Journal of Non-Crystalline Solids 354 (2008) 3793–3798

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Journal of Non-Crystalline Solids

journal homepage: www.elsevier .com/locate / jnoncrysol

Nano- and micro-indentation studies on lithium borate–barium bismuthniobate glasses

Kaushik Das a, C. Karthik b, K.B.R. Varma b, U. Ramamurty a,*

a Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, Indiab Materials Research Center, Indian Institute of Science, Bangalore 560 012, India

a r t i c l e i n f o

Article history:Received 5 December 2007Received in revised form 5 May 2008Available online 24 June 2008

PACS:62.20.de62.20.mj62.20.mm62.25.Mn64.70.kj

Keywords:CrystallizationGlass–ceramicsMechanical propertiesFractureHardnessIndentationMicroindentationBorates

0022-3093/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.jnoncrysol.2008.05.009

* Corresponding author. Tel.: +91 80 2293 3241; faE-mail address: ramu@materials.iisc.ernet.in (U. R

a b s t r a c t

The structure and mechanical properties of multifunctional lithium tetra-borate based glasses and glass–ceramics of the system (100 � x) Li2B4O7�x(BaO–Bi2O3–Nb2O5) with x = 10, 20 and 30 in molar ratio, havebeen characterized. Nano- and micro-indentation techniques were employed to evaluate the elastic mod-ulus, hardness and toughness of the as-cast and annealed glasses. These were complemented withdetailed structural investigations using X-ray diffraction, microscopy (optical, scanning electron and highresolution transmission electron microscopies) and nuclear magnetic resonance spectroscopy. Theseinvestigations reveal a smooth variation of the mechanical properties with composition except for thecomposition corresponding to x = 20. This deviation has been attributed to subtle changes in the glass-structure due to amorphous phase-separation and heat-treatment-assisted nano-crystallization.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Transparent glass–ceramics of the family of lithium tetra-bo-rate based glasses, containing nanocrystalline Aurivillius familyof oxides, have generated much interest recently due to theirpromising multi-functionality in terms of non-linear optical prop-erties and ferroelectric properties [1,2] and hence have potentialapplications in the field of non-linear optical devices, electro-op-tics and ferroelectric random access memory devices [3]. Thougha considerable volume of literature regarding the dependence ofthe functional properties on composition, processing and micro-structures in these glass–ceramics exists [1–3], their mechanicalbehavior has not been examined in detail. A preliminary and onlystudy to date by Gowda et al. [4] shows that elastic moduli oflithium-based borate glasses are directly related to the glassstructure. From the design perspective as well structural integritystand point of view, it is imperative to develop a detailed under-

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x: +91 80 2360 0472.amamurty).

standing of the mechanical and elastic properties of these materi-als and how they depend on composition, processing conditions,etc. Such an understanding is especially important since theseglasses are brittle in nature.

One of the widely used techniques for the evaluation of theelastic, plastic and fracture properties of metals, ceramics, metallicglasses, polymers, piezoelectric ceramics, biological materials andfilms at both ultra-microscopic and macroscopic level is thedepth-sensing instrumented indentation [5–12]. In the presentstudy, instrumented indentation techniques, namely nano-inden-tation and micro-indentation have been used to evaluate themechanical properties such as elastic modulus (E), hardness (H),toughness (Kc), brittleness and elastic recovery of glasses andglass–ceramics belonging to the system (100 � x) Li2B4O7�x(BaO–Bi2O3–Nb2O5) with x = 10, 20 and 30. These experiments werecomplimented by various characterization techniques like scan-ning electron microscopy (SEM) and nuclear magnetic resonance(NMR) to understand the structure and microstructure of theglasses and in turn rationalize the observed trends in mechanicalproperties.

