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Ionic migration effects on the mechanical properties of glass surfaces
Journal of Non-Crystalline Solids 247 (1999) 232±236
Ionic migration eects on the mechanical properties of glass surfaces Gilberto Y. Odo *, Leonardo N. Nogueira, Carlos M. Lepienski Laborat orio de Propriedades Nanomec^ anicas, Dept. de Fõsica UFPR, C. Polit ecnico, Cx. Postal 19.081, CEP, 81.531-990 Curitiba, PR, Brazil
Abstract Modi®cations of glass surfaces were produced by the ionic migration of alkaline ions due to an external electric ®eld. Using aluminum plates as electrodes, hydrogen ions penetrate into samples substituting for alkaline ions. With deposited silver ®lms as the positive electrode, Ag ions penetrate into the glass substituting for Na ions in the depleted region. The hardness and residual stresses at glass surface produced by the ionic migration are measured. Hardness was measured using the nanoindentation technique. The surface stress generated by the penetration of foreign ions was measured using the indentation cracks lengths generated by a Vickers indentation. We observed that presence of an internal hydrogen layer produces a tensile residual stress at the sample surface. A compressive stress was observed as a result of a Ag ions layer near the anode region. The measured stresses are compared with the stresses calculated using a model which considers the existence of a layer of foreign ions with dierent ionic radii at the glass surface. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction It is well known that mechanical properties of glasses are dependent on their surface state. Flaws in the surface and residual stresses are factors that aect crack propagation and fracture. Indentation crack in glass and ceramics are aected by residual stresses at the surface [1]. Surface modi®cations by tempering or chemical exchange aect the mechanical properties of glass [2]. Modi®cations in fracture strength of glasses are observed after an ionic exchange of alkali ions by other ions in the glass surface [3]. The indentation fracture method is widely utilized to determine the mechanical properties of brittle materials [4,5].
* Corresponding author. Tel.: +55-41 361 3279; fax: +55-41 361 3408; e-mail: odo@®sica.ufpr.br
Ionic migration is a process that can be induced by an external electric ®eld. Such migration produces ionic modi®cations at glass surfaces. Alkali ions, as Na in soda±lime glasses, drift under the in¯uence of the electric ®eld leaving a layer depleted of sodium ions in the glass close to the anode. This phenomena aects the properties of glass insulators for high voltage direct current transmission lines, since ionic migration may be a cause of failures in these insulators [6,7]. Glass insulators for high voltage direct current transmission lines are submitted to electric ®elds for long periods and ionic migration can aect their mechanical strength. In this paper we study the eect of ionic migration on the mechanical properties of glass surfaces. Depending on the electrode material, certain types of ions, such as Ag ions or hydrogen from air, can penetrate into the glass surface substitut-
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 0 7 6 - 9
G.Y. Odo et al. / Journal of Non-Crystalline Solids 247 (1999) 232±236
ing almost totally for the Na ions in a near surface layer [8,9]. In chemical exchange processes, a compressive stress appears at the glass surface if the foreign ions have a larger radius than the original alkali ion [3]. This eect was shown by Berg et al. [3], who discussed the mechanical eects of the Na ± Ag chemical exchange process in soda-lime silicate glass occurring in an AgNO3 liquid. Using the indentation fracture method, they observed a compressive stress at the glass surface produced by ionic exchange. Carlson [8] and Kaneko [10] by applying an external electric ®eld to the glass using deposited silver electrodes, showed that for each sodium ion that leaves the near anode region in the glass one silver ion penetrates into the glass. Lepienski et al. [9] applied an external electric ®eld to the glass samples, using aluminum plates as electrodes, and showed that a layer of hydrogen ions is observed in the glass surface near the anode surface. In this case, a space charged region near the surface is formed. They also observed an one to one correspondence in the H±Na exchange in the near surface anode region in the glass. As a ®rst approximation, the sodium ions are swept from the near surface anode region and the Ag or H ions occupy the vacated sites [8±10]. The layer depth, d, is determined by the total charge that ¯ows through the glass, by the expression d
