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Mechanical properties of a TAS fiber: a preliminary study
Journal of Non-Crystalline Solids 316 (2003) 131–137 www.elsevier.com/locate/jnoncrysol
Mechanical properties of a TAS fiber: a preliminary study C. Pouvreau a, M. Drissi-Habti a,1, K. Michel b, B. Bureau b, J.-C. Sangleboeuf C. Boussard-Pledel b, T. Rouxel a, J.-L. Adam b a
a,*
,
LARMAUR UPRES-JE 2310, B^ at. 10B, University of Rennes 1, Campus Beaulieu, 35042 Rennes cedex, France b UMR CNRS 6512, University of Rennes 1, Campus de Beaulieu, 35042 Rennes cedex, France
Abstract The mechanical properties, including elastic moduli, hardness, fracture toughness and tensile strength of a glass fiber in the Te–As–Se system (TAS) were studied. The values for the hardness (1.4 GPa) and the fracture toughness (0.18 p MPa m) show that this glass is both soft and brittle in comparison to glasses from other systems. However, indentation measurements should be interpreted with caution due to an indentation creep phenomenon and to a delayed fracture process. In addition, the effect of treatments in air (relative humidity about 60%) at different temperatures below Tg were investigated. The main result of this study is that the studied TAS glass is sensitive to the presence of humidity, and aging treatments have a pronounced detrimental effect on the strength of the uncoated fibers. 2003 Elsevier Science B.V. All rights reserved. PACS: 61.43.Fs; 62.20.)x; 62.20.Mk
1. Introduction The first optical fibers used in telecommunication were made of pure silica, but these are incompatible with most other glasses because of their very low thermal expansion coefficient, unusually high softening point and very high viscosity. It is thus of paramount interest to develop new glass fiber compositions more suitable to some specific applications, and heavy metal halide glasses open a new realm of possibilities [1]. Besides, in
*
Corresponding author. Tel.: +33-2 23 23 67 18; fax: +33-2 23 23 63 59. E-mail address: jean-christophe.sangleboeuf@univ-rennes1. fr (J.-C. Sangleboeuf). 1 Visiting Professor of the University of Rennes.
comparison to silica, these glasses are advantaged by exceptional optical properties. There is a wide range of actual and potential technological applications for amorphous chalcogenide materials. For instance, chalcogenide glasses can be used in IR-transmitting optical devices because of their transparency over a broad wavelength range (second atmosphere window, 2– 10 lm). However, though a theoretical loss of about 102 dB/km at 4.5 lm has been predicted for glassy GeSe3 [2], attenuation for chalcogenide fibers was found experimentally considerably higher (10–103 at about 3 lm) [3–6]. Therefore, at this time, these glasses seem unsuitable for longdistance trunk communication applications. Nonetheless, for smaller scale applications, chalcogenide glass fibers can be very useful for applications in:
0022-3093/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 1 9 4 5 - 2
C. Pouvreau et al. / Journal of Non-Crystalline Solids 316 (2003) 131–137
• detection of room and hot temperature objects [7], • CO2 laser power transmission for surgery as well as for cutting and/or welding [8], • on-line following of chemical reaction mechanism such as alcoholic fermentation process [9], • remote IR spectroscopy of any organic compounds (environmental, biochemical, medical applications) [10].
10 9
Attenuation (dB/m)
132
8 7 6 5 4 3 2 1 0 2
Moreover, large non-linear effects were observed in chalcogenide glasses, with vð3Þ values about 1012 e.s.u. (which are about one order of magnitude larger than that of semi-conductordoped glasses and two orders of magnitude larger than that of silica), which make these glasses interesting for applications involving high-speed non-linear elements for up-graded light-transmitting devices. However, although the presently studied glasses exhibit attractive optical properties, they suffer from poor mechanical performance [11]. Their hardness ranges typically between 0.39 and 2.35 GPa p and their fracture toughness is below 0.3 p MPa m [12] (Kc ¼ 0:76 MPa m for a standard float glass). In this paper, the preliminary results of mechanical testing investigations conducted on a glass fiber in the Te–As–Se system are reported, with a special focus on the fracture properties and the incidence of aging treatment at different temperatures and humidity levels. Bulk glass specimens of same composition as the fiber were also studied for comparison.
