Plasma-generated silicon oxide coatings of carbon fibres for improved bonding to mineral-based impregnation materials and concrete matrices

Abstract

To improve its bonding behaviour with cementitious materials, the surface of carbon fibre (CF) yarn was modified by depositing a SiOx-coating using a plasma-enhanced, chemical vapour deposition (PECVD) process with hexamethyldisiloxane (HMDSO) as precursor material. After applying the SiOx coating, the fibre was impregnated with a fine-grained cement suspension to process mineral-impregnated carbon fibres (MCF). After advanced hydration, the MCF were investigated in respect of their flexural strength as well as their porosity. Moreover, the MCF were implemented in a fine-grained concrete matrix and assessed with respect to their pull-out behaviour.It was found that the plasma generated SiOx coating led to a significant increase in maximum pull-out force, indicating better bonding between the modified carbon fibre yarns and the concrete matrix. This is attributed to a more pronounced pozzolanic reaction induced by the SiOx-films, promoting the local formation of calcium silicate hydrate (CSH) directly on the multifilament yarns, as supported by thermal gravimetric and microscopic analysis. However, it was also found that the deposition of the SiOx-films onto the CF resulted in more porosity in the impregnated cementitious matrix's structure, as proven by mercury intrusion porosity measurements. The higher porosity induced in turn a decrease in flexural strength for the MCF with high SiOx deposition.It can be concluded that the deposition of a SiOx film onto the CF surfaces via plasma treatment is an innovative, promising approach toward enhancing the interactions between fibre and the cementitious matrix, but the process and material parameters need to be well balanced with regard to the microstructure and mechanical properties of the MCF.