Indentation recovery of soda-lime silicate glasses containing titania, zirconia and hafnia at low temperatures

Materials Science and Engineering A 532 (2012) 456–461

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Indentation recovery of soda-lime silicate glasses containing titania, zirconia and hafnia at low temperatures Mirabbos Hojamberdiev ∗ , Harrie J. Stevens Kazuo Inamori School of Engineering, Alfred University, 2 Pine Street, Alfred, NY 14802, United States

a r t i c l e

i n f o

Article history: Received 31 May 2011 Received in revised form 7 September 2011 Accepted 3 November 2011 Available online 12 November 2011 Keywords: Mechanical characterization Indentation Glass Rapid solidification Indentation recovery

a b s t r a c t From thermal treatment of indentations made on glass, the densification contribution to the total indentation deformation can be estimated. In this work, we studied the physico-mechanical properties and low-temperature indentation recovery of soda-lime silicate glasses with the composition of (in wt%) 73SiO2 –12CaO–15Na2 O–xTiO2 (ZrO2 or HfO2 ), where x is 0, 1, 2, 4 and 8. The obtained results demonstrated that the addition of group IV B metal oxides (TiO2 , ZrO2 and HfO2 ) into soda-lime silicate glass increased density, elastic moduli, Vickers hardness and brittleness due to the higher atomic weight of additives, increased network connectivity, higher strength of individual bonds formed and lower molar volume, respectively. Indentation recovery of glass samples containing 8 wt% TiO2 (G5), 8 wt% ZrO2 (G9) and 8 wt% HfO2 (G13) was comparatively estimated with that of a pure soda-lime silicate glass (G1), as a reference, on the basis of the depth change of indentation impressions at low temperatures, 25 ◦ C and 100 ◦ C, for different periods with a maximum of 48 h. It was found that the higher indentation recovery for all four samples was observed at both temperatures within the first 6 h due to stored strain energy in deformation zone but equilibrium was not reached even after 48 h because of low thermal treatment temperatures, which generally provide a thermodynamic driving force to recover. According to the glass composition, the indentation recovery rate follows the order: G13 < G9 < G5 < G1, which is mainly attributed to the decrease in densification with the increase the size of tetravalent metal cations introduced into soda-lime silicate glass. All the indentations made on the experimental glasses exhibited three common features: elasticity, viscoelasticity and plasticity. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Indentation is the most commonly applied means of testing the mechanical properties of various kinds of materials, including brittle glass, glass-ceramics and ceramics. To date, much effort has been expended in order to develop various indentation techniques, including Vickers, Knoop, Brinell, etc., for evaluating the abovenamed materials’ hardness over a continuous range. By applying an indentation technique, the contribution of indentation-induced densification to total indentation deformation of glass can also be estimated [1]. However, loading a glass with a sharp indenter under high compression ultimately leads to the permanent deformation that may chiefly hinder an elastic restoration, especially at low temperatures. Moreover, there is always some uncertainty associated with the very end of conventional diamond indenter tips, which hinders reliable shallow nanoindentation measurements due to

∗ Corresponding author at: Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan. Tel.: +81 44 945 5323; fax: +81 44 945 5358. E-mail address: mirabbos [email protected] (M. Hojamberdiev). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.11.012

less sufficient control to produce precise indenter geometries and shapes and blunted tips during usage [2]. Plasticity is delayed to a greater indentation depth by indenting a solid material with a spherical indenter, which is favorable method for measuring elastic properties without initiating any significant cracks. Swain and Mencik [3] have successfully applied a spherical indenter and managed to calculate aberrations due to imperfections in the shape. Also, they noted the formation of plastic flow and Hertzian cone cracking in glasses under the spherical indenter with higher load [4]. It is however difficult to manufacture a diamond indenter that conforms to a perfect spherical shape; therefore, the hardened steel ball- and tungsten carbide ball-mounted indenters have been widely used. A Berkovich tip (a three-faceted diamond pyramid) is the easiest tip to come to a sharp point and so is more readily employed in instrumented indentation experiments for investigating the mechanical properties of small volume of material [5]. The most extensively used method to determine the elastic moduli and hardness by nanoindentation was proposed by Oliver and Pharr [6], in which the slope of the unloading curve which is usually nonlinear was used to calculate the elastic moduli and to provide a physically justifiable procedure for determining the penetration depth much below 1 ␮m.

