Wear mechanisms in thermal spray steel coatings

Low carbon steel coatings (0.2% C) deposited on 319 Al alloy substrates by a plasma transfer wire arc (PTWA) process, were subjected to dry sliding tests over a range of loads (5-75 N) and sliding speed (0.2-2.5 m/s). A wear map was constructed to summarize the measured wear rates and mechanisms that control the wear rates. Four regimes were identified for sliding wear under dry atmospheric conditions: at low loads (<20 N) and velocities (<1 m/s), surface oxidation to Fe₂O₃ was the main wear mechanism. The highest wear rates occurred at high loads and low velocities, and were associated with the fracture of the highly deformed steel splat tip. At high loads and velocities the wear rates decreased, as a result of the formation of thicker oxide rich tribo-layers and hardening of the sliding surfaces. The wear rates were the lowest at low load and high velocity conditions, where the surfaces were covered with oxide rich tribo-layers. The effect of atmospheric humidity was also studied by performing sliding wear tests in an environmental test chamber at humidity levels between 10% and 99% RH. As the atmospheric humidity increased above 50% RH, a tribo-polishing process became active, and the wear rates decreased.