L E Pasto et al./Composites: Part B 30(1999)631-646 of a 2D CFCC can be simplified to the collective failure of several minicomposites in parallel. Characterization of minicomposites has also allowed for the determination of in-situ fiber strengths by the analysis of the fracture surface of pulled out fibers(Fig. 8). This is important because the strength of the reinforcing fibers is often affected as a result of high temperatures and aggressive environments often required for the synthesis of the matrix Analysis of load displacement curves obtained during the tensile evaluation of minicomposites have provided vast 0 0.25 0.5 0.75 information about the various micromechanical mechan- isms that are responsible for the tough behavior of CFCCs [11]. Fig. 9 shows the tensile load versus cross-head displa- Fig 10. Tensile load versus displacement curve for Hi-Nicalon"/CVI SiC cement response of a CG-NicalonCVI SiC minicompo- with a 0 I um thick carbon interphas site with a 1.0 um thick carbon fiber coating. The curve non-linear behavior up to the peak load for loads larg and control. It also has a collection of small capacity load than the load required to initiate matrix cracking. The cells and fixturing for the conduction of a wide variety of small jumps in the load in the non-linear region are asso- mechanical tests that include the bending of single ceramic ciated with the occurrence of additional matrix cracks. At fiber [10]. In this case, it was demonstrated that the propor- the peak load, a critical number of fibers in the bundle have tional stress limit, or yield strength was substantially higher failed triggering the failure of all fibers and leading to a for cross-linked PPXTA fibers than for uncross-linked continuous decrease in load bearing capacity with increas- fibers. In addition, it was found that the cross-linked ing displacement. The tail in the curve after the peak load is PPXTA exhibited a large recoverable compressive strain, the result of frictional sliding of the fiber bundle that bridges reminiscent of elastomers, in contrast to the large unrecov- the critical matrix crack as it is being pulled out from the erable strain exhibited by uncross-linked PPXTA fibers [9]. matrix. With the use of micromechanical models, it is The analysis of the fracture surface of a 2D CFCC (Fig. 6) possible, for example, to determine the magnitude of the suggests that the failure process and ultimately the strength interfacial shear stress(an important parameter to predict of these materials is controlled to a large extent by the fiber the mechanical behavior of these materials)from the tail of bundles aligned along the loading direction. This realization the curve or from the distribution of matrix cracks. The tail has prompted work in the characterization of fiber bundles in the curve can only be observed when the magnitude of the and minicomposites to tailor composite properties and to fiber bond strength and the interfacial shear stress are both further develop an understanding of the micromechanical low, and if the stiffness of the load train is large. Otherwise mechanisms responsible for the macroscopic behavior of a sudden load drop follows the peak load as illustrated in the CFCCs. Minicomposites consists of a single fiber tow, tensile load versus displacement curve for Hi-Nicalon containing any where between 500 and 800 filaments coated CVI SiC with a 0. 1 um thick carbon interphase in Fig 10 with the specific interphase and infiltrated with the matrix of In this case, the thinner carbon coating and the larger surface interest. Fig. 7 is a scanning electron micrograph showing roughness of Hi-Nicalon fibers result in a higher inter- the fracture surface of a minicomposite after tensile testing. facial shear stress, in shorter matrix crack spacing and The micrograph illustrates the distribution of fiber pullout fiber pull-out lengths. However, note that the improved ther lengths(which are a direct reflection of the distribution of mal stability and mechanical properties of Hi-Nicalon fiber strengths)and the fracture surface of individual fibers. over CG-Nicalon fibers result in a significantly larger In relation to the micrograph in Fig. 6, the tensile failure tensile strength. In addition to their usefulness in understanding and quan- fy ing the micromechanical mechanisms that are responsi ble for the tough behavior of CFCCs, minicomposites are also used to probe novel fiber coatings and interfacial concepts, an area that continues to be the focus of intense research [12] 3. 1.2. Fiber/matrix interface After the fibers, perhaps the most critical element in CFCCs is the fiber/matrix interface. hereafter referred to as the interface. The interface in CCCs is what makes possible to combine brittle fibers in a brittle matrix to obtain Fig. Il. Schematic of the single fiber push-in and push-out tes a tough composite
