1. Silicon Carbide- Boron Carbide Composites
Zeynep AYGUZER YASAR
Richard A. Haber
CCOMC Meeting
October, 2016

2. Background SiC B4C High hardness [1] Wear and oxidation resistance [1]
Excellent thermal conductivity [1]
Low coefficient of thermal expansion [1]
Low density [1]
High hardness [3]
High melting point [3]
High neutron absorption cross-section [3]
Good chemical stability [3]
Extremely low density [4]
[2]
[5]
1. . D. Ahmoye, "Pressureless Sintering and Mechanical Properties of Silicon Carbon Composites with in-situ Converted Titanium Dioxide to Titanium Carbon," (2010).
2. L. Silvestroni, "Development and characterization of non-oxide ceramic composites for mechanical and tribological applications," (2009)
3. F. Thevenot, "Boron carbide—a comprehensive review," Journal of the European Ceramic Society, 6[4] (1990).
4. B. M. Moshtaghioun, A. L. Ortiz, D. Gómez-García, and A. Domínguez-Rodríguez, "Toughening of super-hard ultra-fine grained B 4 C densified by spark-plasma sintering via SiC addition," Journal of the European Ceramic Society, 33[8] (2013).
5. F. Thevenot, "Boron carbide—a comprehensive review," Journal of the European Ceramic Society, 6[4] (1990).

3. Overview
Some researchers studying SiC discuss the significance of oxygen content in SiC powder, and try to eliminate that surface oxygen layer by adding carbon and boron, or by acid etching the starting powder.
Matthias et al. studied acid treatment on SiC powder to reduce oxygen content to have better mechanical properties. Results showed that, removing oxide layer lead to decreased grain size. [6]
Clegg et al. studied role of carbon on SiC, and oxide layer removal with carbon and boron additives. [7]
However there is no research that addressed the role of oxygen content on microstructure and mechanical properties SiC-B4C composites. For this reason, this issue will be the primary focus of this research.
6.M. Wilhelm, S. Werdenich, and W. Wruss, "Influence of resin content and compaction pressure on the mechanical properties of SiC–Si composites with sub-micron SiC microstructures," Journal of the European Ceramic Society, 21[7] (2001).
7.W. J. Clegg, "Role of Carbon in the Sintering of Boron‐Doped Silicon Carbide," Journal of the American Ceramic Society, 83[5] (2000).

4. Purpose Producing high dense SiC-B4C composites.
Understanding and controlling surface oxygen in micron sized SiC powders.
Understanding the role oxygen plays in the pressure assisted densification of SiC-B4C composites.
Optimizing mechanical properties of SiC-B4C composites.

5. SiC Acid Etching 30 g HC-UF 25 Starck SiC 50% HF
Neutralized with Ammonium hydroxide
Mixing 1hour,4 hours and 24 hours
Dry
Centrifuge and wash with ID water

6. Hardness (1000g load) (GPa)
Sample
Unetched SiC
50%HF-SiC 1 hour etched
50%HF-SiC 4 hours etched
50%HF-SiC 24 hours etched
O2%
1.69
0.6
0.67
0.5
Theoretical density%
93.9
99.9
100.0
99.6
E (GPa)
362
423
432
417
Hardness (1000g load) (GPa)
17.81
26.56
25.83
26.01
98% SiC-0.5% B4C-1.5 %C are mixed and sintered by using SPS (Spark Plasma Sintering) under vacuum at 1950°C.

7. SEM micrographs of etched SiC taken at 2
SEM micrographs of etched SiC taken at 2.5Kx (a) 1 hour etched,(b) 4 hours etched,(c) 24 hours etched A
99.9 %ρth B
100%ρth C
99.6 %ρth

8. HC-UF 25 Starck SiC, HD-20 Starck B4C and carbon lamb black have used as starting material.
Dry mill
B4C from 10-50% at 10% increments plus 1.5%C and SiC will be mixed in a small bottle by dry mill
Wet mill
B4C from 10-50% at 10% increments plus C and SiC will be mixed in ethanol with SiC media by wet milling
Sample matrix
Mixing method
10%B4C-1.5%C-SiC
Dry mill
20%B4C-1.5%C-SiC
30%B4C-1.5%C-SiC
40%B4C-1.5%C-SiC
50%B4C-1.5%C-SiC
Wet (1)mill
10%B4C-1.13%C-SiC
Wet (2) mill
20%B4C-1.13%C-SiC
30%B4C-1%C-SiC
40%B4C-0.94%C-SiC
50%B4C-0.88%C-SiC
Composites will be examined from 10% to 50% B4C and C addition to SiC sintered using SPS under vacuum at 1950°C (5 minutes).
After sintering, density is measured by the Archimedes method, microstructure and mechanical properties will be characterized via FESEM.

9. Theoretical Density & Elastic Modulus vs Boron carbide
With increasing B4C content, elastic modulus decreases.
The difference between two set wet milling is reduce carbon content.
Densities decrease with increasing B4C content.
-Samples were sintered using SPS under vacuum at 1950°C, 50 MPa pressure for 5min.

10. Theoretical Density & Hardness vs Boron carbide
Except Dry mixed, with increasing B4C content, hardness values increase.
-Samples were sintered using SPS under vacuum at 1950°C, 50MPa pressure for 5min.

