Nanoscale plasticity in titania densified alumina ceramics

The present study explores the physics behind the loading rate (dP/dt or $P˙≈1−1000mNs−1$⁠) dependent nanoscale plasticity (NSP) events observed during carefully controlled nanoindentation (NI) experiments on 1, 3, and 5 wt. % Titania Densified Alumina (TDA) ceramics. Characterizations of the TDA ceramics are carried out by x-ray diffraction, field emission scanning electron microscopy (FESEM), and NI techniques. A significant enhancement (∼30%) of the nanohardness of TDA ceramics occur with an enhancement in $P˙$⁠. The results confirm that both the critical load (P_c) at which micro-pop-in or the NSP events initiate and the corresponding critical depth (h_c) are sensitive functions of relative density, size of relatively finer grains, loading rate, and the amount of sintering aids. The experimentally observed empirical power law dependence of all the NSP related parameters on $P˙$ is rationalized theoretically and qualitatively. It is suggested that the shear induced homogeneous dislocation nucleation underneath the nanoindenter may be the main factor contributing to the occurrence of the NSP events at relatively lower loading rates. However, especially at the relatively higher loading rates, the FESEM based evidence and the data obtained from the related NI experiments suggest that there is a more acute interconnection between the homogeneous dislocation nucleation induced profuse occurrence of the NSP events, shear band formations, and microcrack formation in the TDA ceramics. Finally, the design implications of the present results for the development of better alumina ceramics for load and strain tolerant applications are discussed.