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Influence of leucite content on slow crack growth of dental porcelains
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1114–1122
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Influence of leucite content on slow crack growth of dental porcelains Paulo F. Cesar a,∗ , Fabiana N. Soki a , Humberto N. Yoshimura b , Carla C. Gonzaga a , Victor Styopkin c a b c
˜ Paulo, Sao ˜ Paulo, Brazil Department of Dental Materials, School of Dentistry, University of Sao ˜ Paulo, Sao ˜ Paulo, Brazil Laboratory of Metallurgy and Ceramic Materials, Institute for Technological Research of the State of Sao Jendental, Kiev, Ukraine
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. To determine the stress corrosion susceptibility coefficient, n, of seven dental
Received 3 February 2007
porcelains (A: Ceramco I; B: Ceramco-II; C: Ceramco-III; D: d.Sign; E: Cerabien; F: Vitadur-
Received in revised form
Alpha; and G: Ultropaline) after aging in air or artificial saliva, and correlate results with
5 December 2007
leucite content (LC).
Accepted 10 January 2008
Methods. Bars were fired according to manufacturers’ instructions and polished before induction of cracks by a Vickers indenter (19.6 N, 20 s). Four specimens were stored in air/room temperature, and three in saliva/37 ◦ C. Five indentations were made per specimen and crack
Keywords:
lengths measured at the following times: ∼0; 1; 3; 10; 30; 100; 300; 1000 and 3000 h. The stress
Dental porcelain
corrosion coefficient n was calculated by linear regression analysis after plotting crack length
Slow crack growth
as a function of time, considering that the slope of the curve was [2/(3n + 2)]. Microstructural
Fracture toughness
analysis was performed to determine LC.
Microstructure
Results. LC of the porcelains were 22% (A and B); 6% (C); 15% (D); 0% (E and F); and 13% (G).
Mechanical properties
Except for porcelains A and D, all materials showed a decrease in their n values when stored in artificial saliva. However, the decrease was more pronounced for porcelains B, F, and G. Ranking of materials varied according to storage media (in air, porcelain G showed higher n compared to A, while in saliva both showed similar coefficients). No correlation was found between n values and LC in air or saliva. Significance. Storage media influenced the n value obtained for most of the materials. LC did not affect resistance to slow crack growth regardless of the test environment. © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Porcelains are highly esthetic materials used as monolithic restorations (inlays, onlays and veneers) or as veneering materials over core ceramics in bilayered crowns and fixed partial dentures (FPDs). However, due to relatively low fracture tough-
∗
ness, unwanted fracture rates have been reported for such restorations and prostheses in clinical trials [1–3]. The fracture of ceramics in service occurs with little or no plastic deformation when a small flaw or crack propagates in an unstable manner under applied tensile stress (i.e. catastrophic failure) [4]. For a body containing a crack of length c, subjected to a
´ ˜ Paulo, Av. Prof. Lineu Corresponding author at: Departamento de Materiais Dentarios, Faculdade de Odontologia, Universidade de Sao ´ ˜ Paulo, CEP 05508-900, SP, Brazil. Tel.: +55 11 5561 3042; Prestes, 2227 Cidade Universitaria, “Armando Salles de Oliveira”, Sao fax: +55 11 5561 3042. E-mail address: [email protected] (P.F. Cesar). 0109-5641/$ – see front matter © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2008.01.003
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1114–1122
tensile stress , the stress intensity factor at the crack tip is given by: √ KI = Y c
(1)
where Y is a dimensionless constant which depends on the stress mode, shape and dimensions of the material, geometry and length of the crack. Fracture occurs when the critical level of stress intensity factor (KIc ) is reached. When subjected to a stress intensity factor below the critical level (KI < KIc ), the defects in ceramic materials may present a slow and stable growth, mainly in a humid environment such as the oral cavity. This phenomenon is referred as slow or subcritical crack growth (SCG) and leads to strength degradation over time [5]. The presence of water at the tip of a crack under stress results in the rupture of the metallic oxides bonds of the material, with the subsequent formation of hydroxyls. As a consequence of SCG, a defect may reach critical size (under a determined applied stress) and result in fast fracture [6]. The subcritical crack growth is notable for its extreme sensitivity to applied load and it tends also to depend on the concentration of environmental species, temperature and other extraneous variables [7]. The oral environment has many elements that favor SCG in ceramic restorations, such as: (a) water from saliva; (b) water from the luting cement and from dentin tubules; (c) masticatory stresses; (d) stresses associated with differences in the coefficient of thermal expansion of the restoration components; (e) temperature variations; and (f) pH variations [8]. Ceramics containing glassy phase, like dental porcelains, are highly susceptible to SCG [9], therefore the slow growth of defects is expected to be a common finding in such restorations, resulting in a decrease of their strength and in-service reliability [10]. The phenomenon of SCG in ceramic materials can be characterized by the stress corrosion susceptibility coefficient, n, which is calculated using the following empirical power law equation [11]: v=
dc KI = v0 dt KIc
n (2)
where v is the crack velocity at an applied stress intensity factor (KI ), c the crack size, t the time, v0 the critical velocity of the crack at the moment of fracture, and KIc is the fracture toughness. Since KI /KIc < 1, a higher n value means higher resistance to SCG and consequently longer service life. The coefficient n can be measured by direct or indirect methods. In the direct methods, the crack velocity (v) is determined by measuring the crack length in a time interval under different levels of KI . Examples of these techniques are the “double cantilever beam method” and “double torsion” [12]. The main advantage of such techniques is that they allow for determination of v using a large range of KI . However, they have shortcomings such as the need of large specimens with large cracks [11]. The indirect methods are used to determine the SCG parameters by means of flexural strength tests. The most used techniques are the dynamic fatigue test, which determines strength at different stress rates, and the static fatigue test, which measures time to fracture of different static
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loads [9]. The indentation fracture method (IF) is an alternative to the previously mentioned techniques in which the lengths of the cracks generated by a Vickers indenter are measured over time [13]. This method allows for determination of n by means of correlation plots between time and crack size. The microstructure of a ceramic material is known to strongly affect crack propagation and its mechanical properties [14]. In this regard, it has been demonstrated that dental porcelains with higher amounts of leucite are more resistant to fast crack propagation, and have higher fracture toughness due to the phenomenon of crack deflection around leucite particles [15]. Therefore, it is expected that the presence of leucite in the glassy matrix of porcelains will also hinder slow crack propagation. The objective of this study was to determine the stress corrosion susceptibility coefficient, n, of seven dental porcelains after aging in air or artificial saliva, and correlate results with leucite content (LC). The hypotheses to be tested are: (1) the leucite content of the material influences its n value; and (2) the n value will depend on the storage media.
2.