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It is known that a permanent deformation in glass is caused mainly by two different processes during the indentation: volume-conservative (i.e. plastic flow) and volume-contraction (i.e. densification), according to the glass type, either ‘normal’ or ‘anomalous’ [7]. Nevertheless, the formation mechanism of the indentation-induced impressions will affect the fracture toughness of glass, as described by Sawasato et al. [8]. The indentation-induced impressions in glass can fully or partially be recovered on the basis of structural relaxation caused by various environmental factors, namely, temperature, humidity, pressure and atmosphere. MacKenzie [9] reported that the molar volume of vitreous silica densified in the rigid state under compressive stress approached the molar volume of uncompressed vitreous silica, which supports the molecular entanglement model, by annealing at 1000 ◦ C for 1 h. Yoshida et al. [10] investigated the contribution of indentation-induced densification to total indentation deformation beneath a Vickers indenter of binary sodium borate (Na2 O-B2 O3 ) glasses using atomic force microscopy (AFM) and estimated an indentation recovery to be greater than 65% after annealing at Tg × 0.9 K for 2 h for all borate glasses. In another of their works [11], the densification contributions were estimated to be 92% for silica glass, 61% for soda-lime silicate glass and 4.6% for bulk metallic glass at Tg × 0.9 K for 2 h. Generally, the recovering volume of the indentation impressions increases with the increase in annealing temperature. Sawasato et al. [8] studied the volume recovery of Vickers indentation impressions made on soda-lime glass as a function of annealing temperature and time and estimated the ratio of volume recovery to be 27% after annealing at Tg × 0.6 K and 71% after annealing at Tg . Besides, they also noted that the elastic recovery of a diagonal length was more limited than the elastic recovery of the depth due to the difference in the density distribution under the indentation impression. Most recently, Yoshida et al. [12] evaluated the indentation-induced densification of sodalime glass under the diamond indenters with different geometries from the volume recovery of indentations by thermal annealing. They found that with the increase in the inclined face angle, the densification contribution decreased and the shear-flow contribution increased. This significant indenter-geometry dependence of densification in glass can be explained by stress distribution under the indenters. The thermal treatment is of primary interest in order to estimate the ratio of densification to a total indentation deformation of glasses. In the present study, we focus our investigation on sodalime silicate glass, which is the most widely used, with the addition of titania, zirconia and hafnia. Elastic moduli and indentation properties of glasses were determined as a function of additive content. The effect of low temperatures, 25 ◦ C and 100 ◦ C, on the indentation recovery of glasses was studied as function of time up to 48 h. On the basis of the depth change of the indentation impressions at 25 ◦ C and 100 ◦ C, an indentation recovery rate was estimated for each glass sample.

