Composite materials are increasingly utilized to build high performance structures used in aerospace and numerous Department of Defense assets. Composites are usually covered in a variety of different coatings to provide protection from the elements or to impart other special properties such as radar absorption or reduced infrared signatures. When these assets require maintenance, selective removal of these coatings is required without imparting any damage or changes to the material properties of the composite. Atmospheric plasma coating removal offers a one-step process that can eliminate the need for hand sanding, traditional blast media, and toxic chemicals that are used for these maintenance activities today.

The atmospheric plasma coating removal (APCR) process uses atomic oxygen produced from compressed air to chemically etch organic coatings from substrates. In the case of many composites, an organic resin serves as the binding matrix of the composite structure. A potential concern when using atmospheric plasma coating removal on composite substrates is that the atomic oxygen or the moderate thermal energy from the plasma discharge may cause damage to this organic resin, and hence change the material properties of the composite. The goal of this project is to use APCR to selectively remove coatings over a typical graphite/epoxy composite, and study the plasma treated composite substrate material to determine what mechanical and chemical changes, if any, the plasma has imparted to the composite substrate. 

Plasma coating removal was completed using Atmospheric Plasma Solutions’ commercially available PB-7000 APCR system. A three-axis robotic gantry was used to control the plasma treatment parameters. Composite samples prepared and coated by the Air Force Research Laboratory Coating Technology Team were tested in cooperation with Atmospheric Plasma Solutions, Inc., the Air Force Research Laboratory, and Pacific Northwest National Laboratory. The suite of testing included scanning electron microscopy and energy dispersive spectroscopy imaging, Fourier transform infrared spectroscopy, three-dimensional multi-scan optical microscopy, short beam shear (SBS) mechanical testing (ASTM D2344), C-scan ultrasonic inspection, pull-off adhesion testing (ASTM D4541), and differential scanning calorimetry glass transition temperature determination (ASTM E1356).

This study found no evidence suggesting that the carbon fiber composite experiences any detrimental effects after the atmospheric plasma coating removal process. Additional mechanical testing is suggested to better characterize the outermost layers of the composite panels due to some limitations of the SBS analysis used in this study. Glass transition temperature measurements could also be further studied by using dynamic mechanical analysis (DMA) to achieve enhanced resolution of any transitions. Adhesion testing should also be studied further, as the pre-processing steps used in the coating re-application likely eliminated any potential benefits that the plasma treatment imparted to the surface after coating removal.

Lessons learned from this study will serve to inform future studies of the effects of APCR on composite substrates. Further studies should include other composite materials and the additional testing mentioned above. APCR shows great potential for composite materials, but further studies should be completed to ensure that optimal treatment parameters are used for each application.