2. Materials and experimental methods 2.1. Processing The composition of the chalcogenide glass used for the fibers is Te2 As3 Se5 . This particular composition was chosen because of its remarkable properties: large optical window covering the spectral region from 3 to 12 lm (Fig. 1); excellent resistance to devitrification during the drawing process; good durability towards water and solvent corrosion; and a glass transition temperature (Tg ¼ 137 C) suitable to on-line tapering of the
3
4
5
6
7
8
9
10
11
12
Wavelength (µm)
Fig. 1. Attenuation spectrum of a TAS glass fiber.
fiber during the drawing process [13,14]. 99.999 pure raw materials were used for the glass preparation as detailed in a previous paper [13]. Selenium and arsenic were further purified to eliminate the remaining oxygen and hydrogen by the volatilization technique by heating at 240 and 290 C respectively under vacuum for several hours. The required amounts of Te, As and Se are sealed in a silica tube under vacuum and the mixture is maintained at 700 C for 12 h in a rocking furnace to ensure a good homogenization of the liquid. Then the ampoule is quenched in water and annealed near the glass transition temperature. The obtained samples have a 1 cm large diameter and are 10 cm long. For the determination of bulk properties, two batches of glass (batches I and II) with nominally identical compositions were synthesized following this process route. Glass cylinders are heated up to the softening temperature, and flowed to the appropriate diameter by tuning both the temperature (and thus the viscosity) and the drawing speed. No polymer coating was applied on the fiber for the purpose of the understanding of the mechanical behavior of the glass. The diameter of the fiber is about 400 lm. 2.2. Ultrasonic measurements The elastic moduli were calculated from the measurements of the longitudinal (Vl ) and transverse (Vt ) ultrasonic wave velocities with a better than 2% relative error by means of 10 MHz piezoelectric transducers. YoungÕs modulus (E) and
C. Pouvreau et al. / Journal of Non-Crystalline Solids 316 (2003) 131–137
PoissonÕs ratio (m) were derived from the classical elasticity relationships:
Kc ¼ 0:016ðE=H Þ
1=2
P =c3=2 :
133
ð4Þ
where q is the density of the material. Specimens are 5 mm thick and 1 cm in diameter, with mirror finished parallel surfaces. The density was measured at 20 C by means of helium-pycnometry with a relative error better than 0.5.
The topology of the indented surfaces was analysed either with an optical microscope and an atomic force microscope (Digital Instrument, Nanoscope III) operated in both tapping and contact modes, depending on the scale of the observation. The scatter on the fracture toughness data (averaged over 15 measurements) is mainly due to the statistical nature of fracture, which affects the results the more because chalcogenide glasses are remarkably brittle.
2.3. Indentation experiments
2.4. Tensile experiments and statistical treatment
Indentation experiments were performed on a Matsuzawa apparatus equipped with a Vickers diamond indenter on the same specimens as those used for the ultrasonic testing. All characteristics were averaged over measurements on 15 indentations. Specimen surfaces were mirror-polished with alumina suspensions up to 0.25 lm particle size prior to indentation. A 2.94 N load was applied for 20 s. Note also that all measurements were performed in a thermally regulated room, at 20 C. These latter conditions are of primary importance because chalcogenide glasses, having relatively low glass transition temperatures, tend to exhibit temperature- and time-dependent behaviors [15]. MeyerÕs hardness (H ) is defined by
Tensile tests were carried out on a DY30 Adamel Lomarghy testing machine. Fibers were end-tabbed to prevent against sliding and/or indentation. Alignment was optimized on a set of fibers prior to testing. Displacement controlled experiments were conducted on specimens with 10 cm long gauges, using a load cell of 100 N, a crosshead speed of 0.8 mm/min and a setup pre-load of 0.1 N. Specimens were carefully examined after testing using optical and electronic microscopes. Results of statistical tests were based on 30 conclusive tests, i.e. 30 fibers that failed within the gauge length. A statistical analysis was performed using the classical Weibull theory. The probability of failure, Pf , is defined as [17] Z m ð5Þ Pf ¼ 1 exp ððr ru Þ=r0 Þ dV ;
E ¼ qð3Vl 2 4Vt2 Þ=½ðVl 2 =Vt2 Þ 1;
ð1Þ
m ¼ qð3Vl 2 4Vt2 Þ=½2ðVl 2 Vt2 Þ 1;
ð2Þ
H ¼ P =2a2 ;
ð3Þ
where P (N) is the load applied on the indenter and a (m) is the half of the mean size of the two diagonals. The experimental error on H , resulting from the inaccuracy on the measurement of a, is about 0.2 GPa. Note that MeyerÕs hardness, analogous to a mean normal stress, is the most fundamental measure of hardness. The apparent indentation fracture toughness, Kc , of the glass was determined from the mean size of the cracks observed on the surface of the specimen growing away from the indentation corners. Let c be half the distance between two opposite crack tips, the load was carefully chosen to ensure c=a > 2:5, so that a radial-median crack pattern prevails and c is the radius of the half-penny-like cracks. The following equation was used [16]:
where r is the applied stress, r0 is a scaling parameter, ru is a stress level below which the probability of failure is 0, V the volume of the specimen and m the Weibull modulus. Note that ru is generally taken equal to zero (safest assumption) and thus, when the stress is uniform through the specimen volume, Eq. (5) can be rewritten as m
Pf ðV Þ ¼ 1 exp½ðr=r0 Þ
ð6Þ
which further gives lnlnð1=ð1 Pf ÞÞ ¼ m lnðr=r0 Þ:
ð7Þ
The tensile stresses measured on the N specimens are ordered from r1 to rN , with ri 6 riþ1 , and a failure probability Pfi is assigned to each ri , according to
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Pfi ¼ i=ðN þ 1Þ:
ð8Þ
It is then possible to estimate m from the experimental failure stress values and from the corresponding failure probabilities.
