Can we use Micro controllers(Arduino,Rasberry pi) instead of PLC's?

yes, you can use it in some of application but you would have to lot more work on operating infratructure to develop in micro controller while PLC has it own operating infrastructure and you have to just programe on it according to your requirements.

You could but peripheral modules( IO) would have to be designed by your self. Not to mention it would have to meet international regulation before it could have been used for the Industrial scale.