cleaned in acetone and re-annealed at 10 ◦ C above their respective Tg determined using a STA 409CD differential scanning calorimeter (Netzsch-Gerätebau GmbH, Germany). The densities of the glass samples were measured using the Archimedes method, with kerosene as the liquid medium. Elastic moduli (Young’s modulus, bulk modulus, shear modulus and Poisson’s ratio) of the glass samples of known thickness were calculated by measuring their longitudinal and shear wave sound velocities by a pulse-echo method [13] using a TDS420 oscilloscope (Tektronix, USA). Indentation properties (Vickers hardness, fracture toughness, brittleness and fracture surface energy) of the glass samples were measured by a Vickers indentation method using a HMV-2000 Micro Hardness Tester (Shimadzu, Japan) with the load of 200 g applied for 15 s. The Vickers hardness, HV , was obtained using HV = 2Psin(/2)/d2 , where P is the applied load,  is the included angle between the opposite faces of the pyramid and d is the average diagonal of the indentation impression. Brittleness was first defined by Lawn and Marshall [14] in the late 1970s as the ratio of hardness to fracture toughness (B = H/K) and later intensively discussed with an indentation technique by Sehgal and Ito [15]. Brittleness of the glass samples was calculated using B = (1/0.0056)3/2 (C/a)3/2 (P)−1/4 , where B – brittleness, m−1/2 ; C – crack length, m; a – indentation diagonal length, m; P – indentation load, N; the quantity (1/0.0056) has units of N1/6 m−1/3 . The indentation fracture toughness, KC , was calculated using the equation developed by Anstis et al. [16] 3/2 1/2 KC = (E/H) (P/co ), where is a material-independent constant (=0.016 for Vickers produced radial cracks), E is Young’s modulus, H is Vickers hardness, co is the average length of the radial crack. The fracture surface energy, , was calculated using 2 (1 − 2 )/2E], where  is Poisson’s ratio.  = [KIC Indentation experiments were performed on an Instron 5566 universal hydraulic testing machine (Instron, USA) fitted with a 1 mm diameter spherical tungsten carbide ball-mounted indenter. The indenter was driven at a load rate of 0.015 kN/s with a 15 s dwell time at the maximum load of 0.09 kN, which was the optimum load for indenting glass samples without initiating any significant cracks. Before starting the indentation, a correction was accomplished by measuring a displacement drift. Prior to indentation, all the glass samples were carefully cleaned with acetone one more time and dried with a warm air blower in order to remove any debris/dirt on the surface. Just after indentation the samples were directly subjected to two different temperatures, 25 ◦ C and 100 ◦ C, for 0.25, 1, 6, 12, 24 and 48 h in order to investigate their indentation recovery as a function of time. The residual indentations in the glass samples were scanned after above-mentioned times using a laser non-contact profilometer (Zygo Corporation, USA). After defining the surface plane, the area scan yielded the maximum depth of the indentation. Based on the depth change of the indentations at two temperatures, indentation recovery of the glass samples was evaluated as a function of time.

2. Experimental

3. Results and discussion

A series of glass samples were produced by melting 100 g batches of the compositions (in wt%) 73SiO2 – 12CaO–15Na2 O–xTiO2 (ZrO2 or HfO2 ), where x is 0, 1, 2, 4 and 8. Filled platinum crucibles were inserted into a preheated electric furnace at 1000 ◦ C and melted at 1500 ◦ C for 4 h. Melts were poured on a preheated steel plate, held at their respective annealing temperatures for 1 h and then slowly cooled to room temperature. The glass bars were cut into the 25 mm × 20 mm × 3 mm slices by a Buehler IsoMet low-speed saw using a cutting fluid, and the surfaces of glass slices were thoroughly ground and polished using first 180/320/600/800/1200 SiC grit and then 6/3/1/0.25 ␮m diamond paste. The polished glass slices were ultrasonically

To date, the incorporation of titania, zirconia and hafnia into silicate glasses has been of scientific and practical interest because of their beneficial effects on glass properties, namely, thermal, chemical, optical and mechanical. It is known that the rate of indentation recovery mainly depends upon glass composition as well as mechanical properties. In this context, it is essential to first evaluate physico-mechanical properties in the light of which the indentation recovery of each glass sample can be comprehensively discussed. The average values of physico-mechanical properties of the experimental glasses are listed in Table 1 with their respective chemical compositions. As expected, the densities of glass samples increase monotonically with the increase in the amount of titania, zirconia

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Table 1 Chemical compositions (wt%) and physico-mechanical properties of glass samples. Chemical composition and property

G1

G2

G3

G4

G5

G6

G7

G8

G9

G10

G11

G12

G13

SiO2 CaO Na2 O TiO2 ZrO2 HfO2 Density, ±0.008 g/cm3 Young’s modulus, ±6.24 GPa Bulk modulus, ±3.08 GPa Shear modulus, ±2.46 GPa Poisson’s ratio, ±0.017 Vickers hardness, ±0.07 GPa Brittleness, ±0.220 ␮m−1/2 Fracture toughness, ±0.015 MPa m1/2 Molar volume, cm3 Fracture surface energy, J/m2