Materials and methods
Seven dental porcelain powders were used: A, B, C, D, E, F and G (Table 1). Porcelains A, B, C, D and G are indicated for porcelainfused-to-metal and all-ceramic restorations, and porcelains E and F are indicated to be used as veneering material for alumina-based cores. The different types of porcelain were chosen in order to provide a wide variation of leucite contents. Porcelains E and F are silica-rich materials, while the other materials are feldspathic porcelains. Materials E and F had to be chosen to represent leucite-free materials because apparently there are no commercial feldspathic porcelains without leucite. Green specimens (5 mm × 6 mm × 30 mm) were prepared by the vibration–condensation method using a stainless steel mold and sintered in a dental porcelain furnace following the firing schedules recommended by the manufacturers (Table 2). The reproducibility of the specimen preparation was monitored by sintered density measured by Archimedes’ method (the coefficient of variation was 0.5% or less for most of the porcelains, except for that of porcelain E, which was 1.6%). After firing, the specimens were machined to the dimensions of 3 mm × 4 mm × ∼25 mm, following the guidelines in ASTM C 1161 [16]. Then, the 4-mm side was mirror polished using a polishing machine (Ecomet 3, Buehler, Lake Bluff, ILL, USA) with diamond suspensions (45, 15, 6 and 1 m). Radial cracks were generated on polished surfaces with a ˜ Paulo, Vickers microhardness tester MVK-H-3 (Mitutoyo, Sao Brazil) with load of 19.6 N and dwell time of 20 s. This load was chosen in order to generate long radial cracks (c/a > 2.5, where c is the crack length and a is half of the impression diagonal) in all porcelains. Higher loads caused chipping of the porcelain surface near the indentation. It has been demonstrated that Vickers hardness of ceramic materials is not independent of load [17]. Moreover, a previous work showed that the Vickers hardness of a feldspathic porcelain varied significantly when lower loads were used (from 2.0 to 4.9 N), however the hardness values did not vary when higher loads were applied (from 9.8 to 49.0 N) [18]. In the present work, the indentation load was the same for all materials because the effect of the inden-
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Table 1 – Porcelains used in the study (shade A3-dentin) Porcelain
Manufacturer/Brand name
Batch number
A
Dentsply Ceramco R & D, Burlington, USA/Ceramco I
D1738
B
Dentsply Ceramco R & D, Burlington, USA/Ceramco II
D1739
C
Dentsply Ceramco R & D, Burlington, USA/Ceramco Finesse
D1740
D
Ivoclar, Schaan, Liechtenstein/d.Sign
54D2001-12
E
Noritake, Tokyo, Japan/Cerabien
0C902
F
Vita, Bad Sackingen, Germany/Vitadur-Alpha
6335
G
Jendental, Kiev, Ukraine/Ultropaline
UK 3321-09
tation load on the porcelains’ hardness was not previously determined. Crack lengths were measured under optical microscopy at various times over a period of 4 months (∼0; 1; 3; 10; 30; 100; 300, 1000 and 3000 h) in two storage media (air or artificial saliva). For storage in air (∼60% relative humidity and ∼22 ◦ C), four specimens of each material were used, and five indentations were made in each one. Indentations were made in the central area of the polished surface, and the distance between two indentations was at least ten times the size of the radial cracks. Immediately after making each indentation, the crack size was measured and specimens were stored in a plastic box until next measurement. For storage in artificial saliva, three specimens of each material were used and five indentations were made in each one, using the same configuration described above. Indentations were made on the specimen surface with a drop of artificial saliva. This procedure aimed to facilitate penetration of saliva into the radial crack tip, avoiding formation of air bubbles in this region, which could preclude the reactant (saliva) from interacting
Manufacturer’s description High-fusing, leucite-based porcelain, used for metal-ceramic or all ceramic restorations, containing isometric leucite particles. Firing temperature: 1000 ◦ C High-fusing, leucite-based porcelain, used for metal-ceramic or all ceramic restorations, containing leucite particles. Firing temperature: 1000 ◦ C Low-fusing, leucite-based porcelain, used for metal-ceramic or all ceramic restorations, containing fine-grained leucite particles. Firing temperature: 800 ◦ C Low-fusing, leucite-based porcelain, used for metal-ceramic or all ceramic restorations, containing leucite particles and crystals of fluorapatite. Firing temperature: 875 ◦ C High-fusing porcelain to be used with alumina frameworks. Firing temperature: 960 ◦ C High-fusing porcelain to be used with alumina frameworks. Firing temperature: 970 ◦ C High-fusing, leucite-based porcelain. Firing temperature: 930 ◦ C
with the crack tip. Immediately after making the indention, the surface was dried with a paper cloth to allow visualization of crack size. After that specimens were stored at constant temperature (37 ◦ C) in a glass box with artificial saliva with the following composition: 100 mL of KH2 PO4 (2.5 mM); 100 mL of Na2 HPO4 (2.4 mM); 100 mL of KHCO3 (1.5 mM); 100 mL of NaCl (1.0 mM); 100 mL of MgCl2 (0.15 mM); 100 mL of CaCl2 (1.5 mM); and 6 mL of citric acid (0.002 mM). The stress corrosion susceptibility coefficient, n, was determined using the indentation-fracture method proposed by Gupta and Jubb [13], based on the theory of the evolution of radial cracks [19], according to the following relation:
ln c =
2 3n + 2
ln t + I
(3)
where c is the size of radial cracks generated by the Vickers indentor at a time t after the impression, and I is the intercept with the c axis, which depends on the indentation load [13]. Thus, the slope of the curves in the logarithmic plots of
Table 2 – Firing schedules for the porcelains used in the present study Porcelain Drying time (min) Vacuum (mmHg) Initial temperature (◦ C) Heating rate (◦ C/min) Maximum temperature (◦ C) Vacuum off temperature (◦ C) Cooling time (min) Average cooling rate (◦ C/min)
A 9 29 600 55 1000 1000 6 150
B 9 29 600 55 1000 1000 6 150
C
D
E
F
G
9 29 450 55 800 800 6 117
6 29 403 60 875 875 6 129
7 29 600 45 960 960 9 96
9 29 600 60 970 970 6 145
9 29 250 60 930 930 6 138
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1114–1122
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was made on polished blocks of 3 mm × 4 mm × 4 mm of the seven porcelains studied. To reveal the microstructure of the materials, specimens were etched with 2% hydrofluoric acid (HF) solution for 15 s. After that, they were analyzed using a scanning electron microscope (SEM) (Jeol-JSM 6300, Peabody, MA, USA) coupled with an energy dispersive spectroscope (EDS) (Noran Instruments, Middletown, WI, USA). The volume fraction and the size of leucite and pores were determined using an image analyzer software (Leica, QWin, Germany). A semi-quantitative chemical analysis of all materials was also performed by means of X-ray fluorescence (Shimadzu, XRF 1500, Japan).
3.
Fig. 1 – Mean crack size (c) as a function of log time for the seven materials tested in air and saliva up to 3000 h. Regression analysis was performed using only data up to 10 h for all porcelains.
crack size versus time equals 2/(3n + 2). A soda-lime glass slide (3 mm-thick), similar to the one used by Gupta and Jubb [13], was used as reference material in order to verify the reproducibility of the proposed method (see Fig. 1). Fracture toughness was calculated using measurements obtained in air immediately after indenting specimens. The following equation was used [20]:
E 0.5 P 1.5
KIc = 0.016
H
c
(4)
where P is the indentation load, c the size of radial cracks, E the elastic modulus and H is the material hardness. The elastic modulus of each material was determined by the pulse-echo method. Fracture toughness values were analyzed by means of one-way ANOVA. A microstructural analysis was performed in order to determine the crystalline content of each material. This analysis
Results
Crack sizes as a function of time (up to 3000 h) for the seven porcelains studied are presented in Fig. 1 (storage in air and saliva). Both graphs show that crack sizes increased over time; however the increase was more significant in the first hours of the experiment (from ∼0 to 10 h). The decrease in crack velocity above 10 h indicates that the influence of the residual tensile stresses generated by the Vickers indentation decreased significantly after the radial cracks reached a certain length and when the crack tip was far from the elastic–plastic deformation zone. Since slow crack growth is a phenomenon directly associated with the presence of stress fields at the crack tip, the authors decided to use only the data obtained up to 10 h (Fig. 1) to calculate the n values of the dental porcelains studied. Such crack growth stabilization was also observed for the sodalime glass slide in ambient air, and the calculated n value was 25, considering the radial crack sizes measured up to 10 h. This value was close to the ones reported by Gupta and Jubb [13] for similar material (n = 21.8 − 24.8), using measurements made during the first hour after indentation. Therefore, this result validates the decision of using only the data obtained up to 10 h. The soda-lime glass slide used in this study was previously annealed by the manufacturer, and the KIc value obtained for it was 0.70 ± 0.02 MPa m1/2 , which was similar to the value obtained for an annealed sodalime glass using the chevron-notch beam method [21]. This fact indicates that this glass slide did not have significant residual stresses that could have affected the measurements obtained. As for storage in air, cracks measured at 3000 h in porcelains B, F and G increased from 5 to 9% compared to measurements made at ∼0 h. For the other materials (A, C, D, and E), cracks increased from 11 to 16% during the same period. In saliva, the increase in crack length for porcelains A, B, D, E, F, and G varied from 11 to 16% after 3000 h, whereas this variation was 21% for porcelain C. The leucite content, fracture toughness, stress corrosion susceptibility coefficient, and scaling parameter (I) of the seven porcelains studied, are shown in Table 3. The n values determined in air varied significantly among the different porcelains, and the highest value (n = 68, for porcelain G) was 2.7 times higher than the lowest value (n = 25, for porcelain C). As for experiments conducted in artificial saliva at 37 ◦ C, the values of n varied between 30 and 37, except for porce-
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108 107 145 127 134 116 123 117 103 150 129 132 128 114 34 30 19 31 31 37 33
Sal. Air Sal.
