It’s thick steel, about five inches I believe. On top of that is what’s generically called non-skid. It’s a tar-like substance with grains of silica in it, and it’s applied so that it’s very rough and bumpy. It has to be applied in very specific environmental conditions of temperature and humidity, so when new non-skid is applied (always in the ship’s home port or a shipyard, never at sea), it’s often done under tents that have been set up so the environment can be controlled.
What’s interesting is that over the course of a six month or longer deployment, the non-skid is worn away by the impacts of the tailhooks so that you an see the underlying paint or whatever it is that’s between the steel and the non-skid. It gets really slick by the end of the cruise, especially once it gets greasy and if it also becomes wet from rain or sea spray.