Key Engineering Materials Vol. 742

Abstract: Innovative lightweight structures realized by employing assembly injection molding bears high potential. Using assembly injection molding, complex shaped hybrid structures can be manufactured in a precise and fast processing step. Though, especially the interface quality of a hybrid joint is a crucial factor, which determines the overall quality of such lightweight structures. High bonding strength values were achieved between aluminum and multiple polymeric materials with double-lap joints manufactured by employing assembly injection molding. Thereby, the influence of aluminum surface pre-treatments as well as intrinsic adhesion modifications of the polymeric materials were investigated.

Abstract: In this study flax fabrics were treated with polyethylene imine (PEI) and afterwards, in combination with carbon fabrics, integrated into epoxy resin via vacuum infusion process. The influence of the 2 stacking sequences of the fabrics and 2 PEI concentrations were evaluated with regard to the mass fractions of the composite components and mechanical properties of the manufactured composites, namely, flexural and interlaminar shear properties. The results showed that the effect of the surface treatment is dependent on the stacking sequence. Namely, increase of the PEI concentration resulted in a corresponding increase of the mass fraction of the polymer matrix in the case of interchanging arrangement of flax and carbon fabrics. Further remarkable results showed that the same specimen provided the highest values of the supported maximum load after the surface treatment. Influence of the PEI treatment on the strength values with regard to the stacking sequence and polyelectrolyte concentration led to controversial results. Decrease of flexural modulus after the surface treatment was observed in the case of all samples.

Abstract: Multi-material-design offers high potential for weight saving and optimization of engineering structures but inherits challenges as well, especially robust joining methods and long-term properties of hybrid structures. The application of joining techniques like ultrasonic welding allows a very efficient design of multi-material-components to enable further use of material specific advantages and are superior concerning mechanical properties.The Institute of Materials Science and Engineering of the University of Kaiserslautern (WKK) has a long-time experience on ultrasonic welding of dissimilar materials, for example different kinds of CFRP, light metals, steels or even glasses and ceramics. The mechanical properties are mostly optimized by using ideal process parameters, determined through statistical test planning methods.This gained knowledge is now to be transferred to application in aviation industry in cooperation with CTC GmbH and Airbus Operations GmbH. Therefore aircraft-related materials are joined by ultrasonic welding. The applied process parameters are recorded and analyzed in detail to be interlinked with the resulting mechanical properties of the hybrid joints. Aircraft derived multi-material demonstrators will be designed, manufactured and characterized with respect to their monotonic and fatigue properties as well as their resistance to aging.

Abstract: To join components of fiber reinforced polymers (FRP) to metals or FRP, too, bolted joints are used regularly. Reliability, affordability, and disassembly are the drivers for dissemination of this joining technique; despite some disadvantages. For bolted joints of metals it is well known, that a preloading of the bolts increases the efficiency drastically. Therefore, preloaded screws are the standard in mechanical engineering. Prerequisite is a load transfer by friction of the joining partners induced by a preload of the bolt. However, when joining FRP components a preloading of the bolts is usually not considered since a force release is afraid due to creeping of the polymeric matrix. Furthermore, the load transfer by friction of FRP is not investigated sufficiently. The limiting parameters of preloaded bolted joints of different FRP materials are investigated here. Particularly these are the allowed contact pressure at the bolt heads or washers, the coefficient of friction in the joining zone, and the release behavior of the preloading. The load transfer capability of bolted joints under optimized conditions is presented.

Abstract: Intrinsic hybrid laminates are well established since many years in aerospace engineering, e.g. Glass Laminate Aluminium Reinforced Epoxy (GLARE) is widely used as a substitute for aluminium sheets of the outer shell of modern aircrafts. The reduction of density and an increased stiffness by compounding glass fiber and aluminium makes GLARE advantageous. Driven by environmental protection acts and the need for lightweight design material compounds attract more awareness in the automotive engineering as well. Functional components like chassis springs are well predestined for the application of glass fiber reinforced plastics (GFRP). Therefore, an intrinsic hybrid made up by GFRP and a high strength steel has recently been developed and characterized. This investigation sets the focus on the interface between GFRP and steel. Double cantilever beam tests (mode I) and shear tests (mode II) are conducted in order to measure the energy release rate and the shear strength of the considered interface. A variety of surface treatments of steel layer has been characterized by this approach. The results show up that good adhesion can be achieved by silane and titanium dioxide primers, however, the variation within the data is significantly higher than for other surface treatment variants. Furthermore, the increase of the energy release rate by fiber bridging effects is considered as well and an approach for its quantitative description by a power law is presented.

