Effect of Silicon Infiltration on the Mechanical Properties of 2D Cross-ply Carbon-Carbon Composites

Carbon Science Vol. 5, No. 3 September 2004 pp. 108-112 Effect of Silicon Infiltration on the Mechanical Properties of 2D Cross-ply Carbon-Carbon Composites S. R. Dhakate 1,2, T. Aoki 1 and T. Ogasawara 1 1 Advanced Composites Evaluation Technology Center, Japan Aerospace Exploration Agency, Ohsawa, Mitaka, Tokyo 181-0015, Japan 2 Carbon Technology Unit, Engineering Materials Division, National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India e-mail: dhakate@mail.nplindia.ernet.in (Received July 30, 2004; Accepted August 23, 2004) Abstract Effect of silicon infiltration on the bend and tensile strength of 2D cross-ply carbon-carbon composites are studied. It is observed that bend strength higher than tensile strength in both types of composite is due to the different mode of fracture and loading direction. After silicon infiltrations bend and tensile strength suddenly decreases of carbon-carbon composites. This is due to the fact that, after silicon infiltration, silicon in the immediate vicinity of carbon forms the strong bond between carbon and silicon by formation silicon carbide and un-reacted silicon as free silicon. Therefore, these composites consist of three components carbon, silicon carbide and silicon. Due to mismatch between these three components secondary cracks developed and these cracks propagate from 90 oriented plies to 0 oriented plies by damaging the fibers (i.e., in-situ fiber damages). Hence, secondary cracks and in-situ fiber damages are responsible for degradation of mechanical properties of carbon-carbon composites after silicon infiltration which is revealed by microstructure investigation study by scanning electron microscope. Keywords: A. Carbon/Carbon composites, C. Scanning electron microscope, D. Mechanical properties 1. Introduction Carbon/carbon (C/C) composites are among the most promising high temperature materials for utilization in various applications such as heat shields, rocket nozzles, and brake disks etc., [1, 2]. Many of these applications involve exposure to oxidizing atmosphere. However, in these composites oxidation can severely degrade the mechanical properties due to the low susceptibility of carbon to oxygen at temperatures as low as 500 C [3]. Some studies reveals that, even their is 2~5% weight loss due to oxidation, lead to 40~50% decreases in the mechanical properties of C/C composites [4, 5]. To overcome this obstacle with C/C composites, efforts are made by material scientists to develop oxidation resistant C/C composites by incorporating oxidation barrier or inhibitors into them [6-11]. Various experiments conducted on C/C composites to render the oxidation resistance include coating of C/C composites with high temperature resistant ceramic materials like silicon nitride, silica, silicon oxy-carbide and silicon carbide [12]. Coating the C/C composites with oxidation resistant ceramics materials can solve the problem of oxidation provided the coefficient of thermal expansion of the coating material matches that of the composites; otherwise cracks will develop in the composites. The cracks once developed will allow free entry of oxygen into the composites which eventually get oxidized at temperatures as low as 500 C and sudden reduction in the mechanical properties of the composites occurs. To overcome this disadvantage of coating, additional efforts are required to fill the cracks by suitable sealants [13]. It is time consuming and not economically viable process. Therefore, there is need to develop a cost effective process to provide oxidation resistance to C/C composites. One of them is metal infiltration; it is a conventional technique to improve the oxidation resistance of C/C composites [14-16]. This technique has the advantage to fabricate near-net shape components by easily infiltrating the oxidation resistance materials. In the present study silicon infiltrated C/C composites were used to study how the silicon infiltration affects the mechanical properties of C/ C composites. The oxidation behavior of silicon infiltrated C/C composite under oxidizing atmosphere is studied by Ogasawara et al. [17]. It is reported that silicon infiltration does not influence oxidation initiation temperature to higher extent but reduced the total weight loss of C/C composites by 28% up to 1000 C. On the other hand, these composites exhibit excellent oxidation resistance for short term exposure in high enthalpy airflow. 2. Experimental In this study commercially available 2D (0 /90 ) cross-ply

