When solgel hybrid coatings are applied strong van

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When sol–gel hybrid coatings are applied, strong van der Waals bonds between films and metal surface are formed at first and are transformed to stable covalent bonds during drying stage of the films. The following studies show the formation of chemical bonds between the sol–gel layer and different metal surfaces. The surface of aluminum forms oxide layer with OH groups. The thickness and density of OH groups depend on metal alloy composition and pre-treatment process. Al ð OH Þ 3 þ RSi ð OH Þ 3 ! ð HO Þ 2 Al ² O ² Si ð OH Þ 2 R þ H 2 O Organosilane sols can react with OH groups to form chemical bonds. Si–O–Al linkage is responsible for strong interaction of the sol–gel layer with the metal surface [ 6 8 ]. The sol–gel coating is formed from a mixture of zirco- nium isopropoxide and R 0 Si(OR) 3 and is applied on Ti substrate. Chemical bonds Zr–O–Ti form between the thin film and Ti substrate [ 9 ]. Hybrid coatings prepared from aminopropyltriethox- ysilane (APS) and methyltriethoxysilane(MTEOS) are applied on electrogalvanized steel. The coatings adhere to the substrate via the Si–O–Zn linkages [ 10 ]. The improvement of corrosion protection of epoxy resin–silica hybrid coatings results from enhancement of the adhesion strength of the hybrid coatings on CRS cou- pons, which may be attributed to the formation of Fe–O–Si covalent bond at the interface of coating/CRS system [ 11 ]. Sol–gel hybrid coatings with phosphonate functional- ities show an improved corrosion protection as compared with pure silica sol–gel coatings due to the strong chemical bonding of phosphonate groups to the metal substrate [ 12 ]. Hybrid coatings were synthesized by copolymerization of epoxy-siloxane and titanium or zirconium alkoxides. The sol–gel coating system doped with tris (trimethylsilyl)- phosphate revealed improved corrosion protection of the magnesium alloy due to formation of hydrolytically stable Mg–O–P chemical bonds [ 13 ]. All these above studies indicate that the formation of hydrolytically stable M–O–P or M–O–Si chemical bonds is responsible for strong interaction and good adhesion per- formance of the sol–gel layer with the metal surface. Since silane sols offer very good adhesion performance together with high environmental compatibility, silane pre- treatments are also widely used as adhesion promoters for organic coated metals to replace the traditional metal sur- face pre-treatment (for example, chromate conversion coating.), which are considered toxic and hazardous to the environment. Sol mixtures of GPS ( c -glycidoxypropyltrimethox- ysilane) and TEOS were used as adhesion promoters for pre-treatments of copper [ 14 ]. Pre-treatments by ZrO 2 sols prepared from Zr(OBu- n ) 4 are applied on aluminium alloys. The ZrO 2 films were painted with a polyester resin. Thermal aging cycles evi- dence good adhesion properties for ZrO 2 pre-treatments [ 15 ].
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