Acoustic Emission Monitoring of Nanoindentation-Induced Slip and Twinning in Sapphire
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ACOUSTIC EMISSION MONITORING OF NANOINDENTATION-INDUCED SLIP AND TWINNING IN SAPPHIRE Natalia I. Tymiak, Antanas Daugela, Thomas J. Wyrobek, and Oden L. Warren Hysitron, Inc., Minneapolis, MN 55439 ABSTRACT Monitoring with an Acoustic Emission (AE) sensor integrated into an indenter tip was utilized for the evaluation of the earliest stages of indentation-induced plasticity in sapphire single crystal. The evaluated surfaces included basal (C), rhombohedral (R) and two different prismatic orientations (A and M). The differences between the mechanisms of the initial stages of plasticity for the various crystallographic orientations were reflected in the following aspects of AE activity: detection of a specific type of AE waveform that correlated to the presence of linear surface features near the indentation impressions; AE signal associated with the yield point, consisting either of one or two distinct waveforms; and presence or absence of AE signals after the yield point. Moreover, analysis of AE activity revealed loading rate effects on the yield point mechanism for the M plane. The possibility of plasticity onset mechanisms involving both slip and twinning is discussed. INTRODUCTION Applications of sapphire include silicon-on-insulator structures, light-emitting diodes, optical devices, lasers, and missile guidance systems. Evaluation of the earliest stages of plasticity at room temperature presents a special interest for applications where high-speed loading and/or highly localized stresses are involved. While normally regarded as brittle with the brittle-to-ductile transition temperature exceeding 1100 ° C, sapphire does deform plastically at much lower temperatures, provided there is sufficient hydrostatic pressure to suppress fracture. High levels of hydrostatic compression generated during normal indentation and sliding contacts facilitate slip and twinning at room temperature. Room-temperature twinning and twinninginitiated cracking of sapphire pose a significant concern for the mechanical performance of optical devices where sapphire is used as a material for electromagnetic windows [1]. Previous experimental studies demonstrated that slip and twinning patterns in sapphire are highly dependent on the crystallographic orientations of the evaluated surfaces [2-6]. For several orientations including M [2-7], and R [2], linear surface features identified as twins were observed near hardness testing [2,6,7] and micro-indentation impressions [3-5]. The presence of linear surface features correlated to twins visible in TEM images [2]. Twins were also seen in TEM images of Vickers indents into plane A [8]. In contrast, linear surface features were not observed near indentations into plane C [2-4] and twinning was not seen in TEM images of indented areas [2]. With the advent of depth-sensing indentation, extra information regarding the earliest stages of contact-induced plasticity in sapphire became available from load-displacement curves. It has been established that, similar to many metal and ceramic singl
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