Headcrash...
One uses the term Head Crash when the read / write head physically damages the surface of the storage medium.
Normally the read/write head floats on a small air cushion over the magnetic-coated surface of the hard drive. Sometimes a "touchdown" occurs when the read/write head touches the surface of the storage medium.
These "touchdown's" are not uncommon for hard drives. A "touchdown" does not automatically mean that a head crash has occurred. The surface of the storage medium (drive plate) is coated with a special layer. This ensures that such contacts do not lead to the damage of the medium.
Some hard drive manufacturers try to raise their sinking profit margins by using inexpensive materials. Thin film coating is used a lot here. These coatings are less expensive to manufacture but also show a much higher breakdown rate in comparison to other methods.
A head crash occurs when the read/write head hits on the surface of the medium destroying the protective layer and the underlying magnetic coating. The destruction can be so great that the entire coating is destroyed right down to the substrate (plastic, glass or ceramic plate).
Heads crashes can also occur through vibrations, incorrect installation of the hard drive and by extreme temperature changes.