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Determination of the influence of TiO2 on the elastic properties of a mica based glass ceramic by ultrasonic velocity measurements
Journal of Non-Crystalline Solids 351 (2005) 3655–3662 www.elsevier.com/locate/jnoncrysol
Determination of the influence of TiO2 on the elastic properties of a mica based glass ceramic by ultrasonic velocity measurements C.H. Gu¨r, A. Ozturk
*
Middle East Technical University, Metallurgical and Materials Engineering Department, 06531 Ankara, Turkey Received 8 March 2005 Available online 26 October 2005
Abstract The influence of small amount (1 or 2 wt%) of TiO2 additions and crystallization heat treatment on the elastic properties of a mica based glass ceramic have been investigated by ultrasonic velocity measurements. The mica based glass ceramic was prepared through controlled crystallization of a glass in the SiO2, Al2O3, CaO, MgO, K2O and F system. Evidences of TiO2 acting as a nucleating agent in this system was demonstrated. The longitudinal and transversal wave velocities of the as-prepared glass and the mica based glass ceramic were measured by using 5 MHz probes at room temperature. Elastic properties namely; longitudinal modulus, YoungÕs modulus, bulk modulus, and shear modulus were calculated from the ultrasonic velocity values measured and density values obtained experimentally. It has been observed that small amount of TiO2 additions caused a notable but not significant; however, the crystallization heat treatment had a profound effect on the elastic properties of the glass in the system studied. 2005 Elsevier B.V. All rights reserved. PACS: 61.43; 61.43.F; 43.35.A; 62.20.D
1. Introduction The glass ceramics containing mica crystals as the predominant crystalline phase have generated interest in biomedical field especially in the replacement of natural bone and dental restorations since the 1980s. Primary interest in these materials was developed due to their machinability [1,2] and biocompatibility [3]. In addition, they posses appropriate chemical, mechanical, thermal, and physical properties [1–4]. The unique properties result from the uniform, reproducible, fine grained microstructure consisting of easily cleavable and interlocking laminar mica crystals dispersed in a glassy matrix [5,6]. The properties of mica based glass ceramics closely match to those of human enamel [7,8] and can be tailored by changes in composition and heat treatment [5].
*
Corresponding author. Tel.: +90 312 210 5932; fax: +90 312 210 1267. E-mail address: [email protected] (A. Ozturk).
0022-3093/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.09.022
Grossman [9] has shown that mica based glass ceramics in the system K2O–SiO2–MgO–F can be utilized successfully in dental inlays, onlays, and crowns. Some compositions of mica glass ceramics that can be melted and cast into the form of tooth has been marketed under different commercial names such as Dicor [9], Macor [10], and Bioverit [11]. In spite of the good peculiarities of the mica glass ceramics, they are hard to prepare due to their high melting and forming temperatures, large crystallization tendency, phase separation on cooling, and compositional changes due to the fluorine volatilization on melting [4]. Efforts have been continuing to overcome the difficulties in their production and to develop a new mica glass ceramics that will exhibit better performance. Properties of mica based glass ceramics are related to the microstructure developed during heat treatment as well as chemical composition of the parent glass. It is commonly known that the nature of the crystallinity and distribution of the crystalline phase(s) formed during crystallization depends also on the kind and amount of nucleating agent
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used. Several investigations [2,4,12–16] conducted on the role of TiO2 as a nucleating agent in silicate systems have revealed that small addition of TiO2 to many silicate systems has remarkable influence on the crystallization. The role of TiO2 as a nucleating agent in mica based glass ceramics have been overlooked since these materials posses fluorine that promotes nucleation. Recently, Ozturk [17] has presented the evidence of TiO2 acting as volume nucleating catalyst in mica based glass ceramics. The present investigation was undertaken to accomplish two purposes. The first one is to find out the role of TiO2 as nucleating agent in the SiO2–Al2O3–CaO–MgO–K2O–F system. The second purpose is to determine the influence of TiO2 and crystallization heat treatment on the elastic properties of a glass in this system by ultrasonic velocity measurements. Ultrasonic techniques are a versatile tool for investigating the changes in microstructure, deformation process and mechanical properties [18]. The various parameters upon which the elastic moduli of glasses depend can be studied by measuring the ultrasonic wave velocities [19,20]. Although a number of researchers [10,21–26] evaluated the mechanical properties of mica based glass ceramics by destructive techniques, information on the evaluation of elastic properties of these materials by non-destructive techniques has not been reported in the open literature. Thus ultrasonic evaluation of the elastic properties of these materials is of importance from both a scientific and technological point of view. 2. Experimental On the basis of the formation of a mica based glass ceramics, a glass composition presented in Table 1 was taken under consideration. Fluorophlogopite (KMg3AlSi3O10F2) crystals can be formed from this glass upon subjecting a suitable heat treatment [17]. The parent glass was formed by melting appropriate amounts of extra pure grade powders supplied from Merck. TiO2 was introduced to parent glass in the batch. TiO2 additions were made in the amounts of 1 or 2 wt%, of glass. It has been reported [2] that formation of fluorophlogopite crystals is reduced and some TiO2 crystals begin the form when TiO2 additions exceed 2 wt%.
