Mechanical properties of glasses and TiO2 nanocrystal glass composites in BaO–TiO2–B2O3 system

Mechanical properties of glasses and TiO2 nanocrystal glass composites in BaO–TiO2–B2O3 system

Journal of Non-Crystalline Solids 380 (2013) 128–134 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: w...

2MB Sizes 0 Downloads 136 Views

Journal of Non-Crystalline Solids 380 (2013) 128–134

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol

Mechanical properties of glasses and TiO2 nanocrystal glass composites in BaO–TiO2–B2O3 system Gadige Paramesh, K.B.R. Varma ⁎ Materials Research Centre, Indian Institute of Science, Bangalore-560012, India

a r t i c l e

i n f o

Article history: Received 26 April 2013 Received in revised form 6 September 2013 Available online 2 October 2013 Keywords: Borate glasses; TiO2; Nanocrystals; Mechanical properties; Nanoindentation

a b s t r a c t Glasses and glass-nanocrystal (anatase TiO2) composites in BaO–TiO2–B2O3 system were fabricated by conventional melt-quenching technique and controlled heat treatment respectively. Poisson's ratio and Young's moduli were predicted through Makishima–Mackenzie theoretical equation for the as-quenched glasses by taking the four and three coordinated borons into account. Mechanical properties of the glasses and glass-nanocrystal composites were investigated in detail through nanoindentation and microindentation studies. Predicted Young's moduli of glasses were found to be in reasonable agreement with nanoindentation measurements. Hardness and Young's modulus were enhanced with increasing volume fraction of nanocrystallites of TiO2 in glass matrix whereas fracture toughness was found susceptible to the surface features. The results were correlated to the structural units and nanocrystals present in the glasses. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Borate based glass-ceramics have been investigated for nonlinear optic (NLO) device applications owing to their wide transmission window, large laser damage threshold coupled with high nonlinear optic coefficients [1–4]. Different borate structural units influence the physical properties of the material [5,6] that exist depending on the constituent oxides present in the borate network. Glasses in BaO–TiO2–B2O3 (BTBO) system have received particular attention of many researchers due to their large refractive indices and possibility of crystallizing NLO active phases such as BaTi(BO3)2, Ba3Ti3O6(BO3)2, and β-BaB2O4[2,7]. BTBO glasses were initially investigated in the light of fabricating BaTiO3 based glass-ceramics for capacitor and ferroelectric applications [8,9]. Recently authors have reported heterogeneous bulk nanocrystallization of TiO2 (anatase phase) in the BTBO system [10,11]. Transparent glasses comprising TiO2 nanocrystals have exhibited high refractive indices (no N 2). The BTBO glasses were also found to be hydrophobic (contact angle ≈ 90°) in nature [10,12]. Anatase TiO2 has been used for photocatalytic, dye-sensitized solar cells and antimicrobial applications [13–17] in its powder and thin film forms. Thin films and powders suffer from mechanical stability point of view and therefore protective coating or recoating in the case of thin films and reprocessing of powders is necessary, whereas TiO2 nanocrystals dispersed in glass matrix give additional stability as well as combination of new properties [18,19]. In view of functional potentialities of the anatase TiO2 nanocrystal composites in BTBO system, investigations concerning the mechanical properties are important. ⁎ Corresponding author. Tel.: +91 80 2293 2914; fax: +91 80 2360 0683. E-mail address: [email protected] (K.B.R. Varma). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.09.010

