Microstructure-Based Damage Modelling of Plasma Sprayed Ceramic Coatings in SOFC-Layer Systems

Solid oxide fuel cells (SOFC) have developed to promising energy conversion devices for many applications. However, operation conditions under elevated temperatures, especially high temperature regimes and repeated temperature changes imply extreme demands for the used materials, questioning durability and reliability of such devices.

The scrutinized SOFC isolation layer system as shown in Figure 1 consists of a staple of ceramic layers sprayed on a steel substrate (interconnector). The main focus in this project is investigating application induced failure mechanisms under service conditions.

A newly developed ceramic of Mg-spinel type offers self-healing capabilities (Fig. 2). The self-healing effects provide fracture annihilating potential and promising mechanical capabilities. The manufacturing process of these ceramics causes a complex microstructure with structural defects such as pores or debonded splats. Splats are rapidly solidified molten ceramic particles on a substrate. Residual stresses, caused by quenching stresses during manufacturing as well as thermo-mechanical mismatch stresses due to temperature change may lead to crack initiation in the ceramic and subsequent mechanical weakening effects, respectively.

We present a microstructure-based approach to simulate the influence of thermal cycling on the ceramic coating stress evolution. This stress evolution and, thus, the damage behaviour is affected by the above mentioned influences. The numerical model represents a SOFC system which consists of a ceramic layer on a steel substrate and a thermally grown oxide (TGO) interlayer (Fig. 3). Assuming fracture mode I, the load case normal to the crack plane, the focus is set on the evolution of principal stresses in the ceramic microstructure.

The results of micromechanical simulations show how stress localisations due to structural defects such as pores or microcracks heavily influence this stress evolution and, therefore, the crack initiation in the ceramic layer.

We found the directions of the principle stresses in the microstructure facilitate delamination of the layer. Parameter studies show how residual stresses, TGO and interface roughness affect the stress evolution in the ceramic layer. A comparison of the new self-healing ceramic with a standard reference material point out the promising properties of this newly developed material. Detailed microstructure analyses and insights provide an understanding of the main failure mechanisms of the ceramic layer in depth and deliver clear optimization potentials for sprayed ceramic layer systems of SOFCs.