Quick answer: The Physical Material must be assigned where physics actually reads it — the collision setup (or the rendering material’s Phys Material slot). Friction between two surfaces also depends on the project’s Friction Combine Mode.
An “ice” Physical Material with near-zero friction is created and assigned to the ice surface’s material — but objects slide on it the same as on concrete.
Assign Where Physics Reads It
Physics looks for the Physical Material in this order:
- On the collision body / primitive component’s Phys Material Override.
- Otherwise, on the rendering material assigned to the hit face (its Phys Material slot).
If you set it on the material asset but the mesh uses a different material on that face — or has a Phys Material Override of None — physics never sees your ice.
Friction Combine Mode
The friction used in a contact is a combination of both surfaces’ friction. Project Settings → Physics → Friction Combine Mode (Average / Min / Max / Multiply). If it’s Average and the other surface has high friction, your low-friction ice gets averaged up. Use Min if “touch ice = slide” should win.
Per-Surface Override
A Physical Material can override the combine mode for friction and restitution individually — set the ice material to override-with-Min so it’s slippery against anything.
Verifying
Objects slide freely on the ice surface and grip on concrete. The friction difference is obvious. show Collision + a physics debug confirms the right Physical Material on the ice faces.
“Physics reads the Physical Material off collision or the rendering material — and combines two surfaces. Check both the assignment and the combine mode.”
For surface-type gameplay (footstep sounds, decals, friction), drive everything off the Physical Material’s Surface Type — one source of truth per surface.