Functional bulk glass-ceramics for many applications may have to be cut into desired shapes and capable of withstanding thermal/mechanical stresses. This has led to the study of mechanical properties of technologically important glasses and glass nanocrystal composites [20,21] in detail. Mechanical properties of BTBO glasses and those with TiO2 nanocrystals are not reported in the literature to the best of authors' knowledge. Present article reports the mechanical properties of BTBO glasses and glass-nanocrystal composites of TiO2 (anatase phase). Young's moduli of glasses were calculated for base glass compositions using theoretical expressions and the values obtained were compared with that of values extracted from nanoindentation studies. Glasses in the compositions 0.25BaO–0.25TiO2–B2O3 and 0.4BaO–0.4TiO2–B2O3 were fabricated and TiO2 at nanocrystalline scale was obtained by heat treating the glasses at appropriate temperatures. 2. Experimental Glasses in the 0.25BaO–0.25TiO2–B2O3 (BT25) and 0.4BaO–0.4TiO2– B2O3 (BT40) systems were fabricated by the conventional meltquenching technique using reagent grade oxides as described in Ref. [10]. Anatase TiO2 nanocrystal glass composites were obtained by subjecting BT40 glasses to controlled heat treatment at different temperatures, 600, 650 and 700 °C (which are close to the crystallization temperature as predetermined using differential thermal analyses [10,11]), for different durations (in the range of 1–3 h). Density (ρ) of the glasses and glass-nanocrystal composites (GNCCs) were determined using Archimedes principle. For this, samples were weighed in air (wa) and Xylene liquid (wl). ρ (=0.86*wa/(wa − wl) was calculated by taking the Xylene density (0.86 g cm−3) into account. Glasses and

G. Paramesh, K.B.R. Varma / Journal of Non-Crystalline Solids 380 (2013) 128–134

129

Table 1 Density (ρ), molar volume (Vm), packing density (ρp), fraction of borate units ([BO4] and [BO3]) and calculated Young's modulus (Ecal) and Poisson's ratio (νcal) of glasses. Sample

ρ g cm−3 ± 0.005

Vm cm3

[BO4] units

[BO3] units

ρp

BT25 ASQ BT40 ASQ

3.30

38.76

0.44

0.56

0.63 94

0.28

3.32

49

0.41

0.59

0.58 107

0.26

Ecal (GPa)

νcal

GNCCs were ground and polished with abrasive SiC papers of different grit sizes followed by fine Al2O3 powder. For optical mirror finishing, fine CeO2 powder was used.

2.1. NMR studies on as-quenched base glasses Nuclear magnetic resonance (NMR) of 11B nucleus combined with magic angle spinning (MAS) for the solid powder samples of base glasses was recorded to evaluate the fraction of tetrahedral and trigonal borate units in the system. Experiments were carried out on AV-Bruker500 MHz solid state NMR spectrometer built with triple resonance probe. Pulverized sample (50 mg) was tightly packed in cylindrical rotors. Spectra were recorded at 160.419 MHz (magnetic field, Bo = 11.74 T). Sample was rotated with a speed of 8 kHz while recording MAS-NMR spectra to average out other anisotropies present in the powder sample. The chemical shifts of other boron samples are taken by calibrating the chemical shifts of 11B with respect to standard solid sample Borax.

2.2. X-ray powder diffraction X-ray powder diffraction (XRD) studies were carried out to confirm the crystalline phases present in the heat treated samples using Cu Kα radiation (X'Pert PRO, PANalytical). The volume fraction of the crystalline TiO2 phase was determined based on XRD studies in which KCl and polycrystalline TiO2 anatase samples were used as internal standards [22,23]. Sample and reference material were taken in 1:1 weight ratio to perform XRD. Strong peak intensities of the sample and KCl were taken into account to calculate the volume fraction of TiO2 phase (anatase) present in glass. Prior to this, mixture of (1:1 weight ratio)

Fig. 2. Load–displacement curves for the BT25 and BT40 glasses.

