Mechanical Properties of Silicon Carbide Micro-Fibers Alexander J. Wirtz a , Brian J. Jaques b , and Darryl P. Butt b a The College of Idaho, b Boise State University 0 0.5 1 1.5 2 2.5 0 0.2 0.4 0.6 0.8 1 1.2 Stress (GPa) Strain (%) Sample 3 Sample 8 Sample 15 Sample 12 Sample 23 Abstract Background Experimental Procedure Acknowledgements References Results Silicon carbide (SiC) fibers have many notable properties such as low density, high elastic modulus, and high temperature mechanical strength. Such properties make these fibers desirable in engineering products such as aerospace heat-resistant tiles, fiber optics communications, and semiconductor electronics. These fiber were investigated as various processing speeds to increase the carbon to silicon carbide conversion may have effects on mechanical properties. Laser diffraction was used to measure the diameter of each fiber to accurately determine the mechanical properties of the processed fibers. Fracture strength and Young’s modulus were found and evaluated using Weibull statistics relative to processing parameters of the fibers. Creation Process: Carbon fiber tows were heat treated in a silicon- containing gas at different processing speeds to increase the amount of conversion from carbon to silicon carbide. The effects of the processing speeds on the mechanical properties should affect the silicon carbide conversion, and thus, the mechanical properties of the fiber. [3] Laser Diffraction Benefits and Theory: Rapid measurement: 2 minutes per sample Accuracy: ± 0.1 μm [5] Cost: << SEM and other forms of microscopy Bragg’s Law (Equation 1), along with Babinet’s Principle, can be applied to determine the diameter ( ) of a carbon and/or silicon carbide fiber because the diffraction pattern of light around a small fiber mimics the single-slit experiment, with the exception of the light intensity [2] [4] (Figure 1). Variables in Equation 1 are defined in Figure 3. = = − ∆ ACF LLC. provided four different samples for characterization: • No Heat Treatment • 3 in / min Precise cardstock test frames were fabricated using a CNC laser to provide a 1 inch gauge length Individual fibers were fixed to the test frames with cement and a secondary frame was fixed on top of the fiber to provide a more rigid test fixture (Figure 2a). The completed frames were cured for 24 hours before any further testing. Mechanical Testing: Laser Diffraction: SiC Fracture Analysis: Weibull Statistics Laser diffraction was conducted with a 5 mW helium- neon laser system. Images of the diffracted laser nodes were captured and spacing was measured (similar to Figures 3 - 4) Calculations were performed using Bragg’s Law in Equation 1 (variables defined in Figure 3) The diameters measured from diffraction testing are shown in Table 1. 0.2μm/sec Each test frame was mounted in a Shimadzu mechanical testing system (Figure 6). The frame sides were cut to isolate the fiber to the applied tension (Figure 2b). Mechanical testing: 10 N load cell Strain rate of 0.2 μm / sec Young’s modulus (E) and fracture strength (σ f ) were calculated for each fiber. An example stress vs. strain graph for 3 in / min fibers is shown in Figure 5. Single Slit SiC Fiber Figure 1 Equation 1 Figure 2a Figure 4 Table 1 Figure 3 This research was made possible by Boise State University and the National Science Foundation’s Research Experiences for Undergraduates (REU) Program Award DMR-1359344. Special thanks to: Advanced Ceramic Fibers, LLC, Materials in Energy and Sustainability REU/RET Director Rick Ubic, and the Advanced Materials Laboratory research group. 1. Carter, C. Barry, Norton, M. Grant. 2007. Ceramic Materials: Science and Engineering. Pg. 302-305. New York: Springer Science, Business Media, LLC. 2. Halliday, David, Resnick, Robert, Walker, Jearl. (2001). Fundamentals of Physics. Sixth Edition, Pg. 893-896. New York: John Wiley & Sons, Inc. 3. Hinoki, Tatsuya, Lara-Curzio, Edgar, Snead, Lance L. (2001). Mechanical Properties of High Purity SiC Fiber-Reinforced CVI-SiC Matrix Composites. Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN37831. 4. Li, Chi-Tang, Tietz, James V. (1990). Improved accuracy of the laser diffraction technique for diameter measurement of small fibers. Journal of Materials Science, 25, 4694-4698. 5. Meretz, S., Linke, T., Schulz, E., Hampe, A., Hentschel, M. (1992). Diameter measurement of small fibers: laser diffraction and scanning electron microscopy technique results do not differ systematically. Journal of Materials Science Letters, 11, 1471-1472. 6. Summerscales, John. (2014). Composites Design and Manufacture: Reinforcement Fibers. ACMC University of Plymouth, Plymouth University, Plymouth. Conclusions: Weibull modulus for each fiber type was found and can be used to better understand fracture mechanisms and to predict failure probability. Treated fibers show two Weibull moduli which represent two distinct flaws causing failure. Initial Weibull moduli (m 1 ) increase as the processing speed decreases. A sample size of 30 produced a below average Weibull distribution with low confidence. An increased sample size (for each distinct flaw) is recommended for more reliable data. [1] Both f racture strengths and Young’s moduli decrease as processing speeds decrease (more C-SiC conversion), which is expected when compared to literature. [3] [6] ∆ 2 λ = 632.8 ℎ =1 =2 Analogous to • 5 in / min • 1 in / min Figure 2b Fiber Preparation: Diameter Results Fiber Set Average Diameter (μm) Standard Deviation (μm) No H.T. 7.1 ±0.4 5 in / min 7.2 ±0.4 3 in / min 7.5 ±0.3 1 in / min 7.2 ±0.3 Table 1 Figure 6 Figure 5 -5 -4 -3 -2 -1 1 2 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 lnln(1/P s ) ln(σ f ) No H.T. 1 in/min 3 in/min 5 in/min Fiber Set Young’s Modulus (GPa) Fracture Strength (GPa) No H.T. (n=30) 240 ± 57 3.5 ± 0.9 5 in / min (n=30) 210 ± 35 1.6 ± 0.9 3 in / min (n=30) 220 ± 23 1.4 ± 0.7 1 in / min (n=30) 190 ± 37 0.5 ± 0.5 m = 3.7 m 2 = 1.2 m 2 = 1.8 m 1 = 6.9 m 1 = 3.9 m 2 = 0.9 m 1 = 9.4 Table 2 Mechanical Properties: Figure 7 SEM Fractography and Imaging: a) No H.T. fracture surface b) No H.T. fiber surface c) 5 in / min fiber tow d) 3 in / min fracture surfaces c b d Weibull statistics (Figure 7) were used to determine the Weibull modulus (m) by plotting a linear relationship between fracture strength ( σ f ) and probability of survival (P s ). [1] a