Effect of Surface Modification of Carbon Fibers on the Formation of SiO2 Coatings

Abstract

Surface modification of carbon fiber with 1,1-dihydroperoxycyclohexane (DHPC) before application of SiO₂ barrier coating was studied. IR spectroscopy showed presence of the COOH carboxyl groups on the surface of modified carbon fibers. The modified surface has less hydrophobicity due to the presence of polar groups, which contribute to a change in the wettability of the fiber by the sol–gel solution and the adhesion of the oxide coating to the fiber surface. The quality and thickness of the oxide coatings were assessed by scanning electron microscopy. The use of DHPC leads to oxidation of the carbon fiber, making surface longitudinal stripes deeper. To determine the effect of DHPC modification on the properties of the fiber surface, a SiO₂ coating was applied by immersion in a tetraethoxysilane sol. DHPC modification leads to the formation of a uniform SiO₂ coating on the fiber surface. After the heat treatment, the coating thickness on the DHPC–modified fibers is 130 ± 30 nm. Analysis of the IR spectra showed that the addition of DHPC to the sol leads to an increase in the intensity of the Si–O–Si, Si–OH, and OH absorption bands, which are characteristic of the hydrolysis and polycondensation products of tetraethoxysilane. In the wavenumber range of 400–1200 cm^–1, the Si–O–Si fragments form, which is confirmed by the presence of bands of Si–O– deformation bond at ν = 881 and 442 cm^–1. The introduction of DHPC into the sol promotes the formation of a uniform coating on the carbon fibers. The coating thickness on the initial fibers reaches 410±100 nm at room temperature, decreasing to 190±60 nm after heating. The fiber and sol modification contribute to formation of a uniform coating with a thickness of 200 ± 70 nm and 130 ± 30 nm before and after the heating, respectively. DHPC modification of the fiber surface and tetraethoxysilane sol promotes the formation of continuous, uniform, and quality SiO₂ coatings on the surface of carbon fibers.