This project presents: 1) Literature review on adhesive bonding technologies in aviation industry including surface pretreatments (pre-preparation), surface quality assurance, and surface chemistry analysis methods; and 2) Development and study of a novel solid-state electrochemical sensor for surface chemistry analysis of composite surfaces.
The performance of an adhesive bonding is greatly determined by the adherend’s surface pretreatments which could increase surface tension, surface roughness, and change surface chemistry thereby increasing bond strength and durability of polymer composite adhesive joints.
The primary goal of the surface pretreatments is to increase the surface roughness, surface energy, chemical activity, and cleanliness of the composite adherend as much as possible. Methods of surface pretreatments are reviewed in this paper, including: (1) abrasion/solvent cleaning; (2) grit blasting; (3) peel-ply; (4) tear-ply; (5) acid etching/anodizing; (6) corona discharge treatment; (7) plasma treatment; (8) flame treatment; (9) laser treatment; (10) others.
One of the critical issues in aviation industry for an adhesive bonding is to analyze the prepared composite surfaces using a nondestructive inspection (NDI) or nondestructive test (NDT) method to determine whether the quality of surfaces are ready for the following bonding processes.
Existing NDI methods include: (1) Near-Infrared; (2) Electrical potential; (3) Transient thermal NDT; (4) Electrical Impedance Spectroscopy (EIS); (5) Neutron radiography; and (6) X-ray Photoelectron Spectroscopy (XPS). However, up till now, these methods cannot provide definitive analysis or online and in-field analysis. Because of the non-availability of an on-line, in-field NDI method for surface chemistry analysis, excess or inadequate surface treatment and quality control processes may exist in the current aircraft manufacture processes incurring either a high cost or potentially weak adhesive bonds.
Electrochemical reactions usually occur in liquid electrolyte or on conducting electrode but not on non-conducting composite. Conventional electrochemical sensors involve liquid electrolytes which will cause contamination on composite surfaces when they are used for surface chemistry analysis. In this work, we explore an all solid-sate electrochemical sensor technology. Redox pairs or mediators are combined into a solid-state electrolyte, NafionTM. The mediators can pass electrons to or from the composite surfaces causing slight reduction or oxidation of the composite surfaces. The output current in response to cyclic polarization (cyclic potential scanning) is used as the indication of the surface contamination level.
The sensors included a working or sensing electrode with mediated Nafion clusters, Nafion membrane, Pt catalyzed carbon counter electrode, and Ag|AgCl reference electrode. The working electrode and counter electrode were attached to the Nafion membrane from different sides. The sensors were tested on different kinds of surfaces: original, polished, and sulfuric acid treated acrylic samples and pristine peel ply prepared, polished, and sulfuric acid treated composite laminate surface samples. The sensors showed a high sensitivity to the surface contamination. The performances and possible mechanisms related to the electrochemical sensors are discussed.
Author: Ge, Yao
Source: University of Miami