Lenses and other transparent optical materials suffer rapid damage whensubjected to blowing abrasive particulates. The time-scale of these impactevent falls between typical scratch tests (less than 1m/s) and ballistictests (100s of m/s) and has not been studied in depth to date. Polymericlens materials like polycarbonate are usually treated with ascratch-resistant coating, which is commonly silica-based. The coatingprovides some protection, yet is not sufficient at resisting abrasion fromblown sand in most commercial products. We demonstrate that siliconeelastomeric coatings are superior to polycarbonate and silica glass atresisting damage by blown sand particles. Sand abrasion tests were conductedusing a custom-built test apparatus that exposes the sample to 400 microndiameter quartz silica moving at 16.5 m/s (approx. 38 mph). Scanningelectron microscopy revealed the presence of small cracks and pits inpolycarbonate, coated polycarbonate, and silica glass after sand exposure.No such damage was observed in the silicone-coated samples after anidentical exposure.

We speculate that the elastic tensile strain at the surface is an importantpredictor of the material response at the time-scale of the impact. A simplemathematical model was developed using a momentum balance pre- andpost-impact, and was used to approximate the maximum deformation and impacttime-scale. A semispherical interaction volume was used in the model with aradius of 1.5x the particle diameter, determined through profilometryexperiments. The material’s resistance to deformation was measuredexperimentally through a static mechanical test using a spherical indenterto represent the particle. Tensile tests were performed on both materials toidentify the maximum elastic strain.Additionally, dynamic mechanical testswere performed to confirm that the mechanical behavior at long time-scaleswas valid at shorter time-scales of the impacts. DMA curves were shiftedusing the WLF equation. Profilometry and scanning electron microscopy (SEM)imaging were used to confirm the presence or absence of blown-sand induceddamage.