Batches of 50 g were melted in a platinum crucible at 1500 C at normal laboratory conditions without controlling the atmosphere. When melting was complete; the melt was poured into a stainless steel mould to obtain glass blocks. The nominal dimensions of the glass blocks were approximately 50 · 50 · 10 mm as length, width and depth, respectively. The glass blocks were colorless and indicated no unmolten batch materials and crystallinity. For each composition, (i.e. containing no, 1 wt% and 2 wt% TiO2) three glass blocks were formed in order to have sufficient number of specimens for property measurements and evaluations. The glass blocks were annealed at 600 C for 1 h. After annealing, two of the glass blocks from each composition were subjected to a two-step heat treatment, called crystallization, to convert the as-prepared glass blocks into glass ceramic. The nucleation and crystal growth temperatures were 620 C and 940 C, respectively. The blocks were held at each temperature for 8 h. The heating and cooling rates were 3 C/min. Following the crystallization, the surfaces of the blocks were ground and polished mechanically to get smooth, flat and parallel surfaces prior to ultrasonic measurements. Grinding was accomplished with the application of a series of SiC emerald papers, alumina powder solution on a cloth, and finally 0.25 lm diamond paste onto the surfaces. The ultrasonic velocity measurements were conducted on the as-prepared and crystallized (glass ceramic) samples at room temperature. A Panametrics 5052UAX50 analyzer, a Philips PM3365A oscilloscope, and 5 MHz probes were employed for the measurements. Grease was used as coupling medium for longitudinal wave velocity (VL) measurements. The coupling medium for transversal wave velocity (VT) measurements was honey. The velocity of sound was calculated from the equation: V ¼ 2w=t;
ð1Þ
where V, w, and t are the velocity of sound, the thickness of the test object, and the time-of-flight between adjacent backwall signals on the oscilloscope, respectively. Various elastic properties were computed according to the following formulae [20]: L ¼ q.V 2L ;
ð2Þ
G ¼ q.V 2T ; Table 1 Chemical composition of the parent glass Chemicals
SiO2 K2O Al2O3 MgO CaO F2 a
ð3Þ 2 L
K ¼ q=3 ð3V 4V E ¼ ½q.V
Glass composition (wt%) Calculateda
Analyzedb
56.7 17.7 0.6 14.5 3.8 6.2
56.8 18.5 1.7 12.9 4.5 5.6
Calculated values were obtained assuming the batch ingredients were converted into their respective oxides during melting. b Analyzed values were obtained from wet chemical analyses.
2 2 T ð3V L
4V
2 T Þ; 2 2 T Þ=ðV L
ð4Þ V
2 T Þ;
ð5Þ
where L, G, K, E, and q are longitudinal modulus, shear modulus, bulk modulus, YoungÕs modulus of elasticity, and density of the sample, respectively. Following the ultrasonic measurements, the density of the blocks was measured according to the ArchimedesÕ principle using water as the immersion liquid. The density of the specimens was calculated using the formula qglass ¼ m1 .qwater =ðm1 m2 Þ;
ð6Þ
C.H. Gu¨r, A. Ozturk / Journal of Non-Crystalline Solids 351 (2005) 3655–3662
where m1, m2, and qwater are the dry weight of the specimen, the weight of the specimen in water, and the density of water, respectively. The samples were characterized by X-ray diffraction, Rigaku Geigerflex (DMAX/B), and scanning electron microscopy, Jeol 6400, in order to determine the phases formed and the microstructure developed. The images were taken from the freshly fractured surfaces. 3. Results The as-prepared glass blocks were transparent and clear. Wet chemical analyses were performed to determine the chemical composition of the parent glass (the as-prepared glass containing no TiO2) since there is a possibility of fluorine volatilization during glass melting. The analysis results are given in Table 1. Even though there is a slight difference between the intended and analyzed composition of the glass, the both results match up with each other. Therefore, fluorine volatilization occurred throughout the glass making procedure was ignored. The colorless as-prepared glass blocks of different compositions appeared in white (milky) color after the crystallization heat treatment. It was obvious from the appearance of the crystallized blocks that phase separation occurred during cooling and at least two crystalline phases precipitated during crystallization of the parent glass. Crystalline phases formed during crystallization of the glass were identified by X-ray powder diffraction (XRD) analysis. A representative XRD pattern of the crystallized counterpart of the as-prepared glass (glass ceramic) containing 1 wt% TiO2 is shown in Fig. 1. The analyses suggested that fluorophlogopite was the primary crystalline phase. In addition, synthetic mica, wollastonite, and kaliophilite crystals formed during crystallization. The amount of TiO2 additions did not change the number of the crystalline phases precipitated but had an influence on the shape and size of the crystalline phases present. Longitudinal wave velocity (VL) and transversal wave velocity (VT) of the blocks were measured in the as-pre-
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pared and crystallized form. Average VL of the blocks was calculated by taking the mean average of five measurements from an individual block, and then, taking the mean average of velocities of the three blocks having the same TiO2 content and exposed to the same heat treatment. Maximum uncertainty of the data points was calculated as 2% of the determinations. Average values for the VL and VT obtained in this study was compared with the values of VL and VT of the selected glasses and ceramics in Table 2. The variations of VL and VT with TiO2 addition are illustrated in Figs. 2 and 3, respectively, for the as-prepared glass and the mica based glass ceramic. There was not any significant difference in average VL of the parent glass and the mica based glass ceramic obtained from this glass as seen in Fig. 2. When 1 wt% TiO2 was incorporated, average VL increased from 5818 to 5962 m/s for the as-prepared glass; whereas decreased from 5764 to 5723 m/s for the mica based glass ceramic. As TiO2 content was increased from 1 to 2 wt%, average VL for the as-prepared glass decreased from 5962 to 5838 m/s; and for the mica based glass ceramic decreased from 5723 to 5652 m/s. The relative amount of initial Table 2 A comparison of the longitudinal wave velocity (VL) and transversal wave velocity (VT) of the as-prepared glass and mica based glass ceramic investigated in this study with selected glasses and ceramics [27] Material
VL (m/s)
VT (m/s)
As-prepared glassa Mica based glass ceramica Glass, flint Glass, crown, regular Glass, plate Glass, pyrex Glass, quartz Magnesia Porcelain Quartz, fused Quartz, natural
5962 5723 4260 5660 5770 5610 5770 8850 5600 5970 5730
3451 3477 2560 3420 3430 – 3520 – 3400 – –
a
Contains 1 wt% TiO2.
6000 as-prepared glass glass-ceramic
VL (m/s)
5900
5800
5700
5600 0
1
2
wt% TiO2
Fig. 1. X-ray diffraction pattern of a mica based glass ceramic.
Fig. 2. Variation of longitudinal wave velocity with TiO2 addition for the as-prepared glasses and mica based glass ceramics. Error bands indicate the uncertainty of the data points.