73 12 15 – – – 2.506 72.08 42.97 29.53 0.220 5.00 7.242 0.677 23.90 3.02

73 12 15 1 – – 2.520 73.30 45.00 29.83 0.229 5.09 7.378 0.676 23.85 2.95

73 12 15 2 – – 2.533 74.78 47.66 30.19 0.239 5.19 7.583 0.672 23.80 2.84

73 12 15 4 – – 2.560 76.52 49.05 30.85 0.240 5.26 7.728 0.668 23.70 2.74

73 12 15 8 – – 2.610 79.28 52.42 31.77 0.246 5.56 8.058 0.677 23.52 2.71

73 12 15 – 1 – 2.543 76.56 47.62 31.07 0.232 5.57 7.425 0.735 23.80 3.34

73 12 15 – 2 – 2.578 78.81 49.57 31.98 0.233 5.71 7.626 0.735 23.72 3.24

73 12 15 – 4 – 2.642 80.73 51.70 32.56 0.240 5.80 7.870 0.722 23.59 3.04

73 12 15 – 8 – 2.748 83.10 54.46 33.35 0.247 6.16 8.186 0.738 23.50 3.08

73 12 15 – – 1 2.561 76.03 47.09 30.89 0.231 5.78 7.627 0.743 23.97 3.44

73 12 15 – – 2 2.624 76.51 47.78 31.03 0.233 5.87 7.892 0.729 23.95 3.28

73 12 15 – – 4 2.758 79.12 51.11 31.85 0.242 6.06 8.142 0.730 23.82 3.17

73 12 15 – – 8 2.993 84.62 56.22 33.93 0.248 6.30 8.363 0.739 23.74 3.02

and hafnia of higher atomic weight. For instance, the density of a reference sample G1 increases from 2.506 g/cm3 to 2.610 g/cm3 in sample G5 with 8 wt% TiO2 , 2.748 g/cm3 in sample G9 with 8 wt% ZrO2 and 2.993 g/cm3 in sample G13 with 8 wt% HfO2 . The difference in the densities of glass samples is attributed to the increase in atomic weight and the amount of titania, zirconia and hafnia added into soda-lime silicate glasses. As can be seen in Table 1, the introduction of titania, zirconia and hafnia in soda-lime silicate glass progressively leads to the increase in elastic moduli (Young’s modulus, bulk modulus, shear modulus and Poisson’s ratio) and Vickers hardness in the following sequence: reference sample (G1) < TiO2 (G2–G5) < ZrO2 (G6–G9) < HfO2 (G10–G13). The obtained results are consistent with the previously reported data [17]. Nevertheless, a quite similar trend is not observed because the types of glasses in the experiments differ in composition. Compared with the reference sample G1, the G2–G5 glass samples show increasing values for elastic moduli and Vickers hardness with the increase in titania content. This can be considered as a result of structural contraction and high strength in the Ti–O bond due to a higher ionic potential of titanium. It is known that the presence of alkali and alkaline-earth oxides in glass, as oxygen donors, promotes the formation of non-bridging oxygens. The non-bridging oxygens allow the flexible network to better adjust to the incorporation of a larger amount of titanium and to remain homogenous over a broader range of TiO2 content than the TiO2 –SiO2 network without any modifiers. Morsi and ElShennawi [18] speculated that the contraction of the glass structure could also be retarded with increasing titanium content because some titanium ions can enter the structure as network modifiers (i.e. as TiO6 units). They concluded that more titanium ions enter the structure as network formers rather than as network modifiers only at lower titanium content added and that the reverse can be observed at higher titanium content. According to Henderson and Fleet [19], the addition of titanium draws electrons from the bridging oxygens shared with silicon resulting in a slight increase in the effective charge on the Si. This increase suggests that the addition of TiO2 strengthens the already fully polymerized SiO2 network owing to the formation of the strong bond between Si–O and Ti–O. Further increase in the values of elastic properties and Vickers hardness can be noted in samples G6–G9 with ZrO2 and samples G10–G13 with HfO2 . Since their electronegativity scales are comparably lower than that of TiO2 , this tendency can be related to the formation of strong Zr–O and Hf–O bonds. Zirconia and hafnia are known to have similar thermodynamic and chemical properties, in particular, valence, ionic and atomic radii, and atomic volume, and both have larger cationic sizes and higher atomic weights compared