Abstract: Boron doped diamond coatings are used in electrochemistry, due to the high overvoltage for oxygen generation. Niobium is often used as bulk material, but also diamond deposition on titanium was demonstrated. For metallic bulk materials corrosion can take place in case of defects in the diamond coating. This problem can be avoided by using carbon based substrates. Diamond deposition on carbon substrates is difficult, because atomic Hydrogen needed for diamond growth attacks graphitic and amorphous carbon. These reactions have the effects that carbon in the substrate is etched and the amount of atomic hydrogen needed for diamond growth is reduced. To reduce the carbon etching on the substrate, the duration till diamond layer formation should be short. By controlling the diamond deposition conditions, boron addition and seeding with diamond prior to deposition, the formation of diamond coatings on carbon fibre composites (CFC) is possible. Electrochemical measurements of the boron doped diamond coatings verified the excellent electrochemical properties of the samples, e.g. good electrical conductivity, high overvoltage for oxygen and hydrogen but also chemical inertness.

Abstract: Steel tools, which are used in industrial high-throughput processes like injection molding, are susceptible for wear and corrosion due to rapid cyclic temperature and pressure fluctuation as well as the use of abrasive polymers. For the protection of tool surfaces high quality ceramic thin films can be applied by metal-organic chemical vapor deposition (MOCVD). In addition to protective properties ceramic materials like yttria-stabilized zirconia (YSZ) are able to thermally insulate tool surfaces providing a more precise temperature regulation with intent to avoid the formation of surface flaws, e.g. weld lines, in the later plastic parts. At the same time it enables the shortening of cycle times as well as the decrease of energy demands during the molding process. In this work we demonstrate the fabrication of zirconia based thin films and multilayer systems on steel tools with complex 3D surfaces via MOCVD using metal acetylacetonates as precursor materials. Coating development was carried out by measuring the film thicknesses at different parameter settings. The usage of autonomous liquid flow controllers enables the formation of multilayer systems as well as the control of crystallinity by addition of different dopants to the material. For process development substrates were engineered according to tool geometry containing typical cavities and defined cracks with aspect ratios up to 1:60. That application enables the proof of conformity and the verification of homogeneous film thickness distribution. Exploitation of these results offers the coating of tools, which are tested regarding their desired properties by industrial project partners under production conditions.

Abstract: Due to the restriction of passive layers containing Cr⁶⁺ [1], which were characterized by excellent corrosion protection due to their self-healing effect for scratches on metal surfaces, current corrosion protection systems consist of chromium (III) -containing thick layer passivation. Due to their lower hardness, current corrosion protection systems are susceptible to mechanical stress. This is particularly critical at barrel plating of screws, rivets etc. where the manufacturing process leads to damages of the corrosion protection layer and consequently to reduced corrosion resistance.To counter this problem, we point out one approach to install hard particles into the passivation layer. The entrapment of the hard particles into the passivation is detected by Glow Discharge Optical Emission Spectrometry. Comparative investigations in the corrosion chamber prove the improvement of the corrosion protection of steel parts with passivation layers containing hard particles.

Abstract: We investigate the effects of static and dynamic anodic oxidation treatment on the surface chemical composition and functionality of carbon fibers. During static treatment, the electrolytic surface oxidation process is performed on a spatially fixed carbon fiber bundle, while in the dynamic process a moving, continuous carbon fiber tow is oxidized. In both treatment modes electrolytic current density and treatment time were varied. Surface chemical composition and functionality of the resulting carbon fibers were analyzed by x-ray photoelectron spectroscopy. A good agreement between the chemical composition and the functionality of fibers from static and dynamic anodic oxidation treatment is found. This suggests that results from static fiber treatment in a variable, easy to handle laboratory setup can be applied to dynamic anodic oxidation process conditions on a large scale.

Abstract: The topography of a surface consists of structures of different length scales. The surface roughness caused by these structures plays a decisive role in interfacial properties. Atomic Force Microscopy (AFM) can be applied to measure the surface topography with great accuracy and thus facilitates roughness quantification. Here, however, the data reduction poses a challenge. In a conventional approach, surface roughness parameters are evaluated based on averaging height differences, which leads to values dominated by the largest height differences of the surface topography. To quantify contributions of smaller structures to the roughness, a previous study presented a tunable local background correction, which eliminates structures on a larger than selected scale. Therefore, this method only considers surface structures smaller than the chosen scale. A different approach to quantify surface roughness on all length scales covered by AFM measurements uses Fourier transformation of the surface topography to calculate the power spectral density, which describes the amplitudes of different contributing spatial frequencies.In the current study, a new approach based on power spectral density is used to quantify surface roughness parameters as a function of the length scale of contributions to the surface topography. This procedure allows a comprehensive characterization of surface roughness and an intuitive comparison of different surfaces.The usefulness of this method and its compatibility to local background correction is demonstrated by analyzing several commercially available carbon fibers with and without different fiber surface treatments.