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C.H. Gu¨r, A. Ozturk / Journal of Non-Crystalline Solids 351 (2005) 3655–3662 3700 as-prepared glass glass-ceramic
VT (m/s )
3600
3500
3400
3300
3200 0
1
2
wt% TiO2
Fig. 3. Variation of transversal wave velocity with TiO2 addition for the as-prepared glasses and mica based glass ceramics.
increase and later decrease with increasing TiO2 additions in the parent glass was within 2% of the measured values. Average VT obtained in this study was in the range of 3450–3500 m/s for the parent glass and its glass ceramic. VT of the as-prepared glasses and the mica based glass
Fig. 4. SEM micrographs of a mica based glass ceramic containing no TiO2: (a) ·6000 and (b) ·30 000.
ceramics showed a dissimilar behavior with increasing TiO2 addition. When 1 wt% TiO2 was incorporated, VT decreased for the as-prepared glass and increased for the mica based glass ceramic. As TiO2 addition was increased from 1 to 2 wt%, VT of both the as-prepared glass and the mica based glass ceramic decreased. Two of the representative scanning electron microscopy (SEM) micrographs of the glass ceramic sample containing no TiO2 is shown in Fig. 4(a) and (b) for two different magnifications. SEM micrographs of the glass ceramic sample containing 1 wt% TiO2 as nucleating agent is shown in Fig. 5(a) and (b) for the same magnifications. The average grain size and aspect ratio of the crystals were 0.4 ± 0.06 lm and 1.6 ± 0.1 lm, respectively, for the sample containing 1 wt% TiO2 but 0.6 ± 0.12 lm and 2.0 ± 0.15 lm, respectively, for the sample containing no TiO2. The plus and minus signs indicate the standard deviation from the determination points. It has been observed that the aspect ratio and the grain size of the crystals decreased further when TiO2 content was increased from 1 wt% to 2 wt%. Density of the as-prepared glass and the mica based glass ceramic was calculated as a function of TiO2 addition according to Eq. (6). Results are tabulated in Table 3.
Fig. 5. SEM micrographs of a mica based glass ceramic containing 1 wt% TiO2: (a) ·6000 and (b) ·30 000.
C.H. Gu¨r, A. Ozturk / Journal of Non-Crystalline Solids 351 (2005) 3655–3662
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Table 3 Density and elastic properties of the as-prepared glasses and mica based glass ceramics Specimen
TiO2 addition (wt%)
Density (g/cm3)
Longitudinal modulus (GPa)
Elastic modulus (GPa)
Bulk modulus (GPa)
Shear modulus (GPa)
Parent glass Glass ceramic As-prepared glass Glass ceramic As-prepared glass Glass ceramic
no no 1 1 2 2
2.520 2.606 2.528 2.619 2.537 2.629
85.79 89.23 93.07 85.78 89.95 83.99
79.23 72.12 75.92 76.43 74.16 69.89
40.93 51.23 52.92 43.55 50.57 46.73
33.64 30.88 30.11 31.67 29.54 27.95
100
2.65
2.6 as-prepared glass glass-ceramic
2.55
Elastic moduli (GPa)
Density (g/cm3)
L
80 E
60 K
40 G
20 2.5 0
1
2
0
wt% TiO2
0
1
Fig. 6. Variation of density with TiO2 addition for the as-prepared glasses and mica based glass ceramics.
Fig. 7. Variation of elastic moduli with TiO2 addition for the as-prepared glasses. (L; longitudinal modulus, E; YoungÕs modulus, K; bulk modulus and G; shear modulus).
100 L
80
Elastic moduli (GPa)
Determination of the data points was accomplished by taking the mean average of three measurements on individual block and then taking the mean average of the three blocks having the same TiO2 addition and exposed to the same heat treatment. Maximum uncertainty of the data was less than 0.5% of the determination points. The variations of density with TiO2 addition for the asprepared glass and the mica based glass ceramic are shown in Fig. 6. The densities ranged between 2.520 g/cm3 and 2.537 g/cm3 for the as-prepared glasses and 2.606 g/cm3 and 2.629 g/cm3 for the mica based glass ceramics. The density of the as-prepared glass and glass ceramic increased with increasing TiO2 addition in more or less the same manner. The longitudinal modulus (L), YoungÕs modulus (E), bulk modulus (K) and shear modulus (G), of the as-prepared glasses and the mica based glass ceramics were calculated employing Eqs. (1)–(5). Results are presented in Table 3. Maximum uncertainty of the data was calculated as 2% of the determination points. The variations of the longitudinal, elastic, bulk, and shear moduli with TiO2 addition are shown in Fig. 7 for the as-prepared glasses and in Fig. 8 for the mica based glass ceramics. For the as-prepared glasses, the longitudinal and bulk moduli tend to increase, but the shear and YoungÕs moduli tend to decrease with increasing TiO2 additions. For the mica based glass ceramic, the longitudinal and bulk moduli had a tendency to increase, but the shear and YoungÕs moduli had a tendency to decrease with increasing TiO2
2
wt% TiO2
E
60 K
40 G
20
0 0
1
2
wt% TiO2 Fig. 8. Variation of elastic moduli with TiO2 addition for the mica based glass ceramics. (L; longitudinal modulus, E; YoungÕs modulus, K; bulk modulus, and G; shear modulus).