KCl and polycrystalline anatase TiO2 strong peak intensities were used to calculate the reference intensity ratio. 2.3. Electron microscopy The microstructural features of the GNCCs were examined through scanning electron microscopy (SEM) (FESEM, Inspect™ S50). Samples were gold sputtered to avoid charging while imaging. Transmission electron microscopy studies (TEM) were carried out using Tecnai, GF30 operated at 200 kV. For TEM analyses, pulverized samples were dispersed in acetone medium and drop-casted on carbon coated copper grid. Micro/nanostructure features and selected area diffraction (SAD) patterns were recorded. 2.4. Mechanical testing Mechanical properties were investigated using Birkovich nano indenter (Triboindenter of Hysitron, Minneapolis, USA) at a maximum load of 100 mN associated with the loading and unloading rate of 0.4 mN/s (resolution of force and depth measurements were 1 nN and 0.2 nm respectively). Ten indents were taken on each sample. In order to determine the fracture toughness of samples, Vickers micro indentation experiments were carried out at high loads, 2–3 N. Crack length was measured using an optical microscope (Olympus, BX51). 3. Results and discussion 3.1. Structure of the glass and calculation of mechanical parameters As-quenched BT25 and BT40 glasses were found to be amorphous in nature via XRD (not shown). Density (ρ) and molar volume (Vm) of the glasses are given in Table 1. Vm is found to be large for BT40 with small increase in density with increasing BaO and TiO2 content. This is due to the presence of larger Ba2+ ions that result in the expansion of the

Table 2 Nanoindentation hardness (H), elastic modulus (EIT), fracture-toughness (KC) and brittleness (B) of base glasses.

Fig. 1. 11B NMR spectra for as-quenched glasses.

Sample

H (GPa) (±0.2)

EIT (GPa) (±0.2)

KC MPa m1/2 (±0.05)

B μm−1/2 (±0.2)

BT25 ASQ BT40 ASQ

5.4 6.0

84 91

0.73 0.95

7.4 6.3

130

G. Paramesh, K.B.R. Varma / Journal of Non-Crystalline Solids 380 (2013) 128–134

network to adjust within small interstitial sites (of B2O3 network) thereby increasing the molar volume. In BTBO system, Ba2+ ions would create two non-bridging oxygens (NBO) by breaking the B–O–B network and result in converting some of BO3 into BO4 units. TiO6 and TiO4 units of TiO2 are distributed along with the other structural units in network randomly [12]. Therefore, BTBO glasses constitute NBOs which are charge balanced by Ba2+ ions, BO4, BO3, TiO4 and TiO6 units in a random three dimensional network configuration. In borate glasses, fractions of tetrahedral [BO4] as well as trigonal [BO3] structural units influence the properties [24,25]. MAS NMR of 11 B nucleus is a powerful technique in which the resonances of [BO4] and [BO3] could be distinguished due to the large difference in the quadrupolar coupling constants. Therefore 11B MAS NMR spectra were recorded for BT25 and BT40 glasses and shown in Fig. 1. The [BO4] and [BO3] peaks are well resolved for both the glasses in the present experiment. The sharp resonance (due to low quadrupolar broadening) around δiso = 1.2–1.3 ppm corresponds to the symmetric BO4 tetrahedral sites. The broad peak with doublet line shape (due to second-order quadrupole effect) around δiso = 9 ppm and δiso = 14 ppm is associated with the trigonal units [25,26]. Broadening is caused due to the strong quadrupolar interaction in the [BO3] units [27,28]. The multiple Gaussian-peak fit was performed to obtain the area under curve and fraction of [BO4] and [BO3] units were determined [29]. The values obtained are depicted in Table 1. Fraction of [BO4] units directly influences the physical properties such as packing density (ρp) and elastic properties in borate glasses. The Young's modulus (E) of the polycomponent oxide glasses could be predicted using the theoretical equation proposed by Makishima and Mackenzie [30]. E (GPa) is related to the ρp and dissociation energies (Gi) of oxide component i in glass as given below

Fig. 4. X-ray diffraction pattern for the glass and GNCCs.

of the glasses were calculated by taking the fraction of BO3 and BO4 units into account. ρp is given as [32] X

E ¼ 83:6ρp

X

Gi X i

ð1Þ

ρp ¼ ρX

i

where Xi is the mole fraction of component i. Recently Morinaga [31] has applied the above equation to predict Young's moduli of several oxide based glasses. The measured and calculated values were found to be in close agreement for large number of glass compositions. In order to evaluate the E values of BTBO glasses theoretically, ρp values