3. Results 3.1. Mechanical properties on the bulk material YoungÕs modulus, PoissonÕs ratio, MeyerÕs hardness and fracture toughness values are summarized in Table 1 for both nominally identical TAS-glass batches along with the values of a Ge30 Se70 glass (given for comparison). Note that YoungÕs modulus and fracture toughness of the TAS glass compare well with those of the chalcogenide glass, but the presently studied glass is noticeably softer (for comparison, the hardness of a standard window glass is about 6.1 GPa). The hardness was found to decrease significantly with increasing loading time. For instance, H dropped by 13.4% when the loading time was increased from 20 to 100 s. Some lateral cracks were also found to appear with delay, few seconds after complete unloading, and would eventually lead to chipping. These latter observations clearly suggest time-dependent processes, and will be further discussed. 3.2. Tensile strength and statistical analysis The mean tensile strength measured less than 2 h after drawing on the as-drawn TAS fiber, rR ¼ 93 14 MPa, is very low when compared to usual glassy fibers. In the case of silica fibers, Mecholsky [18] reported tensile stresses ranging from 100 to 1000 MPa. Note however that (i) the TAS fiber was not coated for the purpose of this work, and (ii) the fiber drawing process is still at
Fig. 2. Weibull plots: (r) as-drawn fiber, (N) after aging for 2 h at 80 C, (j) after aging for 2 h at 120 C, and ( ) after aging for 24 h at 80 C.
the laboratory scale and there is no doubt better results would be obtained at a larger scale in a cleaner and better controlled environment. The data used for the determination of WeibullÕs modulus are plotted in Fig. 2. The low Weibull modulus (around 7.8 for non-aged TAS fibers) indicates a significant scatter on the tensile strength data and the presence of a relatively wide distribution of the flaw characteristics (see Section 4). 3.3. The role of aging in air or in water Aging treatments were conducted to evaluate the environmental effects on the fiber strength. The first treatments were done in air (55 < Hr < 65%) at 80 and 120 C. The experimental results are plotted in Fig. 2. Two hours at 120 C or 24 h at 80 C have the same effect on the tensile stress: the initial mean stress value is divided by 2 (Table 2). Noteworthy the most critical flaws, i.e. those corresponding to the low fracture stress ranges in Fig. 2, are more sensitive to the aging treatments. This leads to a decrease of WeibullÕs modulus from 7.8
Table 1 YoungÕs modulus, PoissonÕs ratio, hardness and toughness of TAS glass (batches I and II) and Ge30 Se70 glasses TAS (batch I) TAS (batch II) Ge30 Se70
p
E (GPa)
m
Hardness (GPa)
Kc (MPa
18.1 0.3 17.2 0.3 17.9 0.3
0.28 0.3 0.26
1.4 0.2 1.3 0.2 2.0 0.2
0.18 0.05 0.17 0.05 0.2 0.05
m)
C. Pouvreau et al. / Journal of Non-Crystalline Solids 316 (2003) 131–137
135
Table 2 Change of TAS fiber strength due to aging treatments in air (results averaged on 15 experiments) Mean strength (MPa)
As-drawn
Aged 2 h at 80 C
Aged 24 h at 80 C
Aged 2 h at 120 C
94 14
80 14
57 22
54 13
to 2.6 after aging at 80 C for 24 h and suggests that aging in a humid (ambiance) atmosphere increases the severity of the largest flaws (or of the most critical ones). Indeed, TAS fibers aged in hot water (60 C) for 24 h could not sustain any alignment nor any loading in the tensile testing machine anymore. Therefore, heat-treatments in air below Tg have a clear detrimental effect on the strength and on the integrity of the studied TAS fiber, though beneficial annealing effects were expected since none of the constitutive elements of the fiber were supposed to react with water vapor or to be soluble in water in the considered temperature range.