11. Dry mill series microstructure
A –10%B4C-1.5%C-88.5%SiC
B –20%B4C-1.5%C-78.5%SiC
C – 30%B4C-1.5%C-68.5%SiC
D – 40%B4C-1.5%C-58.5%SiC A
98.6 %ρth B
99.7 %ρth
SEM images show that spectromill does not provide uniform mixing
Large pockets of individual components very evident C
98.5 %ρth D
97.8 %ρth

12. Wet(1) mill series microstructure
With increasing B4C content, porosity increases A
99.6 %ρth
99.2 %ρth B E
98.9 %ρth C
98.8 %ρth
98.8 %ρth D
A- 10%B4C-1.5%C-88.5%SiC
B- 20%B4C-1.5%C-78.5%SiC
C- 30%B4C-1.5%C-68.5%SiC
D- 40%B4C-1.5%C-58.5%SiC
E- 50%B4C-1.5%C-58.5%SiC

13. Wet(2) mill series microstructure
A –10%B4C-1.13%C-88.87%SiC
B –20%B4C-1.13%C-78.87%SiC
C –30%B4C-1%C-69%SiC
D –40%B4C-0.94%C-59.06%SiC
E –50%B4C-0.975%C %SiC A
99.8 %ρth
99.7 %ρth B E
98.9 %ρth C
99.1 %ρth
99.7 %ρth D
10%B4C-1.13%C-88.87%SiC sample shows some elongated grains
With increasing B4C content, grains get smaller

14. Unetched SiC-C-B4C Composites
Sample Matrix
10%B4C-0.5%C-89.5%SiC
20%B4C-0.5%C-79.5%SiC
30%B4C-0.5%C-69.5%SiC
40%B4C-0.5%C-59.5%SiC
50%B4C-0.5%C-49.5%SiC
10%B4C-1.0%C-89.0%SiC
20%B4C-1.0%C-79.0%SiC
30%B4C-1.0%C-69.0%SiC
40%B4C-1.0%C-59.0%SiC
50%B4C-1.0%C-49.0%SiC
10%B4C-1.5%C-88.5%SiC
20%B4C-1.5%C-78.5%SiC
30%B4C-1.5%C-68.5%SiC
40%B4C-1.5%C-58.5%SiC
50%B4C-1.5%C-48.5%SiC
10%B4C-2.0%C-98.0%SiC
20%B4C-2.0%C-88.0%SiC
30%B4C-2.0%C-78.0%SiC
HC-UF 25 Starck SiC, HD-20 Starck B4C and carbon lamb black have used as starting material.
Wet mill
B4C from 10-50% at 10% increments plus C and SiC will be mixed in ethanol with SiC media by wet milling.
Composites will be examined from 10% to 50% B4C and C addition to SiC sintered using SPS under vacuum at 1950°C (5 minutes).
After sintering, density is measured by the Archimedes method, microstructure and mechanical properties will be characterized via FESEM.

15. Theoretical Density & Elastic Modulus vs Boron carbide
Elastic modulus decreases with increasing B4C content.
Densities decrease with increasing B4C content.
-Samples were sintered using SPS under vacuum at 1950°C , 50MPa pressure for 5min.

16. Theoretical Density vs Boron carbide and Temperature
-Samples were sintered using SPS under vacuum at 1950°C and 1850°C, 50 MPa pressure for 5min.

17. Theoretical Density & Hardness vs Boron carbide
Hardness values increase when increases B4C content.
-Samples were sintered using SPS under vacuum at 1950°C,50 MPa pressure for 5min.

18. Unetched SiC-C-B4C CompositesA
99.6 %ρth
99.2 %ρth B C
99.1 %ρth
A- 10%B4C-0.5%C-89.5%SiC
B- 30%B4C-0.5%C-69.5%SiC
C- 50%B4C-0.5%C-49.5%SiC

19. Unetched SiC-C-B4C Composites
A- 10%B4C-1%C-89%SiC
B- 20%B4C-1%C-79%SiC
C- 30%B4C-1%C-69%SiC
D- 40%B4C-1%C-59%SiC
E- 50%B4C-1%C-49%SiC A
99.8 %ρth
99.01 %ρth B E
97.7 %ρth
Significant porosity does not occur.
Image A-E B4C content increases. C
97.9 %ρth
97.5 %ρth D

20. Unetched SiC-C-B4C Composites
50%B4C-1%C-49%SiC
Mechanical polishing
Ion milling
Murakami etching agent for 4.5 min in order to highlight grain boundaries
Cross section:
C3 [±30°/3rpm]
5hr mill time, 6kV
Additional Flat milling for 5 min

21. Unetched SiC-C-B4C CompositesA
99.9 %ρth
A- 10%B4C-1.5%C-88.5%SiC
B- 20%B4C-1.5%C-78.5%SiC
C- 30%B4C-1.5%C-68.5%SiC
D- 40%B4C-1.5%C-58.5%SiC
E- 50%B4C-1.5%C-48.5%SiC
99.4 %ρth B E
98.1 %ρth C
98.9 %ρth
98.4 %ρth D

22. Future Work Different sintering temperature will be tried
Different carbon levels will be added
Nano indentation hardness will be examined on dense samples
Fracture toughness will be done on dense samples

23. Ceramic, Composite, and Optical Materials Center
Acknowledgments
Ceramic, Composite, and Optical Materials Center
Thank You