additions. Nevertheless the changes were mostly within the error limits. When a comparison is made between the elastic properties of the parent glass and its corresponding mica based glass ceramic, the mica based glass ceramic had higher longitudinal, shear and bulk moduli but lower YoungÕs
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modulus than the parent glass. For the composition containing 2 wt% TiO2, the mica based glass ceramic higher shear modulus but lower longitudinal, bulk, and YoungÕs moduli than the as-prepared glass. The longitudinal and shear moduli of the parent glass increased when 1 wt% TiO2 was incorporated, and then decreased when TiO2 addition was increased from 1 to 2 wt%. For the mica based glass ceramic, both moduli decreased continuously with increasing TiO2 additions. The YoungÕs modulus of the parent glass decreased continuously with increasing TiO2 additions. However, the YoungÕs modulus of the mica based glass ceramic increased when 1 wt% TiO2 was incorporated, and then decreased when TiO2 addition was increased from 1 to 2 wt%. 4. Discussion In general, for a given composition, the glass had higher VL than the corresponding glass ceramic due to the formation of crystals. The experimental results are in fairly good agreement with the existing theories on scattering by grains. De et al. [27] have observed similar behavior and concluded that normalized stress wave factor (NSWF) decreases because of precipitation of a large number of small crystals which increases the ultrasonic loss due to scattering. The initial increase in VL with increasing TiO2 addition is caused by the structural rearrangement of the glass due to the transfer of the bridging oxygens into non-bridging oxygens. It is anticipated that with addition of small amount of TiO2 glass structure was modified by fixing the broken Si–O–Si bonds through formation of Ti–O–Si bonds that resulted in the formation of TiO4 tetrahedras which function as terminal groups between the SiO4 and AlO4 tetrahedras. However, when TiO2 addition was increased further Si–O–Si or Ti–O–Si bonds were disrupted and Ti–O–Ti bonds were formed. Formation of Ti–O–Ti bonds caused the disruption of other oxide bonds and hence depolymerization of the structure, which resulted in softening of the glass network. The glass structure was ruined resulting in a decrease in VL. Rajendran et al. [18] have reported similar results in their study on elastic properties of the lead containing bizmuth tellurite glasses. For the mica based glass ceramics, a continuous decrease in VL with increasing TiO2 additions was observed. A possible explanation for that is TiO2 acts as a nucleating agent in this material system hence improves both nucleation and growth rates by decreasing the viscosity and surface energy between crystal and glass. With increasing nucleation and growth rates more and more nuclei form causing an increase in the amount of crystallinity and a change in the microstructure as seen in Figs. 4 and 5. The increase in the amount of crystallinity that performs as scattering centers results in a decrease in VL. From the appearance of Figs. 4(a) and 5(a), it can be realized that additions of as small as 1 wt% TiO2 had a notable change in the microstructure of the mica glass cera-
mic studied. It is evident that much more grains were formed and the residual glassy phase was less in the sample containing 1 wt% TiO2. Grains and grain boundaries could easily be distinguished in the sample containing TiO2 while that was not the case for the sample containing no TiO2 as seen in Figs. 4(b) and 5(b). Also, the aspect ratio and the grain size of the grains decreased as TiO2 additions increased. All the mica based glass ceramics containing TiO2 exhibited a microstructure similar to that reported by Vogel [24], Beall [25], and Tzeng et al. [26] for the mica based glass ceramics of comparable compositions. The texture resembled a house of cards as seen in Fig. 4(b). Interlocking platelets of phlogopite crystals were apparent. It has been reported [25] that this house of cards structure manifested the best machinability and mechanical properties. The experimental data obtained in this study was in accord with those given in the literature. As seen in Table 2, the mica based glass ceramic studied has higher VL than flint glass; more or less the same VL with natural quartz; but lower VL than magnesia. Also, it has higher VT than flint glass; more or less the same VT with plate glass; but lower VT than quartz glass. A comparison between Figs. 2 and 3 reveals that for a given composition TiO2 addition has a contrasting effect on VL and VT for the as-prepared glass and the mica based glass ceramic. This is an expected result since longitudinal wave is the wave in which the particle motion of a material is essentially in the same direction as the wave propagation, whereas transversal wave is the wave in which the particle displacement at each point in a material is perpendicular to the direction of propagation [28]. It should also be noted that coupling medium and the surface parallelity of the specimens have a profound effect on the ultrasonic velocity measurements. Grease oil was used as coupling medium for the VL measurements; whereas for the VT measurements coupling medium was honey. For a given composition, the as-prepared glass had higher density than the corresponding glass ceramic due to the formation of crystals. The experimental results are in good agreement with the findings of Kingery et al. [29] who stated that because of the structural considerations glasses usually but not always have a lower density than corresponding crystalline compositions. The increase in density is attributed to the formation of new linkages with the addition of TiO2 that contributed to a volume contraction. As a consequence the density increased. Kingery et al. [29] have reported that the density is a minimum value for the pure network former and increases as modifier ions, which increase the number of atoms present without changing the network much, are added. The density values were comparable with the mica based glass ceramics of similar compositions reported by Kodaira et al. [4] and Tzeng et al. [26]. The observed anomaly in the elastic property curves is attributed to some type of structural change as explained earlier. The bulk modulus of the parent glass and the mica
C.H. Gu¨r, A. Ozturk / Journal of Non-Crystalline Solids 351 (2005) 3655–3662 80
Elastic moduli (GPa)
as-prepared glass glass-ceramic
75
70
65 0
1
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4. Additions of TiO2 up to 2 wt% altered the elastic properties of the parent glass and mica based glass ceramic due to the structural changes. 5. The addition of TiO2 affected the longitudinal and transversal wave velocities of the parent glass and mica based glass ceramic in opposite manner. 6. Elastic properties of the mica based glass ceramic produced in this study were comparable with those of commercially produced glass ceramics. Therefore, it may be an alternative to many commercial dental materials commonly used for dental applications.
2
wt% TiO2 Fig. 9. Variation of elastic modulus with TiO2 addition for the as-prepared glasses and mica based glass ceramics.
based glass ceramic exhibited a reversal with increasing TiO2 additions. Since the bulk modulus represents the energy density and involves both compressive and shear elements in its description, it is not expected the relation to be valid even approximately [20]. A peculiarity is seen between the YoungÕs modulus of the parent glass and that of the mica based glass ceramic. For the composition containing no and 2 wt% TiO2, the mica based glass ceramic possessed consistently lower elastic modulus than the counterparts while there was no significant change for the composition containing 1 wt% TiO2. This behavior is separately shown in Fig. 9. The observed manner is in contrast to the findings of Vogel [24] who studied the elastic modulus of a mica based glass ceramic of similar composition by flexural test. He reported that the elastic modulus of the mica glass ceramic was higher than that of the parent glass. The increase caused by the crystallization was attributed to the interlocking of grains of the crystal phases formed in the microstructure of the mica glass ceramic [24]. Our fundamental understanding on the evaluation of the properties of these materials is as yet limited. However, a possible explanation may be the formation of fluorophlogipite crystals that increase the deflection without changing the load bearing capacity much. In addition, the newly formed crystals act as scattering centers, thereby reduce the efficiency of energy transmission and NSWF. Hence, the elastic modulus decreases. The observed anomaly in the elastic modulus indicates the change in dimensionality in this glass system. 5. Conclusions 1. TiO2 acts both as a network former and glass modifier in the SiO2–Al2O3–CaO–MgO–K2O–F system. 2. TiO2 plays a volume nucleating agent role during the crystallization process in mica based glass ceramics. 3. The crystallization heat treatment had a profound effect on the elastic properties of the glass produced from this system.
Acknowledgements This work was funded and supported in part by Middle East Technical University, METU, Research Fund Projects, Project Number: AFP-2001-08-03-08, and The Scientific and Technical Research Council of Turkey, TUBITAK, Project Number: MISAG-199. The authors are thankful to METU and TUBITAK. Thanks are also extended to Ms Asli Dereli for her help.
