Gas-filled spheres of aluminum oxide
Gas-filled spheres of aluminum oxide are key to the success of the new coating. Source: Fraunhofer Institute for Chemical Technology
“These spheres are hollow and filled with gas,” notes coatings expert Vladislav Kolarik from ICT’s Energetic Systems department. “When the outer side of a part is exposed to temperatures of 1,000°C, these gas-filled spheres reduce temperatures on the part’s inner side to under 600°C.”Kolarik points out that as chemical and petrochemical reactors, gas and steam turbines used for energy generation, combustion chambers, waste incinerator generators and temperature sensors all are subjected to temperatures of up to 1,000°C, there’s no shortage of demand for thermal protection.Most remarkable, he says, is that the heat-insulating layer from hollow aluminum oxide spheres is a very economical solution when compared with conventional thermal barrier techniques — many of which rely on costly ceramic materials.The scientists adapted a process originally designed to protect metallic components from oxidation. “We’ve optimized the technique so that the coat not only retains its oxidation protection, but furthermore protects against heat,” explains Kolarik. The basic coating layer forms when the aluminum particles interact with the metallic component. This is done by depositing aluminum powder on the surface of the metal and heating it to a suitable temperature over several hours. The result is an aluminum-rich topcoat made from the hollow aluminum oxide spheres on the component’s surface that protects against oxidation at high temperature. “Up to now, it never occurred to anyone to use these spheres to produce another coating layer — they were just a waste product,” adds Kolarik.Now the scientists have refined the process so they can produce both coating layers in the required thickness by taking aluminum particles and mixing them with a viscous liquid bonding agent. This yields a substance similar to a paint or slurry, which the scientists then manually paint, spray or brush onto the metallic component. “All that’s left is to add a fair bit of heat,” says Kolarik. But it’s all easier said than done: Kolarik and his team have had to precisely fine-tune the size and size distribution of the aluminum particles, the temperature and duration of the heating stage and the viscosity of bonding agents. “Just like a master chef, the first job was to come up with a winning recipe,” he notes.“We’re currently in the process of putting the findings from the EU [European Union]-funded PARTICOAT project into practice. This involves coating bigger and bigger components without exceeding the temperature limits for each application area. At the same time we’re trying out techniques to automate the whole coating process,” he concludes.The EU’s Framework Program 7 for Research and Technological development supports the PARTICOAT project under its nanoscience, nanotechnologies and new materials production technologies (NMP) segment. In a separate, materials-related development, researchers at the Fraunhofer Institute for Structural Durability and Reliability, Darmstadt, Germany, have developed a versatile bonding technique for lightweight components such as the carbon-fiber-reinforced plastics (CFRPs) used in some process pipeline applications. The technique relies upon new dual core adhesives that harden in two phases. Typical adhesives of this sort undergo an initial phase of hardening triggered by humidity, heat, anaerobic conditions or UV light. Another activator then completes the process. The result is a bond that provides a constant elasticity and the same rigidity at every point along the bonded surfaces. What’s new here is that the first phase, initiated with heat, gives a soft, flexible product. However, the second phase, initiated by UV light, can be contained to very specific areas. Here, the adhesive’s polymer chains crosslink where it is exposed to UV light, creating a localized area with greater rigidity. Thanks to this gradient of rigidity, bond life is extended and CFRP products suffer less in terms of vibration damage. The bond also acts as a damper, reducing noise.
