Embrittlement mechanisms in polymer lenses for protective eyewear

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Protective materials for eyes and face need to be transparent yet impact resistant, which restrict the choice of materials. Polycarbonate (PC) has been a favored choice for goggles, lenses and visors because of its capacity to absorb mechanical energy for impact by ductile deformations. Recently, more advanced transparent polymers have emerged. The materials of particular interest for Revision are transparent polyamides (PA). Ballistic tests at Revision revealed that they were superior to PC in terms of impact resistance. However, they also noted that a second impact on this material often triggered a brittle type of failure associated with a very poor impact resistance. This behavior is problematic, because modern transparent eyewear requires multi-hit capabilities. It also appeared that the deposition of functional coatings to increase hardness and prevent fogging make the situation worse, the material exhibiting brittle behavior and poor impact resistance from the very first impact. The objective of this project is to investigate possible causes for the embrittlement of transparent polyamides on second shot and/or upon deposition of a hard coating, anti-fog coating or both. Several possible causes for the embrittlement will be investigated including changes in microstructure induced by shock waves, residual stresses from first impact, propagation of surface cracks from stress waves, confinement of plastic deformations by the hard coating. Towards the end of the project we will provide recommendations to Revision on how to mitigate or totally suppress the effects of the scenarios presented above. Barthelat's team at McGill has the expertise in material characterization, microstructure, materials testing, mechanics and mechanical behavior required to investigate this problem. This project will significantly benefit our partner, Revision, in advancing their knowledge and application of transparent polyamides. Through this project Revision will not only gain new and unique knowledge in those materials, it will also train its engineers with new knowledge and data. Finally the postdoc trained through this project will join the Canadian work force with a scientific and technical working knowledge in ballistic materials, making him highly competitive on the job market.