Development of Oriented β-Si3N4 for Ballistic Protection

Final Presentation

Lance Blakeman

Advisor: Professor Trice

Ballistic Protection

1) Break projectile using a very hard surface

2) Prevent projectile or fragments from penetrating

3) Absorb the residual energy using soft backing

Marc André Meyers, Marc André Meyers, Dynamic Behavior of MaterialsDynamic Behavior of Materials, John Wiley & Sons, Inc., New York, New York, 1994., John Wiley & Sons, Inc., New York, New York, 1994.

Many ceramics are suitable for portable armor. They have high: hardness, fracture toughness, flexural strength.

low: density

Graphic: ceradyne.com

Hot-Pressed Ballistic MaterialsMaterial Flexural

Strength

[MPa]

Hardness HV [kg/mm2]

Fracture Toughness

[MPa m1/2]

Density

[g/cm3]

Al2O3 460 1600 3.80 3.98

SiC 380-634 2300-2600 2.50-4.30 3.15-3.20

Si3N4 700-930 1800 5.0-6.0 3.10-3.30

B4C 410 3200 2.50 2.48-2.50

SiC 380-634 2300 2.50-4.30 3.17-3.20

TiB2 365 2100 5.47 4.50

All data from Ceradyne.com

Microstructure of Si3N4

• 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 powderImage 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).

Beta Grain Effects

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).

• 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

Project Motivation/Goals

• Precisely aligned, layered β may provide better ballistic protection

Project Goals1) Develop and document practical

methods to create aligned β-Si3N4 in lab

2) Create Samples of aligned β-Si3N4 in layers with 0°/90° (cross-ply) orientation

3) Examine samples using x-ray diffraction, scanning electron microscopy, and mechanical tests

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

11Desiderio Kovar, Bruce King, Rodney Trice, and John Halloran, “Fibrous Monolithic Ceramics,” Desiderio Kovar, Bruce King, Rodney Trice, and John Halloran, “Fibrous Monolithic Ceramics,” J. Am. J. Am. Ceram. SocCeram. Soc., ., 8080 [10] 2471-87 (1997). [10] 2471-87 (1997).

Filament Extrusion

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

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

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

Binder Burnout Program

145°C

250°C

700°C

500°C

370°C

25

125

225

325

425

525

625

725

825

0 20 40 60 80 100

Time Elapsed [hr]

Tem

per

atu

re [

°C]

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 1) Sintering

2) Grain Transformation α→β

3) β grain growth

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)

Results• Five Samples were completed up to the hot

pressing step

Practice Practice sample of sample of 0°/90°0°/90° SiSi33NN44/BN /BN

(Fibrous (Fibrous Monolith)Monolith)

Unaligned SiUnaligned Si33NN44/ /

0°/90°0°/90° Fibrous Fibrous MonolithMonolith2 Samples of 2 Samples of

0°/90° Aligned 0°/90° Aligned SiSi33NN44

Unaligned SiUnaligned Si33NN4 4

(control sample)(control sample)

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.

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.

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

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

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 Mc

I

ASTM Standard C 1421-01b

ASTM Standard C 1161-02c

Acknowledgements

• Dr. Rodney Trice

• Dave Roberts

• Emily Pickens

• Hyun Jun Kim

• National Science Foundation for funding

Questions?

AdditionalGraphics

Source: Rodney W. Trice, Ph.D. thesis.

University of Michigan, 1998.

Fibrous Monoliths

• BN coated Si3N4 filament

• Creates a weak interface• Failure by delamination rather than brittle fracture

Source: http://msewww.engin.umich.edu:81/people/halloran/FM/fm.html