the fiber (Fig. 3). Some aluminum and oxygen rich impurities, about 5–10 lm large, were observed at the fiber surface by means of an EDS analysis in a scanning electron microscope (Fig. 4). Although the source for the presence of these elements is not clear yet, it is suspected that the fiber preform surface was slightly contaminated by the alumina contained in the suspension during the polishing prior to the drawing stage. Assuming that a typical
4. Discussion The continuous increase of the indentation size with the loading time during indentation testing under constant load was previously studied in details in the case of chalcogenide glasses in the Ge–Se system [15]. It was showed that the penetration displacement is the sum of an elastic component, which reaches values as high as 60% of the total displacement, and a creep one leading to a significant decrease of H with an increase of the loading time. At first glance, it is quite surprising that a glass with a Tg of 137 C behaves viscoelastic at room temperature (near 20 C). However, more in depth investigations revealed a strongly non-Newtonian behavior with a shearthinning character, so that the apparent viscosity drops of several orders of magnitude under sharp contact loading. As a consequence, the glass is expected to flow beneath the indenter due to the very high stress. Note that this problem makes questionable the estimation of the hardness and fracture toughness on the basis of simple indentation experiments. The systematic observation of the fracture surfaces show that fracture initiated at the surface of
Fig. 3. Typical fracture surface of a TAS glass fiber.
Fig. 4. MEB micrograph of a surface flaw.
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Fig. 5. Optical pictures of the surface of a TAS fiber (10 ): (a) just after the drawing process and (b) after exposition to water at 60 C for 1 h (10 mm ¼ 100 lm).
surface flaw looks like a surface crack of depth (a) then it is possible to evaluate the flaw depth from the classical IrwinÕs relationship [18]: Kc ¼ 1:122rðpaÞ
1=2
;
ð9Þ
where r is the applied tensile stress. Replacing r by the mean tensile stress at fracture and taking the Kc values from Table 1 results in a mean defect depth value of 0.9 lm which is small compared to surface heterogeneities described above. The tensile strength results would give a maximum flaw depth for the as-drawn fiber of about 1.5 lm. The aging treatments reduces dramatically the strength with a corresponding increase of the maximum flaw depth to 7 lm after 2 h at 120 C and to 32 lm after 24 h at 80 C. However, the highest strength values are little affected (i.e. 1 lm deep flaws do not grow much). Since aging treatments mainly affect the surface, it can hence be concluded that fibers with surface flaws larger than 1 lm will be more sensitive to the treatment than fibers with relatively clean surfaces. In the present study, the fiber specimens exhibiting the highest strength (flaw size of less than 1 lm) were found relatively insensitive (the strength does not change much) to aging treatment in air below 120 C and for less than 24 h (Fig. 5). 5. Conclusion and perspectives The mechanical properties of the studied TAS fiber are close to those of the Ge–Se chalcogenide glasses with the exception of the hardness, which is
very low in the present case: H ¼ 1:4 GPa. However, indentation creep occurs so that hardness is time-dependent and its meaning calls for caution. Aging treatments in air below Tg induce a dramatic decrease of the tensile strength of the fibers. A 57% decrease of the strength is observed after a 2 h treatment at 120 C. This study makes therefore questionable the apparent chemical inertia and durability of the TAS fiber in humid environment. The evidence for a delayed fracture mechanism subsequently to the indentation process also suggests an occurrence of a fatigue phenomenon. Further investigations are required to identify the source for this fatigue process and the chemistry of the reaction between the glass and the humid environment. Besides, more attention should be paid to the preparation of the glass perform and to the fiber drawing conditions.
Acknowledgements One of the authors, M.D.-H., is indebted to the University of Rennes 1 and to the LARMAUR for having been recipient of a visiting professor position.
References [1] J. Lucas, Mater. Sci. Technol. 9 (1991) 455. [2] A.M. Andriesh, J. Non-Cryst. Solids 77 (1985) 1219. [3] T. Kanamori, Y. Terunuma, S. Takahashi, T. Miyashita, J. Non-Cryst. Solids 69 (1985) 231.