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[25] G.H. Beall, US-Patent 3,689,293, 1972. [26] C.M. Tzeng, J.G. Duh, K.H. Chung, C.C. Chan, J. Mater. Sci. 28 (1993) 6127. [27] A. De, K.K. Phani, S. Kumar, J. Mater. Sci. Lett. 6 (1987) 17.
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Determination of the influence of TiO2 on the elastic properties of a mica based glass ceramic by ultrasonic velocity measurements C.H. Gu¨r, A. Ozturk
*
Middle East Technical University, Metallurgical and Materials Engineering Department, 06531 Ankara, Turkey Received 8 March 2005 Available online 26 October 2005
Abstract The influence of small amount (1 or 2 wt%) of TiO2 additions and crystallization heat treatment on the elastic properties of a mica based glass ceramic have been investigated by ultrasonic velocity measurements. The mica based glass ceramic was prepared through controlled crystallization of a glass in the SiO2, Al2O3, CaO, MgO, K2O and F system. Evidences of TiO2 acting as a nucleating agent in this system was demonstrated. The longitudinal and transversal wave velocities of the as-prepared glass and the mica based glass ceramic were measured by using 5 MHz probes at room temperature. Elastic properties namely; longitudinal modulus, YoungÕs modulus, bulk modulus, and shear modulus were calculated from the ultrasonic velocity values measured and density values obtained experimentally. It has been observed that small amount of TiO2 additions caused a notable but not significant; however, the crystallization heat treatment had a profound effect on the elastic properties of the glass in the system studied. 2005 Elsevier B.V. All rights reserved. PACS: 61.43; 61.43.F; 43.35.A; 62.20.D
1. Introduction The glass ceramics containing mica crystals as the predominant crystalline phase have generated interest in biomedical field especially in the replacement of natural bone and dental restorations since the 1980s. Primary interest in these materials was developed due to their machinability [1,2] and biocompatibility [3]. In addition, they posses appropriate chemical, mechanical, thermal, and physical properties [1–4]. The unique properties result from the uniform, reproducible, fine grained microstructure consisting of easily cleavable and interlocking laminar mica crystals dispersed in a glassy matrix [5,6]. The properties of mica based glass ceramics closely match to those of human enamel [7,8] and can be tailored by changes in composition and heat treatment [5].
*
Corresponding author. Tel.: +90 312 210 5932; fax: +90 312 210 1267. E-mail address: [email protected] (A. Ozturk).
0022-3093/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.09.022
Grossman [9] has shown that mica based glass ceramics in the system K2O–SiO2–MgO–F can be utilized successfully in dental inlays, onlays, and crowns. Some compositions of mica glass ceramics that can be melted and cast into the form of tooth has been marketed under different commercial names such as Dicor [9], Macor [10], and Bioverit [11]. In spite of the good peculiarities of the mica glass ceramics, they are hard to prepare due to their high melting and forming temperatures, large crystallization tendency, phase separation on cooling, and compositional changes due to the fluorine volatilization on melting [4]. Efforts have been continuing to overcome the difficulties in their production and to develop a new mica glass ceramics that will exhibit better performance. Properties of mica based glass ceramics are related to the microstructure developed during heat treatment as well as chemical composition of the parent glass. It is commonly known that the nature of the crystallinity and distribution of the crystalline phase(s) formed during crystallization depends also on the kind and amount of nucleating agent
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used. Several investigations [2,4,12–16] conducted on the role of TiO2 as a nucleating agent in silicate systems have revealed that small addition of TiO2 to many silicate systems has remarkable influence on the crystallization. The role of TiO2 as a nucleating agent in mica based glass ceramics have been overlooked since these materials posses fluorine that promotes nucleation. Recently, Ozturk [17] has presented the evidence of TiO2 acting as volume nucleating catalyst in mica based glass ceramics. The present investigation was undertaken to accomplish two purposes. The first one is to find out the role of TiO2 as nucleating agent in the SiO2–Al2O3–CaO–MgO–K2O–F system. The second purpose is to determine the influence of TiO2 and crystallization heat treatment on the elastic properties of a glass in this system by ultrasonic velocity measurements. Ultrasonic techniques are a versatile tool for investigating the changes in microstructure, deformation process and mechanical properties [18]. The various parameters upon which the elastic moduli of glasses depend can be studied by measuring the ultrasonic wave velocities [19,20]. Although a number of researchers [10,21–26] evaluated the mechanical properties of mica based glass ceramics by destructive techniques, information on the evaluation of elastic properties of these materials by non-destructive techniques has not been reported in the open literature. Thus ultrasonic evaluation of the elastic properties of these materials is of importance from both a scientific and technological point of view. 2. Experimental On the basis of the formation of a mica based glass ceramics, a glass composition presented in Table 1 was taken under consideration. Fluorophlogopite (KMg3AlSi3O10F2) crystals can be formed from this glass upon subjecting a suitable heat treatment [17]. The parent glass was formed by melting appropriate amounts of extra pure grade powders supplied from Merck. TiO2 was introduced to parent glass in the batch. TiO2 additions were made in the amounts of 1 or 2 wt%, of glass. It has been reported [2] that formation of fluorophlogopite crystals is reduced and some TiO2 crystals begin the form when TiO2 additions exceed 2 wt%.
