Vibranium material properties

In the MCU Caps shield is pure Vibranium because of licencing so I'll talk in respect to that.

I believe it's a safe bet as pretty much all Vibranium came from Wakanda that BP claws and parts of his suit are made of Vibranium.

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.

EDIT: Some real world science! If we go off Mohs Hardness Scale. We can determine that:

If the two specimens are equal in hardness then they will be relatively ineffective at scratching one another. Small scratches might be produced, or it might be difficult to determine if a scratch was produced.

So from this we can safely assume that "relatively ineffective" allows for scratches to be made however slight they may be. Factor in that caps shield has some kind of paint job on it the scratches we see are entirely plausible given real-world science, cinematic exaggeration and the fact that it's a sci-fi film.

Source: http://geology.com/minerals/mohs-hardness-scale.shtml

Edit: See this answer over on SFF.se on properties of Vibranium-B that I didn't know about! That would explain perfectly how BP was able to cut the shield!

There are two answers to your question: the physics one and the fantasy one.

From a physics perspective, two materials of the same relative hardness can scratch each other, under the right circumstances (involves the direction, the cut, the sharpness, etc.) For example, diamonds can cause scratches on other diamonds. So it's perfectly reasonable that razor-sharp vibranium claws can scratch a flat vibranium surface.

From an in-universe perspective, it's not entirely clear if the shield is pure vibranium or not. Howard Stark says it's made of vibranium, and that it took their entire supply to make. But that doesn't preclude the possibility that they used to vibranium to make an alloy. In fact, it makes it more likely that it's an alloy, rather than just coincidentally having exactly enough to make one normal-sized shield.

In the source comics, the shield isn't pure vibranium. It's a vibranium/adamantium alloy. I don't think Marvel can mention adamantium, since that's so closely associated with Wolverine, but that doesn't mean they couldn't have made a vibranium/steel or vibranium/titanium or vibranium/other-rare-metal alloy to make the shield with.

Let's look at a practical reason.

The Vibranium in Howard Stark's Shield was mined at some undetermined date, and processed by Howard into the shield in the late 1930s early 1940s. It has been frozen for decades, and has received plenty of punishment from physical, electrical/magical, and laser/repulser sources. As far as we know, Howard Stark only had one crack at it. Sure he's a genius, but nothing is done perfectly the first time. He also makes it as a defensive tool.

The Vibranium in T'Challa's claws were mined and processed by Wakandans at some unknown date. Likely made for T'Challa, as his father was not the same size as him. Wakandans have been working with Vibranium for, well, forever, and are typically shown to have technology to outpaces normal modern technology. They are also intended to be used offensively by a warrior.

In the comics and cartoons, Wakandan technology is one if not the most advanced on the Planet. Tony Stark's tech doesn't even come close. Wakandans use vibranium in all their weapons, and have applied technology to it. It is often electrified and has active electronics in it, making it deadlier than normal.

The simple answer is this: The Black Panther's vibranium claws are newer, designed by experienced users for offensive purposes. This is why they can scratch and gouge an older, inferior defensive shield.

At a minimum, the claws are decades more advanced metallurgically than the shield. Worst case, they are decades more advance in metallurgy, active electronics, and built by skilled weapon smiths instead of an amateur technologist.

As Will Smith would say, Cap's shield is the old and busted, T'Challa's claws are the new hotness.

In the comics Black Panthers suit is mostly made from Vibranium, however it is said his claws are made from an isotope called 'Anti-Metal'. According to Wikipedia:

The Black Panther uses vibranium in a micro weave mesh in his uniform that robs incoming objects of their momentum. He also cannot be stabbed, although the costume and the Black Panther can be cut if the attacker slashes along the uniform's grain. Beyond that he uses vibranium in the soles of his boots that allows him to survive a fall of several stories and, if given enough momentum, the Panther can also scale walls or skim across water. The field can be also used offensively to shatter or weaken objects, such as kicking something with the boots. He also used the other variety Anti-Metal, in retractable claws.

Anti-Metal is a "particular isotope of Vibranium, it was dubbed "Anti-Metal" due to its property of dissolving other metals." Panthers claws are made of anti-metal which is a stronger version of Vibranium.

This variation produces vibrations of a specific wavelength that break down the molecular bonds in other metals, causing them to liquefy. If huge quantities of anti-metal are gathered together, the vibrations increase exponentially. Anti-Metal is able to become an artificial and unstable form of the Wakandan variety of vibranium through certain particle bombardments on it.