Acoustic emission monitoring of the earliest stages of contact-induced plasticity in sapphire

Abstract

Monitoring nanoindentation with an acoustic emission (AE) sensor integrated into an indenter tip provided a unique insight into the mechanisms of the initial stages of plasticity for four different sapphire single crystal surface orientations. These orientations included basal, (C (0 0 0 1)); rhombohedral, (R ( 1 ̄ 0 1 2 )); and two prismatic, (A ( 1 2 ̄ 1 0 ) and M ( 1 0 1 ̄ 0 )), planes. AE activity was distinctively different for each plane as evident from the following: detection of or the lack thereof a specific type of AE signal indicative of a sustained process, AE signal associated with the yield point consisting either of one or of two distinct waveforms, and the presence or the absence of AE signals after the yield point. Clear evidence of a loading rate effect on the plasticity onset mechanism was obtained for the M plane where the type of AE signal associated with the yield point changed with increasing loading rate. The experimental findings are discussed in terms of slip or twinning-dominated mechanisms governing the onset of plasticity.