Though the latter images of Law and Luffy are related to the answer of a different D. question, it does raise a good point that not everyone might catch about how Luffy and Law's dynamic works.
Luffy calls Law "Torao" but not because he doesn't actually know his name.
In fact, Luffy knows the full thing! That's where the nickname came from in the first place—the fact that Luffy can't pronounce Trafalgar for the life of him. Torao (or Traffy, for English) is as close as Luffy can get to Trafalgar.
So it's a bit weird to think about how Luffy doesn't call him Law. Luffy clearly loves the guy! He willingly agreed to a shady alliance where he didn't know details of the plan or what it would entail because Luffy liked the guy who was asking him for a hand. Luffy, when first spotting Law after two years, introduces him to his crew like this:
To be named the same as a crewmember that the crew just helped and saved an island for last arc is a bold play by Oda showing us exactly what Luffy was headed towards, considering the standoffish fellow captain nakama. Hell, the official translation makes it clear that Luffy thinks of Law as one of his people by going the extra step and clarifying to English speakers that Law is closer to crew than not now, even if he verbally disagrees.
So why wouldn't he call Law his name? He cares enough to know it! That is the standard Oda himself has set for if Luffy will call them by name! Luffy has put in the effort of caring and remembering Law's name, so it isn't that.
It is because Law doesn't.
Law maintains that distance and only addresses Luffy with "Mugiwara-ya" rather than his name. Not only is it not Luffy's first name, but Law keeps the ambiguous -ya honorific attached.
So Luffy returns that, respecting one of Law's boundaries in his own way by keeping the nickname of his surname. Both of them address the other as peers rather than close companions. It's a bit of a mess, since Law stringently attaches -ya to keep that distance and Luffy has mangled Trafalgar into "Torao", but it works despite being unorthodox. Both of them are ok with that.
It's almost a microcosm of their alliance as a whole—completely removed from the normal idea of a "pirate alliance" actually is. Where they operate in a way that is perceived as weird but it works almost only because of that. In a normal pirate alliance, they would've betrayed each other and worked only for themselves but instead they work together and care about the other. It was continuously hammered in that pirate alliances end in betrayal and it didn't happen. Even when Law tries to break the alliance, Luffy doesn't let him because this isn't your normal pirate alliance. It's the Straw Heart Pirate alliance, and that means there's going to be chaos and madness but we're getting through this together no matter what. It works because they're both weird, disrespectful pirates who care more about doing their own thing rather than fitting into the norm.