V i xi

i

ð2Þ

Mi xi

i

where xi is mole fraction of constituent oxide, Mi is molecular weight and Vi is the packing density parameter of oxide MpOq with ionic radii Rm and RO defined as Vi ¼

  4 3 3 πN pRM þ qRO 3 Av

ð3Þ

where NAv is Avogadro number. The ρp values are given in Table 1. ρp of BT25 is greater than BT40 glass due to its small Vm. Eq. (1) was used to calculate the E values. Dissociation energies of oxide components present in the BTBO system are G(TiO2) = 86.61, G(BaO) = 40.6, G3 ([BO3]) = 16.32 and G4 ([BO4]) = 77.8 kJ cm−3 [30]. Fraction of BO4 (N4) units obtained from NMR experiments is used to calculate the effective dissociation energy of B2O3 (GB) for the present glass systems using the following relation GB ¼ N4 ðG4 −G3 Þ þ G3

ð4Þ

Table 3 Density (ρ), volume fraction (x), particle size (dSEM) and crystallite size (dScherror) of GNCCs.

Fig. 3. SEM image of indentation cracks produced along the corners of Vicker's indenter.

Sample

ρ (g cm−3) ± 0.005

x from density % (±1)

x from XRD % (±1)

dSEM (nm) ± 2

dScherror (nm)

BT40 HT600 BT40 HT650/1 h BT40 HT650/3 h BT40 HT700/1 h

3.35

6



8



3.36

8

4

25

15

3.38

12

5

35

24

3.39

14

10

85

47

G. Paramesh, K.B.R. Varma / Journal of Non-Crystalline Solids 380 (2013) 128–134

Calculated Young's modulus (Ecal) parameters are found to be 94 and 107 GPa for BT25 and BT40 glasses respectively. Though BT40 has small packing density due to large molar volume, the Young's modulus is found to be higher than that of BT25. This is attributed to higher TiO2 content (high dissociation energy), large number of TiO6 octahedra present in the network of BT40. If the system contains higher number of NBOs, they facilitate easier atomic displacements when stress is applied and result in small elastic modulus. Poisson's ratio (ν) of glasses is directly related to the packing density of the glass with the following equation proposed by Makishima and Mackenzie [33] v ¼ 0:5−

1 7:2ρp

ð5Þ

Calculated νcal values using the above equation (Eq. (5)) for BT25 and BT40 are 0.28 and 0.26 respectively. BT40 having more open structure i.e. small ρp resulted in low ν as compared to that of BT25 glass. The calculated values are consistent as ν values for most of the oxide glasses lie in the range of 0.2–0.3. 3.2. Nano/micro indentation mechanical parameters of glasses Nanoindentation experiments were carried out on BT25 and BT40 glass systems for evaluation of hardness (H) and E. Load/unload vs.

131

displacement curves for the BT25 and BT40 glasses are shown in Fig. 2. Oliver and Pharr [34,35] method was invoked to analyze the load/unload–displacement curves to obtain E and H values. Indentation Young's modulus (EIT) of the material calculated from the experiments using the following expression     1−v2 1−v2i 1 ¼ þ Er EIT Ei

ð6Þ

where Er is the reduced modulus obtained from load–displacement curves and Ei = 1140 GPa, νi = 0.07 are Young's modulus and Poisson's ratio of indenter material (diamond). Here ν values for glasses obtained from Eq. (5) were used to deduce the EIT values of the glasses. Obtained H and EIT values are listed in Table 2. H and E values are found to be moderately higher for BT40 glass, which again could be attributed to the large TiO2 content associated with TiO6 octahedral units. The calculated E values in the previous section and values obtained from nanoindentation were found to be in reasonable agreement. However, calculated E values are found to be slightly higher than those measured (11% high for BT25 whereas 18% for BT40) values. The theoretical equation is based on the dissociation energies of constituent oxides and packing density. However, the dissociation energy of the oxide components in a compound

Fig. 5. TEM images for the (a) as-quenched glass and heat treated samples at (b) 650 °C/1 h and (c) 650 °C/3 h. Insets correspond to SAD patterns. (d) High resolution image for the sample in (c).