Q ; Ns Aq
1
where Q is the total charge passing through the sample, q the charge of the sodium ions, A the electrode area, and Ns the concentration of sodium ions [8,9]. The penetration of dierent sized ions in the material can produce a residual stress at the surface sample. Miltat [11] describes a model to calculate the stress ®eld induced by this eect. If a foreign ion of dierent size substitute for another ion in the material, the induced compressive or tensile stress at the surface can be calculated by E e; 2 rs 1ÿm
233
where E is the elastic modulus, m the Poisson modulus, and e the linear dilatation due to the volume modi®cation, DV, induced by the ionic exchange, and it is given by e
DV ; 3V
3
where V is the volume of the original ion. The toughness of the glass can be measured by determining the lengths of radial cracks induced by a Vickers diamond indentation [12]. In brittle materials, the indentation by Vickers tip generates radial cracks and the toughness is determined by the expression KIc vr
P c3=2 0
;
4
where vr is the residual stress factor, P the indentation peak load and c0 the radial crack length. If a compressive residual stress occurs at the surface, the radial crack length is less than for a nonstressed surface. For tensile residual stress, the crack length is longer. To determine the residual stress in a thin stressed layer at the glass surface several models were proposed based on the measurement of the half-penny radial crack lengths produced at the corners of Vickers indentations [4]. In this case K I K r K a vr
P /ra c1=2 ; c3=2
5
where Kr is the stress intensity factor due to the indentation, considering the half-penny radial cracks to be center-loaded by a point load at the deformation zone, Ka is the residual stress intensity factor due to the residual stress at the glass surface, ra is the compressive or tensile stress at the surface, / is the crack geometry calculated from the crack shape and c is the radial crack length, measured from the impression center, in the stressed conditions. At the equilibrium, after the complete growth and stabilization of the crack, KI KIc . Then the residual stress can be measured by r KIc
1 ÿ c0 =c /c1=2
3=2
:
6
For surface stresses caused by modi®cations only in a thin layer of depth d in the material surface,
234
G.Y. Odo et al. / Journal of Non-Crystalline Solids 247 (1999) 232±236
Lawn [5] proposed the following expression to determine the residual stress: r KIc
1 ÿ c0 =c3=2 : 1=2 /d 1=2 2 ÿ d=c
7
Since Lawn [5] only considered surface half-penny cracks and the stress state can be simpli®ed as plane stress, it was assumed / p1=2 [1]. 2. Experimental procedure The samples were soda-lime silicate glass used for microscopy slices, 1.2 mm thick. The glass composition is: 72.9% of SiO2 , 12.6% of Na2 O, 7.9% of CaO, 2.4% of MgO, 2.2% of Al2 O3 and 2% other compounds (in wt%). Two kinds of electrode were used: (a) thermal evaporated vacuum deposited silver metal ®lms on the glass; (b) aluminum plates pressed against the glass samples. In both cases, the electrode area was 2 cm2 . Using silver electrodes the applied electrical ®eld was typically 50 V/mm at a temperature of about 150°C. The current that ¯ows through the samples was recorded and the total charge calculated by integrating the current. Using this procedure and Eq. (1), samples with exchanged sodium± silver ion layers with calculated thickness of 1, 3 and 5 lm were produced. After the electric ®eld application, the silver electrodes were dissolved by nitric acid. For the case of aluminum plate electrodes, the applied electric ®eld was about 1000 V/ mm, at a temperature of 150°C. In this case, samples with exchanged sodium±hydrogen ion layers with calculated thickness of 1, 3 and 5 lm were produced. The indentations were performed by using a Vickers indenter diamond tip and loads of 5 N, with a 30 s dwell time. The lengths of the radial cracks produced by the indentation were measured using an optical microscope, approximately 20 min after each indentation to minimize the slow crack growth eect. A ®xed load, P 5 N, was used for all tests. This was the smallest load for which the crack presented always the well formed typical radial pattern. The measurements of hardness were performed using the technique described by Oliver and Pharr