Batches of 50 g were melted in a platinum crucible at 1500 C at normal laboratory conditions without controlling the atmosphere. When melting was complete; the melt was poured into a stainless steel mould to obtain glass blocks. The nominal dimensions of the glass blocks were approximately 50 · 50 · 10 mm as length, width and depth, respectively. The glass blocks were colorless and indicated no unmolten batch materials and crystallinity. For each composition, (i.e. containing no, 1 wt% and 2 wt% TiO2) three glass blocks were formed in order to have sufficient number of specimens for property measurements and evaluations. The glass blocks were annealed at 600 C for 1 h. After annealing, two of the glass blocks from each composition were subjected to a two-step heat treatment, called crystallization, to convert the as-prepared glass blocks into glass ceramic. The nucleation and crystal growth temperatures were 620 C and 940 C, respectively. The blocks were held at each temperature for 8 h. The heating and cooling rates were 3 C/min. Following the crystallization, the surfaces of the blocks were ground and polished mechanically to get smooth, flat and parallel surfaces prior to ultrasonic measurements. Grinding was accomplished with the application of a series of SiC emerald papers, alumina powder solution on a cloth, and finally 0.25 lm diamond paste onto the surfaces. The ultrasonic velocity measurements were conducted on the as-prepared and crystallized (glass ceramic) samples at room temperature. A Panametrics 5052UAX50 analyzer, a Philips PM3365A oscilloscope, and 5 MHz probes were employed for the measurements. Grease was used as coupling medium for longitudinal wave velocity (VL) measurements. The coupling medium for transversal wave velocity (VT) measurements was honey. The velocity of sound was calculated from the equation: V ¼ 2w=t;
ð1Þ
where V, w, and t are the velocity of sound, the thickness of the test object, and the time-of-flight between adjacent backwall signals on the oscilloscope, respectively. Various elastic properties were computed according to the following formulae [20]: L ¼ q.V 2L ;
ð2Þ
G ¼ q.V 2T ; Table 1 Chemical composition of the parent glass Chemicals
SiO2 K2O Al2O3 MgO CaO F2 a
ð3Þ 2 L
K ¼ q=3 ð3V 4V E ¼ ½q.V
Glass composition (wt%) Calculateda
Analyzedb
56.7 17.7 0.6 14.5 3.8 6.2
56.8 18.5 1.7 12.9 4.5 5.6
Calculated values were obtained assuming the batch ingredients were converted into their respective oxides during melting. b Analyzed values were obtained from wet chemical analyses.
2 2 T ð3V L
4V
2 T Þ; 2 2 T Þ=ðV L
ð4Þ V
2 T Þ;
ð5Þ
where L, G, K, E, and q are longitudinal modulus, shear modulus, bulk modulus, YoungÕs modulus of elasticity, and density of the sample, respectively. Following the ultrasonic measurements, the density of the blocks was measured according to the ArchimedesÕ principle using water as the immersion liquid. The density of the specimens was calculated using the formula qglass ¼ m1 .qwater =ðm1 m2 Þ;
ð6Þ
C.H. Gu¨r, A. Ozturk / Journal of Non-Crystalline Solids 351 (2005) 3655–3662
where m1, m2, and qwater are the dry weight of the specimen, the weight of the specimen in water, and the density of water, respectively. The samples were characterized by X-ray diffraction, Rigaku Geigerflex (DMAX/B), and scanning electron microscopy, Jeol 6400, in order to determine the phases formed and the microstructure developed. The images were taken from the freshly fractured surfaces. 3. Results The as-prepared glass blocks were transparent and clear. Wet chemical analyses were performed to determine the chemical composition of the parent glass (the as-prepared glass containing no TiO2) since there is a possibility of fluorine volatilization during glass melting. The analysis results are given in Table 1. Even though there is a slight difference between the intended and analyzed composition of the glass, the both results match up with each other. Therefore, fluorine volatilization occurred throughout the glass making procedure was ignored. The colorless as-prepared glass blocks of different compositions appeared in white (milky) color after the crystallization heat treatment. It was obvious from the appearance of the crystallized blocks that phase separation occurred during cooling and at least two crystalline phases precipitated during crystallization of the parent glass. Crystalline phases formed during crystallization of the glass were identified by X-ray powder diffraction (XRD) analysis. A representative XRD pattern of the crystallized counterpart of the as-prepared glass (glass ceramic) containing 1 wt% TiO2 is shown in Fig. 1. The analyses suggested that fluorophlogopite was the primary crystalline phase. In addition, synthetic mica, wollastonite, and kaliophilite crystals formed during crystallization. The amount of TiO2 additions did not change the number of the crystalline phases precipitated but had an influence on the shape and size of the crystalline phases present. Longitudinal wave velocity (VL) and transversal wave velocity (VT) of the blocks were measured in the as-pre-
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pared and crystallized form. Average VL of the blocks was calculated by taking the mean average of five measurements from an individual block, and then, taking the mean average of velocities of the three blocks having the same TiO2 content and exposed to the same heat treatment. Maximum uncertainty of the data points was calculated as 2% of the determinations. Average values for the VL and VT obtained in this study was compared with the values of VL and VT of the selected glasses and ceramics in Table 2. The variations of VL and VT with TiO2 addition are illustrated in Figs. 2 and 3, respectively, for the as-prepared glass and the mica based glass ceramic. There was not any significant difference in average VL of the parent glass and the mica based glass ceramic obtained from this glass as seen in Fig. 2. When 1 wt% TiO2 was incorporated, average VL increased from 5818 to 5962 m/s for the as-prepared glass; whereas decreased from 5764 to 5723 m/s for the mica based glass ceramic. As TiO2 content was increased from 1 to 2 wt%, average VL for the as-prepared glass decreased from 5962 to 5838 m/s; and for the mica based glass ceramic decreased from 5723 to 5652 m/s. The relative amount of initial Table 2 A comparison of the longitudinal wave velocity (VL) and transversal wave velocity (VT) of the as-prepared glass and mica based glass ceramic investigated in this study with selected glasses and ceramics [27] Material
VL (m/s)
VT (m/s)
As-prepared glassa Mica based glass ceramica Glass, flint Glass, crown, regular Glass, plate Glass, pyrex Glass, quartz Magnesia Porcelain Quartz, fused Quartz, natural
5962 5723 4260 5660 5770 5610 5770 8850 5600 5970 5730
3451 3477 2560 3420 3430 – 3520 – 3400 – –
a
Contains 1 wt% TiO2.
6000 as-prepared glass glass-ceramic
VL (m/s)
5900
5800
5700
5600 0
1
2
wt% TiO2
Fig. 1. X-ray diffraction pattern of a mica based glass ceramic.
Fig. 2. Variation of longitudinal wave velocity with TiO2 addition for the as-prepared glasses and mica based glass ceramics. Error bands indicate the uncertainty of the data points.
