The stringent performance requirements for tactical vehicles, aircrafts and munition system currently necessitates the use of chemicals, such as hexavalent chromium, in the manufacturing process that pose significant environmental and health hazards. These chemicals enter the waste stream and can pose long term environmental hazards and add to life-cycle costs which include disposal and environmental remediation costs for the Department of Defense (DOD). The technical objective for this proposed research project, WP-2742, is to develop a single step, lab-scale, portable and scalable atmospheric plasma-based technology that can be used to both clean a metallic substrate and deposit a zirconia-silica gradient conversion coating in a single-step, chemical free process that will be optimized to adhere to the substrate, provide corrosion barrier properties and promote adhesion of subsequent organic coatings.
The proposed effort will replace chemical based cleaning, degreasing and conversion coating processes with a new single-step atmospheric plasma based system. The process has been termed Coatings with Gradient in Composition using Atmospheric Plasma (CGCAP). The proposed effort will replace the cleaning/degreasing steps by modifying the chemistry in atmospheric plasma, assisted by lasers, for cleaning, activation and surface texturing of the aluminum substrate. The chemical conversion step will be replaced by the deposition of zirconia and silica using atmospheric plasma. During this process, molecular scale mixing and reactions of the zirconia-silica-organosilane precursors in the atmospheric plasma environment will provide nanoscale multi-functionality. Composition gradients will be produced in order to take advantage of the strong binding to the aluminum substrate and good corrosion barrier properties provided by the zirconia and strong adhesion to the subsequent organic coatings from the silica and organosilanes.
This proposed work will use an integrated computational materials engineering (ICME) based approach to connect the processing, materials and performance properties through modeling and simulation in order to increase the understanding of the system, which will guide selection of materials and processing conditions to increase the speed at which it will be developed and optimized The adhesion and corrosion protection performance will be measured using relevant American Society for Testing and Materials (ASTM) or military standards (MIL-STD) and detailed characterization of the CGCAP coatings using X-ray diffraction, Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM), Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), and grazing angle X-ray Diffraction (XRD) will be performed. The mechanical properties and adhesion of the coating to the substrate and paints will be tested. A life-cycle cost assessment will be performed in conjunction with small business partner and industry as the laboratory scale setup and process parameters are optimized. The optimized process for different size coupons and flat vs. curved surface will be tested as part of lab-scale feasibility and scalability study.
Current chemical processes require multiple steps including cleaning, degreasing, deoxidizing and anodizing/chemical conversion coating to impart the necessary surface modification all while using chemicals with significant environmental and health hazards. Chemicals such as hexavalent chromium and phosphates in the manufacturing process pose significant environmental and health hazards. An atmospheric plasma based method is proposed that is free from chemical waste and designed to eliminate contaminants using a microwave plasma based application system. While reducing the environmental hazards and the costs are important missions of the Strategic Environmental Research and Development Program (SERDP), ultimately, new processes must sustain, if not enhance, current mission capabilities to be adopted by DOD. The team, consisting of plasma and coating/corrosion experts, designed the process to add additional corrosion protections to the surface layer. This is because the nanoscale control of the morphology and chemistry allowed by the plasma process can be used to add corrosion control mechanisms after cleaning and activation of the surface. Using multiscale modeling and simulations, an increased fundamental understanding of plasma deposition and coating growth will be used to guide the development of a robust technology platform for the DoD in different application domains.