C. Pouvreau et al. / Journal of Non-Crystalline Solids 316 (2003) 131–137 [4] J.A. Savage, Infrared Optical Materials and their Anti-reflection Coatings, Adam Hilger, Bristol, UK, 1987. [5] S. Shibata, M. Horiguchi, K. Jinguji, S. Mitachi, T. Kanamori, T. Manabe, Electron. Lett. 17 (1981) 775. [6] J. Nishii, S. Morimoto, R. Yokota, T. Yamagishi, J. NonCryst. Solids 95 (1987) 641. [7] F. Gilbert, F.F. Ardouin, P. Morillon, K. Le Foulgoc, X.H. Zhang, C. Blanchetiere, H.L. Ma, J. Lucas, Proc. SPIE Fiber Optic and Laser Sensor XIV 2839 (1996) 239. [8] S. Hocde, C. Boussard-Pledel, G. Fonteneau, D. Lecoq, H.L. Ma, J. Lucas, J. Non-Cryst. Solids 274 (2000) 17. [9] C. Blanchetiere, K. Le Foulgoc, H.L. Ma, X.H. Zhang, J. Lucas, J. Non-Cryst. Solids 184 (1995) 200. [10] S. Hocde, O. Loreal, O. Sire, B. Turlin, C. Boussard-Pledel, D. Lecoq, B. Bureau, G. Fonteneau, C. Pigeon, P. Leroyer,
[11] [12] [13] [14] [15] [16] [17] [18]
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J. Lucas, Proc. SPIE Biomonitoring Endoscopy Technol. 4158 (2000) 49. J.-P. Guin, Doctorate thesis, Rennes, October 2001. J.-P. Guin, T. Rouxel, J.-C. Sanglebœuf, I. Melsco€et, J. Lucas, J. Am. Ceram. Soc. 85 (6) (2002) 1545. X.H. Zhang, H.L. Ma, C. Blanchetiere, J. Lucas, J. NonCryst. Solids 161 (1993) 327. S. Hocde, C. Boussard-Pledel, G. Fonteneau, J. Lucas, Solid State Sci. 3 (2001) 279. J.P. Guin, T. Rouxel, V. Kerivyn, J.C. Sangleboeuf, I. Serre, J. Lucas, J. Non-Cryst. Solids 298 (2002) 260. G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, J. Am. Ceram. Soc. 64 (9) (1981) 533. W. Weibull, J. Appl. Mech. 9 (1951) 293. J.J. Mecholsky, Engineered Materials Handbook, vol. 4, ASM International, 1991, p. 663.
Mechanical properties of a TAS fiber: a preliminary study C. Pouvreau a, M. Drissi-Habti a,1, K. Michel b, B. Bureau b, J.-C. Sangleboeuf C. Boussard-Pledel b, T. Rouxel a, J.-L. Adam b a
a,*
,
LARMAUR UPRES-JE 2310, B^ at. 10B, University of Rennes 1, Campus Beaulieu, 35042 Rennes cedex, France b UMR CNRS 6512, University of Rennes 1, Campus de Beaulieu, 35042 Rennes cedex, France
Abstract The mechanical properties, including elastic moduli, hardness, fracture toughness and tensile strength of a glass fiber in the Te–As–Se system (TAS) were studied. The values for the hardness (1.4 GPa) and the fracture toughness (0.18 p MPa m) show that this glass is both soft and brittle in comparison to glasses from other systems. However, indentation measurements should be interpreted with caution due to an indentation creep phenomenon and to a delayed fracture process. In addition, the effect of treatments in air (relative humidity about 60%) at different temperatures below Tg were investigated. The main result of this study is that the studied TAS glass is sensitive to the presence of humidity, and aging treatments have a pronounced detrimental effect on the strength of the uncoated fibers. 2003 Elsevier Science B.V. All rights reserved. PACS: 61.43.Fs; 62.20.)x; 62.20.Mk
1. Introduction The first optical fibers used in telecommunication were made of pure silica, but these are incompatible with most other glasses because of their very low thermal expansion coefficient, unusually high softening point and very high viscosity. It is thus of paramount interest to develop new glass fiber compositions more suitable to some specific applications, and heavy metal halide glasses open a new realm of possibilities [1]. Besides, in
*
Corresponding author. Tel.: +33-2 23 23 67 18; fax: +33-2 23 23 63 59. E-mail address: jean-christophe.sangleboeuf@univ-rennes1. fr (J.-C. Sangleboeuf). 1 Visiting Professor of the University of Rennes.
comparison to silica, these glasses are advantaged by exceptional optical properties. There is a wide range of actual and potential technological applications for amorphous chalcogenide materials. For instance, chalcogenide glasses can be used in IR-transmitting optical devices because of their transparency over a broad wavelength range (second atmosphere window, 2– 10 lm). However, though a theoretical loss of about 102 dB/km at 4.5 lm has been predicted for glassy GeSe3 [2], attenuation for chalcogenide fibers was found experimentally considerably higher (10–103 at about 3 lm) [3–6]. Therefore, at this time, these glasses seem unsuitable for longdistance trunk communication applications. Nonetheless, for smaller scale applications, chalcogenide glass fibers can be very useful for applications in:
0022-3093/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 1 9 4 5 - 2
C. Pouvreau et al. / Journal of Non-Crystalline Solids 316 (2003) 131–137
• detection of room and hot temperature objects [7], • CO2 laser power transmission for surgery as well as for cutting and/or welding [8], • on-line following of chemical reaction mechanism such as alcoholic fermentation process [9], • remote IR spectroscopy of any organic compounds (environmental, biochemical, medical applications) [10].