with silicon. As Lin et al. [20] assumed, zirconium and hafnium may also enter the Si position in the Q species and may cause longer average T–O (T = Si, Ti, Zr and Hf) bonds and the polymerization of a network via the following reaction [21]: Si–O–M–O–Si + Ti–O–Ti ↔ Si–O–Si + Ti–O–M–O–Ti, where M – metal cations, Ti may be replaced by Zr or Hf. The brittleness of glass samples was also determined in this work. From Table 1 it can be seen that the brittleness value increases with increasing elastic moduli and Vickers hardness in the following order: G1 (7.242 ␮m−1/2 ) < G5 (8.058 ␮m−1/2 ) < G9 (8.186 ␮m−1/2 ) < G13 (8.363 ␮m−1/2 ). Also, the brittleness value closely correlates with the increase in density and the decrease in molar volume of glass samples. In general, higher molar volume implies that the glass structure is more open and deformation proceeds easily; a large deformation thus results in the suppression of a median cracking and a lowering of brittleness. Since higher molar volume (27.32 cm3 ) of silica glass provides higher brittleness (∼9 ␮m−1/2 ), this concept cannot be applied for all glasses [15]. Kurkjian et al. [22] reported that the Si-based glasses with the densities approaching that of silica glass are considered to be anomalous. According to the classification made by Sehgal and Ito [23], our experimental glasses are considered to be in a ‘large density case’ in which high brittleness may be caused by a lack of densification in the presence of plastic flow. In contrast, the lowest brittleness values were reported, so far, to be 1.2 ␮m−1/2 for 100% B2 O3 sample and 4.7 ␮m−1/2 for the 76SiO2 –14Li2 O–10 K2 O glass owing to the simultaneous existence of densification and plastic flow [23–25]. Normally, the basic glass consists of alkali-rich pathways with isolated silica-rich regions, and adding titania, zirconia and hafnia may change this structure, i.e. making it less connected. Probably, this may cause the fracture toughness and fracture surface energy to decrease because the glass becomes more rigid. The fracture surface energy is the energy required to generate Orowan stress that can break the bond and create two new surfaces due to the fracture. The calculated fracture surface energies are consistent with fracture toughness and brittleness of glass samples. As a representative, Fig. 1 shows the optical micrographs (a), 3Dlandscapes (b) and traces (c) of the unindented (left) and indented (right) glass sample G13 taken by using a Zygo laser non-contact profilometer. It is visible that the glass samples are well polished and have sufficient surface finish. As can be seen in Fig. 1b and c, almost no material is pushed out to form a pile up at the rim of the indentation. Generally, the indentations made on anomalous silica glass have sinking-in walls, but not pile-ups, resulted from

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Fig. 1. Optical micrographs (a), 3D-landscapes (b) and traces (c) of the unindented (left) and indented (right) glass sample G13.

the reorganization of the silica tetrahedral structural units [SiO4 ] by changing the angle of the Si–O–Si bond without breaking it during the loading. Since the glass samples are not anomalous, the reason might be related to the lower stress under the applied load of 0.09 kN and the geometry of the indenter applied. Varshneya and Mauro [26] reported a similar behavior for a binary chalcogenide glass with the covalent coordination number of 2.4 and was considered to be as the similarity of a binary chalcogenide with ‘anomalous’ high silica glasses. Also, they observed the formation of pile-ups along with a ring-like and radial cracking in chalcogenide glasses with lower and higher coordination numbers, respectively, under the same load and concluded that the shear-dominated flow would mostly precede compaction in the glass containing a significant fraction of non-bridging oxygen. This shearing along the intermolecular van der Waals force is easier giving rise to an extensive cracking. Above 0.09 kN load, a ring crack with slight damages, weak radial vent cracks and outer ring crack were observed around the indentation; below 0.09 kN load the indentations were inappreciable to be detected by a laser non-contact profilometer for all the glass samples. Therefore, 0.09 kN was further applied as an optimum load in order to avoid the formation of significant cracks that would considerably affect the estimation of indentation recovery of glass samples. To investigate the effect of titania, zirconia and hafnia on the indentation recovery of soda-lime silicate glass, glass samples (G5, G9 and G13) containing highest amount of titania, zirconia and hafnia were submitted to the indentation experiments along with the reference sample (G1). Just after indentation, the indented glass samples were immediately subjected to two different temperatures

