Strain-Dependent Plasticity Evolution of Window Glass Dong-Hyun Lee, In-Chul Choi, Moo-Young Seok, Yakai Zhao, Jung-A Lee, and Jae-il Jang † Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, South Korea How the applied strain can affect the plasticity evolution of window glass was systematically explored through a series of nanoindentations with various sharp indenters. It was revealed that, as the strain increases, the contribution of shear flow to total plasticity becomes larger, whereas that of densification gets smaller. The results are discussed in terms of the sequence in which each mechanism plays and the detailed mechanism of shear flow. I. Introduction C ONVENTIONAL oxide glasses are brittle at ambient temper- ature and fail in a catastrophic manner under tensile or bending stresses. However, it has been reported that they can show a considerable plasticity under a certain mechanical environment since Taylor’s first report in 1949 that indenta- tion with a sharp indenter lefts permanent impression on the glass surface. 1 Due to the absence of crystalline defects such as dislocations, the mechanism of the plastic deforma- tion in oxide glasses should be different from that in crys- talline ceramics. The possible mechanism found earliest was densification, that is, a permanent volume contraction under compressive stresses. 2 This is possible because amor- phous materials have more open structure than chemically equivalent crystals and the structure can be condensed into a more close-packed arrangement by external loading. 3 The densified region is known to be recovered by annealing treatment. 4 The fact that apparent activation energy of the recovery in the indented glass is close to that in hydrostati- cally densified glass 5 suggests that the densification plays an important role in the plastic deformation underneath a indenter possibly due to the existence of indentation core under hydrostatic pressure. 6 The extent of densification varies with glass composition, especially concentration of network modifiers such as Na 2 O or CaO 7,8 ; glasses having more modifiers exhibit smaller densification, because the modifiers occupy the free space which can shrink by exter- nal stress. In 1970, Peter 7 argued that the permanent densification cannot fully explain indentation behavior of oxide glasses; for example, the presence of material pile-up around hard- ness impression and the slip lines below the indentation indi- cates that there is also some contribution of shear flow to the indentation-induced plasticity. 7,9–11 Now, it is well accepted that plastic deformation in oxide glasses is caused by both densification and shear flow. Nevertheless, how the contribu- tion of each mechanism can be affected by mechanical envi- ronment is not yet fully understood. Especially, only limited efforts have been made for analyzing the contribution of shear flow. A good first step for addressing this issue can be to ana- lyze the influence of applied strain on the contribution of each mechanism to the indentation-induced plasticity. With continuum mechanics concept, the strains underneath a sharp indenter are unique and independent of indentation load or displacement due to the so-called geometrical self- similarity of the sharp tip. A way to overcome this difficulty in applying different strain is varying the sharpness of inden- ter. Generally, sharper indenters with smaller indenter angles induce larger strains in the material due to the larger volume of displaced material. 12–15 The indenter sharpness depen- dency of deformation mechanism in metallic materials has been previously reported. 16,17 With this in mind, here we sys- tematically explore how the applied strain affect the indenta- tion-induced plasticity evolution of window glass (soda-lime silicate glass) through a series of nanoindentation tests using five different three-sided pyramidal indenters having a variety of sharpness. II. Experimental Procedure Nanoindentation tests were performed on a commercial win- dow glass using a Nanoindenter-XP (formerly MTS; now Agilent, Oak Ridge, TN) with five different three-sided pyra- midal indenters having a centerline-to-face angle w of 35.3° (cube-corner indenter), 50°, 65.3° (Berkovich indenter), 75°, and 85°. The sample was loaded to the maximum load, P max , at a constant loading rate, dP/dt, of 10 mN/s. More than 30 tests were performed for each condition. To support the analysis of the sharp indentations, nanoindentations with a spherical tip (whose radius, R, was determined as 6.38 lm by Hertzian contact analysis 18 ) were additionally made at 50 and 75 mN. The indented samples were annealed at 753 K (~0.9T g where T g is the glass transition temperature) for 2 h in an electric furnace. Based on previous reports, 4,8 one may expect that this annealing condition allows a nearly complete recov- ery of the densified region, and only the shear flow contribu- tion remains after the annealing. Before and after annealing, hardness impression morphologies were imaged using both a field-emission scanning electron microscopy (FE-SEM), JSM- 6330F (JEOL Ltd., Tokyo, Japan), and an atomic force microscopy (AFM), XE-100 (Park System, Suwon, Korea). Prior to taking SEM images, thin gold coating was applied to the indented surface to avoid charging. III. Results and Discussion Figure 1 shows representative load-displacement (P–h) curves from nanoindentations made with various indenters. As one may expect, maximum h (h max ) increases with decreasing w (or increasing sharpness). While the indenter having w = 85° exhibits purely elastic contact, as evidenced by the fact that the loading and unloading curves are identical, all other ind- enters (w = 35.3°, 50°, 65.3°, and 75°) left the residual h (h r ) after unloading. Difference in the ratio of h r /h max for each indenter (e.g., ~0.747 for 35.3° and ~0.253 for 75°) indicates that indeed different level of plastic deformation occurs in the window glass. T. Rouxel—contributing editor Manuscript No. 35159. Received June 16, 2014; revised September 1, 2014; approved September 2, 2014. † Author to whom correspondence should be addressed. e-mail: jijang@hanyang.ac.kr 186 J. Am. Ceram. Soc., 98 [1] 186–189 (2015) DOI: 10.1111/jace.13266 © 2014 The American Ceramic Society J ournal