 wt.% of calcium silicon decrease the pressing temperature by 200 °C, refine the structure, and improve the density and mechanical properties of titan diboride materials.     

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.     

Fatigue in selectively fiber-reinforced titanium matrix composites

Many applications of the Ti alloy matrix composites (TMCs) reinforced with SiC fibers are expected to use the selective reinforcement concept in order to optimize the processing and increase the cost-effectiveness. In this work, unnotched fatigue behavior of a Ti-6Al-4V matrix selectively reinforced with SCS-6 SiC fibers has been examined. Experiments have been conducted on two different model panels. Results show that the fatigue life of the selectively reinforced composites is far inferior to that of the all-TMC panel. The fatigue life decreases with the decreasing effective fiber volume fraction. Suppression of multiple matrix cracking in the selectively reinforced panels was identified as the reason for their lack of fatigue resistance. Fatigue endurance limit as a function of the clad thickness was calculated using the modified Smith-Watson-Topper (SWT) parameter and the effective fiber volume fraction approach. The regime over which multiple matrix cracking occurs is identified using the bridging fiber fracture criterion. A fatigue failure map for the selectively reinforced TMCs is constructed on the basis of the observed damage mechanisms. Possible applications of such maps are discussed.     

无压烧结SiC陶瓷研究进展

高硬度、高强度、耐腐蚀、耐磨损和高弹性模量等特性使SiC陶瓷成为航空航天、电力电子和机械工业等领域不可或缺的材料。然而SiC陶瓷超高的合成温度和难以烧结致密的特性限制了它的生产。近年来随着SiC陶瓷烧结机理的深入研究及添加剂的多样化,采用无压烧结工艺制备高性能SiC陶瓷材料已逐渐成为最常用的方法。本文主要阐述了近年来无压烧结SiC工艺添加剂的研究进展,综述了添加剂及其用量、烧结温度和时间对无压烧结SiC陶瓷性能的影响。     

Strength of Al-Zn-Mg-Cu matrix composite reinforced with SiC particles

The AA7075 alloys reinforced with SiC and without SiC particles were fabricated by a pressureless infiltration method, and then, their tensile properties and microstructures were analyzed. The spontaneous infiltration of molten metal at 800 °C for 1 hour under a nitrogen atmosphere made it possible to fabricate 7075 Al matrix composite reinforced with SiC, as well as a control 7075 Al without SiC. A significant strengthening even in the control alloy occurred due to the formation of in-situ AlN particle even without an addition of SiC particles. Composite reinforced with SiC particles exhibited higher strength values than the control alloy in all aging conditions (underaged (UA), peak-aged (PA), and overaged (OA)), as well as a solution treated condition. Spontaneous infiltration was further prompted owing to the combined effect of both Mg and Zn. This may lead to an enhancement of wetting between the molten alloy and the reinforcement. Consequently, strength improvement in a composite may be attributed to good bond strength via enhancement of wetting. The grain size of the control alloy is greatly decreased to about 2.5 μm compared to 10 μm for the commercial alloy. In addition, the grain size in the composite is further decreased to about 2 μm. These grain refinements contributed to strengthening of the control alloy and the composite.     

Analysis of shear stress distribution in pushout process of fiber-reinforced ceramics

The interfacial shear stress distribution of a thin specimen of SiC fiber-reinforced glass matrix composite (fiber volume fraction of 0.1, 0.5 and 0.7) during a fiber pushout process was subjected to finite element analysis using a three concentric axisymmetrical model which consisted of fiber, matrix, and composite. A stress criterion was used to determine interface debonding. Effects of thermally-induced stress and a post debond sliding process at the interface were also included in the analysis. The analytical result showed that shear stress near the specimen surface was introduced during the specimen preparation process. Before the interfacial debonding, the distribution of shear stress during the pushout test was affected by the existence of thermally-induced stress in the specimen. The interfacial shear debonding initiated ≈ 30 μm below the pushing surface and the sliding at the debonded interface proceeded in the direction of both the pushing surface and back surface from the peak shear position; the debonding from the back surface initiated just before the complete debonding of the interface. The pushout load-displacement curve near the origin was straight, however, after the existence of interface sliding at the debonded interface, the curve exhibited non-linearity with the increase in applied load up to the complete debonding at the interface. This debonding process was essentially independent of the fiber volume fraction. The results indicate that the total of thermally-induced stress in the specimen and shear stress distribution generated by applied load are important for the initiation of debonding and the frictional sliding process of the thin specimen pushout test.     

SiC纤维增强TB8复合材料工艺对组织影响规律

通过箔-纤维-箔法制备了SiC纤维增强TB8复合材料,采用光学电子显微镜（OM）、扫描电镜（SEM）和电子探针（EPMA）对复合材料的微观组织进行表征与分析,研究了真空热压复合时压力、温度和时间等工艺参数对SiC纤维增强TB8复合材料微观组织的影响规律。结果表明：压力显著地影响着复合材料基体与基体以及纤维与基体的结合,而温度对纤维基体界面反应情况影响较大。通过热压工艺的优化,可以有效控制界面反应层厚度,获得组织优良的SiCf/ TB8复合材料。     

Multiple matrix cracking in a fiber-reinforced titanium matrix composite under high-cycle fatigue

An experimental study of multiple matrix cracking in a fiber-reinforced titanium alloy has been conducted. The focus has been on the effects of stress amplitude on the saturation crack density and the effects of crack density on hysteresis behavior. Comparisons have been made with predictions based on unit cell models, assuming the sliding resistance of the interface to be characterized by a constant interfacial shear stress. In addition, independent measurements of the sliding stress have been made using fiber pushout tests on both pristine and fatigued specimens. D.P. WALLS, Graduate Student, formerly with the Materials Department, University of California, Santa Barbara     

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.     

Transverse cracking in fiber-reinforced brittle matrix,cross-ply laminates

The topic addressed in this paper is transverse cracking in the matrix of the 90° layers of a cross-ply laminate loaded in tension. Several aspects of the problem are considered, including conditions for the onset of matrix cracking, the evolution of crack spacing, the compliance of the cracked laminate, and the overall strain contributed by residual stress when matrix cracking occurs. The heart of the analysis is the plane strain problem for a doubly periodic array of cracks in the 90° layers. A fairly complete solution to this problem is presented based on finite element calculations. In addition, a useful, accurate closed form representation is also included. This solution permits the estimation of compliance change and strain due to release of residual stress. It can also be used to predict the energy release rate of cracks tunneling through the matrix. In turn, this energy release rate can be used to predict both the onset of matrix cracking and the evolution of crack spacing in the 90° layers as a function of applied stress. All these results are used to construct overall stress-strain behavior of a laminate undergoing matrix cracking in the presence of initial residual stress.