rigidbody collision and forces

 using UnityEngine;
 using System.Collections;
 
 public class moveBall : MonoBehaviour {
     // Use this for initialization
     void Start () {
         rigidbody.velocity = new Vector3(-3, 0, 6);
     }
     
     // Update is called once per frame
     void Update () {
         Debug.Log(rigidbody.velocity);
     }
     
     void OnCollisionEnter(Collision collision){
         if(collision.gameObject.name == "barrier"){
             rigidbody.AddForce( new Vector3(0,0,collision.relativeVelocity.z), ForceMode.VelocityChange );
         }
         if(collision.gameObject.name == "barrier2"){
             rigidbody.AddForce( new Vector3(0,0,collision.relativeVelocity.z), ForceMode.VelocityChange );
         }
     }
 }

This is the code im using to move a ball around a field. In the collision code I am reversing the direction on the z axis.

However the ball slows down upon each collision. I want to stop this.

So if anyone knows how to do this or a better way of doing it completely id appreciate it. It is a 2d pong style game.

Thanking You