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In Civil War we see Black Panther scratches Cap's shield with his claws, even though both are made of vibranium. How is this possible, especially given that Cap's shield is made of a vibranium alloy which is even stronger than simply using Vibranium?
My Question is different from How were bullets able to dent Captain America's Shield? because the conclusion to that question was that the bullets don't dent the shield, whereas in this case the claws do scratch the shield.
Simple answer. In the MCU, Cap's shield is only made out of Vibranium, not an alloy, as in the comics.
"No, no, that's just a prototype."
"What's it made of?"
"Vibranium. It's stronger than steel and a third of the weight. It's completely vibration absorbent."
"How come it's not standard issue?"
"That's the rarest metal on Earth. What you're holding there, that's all we've got."
―Howard Stark and Steve Rogers
Thus, we have two items that are likely made of metals of the same toughness and hardness (there's no information on what other processing might be done with Vibranium). And that's assuming that Wakanda hasn't made any improvements on the process in the last several decades.
It's entirely possible to scratch strong metals, even with weaker materials. One example is using water to cut metal. It's about concentration of force, not strength. See this question on the physics Stack Exchange: Is it possible to cut harder material with a less hard material?
The pressure that a water jet can produce during cavitation; the momentum of.a soft sand grain burrowing into a hard coating; the heat generated when laser light hits a surface; the heat of electrons decelerating as they hit an anode - all these and more can be mechanisms of material removal which can be used to shape an object - "cut" it. Typically the mechanism is that local pressure, while exceeding the yield stress of the "soft" material, cannot be dissipated because of the speed of impact.
This exact question was also already asked on the movies Stack Exchange: Vibranium material properties.
We can clearly see the claws are sharpened to blades, the shield is a perfectly flat surface. The claws would rightfully so scratch the shield as they are the same density but the blades are sharp points.
If it helps think of it in terms of real world glass. You can sharpen glass to a point where it can cut through other glass not too dissimilar to a diamond cutting through glass.
Here's another related question: How does Vibranium work?
Whether normal Vibranium can be converted into Anti-metal is unknown, but the Wakandans do have small supplies of this Savage Land Vibranium on hand and have equipped the Black Panther with it.
The material coats his claws and often other weapons he uses, allowing him to tear through metallic substances with little effort.
There is also a ton of info in these questions:
It is possible (though difficult) for an something to scratch something with a similar hardness, although it is likely that Black Panther's claws suffered some measure of damage as well.
Though it wasn't explicitly stated, Black Panther's claws are made of Antarctic Vibranium or "Anti-metal", which is the opposite of Wakandan Vibranium. While Vibranium from Wakanda strengthens vibrations, Antarctic Vibranium weakens them.
Given that Cap's shield has three colours and does not appear to be painted I think it is reasonable to assume there are at least three metals used that are at least somewhat different.
Given they say the shield is Vibranium, not a Vibranium alloy, they may be only talking about the core of the shield, and only an outer layer on the shield that is scratched.
edit, if not then it is painted and the paint is being scratched.
