Something along the lines of

var cubeStrength : float;

function OnCollisionEnter() {

 cubeStrength --;

if (cubeStrength <= 0) {

   Destroy (gameObject);

} }

Simply apply this code to the cube, set a strength in the inspector window (higher number for stronger material).

Each time you hit the cube the cubeStrength will be decreased by one point. Once the value hits 0 it will be destroyed.