Kay Y. Blohowiak

Testing and evaluation of nonchromated coating systems for aerospace applications

Abstract The advanced corrosion resistant aircraft coatings program (ACRAC) is part of the Air Force strategy to improve performance and reduce environmental impact of coatings used on Air Force weapon systems. The program addresses the Air Force near and mid-term strategies to eliminate chromate corrosion inhibitors and reduce steps in the outer mold line coating process. Evaluation of the coating process (surface preparation, conversion coating, primer, topcoat) as a system is a key feature of the ACRAC program. Results to date indicate that the current-state-of-the-art nonchromated coating systems are significantly less effective than those with chromate. A chromate conversion coating is required for the nonchromate primer system to meet minimum requirements. Sol–gel-process based conversion coatings can replace chromate conversion coatings provided a chromated primer is used. Several approaches to incorporating inhibitors into sol–gel coatings are discussed. Electrochemical methods for testing coating performance are discussed and a new procedure based on impedance spectroscopy for evaluating active damage repair is presented.

Effect of Surface Morphology on Crack Growth at a Sol-Gel Reinforced Epoxy/Aluminum Interface

The Boeing sol-gel conversion coating (Boegel-EPII), derived from an acid-catalyzed aqueous solution of organofunctional silane and zirconium alkoxide precursors, is being used as an adhesion promoter for adhesive bonding and painting applications in the aerospace industry. A unique advantage of the sol-gel process is that strong and durable bonds are produced without the hazardous chemical usage and rinse-water requirements of conventional anodizing or etching processes. In this study, a fracture mechanics method was used to investigate the adhesion properties of sol-gel-reinforced epoxy/aluminum joints. The Hugh Brown asymmetric double cantilever beam (ADCB) wedge test was employed, which allowed the measurements of the critical energy-release rate, subcritical crack-growth kinetics, and threshold energy-release rate on a single sample in a reasonably short period of time. These experiments were carried out with aluminum substrates on which the surface morphology was systematically varied by polishing, sanding, grit-blasting, and chemical etching. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to identify the locus of failure. The surface morphology of the substrates was characterized with SEM, optical profilometry, and spreading kinetics. The macrorough structures drive the crack to within a thin epoxy layer close to the polymer/metal interface, which enhances the initial strength of the sol-gel-reinforced interface. The microroughness of the substrate is, however, more effective than the macroroughness in enhancing the durability. Lastly, an attempt has been made to correlate the energy-release rate with the fractal dimension for sol-gel-reinforced joints with macrorough substrates.

Fracture behavior of an epoxy/aluminum interface reinforced by sol–gel coatings

The strengths of epoxy/aluminum joints reinforced with a zirconium-silicon based sol–gel adhesion promoter were investigated using an ADCB (Asymmetric Double Cantilever Beam) wedge test. The fracture energies and loci of failure of these joints were shown to depend upon the mixity of the normal and shear modes of stress acting at the crack. The ADCB geometry enabled the crack to propagate along the epoxy/aluminum interfaces so that the effect of surface pretreatment and the processing conditions of the adhesion promoter on adhesion strength could be directly evaluated. The dry strength of these joints depends on the thickness of the sol–gel film derived from different concentrations of the precursors. Thinner films are more fully crosslinked and thus give higher adhesion strengths than those obtained with thicker films. The differences in the wet strengths of the sol–gel reinforced joints for various surface pretreatments suggest that the sol–gel films are subject to moisture degradation with certain surface pretreatments. The loci of failure of many of these joints alternate between the sol–gel/aluminum and epoxy/sol–gel interfaces. This behavior is similar to that observed more generally in adhesively-bonded joints tested in DCB (Double Cantilever Beam) geometry. The brittle versus ductile behavior associated with the failure process reveals important information about how the sol–gel films affect the adhesion strength.

Effect of Processing Conditions on Adhesion Performance of a Sol-Gel Reinforced Epoxy/Aluminum Interface

The Boeing sol–gel process (Boegel-EPII) is a surface preparation method for metallic substrates for adhesive bonding and painting applications. This paper describes an investigation into the effect of processing conditions on adhesion strength and durability of a sol–gel reinforced, rubber toughened epoxy/aluminum joint. Using an asymmetric double cantilever beam (ADCB) wedge test, the adhesion performance of the sol–gel reinforced epoxy/aluminum joint in a humid environment was measured as a function of sol–gel processing conditions. The sol–gel drying time, concentration and drying humidity all have an effect on adhesion performance. Prolonged drying led to a decrease in fracture energies. The critical and threshold fracture energies show different trends as sol–gel concentration varies, and better adhesion performance was observed for sol–gel dried at higher humidity compared to lower humidity. The failure modes and mechanisms were studied by XPS and SEM. Analysis of locus of failure revealed that the observed trends for adhesion performance can be explained in terms of interdiffusion of the sol–gel film and epoxy. The diffusion of the epoxy into the sol–gel layer is hypothesized to strongly depend on the degree of condensation of the sol–gel film and is directly affected by the sol–gel processing conditions.