Effect of Hydrogen on Strength of Nanoceramics
The effects of hydrogen on strength of nanoceramics are theoretically studied. Within the approach suggested, the effects of hydrogen on the principle fracture processes that control strength - nanocrack generation and the growth of pre-existent cracks - in deformed nanoceramics containing hydrogen are analyzed. The conditions at which nanocrack growth near disclination dipoles is energetically favored in nanoceramics α-Al2O3 (corund) and 3C-SiC (the cubic phase of silicon carbide) are revealed, and the equilibrium lengths of such nanocracks are calculated. It is shown that the equilibrium lengths of nanocracks increase in the presence of hydrogen and can be close to the grain size. As a consequence, such nanocracks can merge resulting in brittle fracture of nanoceramics. Also, the effects of dislocations with large Burgers vectors, which form in the course of grain boundary sliding, and hydrogen impurities on the propagation of pre-existent cracks in nanoceramics are analysed. It is demonstrated that grain boundary sliding leads to an increase of the values of the critical stress intensity factor by 10 to 30 percent, whereas, in contrast, the presence of hydrogen can reduce these values by a factor of 1.5 and more. Thus, it is demonstrated that hydrogen promotes the embrittlement of nanoceramics.