Anti-fogging technique for protection of optical grating scales

Abstract

The combined technique consisting of CF4/O2 plasma nanotexturing of the glass surface and the deposition of the SiOx containing DLC film on the top of the grating scale followed by O2 plasma treatment was designed for the long-lasting protection of the optical grating scales from fogging. Hierarchical nanotexturing increased the average surface roughness Ra from 0.8 nm to 5.1 nm and considerably decreased the wetting angle from 11° to <1° leading to the superhydrophilic silicate glass surface. XPS analysis of the DLC film before and after O2 plasma treatment shows that even short exposure to O2 plasma produces oxidized silicon on the surface of the DLC film. O2 plasma treatment time of 180 s leads to the increase of the carbon CC peak (sp2 hybridization) intensity from 15% to 32%, and a decrease of the carbon C-C peak (sp3 hybridization) intensity from 67% to 45%. The wetting angle of the plasma nanotextured and DLC-coated optical grating scale retains superhydrophilic properties and optical transmittance after long-term storage. Real-time fogging testing confirms the fast appearance of a continuous transparent water film on the surface of the optical grating scale at the beginning of water condensation process. The proposed technique ensures long-lasting operation of the encoder with the optical grating scale below dew point temperature.