[13], through the analysis of the loading/unloading curves obtained with a nanoindenter. The loads varied from 0.5 to 400 mN to obtain information in a range of penetration depths. For each load, 10 indents were performed. 3. Results In Fig. 1 the hardness is shown as a function of the tip penetration depth for the samples submitted to ionic migration using aluminum foils in contact with the glass as electrodes. With this electrode, hydrogen ions penetrate into the glass replacing the sodium ions in a near surface layer at the anode region. The results for samples with ionic migration modi®ed layers with calculated thickness of 1, 3 and 5 lm are presented. Hardness is less in the aected region than in the sample without modi®cations. For the sample with 1 lm of modi®ed layer, the hardness at surface is less than in the original glass surface. At penetrations greater than 1 lm the hardness tends to those obtained for unmodi®ed surfaces. Loads to obtain penetration depths greater than 1.6 lm were not used. For the samples with an ionic migration layer thickness of 3 and 5 lm, the hardness was less at a penetration depth of 1.0 lm. In Fig. 2 the hardness as a function of tip penetration depth is presented for the samples in
Fig. 1. Hardness as a function of tip penetration, obtained by nanoindentation, for glass samples submitted to ionic migration using aluminum foils as electrodes. The lines in the graph only connect the experimental points for each sample, and do not represent a ®tting.
G.Y. Odo et al. / Journal of Non-Crystalline Solids 247 (1999) 232±236
Fig. 2. Hardness as a function of tip penetration, obtained by nanoindentation, for glass samples submitted to ionic migration using silver ®lms as electrodes. The lines in the graph only connect the experimental points for each sample, and do not represent a ®tting.
which the electric ®eld was applied using silver deposited electrodes. With this kind of electrode, the penetration of silver ions replaces the sodium ions in the glass. The Ag±Na ion exchanged layers with calculated thickness of 1, 3 and 5 lm were indented. The results for the samples with modi®ed layers with depths of 1 and 3 lm are presented in Fig. 2. The hardness in samples with a 5 lm ionic migration layer depth had a greater dispersion in the hardness and the results were not included in the graph. It can be observed in Fig. 2 that alterations in hardness due to the presence of the silver layer at the surface are observed. Surface residual stress induced by the ionic migration was calculated by the measurements of the radial cracks lengths generated by the Vickers indentation, using the lengths obtained for the
235
modi®ed and the pristine glass surfaces. The results are summarized in Table 1. The calculations were performed using KIc 0.75 MPa m1=2 as the fracture toughness for glass [8]. Residual stresses for glasses with ionic migration performed using silver ®lm electrodes increases with the thickness of the layers modi®ed by Ag ions. For the samples with hydrogen ions replacing Na , the residual stress decreased with the depth of the modi®ed layer. The dispersion observed in the measured stresses is normally observed when this type of experimental procedure is used.
4. Discussion It is well established [14] that the surface hardness increases if the surface is under compression and decreases if the surface is submitted to a tensile stress. Analyzing the hardness and residual stresses for the samples on which the electric ®eld was applied using aluminum plates, a residual tensile stress and a decrease in the hardness of the aected region were observed. The hardness showed the expected behavior. However, the residual stress decreased as the amount of hydrogen in the glass increased. Optical microscopy of 5 lm depth sample, showed a large number of cracks at the surface. These cracks were not observed in the 1 lm depth layer sample. The smaller residual stresses could be attributed to a stress relaxation process. The residual stresses generated during the ionic migration process could relax due to
Table 1 Residual stress for the samples submitted to ionic migration Residual stress, r (MPa)
Electrode
Penetrating ion
Applied load, P (N)
