Winter Soldier's Prosthetic Arm was a cybernetic implant attached to the Winter Soldier's body to be used in place of his missing left arm.
Creation of the Winter Soldier
Following Bucky Barnes' fall from Arnim Zola's speeding train, he suffered injuries that included the loss of his left arm. The prosthetic arm was only a portion of the reconstruction efforts HYDRA used in turning Barnes into their deadly assassin. The construction of the arm presumably took place in secret HYDRA facilities in the Soviet Union, following World War II. With the communist red star imprinted on the arm, the brainwashed Barnes eventually became known as the Winter Soldier.
On December 16, 1991, HYDRA ordered the Winter Soldier to steal a case of the Super Soldier Serum and assassinate its owners, Howard and Maria Stark. During his mission, he used the strength of his bionic arm to punch Howard Stark to death, and then staged his body to look like he died from the force of a car crash.
Training the Winter Soldiers
After his assassination mission, he began sparring with the newly enhanced Winter Soldiers with the Super Soldier Serum obtained from his mission. The Winter Soldier fought one of the Winter Soldiers, Josef, in a training exercise. However, despite his bionic arm, Josef managed to overpower and defeat the Winter Soldier. Suddenly, the Winter Soldiers rebelled against their handlers, and he used his prosthetic arm to protect and escort Colonel Vasily Karpov to safety.
As Nick Fury had seemingly successfully evaded his would-be HYDRA assassins in his SUV, the Winter Soldier appeared on the road and used a Magnetic Disk Grenade to crash his SUV. As the Winter Soldier approached, he used his bionic arm to rip off the door of the armored S.H.I.E.L.D. SUV, but Fury had escaped by digging into the ground and escaping to Steve Rogers' apartment.
Later that night, the Winter Soldier tracked Fury down at Rogers' apartment and shot him through the chest, leading Rogers to chase him across rooftops. Rogers caught up to him on a rooftop and threw his shield at him. However, Barnes caught his shield with his bionic arm and threw it back at him, allowing Barnes to escape.
During his fight with Captain America in Washington, D.C. following his attempt to kill Nick Fury, the Winter Soldier's arm was damaged by Captain America's Shield. The arm, which is at least partially mechanical, was repaired later by HYDRA doctors in the Ideal Federal Savings Bank, a HYDRA facility run by Alexander Pierce.
During the battle in Washington D.C., Barnes killed several S.H.I.E.L.D. personnel at the Triskelion and used his prosthetic arm as a shield to block gunfire. He later used his arm to punch a hole through the window of a Quinjet and steal it to fly towards Captain America and Falcon.
On the third Insight Helicarrier, Barnes ambushed the two heroes and used his prosthetic arm to throw Falcon into the air. Later, as Captain America and the Winter Soldier fought each other inside the Helicarrier, Barnes used his bionic arm to block his shield attacks and to punch Captain America. The strength of his arm, in combination with his naturally enhanced strength, was used to force a knife into Captain America's shoulder and to pin his arm. As Rogers tried to climb up to install the last chip, Barnes used his unbroken robotic arm to mortally shoot Rogers. As the Helicarriers collapsed at the Triskelion, Barnes later used the arm to rescue Rogers from drowning in the Potomac River.
After the Bombing of the Vienna International Centre, Barnes was presumed to be the culprit and the Joint Counter Terrorist Centre prepared an operation to kill him in Bucharest. Captain America fought alongside him to keep him alive. During the breach, Barnes shielded himself with his arm to block gunfire and to throw Captain America against a Joint Counter Terrorist Centre combatant. Whilst trying to escape to the lower levels of the apartment, he lept from the top floors and grabbed the metal railings with his prosthetic arm to break his fall.
As the chase continues into the underground tunnels, Barnes used his bionic arm to grab a speeding motorcycle and ride it himself. However, he was eventually captured and imprisoned at the Joint Counter Terrorism Centre in Berlin.
The Winter Soldier Escapes
After Helmut Zemo used Barnes' command words to brainwash him in the sublevels, he incapacitated Sam Wilson and used the strength of his bionic arm to throw Rogers into an elevator shaft. As he made his way upstairs, he fought Black Widow, Sharon Carter, Tony Stark with his Iron Man Gauntlet, and T'Challa, but he successfully fled to the roof where he was about to escape with a helicopter. However, after Rogers helped disable the helicopter, Barnes tried to choke Rogers to death with his robotic arm, but he was knocked unconscious as they fell into the river. Rogers and Wilson eventually took Barnes to an abandoned warehouse, locking his prosthetic arm in a large industrial vice to learn more about the HYDRA Siberian Facility and the Winter Soldiers.
Barnes used his prosthetic arm to defend himself from Tony Stark's allies at the Flughafen Leipzig-Halle. Intercepted by Spider-Man, Barnes found the strength of his arm matched when an attempt to land a strike was countered by the enhanced youth who casually voiced his awe that Barnes had a metal arm before they were separated by the arrival of Falcon. Later, when the two Avengers factions met in a skirmish, Barnes was engaged by Black Panther, who almost overpowered him due to his use of the vibranium suit, Panther Habit.
The arm was destroyed by Tony Stark's unibeam when Stark was manipulated by Helmut Zemo to attack Rogers and Barnes. The arm was not replaced even after the battle, and the remaining shoulder of the arm was removed before Barnes chose to be frozen until a cure for his brainwashing can be found.
The bionic arm gives him strength equal, if not superior to that of Captain America, and comparable to that of Iron Man. The Winter Soldier was able to rip open a S.H.I.E.L.D. armor SUV, rip apart Sam Wilson's EXO-7 Falcon, and smash concrete by punching it. With his bionic arm supporting his already-enhanced natural strength, the Winter Soldier was able to overpower Captain America in a fight.
The limits of the bionic arm's strength were realized during the Avengers Civil War. While the arm was stronger than a natural one, it was not as flexible. This was seen when the Winter Soldier found himself matched by Spider-Man, an enhanced youth, who was able to counter his blow due to his superhuman strength and bend back the appendage with ease. The arm was later almost overpowered by the Black Panther whose own enhanced strength combined with his vibranium suit, the Panther Habit, allowed him to push back the force of the bionic arm while exploiting its limited mobility. The arm proved inferior to the gauntlets of the Mark XLVI as Stark was easily able to counter it before disintegrating the limb with his unibeam.