and control. It also has a collection of small capacity load cells and fixturing for the conduction of a wide variety of mechanical tests that include the bending of single ceramic fiber [10]. In this case, it was demonstrated that the proportional stress limit, or yield strength was substantially higher for cross-linked PPXTA fibers than for uncross-linked fibers. In addition, it was found that the cross-linked PPXTA exhibited a large recoverable compressive strain, reminiscent of elastomers, in contrast to the large unrecoverable strain exhibited by uncross-linked PPXTA fibers [9]. The analysis of the fracture surface of a 2D CFCC (Fig. 6) suggests that the failure process and ultimately the strength of these materials is controlled to a large extent by the fiber bundles aligned along the loading direction. This realization has prompted work in the characterization of fiber bundles and minicomposites to tailor composite properties and to further develop an understanding of the micromechanical mechanisms responsible for the macroscopic behavior of CFCCs. Minicomposites consists of a single fiber tow, containing anywhere between 500 and 800 filaments coated with the specific interphase and infiltrated with the matrix of interest. Fig. 7 is a scanning electron micrograph showing the fracture surface of a minicomposite after tensile testing. The micrograph illustrates the distribution of fiber pullout lengths (which are a direct reflection of the distribution of fiber strengths) and the fracture surface of individual fibers. In relation to the micrograph in Fig. 6, the tensile failure of a 2D CFCC can be simplified to the collective failure of several minicomposites in parallel. Characterization of minicomposites has also allowed for the determination of in-situ fiber strengths by the analysis of the fracture surface of pulled out fibers (Fig. 8). This is important because the strength of the reinforcing fibers is often affected as a result of high temperatures and aggressive environments often required for the synthesis of the matrix. Analysis of load displacement curves obtained during the tensile evaluation of minicomposites have provided vast information about the various micromechanical mechanisms that are responsible for the tough behavior of CFCCs [11]. Fig. 9 shows the tensile load versus cross-head displacement response of a CG-Nicalone CVI SiC minicomposite with a 1.0 mm thick carbon fiber coating. The curve non-linear behavior up to the peak load for loads larger than the load required to initiate matrix cracking. The small jumps in the load in the non-linear region are associated with the occurrence of additional matrix cracks. At the peak load, a critical number of fibers in the bundle have failed triggering the failure of all fibers and leading to a continuous decrease in load bearing capacity with increasing displacement. The tail in the curve after the peak load is the result of frictional sliding of the fiber bundle that bridges the critical matrix crack as it is being pulled out from the matrix. With the use of micromechanical models, it is possible, for example, to determine the magnitude of the interfacial shear stress (an important parameter to predict the mechanical behavior of these materials) from the tail of the curve or from the distribution of matrix cracks. The tail in the curve can only be observed when the magnitude of the fiber bond strength and the interfacial shear stress are both low, and if the stiffness of the load train is large. Otherwise, a sudden load drop follows the peak load as illustrated in the tensile load versus displacement curve for Hi-Nicalone CVI SiC with a 0.1 mm thick carbon interphase in Fig. 10. In this case, the thinner carbon coating and the larger surface roughness of Hi-Nicalone fibers result in a higher interfacial shear stress, in shorter matrix crack spacing and fiber pull-out lengths. However, note that the improved thermal stability and mechanical properties of Hi-Nicalone over CG-Nicalone fibers result in a significantly larger tensile strength. In addition to their usefulness in understanding and quantifying the micromechanical mechanisms that are responsible for the tough behavior of CFCCs, minicomposites are also used to probe novel fiber coatings and interfacial concepts, an area that continues to be the focus of intense research [12]. 3.1.2. Fiber/matrix interface After the fibers, perhaps the most critical element in CFCCs is the fiber/matrix interface, hereafter referred to as the interface. The interface in CFCCs is what makes possible to combine brittle fibers in a brittle matrix to obtain a tough composite. 636 A.E. Pasto et al. / Composites: Part B 30 (1999) 631–646 Fig. 10. Tensile load versus displacement curve for Hi-Nicalone/CVI SiC with a 0.1 mm thick carbon interphase. Fig. 11. Schematic of the single fiber push-in and push-out test
