It's said that Wolverine's Adamantium claws are indestructible. However, when he rubs his claws against each other, it creates sparks. Don't sparks represent damage/chipping?
Are Wolverine's claws truly indestructible?
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It should be clarified that there are at least three grades of adamantium in the comics.
Firstly, there's proto-adamantium. This alloy was created by US Government metallurgist, Myron MacLain during World War II, and was bonded with Wakandan vibranium to create Captain America's shield. MacLain never managed to reproduce this metal though, so it only exists in Cap's shield, and is considered the strongest version of adamantium.
Secondly, there's true adamantium (also known as primary adamantium). After creating proto-adamantium, Myron MacLain spent decades unsuccessfully trying to reproduce it. Eventually though, his efforts led to the creation of another alloy, true adamantium, which is considered the strongest reproducable metal on Earth. This is the type of adamantium which was used to lace Wolverine's bones.
Thirdly, there's secondary adamantium. In-universe, secondary is a lower grade alloy which isn't as strong as true adamantium, but is still far stronger than titanium, and significantly cheaper to produce than true adamantium, making it more cost-effective for many purposes. Out-of-universe, secondary adamantium was created to retcon past instances of adamantium being damaged, and reaffirm the extreme durability of true adamantium.
Technically, there's also a fourth type of adamantium known as adamantium beta. When Wolverine's bones were laced with true adamantium, his mutant physiology apparently changed the metal on a molecular level so that it wouldn't interfere with his production of red blood cells. There's never been any indication that this affected the durability of the metal though, so although the metal on Wolverine's bones is now classed as adamantium beta rather than true adamantium, it doesn't make any practical difference in relation to your question. Adamantium beta is, as far as we know, equal in durability to true adamantium.
So far as the durability of true adamantium goes, the word "indestructible" is used in the relative sense, much as "invulnerable" is for Superman. True adamantium isn't literally indestructible, since it can be damaged by sufficient force, but the force required is so great that true adamantium is practically indestructible for most intents and purposes. This is what the Official Handbook of the Marvel Universe Vol 2 #1 has to say about the durability of true adamantium:
True Adamantium is nearly as strong as Captain America's shield, and is, for all practical intents and purposes, indestructible. The degree of impermeability varies directly with the thickness of the Adamantium. A direct blow from Thor's hammer will only slightly dent a solid cylinder of True Adamantium (see Thor). A sufficient mass of Adamantium could survive a direct hit from a nuclear weapon.
For what it's worth, we have seen Wolverine's claws damaged on a few occasions, but always in alternate realities or futures.
The Earth-616 version of Wolverine has taken blows from Thor's hammer and the Hulk, and as far as we know, the adamantium on his bones wasn't damaged by any of them.