Tape Casting of Ceramic Armor Composites for Ballistic TestingRoland Bruyns - Purdue Graduate Student - Materials Engineering
Tape casting is a process for producing thin formable ceramic strip which can then be formed and sintered to achieve desired properties.
The main objective of this project is to produce lightweight damage tolerant armor which resists or prevents penetration by projectiles. Tapes of alumina and alumina combined with 20% partially stabilized zirconia were produced and evaluated by ballistic testing.
Ceramics are chosen for this application because of their low density, high hardness, and resistance to intermediate velocity projectiles ( approx. 700 m/sec ).
The tape casting process involves the deposition of a slip onto a carrier (in our case mylar film) and passing this deposition at constant velocity beneath a "doctor blade" to produce a uniform deposit. The slip is a mixture of powdered ceramic, DI water, and a dispersant. This is ball milled and filtered to ensure no agglomerations. A polymer binder is then added and blended by magnetic stirring. The slip is then tape cast and allowed to dry for 4 hours resulting in "green tape". After drying a very slow heat up is applied to allow all of the binder to burn out prior to sintering. Sintered tape achieves 98-99% of theoretical density.
Tapes were cast of pure alumina, Alumina with 20% partially (ceria) stabilized Zirconia, and the same blend except layered with tissue to attempt to deflect crack propogation during testing. The addition of the partially stabilized zirconia was expected to also inhibit crack propogation since destabilization of the system (by impact) should induce a phase transformation promoting expansion and putting the ceramic into compression.
Balistic testing was conducted by firing hardend tool steel "bullets" into a solid block of Aluminum alloy 6061-t6. The depth of penetration of such bullets could readily be measured by x-raying the block after the test. Tests were then repeated with 6.4mm thick specimens of the tape cast ceramics placed in the path of the bullet immediately in front of the aluminum block.
The results of the ballistic tests were quite scattered, but tended to show that the "pure alumina" did the best job of stopping the projectile. Both versions of the mixed ceremic (layered) and (monolythic) caused the bullet to fracture but did not prevent penetration into the aluminum block. "Depth of Penetration" was the main measurment for the testing.
The results being somewhat contrary to expectation, it will be necessary in future work to determine if the expected transformation of the PS zirconia actually took place or not.