alex wirtz poster-sic fibers poster final

Mechanical Properties of Silicon Carbide Micro-Fibers

Alexander J. Wirtza, Brian J. Jaquesb, and Darryl P. Buttb

aThe College of Idaho, bBoise State University

0

0.5

1

1.5

2

2.5

0 0.2 0.4 0.6 0.8 1 1.2

Stre

ss (

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 (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.

𝒂𝒔𝒊𝒏𝜽 = 𝒏𝝀

𝒂 = 𝒏𝝀

𝐬𝐢𝐧 𝒕𝒂𝒏−𝟏 ∆𝒁𝟐𝑳

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 (σf)

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 (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]

𝜃 𝐿

∆𝑍

2

𝐻𝑒𝑁𝑒 𝐿𝑎𝑠𝑒𝑟

𝑆𝑖𝐶 𝐹𝑖𝑏𝑒𝑟

λ = 632.8 𝑛𝑚

𝐿𝑖𝑔ℎ𝑡 𝐼𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦

𝑛 = 1 𝑛 = 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(σf)

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

m2 = 1.2

m2 = 1.8

m1 = 6.9 m1 = 3.9

m2 = 0.9

m1 = 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 (σf) and probability of survival (Ps). [1]

a