(25 ◦ C and 100 ◦ C) for 0.25, 1, 6, 12, 24 and 48 h. Clearly, the initial depths of the indentations vary notably according to the glass composition and straightly correlate with Vickers hardness and elastic moduli, i.e., the indentation made on glass with higher Vickers hardness and elastic moduli has shallow depth, or vice versa, due mainly to the rigidity of the glass structure that can resist ˇ deformation. According to Bridgman and Simon [27], the densification effect decreases with increasing the size of tetravalent metal cations present in glass. The change in indentation depth of glass samples at 25 ◦ C and 100 ◦ C is plotted in Fig. 2 as a function of time. Higher indentation recovery of glass samples was observed at 100 ◦ C within the first 6 h compared to those maintained at 25 ◦ C. The reason may be higher thermal treatment at 100 ◦ C that might have a negligible effect on structural relaxation of glasses. Sawasato et al. [8] observed a very fast initial indentation recovery within the first 15 min at different treatment temperatures. The initial restoration was not measured in this study. It is assumed that there might have been indeed a restoration in the indentations made on the glass samples within that period resulting from the stretching of interatomic bonds when the indentations were first made because the indentation has a strong ability to recover easily due to the instant spring back of the bonds to normal lengths on the basis of restoring force, particularly under a lower load for a shorter time [28]. Note that the indentations with extensive cracks made at >0.09 kN load were not much disturbed by thermal treatment at 100 ◦ C due to a large fraction of permanent compaction resulting from the rearrangement of network structure, the movement of modifier ions and the formation of network cavities.

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52

-20

48

0.150

o

100 C

0.125

Total recovery [%]

Indentation depth [nm]

44 -25

-30

40 0.100

36 32 28 24 20

-35

o

25 C

0.050

16 0.220

25ºC -40 0

10

20

30

40

50

Time [h] -15

-20 Indentation depth [nm]

o 100 C 0.075

o

25 C

Recovery rate

460

-25

-30

-35

100ºC -40 0

10

20

30

40

50

Time [h] Fig. 2. Change in indentation depth of glass samples at 25 ◦ C and 100 ◦ C as a function of time. Keys:  – sample G1,  – sample G5, ♦ – sample G9 and  – sample G13.

Certainly, this permanent compaction could also be recovered partially at higher temperatures by eliminating the effect of viscous flow. For instance, 99% recovery of hydrostatically densified silica glass was achieved at 1000 ◦ C for <1 h [9]. On the contrary, the indentations made on the lead-phosphate and lead-borate glasses do not show any variations when thermally treated [29]. Because these glasses with heavy cations generally do not present any thermoelastic effects, whereas silica glasses with low coordination number evidently show. In this case, indentation recovery of glass samples is delayed and even slowed with the increase in the size of the tetravalent metal cations added at both temperatures in the following order: G1 > G5 (8 wt% TiO2 ) > G9 (8 wt% ZrO2 ) > G13 (8 wt% HfO2 ), owing to the different contributions of densification in each glass. Most recently, anelastic indentation recovery of bioglass was studied at room temperature [30]. It was experimentally proved that the recovery rate of the indentation depth at room temperature was considered a power-function of the indentation load, i.e., a driving force for the recovery was the stored strain energy over the anelastic indentation zone. The highly densified indentations with high-stressed bonds can also be relaxed at a relatively low temperature because this high-stressed state would be thermally unstable, and a less-densified region needs higher temperatures in order to initiate a faster indentation recovery [8]. Perriot et al. [31] explored the densification of silica glass by Vickers indentation using microprobe Raman spectroscopy. On the basis of the peak shift of D2, they found that the bottom of the indentation had a higher density (16%) compared to the near surface of the glass sample (10%). Similarly,

0.244 0.246 0.248 0.250 Poisson's ratio

Fig. 3. Total recovery and recovery rate of indentations made on glass samples as a function of Poisson’s ratio. Key:  – recovery rate;  – total recovery.

