Washcoat Nanoparticle-Driven Microcrack Enlargement in a Coated Catalyst

Nanoparticles are avoided in coating particulate filters made from microcracked ceramics like aluminum titanate (AT) to prevent thermal stress damage during manufacturing despite limited understanding of the process. This study, first in this field, investigated the effect of fines penetration (D₅₀: 21 nm) from washcoating on the thermomechanical properties and long-term durability of a microcracked AT honeycomb filter substrate. Through a combination of bespoke SEM, FIB-SEM, and TEM methodologies developed, visual evidence of nanoparticles from the washcoat penetrating microcracks in the substrate has been obtained. The impact of this penetration upon the substrate's mechanical properties has been successfully evaluated using dynamic stiffness measurements through a high-temperature impulse excitation technique (IET), providing an alternative detection methodology to time-consuming and expensive microscopy measurements for product development. The IET measurements revealed that the fines penetration hinders the closing of microcracks, leading to their enlargement. These microstructural changes are not indicated by strength testing, as the coating increased the substrate strength. However, IET experiments confirmed that the effect of the penetration has been shown to stabilize after three thermal cycles, ensuring the structural stability of these substrates even after applying a coating with nanoparticles.