Layer depth, d (lm)
Crack length modi®ed surface, c (lm)
Crack length reference region, c0 (lm)
Al plate
H
5 5 5
1 3 5
66 6 66 3 64 4
60 5
31 23 17 9 99
Ag evaporated ®lm
Ag
5
1
57 2
60 5
ÿ19 11
5 5
3 5
44 3 35 5
ÿ87 22 ÿ142 50
236
G.Y. Odo et al. / Journal of Non-Crystalline Solids 247 (1999) 232±236
the growth of existing surface ¯aws that generated the observed cracks. Then, the cracks formed during the indentation did not extend as expected due to the fact the surface stresses were relaxed by the crack growth of existing ¯aws. The residual stresses were calculated using a constant vr . However it depends on the hardness and elastic modulus [4]. The hardness for ionic migration samples diered from those observed in virgin samples. Considering this eect, vr shows a maximum increase of 6%, in the region of the exchanged ions with thickness d. As d c, and the dispersion in the calculated stresses is larger, the modi®cation of hardness in the calculated stresses can be disregarded. For the samples with penetration of silver ions, no alteration in hardness was observed when comparing the region modi®ed by ionic migration with a virgin sample. It was expected the hardness would be larger in this region since a compressive stress is observed by the indentation fracture method. Optical microscopy of the glass surface in the modi®ed region, after the ionic migration with silver electrodes, showed the presence of pits in the anode region. These pits may be due to a variable concentration of silver ions in the glass surface. The number of pits increases with the total charge that circulates through the sample. By observing the surface, the residual stress, and hardness, we infer that stress increases with increasing Ag content. As the number of pits increases at the surface they aect crack propagation and residual stress. The hardness as measured by nanoindentation was not aected as the indentations were made in regions with smaller numbers of pits. However, some indentations were probably in a region with pits, and the larger deviations from the typical load vs displacement were eliminated from the calculation of hardness. The mean hardness is only that for the relatively pit free region. In the case of the 5 lm layer sample, the number of pits increased and the dispersion in the indentation hardness indicates that the probability of indenting in a pit increases. The residual stress calculated by using the Eqs. (2) and (3) assuming the substitution of a Na ion, with radius of 0.098 nm, by an Ag ion of radius of 0.123 nm gave 320 MPa. Comparing
320 MPa with the measured 142 MPa, we note that some stress relaxation may occur after the penetration of the Ag ions. Another possibility is that the eective volume occupied by each ion diers from the ionic radius. 5. Conclusions The ionic migration induced by the external electric ®eld produced residual stresses in the anode region. The residual stress is compressive in the case for which a silver layer is used as electrode. This compressive stress is due to silver ions replacing sodium ions as they penetrate into interior of a sample. The measured residual stresses are attributed to the relaxation of stresses by the growth of existing ¯aws in the glass. Acknowledgements This work was supported ®nancially by PADCT, CNPq and FINEP. References [1] J. Salomonson, K. Zeng, D. Rowclie, Acta Mater. 44 (1996) 543. [2] M. Chaudhri, M.A. Phillips, Philos. Mag. A 62 (1990) 1. [3] K.J. Berg, P. Grau, D. Nowak-Wozny, M. Petzold, M. Suszynska, Mater. Chem. Phys. 40 (1995) 131. [4] B. Lawn, Fracture of Brittle Solids, 2nd ed., ch. 8, Cambridge University, New York, 1995, pp. 249±306. [5] B.R. Lawn, E.R. Fuller Jr., J. Mater. Sci. 19 (1984) 4061. [6] D. Decker, G. Marrone, K. Naito, C.A.O. Peixoto, J.P. Reynders, Electra 153 (1994) 22. [7] C.A.O. Peixoto, L. Pargamin, G.E. Marrone, G. Carrara, IEEE Trans. Power Delivery 3 (1988) 776. [8] D.E. Carlson, K.W. Hang, G.F. Stockdale, J. Am. Ceram. Soc. 55 (1972) 337. [9] C.M. Lepienski, J.A. Giacometti, G.F.L. Ferreira, F.L. Freire Jr., C.A. Achete, J. Non-Cryst. Solids 159 (1993) 204. [10] T. Kaneko, J. Non-Cryst. Solids 120 (1990) 188. [11] J. Miltat, Philos. Mag. A 55 (1987) 543. [12] G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, J. Am. Ceram. Soc. 64 (1981) 9. [13] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [14] T.Y. Tsui, W.C. Oliver, G.M. Pharr, J. Mater. Res., 1995.