10 9
Attenuation (dB/m)
132
8 7 6 5 4 3 2 1 0 2
Moreover, large non-linear effects were observed in chalcogenide glasses, with vð3Þ values about 1012 e.s.u. (which are about one order of magnitude larger than that of semi-conductordoped glasses and two orders of magnitude larger than that of silica), which make these glasses interesting for applications involving high-speed non-linear elements for up-graded light-transmitting devices. However, although the presently studied glasses exhibit attractive optical properties, they suffer from poor mechanical performance [11]. Their hardness ranges typically between 0.39 and 2.35 GPa p and their fracture toughness is below 0.3 p MPa m [12] (Kc ¼ 0:76 MPa m for a standard float glass). In this paper, the preliminary results of mechanical testing investigations conducted on a glass fiber in the Te–As–Se system are reported, with a special focus on the fracture properties and the incidence of aging treatment at different temperatures and humidity levels. Bulk glass specimens of same composition as the fiber were also studied for comparison.
2. Materials and experimental methods 2.1. Processing The composition of the chalcogenide glass used for the fibers is Te2 As3 Se5 . This particular composition was chosen because of its remarkable properties: large optical window covering the spectral region from 3 to 12 lm (Fig. 1); excellent resistance to devitrification during the drawing process; good durability towards water and solvent corrosion; and a glass transition temperature (Tg ¼ 137 C) suitable to on-line tapering of the
3
4
5
6
7
8
9
10
11
12
Wavelength (µm)
Fig. 1. Attenuation spectrum of a TAS glass fiber.
fiber during the drawing process [13,14]. 99.999 pure raw materials were used for the glass preparation as detailed in a previous paper [13]. Selenium and arsenic were further purified to eliminate the remaining oxygen and hydrogen by the volatilization technique by heating at 240 and 290 C respectively under vacuum for several hours. The required amounts of Te, As and Se are sealed in a silica tube under vacuum and the mixture is maintained at 700 C for 12 h in a rocking furnace to ensure a good homogenization of the liquid. Then the ampoule is quenched in water and annealed near the glass transition temperature. The obtained samples have a 1 cm large diameter and are 10 cm long. For the determination of bulk properties, two batches of glass (batches I and II) with nominally identical compositions were synthesized following this process route. Glass cylinders are heated up to the softening temperature, and flowed to the appropriate diameter by tuning both the temperature (and thus the viscosity) and the drawing speed. No polymer coating was applied on the fiber for the purpose of the understanding of the mechanical behavior of the glass. The diameter of the fiber is about 400 lm. 2.2. Ultrasonic measurements The elastic moduli were calculated from the measurements of the longitudinal (Vl ) and transverse (Vt ) ultrasonic wave velocities with a better than 2% relative error by means of 10 MHz piezoelectric transducers. YoungÕs modulus (E) and
C. Pouvreau et al. / Journal of Non-Crystalline Solids 316 (2003) 131–137
PoissonÕs ratio (m) were derived from the classical elasticity relationships:
Kc ¼ 0:016ðE=H Þ
1=2
P =c3=2 :
133
ð4Þ
where q is the density of the material. Specimens are 5 mm thick and 1 cm in diameter, with mirror finished parallel surfaces. The density was measured at 20 C by means of helium-pycnometry with a relative error better than 0.5.
The topology of the indented surfaces was analysed either with an optical microscope and an atomic force microscope (Digital Instrument, Nanoscope III) operated in both tapping and contact modes, depending on the scale of the observation. The scatter on the fracture toughness data (averaged over 15 measurements) is mainly due to the statistical nature of fracture, which affects the results the more because chalcogenide glasses are remarkably brittle.
