Damage resistance and anticorrosion properties of nanosilica-filled epoxy-resin composite coatings
Introduction
Polymer composites are subjected in many applications as adhesives and matrix resins, epoxy resin being one of the most common polymer matrices that are widely used to protect steel reinforcements in concrete structures [1], [2]. It has excellent mechanical properties, chemical resistance, good electrical insulating properties and strong adhesion to heterogeneous substrates. Epoxy coatings not only reduce the corrosion of a metallic substrate by providing an effective physical barrier between the metal and the environment containing an aggressive species, such as an enhanced chloride-ion concentration, O2 or H+, they also serve as a reservoir for corrosion inhibitors that help the steel surface to resist attack from aggressive species.
The practical use of epoxy coatings in industry, however, is seriously limited by poor impact resistance and stress cracking resistance due to a highly cross-linked structure [3] as well as by susceptibility to damage by surface abrasion and wear [4]. To overcome this drawback, researchers have made numerous attempts to improve the properties of epoxy by adding various nanofillers [5], [6], [7], [8], [9], [10], [11], [12], [13]. They studied the favourable effects of particle size, volume fraction and the quality of the dispersion on the mechanical response of the polymer composites [6], [14], [15], [16], [17], [18], [19], [20], [21]. In addition, a lot of attention has been paid to epoxy coatings containing nanoparticles that show a significantly improved barrier performance for corrosion protection by decreasing the porosity [22].
There is, however, still a lack of knowledge of the fracture mechanisms in particulate (i.e., silica)/epoxy composite coatings on various substrates under applied load. As commonly used surface-sensitive diagnostic methods like scanning electron microscopy (SEM) are insufficient tools for the detection of all possible failure sources and for establishing a direct correlation between the applied strain and delamination, a different approach is needed to gain a full insight into crack formation, progress and delamination. Acoustic emission (AE) analysis during tensile, fatigue or three-point bending provides a suitable method to obtain the strain dependence of crack formation and growth. So far AE studies mostly investigated damage resistance of fibre (i.e., carbon) or textile/epoxy composites and metallic coatings on metallic substrates. Here AE signals were used to determine the quality of coatings [23] and to relate the spectral signature of AE signals to a specific type of failure such as matrix cracking, fibre breakage, debonding, fibre pull-out or delamination [24].
In this study we evaluated the influence of incorporation of silica nanoparticles in an epoxy matrix on the damage resistance of 50 μm thick silica/epoxy coatings adsorbed on duplex stainless steel (DSS 2205) substrate. Mechanical properties of coatings were studied with Vickers hardness test. To analyze the crack formation and delamination of the coatings, three-point-bending tests were performed together with acoustic emission analysis to detect transient stress waves propagating in the coating as a consequence of bending deformation. Contact angle/surface energy measurements gave us insight into the surface and wetting characteristics of silica/epoxy coatings in comparison to pure epoxy coatings. Finally, the anticorrosion behaviour of the coatings was evaluated with potentiodynamic measurements.
Section snippets
Materials
Duplex stainless steel DSS 2205 (22.7% Cr, 5.7% Ni, 2.57% Mo, 1.37% Mn, 0.38% Si, 0.032% P, 0.03% C, 0.001% S in mass fraction) was used as a substrate.
Epoxy resin (Epikote 828LVEL, Momentive Specialty Chemicals B.V.) was mixed with a hardener 1,2-Diaminocyclohexane (Dytek DCH-99, Invista Nederland B.V.) in the ratio 100: 15.2 wt.% and used as the matrix in the composite.
Composite reinforcing silica (SiO2) nanoparticles with a mean diameter of 130 nm were synthesized following the
Vickers hardness test
Typical Vickers indents in 50-μm-thick pure epoxy and 130-nm silica/epoxy coating under a 0.5 kg load are presented in Fig. 3. It is clear that the indents in the epoxy coating (Fig. 3a) are consistently larger than those in the silica/epoxy coating (Fig. 3b) under the same loading conditions. In addition, a star-shaped Vickers impression was observed in the silica/epoxy coating, whereas the pure epoxy coating exhibits a pyramidal impression. The former also appears to exhibit a much greater
Conclusions
130-nm silica particles were homogeneously dispersed in an epoxy matrix at a concentration of 2 wt.% and the mixture was successfully adsorbed on duplex stainless steel of type DSS 2205 in a form of a 50-μm coating. The epoxy coating modified with silica nanoparticles showed a significantly enhanced hardness of ∼30% compared to the pure epoxy coating. However, a ∼4% increase in the fracture toughness upon adding silica into the epoxy coating was not very pronounced.
We have outlined an
Acknowledgements
The authors gratefully acknowledge The Research Foundation – Flanders (FWO) for the financial support enabling research work at K. U. Leuven. This work was partly carried out within the framework of the Slovenian programme P2-0132, “Fizika in kemija površin kovinskih materialov” of the Slovenian Research Agency, whose support is gratefully acknowledged by M. Conradi and A. Kocijan. I. Verpoest is holder of the Toray Chair in Composite Materials at K. U. Leuven.
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