Study of the evolution of nanopores and microstructural degradation of fine-grained YSZ–NiO(Ni) anode materials in a hydrogen sulfide containing atmosphere

The formation of solid oxide fuel cell (SOFC) anode structure resistible against degradation in operating environments is a relevant task while improving SOFC models. This work is aimed at studying the effect of hydrogen sulfide content in a high-temperature (600 °C) hydrogenous atmosphere on the evolution of nanopores and microstructural degradation, as well as strength, microhardness, and crack growth resistance of a fine-grained YSZ–NiO(Ni) anode materials for SOFCs. To obtain the corresponding YSZ–NiO(Ni) cermet structure, specimens of the YSZ–NiO anode ceramics were reduced in Ar–5 vol% H 2 mixture for 4 h at 600 °C under a pressure of 0.15 MPa. A part of reduced specimens was aged in “hydrogen sulfide in Ar–5 vol% H 2 mixture” atmosphere for 4 h at 600 °C under a pressure of 0.1 MPa. The atmosphere contained 7 or 18 vol% H 2 S. After aging, the physical and mechanical behaviors of specimens were studied. The following investigation methods were utilized: strength test under three-point bending, Vickers microhardness test, fracture toughness test by indentation method, SEM, EDX, and STEM. It was revealed that the atmosphere containing up to 7 vol% H 2 S had a slight effect on the strength and electrical conductivity of the YSZ–NiO(Ni) cermet. In contrast to this, an increased content of hydrogen sulfide (18 vol% H 2 S) caused some changes in the YSZ–Ni cermet microstructure. As compared to the preconditioned cermet, strength of the aged cermet decreased by almost 30%. In contrast to this, microhardness and fracture toughness of the aged cermet were only lowered by about 10%. To explain this difference, several fracture micromechanisms were suggested: (1) the localization of stresses in the crack tip vicinity (so-called process zone) during indentation that promotes further crack growth until touching a “material–pore” interface where stress relaxation occurs causing crack retardation (the case of Vickers indentation test); (2) a decrease in the integral cross-section area due to pores in the material bulk when expanding ultimate stress zone that results in a sharp decrease in strength of the YSZ–NiO(Ni) cermet aged in a hydrogen sulfide containing atmosphere (the case of strength test). It was found that in the case of increased content of H 2 S (18 vol%) in a hydrogenous atmosphere, microstructure degradation due to the nanopores formation and weakening of the “zirconia–nickel” bonds occurs promoting low-energy intergranular fracture along boundaries