Table 1Table showing the average elastic modulus and hardness for the glasses and glass–ceramics

Sample E (GPa) H (GPa)

Li2B4O7 71.6 ± 2.5 4.0 ± 0.5x = 10 AQ 103.0 ± 9.2 6.8 ± 0.4x = 10 HT 114.6 ± 3.6 6.9 ± 0.5x = 20 AQ 115.3 ± 5.2 4.5 ± 0.7x = 20 HT 138.8 ± 9.4 5.3 ± 0.3x = 30 AQ 100.1 ± 5.1 8.3 ± 0.1x = 30 HT 119.0 ± 8.1 9.5 ± 1.3

3794 K. Das et al. / Journal of Non-Crystalline Solids 354 (2008) 3793–3798

2. Materials and experiments

The glasses under investigation which belong to the system(100 � x)Li2B4O7�x(BaO–Bi2O3–Nb2O5) have been prepared via theconventional melt-quenching technique using analytical gradeBaCO3, Bi2O3, Nb2O5, Li2CO3 and H3BO3. The procedure involvedwas melting of the stoichiometric mixture of the above mentionedoxides at 1200 �C for 30 min followed by splat-quenching, as de-scribed elsewhere [1]. Splat-quenching leads to the formation oftransparent glassy samples, some of which were subjected to anannealing heat-treatment, whereby they were maintained at a tem-perature of 510 �C for 1 h. This temperature is in the vicinity of thefirst crystallization temperature as determined by thermal analysisand thus produces very-fine size (�40 nm) crystallites, dispersedwithin an amorphous matrix, the structure of which has beeninvestigated using transmission electron microscopy (TEM) [1].

Two sets of samples, namely: (a) as-quenched (AQ) glasses ofpristine Li2B4O7 (LBO) and the compositions corresponding tox = 10, 20 and 30 and (b) heat-treated (HT) (510 �C for 1 h) glass–ceramics of compositions x = 10, 20 and 30 were examined. Thesample-dimensions were approximately 30 � 30 � 2 mm3. Theywere mounted on cold-setting polymer and polished first withSiC paper from grade 1200 to 2500 and then polished up to a1 lm finish with diamond paste, to obtain mirror-finish, as is re-quired for nano-indentation. The test-samples (glasses and glass–ceramics) have always been stored in a vacuum desiccators, andthe samples have come in contact with air for a small amount oftime (2–3 h) during nano-indentation. Moreover, since the sampleshave never come in direct contact with water, it can be expectedthat the diffusion kinetics during atmospheric exposure governingwater contamination would be negligible, so as to have any insig-nificant effect on the variation of mechanical properties. In addi-tion to that, repeat-experiments on the samples, which wereconducted over a range of 6 months, did not show any significantvariation in the mechanical properties. Hence any possible humid-ity effects on the measured mechanical response can be safely ru-led out.

X-ray diffraction (XRD) studies of the glasses and glass–ceram-ics were conducted using CuKa radiation at room temperature toexamine their amorphous and/or crystalline nature. Scanning elec-tron microscopy was employed to study the microstructure of theAQ and HT glass–ceramics. In this case, the surface was coatedwith a thin film of gold and images were captured in the secondaryelectron (SE) mode, with 10 kV operating voltage. The distributionof the constituent elements in the glass–ceramics was also exam-ined by SEM X-ray mapping (FEI Sirion SEM) in SE mode. SEM(JSM-840) was also employed to obtain the high resolution elec-tron microscopy images. Besides, nuclear magnetic resonance(NMR) spectroscopy of the AQ glasses has been conducted toexamine the change, if any, in B-coordination with increase in con-tent of the modifier oxides.

Nano-indentation experiments (Hysitron Triboindenter), werecarried out at room temperature, in air using a Berkovich diamond

indenter. The load–displacement measurements were made underload-control with the maximum indentation force ranging be-tween 1 and 3 mN, depending on the surface roughness. The load-ing and unloading rates were 200 lN/s, with 15 s dwell time at themaximum force. Oliver and Pharr method [5,6] was employed toanalyze the load vs. displacement plots, which gives the E and Hof the sample indented. Here, a Poisson’s ratio, m = 0.20 has beenassumed for all the glasses and glass–ceramics. Note that most bo-rate glasses have been found to have m ranging between 0.20 and0.25 and it has also been found that for a specific system of borateglasses, m varies negligibly with composition [4]. A minimum of 10indentations were performed for each case.

