Lenses and other transparent optical materials suffer rapid damage when
subjected to blowing abrasive particulates. The time-scale of these impact
event falls between typical scratch tests (less than 1m/s) and ballistic
tests (100s of m/s) and has not been studied in depth to date. Polymeric
lens materials like polycarbonate are usually treated with a
scratch-resistant coating, which is commonly silica-based. The coating
provides some protection, yet is not sufficient at resisting abrasion from
blown sand in most commercial products. We demonstrate that silicone
elastomeric coatings are superior to polycarbonate and silica glass at
resisting damage by blown sand particles. Sand abrasion tests were conducted
using a custom-built test apparatus that exposes the sample to 400 micron
diameter quartz silica moving at 16.5 m/s (approx. 38 mph). Scanning
electron microscopy revealed the presence of small cracks and pits in
polycarbonate, coated polycarbonate, and silica glass after sand exposure.
No such damage was observed in the silicone-coated samples after an
identical exposure.
We speculate that the elastic tensile strain at the surface is an important
predictor of the material response at the time-scale of the impact. A simple
mathematical model was developed using a momentum balance pre- and
post-impact, and was used to approximate the maximum deformation and impact
time-scale. A semispherical interaction volume was used in the model with a
radius of 1.5x the particle diameter, determined through profilometry
experiments. The material’s resistance to deformation was measured
experimentally through a static mechanical test using a spherical indenter
to represent the particle. Tensile tests were performed on both materials to
identify the maximum elastic strain.Additionally, dynamic mechanical tests
were performed to confirm that the mechanical behavior at long time-scales
was valid at shorter time-scales of the impacts. DMA curves were shifted
using the WLF equation. Profilometry and scanning electron microscopy (SEM)
imaging were used to confirm the presence or absence of blown-sand induced
damage.