32 51 25 30 38 62 68 0.93 (0.12) a 0.89 (0.11) a 0.64 (0.06) c 0.77 (0.06) b 0.78 (0.06) b 0.73 (0.15) b 0.84 (0.11) a 69.7 (0.7) d 69.7 (2.0) d 72.6 (1.4) b 71.0 (0.4) c 71.1 (0.3) c 66.5 (0.5) e 74.4 (2.4) a
Fig. 2 – Stress corrosion coefficient (n) vs. leucite content for the seven porcelains stored in air and saliva.
For each column, values followed by the same letter are not statistically different (p < 0.05).
2.435 (0.003) e 2.483 (0.006) c 2.506 (0.013) b 2.550 (0.003) a 2.409 (0.038) f 2.359 (0.004) g 2.457 (0.060) d 5.0 (2.3) a 4.4 (2.2) b 3.5 (1.4) c 1.9 (0.5) d – – 1.4 (0.4) f 4.7 (5.6) ab 4.2 (6.0) b 6.4 (9.3) a 5.8 (7.8) ab 5.0 (3.5) ab 6.3 (9.3) ab 5.9 (8.9) ab A B C D E F G
0.77 (0.26) ab 1.03 (0.25) a 0.81 (0.21) a 0.75 (0.24) ab 0.27 (0.14) b 0.54 (0.22) ab 0.78 (0.41) ab
22 (2) a 22 (2) a 6 (1) c 15 (1) b 0 0 13 (1) b
Air
n KIc (MPa m1/2 ) E (GPa) (g/cm3 ) PS (m) LC (vol.%) Pore size (m) Porosity (vol.%) Porcelain
Table 3 – Porosity, pore size, leucite content (LC), leucite particle size (PS), density (), elastic modulus (E), fracture toughness (KIc ), stress corrosion susceptibility coefficient (n), and scaling parameter (I) of the seven porcelains tested (standard deviations in parenthesis)
I (m)
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lain C, which showed the lowest value (n = 19). Most materials (porcelains A and D are the exceptions) showed a decrease in their n values when stored in artificial saliva. However, the decrease was more evident in porcelains B, F, and G. In addition, ranking of materials varied according to storage media. For example, in air, porcelain G showed higher n compared to porcelain A, whereas in saliva both showed similar coefficients. No correlation was found between n values and KIc or leucite content (LC) in any of the storage media, as demonstrated in Figs. 2 and 3. With respect to the relationship between I and LC, it is observed in Fig. 4 that despite the low r2 (0.42), the I values tended to fall off with the increase in leucite content. SEM analysis (Fig. 5) showed the presence of leucite particles in porcelains A, B, C, D and G. For all materials, the distribution of the leucite particles in the glassy matrix was heterogeneous and the particles were grouped in clusters. The leucite particles of porcelains A, B, C and G were equiaxial, while those of porcelain D had a dendritic morphology. Porcelain D also presented fine particles dispersed in some regions of the glassy matrix, most likely of fluorapatite since the EDS analysis of these particles showed the presence of fluorine.
Fig. 3 – Stress corrosion coefficient (n) vs. fracture toughness (KIc ) for the seven porcelains stored in air and saliva.
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1114–1122
4.
Fig. 4 – Scaling parameter (I) vs. leucite content (LC) for the seven porcelains stored in air and saliva.
With respect to mean particle size, porcelains A and B showed the largest sizes, whereas porcelain C presented an intermediary particle size. Porcelains D and G presented the smallest particle sizes (Table 3). Table 3 shows the results obtained for density, elastic modulus and porosity. Low levels of porosity (1% or less) were detected in all materials and pore sizes varied from 4.2 to 6.4 m. Table 4 shows chemical composition (in wt%) of all porcelains studied. Porcelain E and F presented the highest percentages of SiO2 (76.6 and 68.1%, respectively). The amount of this component in the other porcelains varied from 58.2 to 65.0%. Porcelain A, B, D, F and G showed the highest percentages of Al2 O3 (varying from 13.1 to 16.4%), and porcelains C and E showed the lowest amounts (9.6 and 8.8%, respectively). Porcelain E showed the lowest amount of K2 O (6.7%), while the other materials showed values varying from 10.0 to 13.0% for this component. The amount of Na2 O was similar for all materials (varied from 3.3 to 5.1%) and the amount of CaO varied from 0.9 to 3.1%. ZrO2 was only found in small quantities (less than 2.5%) in porcelains D, E, F and G. Similarly, only a small amount of MgO was found in porcelains A, C, E and G. Other components such as Fe2 O3, TiO2 , Y2 O3 , ZnO, BaO, CeO2 and Pb2 O5 were also found in the materials but they are not displayed in Table 4 since their concentrations were very low (less than 1%).
Table 4 – Semi-quantitative chemical analysis by X-ray fluorescence (in weight%) for all porcelains studied Component
SiO2 Al2 O3 K2 O Na2 O CaO ZrO2 MgO
Porcelain A
B
C
D
E
F
G
65.0 14.4 11.5 4.5 2.2 0.0 1.0
62.1 13.9 12.4 3.4 3.1 0.0 0.0
62.2 9.6 12.1 5.0 3.1 0.0 2.4
58.2 13.1 10.9 4.4 3.0 2.4 0.0
76.6 8.8 6.7 3.5 0.9 1.6 0.4
68.1 15.1 10.0 3.3 2.6 0.6 0.0
60.8 16.4 13.0 5.1 2.9 0.4 0.5
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Discussion
The n values obtained in this study for specimens stored in saliva at 37 ◦ C (from 19 to 37, Table 3) are consistent with those obtained by Gupta and Jubb [13] for soda-lime glass in water at room temperature (from 14.7 to 19.2), and with other studies [8,22] that used the dynamic fatigue test (from 15 to 29) with dental porcelains in water at 37 ◦ C. Another study [23] that used the indentation fracture technique (IF) showed higher n values (varying from 30 to 68) for dental porcelains stored in water at room temperature for 5 months. However the authors did not mention if crack size stabilization occurred, so the high values reported may be related to measurements made after cracks stopped growing. The first hypothesis of this work was rejected since results showed that leucite content of dental porcelains does not influence their susceptibility to slow crack growth (Fig. 2). It was expected that the presence of leucite (mainly in porcelains A, B, D, and G) would lead to higher values of n since it has been shown that second phase particles hinder crack propagation by means of crack deflection [14]. Deflection occurs when a crack changes its direction of propagation after meeting a second phase particle. This change in the propagation path diminishes the stress intensity factor at the crack tip. Crack deflection around leucite particles most likely occurs when the crack comes upon tangential compressive stresses, and is subsequently guided around the particles by radial tensile stresses. These compressive and tensile stress fields are created around the leucite-glass interface during cooling because leucite particles contract more rapidly than the matrix glass [24]. However, the results of the present study suggest that this toughening mechanism acts only during fast crack propagation, i.e. when the stress intensity at the crack tip has already reached the critical level. The inefficiency of leucite in hindering slow crack propagation could be explained by the fact that the tensile stresses around the leucite/glassy matrix interface also cause an increase in inter-atomic spacing, weakening inter-atomic bonding and making this region more sensitive to the effects of water [25]. On the other hand, when crack propagates at supersonic velocities, this deleterious effect cannot be noted since water is unable to react with the crack tip. The chemical composition of porcelains studied was also determined (Table 4) in order to investigate the influence of this factor on slow crack propagation. The relatively high n values obtained in saliva for porcelains without leucite (E and F) may be related to the presence of Al2 O3 in their glassy matrix. For all other porcelains, significant fractions of the Al2 O3 content detected in their composition were used to form leucite (KAlSi2 O6 ). Given that porcelains E and F are only vitreous, the Al2 O3 content found in these materials (8.8 and 15.1%, respectively) is in their glassy matrix, and may lead to the observed resistance to slow crack propagation, since Al2 O3 enhances inter-atomic bonding and increases surface energy [26]. In order to verify the effect of the leucite content in the properties of feldspathic porcelains, a feldspathic glass would have represented a better control at 0% level, instead of using silica-rich materials (E and F). However, the results showed that the n values of the feldspathic porcelains did not increase with the increase in leucite content when the results obtained
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Fig. 5 – SEM Micrographs of the polished surfaces of the porcelains etched with hydrofluoric acid.
