Nanoindentation and adhesion of sol-gel-derived hard coatings on polyester
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MATERIALS RESEARCH
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Nanoindentation and adhesion of sol-gel-derived hard coatings on polyester C.M. Chan, G.Z. Cao,a) H. Fong, and M. Sarikaya Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195
T. Robinson and L. Nelson Korry Electronics, Co., Seattle, Washington 98109 (Received 2 August 1999; accepted 29 October 1999)
We investigated sol-gel-derived silica-based hard coatings on modified polyester substrates. The silica network was modified by incorporating an organic component and adding transition metal oxides. These modifications resulted in tailored thermal, optical, and mechanical properties of the coatings. Various low-temperature densification techniques were studied including sol-preparation procedure, enhanced solvent evaporation, ultraviolet irradiation, and low-temperature heating (below 150 °C). Oxygen plasma etching was applied to improve the adhesion of the sol-gel coatings on the plastic surface. Nanoindentation analysis revealed that the coatings have a surface hardness up to 2.5 ± 0.27 GPa and an elastic modulus up to 13.6 ± 0.4 GPa, approximately an order of magnitude higher than that of the plastic surface.
I. INTRODUCTION
The many desired features of plastics such as light weight, formability, high impact strength, and ductility have allowed them to be widely used in many optical applications. However, plastics have poor abrasion resistance, which means they are readily scratched, leading to decreased optical transparency. One approach is to apply abrasion-resistant coatings so as to improve their surface mechanical properties such as abrasion resistance. Vapor-phase techniques, such as evaporation, sputtering, and chemical vapor deposition (CVD) (e.g., Refs. 1–3), produce high-quality films with high density and high mechanical properties, but they are expensive, require sophisticated equipment, and often require substrates to be maintained at high temperatures. In addition, uniform coatings of complex-shaped substrates are difficult with such line-of-sight vacuum techniques. Solution deposition is an alternative approach for applying oxide films to temperature-sensitive substrates. For example, the biomimetic synthesis (e.g., Refs 4–8) has two important characteristics: (i) control of solution conditions, including ion concentration (supersaturation levels), pH, and temperature; and (ii) the use of functionalized interfaces to promote mineralization at the substrate surface. By controlling the surface energy of the substrate along with the solution supersaturation, depoa)
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J. Mater. Res., Vol. 15, No. 1, Jan 2000 Downloaded: 16 Jul 2014
sition conditions can be established that favor heterogeneous growth of the thin film on the substrate without bulk particle precipitation (homogeneous growth). The major drawback of this approach is the need of functionalized surface. Langmuir films and self-assembled mon
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