Development of Oriented

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Development of Oriented ß-Si3N4 for Ballistic
ProtectionFinal Presentation

• Lance Blakeman
• Advisor Professor Trice

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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.
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Hot-Pressed Ballistic Materials
Material 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
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Microstructure of Si3N4

• a Grains
• Equiaxed dimensions in all directions
• 99 of grains in typical powder sample following
  formation
• ß Grains
• Hexagonal rods. Can grow, be elongated further
• 1 of grains in a typical sample of powder

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).
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Beta Grain Effects

• In Si3N4, elongated ß grains have been found to
  greatly increase fracture toughness
• ß 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).
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Project Motivation/Goals

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

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Experimental Procedure

• Use similar procedures developed for work with
  fibrous monolithic ceramics and alignment1
• Start With Si3N4 Powder (contains a and ß 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).
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Filament Extrusion
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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

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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 170C
• 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

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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

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Hot Pressing

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

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a?ß Grain Transformation

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

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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
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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.

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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.

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Future Work

• Hot-pressing all samples
• Machining Samples
• X-Ray Diffraction
• to verify a?ß transformation
• to look for contaminants like SiC
• to verify a high degree of grain alignment of
  ß-Si3N4 grains

a-Si3N4 ß-Si3N4
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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
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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
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Acknowledgements

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

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Questions?
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AdditionalGraphics
Source Rodney W. Trice, Ph.D. thesis.
University of Michigan, 1998.
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Fibrous Monoliths
Source http//msewww.engin.umich.edu81/people/ha
lloran/FM/fm.html

• BN coated Si3N4 filament
• Creates a weak interface
• Failure by delamination rather than brittle
  fracture