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VT (m/s )
3600
3500
3400
3300
3200 0
1
2
wt% TiO2
Fig. 3. Variation of transversal wave velocity with TiO2 addition for the as-prepared glasses and mica based glass ceramics.
increase and later decrease with increasing TiO2 additions in the parent glass was within 2% of the measured values. Average VT obtained in this study was in the range of 3450–3500 m/s for the parent glass and its glass ceramic. VT of the as-prepared glasses and the mica based glass
Fig. 4. SEM micrographs of a mica based glass ceramic containing no TiO2: (a) ·6000 and (b) ·30 000.
ceramics showed a dissimilar behavior with increasing TiO2 addition. When 1 wt% TiO2 was incorporated, VT decreased for the as-prepared glass and increased for the mica based glass ceramic. As TiO2 addition was increased from 1 to 2 wt%, VT of both the as-prepared glass and the mica based glass ceramic decreased. Two of the representative scanning electron microscopy (SEM) micrographs of the glass ceramic sample containing no TiO2 is shown in Fig. 4(a) and (b) for two different magnifications. SEM micrographs of the glass ceramic sample containing 1 wt% TiO2 as nucleating agent is shown in Fig. 5(a) and (b) for the same magnifications. The average grain size and aspect ratio of the crystals were 0.4 ± 0.06 lm and 1.6 ± 0.1 lm, respectively, for the sample containing 1 wt% TiO2 but 0.6 ± 0.12 lm and 2.0 ± 0.15 lm, respectively, for the sample containing no TiO2. The plus and minus signs indicate the standard deviation from the determination points. It has been observed that the aspect ratio and the grain size of the crystals decreased further when TiO2 content was increased from 1 wt% to 2 wt%. Density of the as-prepared glass and the mica based glass ceramic was calculated as a function of TiO2 addition according to Eq. (6). Results are tabulated in Table 3.
Fig. 5. SEM micrographs of a mica based glass ceramic containing 1 wt% TiO2: (a) ·6000 and (b) ·30 000.
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Table 3 Density and elastic properties of the as-prepared glasses and mica based glass ceramics Specimen
TiO2 addition (wt%)
Density (g/cm3)
Longitudinal modulus (GPa)
Elastic modulus (GPa)
Bulk modulus (GPa)
Shear modulus (GPa)
Parent glass Glass ceramic As-prepared glass Glass ceramic As-prepared glass Glass ceramic
no no 1 1 2 2
2.520 2.606 2.528 2.619 2.537 2.629
85.79 89.23 93.07 85.78 89.95 83.99
79.23 72.12 75.92 76.43 74.16 69.89
40.93 51.23 52.92 43.55 50.57 46.73
33.64 30.88 30.11 31.67 29.54 27.95
100
2.65
2.6 as-prepared glass glass-ceramic
2.55
Elastic moduli (GPa)
Density (g/cm3)
L
80 E
60 K
40 G
20 2.5 0
1
2
0
wt% TiO2
0
1
Fig. 6. Variation of density with TiO2 addition for the as-prepared glasses and mica based glass ceramics.
Fig. 7. Variation of elastic moduli with TiO2 addition for the as-prepared glasses. (L; longitudinal modulus, E; YoungÕs modulus, K; bulk modulus and G; shear modulus).
100 L
80
Elastic moduli (GPa)
Determination of the data points was accomplished by taking the mean average of three measurements on individual block and then taking the mean average of the three blocks having the same TiO2 addition and exposed to the same heat treatment. Maximum uncertainty of the data was less than 0.5% of the determination points. The variations of density with TiO2 addition for the asprepared glass and the mica based glass ceramic are shown in Fig. 6. The densities ranged between 2.520 g/cm3 and 2.537 g/cm3 for the as-prepared glasses and 2.606 g/cm3 and 2.629 g/cm3 for the mica based glass ceramics. The density of the as-prepared glass and glass ceramic increased with increasing TiO2 addition in more or less the same manner. The longitudinal modulus (L), YoungÕs modulus (E), bulk modulus (K) and shear modulus (G), of the as-prepared glasses and the mica based glass ceramics were calculated employing Eqs. (1)–(5). Results are presented in Table 3. Maximum uncertainty of the data was calculated as 2% of the determination points. The variations of the longitudinal, elastic, bulk, and shear moduli with TiO2 addition are shown in Fig. 7 for the as-prepared glasses and in Fig. 8 for the mica based glass ceramics. For the as-prepared glasses, the longitudinal and bulk moduli tend to increase, but the shear and YoungÕs moduli tend to decrease with increasing TiO2 additions. For the mica based glass ceramic, the longitudinal and bulk moduli had a tendency to increase, but the shear and YoungÕs moduli had a tendency to decrease with increasing TiO2
2
wt% TiO2
E
60 K
40 G
20
0 0
1
2
wt% TiO2 Fig. 8. Variation of elastic moduli with TiO2 addition for the mica based glass ceramics. (L; longitudinal modulus, E; YoungÕs modulus, K; bulk modulus, and G; shear modulus).
