Thermal durability of porous silicon carbide ceramics

Ever tightening automotive emissions legislation has led to include particulate matter (PM) limits, by both mass and number. The most recognised abatement method today is the use of particulate filter substrates, primarily made of specialist porous engineering ceramics. Although porous ceramics make highly effective filters their thermochemical durability over a vehicle’s life time is paramount. This thesis focuses on porous Silicon Carbide (SiC) due to its current market dominance and quantifies the development and application of several methods to assess substrate thermochemical durability.

This work presents an extensive study on the strength of porous SiC, a novel thermal shock test method using infra-red (IR) irradiation with temperature gradient measurement to the sub millimetre level. Furthermore, high temperature and ash corrosion characteristics were also quantified both with and without thermal shock treatments.

The onset of thermal shock damage of SiC was found to be 60°C/mm this was attributable to isolated grain boundary fractures acting as a pore joining mechanism increasing overall flaw size. The critical temperature difference was found to be 97°C/mm at which complete wall strength was lost. A crack prediction model was developed using Monte-Carlo methods. This allowed the prediction of crack formation on a microstructural grain to grain level, accounting for material variation. This predicted the average strength reduction to within 6% of experimental values.

High temperature ageing showed SiC to reduce in strength at 1000°C due to increased microcrack formation as a result of SiO2 phase transformation. Temperatures from 1200°C to 1400°C showed strength to be fully restored due to SiO2 growth closing and blunting of microcracks. High temperature treatments <1200°C post thermal shock, showed small fractures could be healed. Ash ageing showed to start attacking SiC at 1200°C however at lower temperatures ≤1000°C could be beneficial to shock durability.