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After the Sokovia Accords affair and the infamous fight in the abandoned Hydra base in Siberia, Bucky and Steve went into hiding together for a while, taking refuge in Wakanda thanks to King T’Challa’s help. Once there, the one-armed fugitive was put in cryo-sleep until Princess Shuri and her team could figure out a way to reverse the brainwashing and mind-control techniques used on his mind to turn him into the Winter Soldier.
During that time, the mess left by the Avengers’ internal dispute and consequent breakup was cleaned up and an agreement was reached between all involved parties. New accords were written, signed, and put into use. Those turned the Avengers into a semi-private organization partially overseen by the UN. So, with the team officially reinstated, Steve pulled Bucky out of the ice in order to take him back to America, knowing that what awaited them there would be only unpleasant at first.
Barnes was subjected to a lot of different psychological evaluations that aimed to determine if he was functional enough to be released under the care of the team of superheroes. After that came a few months’ worth of research, experiments, and tests that culminated in the former assassin being completely mind-control-free. Only then he was given a new cybernetic arm, although it was modeled after the original one, and thus why it had a red star painted on the shoulder.
Even when it did upset him to have such a blatant reminder of his past as Hydra’s puppet, and the fact that he knew it wouldn’t bode well with his attempt to rebuild his life and not associate with the former Nazi organization anymore, Bucky didn’t complain about it.
For a long time, he even had the suspicion that the person responsible for that added detail was none other than Tony Stark, as a sort of petty revenge for what went down in Europe all those months ago.
Despite that, Buck couldn’t really blame Iron Man for still being mad at him. He also harbored a lot of hatred towards himself because of his actions as Winter Soldier.
Nevertheless, things between the Sergeant and the engineer started to slowly change after one night, when Bucky sought out the older (younger) man for help. He showed up without warning at Tony’s private lab on the compound, even while knowing that such an action wouldn’t be taken kindly.
Still, he had to do it, because while he didn’t trust Tony that much (especially since he was the one who blew his arm off, to begin with, though he understands why the other did it), he trusted even less the staff at the compound since they’re all strangers.
So, Buck surprised a reasonably alarmed Tony when he appeared seated at one of the latter’s lab tables. Bucky looked embarrassed, anxious, scared, and apologetic all at the same time, but he said nothing as he held out his metal arm towards Stark. Only when he registered the other’s confused and questioning expression did he dare to speak up.
“I don’t know how to fix it myself,” said quietly the younger (older) man.
Barnes had messed up the internal mechanism of the arm earlier that day while training with Steve but refused to admit to being bothered by it. He didn’t want to worry his best friend, nor to make him have to reach out to Stark for help. If anybody was going to do that, it would have to be him.
For a long moment, Tony said nothing, looking instead at the man half-shrouded in darkness while wondering if he was for real. “Why come to me?” Stark had asked, skeptical about him being the one Barnes would seek out for help instead of anyone else.
“You could be here to murder me, for all I know,” he said, entertaining the idea that it could be all a ruse to finish him off for once and for all.
Because, despite knowing that the other man was freed from the brainwashing he was subjected to, the genius billionaire still feared that the former assassin could somehow revert to his Winter Soldier mode.
“You are the only one I trust to either kill me or help me. I know you won’t try to use me as a weapon or torture me.”
The honest admission was muttered so quietly that Stark almost couldn’t hear it, yet he understood nonetheless.
Only after that did he tell the other man to wait patiently while he gathered the tools he needed, not without loudly instructing F.R.I.D.A.Y. to let Pepper know his whereabouts and with whom he was. Just as an obvious warning to Barnes that there would be proof and witnesses that time around that he couldn’t get rid of even if he tried. If he ended up murdering Tony, that is.
Once ready, Tony ran a basic diagnosis of the arm in order to figure out the problem. In the meantime, Bucky kept as still and quiet as he could, while appearing as relaxed as possible. The last thing he wanted was to scare the other man or give him reason to feel threatened.
After all, Stark was helping him despite everything he –unintentionally– did to him. And half an hour later, the problem was fixed and Stark was already walking the Sarge out of his lab, telling him half-jokingly and half-seriously to never show up unannounced again. At that, Bucky wholeheartedly promised to not do so unless it was imperious. Then, he bid Stark farewell and disappeared from sight.
Following that episode, things went quiet once more between those two. Thankfully, the metal arm didn’t act up again at least. However, one day and seemingly out of the blue, Bucky was called into Stark’s lab again. There, he found Tony, who told him –in his usual lofty, uncaring, and nonchalant style– that he had something for him.
It turned out that after the fateful night, Tony couldn’t stop thinking about upgrading the cybernetic prosthetic. Although, he emphasized the fact that it wasn’t out of concern for Barnes but rather his engineering instinct kicking in (which, honestly, didn’t sound believable, but Bucky said nothing about it).
According to Tony, he didn’t do it for Barnes; not even for Rogers, since he was still mad at the Captain for choosing the Sergeant over him back in Siberia. No, he did it for the team… Or, at least, that’s what he tried to convince himself and Bucky of.
Unfortunately, he didn’t quite succeed at that. But that’s fine because it meant that he was self-conscious enough to realize that he was already beginning to forgive Barnes as well as to move on from his harbored hatred towards the man.
Going back to the matter at hand (no pun intended), Stark explained that since the second arm was modeled after the first one –and despite all the upgrades N°1 underwent while Buck was still under Hydra’s control–, he felt it wasn’t modernized enough for his liking. And given that Bucky had joined the team and would be working on missions with other Avengers, the engineer considered that not having an arm up to par with some of the other heroes’ state-of-the-art technology wasn’t going to work.
That’s why he made a new arm from scratch. One which biggest difference, compared to the one that was being used by Bucky at that time, was the fact that it’s made of vibranium (courtesy of one generous King of Wakanda).
Also, besides the lack of a taunting red star, there were other perks and upgrades incorporated into the new arm as well. The first biggest upgrade was a variety of different sensors that allowed him to pass through security measures, such as metal detectors without setting them off, and served to conceal other metal objects (like weapons) from being detected. And the second most important one was nanotech that could create holograms to make the arm look and feel like a normal human arm.
The idea behind those upgrades was to facilitate stealth-orientated missions, although the hologram one was also intended for Bucky to use whenever, if he wanted to. Still, he never made use of it since it upset him too much and it increased his body dysphoria. ‘Cause he could still feel on the inside that it was just metal. But besides that, he’s deeply grateful for Stark’s gesture, especially because he knew that there was no need nor obligation for him to do it in the first place.
On Tony’s part, he felt a little worried about giving the war veteran such an upgrade that could make things difficult for everyone if the other chose or was forced to turn against them again. Yet the more sadistic and vengeful side of him was kind of hoping for a reason to get the retribution he was denied in Russia. Nevertheless, that’s just a twisted little fantasy of his, not an actual wish, so it didn’t really count.
Ultimately, though, Tony justified the entire gesture to himself by thinking that it was all done for the greater good and taking the team’s (and Steve’s, even if he didn’t want to admit it) interest into account. If anybody asked, he would say just that. Fortunately for him, nobody did (even when some of them wanted to force him to spit out the truth).
Mainly because they knew he would only lie to their faces, and they all knew better than to do that. The real reason still is that he truly cares about all of them, no matter what. After all, the Avengers are his family.