132

G. Paramesh, K.B.R. Varma / Journal of Non-Crystalline Solids 380 (2013) 128–134

would be different as bond strength and environment of the ions will be altered in the network. Vicker's micro-indentation was employed to evaluate the fracture toughness (KC) of the glasses under study. SEM micrograph of the indenter impression which produced cracks is depicted in Fig. 3. Radial cracks were propagated along the indenter corners in half-penny configuration [36–38]. The crack length (C) gives the resistance of the sample to fracture. Various models were developed to estimate KC. The following equation [39,40] which has been standardized for ceramics, glasses and glass-ceramics is used to estimate the KC of the glasses,  1   E 2 P K C ¼ 0:018 3 H C2

ð7Þ

where P is the applied load, E and H are Young's modulus and hardness of the material. E and H obtained from nanoindentation were used. Brittleness parameter (B) which is the ratio between the H and KC is also evaluated and given in Table 2. BT40 glass also exhibited higher fracture toughness apart from the other mechanical properties. Usually glasses with high [BO3] units i.e. low [BO4]/[BO3] ratio in the system gives rise to large fracture toughness [41]. Due to more open structure of BT40 (less ρp and small [BO4]/[BO3] ratio), it relaxes the applied load or given strain energy through the displacement of structural units to neighbourhood positions. This could impart higher fracture toughness to the BT40 system. Mechanical strength (σ) of the glasses has been explained based on the well-known expression derived by Griffith, which is as follows σ¼

rffiffiffiffiffiffiffiffiffi 2Eγ πC

ð8Þ

where γ is the fracture surface energy. It is evident that higher strength is expected in glasses which show higher E (high for BT40 glass) or γ and smaller C. 3.3. Structural and microstructural features of GNCCs BT40 glasses were heat treated in the vicinity of crystallization exotherm of glass (BT40) to grow nanocrystals in the matrix. The XRD patterns for the as-quenched and heat treated samples at different temperatures and durations are shown in Fig. 4. The XRD patterns obtained for as-quenched glass showed broad peaks confirming the amorphous nature or short-range order associated with the glasses. Subjecting the glasses to the heat treatment at 600 °C for 1 h, peak broadness reduced and heat treated sample at 650 °C for 1 h gave rise to diffraction peaks corresponding to the anatase TiO2 phase. The background broad humps that are noticed over here suggest the presence of residual glass phase in heat treated samples and evolution of nanocrystals within the glass matrix. The intensity of the diffraction peaks increased and full width of half maximum (FWHM) decreased for the samples heat treated for longer durations (650 °C for 3 h) and higher temperature, 700 °C (Fig. 4). This indicates the increase in volume fraction and size of crystallites. Scherrer formula was used to calculate the crystallite size (d) in GNCCs from XRD peaks and values obtained therein are given in Table 3. The structure of the glass and GNCCs is characterized using TEM for more insights. TEM micrographs and the SAD patterns are shown in Fig. 5 for base glasses and heat treated samples. As-quenched sample's SAD pattern exhibited diffuse rings (Fig. 5a) which confirms the amorphous nature. Whereas for heat treated samples (at 650 °C for 1 h and 3 h), the evolution of nanocrystals are observed from TEM images which are depicted in Fig. 5b and c. Apparent diffraction rings detected

Fig. 6. SEM micrographs of anatase crystallized glasses heat treated at (a) 600 °C/1 h, (b) 650 °C/1 h, (c) 650 °C/3 h and (d) 700 °C/1 h.