In order to measure the indentation fracture toughness, Kc, ofthe glasses and glass–ceramics, micro-indentation experimentswere performed (CSM Micro-indenter), so as to produce half-pen-ny crack configurations. The micro-indentations were performed atroom temperature using a Vickers indenter, in load control mode,using maximum loads of 1–3 N, with loading and unloading ratesof 1 N/min. The half-penny cracks produced by Vickers micro-indentation were measured using optical microscopes (Carl Zeissand Olympus), in the bright field mode. SEM was also employedfor higher magnification imaging of the indentation impressionsand the crack-morphologies. The model proposed by Lawn andco-workers [13–16] was employed for calculating Kc.

3. Results

3.1. Structural characterization

Figs. 1a and b show the XRD scans obtained on the AQ and heat-treated glasses, respectively. The XRD spectra (Fig. 1(a)) for the AQshow broad humps, which confirm their amorphous nature. TheXRD spectra of the HT samples (Fig. 1(b)) show broad peaks,revealing the very-fine size of the crystallized phase.

The HREM image of the heat-treated glass–ceramic correspond-ing to the composition x = 30 is shown in Fig. 2, which clearly indi-cates the presence of nanocrystallites dispersed in an amorphousmatrix. The SEM images of the AQ samples as well as the x = 10HT samples are featureless, however the SEM images of x = 30glass–ceramics, as shown in Figs. 3(a) and (b), indicate to a distinctpatterning with dark dotted regions aligned between brighterbands. However, X-ray mapping of the constituent elements inthe glass and glass–ceramics show no elemental segregation inthe microstructure.

The NMR study was conducted with the primary aim to exam-ine whether any change in the B coordination was occurring in thesystem. The NMR spectra of the AQ glasses, obtained using 11B asthe nucleus, are shown in Fig. 4. On comparison, the chemicalshifts of the AQ glasses are similar; however, two sharp peaksare observed at around 42.844 ppm for the x = 20 AQ glass, asshown in Fig. 5. Such chemical shifts are absent in the NMR spectraof the other samples under study.

3.2. Mechanical properties

Table 1 summarizes the mechanical properties of various mate-rials examined in this work. Fig. 6 shows the variation of E as afunction of composition. The mean value of E increases from�70 GPa for pristine LBO glass to �115 GPa for x = 20 AQ glass,after which it falls to a value of �100 GPa for the x = 30 glass.The glass–ceramics also show a similar trend, with E reaching amaximum value of �140 GPa for the x = 20 HT specimen and thendecreasing to �120 GPa for the x = 30 specimen. For all the compo-sitions, the glass–ceramics have higher E (�10–20%) than their cor-responding AQ values.

Fig. 1. XRD patterns of (a)AQ glasses and (b) glass–ceramics.

Fig. 2. HREM image of x = 30 glass–ceramics.

K. Das et al. / Journal of Non-Crystalline Solids 354 (2008) 3793–3798 3795

The variation of H as a function of mol% BaO–Bi2O3–Nb2O5

(BBN) is plotted in Fig. 7. As seen, H initially increases to a valueof �7 GPa for the x = 10 AQ specimen, from �4 GPa of pristineLBO glass. However, the hardness exhibits a minimum for thex = 20 AQ glasses, decreasing to a value of �4.5 GPa. H is againfound to increase for the AQ glasses, attaining a value of �8 GPa.The H values of the glass–ceramic reveals a similar zigzag trend,exhibiting a minimum at x = 20 and similar to the previous obser-vation as in the case of E, the H of the HT samples have always beenfound to be greater than that for the corresponding AQ glasses.

Fig. 8 shows a typical optical image of the half-penny crack-con-figuration obtained by Vickers micro-indentation. The cracksformed in almost all the samples are characterized by the presenceof radial cracks, which are initiated from the four corners of theVickers indentation. In some cases, as found in x = 20 samples(Fig. 9), shallow lateral and/or median cracks have also been ob-served, but in no case are secondary radial cracks observed.

The crack-lengths measured from the samples have been usedto calculate the indentation fracture toughness of the samples,using the model proposed by Lawn and co-workers [13–16],according to which, the fracture toughness of an indented materialis given by

KC ¼ nRV

EH

� �12 P

c0

� �32

; ð1Þ

where nRV is an empirical material independent constant having a

value 0.016 ± 0.004, P is the peak load and c0 is the equilibriumcrack-length. Fig. 10 shows the variation of Kc, computed usingthe above equation, with composition. It is observed that Kc of thepristine LBO glass is the highest (�0.7 MPa m0.5) and toughnessdeteriorates on addition of the network modifying oxides. However,the toughness of the x = 20 AQ glass has been found to be muchhigher (�0.5 MPa m0.5) compared to the AQ samples of other com-positions (�0.3–0.25 MPa m0.5). However, for all the compositions,heat-treatment is found to have lower Kc with the trend in the HTsamples similar to that observed in the AQ samples, with a maxi-mum toughness at x = 20.

