In vitro wear tests of the dual-layer grid blasting-plasma polymerized superhydrophobic coatings on stainless steel orthodontic substrates

Highlights: • A superhydrophobic the dual-layer grid blasting-plasma polymerized (GB-PP) layer coated on orthodontic stainless steel substrates • In vitro wearing tests determine the durability of the GB-PP superhydrophobic caotings • Hydrophobicity mainly afforded by the flourocarbon layer of the GB-PP coatings -- Abstract: Dental stainless-steel archwires, which are frequently used in orthodontics and dentofacial orthopedics, may accumulate food debris, promote bacterial overgrowth, and subsequently result in dental caries. A dual-layer, grid-blasting, plasma-polymerized (GB-PP) superhydrophobic coating was developed in a previous work by changing the micro- and nano-structured surface morphology on AISI 304 stainless-steel substrates. In the present study, in vitro wear tests were performed on artificial saliva that mimicked tooth-brushing, peanut-chewing, and nougat-chewing modes to determine the durability of the superhydrophobic layer. Experimental results revealed that the water contact angle (WCA) of all specimens in the different wear modes decreased with the increase in wearing times. However, the WCA of the GB-PP coated specimens still exceeded 90°. This finding indicates that the deposited coating might have retained its hydrophobic characteristics even with toothbrush-cleaning action or food chewing after some time. The surface morphology obtained from a field-emission scanning electron microscope and the spectra from an energy-dispersive spectrometer showed that the distribution of the C element was uniform on the original surface of the GB-PP coating, but a portion of the C-rich areas was exposed after the tooth-brushing, peanut-chewing, and nougat-chewing wear tests. Fourier transform infrared spectroscopy revealed that CF_x groups still existed on the substrate surfaces after the wear tests. Peanut chewing caused more damage to the superhydrophobic surface than nougat chewing did because the carbohydrate, protein, and oil ingredients in peanut and nougat could have been transferred to the surface, thus masking a part of the fluorocarbon layer. However, the GB-PP coatings deposited on medical-purpose, stainless-steel substrates exhibited good durability after tooth-brushing and nougat-chewing wear tests.