Ionic migration eects on the mechanical properties of glass surfaces Gilberto Y. Odo *, Leonardo N. Nogueira, Carlos M. Lepienski Laborat orio de Propriedades Nanomec^ anicas, Dept. de Fõsica UFPR, C. Polit ecnico, Cx. Postal 19.081, CEP, 81.531-990 Curitiba, PR, Brazil
Abstract Modi®cations of glass surfaces were produced by the ionic migration of alkaline ions due to an external electric ®eld. Using aluminum plates as electrodes, hydrogen ions penetrate into samples substituting for alkaline ions. With deposited silver ®lms as the positive electrode, Ag ions penetrate into the glass substituting for Na ions in the depleted region. The hardness and residual stresses at glass surface produced by the ionic migration are measured. Hardness was measured using the nanoindentation technique. The surface stress generated by the penetration of foreign ions was measured using the indentation cracks lengths generated by a Vickers indentation. We observed that presence of an internal hydrogen layer produces a tensile residual stress at the sample surface. A compressive stress was observed as a result of a Ag ions layer near the anode region. The measured stresses are compared with the stresses calculated using a model which considers the existence of a layer of foreign ions with dierent ionic radii at the glass surface. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction It is well known that mechanical properties of glasses are dependent on their surface state. Flaws in the surface and residual stresses are factors that aect crack propagation and fracture. Indentation crack in glass and ceramics are aected by residual stresses at the surface [1]. Surface modi®cations by tempering or chemical exchange aect the mechanical properties of glass [2]. Modi®cations in fracture strength of glasses are observed after an ionic exchange of alkali ions by other ions in the glass surface [3]. The indentation fracture method is widely utilized to determine the mechanical properties of brittle materials [4,5].
* Corresponding author. Tel.: +55-41 361 3279; fax: +55-41 361 3408; e-mail: odo@®sica.ufpr.br
Ionic migration is a process that can be induced by an external electric ®eld. Such migration produces ionic modi®cations at glass surfaces. Alkali ions, as Na in soda±lime glasses, drift under the in¯uence of the electric ®eld leaving a layer depleted of sodium ions in the glass close to the anode. This phenomena aects the properties of glass insulators for high voltage direct current transmission lines, since ionic migration may be a cause of failures in these insulators [6,7]. Glass insulators for high voltage direct current transmission lines are submitted to electric ®elds for long periods and ionic migration can aect their mechanical strength. In this paper we study the eect of ionic migration on the mechanical properties of glass surfaces. Depending on the electrode material, certain types of ions, such as Ag ions or hydrogen from air, can penetrate into the glass surface substitut-
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 0 7 6 - 9
G.Y. Odo et al. / Journal of Non-Crystalline Solids 247 (1999) 232±236
ing almost totally for the Na ions in a near surface layer [8,9]. In chemical exchange processes, a compressive stress appears at the glass surface if the foreign ions have a larger radius than the original alkali ion [3]. This eect was shown by Berg et al. [3], who discussed the mechanical eects of the Na ± Ag chemical exchange process in soda-lime silicate glass occurring in an AgNO3 liquid. Using the indentation fracture method, they observed a compressive stress at the glass surface produced by ionic exchange. Carlson [8] and Kaneko [10] by applying an external electric ®eld to the glass using deposited silver electrodes, showed that for each sodium ion that leaves the near anode region in the glass one silver ion penetrates into the glass. Lepienski et al. [9] applied an external electric ®eld to the glass samples, using aluminum plates as electrodes, and showed that a layer of hydrogen ions is observed in the glass surface near the anode surface. In this case, a space charged region near the surface is formed. They also observed an one to one correspondence in the H±Na exchange in the near surface anode region in the glass. As a ®rst approximation, the sodium ions are swept from the near surface anode region and the Ag or H ions occupy the vacated sites [8±10]. The layer depth, d, is determined by the total charge that ¯ows through the glass, by the expression d
