Enhanced slurry erosion resistance of low friction and highly hydrophobic halloysite-fluoro-polysilsesquioxane coatings

Abstract

Multifunctional coatings were obtained by a sol-gel co-condensation of a fluorinated silane and tetraethoxysilane over activated Halloysite nanotubes (hHNT). They were applied onto epoxy based substrates (neat epoxy or hHNT loaded) with different curing conditions. The coatings characterization showed low friction and high hydrophobicity. The coefficient of friction against stainless steel was 0.1, according to the nanoscratch test and the advancing contact angles reached values in the range 135-145°. The materials were subjected to a slurry erosion test under high speed flow condition and wettability was used to characterize the surface integrity over erosion-cumulative time periods. Fluorine containing coatings exhibited an outstanding resistance to the high speed slurry erosion (HSSE). After one hour of HSSE test, average advancing contact angles of 145° were measured for the fluorinated samples. The high hydrophobicity of the coatings showed a correlation with the erosion resistance, while the substrate showed no effect. An explanation of the cushioning effect of hydrophobicity upon erosion wear is given, based on wall-detached eddies on the boundary layer.