Development of Oriented β -Si 3 N 4 for Ballistic Protection Final Presentation

1. Development of Oriented β-Si3N4 for Ballistic ProtectionFinal Presentation Lance Blakeman Advisor: Professor Trice

2. Ballistic Protection • Break projectile using a very hard surface • Prevent projectile or fragments from penetrating • Absorb the residual energy using soft backing Graphic: ceradyne.com Many ceramics are suitable for portable armor. They have high: hardness, fracture toughness, flexural strength. low: density Marc André Meyers, Dynamic Behavior of Materials, John Wiley & Sons, Inc., New York, New York, 1994.

3. Hot-Pressed Ballistic Materials All data from Ceradyne.com

4. α Grains Equiaxed – dimensions ≈ in all directions 99% of grains in typical powder sample following formation β Grains Hexagonal rods. Can grow, be elongated further 1% of grains in a typical sample of powder Microstructure of Si3N4 Image Source: R.W. Trice and J.W. Halloran, “Mode I Fracture Toughness of a Small-Grained Silicon Nitride: Orientation, Temperature, and Crack Length Effects,” J. Am. Ceram. Soc., 82 [10] 2633-40 (1999).

5. Beta Grain Effects • In Si3N4, elongated β grains have been found to greatly increase fracture toughness • β grains tend to deflect cracks or display frictional bridging rather than being cut by cracks • Toughening mechanisms expend more energy Micrograph Source: Rodney W. Trice and John W. Halloran, “Mode 1 Fracture of a Small-Grained Silicon Nitride,” J. Am. Ceram. Soc., 82 [10] 2633-40 (1999).

6. Project Motivation/Goals • Precisely aligned, layered β may provide better ballistic protection Project Goals • Develop and document practical methods to create aligned β-Si3N4 in lab • Create Samples of aligned β-Si3N4 in layers with 0°/90° (cross-ply) orientation • Examine samples using x-ray diffraction, scanning electron microscopy, and mechanical tests

7. Experimental Procedure • Use similar procedures developed for work with fibrous monolithic ceramics and alignment1 • Start With Si3N4 Powder (contains α and β grains) • Si3N4 powder is combined with polymer binder in 50 vol% / 50 vol% mixture • Alumina and Yttria added as sintering aids • 92g Si3N4 : 6 g Y2O3 : 2 g Al2O3 1Desiderio Kovar, Bruce King, Rodney Trice, and John Halloran, “Fibrous Monolithic Ceramics,” J. Am. Ceram. Soc., 80 [10] 2471-87 (1997).

8. Filament Extrusion

9. Making Filament Sheets • Filament Winding • Adhering Filaments Together • Hair spray used • Super glue used to repair breaks and make splices • Finished ribbon cut into circular plies that will fit within die

10. Warm Pressing • Plies are stacked in 2½” diameter cylindrical die at desired angle. • Release agent – Polyethylene Glycol 6,000 applied to die • Die heated to 170°C • Sample is pressed with load frame • Causes filaments to adhere to one another • ≈2.5 MPa of pressure is applied via axial force using a load frame

11. Binder Burnout • Polymer and other hydrocarbon contaminants (hairspray, release agent, etc.) are removed through combustion • Slow burn prevents distortion from CO2 which would disrupt alignment

12. Hot Pressing • Specifics • Graphite dies used to apply heat and uniaxial load for 1-4 hours • Heated to 1750°C (600°C/hr) • Pressure of 25 MPa (1.75 hr) • Nitrogen Atmosphere • Part needs to be machined into desired shape afterwards • Purposes • Sintering • Grain Transformation α→β • β grain growth

13. α→β Grain Transformation • Occurs under high temperature, high pressure, low oxygen conditions. • Sintering agents interact with silica to form a liquid • α has greater solubility, more unstable. • This drives it into solution to precipitate as more stable, less soluble β grains. • New β grains will align themselves with the preexisting aligned grains (Seeds)

14. Results • Five Samples were completed up to the hot pressing step • Practice sample of 0°/90°Si3N4/BN (Fibrous Monolith) • Unaligned Si3N4 (control sample) • Unaligned Si3N4/ 0°/90°Fibrous Monolith • 2 Samples of 0°/90° Aligned Si3N4

15. Factors Investigated • Filament winding techniques • Minimizing damage in cutting plies • Discovering adequate warm pressing pressures for various samples • Controlling warm pressing pressures • Determining adequate warm pressing temperatures • Minimizing damage in removing sample from die • Documentation of how to produce these samples successfully in the Purdue MSE labs was produced.

16. Current Status • Hot press thermocouple was replaced • Still is a problem in the Honeywell Digital Control Programmer (DCP-700) that controls temperature • Tried to swap some boards with the controller for pressure. Failed to locate problem. Slightly different models. • Sent in for repair on July 7, no parts available. • Currently exploring options to replace/repair controller • These samples must be hot pressed. Pressureless sintering will not work for these particular samples.

17. Future Work • Hot-pressing all samples • Machining Samples • X-Ray Diffraction • to verify α→β transformation • to look for contaminants like SiC • to verify a high degree of grain alignment of β-Si3N4 grains α-Si3N4 β-Si3N4

18. Future Work • SEM to observe grain alignment directly • Vickers Hardness Testing • ASTM Standard C 1327-99 • Apply known load using diamond indenter • Measure indentations • ASTM Standard C 1327-99

19. Future Work • Fracture Toughness • ASTM C 1421-01b • precrack is introduced into specimen and is propagated by loading in three point fixture • Flexural Strength • ASTM C 1161-02c • specimen loaded to fracture in four point fixture • uses • ASTM Standard C 1421-01b • ASTM Standard C 1161-02c

20. Acknowledgements • Dr. Rodney Trice • Dave Roberts • Emily Pickens • Hyun Jun Kim • National Science Foundation for funding

21. Questions?

22. AdditionalGraphics Source: Rodney W. Trice, Ph.D. thesis. University of Michigan, 1998.

23. Fibrous Monoliths Source: http://msewww.engin.umich.edu:81/people/halloran/FM/fm.html • BN coated Si3N4 filament • Creates a weak interface • Failure by delamination rather than brittle fracture