Q ; Ns Aq
1
where Q is the total charge passing through the sample, q the charge of the sodium ions, A the electrode area, and Ns the concentration of sodium ions [8,9]. The penetration of dierent sized ions in the material can produce a residual stress at the surface sample. Miltat [11] describes a model to calculate the stress ®eld induced by this eect. If a foreign ion of dierent size substitute for another ion in the material, the induced compressive or tensile stress at the surface can be calculated by E e; 2 rs 1ÿm
233
where E is the elastic modulus, m the Poisson modulus, and e the linear dilatation due to the volume modi®cation, DV, induced by the ionic exchange, and it is given by e
DV ; 3V
3
where V is the volume of the original ion. The toughness of the glass can be measured by determining the lengths of radial cracks induced by a Vickers diamond indentation [12]. In brittle materials, the indentation by Vickers tip generates radial cracks and the toughness is determined by the expression KIc vr
P c3=2 0
;
4
where vr is the residual stress factor, P the indentation peak load and c0 the radial crack length. If a compressive residual stress occurs at the surface, the radial crack length is less than for a nonstressed surface. For tensile residual stress, the crack length is longer. To determine the residual stress in a thin stressed layer at the glass surface several models were proposed based on the measurement of the half-penny radial crack lengths produced at the corners of Vickers indentations [4]. In this case K I K r K a vr
P /ra c1=2 ; c3=2
5
where Kr is the stress intensity factor due to the indentation, considering the half-penny radial cracks to be center-loaded by a point load at the deformation zone, Ka is the residual stress intensity factor due to the residual stress at the glass surface, ra is the compressive or tensile stress at the surface, / is the crack geometry calculated from the crack shape and c is the radial crack length, measured from the impression center, in the stressed conditions. At the equilibrium, after the complete growth and stabilization of the crack, KI KIc . Then the residual stress can be measured by r KIc
1 ÿ c0 =c /c1=2
3=2
:
6
For surface stresses caused by modi®cations only in a thin layer of depth d in the material surface,
234
G.Y. Odo et al. / Journal of Non-Crystalline Solids 247 (1999) 232±236
Lawn [5] proposed the following expression to determine the residual stress: r KIc
1 ÿ c0 =c3=2 : 1=2 /d 1=2 2 ÿ d=c
7
Since Lawn [5] only considered surface half-penny cracks and the stress state can be simpli®ed as plane stress, it was assumed / p1=2 [1]. 2. Experimental procedure The samples were soda-lime silicate glass used for microscopy slices, 1.2 mm thick. The glass composition is: 72.9% of SiO2 , 12.6% of Na2 O, 7.9% of CaO, 2.4% of MgO, 2.2% of Al2 O3 and 2% other compounds (in wt%). Two kinds of electrode were used: (a) thermal evaporated vacuum deposited silver metal ®lms on the glass; (b) aluminum plates pressed against the glass samples. In both cases, the electrode area was 2 cm2 . Using silver electrodes the applied electrical ®eld was typically 50 V/mm at a temperature of about 150°C. The current that ¯ows through the samples was recorded and the total charge calculated by integrating the current. Using this procedure and Eq. (1), samples with exchanged sodium± silver ion layers with calculated thickness of 1, 3 and 5 lm were produced. After the electric ®eld application, the silver electrodes were dissolved by nitric acid. For the case of aluminum plate electrodes, the applied electric ®eld was about 1000 V/ mm, at a temperature of 150°C. In this case, samples with exchanged sodium±hydrogen ion layers with calculated thickness of 1, 3 and 5 lm were produced. The indentations were performed by using a Vickers indenter diamond tip and loads of 5 N, with a 30 s dwell time. The lengths of the radial cracks produced by the indentation were measured using an optical microscope, approximately 20 min after each indentation to minimize the slow crack growth eect. A ®xed load, P 5 N, was used for all tests. This was the smallest load for which the crack presented always the well formed typical radial pattern. The measurements of hardness were performed using the technique described by Oliver and Pharr
