Surface Modification of Glass Fibers by Oxidized Plasma Coatings to Improve Interfacial Shear Strength in GF/Polyester Composites V. Cech , 1 A. Knob, 1 T. Lasota, 2 J. Lukes, 3 L.T. Drzal 4 1 Institute of Materials Chemistry, Faculty of Chemistry, Brno University of Technology, Purkynova 118, CZ-612 00, Brno, Czech Republic 2 Honeywell spol.s.r.o., Turanka 100, CZ-627 00, Brno, Czech Republic 3 Department of Mechanics, Biomechanics and Mechatronics, Czech Technical University in Prague, Technicka 4, CZ-166 07 Prague 6, Czech Republic 4 Composite Materials and Structures Center, Michigan State University, East Lansing, Michigan 48824-1226 Plasma-polymerized films (interlayers) of tetravinylsi- lane in mixture with oxygen gas (oxygen fraction 0-0.71) were coated on glass fibers (GF) used as reinforcements in GF/polyester composite. Oxygen atoms of increased concentration (0-18 at.%) were partly incorporated into the plasma polymer network, forming SiAOAC/CAOAC bonding species and partly forming side polar (hydroxyl, carbonyl) groups with enhanced oxygen fraction. The amount of oxygen in plasma coatings influenced the Young’s modulus, interfacial adhesion, and surface free energy of the interlayer. To determine the interfacial shear strength, a microindentation test was imple- mented for individual glass fibers on a cross-section of GF/polyester composite. The interfacial shear strength for oxidized plasma coatings was up to 21% higher than that for the non-oxidized interlayer, indicating a direct chemical effect of oxygen atoms on interphase proper- ties. The interphase shear failure was controlled by the shear strength at the interlayer/fiber interface as follows from experimental and model data. POLYM. COMPOS., 00:000–000, 2017. V C 2017 Society of Plastics Engineers INTRODUCTION Fiber-reinforced polymer composites combine the high strength of stiff reinforcements with the plastic polymer matrix allowing them to be formed into complex shapes. However, composite performance is controlled not only by the mechanical properties of these constituents but also by the interface (two-dimensional contact area) between the fiber and the matrix. It is suspected that interfaces are the most important parts of composites (Yann Le Petitcorps, Interfacial Phenomena in Composite Materials, Arcachon 2001). In most cases, the fiber and polymer matrix are materials of distinct chemical and physical properties, and the Young’s modulus of the fiber differs markedly from that of the matrix. Adhesion bonding between the fiber sur- face and the matrix must be sufficient to ensure stress trans- fer from the matrix to the load-bearing fiber. However, improvement in interface bond strength at bare fibers can reduce the impact fracture toughness of reinforced compo- sites. Fiber-coating appears to be one of the most effective methods of achieving both high strength and high fracture toughness, when an appropriate interlayer (coating) mate- rial is chosen [1, 2]. In the case of glass fibers, sizing (func- tional coating) is deposited by a wet chemical process to improve compatibility, wettability, and chemical bonding between the fiber and the polymer matrix. Silane coupling agent, chemically reactive with both the fiber and the matrix, is responsible for chemical bonding as a crucial con- stituent of sizing [3]. An interphase region is formed around the fiber surface including the polysiloxane interlayer inter- diffused with polymer matrix [3, 4]. The interphase is a very complex (three-dimensional) material formation; the composition, structure, and properties vary across the inter- mediate region (from less than 100 nm to several microns thick [4–7]) as the interphase is of varying thickness and uniformity [8]. Another problem is that molecules of silane coupling agent tend to self-condense, forming siloxane oligomers rather than complete siloxane bonding with the Correspondence to: V. Cech; e-mail: cech@fch.vut.cz Contract grant sponsor: Czech Science Foundation; contract grant number: 16-09161S. DOI 10.1002/pc.24573 Published online in Wiley Online Library (wileyonlinelibrary.com). V C 2017 Society of Plastics Engineers POLYMER COMPOSITES—2017