On top of the substrate material described earlier that forms the base of the platter there is an extremely thin layer of magnetic material and this is actually where the data is stored. In the past, an oxide media was typically used. This actually refers to iron oxide, or rust, and was usually referred to as some type of high performance material. This oxide media was actually little bits of rust that were attached to the surface of the platter. If you look at the surface of an old platter its actually light brown in color. While oxide media is a low-cost material it does have some downfalls. Since it is a relatively soft material it can easily be damaged from coming in contact with a read write head. In addition to the soft material make-up, it also was only practical for low-density storage.
In todays hard drives platters are covered with a very thin film media of magnetic material. There are a couple of different manufacturing processes that can be used to apply this material to the platters. The first is electroplating; most people are familiar with this term and its a similar process to that used in electroplating jewelry. The second method is known as sputtering. This uses a vapor-deposition method to put down an exceptionally thin layer of magnetic material on the platter. Despite the increased cost of this method it is preferred over electroplating due to the ever increasing need for a more uniform and smooth surface on the platter for the sake of performance. In addition, the thin film media has a far greater magnetic property which allows it to hold more data in the same amount of space. Lastly, the thin film media is less vulnerable to damage since it is a more hard-wearing material.
A layer of carbon is applied on top of the magnetic media on the surface of the platter as a protective covering; this is topped by an ultra thing layer of lubrication. All of this is in an effort to shield the disk from any damage that may occur through unintentional contact from the heads or any other foreign substance that may get in the hard drive.