Both materials proved successful in previous missions to space. AVCOAT, which is manufactured directly onto the spacecraft and has an embedded honeycomb-like material, was used for the original Apollo capsules. PICA, or Phenolic Impregnated Carbon Ablator, which is manufactured in blocks and attached to the vehicle after fabrication, was used on Stardust, NASA's first unmanned space mission dedicated solely to exploring a comet.
Space-X is working on PICA variants PICA-X.
The "X" stands for the SpaceX-developed variants of the rigid, lightweight material, which has several improved properties and greater ease of manufacture.
"We tested three different variants developed by SpaceX," said Tom Mueller, VP of Propulsion, SpaceX. "Compared to the PICA heat shield flown successfully on NASA's Stardust sample return capsule, our SpaceX versions equaled or improved the performance of the heritage material in all cases."
The Dragon capsule will enter the Earth's atmosphere at around 7 kilometers per second (15,660 miles per hour), heating the exterior of the shield to up to 1850 degrees Celsius. However, just a few inches of the PICA-X material will keep the interior of the capsule at room temperature.
In January 2006, NASA's Stardust sample return capsule, equipped with a PICA heat shield, set the record for the fastest reentry speed of a spacecraft into Earth's atmosphere - experiencing 12.9 kilometers per second (28,900 miles per hour). SpaceX's Dragon spacecraft will return at just over half of that speed, and will experience only one tenth as much heating.
PICA is a modern TPS [Thermal protection systems] material and has the advantages of low density (much lighter than carbon phenolic) coupled with efficient ablative capability at high heat flux. Stardust's heat shield (0.81 m base diameter) was manufactured from a single monolithic piece sized to withstand a nominal peak heating rate of 1200 W/cm^2
Atmospheric re-entry at wikipedia