2.3. Indentation experiments
2.4. Tensile experiments and statistical treatment
Indentation experiments were performed on a Matsuzawa apparatus equipped with a Vickers diamond indenter on the same specimens as those used for the ultrasonic testing. All characteristics were averaged over measurements on 15 indentations. Specimen surfaces were mirror-polished with alumina suspensions up to 0.25 lm particle size prior to indentation. A 2.94 N load was applied for 20 s. Note also that all measurements were performed in a thermally regulated room, at 20 C. These latter conditions are of primary importance because chalcogenide glasses, having relatively low glass transition temperatures, tend to exhibit temperature- and time-dependent behaviors [15]. MeyerÕs hardness (H ) is defined by
Tensile tests were carried out on a DY30 Adamel Lomarghy testing machine. Fibers were end-tabbed to prevent against sliding and/or indentation. Alignment was optimized on a set of fibers prior to testing. Displacement controlled experiments were conducted on specimens with 10 cm long gauges, using a load cell of 100 N, a crosshead speed of 0.8 mm/min and a setup pre-load of 0.1 N. Specimens were carefully examined after testing using optical and electronic microscopes. Results of statistical tests were based on 30 conclusive tests, i.e. 30 fibers that failed within the gauge length. A statistical analysis was performed using the classical Weibull theory. The probability of failure, Pf , is defined as [17] Z m ð5Þ Pf ¼ 1 exp ððr ru Þ=r0 Þ dV ;
E ¼ qð3Vl 2 4Vt2 Þ=½ðVl 2 =Vt2 Þ 1;
ð1Þ
m ¼ qð3Vl 2 4Vt2 Þ=½2ðVl 2 Vt2 Þ 1;
ð2Þ
H ¼ P =2a2 ;
ð3Þ
where P (N) is the load applied on the indenter and a (m) is the half of the mean size of the two diagonals. The experimental error on H , resulting from the inaccuracy on the measurement of a, is about 0.2 GPa. Note that MeyerÕs hardness, analogous to a mean normal stress, is the most fundamental measure of hardness. The apparent indentation fracture toughness, Kc , of the glass was determined from the mean size of the cracks observed on the surface of the specimen growing away from the indentation corners. Let c be half the distance between two opposite crack tips, the load was carefully chosen to ensure c=a > 2:5, so that a radial-median crack pattern prevails and c is the radius of the half-penny-like cracks. The following equation was used [16]:
where r is the applied stress, r0 is a scaling parameter, ru is a stress level below which the probability of failure is 0, V the volume of the specimen and m the Weibull modulus. Note that ru is generally taken equal to zero (safest assumption) and thus, when the stress is uniform through the specimen volume, Eq. (5) can be rewritten as m
Pf ðV Þ ¼ 1 exp½ðr=r0 Þ
ð6Þ
which further gives lnlnð1=ð1 Pf ÞÞ ¼ m lnðr=r0 Þ:
ð7Þ
The tensile stresses measured on the N specimens are ordered from r1 to rN , with ri 6 riþ1 , and a failure probability Pfi is assigned to each ri , according to
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C. Pouvreau et al. / Journal of Non-Crystalline Solids 316 (2003) 131–137
Pfi ¼ i=ðN þ 1Þ:
ð8Þ
It is then possible to estimate m from the experimental failure stress values and from the corresponding failure probabilities.
3. Results 3.1. Mechanical properties on the bulk material YoungÕs modulus, PoissonÕs ratio, MeyerÕs hardness and fracture toughness values are summarized in Table 1 for both nominally identical TAS-glass batches along with the values of a Ge30 Se70 glass (given for comparison). Note that YoungÕs modulus and fracture toughness of the TAS glass compare well with those of the chalcogenide glass, but the presently studied glass is noticeably softer (for comparison, the hardness of a standard window glass is about 6.1 GPa). The hardness was found to decrease significantly with increasing loading time. For instance, H dropped by 13.4% when the loading time was increased from 20 to 100 s. Some lateral cracks were also found to appear with delay, few seconds after complete unloading, and would eventually lead to chipping. These latter observations clearly suggest time-dependent processes, and will be further discussed. 3.2. Tensile strength and statistical analysis The mean tensile strength measured less than 2 h after drawing on the as-drawn TAS fiber, rR ¼ 93 14 MPa, is very low when compared to usual glassy fibers. In the case of silica fibers, Mecholsky [18] reported tensile stresses ranging from 100 to 1000 MPa. Note however that (i) the TAS fiber was not coated for the purpose of this work, and (ii) the fiber drawing process is still at
Fig. 2. Weibull plots: (r) as-drawn fiber, (N) after aging for 2 h at 80 C, (j) after aging for 2 h at 120 C, and ( ) after aging for 24 h at 80 C.
the laboratory scale and there is no doubt better results would be obtained at a larger scale in a cleaner and better controlled environment. The data used for the determination of WeibullÕs modulus are plotted in Fig. 2. The low Weibull modulus (around 7.8 for non-aged TAS fibers) indicates a significant scatter on the tensile strength data and the presence of a relatively wide distribution of the flaw characteristics (see Section 4). 3.3. The role of aging in air or in water Aging treatments were conducted to evaluate the environmental effects on the fiber strength. The first treatments were done in air (55 < Hr < 65%) at 80 and 120 C. The experimental results are plotted in Fig. 2. Two hours at 120 C or 24 h at 80 C have the same effect on the tensile stress: the initial mean stress value is divided by 2 (Table 2). Noteworthy the most critical flaws, i.e. those corresponding to the low fracture stress ranges in Fig. 2, are more sensitive to the aging treatments. This leads to a decrease of WeibullÕs modulus from 7.8
Table 1 YoungÕs modulus, PoissonÕs ratio, hardness and toughness of TAS glass (batches I and II) and Ge30 Se70 glasses TAS (batch I) TAS (batch II) Ge30 Se70
p
E (GPa)
m
Hardness (GPa)
Kc (MPa
18.1 0.3 17.2 0.3 17.9 0.3
0.28 0.3 0.26
1.4 0.2 1.3 0.2 2.0 0.2
0.18 0.05 0.17 0.05 0.2 0.05
m)
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135
Table 2 Change of TAS fiber strength due to aging treatments in air (results averaged on 15 experiments) Mean strength (MPa)
As-drawn
Aged 2 h at 80 C
Aged 24 h at 80 C
Aged 2 h at 120 C
94 14
80 14
57 22
54 13
to 2.6 after aging at 80 C for 24 h and suggests that aging in a humid (ambiance) atmosphere increases the severity of the largest flaws (or of the most critical ones). Indeed, TAS fibers aged in hot water (60 C) for 24 h could not sustain any alignment nor any loading in the tensile testing machine anymore. Therefore, heat-treatments in air below Tg have a clear detrimental effect on the strength and on the integrity of the studied TAS fiber, though beneficial annealing effects were expected since none of the constitutive elements of the fiber were supposed to react with water vapor or to be soluble in water in the considered temperature range.
