Structural coloration stemming from microstructure-induced light interference has been recognized as a promising surface colorizing technology, based on its potential in a wide array of applications, including high-definition displays, anti-counterfeiting, refractive index sensing, and photonic gas and vapor sensing. Vibration-assisted ultraprecision texturing using diamond tools has emerged as a high-efficiency and cost-effective machining method for colorizing metallic and ductile surfaces by creating near-wavelength microstructures. Although theoretically possible, it is extremely challenging to apply the vibration-assisted texturing technique directly to colorize non-metallic and brittle materials (e.g., silicon and acrylic polymers) with high-quality, crack-free microstructures owing to the intrinsic brittleness of these materials. The most challenging part of vibration texturing for structural coloration on non-metallic materials is to generate a predictable grating geometry with a high aspect ratio (grating depth over spacing), while maintaining suitable cutting conditions for a good surface finish.
In a new paper published in Light: advanced manufacturing, a team of scientists, led by Professor Ping Guo from Northwestern University, USA and Professor Jianjian Wang from Tsinghua University, China, and co-workers have explored the feasibility and addressed the challenges of vibration-assisted texturing of non-metallic and brittle materials. To achieve the maximum feature aspect ratio, the vibration trajectories were optimized to minimize tool–workpiece impact and interference and maintain high-quality cutting conditions. High-spatial-frequency grating-type microstructures with spacings ranging from 0.75 to 4 μm were successfully fabricated on a silicon surface in the ductile regime with optimized elliptical trajectories. The successful structural coloration of silicon and acrylic polymers was demonstrated. Polychromatic images were rendered with controllable grating spacings at each pixel location. In summary, they verified the hypothesis that elliptical vibration cutting can be utilized to generate grating-type microstructures directly on brittle materials, where each grating is created in one vibration cycle. These scientists summarize the principle of ductile-regime texturing:
“Elliptical vibration-assisted texturing typically utilizes a relatively large nominal cutting velocity. The scallops formed by the overlapping tool trajectories create conjunctive microstructures on the material surface. Because each grating feature is machined in a separate vibration cycle, the trajectory amplitudes must be carefully determined, to ensure that (1) the material is removed without damage to the surface and (2) a high aspect ratio can be achieved with minimal interference between the tool flank and workpiece.”
“Unlike general 1D vibration-assisted texturing, the elliptical (2D) vibration cutting process can provide a return motion in the cutting direction, to generate high-aspect-ratio features with extremely small spacings on silicon surfaces.” they added.
“Each vibration cycle generates a discrete surface feature during texturing. The microstructure generation rate is equal to the vibration frequency. Hence, this texturing process can potentially lead to the high-efficiency texturing of gratings using a high-frequency vibration tool.” the scientists forecast.