This is usually used, when a character gets transformed into a robot, or just basically becomes a machine version of themselves. It also counts, and this is the case for most stories featured here, is when a person wears something that makes them appear / behave like a robot. Everything that features, like a robot suit or something, is part of this here as well.
The White Rose by jarjarjarjarjar
This absolutely fantastic nanomachines mechanization story by jarjarjarjarjar follows quite the genius inventor John, many years in the future. Even though John has a genius mind, his body has betrayed him all his life, and he’s barely capable of surviving without all kinds of medical supplies, breathing apparatus, and a special suit that keeps up his blood circulation. Having made a lot of scientific research on nanomachines, John had always the vision of using them to maybe fix problems within his body. Sadly though, nearly every government on earth immediately banned the use of nanomachines on biological material, understandable being very concerned about their potential damage. As a result, all nanomachines manufactured had that block integrated on their very base level, making it impossible to somehow reverse that. But there actually had been a set of nanomachines that didn’t have that blocker. Some very first generations hadn’t been made with that blockade, and John had managed to get his hands on some of them. The issue was just that their memory was so small, they head to be reprogrammed every 24 hours. With his obsession of fixing his body and having the strong desire to become the ideal partner for his caretaker Laura, John goes further and further to somehow transform himself into what Laura had always dreamed of… The White Rose by jarjarjarjarjar Part I DeviantArt Even after so many years John still struggled to get to sleep. The machines that kept him alive also kept him awake. As the respirator forced air into his chest there was a slight whine from the pump, followed by a slight whistle as the air was forced out again. The lights that served as status indicators on the devices were just bright enough to bother him even with his eyes closed. ...