Plasteel

Plasteel is a type of advanced synthetic material that has the consistency of plastic but the tensile strength of a steel alloy that was developed during the Dark Age of Technology but long used by the Imperium of Mankind in the construction of many types of personal infantry armour.
Heavy-duty plasteel was combined with the heat-resistant ceramic material called ceramite to craft the plating of Terminator Armour when the Adeptus Mechanicus still maintained the technical knowledge required to manufacture it.