the surfaces of the indentations made on experimental glass samples were not completely affected by thermal treatment compared to the bottom of the indentation. According to Rouxel et al. [32], the ratio between the total penetration displacement and the vertical projection of the contact area depends on the glass composition and seems to correlate with Poisson’s ratio. As speculated in the previous report [11], Poisson’s ratio is very sensitive to the excess volume in the glass network and can be used as a suitable parameter to explain the compositional effect on the indentations recovery of glass samples. The total recovery (in %) and recovery rate of the indentations made on glass samples are shown in Fig. 3 as a function of Poisson’s ratio. The results obtained conform to the data previously reported for silica, soda-lime silicate, oxynitride and bulk metallic glasses [11,33]. As shown in Fig. 3, the total indentation recovery and recovery rate significantly decrease with the increase in Poisson’s ratio for all glass samples treated at 25 ◦ C and 100 ◦ C for various periods. The total indentation recovery of the indentations made on glass samples under the current experimental conditions can take place in the following order: 20.7% (G13) < 21.2% (G9) < 24.7% (G5) < 27.1% (G1) at 25 ◦ C and 35.6% (G13) < 38.7% (G9) < 43.2% (G5) < 48.9% (G1) at 100 ◦ C. The difference in total indentation recovery of the indentations at two temperatures can be related to the different activation energies, which are normally required in the temporary molecular entanglement of the silica glass network. By applying thermal treatment at 25 ◦ C and 100 ◦ C, the recovery of the indentations made on glass samples using a Brinell indenter is not much close to that of the Vickers indentations of glasses annealed at Tg × 0.9 [11]. Nevertheless, the Brinell indenter with a spherical tip did not cause glass samples to reach a critical Hookean point at 0.09 kN load, so all the indentations showed more elasticity and viscoelasticity than plasticity. The deformation energies for silicate glasses during loading using the Vickers indentation technique were estimated by Suzuki et al. [34] and found to be 139 kJ/mol for total deformation, 96 kJ/mol for elastic deformation and 43 kJ/mol for plastic deformation, which is very close to the activation energy (46–54 kJ/mol) for the recovery of densification in silica glass reported so far. It was also noted that all these deformation energies increased with decreasing a penetration depth. The indentation recovery rate of glass samples was estimated using Log d = − mlog t + K [35], where m and K are constants. The indentation recovery rate, or slope m, is greater at first 6 h, after which it slows down drastically for longer period, due to the high stress induced by indentation. Levengood and Vong [35] studied the influence of the loading time on the recovery rate of an indentation, and it was found that the longer loading time could cause a slow recovery rate due to the localized deformation in the

M. Hojamberdiev, H.J. Stevens / Materials Science and Engineering A 532 (2012) 456–461

center of the indentation. Since our experimental glasses are considered to behave as ‘normal’, three common features: elasticity, viscoelasticity and plasticity, can be observed during the indentation experiments. This assumption is based on the fact that our glass samples contain a substantial amount of network modifiers and intermediates, which tend to promote shear-dominated plastic flow originating from the slip motion of planar structures or from the redistribution of non-bridging oxygens in glass [10] rather than densification. Peter [36] also explained that all the glasses can be densified, and the densification contribution to the total indentation deformation can vary with glass composition. 4. Conclusions In this work, the physico-mechanical properties and indentation recovery of soda-lime silicate glasses with the composition (in wt%) of 73SiO2 –12CaO–15Na2 O–xTiO2 (ZrO2 or HfO2 ), where x is 0, 1, 2, 4 and 8, were investigated. The obtained results demonstrated that the addition of titania, zirconia and hafnia in soda-lime silicate glass increased density, elastic moduli, Vickers hardness and brittleness due to the higher atomic weight of additives, increased network connectivity, higher strength of individual bonds formed and lower molar volume, respectively. Indentation recovery of glasses containing 8 wt% TiO2 (G5), 8 wt% ZrO2 (G9) and 8 wt% HfO2 (G13) was comparatively estimated with that of a pure soda-lime silicate glass (G1), as a reference, on the basis of the depth change of the indentation impressions at two temperatures, 25 ◦ C and 100 ◦ C, for different periods with a maximum of 48 h. It was found that the higher indentation recovery for all four samples was observed at both temperatures within the first 6 h due to stored strain energy in deformation zone but equilibrium was not reached even after 48 h because of low thermal treatment temperatures, which generally provide a thermodynamic driving force to recover. According to the glass composition, the indentation recovery rate followed the order: G1 > G5 (8 wt% TiO2 ) > G9 (8 wt% ZrO2 ) > G13 (8 wt% HfO2 ), which is mainly attributed to the decrease in densification with the increase the size of tetravalent metal cations introduced into sodalime silicate glass. The total indentation recovery of glass samples was in the range of 20.7–27.1% at 25 ◦ C and 35.6–48.9 at 100 ◦ C. All the indentations made on the experimental glasses exhibited three common features: elasticity, viscoelasticity and plasticity. Acknowledgements MH would like to thank the Fulbright Scholar Program for the award of a research scholarship under which the present study was

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