the fiber (Fig. 3). Some aluminum and oxygen rich impurities, about 5–10 lm large, were observed at the fiber surface by means of an EDS analysis in a scanning electron microscope (Fig. 4). Although the source for the presence of these elements is not clear yet, it is suspected that the fiber preform surface was slightly contaminated by the alumina contained in the suspension during the polishing prior to the drawing stage. Assuming that a typical
4. Discussion The continuous increase of the indentation size with the loading time during indentation testing under constant load was previously studied in details in the case of chalcogenide glasses in the Ge–Se system [15]. It was showed that the penetration displacement is the sum of an elastic component, which reaches values as high as 60% of the total displacement, and a creep one leading to a significant decrease of H with an increase of the loading time. At first glance, it is quite surprising that a glass with a Tg of 137 C behaves viscoelastic at room temperature (near 20 C). However, more in depth investigations revealed a strongly non-Newtonian behavior with a shearthinning character, so that the apparent viscosity drops of several orders of magnitude under sharp contact loading. As a consequence, the glass is expected to flow beneath the indenter due to the very high stress. Note that this problem makes questionable the estimation of the hardness and fracture toughness on the basis of simple indentation experiments. The systematic observation of the fracture surfaces show that fracture initiated at the surface of
Fig. 3. Typical fracture surface of a TAS glass fiber.
Fig. 4. MEB micrograph of a surface flaw.
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Fig. 5. Optical pictures of the surface of a TAS fiber (10 ): (a) just after the drawing process and (b) after exposition to water at 60 C for 1 h (10 mm ¼ 100 lm).
surface flaw looks like a surface crack of depth (a) then it is possible to evaluate the flaw depth from the classical IrwinÕs relationship [18]: Kc ¼ 1:122rðpaÞ
1=2
;
ð9Þ
where r is the applied tensile stress. Replacing r by the mean tensile stress at fracture and taking the Kc values from Table 1 results in a mean defect depth value of 0.9 lm which is small compared to surface heterogeneities described above. The tensile strength results would give a maximum flaw depth for the as-drawn fiber of about 1.5 lm. The aging treatments reduces dramatically the strength with a corresponding increase of the maximum flaw depth to 7 lm after 2 h at 120 C and to 32 lm after 24 h at 80 C. However, the highest strength values are little affected (i.e. 1 lm deep flaws do not grow much). Since aging treatments mainly affect the surface, it can hence be concluded that fibers with surface flaws larger than 1 lm will be more sensitive to the treatment than fibers with relatively clean surfaces. In the present study, the fiber specimens exhibiting the highest strength (flaw size of less than 1 lm) were found relatively insensitive (the strength does not change much) to aging treatment in air below 120 C and for less than 24 h (Fig. 5). 5. Conclusion and perspectives The mechanical properties of the studied TAS fiber are close to those of the Ge–Se chalcogenide glasses with the exception of the hardness, which is
very low in the present case: H ¼ 1:4 GPa. However, indentation creep occurs so that hardness is time-dependent and its meaning calls for caution. Aging treatments in air below Tg induce a dramatic decrease of the tensile strength of the fibers. A 57% decrease of the strength is observed after a 2 h treatment at 120 C. This study makes therefore questionable the apparent chemical inertia and durability of the TAS fiber in humid environment. The evidence for a delayed fracture mechanism subsequently to the indentation process also suggests an occurrence of a fatigue phenomenon. Further investigations are required to identify the source for this fatigue process and the chemistry of the reaction between the glass and the humid environment. Besides, more attention should be paid to the preparation of the glass perform and to the fiber drawing conditions.
Acknowledgements One of the authors, M.D.-H., is indebted to the University of Rennes 1 and to the LARMAUR for having been recipient of a visiting professor position.
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