You need your floor collider also have a 0 bounce material. Note that you don't need a rigidbody to set physics material. It is the collider which provide this property.
I had this issue as well and solved it by removing any positive velocity on every fixed update should there be any. I wrote a short blog post about it here Heavy things should not bounce!
Well, You could disable physic material if cube collides with Plane or you could manually set bounciness to 0 in script when cube collides with plane...
actually I just had what sounds like a similar issue because I wasn’t paying attention. I had a cube that i resized larger than it’s box collider size. I had the floor and cube’s physics material set to no bounce, but it did bounce for an instant according to debug.log outputting oncollide events.
I was having this issue in a “Coin Pusher” game scenario. When dropping coins on the pusher, which is moved via script, the coins would fly off of it. I tried a bunch of things, but what finally fixed it was the “Is Kinematic” setting on the Pusher. Coins are still just “regular” rigid bodies with colliders, but since the Pusher’s moved via script, IsKinematic was needed.