Adamantine Weapon

You touch a nonmagical weapon. Until the spell ends, that weapon becomes adamantine and attacks with the weapon score a critical hit on a roll of 19 or 20. 

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the weapon's attacks score a critical hit on a roll of 18-20. When you use a spell slot of 5th level or higher, the weapon's attacks score a critical hit on a roll of 17-20.