How do you flip a card with smooth rotation?

Hello everyone!

I'm making a game which you flip cards. My camera points at the top showing the cards. When i click on a card that one flips but i want to add a realistic flip effect (smooth rotation of the card object) when the flip happens. Here is what i tried but it doesn't work:

 public var card_1: GameObject;
 card_1 = GameObject.Find("card_1");
 
 function OnMouseDown () {
   card_1.transform.Rotate(Vector3(0,0,180) * Time.deltaTime);
 }

The js code is attached to the card object.

Any suggestions? Thanx in advance!