G. Paramesh, K.B.R. Varma / Journal of Non-Crystalline Solids 380 (2013) 128–134

in SAD patterns ascertain the crystalline nature. All the rings were indexed to anatase TiO2 phase by calculating the inter-planar spacing from the ring radius and compared it with standard XRD values. The regular fringe pattern in high resolution TEM image, as shown in Fig. 5d, corroborated the presence of nanocrystallites within the glass matrix. The microstructural characteristics as revealed by scanning electron microscope for the GNCCs are shown in Fig. 6. Fine nano particles associated with spherical morphology are witnessed in the micrographs. Samples heat treated at 650 °C showed micro flaws and porosity (Fig. 6b and c) whereas the 700 °C heat treated sample (SEM image shown in Fig. 6d) had densely populated particulates. Average particle size was also measured using SEM. Density obtained for GNCCs has increased with heat treatment temperature. The increase in density of GNCCs is directly related to the crystal growth and their volume fraction. Volume fraction (x) of the crystallites in the matrix is calculated from the density data using the following equation [42]

x ¼ 100 

1 1 − ρg ρτ 1 1 − ρg ρc

! !

ð9Þ

where ρg is density of the as-quenched glass, ρτ is the density of the sample heat treated at certain temperature for time τ and ρc is the

133

density of the crystalline phase (for anatase TiO2, ρc = 3.8 g cm−3.). The x values that are obtained using Eq. (9) are tabulated in Table 3. The x values were also estimated from the XRD data by taking the intensity of the strong diffraction peak into account using the following formula [23] x¼

   xR I IR ki

ð10Þ

where xR is the percentage of reference material (KCl in the present case) and ki corresponds to the reference intensity ratio of the strongest diffraction lines of the mixture of polycrystalline anatase TiO2 powder and KCl. I and IR are the intensities of diffraction peaks of the mixture of GNCCs (pulverized) and KCl respectively. x values obtained from the Eq. (10) are given in Table 3. x values obtained from XRD data were found to be lower than those calculated by density measurements. This difference may be due to change in density of the glass matrix with crystallization and the net density of glass would be less than that of the base glass density, ρg used in Eq. (10) to estimate x. However, if one looks into the glass composition 0.4BaO–0.4TiO2–B2O3, TiO2 is ≈ 22%. Since TiO2 is the crystallized phase in heat treated samples, large portion of the matrix remains in glassy state in which fine nanocrystals of TiO2 were dispersed. Therefore, the experimentally determined x values at different heat treatments are in agreement within the experimental errors as compared to the total TiO2 present in the base glass composition. Density, x and d values of the GNCCs are summarized in Table 3.

Fig. 7. Post-mortem nanoindentation SEM micrographs for the (a) as-quenched and heat treated at (b) 650 °C/1 h, (c) 650 °C/3 h and (d) 700 °C/1 h samples.

134

G. Paramesh, K.B.R. Varma / Journal of Non-Crystalline Solids 380 (2013) 128–134

Mechanical properties of the GNNCs were also studied and presented below.

Table 4 Hardness (H), Young's modulus (EIT), fracture toughness (KC) and brittleness (B) of GNCCs. Sample

