From Halopedia, the Halo wiki
Polycarbonates are a particular group of thermoplastic polymers. They are easily worked, moulded, and thermoformed; as such, these plastics are very widely used in the modern chemical industry. Their interesting features (temperature resistance, impact resistance and optical properties) position them between commodity plastics and engineering plastics.
The M12 Chaingun Warthog uses polycarbonate as a ballistic armor, built upon a titanium frame.