Yes you can join in defense service after completing B. Tech in Mechanical. To join Indian defense services you have to pass combined defense services examination organized by UPSC two times in a year. After passing CDS examination you will be able to join Indian army, Indian air force and navy.

The defense force would include forces such as:

Indian Army
Indian Navy
Indian Air Force

Here I am providing you the basic eligibility required for the CDS exam. This is as follows:

IMA & OTA:

The applicant has to pass graduation in any stream from any well recognized university …
Degree of a recognized University or equivalent

Naval Academy B.Sc. with Physics & Mathematics or Bachelor of Engg
Air Force Academy B.Sc. with Physics and/or Mathematics or Bachelor of Engg

Age-limits:
19-24 Years Indian Military Academy
19-22 Years Naval Academy
19-23 Years Air Force Academy
19-25 Years Officers’ Training Academy

This exam will have:

Written exam
Interview
Medical test