Microstructure, hardness, and fracture toughness of suspension plasma sprayed yttria-stabilized zirconia electrolytes on stainless steel substrates

• Porosity in as-deposited YSZ coatings decreases with increasing torch power.
• Porosity in the YSZ coatings decreases with decreasing stand-off distance.
• Fracture toughness of coatings made at an optimal condition is 1.5 ± 0.15 MPa m1/2.
• Fracture toughness of plasma coatings is comparable to that of tape cast coatings.
• Interfacial fracture toughness is greater than the fracture toughness of coatings.

Recent research interest in solid oxide fuel cells (SOFCs) has grown towards effective manufacturing processes that are compatible with low-cost materials such as stainless steel structural supports. Suspension plasma spraying is a promising technique that rapidly produces a fully consolidated, thin electrolyte layer with good microstructure control and without any post-deposition heat treatment. In the present study, suspension plasma spraying was used to deposit yttria-stabilized zirconia electrolyte coatings on porous stainless steel substrates using a range of spray conditions. Mechanical properties of the coatings, such as hardness and fracture toughness, were characterized. These properties can affect the durability of SOFC electrolytes, coupled with the thermal stresses generated during operation. Coating toughness and hardness were observed to be dependent on the torch power and stand-off distance during fabrication, which affect coating microstructural features such as porosity, microcracks, and segmentation cracks. The fracture toughness measured using an indentation technique of the coatings produced with a torch power of 133 kW and a stand-off distance of 90 mm was found to be 1.5 ± 0.15 MPa m1/2.