109 Effect of Silicon Infiltration on the Mechanical Properties of 2D Cross-ply Carbon-Carbon Composites C/C and silicon infiltrated carbon-carbon (C/C-Si) composites were procured from NGK insulators, Ltd., Japan. The silicon infiltrated composites were fabricated by a preformed yarn method; this process consists of three manufacturing steps [18]. (1) Preformed yarn was made by packing carbon fibers (T700 PAN based, Toray Co. Ohtsu, Japan), carbon powder and carbon precursor (phenolic resin) in a polymer tube (straw). Unidirectional prepreg sheets made of the preformed yarns were stacked for 0 /90 cross-ply configuration which consists of 45% fiber volume. (2) The prepreg laminates were heat treated in an inert atmosphere and substrate C/C was formed. The substrate C/C composite include many voids and matrix cracks (Figure 2a and b) in transverse (90 ) fiber bundles. (3) The substrate C/C composites were immersed in molten silicon at elevated temperature, and the voids and matrix cracks were infiltrated with metal silicon by capillary attraction. The details of the silicon infiltrated composite was given in Table 1. The mechanical properties (tensile and bend strength) of these composites were measured. The bend strength was measured by four point bending method on an Instron testing machine (model 4482) with span length 100 mm (dimension of composite test specimen 120 mm 9 mm 3 mm) and loading speed 0.5 mm/min. The tensile strength was measured on a servohydraulic testing system (Model 8501, Instron Corporation, USA) at room temperature. The dog-bone type specimens for the tensile test were machined from composites plate such that the loading direction was parallel to the fiber direction of one of the plies. Figure1 shows geometry of tensile test specimen and gripping area of test specimen is protected by thick paper tabs. The tensile specimens had gauge length of 30 mm, thickness 6 mm, width 6 mm and overall length 160 mm. The tension was applied at constant speed 0.5 mm/min. The microstructure of the composites was studied by field emission scanning electron microscope to understand how the silicon infiltration affects the microstructure of C/C composites and the ultimate properties of the composite. 3. Results and Discussion Fig. 2. Scanning electron micrographs of C/C composites (a) Cross section view show the cracks in 90 oriented fiber plies, (b) Crack between 90 and 0 oriented fiber plies. cracks and porosity developed during the pyrolysis of the composite in the 90 oriented ply due to shrinkage of the matrix and thermal contraction between the fibers and the matrix. These cracks further propagate at the interface between the 0 and 90 layers due to mismatch of coefficient of thermal expansion (CTE) between them is shown in Figure 2b. Figure 3a of silicon infiltrated C/C-Si composite shows cracks and porosity developed in C/C filled by silicon, and silicon in the immediate vicinity of carbon, in the form of silicon carbide and the remaining silicon as free silicon in the composites. The silicon infiltrated C/C composite consists of carbon, silicon carbide and un-reacted free silicon; there Figure 2 shows scanning electron micrographs of a C/C composite. In Figure 2a the cross section view shows the Table 1. Composition of Silicon infiltrated carbon/carbon composites (C/C-Si) Fig. 1. Tensile test specimen geometry (All dimensions are in mm). Composition (mass %) Free carbon SiC Free Silicon Porosity (Vol.%) Density (kg/m3) Fiber volume fraction (%) 75 23 2 1.5 2.0 40~50

110 S. R. Dhakat et al. / Carbon Science Vol. 5, No. 3 (2004) 108-112 Fig. 3. Scanning electron micrographs of C/C-Si composites (a) Cross section view show the cracks filled, (b) Secondary crack and fiber damage after silicon infiltration. Fig. 5. (a) Load-displacement curve of composites after bend test (B-C/C & B-C/C-Si) and (b) Stress-strain curve after tensile (T-CCA1 & T-C/C-Si) test. Fig. 4. Bend and tensile strength of composites. composition is given in Table 1. In these composites due to CTE mismatch between three components, secondary cracks are developed; these cracks are not just developed in the 90 oriented plies but they pass through the 0 oriented ply with damage to the fibers, as shown in Figure 3b. Generally it is observed that in C/C composites, the cracks developed in the 90 fiber bundles are propagated through the interfaces between the 90 and 0 oriented plies due to low bonding strength between these laminates [18] as a cause CTE mismatch between them. In C/C-Si composites, the cracks between the 90 and 0 oriented plies are also filled by silicon and the formation of SiC between them improves the bonding strength between the laminates, and as a result secondary cracks propagate straight through the 0 plies. Figure 3b also shows broken carbon fibers and as a consequence of secondary cracks propagate straight by damaging the 0 plies. In some of the areas in silicon infiltrated composites (Figure 3b), silicon infiltrated in the immediate vicinity of fibers, carbon fibers are partly consumed in the formation of silicon carbide. This shows that the silicon infiltration develops in-situ damage in the C/C composite. Figure 4 shows the bend and tensile strength of the composites. The bend strength of C/C is considerably greater than the tensile strength. The higher value of bend strength is due to the different mode of fracture and loading direction. During bending test, upper most ply of 2D composites are under direct contact of load, and micro-cracking take place from tensile loading direction and cracks travel from tensile to compressive loading side if the cracks are not change the propagation path. On the other hand, in tensile test complete fibers oriented in loading (0 ) direction are under stress. On the application of stress, matrix micro-cracking take place and cracks propagated to perpendicular direction to the fiber oriented to parallel to loading direction and composites get damage. The bend strength of the C/C composite is higher as compared to C/CSi composites and the strength decreases suddenly from 290 MPa to 120 MPa after silicon infiltration. Also in the same fashion, the tensile strength of the C/C-Si composite is

