BS in Materials Engineering
Materials Engineering Department
The improved thermal properties of metal-substrate catalytic converters enable effective catalysis sooner after ignition than traditional ceramic monoliths, resulting in less harmful emissions. ACAT Global, a manufacturer of stainless steel catalytic converters, uses ferritic stainless steel substrates for low temperature catalytic converters for diesel automobiles. Production begins with an oxidizing heat treatment to increase surface roughness. The success of the second production step, the application of catalyst-containing ceramic slurry (washcoat), depends on the surface roughness that results from oxidation. Formation of the primary oxide of interest, alumina (Al2O3), is controlled by the stainless steel foil composition, oxidation temperature, and oxidation time. ACAT intends to use a specific foil, so composition will be constant. Temperature will also be constant; 900°C is most likely to result in growth of rough alumina since diffusion of aluminum to the surface is faster than the diffusion of oxygen into the material at that temperature. This project focuses on the effect of the last parameter, treatment time, on oxide morphology and roughness. Samples were removed at hour intervals of a six hour 900°C hold. Qualitative assessment of surface morphology was performed using SEM, revealing that “rough” oxide resulted from treatment times exceeding three hours at temperature. Additionally, SEM images revealed an increase in the frequency and magnitude of surface features with increasing treatment time and a decrease in the frequency and size of dark regions of smooth oxide. Profilometry was used to collect Ra roughness values of the rough oxide samples. Average profile roughness increased from 217 nm for the untreated foil to a maximum value of 564 nm for the five hour sample. The decrease in Ra from five to six hour samples was unexpected and could be a consequence of limited vertical resolution of the profilometer settings. Oxidation times of four, five, and six hours appear to have the rough morphology preferred for washcoat adhesion. Specific interactions of the washcoat with features of various heights and lateral spacings should be assessed to determine ideal oxidation time.