Effect of aluminum titanate on the fracture mechanism of the nickel–zirconia system under uniaxial and biaxial strength testing conditions

Solid Oxide Fuel Cell (SOFC) anodes were fabricated with NiO–YSZ and 0–10 wt% aluminum titanate (Al2TiO5, ALT) addition. Samples were manufactured using a tape casting procedure to a thickness of approximately 500 µm. A remarkable enhancement of the mechanical strength, up to 166%, was found when compared to the samples without ALT addition. The development of secondary phases was observed proportional to the amount of aluminum titanate. Mechanical properties evaluation was conducted using uniaxial and biaxial strength testing. Weibull statistics was used for mechanical properties analysis and an advanced statistical analysis was employed to identify the existence of multiple flaw populations. Fractography was performed on selected samples to elucidate the fracture mechanisms. Biaxial testing was characterized by high mechanical strength with lower Weibull moduli; whereas, uniaxial testing showed lower mechanical strength and higher Weibull moduli. It was found that the addition of 0%, 5% and 10% ALT to Ni–YSZ samples exhibit a fracture mechanism that is dependent on one flaw population namely porosity and mechanical strength of secondary phases, respectively. Samples without the addition of ALT were characterized by intergranular fracture while transgranular fracture was found for the addition of 5% and 10% ALT to Ni–YSZ samples. For the addition of 1% ALT to Ni–YSZ samples both flaw populations were identified. Analysis of the fracture surfaces revealed the simultaneous presence of intergranular and transgranular features. It is proposed that secondary phases, developed by ALT addition, increase the mechanical strength of the material shifting the fracture mechanism from intergranular to transgranular.