The effect of humidity on the sliding wear of plasma transfer wire arc thermal sprayed low carbon steel coatings

Low carbon steel coatings were applied on 319 Al alloy substrates using a plasma transfer wire arc (PTWA) type thermal spraying process. A pin on disc type wear tester placed in an environmental test chamber was used for the wear tests. Tests were performed in an atmospheres with various humidity levels in the range of 10-99% RH. At low load (<20 N) and sliding velocity (<1.0 m/s) tests, an Fe2O3 rich compound formed on the worn surfaces as a result of oxidative wear. As the atmospheric humidity increased, a tribo-polishing process became active, and the wear rates decreased with increasing humidity. SEM metallography indicated that the microstructures of samples tested at this condition exhibited highly polished metallic contact areas with the compacted hydrated oxide layers entrapped in non-contact areas. By increasing the load in atmospheres with ≤50% RH wear rates increased. The high wear rates were associated with the fracture and fragmentation of the edges of splats. At 50 N 0.5-m/s wear rates started to decrease above 50% RH and also the COF started to decrease. The dominant wear mechanism was a chemical–mechanical polishing process. The decrease in the wear rates started at higher relative humidity levels, e.g. 85% RH at 50 N and 2 m/s as the testing conditions became more severe.