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Manufacturing techniques and their impact on the properties, macro- and microstructure of carbon-bonded alumina foamsThursday (08.10.2020) 09:50 - 10:10
Carbon-bonded alumina foams are typically used as filters for the filtration of steel melts due to their low wettability and their good resistance towards thermal shock and corrosion. Therefore, the continuous improvement of their manufacturing quality is of high interest. For this purpose, different manufacturing routes for the preparation of carbon-bonded alumina foams by means of Schwartzwalder’s replica technique were investigated. Al2O3-C-slurries were prepared by either high-shear mixing or ball milling to evaluate the influence of resulting slurry qualities on the properties of the manufactured foam. For the application of the slurry onto the polyurethane template, centrifugation, rolling or spray coating were conducted. The result were foam samples with two coating layers, having a variety of combinations of ball milled or shear mixed slurries applied by aforementioned techniques. All foam samples were heat-treated in a coke-filled steel retort for 3 hours at 800°C.
Subsequently, the mechanical properties, macrostructure and microstructure of the samples were evaluated by means of cold crushing tests, µCT-analysis, SEM and mercury intrusion porosimetry, respectively.
The largest influence on the microstructure was seen in the mixing step. While coatings prepared by shear mixing showed a coarse microstructure with larger material pores, all of the ball milled layers had a finer structure with significantly smaller material pores. The different sizes of the material pores result from the differently milled grains of the tar pitch binder, which melt during the heat treatment, the ball-milled grains being significantly smaller.
The highest mechanical strength was determined for samples that had received a second coating of ball milled slurry by means of centrifugation, pointing out the higher homogeneity and finer structure of this coating, while the lowest mechanical strength was measured for samples exclusively manufactured by spray-coating. This was correlated to a higher inhomogeneity of the spray-coating process, frequently resulting in thinner struts in the foam’s center.