Effect of Silicon Infiltration on the Mechanical Properties of 2D Cross-ply Carbon-Carbon Composites decreased, tensile strength decreased from 235 MPa of the C/C composite to 41 MPa of the C/C-Si composites. The extent of decrease in tensile strength is higher as compared to bend strength in C/C-Si composites. Figure 5a, shows the load -displacement curve of composites observed during bend test. In C/C composites during the bend test, the load increased linearly (with increasing the load; displacement also increased without any deviation in load-displacement curve) up to a maximum load (Figure 5a, B-C/C curve), thereafter load drop is observed. This indicates that in C/C some nonlinearity precedes an abrupt load drop, on the assumption that the material behaves elastically. Initially load increases linearly up to a maximum breaking load, micro-cracking takes place in the tension side and these cracks propagate parallel to the 90 plies and thereafter, the crack path changes due to the different stress concentration center (weak bonding between 90 and 0 plies, cracks and voids developed due to the thermal mismatch between fiber direction, and due to the shrinkage of the matrix), these cracks propagate through the 0 plies. However, in the case of C/C-Si composites (Fig. 5a, B-C/C-Si curve), on the application of load, the secondary cracks developed after silicon infiltration, the fracture would initiate at the tip of prestressed secondary cracks (secondary cracks generated after silicon infiltration, which are prestressed by residual stresses due to the mismatch between coefficient of thermal expansion of carbon, silicon and silicon carbide) perpendicular to the 0 plies fiber surface (Figure 3b, secondary cracks developed in 90 plies pass straight through the 0 plies). However, in such composites, energy at the tip of the notches or cracks is smaller than the bonding energy between the fibers and the matrix. The crack therefore does not stop at the fiber-matrix interfaces and does not change the direction of crack propagation but passes straight through the 0 and 90 oriented plies causing these composites to fail at lower value of load and as a result decreases the flexural strength. Figure 4b, shows the stress-strain curves of composites observed during tensile tests. During a tensile test in C/C composite the load increased linearly up to a maximum (Fig. 5b, T-C/CA1 curve). However, in C/C-Si composites the load increased almost linearly up to a maximum (Fig. 5b, TC/C-Si), but non-linearity is observed which is due to change in stress concentration center i.e., secondary cracks developed in infiltrated area and nearby area that changes the path of crack propagation during the test after silicon infiltration. On silicon infiltration, silicon fills only the preexisting cracks and the porosity in the C/C composites. It does not change the existing fiber-matrix bonding of the whole composite structure (Bonding between 0 and 90 oriented fibers and fiber-matrix bonding within the 0 and 90 plies). The mainly strong interactions develop between the silicon infiltrated areas due to the formation of silicon carbide in the form carbon-silicon bonds. As a consequence, during tensile fracture some weakly bonded fibers are not broken or 111 Fig. 6. Fracture surface of C/C-Si composite (a) At low magnification, (b) At higher Magnification. damaged during the test in one plane. As a result the loaddisplacement curve shows some nonlinearity in the C/C-Si composites. This is confirmed by observing the fracture surface of composites by SEM. Figure 6 shows the fracture surface of C/C-Si composites at low magnification; it shows the composite fracture in a single plane (showing more or less, a smooth surface). On higher magnification it was observed that some fiber pull out takes place. During the tensile test the ultimate value of load and displacement decreased more as compared to the bend test values. This decrease is due to the secondary cracks occurring perpendicular to the tension direction (in between 90 oriented fibers) and these cracks are concentrated by stresses, at a very small tensile load, these cracks propagate very easily in C/C-Si composites perpendicular to 0 oriented plies and composites fracture at less than expected load. 4. Conclusion The bend strength of C/C and C/C-Si composites is higher than tensile strength is due to different mode of fracture and loading direction. Both bend and tensile strength decreases suddenly after silicon infiltration in C/C composites. The sudden decreases in strength are related to the development

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