According to Lawn and Marshall [16], the ratio Kc/H can be viewedas a governing factor determining ductility vs. brittleness in a con-tact problem and the inverse quantity H/Kc has been termed as theindex of brittleness. In Fig. 11, the factor H/Kc has been plotted as afunction of composition. It clearly shows the pristine LBO glass tobe least brittle and brittleness is found to increase on addition ofBBN, with a trend similar to that observed in the case of hardness.However, as has been observed in the case of Kc, heat-treatmentleads to increased brittleness for all the compositions tested.

4. Discussion

The mechanical properties like E, H and Kc, obtained for variouscompositions of the glasses and glass–ceramics have been found todeviate from a smooth trend at x = 20. It is well known that E is afunction of the compactness or density of the glass-structure[17,18]. To ascertain this, the density of the AQ and HT specimenshas been measured using specific gravity method with xylene asthe immersion fluid, at room temperature and pressure [19]. A plotof the density, q, of the glass and glass–ceramics as a function ofcomposition is given in Fig. 12, which shows a monotonic increasein q with BBN content. Thus, the increase in E on increasing thecontent of BBN could be ascribed to increase compactness of theglass structure. Heat-treatment of the AQ glasses leads to an in-crease in density. This increase can be attributed to the high den-sity of the crystallized phase (�7.99 gm/cc) compared to that ofthe pristine LBO glass (�2.44 gm/cc) and thus rationalizes the in-crease in E of HT glass–ceramics over that of the AQ glass.

Fig. 3. SEM images of x = 30 glass–ceramics (a) at 1355 X and (b) at 5422 X.

Fig. 4. NMR spectra of AQ glasses of varying compositions.

Fig. 5. NMR spectra of x = 20 AQ glass.

Fig. 6. Variation of Elastic Modulus with composition and heat-treatment.

Fig. 7. Variation of hardness with composition for both AQ and HT samples.

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However, it is also observed that q, in this case, has a non-lineardependence on composition and the deviation from linearity is ob-served beyond x = 20. This deviation from linearity is possibly dueto phase separation and crystallization. The presence of crystallitesin the HT samples as well as the amorphous nature of the AQ

Fig. 8. Half-penny cracks formed during Vickers indentation of x = 10 AQ glass,showing radial cracking at the four corners of the indentation impression.

Fig. 9. Half-penny cracks produced in x = 20 glass–ceramic, showing radial cracksfrom the four corners, as well as shallow lateral and median cracks, as is evidentfrom the bright curved regions between the radial cracks, adjacent to the Vickersimpression.

Fig. 10. Variation of fracture toughness with composition and heat-treatment.

Fig. 11. Variation of Brittleness with composition and heat-treatment.

Fig. 12. Variation of density with composition and heat-treatment.

K. Das et al. / Journal of Non-Crystalline Solids 354 (2008) 3793–3798 3797

glasses have been reconfirmed by the XRD analysis. The crystal-lized phase was identified to be a fluorite-like BaBi2Nb2O9 and in-dexed accordingly. HREM studies also confirmed the presence ofnanocrystallites. It is worthwhile to note that the patterningobserved during SEM investigation of the HT samples with compo-sition x = 30, clearly points towards the possibility of phase-separa-tion. Besides, TEM analysis of the x = 30 AQ glasses [1] shows theexistence of some irregular patches; but the absence of any diffrac-tion pattern corresponding to any crystalline phase leads to theinference that they are short-range ordered.

In Fig. 13, the ratio of the reversible work to the total work, asobtained from the load–displacement curve, is plotted as a func-tion of composition. The elastic recovery values range between0.4 and 0.6 and show a trend similar to that observed in hardness,with a drop in elastic recovery at x = 20. This observation suggeststhat of all the samples tested the x = 20 glasses and glass–ceramicsundergo the highest amount of plastic deformation during inden-tation among all the tested compositions, which in turn indicatesto a possible change in the glass-structure.

