X-ray photoelectron spectroscopic investigation of the durability of siloxane plasma polymer coatings in a steam atmosphere

Thin-film plasma polymer coatings were deposited on to titanium, stainless steel and 70/30 copper-nickel substrates from hexamethyldisiloxane (HMDSO) as potential permanent promoters of dropwise condensation of steam. Long-term durability condensation tests showed that HMDSO films on titanium exposed to low-velocity steam performed well with some lifetimes in excess of 12 000 h. X-ray photoelectron spectroscopy (XPS) was used to monitor the dependence of chemical composition of the films on the deposition conditions and it was shown that composition was not critically dependent on deposition conditions. Additionally, XPS was used to monitor chemical composition before and after exposure of specimens to steam in an attempt to identify the failure mechanisms for films which had ceased to sustain dropwise condensation over their entire surface. Furthermore, short tube sections were coated with plasma-polymerized HMDSO and exposed to steam at elevated velocities. In this case the durability was poor and this is attributed to water droplet impingement erosion on the tube surfaces. This was confirmed by a subsequent test where no water droplets were present in the steam flow and the coating lifetime was seen to increase.