Optical Coating Properties for Enhanced Blown-Sand Abrasion Resistance
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Optical Coating Properties for Enhanced Blown-Sand Abrasion Resistance Christopher Drew, Suzanne Bosselman, and David Ziegler US Army, Natick Soldier Research, Development, and Engineering Center, Natick, MA, 01760
ABSTRACT 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 timescales 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. INTRODUCTION Impact-resistant eyewear lenses are commonly made of polycarbonate that is very prone to scratches and other surface damage that can quickly give rise to a large amount of scattered light at the expense of the transmitted light intensity1. As a consequence, nearly all polycarbonate lenses are coated with some type of protective coating. We have found that while most coatings generally hold up against slow (
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