PhD in mechanical engineering will plant any number of opportunities firmly before you, and it'll be up to you to reach out and take them. It's no easy road to travel, though; getting a mechanical engineering degree at the doctoral level will require tremendous amounts of work and effort over a significant amount of time, as do all PhDs.
A PhD in mechanical engineering would require a strong knowledge of scientific and mathematic principles, specifically in areas of physics and geometry, among others, along with a strong knowledge of ways to apply those principles to real-world problems. Taking undergraduate courses which could provide this kind of background is a good idea for those who are interested in the PhD in mechanical engineering.