The driving force for sintering in powder metallurgy is the storage of mechanical energy - in the form of surface energy - on the surface of the powder particles during the preparation of the powder.

The result is the formation of one or more of stresses, deformations, heterogeneous structures, or alloy/compound layers on the surface.

This is the driving force behind sintering.

The above is my understanding, you can also ask your college teacher. :)