for porcelains E and F are disregarded (Fig. 2). Moreover, no correlation was found between the n value and leucite particle size (Table 3). As to porosity, no correlation was found between this variable and the n values of the porcelains. Porosity may
be an important factor in the response of a porcelain to slow crack growth, since open pores in the surface of the material act as stress concentrators and facilitate the uptake of water from the ambient. However, it is likely that the low porosity
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1114–1122
detected in the porcelains studied (0.3–1.0%) was not enough to influence the n values measured. The absence of correlation between fracture toughness and stress corrosion coefficient (Fig. 3) indicates that materials with higher resistance to fast crack propagation do not necessarily have higher resistance to slow crack growth. This finding may be explained by the fact that slow crack growth is strongly dependent on extraneous variables such as concentration of active environmental species, while fast fracture is environment-insensitive and usually associated with the kinetics of intrinsic bond rupture [27]. However, it should be noted that porcelain C presented both the lowest n and KIc values, indicating that in this material these two variables may be related to each other. Observation of the scaling parameter I, which is also the radial crack size at 1 h after indentation (Table 3), indicates that crack sizes varied considerably among porcelains studied from 103 to 150 m. Such variation leads to the observed variation in fracture toughness values, since crack sizes are used to calculate this property (Eq. (4)). Therefore, materials with shorter cracks as A, B, and G, showed the highest KIc values, while porcelain C showed both the largest cracks and the lowest fracture toughness. Crack sizes seem to be governed by material microstructure, since porcelains with higher leucite content tended to present shorter cracks, and higher fracture toughness, as observed in Fig. 4. In fact, such relationship has already been demonstrated elsewhere [14]. The second hypothesis of the study was accepted since it was observed that storage media affected the n values obtained for most of the materials (except porcelains A and D). The decrease in the n values after storage in saliva may be explained by the fact that the velocity of crack propagation in vitreous materials made of SiO2 is strongly influenced by the relative humidity of the environment [28]. Also, it has been shown that the crack velocity increases as a function of the hydroxyl concentration, suggesting that these molecules break chemical bonds of the material at the crack tip, allowing for crack extension [29]. In accordance with that, Gupta and Jubb [13] also observed a decrease of about 30% in the n values of soda-lime glass tested in water in comparison to testing in air. An important implication of this fact is that the occurrence of slow crack growth in water will ultimately lead to a decrease in strength of these materials as observed by previous works [30,31]. In addition to the decrease of stress corrosion coefficients in saliva, it is important to note that the variation in n values was much higher in air than in saliva. While in air these values varied from 25 to 68, in saliva they were around 33 (except for porcelain C). This fact points out the importance of testing porcelains in saliva at 37 ◦ C in order to obtain stress corrosion coefficient values, since storage medium plays an important role in the final outcome. Residual stresses present in the porcelain surface affect the apparent fracture toughness values obtained by the Indentation Fracture method when the material is subject to fast cooling, as observed in thermal tempering procedures [32]. In the present study, high cooling rates were used during the firing cycle (Table 2) and no annealing was performed, indicating that the sintered specimens probably had residual stresses to some extent (mainly compression stresses near the surface). However, previous investigations from the authors’ laboratory
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(unpublished data) showed that careful removal of a layer of about 0.4 mm of the specimen surface can eliminate most of the residual stresses due to rapid cooling. In this regard, machining of the specimens in this study strictly followed the guidelines in ASTM C 1161 [16]. This procedure removed a layer of at least 0.6 mm from the specimen surface. In addition, the polishing procedure was carried out carefully in order to avoid the introduction of residual stresses. Therefore, it is not likely that the results shown here were affected by residual stresses induced due to fast cooling. To verify this assumption, an additional experiment was carried out using two polished specimens that had already been used to determine n values. The two materials chosen (A and C) were feldspathic porcelains with different sintering temperatures and leucite content. Specimens were annealed at their glass transition temperatures, Tg , for 30 min, following slow cooling (1 ◦ C/min) to minimize any possibility of introduction of residual stresses. The results showed that the KIc values obtained for porcelains A and C after the annealing treatment (1.07 ± 0.13 e 0.69 ± 0.06 MPa m1/2 , respectively) were similar to those measured in the polished surfaces that were not annealed (Table 3), indicating that compressive residual stresses did not affect the KIc results. These results indicate that the method used in the present study to produce porcelain specimens removed most of the residual stresses generated during the sintering cycle. It is important to consider that the results of the present study are limited since it is an in vitro experiment. Additional clinical aspects need to be considered with respect to slow crack growth, such as the fact that the defects originating the fracture of monolithic restorations are located at the internal surface of the restoration, where the cement is applied and hoop stresses are present [4,10,33]. Therefore, the ambient in which slow crack growth occurs is far more complex than the one used in the present study. On the other hand, the experimental set up used here (porcelain immersed in saliva at 37 ◦ C) is more similar to the clinical situation found in FPDs connectors. In this case, the connector is totally immersed in saliva and crack will grow on its gingival side, where the fracture origin is usually located [34,35]. Other factors that may influence crack growth of porcelains in the clinical situation are: (a) moisture from the luting cement; (b) fatigue stresses from masticatory forces [36]; (c) handling and design of the restoration; and (d) thickness of the restoration [37]. Moreover, crack healing or blunting of the crack tip may occur during prolonged non-stress periods [22]. Hence, a caution is suggested that direct extrapolation of the results of this research to clinical applications could be misleading.
5.
Conclusion
After determining the stress corrosion coefficient (n) of seven dental porcelains with varied leucite contents in different environments (air or saliva), it was possible to conclude that storage media influenced the n values obtained for most materials (specimens stored in saliva showed lower n values). Moreover, it was found that leucite content did not affect resistance to slow crack growth of porcelains tested regardless of the test environment.
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d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1114–1122
Acknowledgments The authors acknowledge the Brazilian agencies FAPESP and CNPq for the financial support of the present research.
references
[1] Haselton DR, Diaz-Arnold AM, Hillis SL. Clinical assessment of high-strength all-ceramic crowns. J Prosthet Dent 2000;83:396–401. [2] Kelly JR, Tesk JA, Sorensen JA. Failure of all-ceramic fixed partial dentures in vitro and in vivo: analysis and modeling. J Dent Res 1995;74:1253–8. [3] McLaren EA, White SN. Survival of in-ceram crowns in a private practice: a prospective clinical trial. J Prosthet Dent 2000;83:216–22. [4] Thompson JY, Anusavice KJ, Naman A, Morris HF. Fracture surface characterization of clinically failed all-ceramic crowns. J Dent Res 1994;73:1824–32. [5] Myers ML, Ergle JW, Fairhurst CW, Ringle RD. Fatigue failure parameters of IPS-empress porcelain. J Dent Res 1994;73, 136–136. [6] Ritter JE. Predicting lifetimes of materials and material structures. Dent Mater 1995;11:142–6. [7] De Aza AH, Chevalier J, Fantozzi G, Schehl M, Torrecillas R. Crack growth resistance of alumina, zirconia and zirconia toughened alumina ceramics for joint prostheses. Biomaterials 2002;23:937–45. [8] Morena R, Beaudreau GM, Lockwood PE, Evans AL, Fairhurst CW. Fatigue of dental ceramics in a simulated oral environment. J Dent Res 1986;65:993–7. [9] Watchman Jr JB. Mechanical properties of ceramics. New York: John Wiley & Sons; 1996. [10] Guazzato M, Albakry M, Ringer SP, Swain MV. Strength, fracture toughness and microstructure of a selection of all-ceramic materials. Part I. Pressable and alumina glass-infiltrated ceramics. Dent Mater 2004;20:441–8. [11] Green DJ. An introduction to the mechanical properties of ceramics. Cambridge: University Press; 1998. [12] Wiederhorn SM. Subcritical crack growth in ceramics. New York: Plenum Press; 1974. [13] Gupta PK, Jubb NJ. Post-indentation slow growth of radial cracks in glasses. J Am Ceram Soc 1981;64:112–4. [14] Cesar PF, Yoshimura HN, Miranda WG, Okada CY. Correlation between fracture toughness and leucite content in dental porcelains. J Dent 2005;33:721–9. [15] Yoshimura HN, Cesar PF, Miranda Jr WG, Gonzaga CC, Okada CY. Fracture toughness of dental porcelains evaluated by IF, SCF, and SEPB methods. J Am Ceram Soc 2005;88: 1680–3. [16] ASTM C. Flexural strength of advanced ceramics at ambient temperature; 2002.
