Development of Oriented β-Si 3 N 4 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, Dynamic Behavior of Materials, John Wiley & Sons, Inc., New York, New York, Many ceramics are suitable for portable armor. They have high: hardness, fracture toughness, flexural strength. low: density Graphic: ceradyne.com

Hot-Pressed Ballistic Materials MaterialFlexural Strength [MPa] Hardness HV [kg/mm 2 ] Fracture Toughness [MPa m 1/2 ] Density [g/cm 3 ] Al 2 O SiC Si 3 N B4CB4C SiC TiB All data from Ceradyne.com

Microstructure of Si 3 N 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 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] (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] (1999). In Si 3 N 4, 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 Goals 1)Develop and document practical methods to create aligned β-Si 3 N 4 in lab 2)Create Samples of aligned β- Si 3 N 4 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 alignment 1 Start With Si 3 N 4 Powder (contains α and β grains) Si 3 N 4 powder is combined with polymer binder in 50 vol% / 50 vol% mixture Alumina and Yttria added as sintering aids –92g Si 3 N 4 : 6 g Y 2 O 3 : 2 g Al 2 O 3 1 Desiderio Kovar, Bruce King, Rodney Trice, and John Halloran, “Fibrous Monolithic Ceramics,” J. Am. Ceram. Soc., 80 [10] (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 CO 2 which would disrupt alignment

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 sample of 0°/90° Si 3 N 4 /BN (Fibrous Monolith) Unaligned Si 3 N 4 / 0°/90° Fibrous Monolith 2 Samples of 0°/90° Aligned Si 3 N 4 Unaligned Si 3 N 4 (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-Si 3 N 4 grains α-Si 3 N 4 β-Si 3 N 4

Future Work SEM to observe grain alignment directly Vickers Hardness Testing –ASTM Standard C –Apply known load using diamond indenter –Measure indentations ASTM Standard C

Future Work Fracture Toughness –ASTM C b –precrack is introduced into specimen and is propagated by loading in three point fixture Flexural Strength –ASTM C c –specimen loaded to fracture in four point fixture –uses ASTM Standard C b ASTM Standard C c

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

Questions?

Additional Graphics Source: Rodney W. Trice, Ph.D. thesis. University of Michigan, 1998.

Fibrous Monoliths BN coated Si 3 N 4 filament Creates a weak interface Failure by delamination rather than brittle fracture Source: ple/halloran/FM/fm.html