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Effect of humidity on indentation crack growth in lead-free ferroelectric ceramics

The effect of humidity on growth of unloaded indentation crack in KNN free-lead ferroelectric ceramics has been investigated. The results showed that crack growth of unloaded indentation in lead-free ferroelectric ceramics could occur in humid air of 70% and 90%RH without electric field and mechanical stresses, but did not in air with RH≤30%. The growth of indentation crack could occur in dry air when the field was larger than the threshold field Eth(y)=0.01EC (normal to the poling direction) or Eth(z)=0.05EC (parallel to the poling direction) and the larger the field, the shorter the incubation time. The increment of crack growth in humid air under sustained field, Δc, was composed of three parts, i.e., Δc=Δc1 + Δc2 + Δc12, where Δc1 was the increment in humid air without field, Δc2 that in dry air under sustained field and Δc12 that induced by combined effect of electric field and humidity because of humidity promoting domain switching. Crack growth of unloaded indentation could occur during hydrogen charging and the threshold stress intensity factor of hydrogen-induced delayed cracking, KIH, as well as the fracture toughness of hydrogenated ceramics, KIC(H), decreased with increasing hydrogen concentration.