[17] Quinn G, Green P, Xu K. Cracking and the indentation size effect for Knoop hardness of glasses. J Am Ceram Soc 2003;86:441–8. [18] Cesar P, Gonzaga C, Okada C, Molisani A, Santana W, Goldenstein H, et al. Variables that affect the indentation fracture testing (IF) of a dental porcelain. Mater Sci Forum 2003;416:663–8. [19] Lawn BR, Evans AG, Marshall DB. Elastic–plastic indentation damage in ceramics—the median-radial crack system. J Am Ceram Soc 1980;63:574–81. [20] Anstis GR, Chantikul P, Lawn BR, Marshall DB. A critical-evaluation of indentation techniques for measuring fracture-toughness. Part 1. Direct crack measurements. J Am Ceram Soc 1981;64:533–8. [21] Wang H, Isgro G, Pallav P, Feilzer A, Chao Y. Influence of test methods on fracture toughness of a dental porcelain and a soda lime glass. J Am Ceram Soc 2005;88:2868–73. [22] Fairhurst CW, Lockwood PE, Ringle RD, Twiggs SW. Dynamic fatigue of feldspathic porcelain. Dent Mater 1993;9:269–73. [23] Ali SM. Prediction of the service life of dental porcelains by the measurements of post-indentation slow crack growth. Dent Mater J 1993;12:45–53. [24] Denry IL, Mackert Jr JR, Holloway JA, Rosenstiel SF. Effect of cubic leucite stabilization on the flexural strength of feldspathic dental porcelain. J Dent Res 1996;75:1928–35. [25] Michalske TA, Bunker BC. Slow fracture model based on strained silicate structures. J Appl Phys 1984;56:2686–93. [26] Doremus RH. Glass Science. 2nd ed. New York: John Wiley & Sons; 1994, 339 pp. [27] Wan KT, Lathabai S, Lawn BR. Crack velocity functions and tresholds in brittle solids. J Eur Ceram Soc 1990;6:259–68. [28] Wiederhorn SM. Influence of water vapor on crack propagation in soda-lime glass. J Am Ceram Soc 1967;50:407–15. [29] Wiederhorn SM. Chemical interpretation of static fatigue. J Am Ceram Soc 1972;55:81–5. [30] Sherrill CA, O’Brien WJ. Transverse strength of aluminous and feldspathic porcelain. J Dent Res 1974;53:683–90. [31] Southan DE, Jorgensen KD. The endurance limit of dental porcelain. Aust Dent J 1974;19:7–11. [32] DeHoff P, Anusavice K, Vontivillu S. Analysis of tempering stresses in metal ceramic disks. J Dent Res 1996;75:743–51. [33] Quinn JB, Quinn GD, Kelly JR, Scherrer SS. Fractographic analyses of three ceramic whole crown restoration failures. Dent Mater 2005;21:920–9. [34] Guazzato M, Proos K, Sara G, Swain MV. Strength, reliability, and mode of fracture of bilayered porcelain/core ceramics. Int J Prosthodont 2004;17:142–9. [35] White SN, Miklus VG, McLaren EA, Lang LA, Caputo AA. Flexural strength of a layered zirconia and porcelain dental all-ceramic system. J Prosthet Dent 2005;94:125–31. [36] White SN. Mechanical fatigue of a feldspathic dental procelain. Dent Mater 1993;9:260–4. [37] Kelly JR. Clinically relevant approach to failure testing of all-ceramic restorations. J Prosthet Dent 1999;81:652–61.
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Influence of leucite content on slow crack growth of dental porcelains Paulo F. Cesar a,∗ , Fabiana N. Soki a , Humberto N. Yoshimura b , Carla C. Gonzaga a , Victor Styopkin c a b c
˜ Paulo, Sao ˜ Paulo, Brazil Department of Dental Materials, School of Dentistry, University of Sao ˜ Paulo, Sao ˜ Paulo, Brazil Laboratory of Metallurgy and Ceramic Materials, Institute for Technological Research of the State of Sao Jendental, Kiev, Ukraine
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. To determine the stress corrosion susceptibility coefficient, n, of seven dental
Received 3 February 2007
porcelains (A: Ceramco I; B: Ceramco-II; C: Ceramco-III; D: d.Sign; E: Cerabien; F: Vitadur-
Received in revised form
Alpha; and G: Ultropaline) after aging in air or artificial saliva, and correlate results with
5 December 2007
leucite content (LC).
Accepted 10 January 2008
Methods. Bars were fired according to manufacturers’ instructions and polished before induction of cracks by a Vickers indenter (19.6 N, 20 s). Four specimens were stored in air/room temperature, and three in saliva/37 ◦ C. Five indentations were made per specimen and crack
Keywords:
lengths measured at the following times: ∼0; 1; 3; 10; 30; 100; 300; 1000 and 3000 h. The stress
Dental porcelain
corrosion coefficient n was calculated by linear regression analysis after plotting crack length
Slow crack growth
as a function of time, considering that the slope of the curve was [2/(3n + 2)]. Microstructural
Fracture toughness
analysis was performed to determine LC.
Microstructure
Results. LC of the porcelains were 22% (A and B); 6% (C); 15% (D); 0% (E and F); and 13% (G).
Mechanical properties
Except for porcelains A and D, all materials showed a decrease in their n values when stored in artificial saliva. However, the decrease was more pronounced for porcelains B, F, and G. Ranking of materials varied according to storage media (in air, porcelain G showed higher n compared to A, while in saliva both showed similar coefficients). No correlation was found between n values and LC in air or saliva. Significance. Storage media influenced the n value obtained for most of the materials. LC did not affect resistance to slow crack growth regardless of the test environment. © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Porcelains are highly esthetic materials used as monolithic restorations (inlays, onlays and veneers) or as veneering materials over core ceramics in bilayered crowns and fixed partial dentures (FPDs). However, due to relatively low fracture tough-
∗
ness, unwanted fracture rates have been reported for such restorations and prostheses in clinical trials [1–3]. The fracture of ceramics in service occurs with little or no plastic deformation when a small flaw or crack propagates in an unstable manner under applied tensile stress (i.e. catastrophic failure) [4]. For a body containing a crack of length c, subjected to a
´ ˜ Paulo, Av. Prof. Lineu Corresponding author at: Departamento de Materiais Dentarios, Faculdade de Odontologia, Universidade de Sao ´ ˜ Paulo, CEP 05508-900, SP, Brazil. Tel.: +55 11 5561 3042; Prestes, 2227 Cidade Universitaria, “Armando Salles de Oliveira”, Sao fax: +55 11 5561 3042. E-mail address: [email protected] (P.F. Cesar). 0109-5641/$ – see front matter © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2008.01.003
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1114–1122
tensile stress , the stress intensity factor at the crack tip is given by: √ KI = Y c
(1)
where Y is a dimensionless constant which depends on the stress mode, shape and dimensions of the material, geometry and length of the crack. Fracture occurs when the critical level of stress intensity factor (KIc ) is reached. When subjected to a stress intensity factor below the critical level (KI < KIc ), the defects in ceramic materials may present a slow and stable growth, mainly in a humid environment such as the oral cavity. This phenomenon is referred as slow or subcritical crack growth (SCG) and leads to strength degradation over time [5]. The presence of water at the tip of a crack under stress results in the rupture of the metallic oxides bonds of the material, with the subsequent formation of hydroxyls. As a consequence of SCG, a defect may reach critical size (under a determined applied stress) and result in fast fracture [6]. The subcritical crack growth is notable for its extreme sensitivity to applied load and it tends also to depend on the concentration of environmental species, temperature and other extraneous variables [7]. The oral environment has many elements that favor SCG in ceramic restorations, such as: (a) water from saliva; (b) water from the luting cement and from dentin tubules; (c) masticatory stresses; (d) stresses associated with differences in the coefficient of thermal expansion of the restoration components; (e) temperature variations; and (f) pH variations [8]. Ceramics containing glassy phase, like dental porcelains, are highly susceptible to SCG [9], therefore the slow growth of defects is expected to be a common finding in such restorations, resulting in a decrease of their strength and in-service reliability [10]. The phenomenon of SCG in ceramic materials can be characterized by the stress corrosion susceptibility coefficient, n, which is calculated using the following empirical power law equation [11]: v=
dc KI = v0 dt KIc
n (2)
where v is the crack velocity at an applied stress intensity factor (KI ), c the crack size, t the time, v0 the critical velocity of the crack at the moment of fracture, and KIc is the fracture toughness. Since KI /KIc < 1, a higher n value means higher resistance to SCG and consequently longer service life. The coefficient n can be measured by direct or indirect methods. In the direct methods, the crack velocity (v) is determined by measuring the crack length in a time interval under different levels of KI . Examples of these techniques are the “double cantilever beam method” and “double torsion” [12]. The main advantage of such techniques is that they allow for determination of v using a large range of KI . However, they have shortcomings such as the need of large specimens with large cracks [11]. The indirect methods are used to determine the SCG parameters by means of flexural strength tests. The most used techniques are the dynamic fatigue test, which determines strength at different stress rates, and the static fatigue test, which measures time to fracture of different static
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loads [9]. The indentation fracture method (IF) is an alternative to the previously mentioned techniques in which the lengths of the cracks generated by a Vickers indenter are measured over time [13]. This method allows for determination of n by means of correlation plots between time and crack size. The microstructure of a ceramic material is known to strongly affect crack propagation and its mechanical properties [14]. In this regard, it has been demonstrated that dental porcelains with higher amounts of leucite are more resistant to fast crack propagation, and have higher fracture toughness due to the phenomenon of crack deflection around leucite particles [15]. Therefore, it is expected that the presence of leucite in the glassy matrix of porcelains will also hinder slow crack propagation. The objective of this study was to determine the stress corrosion susceptibility coefficient, n, of seven dental porcelains after aging in air or artificial saliva, and correlate results with leucite content (LC). The hypotheses to be tested are: (1) the leucite content of the material influences its n value; and (2) the n value will depend on the storage media.