The chemical shifts in the NMR spectra are signatures of thebonding environment of a substance. Hence, the absence of any

Fig. 13. Variation of elastic recovery with composition and heat-treatment.

3798 K. Das et al. / Journal of Non-Crystalline Solids 354 (2008) 3793–3798

chemical shift clearly showed that the B-coordination remains un-changed with the variation in composition. However, the small yetsharp peaks at �42.8 ppm clearly indicates a subtle change in thebonding environment of the B in the x = 20 specimens.

An additional possibility is the nucleation of nano-scale clustersof crystalline phases within the glassy matrix due to indentationpressure, a phenomenon reported in experimental [20] as well asnumerical modeling [21] of indentation studies of metallic glasses.This distribution will then give rise to a compressive stress-fieldwithin the material, which in turn is expected to reduce hardnessand raise the fracture toughness in the x = 20 samples. However,further addition of BBN increases the residual stresses to such anextent that phase-separation ensues at x = 30.

The structure and bonding characters of the oxide glasses differsignificantly from those of metallic glasses where some oxides playthe role of network formers (improving the mechanical properties)while others act as intermediate and/or network-modifier oxides(weakening the bonds within the glass network, leading to reduc-tion in mechanical properties) [17,18]. Nevertheless, it is instruc-tive to draw analogy with the mechanical property variations inamorphous alloys, a widely studied subject.

In case of amorphous alloys, the E, Kc and H are generally foundto increase with partial crystallization [22], with a precipitous dropin fracture strength occurring when the volume fraction of thecrystalline phase exceeds a critical value [23]. Percolation theoryhas been successfully used to explain the mechanical propertiesof the partially crystallized amorphous alloys [24]. In light of this,it is probable that increase in the volume fraction of the short-range ordered domains in the AQ glasses and the crystalline phasein the HT glass–ceramics reaches the percolation threshold �x = 20composition, where a deviation from trend is observed. However,phase separation, which occurs �x = 30 composition, contributessignificantly to the mechanical properties, as is evident from thevariation in hardness and fracture toughness.

The fact that the trend observed in the E is significantly differentfrom that observed in H and Kc is not surprising since it is a phe-nomenon commonly in amorphous materials [24,25]. This is dueto the fact that sensitivity of E and H to the morphology andarrangement of the different phases constituting the microstruc-ture are different [24]. The variation of Kc (and hence B) in oxide

glasses is attributed to the variation of the crack-driving force,which is characterized by the ratio of E to H [26]. The trends ob-served in this study reconfirm the above finding. Besides, the ab-sence of shear bands around the Vickers indentation in the AQ aswell as HT samples indicates the negligible proportion of plasticdeformation, which is evident from the variation of B as well aselastic recovery.

5. Summary

The mechanical properties of glasses and glass–ceramics of thefamily (100 � x) Li2B4O7�x(BaO–Bi2O3–Nb2O5) where x = 0, 10, 20and 30, have been evaluated by instrumented indentation. Theelastic modulus and hardness are found to increase with increasein content of BBN, with a maximum in E and a minimum in H forthe 20 mol% samples. The Kc however decreases on addition ofBBN and in this case the Kc of the x = 20 sample is found to begreater than that of the x = 10 and 30 samples.

The presence of nanocrystallites in the HT samples leads to animprovement in the elastic modulus and hardness of the glass–ceramics, but causes deterioration in the fracture toughness, whencompared to the AQ samples. The combined effects of phase-sepa-ration as well as indentation-pressure assisted crystallization arethought to be possible reasons contributing to the observed re-sponse of the x = 20 samples. Chemical inhomogeneity is unlikelyto control the mechanical properties of these materials.

The NMR spectra shows that there are subtle changes in thebonding environment in the x = 20 glasses, compared to that ofthe other glasses. The exact nature of bonding in the glasses shouldbe a subject for further investigation, but the fact that subtlechanges in structure have enormous consequences on the mechan-ical properties, leads us to conclude that this subtle structuralchange in the x = 20 sample is responsible for its out-of-trendbehaviour.

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