[13], through the analysis of the loading/unloading curves obtained with a nanoindenter. The loads varied from 0.5 to 400 mN to obtain information in a range of penetration depths. For each load, 10 indents were performed. 3. Results In Fig. 1 the hardness is shown as a function of the tip penetration depth for the samples submitted to ionic migration using aluminum foils in contact with the glass as electrodes. With this electrode, hydrogen ions penetrate into the glass replacing the sodium ions in a near surface layer at the anode region. The results for samples with ionic migration modi®ed layers with calculated thickness of 1, 3 and 5 lm are presented. Hardness is less in the aected region than in the sample without modi®cations. For the sample with 1 lm of modi®ed layer, the hardness at surface is less than in the original glass surface. At penetrations greater than 1 lm the hardness tends to those obtained for unmodi®ed surfaces. Loads to obtain penetration depths greater than 1.6 lm were not used. For the samples with an ionic migration layer thickness of 3 and 5 lm, the hardness was less at a penetration depth of 1.0 lm. In Fig. 2 the hardness as a function of tip penetration depth is presented for the samples in
Fig. 1. Hardness as a function of tip penetration, obtained by nanoindentation, for glass samples submitted to ionic migration using aluminum foils as electrodes. The lines in the graph only connect the experimental points for each sample, and do not represent a ®tting.
G.Y. Odo et al. / Journal of Non-Crystalline Solids 247 (1999) 232±236
Fig. 2. Hardness as a function of tip penetration, obtained by nanoindentation, for glass samples submitted to ionic migration using silver ®lms as electrodes. The lines in the graph only connect the experimental points for each sample, and do not represent a ®tting.
which the electric ®eld was applied using silver deposited electrodes. With this kind of electrode, the penetration of silver ions replaces the sodium ions in the glass. The Ag±Na ion exchanged layers with calculated thickness of 1, 3 and 5 lm were indented. The results for the samples with modi®ed layers with depths of 1 and 3 lm are presented in Fig. 2. The hardness in samples with a 5 lm ionic migration layer depth had a greater dispersion in the hardness and the results were not included in the graph. It can be observed in Fig. 2 that alterations in hardness due to the presence of the silver layer at the surface are observed. Surface residual stress induced by the ionic migration was calculated by the measurements of the radial cracks lengths generated by the Vickers indentation, using the lengths obtained for the
235
modi®ed and the pristine glass surfaces. The results are summarized in Table 1. The calculations were performed using KIc 0.75 MPa m1=2 as the fracture toughness for glass [8]. Residual stresses for glasses with ionic migration performed using silver ®lm electrodes increases with the thickness of the layers modi®ed by Ag ions. For the samples with hydrogen ions replacing Na , the residual stress decreased with the depth of the modi®ed layer. The dispersion observed in the measured stresses is normally observed when this type of experimental procedure is used.
4. Discussion It is well established [14] that the surface hardness increases if the surface is under compression and decreases if the surface is submitted to a tensile stress. Analyzing the hardness and residual stresses for the samples on which the electric ®eld was applied using aluminum plates, a residual tensile stress and a decrease in the hardness of the aected region were observed. The hardness showed the expected behavior. However, the residual stress decreased as the amount of hydrogen in the glass increased. Optical microscopy of 5 lm depth sample, showed a large number of cracks at the surface. These cracks were not observed in the 1 lm depth layer sample. The smaller residual stresses could be attributed to a stress relaxation process. The residual stresses generated during the ionic migration process could relax due to
Table 1 Residual stress for the samples submitted to ionic migration Residual stress, r (MPa)
Electrode
Penetrating ion
Applied load, P (N)
Layer depth, d (lm)
Crack length modi®ed surface, c (lm)