2.
Materials and methods
Seven dental porcelain powders were used: A, B, C, D, E, F and G (Table 1). Porcelains A, B, C, D and G are indicated for porcelainfused-to-metal and all-ceramic restorations, and porcelains E and F are indicated to be used as veneering material for alumina-based cores. The different types of porcelain were chosen in order to provide a wide variation of leucite contents. Porcelains E and F are silica-rich materials, while the other materials are feldspathic porcelains. Materials E and F had to be chosen to represent leucite-free materials because apparently there are no commercial feldspathic porcelains without leucite. Green specimens (5 mm × 6 mm × 30 mm) were prepared by the vibration–condensation method using a stainless steel mold and sintered in a dental porcelain furnace following the firing schedules recommended by the manufacturers (Table 2). The reproducibility of the specimen preparation was monitored by sintered density measured by Archimedes’ method (the coefficient of variation was 0.5% or less for most of the porcelains, except for that of porcelain E, which was 1.6%). After firing, the specimens were machined to the dimensions of 3 mm × 4 mm × ∼25 mm, following the guidelines in ASTM C 1161 [16]. Then, the 4-mm side was mirror polished using a polishing machine (Ecomet 3, Buehler, Lake Bluff, ILL, USA) with diamond suspensions (45, 15, 6 and 1 m). Radial cracks were generated on polished surfaces with a ˜ Paulo, Vickers microhardness tester MVK-H-3 (Mitutoyo, Sao Brazil) with load of 19.6 N and dwell time of 20 s. This load was chosen in order to generate long radial cracks (c/a > 2.5, where c is the crack length and a is half of the impression diagonal) in all porcelains. Higher loads caused chipping of the porcelain surface near the indentation. It has been demonstrated that Vickers hardness of ceramic materials is not independent of load [17]. Moreover, a previous work showed that the Vickers hardness of a feldspathic porcelain varied significantly when lower loads were used (from 2.0 to 4.9 N), however the hardness values did not vary when higher loads were applied (from 9.8 to 49.0 N) [18]. In the present work, the indentation load was the same for all materials because the effect of the inden-
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Table 1 – Porcelains used in the study (shade A3-dentin) Porcelain
Manufacturer/Brand name
Batch number
A
Dentsply Ceramco R & D, Burlington, USA/Ceramco I
D1738
B
Dentsply Ceramco R & D, Burlington, USA/Ceramco II
D1739
C
Dentsply Ceramco R & D, Burlington, USA/Ceramco Finesse
D1740
D
Ivoclar, Schaan, Liechtenstein/d.Sign
54D2001-12
E
Noritake, Tokyo, Japan/Cerabien
0C902
F
Vita, Bad Sackingen, Germany/Vitadur-Alpha
6335
G
Jendental, Kiev, Ukraine/Ultropaline
UK 3321-09
tation load on the porcelains’ hardness was not previously determined. Crack lengths were measured under optical microscopy at various times over a period of 4 months (∼0; 1; 3; 10; 30; 100; 300, 1000 and 3000 h) in two storage media (air or artificial saliva). For storage in air (∼60% relative humidity and ∼22 ◦ C), four specimens of each material were used, and five indentations were made in each one. Indentations were made in the central area of the polished surface, and the distance between two indentations was at least ten times the size of the radial cracks. Immediately after making each indentation, the crack size was measured and specimens were stored in a plastic box until next measurement. For storage in artificial saliva, three specimens of each material were used and five indentations were made in each one, using the same configuration described above. Indentations were made on the specimen surface with a drop of artificial saliva. This procedure aimed to facilitate penetration of saliva into the radial crack tip, avoiding formation of air bubbles in this region, which could preclude the reactant (saliva) from interacting
Manufacturer’s description High-fusing, leucite-based porcelain, used for metal-ceramic or all ceramic restorations, containing isometric leucite particles. Firing temperature: 1000 ◦ C High-fusing, leucite-based porcelain, used for metal-ceramic or all ceramic restorations, containing leucite particles. Firing temperature: 1000 ◦ C Low-fusing, leucite-based porcelain, used for metal-ceramic or all ceramic restorations, containing fine-grained leucite particles. Firing temperature: 800 ◦ C Low-fusing, leucite-based porcelain, used for metal-ceramic or all ceramic restorations, containing leucite particles and crystals of fluorapatite. Firing temperature: 875 ◦ C High-fusing porcelain to be used with alumina frameworks. Firing temperature: 960 ◦ C High-fusing porcelain to be used with alumina frameworks. Firing temperature: 970 ◦ C High-fusing, leucite-based porcelain. Firing temperature: 930 ◦ C
with the crack tip. Immediately after making the indention, the surface was dried with a paper cloth to allow visualization of crack size. After that specimens were stored at constant temperature (37 ◦ C) in a glass box with artificial saliva with the following composition: 100 mL of KH2 PO4 (2.5 mM); 100 mL of Na2 HPO4 (2.4 mM); 100 mL of KHCO3 (1.5 mM); 100 mL of NaCl (1.0 mM); 100 mL of MgCl2 (0.15 mM); 100 mL of CaCl2 (1.5 mM); and 6 mL of citric acid (0.002 mM). The stress corrosion susceptibility coefficient, n, was determined using the indentation-fracture method proposed by Gupta and Jubb [13], based on the theory of the evolution of radial cracks [19], according to the following relation:
ln c =
2 3n + 2
ln t + I
(3)
where c is the size of radial cracks generated by the Vickers indentor at a time t after the impression, and I is the intercept with the c axis, which depends on the indentation load [13]. Thus, the slope of the curves in the logarithmic plots of
Table 2 – Firing schedules for the porcelains used in the present study Porcelain Drying time (min) Vacuum (mmHg) Initial temperature (◦ C) Heating rate (◦ C/min) Maximum temperature (◦ C) Vacuum off temperature (◦ C) Cooling time (min) Average cooling rate (◦ C/min)
A 9 29 600 55 1000 1000 6 150
B 9 29 600 55 1000 1000 6 150
C
D
E
F
G
9 29 450 55 800 800 6 117
6 29 403 60 875 875 6 129
7 29 600 45 960 960 9 96
9 29 600 60 970 970 6 145
9 29 250 60 930 930 6 138
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1114–1122
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was made on polished blocks of 3 mm × 4 mm × 4 mm of the seven porcelains studied. To reveal the microstructure of the materials, specimens were etched with 2% hydrofluoric acid (HF) solution for 15 s. After that, they were analyzed using a scanning electron microscope (SEM) (Jeol-JSM 6300, Peabody, MA, USA) coupled with an energy dispersive spectroscope (EDS) (Noran Instruments, Middletown, WI, USA). The volume fraction and the size of leucite and pores were determined using an image analyzer software (Leica, QWin, Germany). A semi-quantitative chemical analysis of all materials was also performed by means of X-ray fluorescence (Shimadzu, XRF 1500, Japan).
3.