H (GPa) ± 0.2

EIT (GPa) ± 0.2

KC (MPa m1/2) ± 0.05

B (μm−1/2) ± 0.2

BT40 HT650/1 h BT40 HT650/3 h BT40 HT700/1 h

6.3 8.4 8.4

74 101 106

0.68 0.57 1.25

9.2 14.7 6.7

3.4. Mechanical properties of GNCCs Mechanical properties of the GNCCs were obtained through nano and micro indentation studies using similar procedure as described previously in the case of base glasses (Section 3.2). Post-mortem SEM micrographs of nanoindentation for base glass as well as GNCCs are shown in Fig. 7. Three sided pyramid (Birkovich) impressions having equal sides are seen in the micrographs. Existence of microcracks on surface of the base glasses can be noticed. These microcracks are being present prior to the indentation as seen in SEM (not shown) and not produced due to nanoindentation. Brittle solids like glasses and glass-ceramics are prone to form surface flaws (while grinding and polishing). GNCCs showed fewer microcracks suggesting the improved mechanical stability. Post-mortem nanoindentation SEM micrographs indicate no severe chipping or damage to the samples. This was further confirmed by the absence of pop-in events in the load–displacement curves (plotted in Fig. 8), implying the elastic deformation of the samples under study. Oliver and Pharr method was employed to extract the mechanical parameters of GNCCs. Poisson's ratio of the base glass was taken in order to evaluate the Young's moduli of GNCCs. All the parameters are summarized in Table 4. It was found that there is an increase in H and EIT of the GNCCs with the increasing heat treatment temperature. This is attributed to the increasing density of the GNCCs due to particulates present in the glass. It is generally observed that the mechanical properties of glasses would be improved upon partial crystallization [43]. KC of the GNCCs is found to be less than that of the base glass for the samples heat treated at 650 °C; whereas the one heat treated at 700 °C is found to show large KC = 1.25 MPa m1/2 and low B. This behavior is attributed to the sensitivity of KC to the surface features such as presence of flaws, pores and internal stresses. The microstructure of the samples heat treated at low temperature is found to possess the microflaws and pores in the system (Fig. 6b and c). The samples heat treated at 700 °C (Fig. 6d) had large volume fraction of the crystallites accompanied by better surface features. Increase in x and d of the particulates, mean free path or spacing between crystallites decreases which affect the mechanical strength of the GNCCs due to impediment of micro flaws propagation in the glassy region [44]. Smaller particulates present in the glass matrix deflect the crack propagation due to higher strength of the crystalline phase, in the present case TiO2 crystallites. However, the residual stresses present in the composites due to thermal expansion mismatch of particulates and the glass matrix along with the surface energy difference influence the hardness and toughness of the specimens [45,46].

Fig. 8. Load–displacement curves for GNCCs.