A.E. Pasto et al. /Composites: Part B 30(1999)631-646 637 Fig. 12. The single-fiber push-out test has a single fiber pushed into the matrix using a small flat-bottomed diamond indenter attached to the load cell in the ITs until the fiber protrudes from the bottom of the sample(Fig. 13) Although in the case of polymer and metallic matrix have been determined in various ways [ 13, 14]. Among the composites, a strong bond is sought between the matrix various indentation techniques, the single fiber push-in and and the reinforcement, the opposite is desired with push-out tests are without a doubt the most popular because CFCCs. Although a certain degree of bonding and frictional of the relative simplicity in their conduction. These tests are iding is required in CFCCs to allow for load transfer schematically described in Fig. 11 between fibers and matrix, the most important characteristic In the case of the single-fiber push-out tests, a single fiber of the interface has to be its ability to deflect cracks that is pushed into the matrix using a small flat-bottomed propagate in the matrix which can only be obtained with an diamond indenter(Fig. 12)attached to the load cell in the interface possessing low toughness ITS(Fig 3)until the fiber protrudes from the bottom of the The two parameters that best quantify the micromech sample(Fig. 13) nical efficiency of interfaces in CFCCs, namely the fiber During the test, both the load and the displacement of the bond strength and the interfacial shear stress or sliding stress fiber surface with respect to the surface of the sample ar monitored as illustrated in Fig. 14 for the case of a CG- Nicalon fiber in a CVI SiC matrix. By the application of micromechanical models it is possible to obtain several parameters from the experimental stress versus fiber-end displacement curves as illustrated in Fig. 15[131 A variation of the push-out tests that has been used to study the wear characteristics of the interface is the single- fiber push-back test. Fig. 16 is a record of the stress versus fiber-end displacement obtained during a push-back test 2000 3 Fig. 14. Plot of the load and displacement of the fiber surface with respect to Fig 13 Fiber protruding from the bottom of a specimen after being pushed the surface of the sample for the case of a CG-Nicalon fiber in a CVI SiC
Although in the case of polymer and metallic matrix composites, a strong bond is sought between the matrix and the reinforcement, the opposite is desired with CFCCs. Although a certain degree of bonding and frictional sliding is required in CFCCs to allow for load transfer between fibers and matrix, the most important characteristic of the interface has to be its ability to deflect cracks that propagate in the matrix which can only be obtained with an interface possessing low toughness. The two parameters that best quantify the micromechanical efficiency of interfaces in CFCCs, namely the fiber bond strength and the interfacial shear stress or sliding stress have been determined in various ways [13,14]. Among the various indentation techniques, the single fiber push-in and push-out tests are without a doubt the most popular because of the relative simplicity in their conduction. These tests are schematically described in Fig. 11. In the case of the single-fiber push-out tests, a single fiber is pushed into the matrix using a small flat-bottomed diamond indenter (Fig. 12) attached to the load cell in the ITS (Fig. 3) until the fiber protrudes from the bottom of the sample (Fig. 13). During the test, both the load and the displacement of the fiber surface with respect to the surface of the sample are monitored as illustrated in Fig. 14 for the case of a CGNicalone fiber in a CVI SiC matrix. By the application of micromechanical models it is possible to obtain several parameters from the experimental stress versus fiber-end displacement curves as illustrated in Fig. 15 [13]. A variation of the push-out tests that has been used to study the wear characteristics of the interface is the single- fiber push-back test. Fig. 16 is a record of the stress versus fiber-end displacement obtained during a push-back test A.E. Pasto et al. / Composites: Part B 30 (1999) 631–646 637 Fig. 12. The single-fiber push-out test has a single fiber pushed into the matrix using a small flat-bottomed diamond indenter attached to the load cell in the ITS until the fiber protrudes from the bottom of the sample (Fig. 13). Fig. 13. Fiber protruding from the bottom of a specimen after being pushed out. Fig. 14. Plot of the load and displacement of the fiber surface with respect to the surface of the sample for the case of a CG-Nicalone fiber in a CVI SiC matrix