Fig. 1 – Mean crack size (c) as a function of log time for the seven materials tested in air and saliva up to 3000 h. Regression analysis was performed using only data up to 10 h for all porcelains.
crack size versus time equals 2/(3n + 2). A soda-lime glass slide (3 mm-thick), similar to the one used by Gupta and Jubb [13], was used as reference material in order to verify the reproducibility of the proposed method (see Fig. 1). Fracture toughness was calculated using measurements obtained in air immediately after indenting specimens. The following equation was used [20]:
E 0.5 P 1.5
KIc = 0.016
H
c
(4)
where P is the indentation load, c the size of radial cracks, E the elastic modulus and H is the material hardness. The elastic modulus of each material was determined by the pulse-echo method. Fracture toughness values were analyzed by means of one-way ANOVA. A microstructural analysis was performed in order to determine the crystalline content of each material. This analysis
Results
Crack sizes as a function of time (up to 3000 h) for the seven porcelains studied are presented in Fig. 1 (storage in air and saliva). Both graphs show that crack sizes increased over time; however the increase was more significant in the first hours of the experiment (from ∼0 to 10 h). The decrease in crack velocity above 10 h indicates that the influence of the residual tensile stresses generated by the Vickers indentation decreased significantly after the radial cracks reached a certain length and when the crack tip was far from the elastic–plastic deformation zone. Since slow crack growth is a phenomenon directly associated with the presence of stress fields at the crack tip, the authors decided to use only the data obtained up to 10 h (Fig. 1) to calculate the n values of the dental porcelains studied. Such crack growth stabilization was also observed for the sodalime glass slide in ambient air, and the calculated n value was 25, considering the radial crack sizes measured up to 10 h. This value was close to the ones reported by Gupta and Jubb [13] for similar material (n = 21.8 − 24.8), using measurements made during the first hour after indentation. Therefore, this result validates the decision of using only the data obtained up to 10 h. The soda-lime glass slide used in this study was previously annealed by the manufacturer, and the KIc value obtained for it was 0.70 ± 0.02 MPa m1/2 , which was similar to the value obtained for an annealed sodalime glass using the chevron-notch beam method [21]. This fact indicates that this glass slide did not have significant residual stresses that could have affected the measurements obtained. As for storage in air, cracks measured at 3000 h in porcelains B, F and G increased from 5 to 9% compared to measurements made at ∼0 h. For the other materials (A, C, D, and E), cracks increased from 11 to 16% during the same period. In saliva, the increase in crack length for porcelains A, B, D, E, F, and G varied from 11 to 16% after 3000 h, whereas this variation was 21% for porcelain C. The leucite content, fracture toughness, stress corrosion susceptibility coefficient, and scaling parameter (I) of the seven porcelains studied, are shown in Table 3. The n values determined in air varied significantly among the different porcelains, and the highest value (n = 68, for porcelain G) was 2.7 times higher than the lowest value (n = 25, for porcelain C). As for experiments conducted in artificial saliva at 37 ◦ C, the values of n varied between 30 and 37, except for porce-
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108 107 145 127 134 116 123 117 103 150 129 132 128 114 34 30 19 31 31 37 33
Sal. Air Sal.
32 51 25 30 38 62 68 0.93 (0.12) a 0.89 (0.11) a 0.64 (0.06) c 0.77 (0.06) b 0.78 (0.06) b 0.73 (0.15) b 0.84 (0.11) a 69.7 (0.7) d 69.7 (2.0) d 72.6 (1.4) b 71.0 (0.4) c 71.1 (0.3) c 66.5 (0.5) e 74.4 (2.4) a
Fig. 2 – Stress corrosion coefficient (n) vs. leucite content for the seven porcelains stored in air and saliva.
For each column, values followed by the same letter are not statistically different (p < 0.05).
2.435 (0.003) e 2.483 (0.006) c 2.506 (0.013) b 2.550 (0.003) a 2.409 (0.038) f 2.359 (0.004) g 2.457 (0.060) d 5.0 (2.3) a 4.4 (2.2) b 3.5 (1.4) c 1.9 (0.5) d – – 1.4 (0.4) f 4.7 (5.6) ab 4.2 (6.0) b 6.4 (9.3) a 5.8 (7.8) ab 5.0 (3.5) ab 6.3 (9.3) ab 5.9 (8.9) ab A B C D E F G
0.77 (0.26) ab 1.03 (0.25) a 0.81 (0.21) a 0.75 (0.24) ab 0.27 (0.14) b 0.54 (0.22) ab 0.78 (0.41) ab
22 (2) a 22 (2) a 6 (1) c 15 (1) b 0 0 13 (1) b
Air
n KIc (MPa m1/2 ) E (GPa) (g/cm3 ) PS (m) LC (vol.%) Pore size (m) Porosity (vol.%) Porcelain
Table 3 – Porosity, pore size, leucite content (LC), leucite particle size (PS), density (), elastic modulus (E), fracture toughness (KIc ), stress corrosion susceptibility coefficient (n), and scaling parameter (I) of the seven porcelains tested (standard deviations in parenthesis)
I (m)
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1114–1122
lain C, which showed the lowest value (n = 19). Most materials (porcelains A and D are the exceptions) showed a decrease in their n values when stored in artificial saliva. However, the decrease was more evident in porcelains B, F, and G. In addition, ranking of materials varied according to storage media. For example, in air, porcelain G showed higher n compared to porcelain A, whereas in saliva both showed similar coefficients. No correlation was found between n values and KIc or leucite content (LC) in any of the storage media, as demonstrated in Figs. 2 and 3. With respect to the relationship between I and LC, it is observed in Fig. 4 that despite the low r2 (0.42), the I values tended to fall off with the increase in leucite content. SEM analysis (Fig. 5) showed the presence of leucite particles in porcelains A, B, C, D and G. For all materials, the distribution of the leucite particles in the glassy matrix was heterogeneous and the particles were grouped in clusters. The leucite particles of porcelains A, B, C and G were equiaxial, while those of porcelain D had a dendritic morphology. Porcelain D also presented fine particles dispersed in some regions of the glassy matrix, most likely of fluorapatite since the EDS analysis of these particles showed the presence of fluorine.
Fig. 3 – Stress corrosion coefficient (n) vs. fracture toughness (KIc ) for the seven porcelains stored in air and saliva.
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1114–1122
4.
Fig. 4 – Scaling parameter (I) vs. leucite content (LC) for the seven porcelains stored in air and saliva.
With respect to mean particle size, porcelains A and B showed the largest sizes, whereas porcelain C presented an intermediary particle size. Porcelains D and G presented the smallest particle sizes (Table 3). Table 3 shows the results obtained for density, elastic modulus and porosity. Low levels of porosity (1% or less) were detected in all materials and pore sizes varied from 4.2 to 6.4 m. Table 4 shows chemical composition (in wt%) of all porcelains studied. Porcelain E and F presented the highest percentages of SiO2 (76.6 and 68.1%, respectively). The amount of this component in the other porcelains varied from 58.2 to 65.0%. Porcelain A, B, D, F and G showed the highest percentages of Al2 O3 (varying from 13.1 to 16.4%), and porcelains C and E showed the lowest amounts (9.6 and 8.8%, respectively). Porcelain E showed the lowest amount of K2 O (6.7%), while the other materials showed values varying from 10.0 to 13.0% for this component. The amount of Na2 O was similar for all materials (varied from 3.3 to 5.1%) and the amount of CaO varied from 0.9 to 3.1%. ZrO2 was only found in small quantities (less than 2.5%) in porcelains D, E, F and G. Similarly, only a small amount of MgO was found in porcelains A, C, E and G. Other components such as Fe2 O3, TiO2 , Y2 O3 , ZnO, BaO, CeO2 and Pb2 O5 were also found in the materials but they are not displayed in Table 4 since their concentrations were very low (less than 1%).