additions. Nevertheless the changes were mostly within the error limits. When a comparison is made between the elastic properties of the parent glass and its corresponding mica based glass ceramic, the mica based glass ceramic had higher longitudinal, shear and bulk moduli but lower YoungÕs
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modulus than the parent glass. For the composition containing 2 wt% TiO2, the mica based glass ceramic higher shear modulus but lower longitudinal, bulk, and YoungÕs moduli than the as-prepared glass. The longitudinal and shear moduli of the parent glass increased when 1 wt% TiO2 was incorporated, and then decreased when TiO2 addition was increased from 1 to 2 wt%. For the mica based glass ceramic, both moduli decreased continuously with increasing TiO2 additions. The YoungÕs modulus of the parent glass decreased continuously with increasing TiO2 additions. However, the YoungÕs modulus of the mica based glass ceramic increased when 1 wt% TiO2 was incorporated, and then decreased when TiO2 addition was increased from 1 to 2 wt%. 4. Discussion In general, for a given composition, the glass had higher VL than the corresponding glass ceramic due to the formation of crystals. The experimental results are in fairly good agreement with the existing theories on scattering by grains. De et al. [27] have observed similar behavior and concluded that normalized stress wave factor (NSWF) decreases because of precipitation of a large number of small crystals which increases the ultrasonic loss due to scattering. The initial increase in VL with increasing TiO2 addition is caused by the structural rearrangement of the glass due to the transfer of the bridging oxygens into non-bridging oxygens. It is anticipated that with addition of small amount of TiO2 glass structure was modified by fixing the broken Si–O–Si bonds through formation of Ti–O–Si bonds that resulted in the formation of TiO4 tetrahedras which function as terminal groups between the SiO4 and AlO4 tetrahedras. However, when TiO2 addition was increased further Si–O–Si or Ti–O–Si bonds were disrupted and Ti–O–Ti bonds were formed. Formation of Ti–O–Ti bonds caused the disruption of other oxide bonds and hence depolymerization of the structure, which resulted in softening of the glass network. The glass structure was ruined resulting in a decrease in VL. Rajendran et al. [18] have reported similar results in their study on elastic properties of the lead containing bizmuth tellurite glasses. For the mica based glass ceramics, a continuous decrease in VL with increasing TiO2 additions was observed. A possible explanation for that is TiO2 acts as a nucleating agent in this material system hence improves both nucleation and growth rates by decreasing the viscosity and surface energy between crystal and glass. With increasing nucleation and growth rates more and more nuclei form causing an increase in the amount of crystallinity and a change in the microstructure as seen in Figs. 4 and 5. The increase in the amount of crystallinity that performs as scattering centers results in a decrease in VL. From the appearance of Figs. 4(a) and 5(a), it can be realized that additions of as small as 1 wt% TiO2 had a notable change in the microstructure of the mica glass cera-
mic studied. It is evident that much more grains were formed and the residual glassy phase was less in the sample containing 1 wt% TiO2. Grains and grain boundaries could easily be distinguished in the sample containing TiO2 while that was not the case for the sample containing no TiO2 as seen in Figs. 4(b) and 5(b). Also, the aspect ratio and the grain size of the grains decreased as TiO2 additions increased. All the mica based glass ceramics containing TiO2 exhibited a microstructure similar to that reported by Vogel [24], Beall [25], and Tzeng et al. [26] for the mica based glass ceramics of comparable compositions. The texture resembled a house of cards as seen in Fig. 4(b). Interlocking platelets of phlogopite crystals were apparent. It has been reported [25] that this house of cards structure manifested the best machinability and mechanical properties. The experimental data obtained in this study was in accord with those given in the literature. As seen in Table 2, the mica based glass ceramic studied has higher VL than flint glass; more or less the same VL with natural quartz; but lower VL than magnesia. Also, it has higher VT than flint glass; more or less the same VT with plate glass; but lower VT than quartz glass. A comparison between Figs. 2 and 3 reveals that for a given composition TiO2 addition has a contrasting effect on VL and VT for the as-prepared glass and the mica based glass ceramic. This is an expected result since longitudinal wave is the wave in which the particle motion of a material is essentially in the same direction as the wave propagation, whereas transversal wave is the wave in which the particle displacement at each point in a material is perpendicular to the direction of propagation [28]. It should also be noted that coupling medium and the surface parallelity of the specimens have a profound effect on the ultrasonic velocity measurements. Grease oil was used as coupling medium for the VL measurements; whereas for the VT measurements coupling medium was honey. For a given composition, the as-prepared glass had higher density than the corresponding glass ceramic due to the formation of crystals. The experimental results are in good agreement with the findings of Kingery et al. [29] who stated that because of the structural considerations glasses usually but not always have a lower density than corresponding crystalline compositions. The increase in density is attributed to the formation of new linkages with the addition of TiO2 that contributed to a volume contraction. As a consequence the density increased. Kingery et al. [29] have reported that the density is a minimum value for the pure network former and increases as modifier ions, which increase the number of atoms present without changing the network much, are added. The density values were comparable with the mica based glass ceramics of similar compositions reported by Kodaira et al. [4] and Tzeng et al. [26]. The observed anomaly in the elastic property curves is attributed to some type of structural change as explained earlier. The bulk modulus of the parent glass and the mica
C.H. Gu¨r, A. Ozturk / Journal of Non-Crystalline Solids 351 (2005) 3655–3662 80
Elastic moduli (GPa)
as-prepared glass glass-ceramic
75
70
65 0
1
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4. Additions of TiO2 up to 2 wt% altered the elastic properties of the parent glass and mica based glass ceramic due to the structural changes. 5. The addition of TiO2 affected the longitudinal and transversal wave velocities of the parent glass and mica based glass ceramic in opposite manner. 6. Elastic properties of the mica based glass ceramic produced in this study were comparable with those of commercially produced glass ceramics. Therefore, it may be an alternative to many commercial dental materials commonly used for dental applications.
2
wt% TiO2 Fig. 9. Variation of elastic modulus with TiO2 addition for the as-prepared glasses and mica based glass ceramics.
based glass ceramic exhibited a reversal with increasing TiO2 additions. Since the bulk modulus represents the energy density and involves both compressive and shear elements in its description, it is not expected the relation to be valid even approximately [20]. A peculiarity is seen between the YoungÕs modulus of the parent glass and that of the mica based glass ceramic. For the composition containing no and 2 wt% TiO2, the mica based glass ceramic possessed consistently lower elastic modulus than the counterparts while there was no significant change for the composition containing 1 wt% TiO2. This behavior is separately shown in Fig. 9. The observed manner is in contrast to the findings of Vogel [24] who studied the elastic modulus of a mica based glass ceramic of similar composition by flexural test. He reported that the elastic modulus of the mica glass ceramic was higher than that of the parent glass. The increase caused by the crystallization was attributed to the interlocking of grains of the crystal phases formed in the microstructure of the mica glass ceramic [24]. Our fundamental understanding on the evaluation of the properties of these materials is as yet limited. However, a possible explanation may be the formation of fluorophlogipite crystals that increase the deflection without changing the load bearing capacity much. In addition, the newly formed crystals act as scattering centers, thereby reduce the efficiency of energy transmission and NSWF. Hence, the elastic modulus decreases. The observed anomaly in the elastic modulus indicates the change in dimensionality in this glass system. 5. Conclusions 1. TiO2 acts both as a network former and glass modifier in the SiO2–Al2O3–CaO–MgO–K2O–F system. 2. TiO2 plays a volume nucleating agent role during the crystallization process in mica based glass ceramics. 3. The crystallization heat treatment had a profound effect on the elastic properties of the glass produced from this system.
Acknowledgements This work was funded and supported in part by Middle East Technical University, METU, Research Fund Projects, Project Number: AFP-2001-08-03-08, and The Scientific and Technical Research Council of Turkey, TUBITAK, Project Number: MISAG-199. The authors are thankful to METU and TUBITAK. Thanks are also extended to Ms Asli Dereli for her help.
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