Mechanical Properties of Silicon Carbide Micro-Fibers
Alexander J.
a
Wirtz ,
Brian J.
b
Jaques ,
aThe College of Idaho, bBoise State University
Experimental Procedure
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.
Background
Creation Process:
 Carbon fiber tows were heat treated in a siliconcontaining 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 (𝑎) 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.
Figure 1
Analogous to
Mechanical Properties:
Fiber Preparation:
Figure 2b
Figure 2a
Results
0.2μm/sec
 ACF LLC. provided four different samples for
characterization:
• No Heat Treatment • 5 in/min
• 3 in/min
• 1 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.
Laser Diffraction:
 Laser diffraction was conducted with a 5 mW heliumneon 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.
𝐿𝑖𝑔ℎ𝑡
𝐼𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦
𝐬𝐢𝐧
Fiber Set
Table 1
No H.T.
5 in/min
3 in/min
1 in/min
Average Diameter (μm)
7.1
7.2
7.5
7.2
m2 = 0.9
𝑛=2
∆𝑍
2
𝐿
m2 = 1.8
-2
-3
𝜃
-4
m1 = 3.9
m = 3.7
m1 = 6.9
-5
𝑆𝑖𝐶 𝐹𝑖𝑏𝑒𝑟
-2.5
-2.0
-1.5
-1.0
-0.5
λ = 632.8 𝑛𝑚
a) No H.T. fracture surface b) No H.T. fiber surface
c) 5 in/min fiber tow
d) 3 in/min fracture surfaces
a
b
c
d
Figure 3
 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 (σf)
were calculated for each fiber.
 An example stress vs. strain graph for 3 in/min
fibers is shown in Figure 5.
Sample 3
Sample 8
Sample 15
Sample 12
Sample 23
0.5
1.0
1.5
2.0
Conclusions:
SEM Fractography and Imaging:
𝐻𝑒𝑁𝑒
𝐿𝑎𝑠𝑒𝑟
0.0
ln(σf)
Figure 7
Mechanical Testing:
1.5
m2 = 1.2
No H.T.
1 in/min
3 in/min
5 in/min
m1 = 9.4
Standard Deviation (μm)
±0.4
±0.4
±0.3
±0.3
Table 2
-1
Figure 4
Stress (GPa)
Equation 1
3.5 ± 0.9
1.6 ± 0.9
1.4 ± 0.7
0.5 ± 0.5
𝑛=1
 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 (m1) 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 fracture strengths and Young’s moduli
decrease as processing speeds decrease (more
C-SiC conversion), which is expected when
compared to literature. [3] [6]
References
1.
2.
3.
4.
1
5.
∆𝒁
𝟐𝑳
240 ± 57
210 ± 35
220 ± 23
190 ± 37
1
𝒏𝝀
𝒕𝒂𝒏−𝟏
No H.T. (n=30)
5 in/min (n=30)
3 in/min (n=30)
1 in/min (n=30)
2
Diameter Results
SiC Fiber
𝒂=
Fracture Strength (GPa)
Table 1
2
𝒂𝒔𝒊𝒏𝜽 = 𝒏𝝀
Young’s Modulus (GPa)
Weibull statistics (Figure 7) were used to determine the Weibull modulus (m) by plotting
a linear relationship between fracture strength (σf) and probability of survival (Ps). [1]
2.5
Single Slit
Fiber Set
SiC Fracture Analysis: Weibull Statistics
lnln(1/Ps)
Abstract
and Darryl P.
b
Butt
0.5
6.
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Li, Chi-Tang, Tietz, James V. (1990). Improved accuracy of the laser diffraction technique for diameter measurement of small fibers. Journal of Materials
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Acknowledgements
0
0
0.2
0.4
0.6
Strain (%)
0.8
1
Figure 5
1.2
Figure 6
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.