Table 4 – Semi-quantitative chemical analysis by X-ray fluorescence (in weight%) for all porcelains studied Component
SiO2 Al2 O3 K2 O Na2 O CaO ZrO2 MgO
Porcelain A
B
C
D
E
F
G
65.0 14.4 11.5 4.5 2.2 0.0 1.0
62.1 13.9 12.4 3.4 3.1 0.0 0.0
62.2 9.6 12.1 5.0 3.1 0.0 2.4
58.2 13.1 10.9 4.4 3.0 2.4 0.0
76.6 8.8 6.7 3.5 0.9 1.6 0.4
68.1 15.1 10.0 3.3 2.6 0.6 0.0
60.8 16.4 13.0 5.1 2.9 0.4 0.5
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Discussion
The n values obtained in this study for specimens stored in saliva at 37 ◦ C (from 19 to 37, Table 3) are consistent with those obtained by Gupta and Jubb [13] for soda-lime glass in water at room temperature (from 14.7 to 19.2), and with other studies [8,22] that used the dynamic fatigue test (from 15 to 29) with dental porcelains in water at 37 ◦ C. Another study [23] that used the indentation fracture technique (IF) showed higher n values (varying from 30 to 68) for dental porcelains stored in water at room temperature for 5 months. However the authors did not mention if crack size stabilization occurred, so the high values reported may be related to measurements made after cracks stopped growing. The first hypothesis of this work was rejected since results showed that leucite content of dental porcelains does not influence their susceptibility to slow crack growth (Fig. 2). It was expected that the presence of leucite (mainly in porcelains A, B, D, and G) would lead to higher values of n since it has been shown that second phase particles hinder crack propagation by means of crack deflection [14]. Deflection occurs when a crack changes its direction of propagation after meeting a second phase particle. This change in the propagation path diminishes the stress intensity factor at the crack tip. Crack deflection around leucite particles most likely occurs when the crack comes upon tangential compressive stresses, and is subsequently guided around the particles by radial tensile stresses. These compressive and tensile stress fields are created around the leucite-glass interface during cooling because leucite particles contract more rapidly than the matrix glass [24]. However, the results of the present study suggest that this toughening mechanism acts only during fast crack propagation, i.e. when the stress intensity at the crack tip has already reached the critical level. The inefficiency of leucite in hindering slow crack propagation could be explained by the fact that the tensile stresses around the leucite/glassy matrix interface also cause an increase in inter-atomic spacing, weakening inter-atomic bonding and making this region more sensitive to the effects of water [25]. On the other hand, when crack propagates at supersonic velocities, this deleterious effect cannot be noted since water is unable to react with the crack tip. The chemical composition of porcelains studied was also determined (Table 4) in order to investigate the influence of this factor on slow crack propagation. The relatively high n values obtained in saliva for porcelains without leucite (E and F) may be related to the presence of Al2 O3 in their glassy matrix. For all other porcelains, significant fractions of the Al2 O3 content detected in their composition were used to form leucite (KAlSi2 O6 ). Given that porcelains E and F are only vitreous, the Al2 O3 content found in these materials (8.8 and 15.1%, respectively) is in their glassy matrix, and may lead to the observed resistance to slow crack propagation, since Al2 O3 enhances inter-atomic bonding and increases surface energy [26]. In order to verify the effect of the leucite content in the properties of feldspathic porcelains, a feldspathic glass would have represented a better control at 0% level, instead of using silica-rich materials (E and F). However, the results showed that the n values of the feldspathic porcelains did not increase with the increase in leucite content when the results obtained
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Fig. 5 – SEM Micrographs of the polished surfaces of the porcelains etched with hydrofluoric acid.
for porcelains E and F are disregarded (Fig. 2). Moreover, no correlation was found between the n value and leucite particle size (Table 3). As to porosity, no correlation was found between this variable and the n values of the porcelains. Porosity may
be an important factor in the response of a porcelain to slow crack growth, since open pores in the surface of the material act as stress concentrators and facilitate the uptake of water from the ambient. However, it is likely that the low porosity
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1114–1122
detected in the porcelains studied (0.3–1.0%) was not enough to influence the n values measured. The absence of correlation between fracture toughness and stress corrosion coefficient (Fig. 3) indicates that materials with higher resistance to fast crack propagation do not necessarily have higher resistance to slow crack growth. This finding may be explained by the fact that slow crack growth is strongly dependent on extraneous variables such as concentration of active environmental species, while fast fracture is environment-insensitive and usually associated with the kinetics of intrinsic bond rupture [27]. However, it should be noted that porcelain C presented both the lowest n and KIc values, indicating that in this material these two variables may be related to each other. Observation of the scaling parameter I, which is also the radial crack size at 1 h after indentation (Table 3), indicates that crack sizes varied considerably among porcelains studied from 103 to 150 m. Such variation leads to the observed variation in fracture toughness values, since crack sizes are used to calculate this property (Eq. (4)). Therefore, materials with shorter cracks as A, B, and G, showed the highest KIc values, while porcelain C showed both the largest cracks and the lowest fracture toughness. Crack sizes seem to be governed by material microstructure, since porcelains with higher leucite content tended to present shorter cracks, and higher fracture toughness, as observed in Fig. 4. In fact, such relationship has already been demonstrated elsewhere [14]. The second hypothesis of the study was accepted since it was observed that storage media affected the n values obtained for most of the materials (except porcelains A and D). The decrease in the n values after storage in saliva may be explained by the fact that the velocity of crack propagation in vitreous materials made of SiO2 is strongly influenced by the relative humidity of the environment [28]. Also, it has been shown that the crack velocity increases as a function of the hydroxyl concentration, suggesting that these molecules break chemical bonds of the material at the crack tip, allowing for crack extension [29]. In accordance with that, Gupta and Jubb [13] also observed a decrease of about 30% in the n values of soda-lime glass tested in water in comparison to testing in air. An important implication of this fact is that the occurrence of slow crack growth in water will ultimately lead to a decrease in strength of these materials as observed by previous works [30,31]. In addition to the decrease of stress corrosion coefficients in saliva, it is important to note that the variation in n values was much higher in air than in saliva. While in air these values varied from 25 to 68, in saliva they were around 33 (except for porcelain C). This fact points out the importance of testing porcelains in saliva at 37 ◦ C in order to obtain stress corrosion coefficient values, since storage medium plays an important role in the final outcome. Residual stresses present in the porcelain surface affect the apparent fracture toughness values obtained by the Indentation Fracture method when the material is subject to fast cooling, as observed in thermal tempering procedures [32]. In the present study, high cooling rates were used during the firing cycle (Table 2) and no annealing was performed, indicating that the sintered specimens probably had residual stresses to some extent (mainly compression stresses near the surface). However, previous investigations from the authors’ laboratory
1121
(unpublished data) showed that careful removal of a layer of about 0.4 mm of the specimen surface can eliminate most of the residual stresses due to rapid cooling. In this regard, machining of the specimens in this study strictly followed the guidelines in ASTM C 1161 [16]. This procedure removed a layer of at least 0.6 mm from the specimen surface. In addition, the polishing procedure was carried out carefully in order to avoid the introduction of residual stresses. Therefore, it is not likely that the results shown here were affected by residual stresses induced due to fast cooling. To verify this assumption, an additional experiment was carried out using two polished specimens that had already been used to determine n values. The two materials chosen (A and C) were feldspathic porcelains with different sintering temperatures and leucite content. Specimens were annealed at their glass transition temperatures, Tg , for 30 min, following slow cooling (1 ◦ C/min) to minimize any possibility of introduction of residual stresses. The results showed that the KIc values obtained for porcelains A and C after the annealing treatment (1.07 ± 0.13 e 0.69 ± 0.06 MPa m1/2 , respectively) were similar to those measured in the polished surfaces that were not annealed (Table 3), indicating that compressive residual stresses did not affect the KIc results. These results indicate that the method used in the present study to produce porcelain specimens removed most of the residual stresses generated during the sintering cycle. It is important to consider that the results of the present study are limited since it is an in vitro experiment. Additional clinical aspects need to be considered with respect to slow crack growth, such as the fact that the defects originating the fracture of monolithic restorations are located at the internal surface of the restoration, where the cement is applied and hoop stresses are present [4,10,33]. Therefore, the ambient in which slow crack growth occurs is far more complex than the one used in the present study. On the other hand, the experimental set up used here (porcelain immersed in saliva at 37 ◦ C) is more similar to the clinical situation found in FPDs connectors. In this case, the connector is totally immersed in saliva and crack will grow on its gingival side, where the fracture origin is usually located [34,35]. Other factors that may influence crack growth of porcelains in the clinical situation are: (a) moisture from the luting cement; (b) fatigue stresses from masticatory forces [36]; (c) handling and design of the restoration; and (d) thickness of the restoration [37]. Moreover, crack healing or blunting of the crack tip may occur during prolonged non-stress periods [22]. Hence, a caution is suggested that direct extrapolation of the results of this research to clinical applications could be misleading.
5.
Conclusion
After determining the stress corrosion coefficient (n) of seven dental porcelains with varied leucite contents in different environments (air or saliva), it was possible to conclude that storage media influenced the n values obtained for most materials (specimens stored in saliva showed lower n values). Moreover, it was found that leucite content did not affect resistance to slow crack growth of porcelains tested regardless of the test environment.
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Acknowledgments The authors acknowledge the Brazilian agencies FAPESP and CNPq for the financial support of the present research.
references
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