Well, from the FAA site
Do I have to take any tests to become a certified mechanic? Yes. You have to take both oral and practical tests. There is a fee for the test. The oral and practical tests cover 43 technical subjects. Typically tests for one certificate--airframe or power plant--takes about 8 hours. "
Since the FAA uses the term 'certified mechanic' one can presume that that is the correct usage, and not the term 'certificated mechanic' a term that the FAA does NOT use.