Crack length reference region, c0 (lm)
Al plate
H
5 5 5
1 3 5
66 6 66 3 64 4
60 5
31 23 17 9 99
Ag evaporated ®lm
Ag
5
1
57 2
60 5
ÿ19 11
5 5
3 5
44 3 35 5
ÿ87 22 ÿ142 50
236
G.Y. Odo et al. / Journal of Non-Crystalline Solids 247 (1999) 232±236
the growth of existing surface ¯aws that generated the observed cracks. Then, the cracks formed during the indentation did not extend as expected due to the fact the surface stresses were relaxed by the crack growth of existing ¯aws. The residual stresses were calculated using a constant vr . However it depends on the hardness and elastic modulus [4]. The hardness for ionic migration samples diered from those observed in virgin samples. Considering this eect, vr shows a maximum increase of 6%, in the region of the exchanged ions with thickness d. As d c, and the dispersion in the calculated stresses is larger, the modi®cation of hardness in the calculated stresses can be disregarded. For the samples with penetration of silver ions, no alteration in hardness was observed when comparing the region modi®ed by ionic migration with a virgin sample. It was expected the hardness would be larger in this region since a compressive stress is observed by the indentation fracture method. Optical microscopy of the glass surface in the modi®ed region, after the ionic migration with silver electrodes, showed the presence of pits in the anode region. These pits may be due to a variable concentration of silver ions in the glass surface. The number of pits increases with the total charge that circulates through the sample. By observing the surface, the residual stress, and hardness, we infer that stress increases with increasing Ag content. As the number of pits increases at the surface they aect crack propagation and residual stress. The hardness as measured by nanoindentation was not aected as the indentations were made in regions with smaller numbers of pits. However, some indentations were probably in a region with pits, and the larger deviations from the typical load vs displacement were eliminated from the calculation of hardness. The mean hardness is only that for the relatively pit free region. In the case of the 5 lm layer sample, the number of pits increased and the dispersion in the indentation hardness indicates that the probability of indenting in a pit increases. The residual stress calculated by using the Eqs. (2) and (3) assuming the substitution of a Na ion, with radius of 0.098 nm, by an Ag ion of radius of 0.123 nm gave 320 MPa. Comparing
320 MPa with the measured 142 MPa, we note that some stress relaxation may occur after the penetration of the Ag ions. Another possibility is that the eective volume occupied by each ion diers from the ionic radius. 5. Conclusions The ionic migration induced by the external electric ®eld produced residual stresses in the anode region. The residual stress is compressive in the case for which a silver layer is used as electrode. This compressive stress is due to silver ions replacing sodium ions as they penetrate into interior of a sample. The measured residual stresses are attributed to the relaxation of stresses by the growth of existing ¯aws in the glass. Acknowledgements This work was supported ®nancially by PADCT, CNPq and FINEP. References [1] J. Salomonson, K. Zeng, D. Rowclie, Acta Mater. 44 (1996) 543. [2] M. Chaudhri, M.A. Phillips, Philos. Mag. A 62 (1990) 1. [3] K.J. Berg, P. Grau, D. Nowak-Wozny, M. Petzold, M. Suszynska, Mater. Chem. Phys. 40 (1995) 131. [4] B. Lawn, Fracture of Brittle Solids, 2nd ed., ch. 8, Cambridge University, New York, 1995, pp. 249±306. [5] B.R. Lawn, E.R. Fuller Jr., J. Mater. Sci. 19 (1984) 4061. [6] D. Decker, G. Marrone, K. Naito, C.A.O. Peixoto, J.P. Reynders, Electra 153 (1994) 22. [7] C.A.O. Peixoto, L. Pargamin, G.E. Marrone, G. Carrara, IEEE Trans. Power Delivery 3 (1988) 776. [8] D.E. Carlson, K.W. Hang, G.F. Stockdale, J. Am. Ceram. Soc. 55 (1972) 337. [9] C.M. Lepienski, J.A. Giacometti, G.F.L. Ferreira, F.L. Freire Jr., C.A. Achete, J. Non-Cryst. Solids 159 (1993) 204. [10] T. Kaneko, J. Non-Cryst. Solids 120 (1990) 188. [11] J. Miltat, Philos. Mag. A 55 (1987) 543. [12] G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, J. Am. Ceram. Soc. 64 (1981) 9. [13] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [14] T.Y. Tsui, W.C. Oliver, G.M. Pharr, J. Mater. Res., 1995.