Available online at www.sciencedirect.com Journal of the European Ceramic Society 31 (2011) 1865–1871 Nanoscale elastic–plastic deformation and stress distributions of the C plane of sapphire single crystal during nanoindentation W.G. Mao a,b , Y.G. Shen a,∗ , C. Lu c a Department of Manufacturing Engineering and Engineering Management (MEEM), City University of Hong Kong, Kowloon, Hong Kong b Key Laboratory of Low Dimensional Materials and Application Technology, Ministry of Education, Xiangtan University, Hunan 411105, China c Department of Mechanical Engineering, Curtin University, Perth, WA 6845, Australia Received 31 August 2010; received in revised form 27 January 2011; accepted 7 April 2011 Available online 4 May 2011 Abstract The nanoscale elastic–plastic characteristics of the C plane of sapphire single crystal were studied by ultra-low nanoindentation loads with a Berkovich indenter within the indentation depth less than 60 nm. The smaller the loading rate is, the greater the corresponding critical pop-in loads and the width of pop-in extension become. It is shown that hardness obviously exhibits the indentation size effect (ISE), which is 46.7 ± 15 GPa at the ISE region and is equal to 27.5 ± 2 GPa at the non-ISE region. The indentation modulus of the C plane decreases with increasing the indentation depth and equals 420.6 ± 20 GPa at the steady-state when the indentation depth exceeds 60 nm. Based on the Schmidt law, Hertzian contact theory and crystallography, the possibilities of activation of primary slip systems indented on the C surface and the distributions of critical resolved shear stresses on the slip plane were analyzed. © 2011 Elsevier Ltd. All rights reserved. Keywords: Single crystal sapphire; Nanoindentation; Critical resolved shear stress; Multiple pop-in events; Mechanical properties 1. Introduction Sapphire (-Al 2 O 3 ) is an important crystal material due to its high hardness, chemical inertness, superior mechanical performance, and thermodynamic stability. 1–3 Generally, sap- phire is brittle and its brittle-to-ductile transition temperature is ∼1373 K. However, the plastic deformation of sapphire crystals may occur under low loads at room temperature. 4 Recently, the evaluation of the initial stages of plasticity and elastic–plastic deformation properties of sapphire with different surface orien- tations at room temperature have been extensively investigated by nanoindentation tests. 4–12 These studies show that single and/or multiple displacement discontinuity (pop-in) events dur- ing loading are found to distinguish between the fully elastic and the elastic-plastic regimes, associated with the nucleation of dislocations. As we know, the C plane (0 0 0 1) of sapphire is gen- erally selected as a calibration medium during nanoindentation 13 and substrate in the preparation of many kinds of functional ∗ Corresponding author. Tel.: +852 2784 4658; fax: +852 2788 8423. E-mail address: meshen@cityu.edu.hk (Y.G. Shen). thin film/substrate systems. 14,15 It is necessary to study the mechanical properties of the C plane under different size scales and measurement conditions. Although many studies have been done on elastic-plastic behaviors of the C plane by micro- /nano-indentation with relatively large Berkovich or spherical indenters, the interpretation of indentation results obtained at room temperature is not straightforward because these inden- tation measurements are strongly affected by factors such as the anisotropic elasticity of sapphire, surface roughness, the radius and shape of indenter tip, loading rate and indentation depth. 4–8 In this paper, the nanoscale elastic/plastic deformation and mechanical properties of the C plane have been systemati- cally studied by depth-sensing nanoindentation experiments at room temperature. The contact area function and radius of the Berkovich tip were carefully calibrated under very small inden- tation depth by nanoindentation and atomic force microscopy (AFM) instruments. The mechanical properties and surface deformation mechanism of sapphire crystal indented on the C plane were analyzed under the anisotropic elastic character- istic and small indentation depth. The critical resolved shear stresses (CRSS) at slip planes were evaluated with the aid of the Hertzian contact theory, compared with the corresponding the- 0955-2219/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2011.04.012