4. Conclusions Mechanical properties of glasses and GNCCs in BaO–TiO2–B2O3 system were investigated through nano and micro indentation studies. Tetrahedral and trigonal borate units present in glasses were estimated through NMR studies. Predicted and indentation elastic moduli of glasses are in reasonable agreement. BT40 glass was found to possess improved mechanical properties. The GNCCs showed enhanced mechanical behavior as compared to that of the base glasses. The results obtained were correlated with the structural and microstructural features of the glasses and GNCCs. References [1] T. Honma, Y. Benino, J. Am. Ceram. Soc. 88 (2005) 989. [2] S. Kosaka, Y. Benino, T. Fujiwara, V. Dimitrov, T. Komatsu, J. Solid State Chem. 178 (2005) 2067. [3] G.S. Murugan, K.B.R. Varma, J. Mater. Chem. 12 (2002) 1426. [4] R. Vaish, V. Rodriguez, M. Maglione, J. Etourneau, K.B.R. Varma, Int. J. Appl. Glas. Sci. 1 (2010) 350. [5] P. Becker, Adv. Mater. 10 (1998) 979. [6] T. Sasaki, Y. Mori, M. Yoshimura, Y.K. Yap, T. Kamimura, Mater. Sci. Eng., R 30 (2000) 1. [7] C.A.C. Feitosa, V.R. Mastelaro, A.R. Zanatta, A.C. Hernandes, E.D. Zanotto, Opt. Mater. 28 (2006) 935. [8] A. Bhargava, R.L. Snyder, R.A. Condrate Sr., Mater. Lett. 7 (1988) 185. [9] A. Bhargava, J.E. Shelby, R.L. Snyder, J. Non-Cryst. Solids 102 (1988) 136. [10] G. Paramesh, K.B.R. Varma, J. Am. Ceram. Soc. 95 (2012) 2876. [11] G. Paramesh, K.B.R. Varma, Adv. Mater. Res. 622–623 (2013) 950. [12] G. Paramesh, K.B.R. Varma, Int. J. Appl. Glas. Sci. 4 (2013) 248. [13] M. Grätzel, J. Photochem. Photobiol. C 4 (2003) 145. [14] M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, Renew. Sustain. Energy Rev. 11 (2007) 401. [15] G. Fu, P.S. Vary, C.-T. Lin, J. Phys. Chem. B 109 (2005) 8889. [16] T. Watanabe, A. Nakajima, R. Wang, M. Minabe, S. Koizumi, A. Fujishima, K. Hashimoto, Thin Solid Films 351 (1999) 260. [17] K. Hashimoto, H. Irie, A. Fujishima, Jpn. J. Appl. Phys. 44 (2005) 8269. [18] H. Masai, T. Toda, Y. Takahashi, T. Fujiwara, Appl. Phys. Lett. 94 (2009) 151910. [19] H. Masai, T. Fujiwara, H. Mori, Appl. Phys. Lett. 92 (2008) 141902. [20] K. Shinozaki, T. Honma, T. Komatsu, J. Non-Cryst. Solids 358 (2012) 1863. [21] S. Striepe, M. Potuzak, M.M. Smedskjaer, J. Deubener, J. Non-Cryst. Solids 362 (2013) 40. [22] F.H. Chung, J. Appl. Crystallogr. 7 (1974) 519. [23] A.M. Rodrigues, J.M.R. Mercury, V.S. Leal, A.A. Cabral, J. Non-Cryst. Solids 362 (2013) 114. [24] P.J. Bray, J.O. Edwards, J.G. O'Keefe, V.F. Ross, I. Tatsuzaki, J. Chem. Phys. 35 (1961) 435. [25] V.K. Michaelis, P.M. Aguiar, S. Kroeker, J. Non-Cryst. Solids 353 (26) (2007) 2582. [26] L. van Wullen, W. Muller-Warmuth, Solid State Nucl. Magn. Reson. 2 (1993) 279. [27] N.J. Clayden, S. Esposito, A. Aronne, P. Pernice, J. Non-Cryst. Solids 249 (1999) 99. [28] S. Kroeker, J.F. Stebbins, Inorg. Chem. 40 (2001) 6239. [29] T. Takaishi, J. Jin, T. Uchino, T. Yoko, J. Am. Ceram. Soc. 83 (2000) 2543. [30] A. Makishima, J.D. Mackenzie, J. Non-Cryst. Solids 12 (1973) 35. [31] S. Inaba, S. Fujino, K. Morinaga, J. Am. Ceram. Soc. 82 (12) (1999) 3501. [32] S. Inabaw, S. Fujino, J. Am. Ceram. Soc. 93 (2010) 217. [33] A. Makishima, J.D. Mackenzie, J. Non-Cryst. Solids 17 (1975) 147. [34] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [35] W.C. Oliver, G.M. Pharr, J. Mater. Res. 19 (2004) 3. [36] B.R. Lawn, M.V. Swain, J. Mater. Sci. 10 (1975) 113. [37] B.R. Lawn, A.G. Evans, D.B. Marshall, J. Am. Ceram. Soc. 63 (1980) 574. [38] T.J. Lardner, J.E. Ritter, M.L. Shiao, M.R. Lin, Int. J. Fract. 44 (1990) 133. [39] Japanese Industrial Standards, Report 1607, Nippon Kikaku Kyokai, Tokyo, Japan, 1990. 7. [40] F. Torres, Y. Benino, T. Fujiwara, T. Komatsu, Mater. Res. Bull. 39 (2004) 1431. [41] T. Watanabe, K. Muratsubaki, Y. Benino, H. Saitoh, T. Komatsu, J. Mater. Sci. 36 (2001) 2427. [42] A. Karamanov, M. Pelino, J. Eur. Ceram. Soc. 19 (1999) 649. [43] P.W. Mcmillan, Glass-Ceramics, second ed. Academic Press, London, 1979. [44] K.J. Anusavice, N. Zhang, J. Am. Ceram. Soc. 80 (1997) 1353. [45] N. Miyata, H. Jinno, J. Mater. Sci. 17 (1982) 547. [46] N. Miyata, H. Jinno, J. Mater. Sci. 16 (1982) 2205.