d e n t a l m a t e r i a l s 3 2 ( 2 0 1 6 ) 870–878 Available online at www.sciencedirect.com ScienceDirect jo ur nal ho me pag e: www.int l .e lsev ierhea l th .com/ journa ls /dema Microstructure characterization and SCG of newly engineered dental ceramics Nathália de Carvalho Ramosa, Tiago Moreira Bastos Camposb, Igor Siqueira de La Paza, João Paulo Barros Machadoc, Marco Antonio Bottinoa, Paulo Francisco Cesard, Renata Marques de Meloa,∗ a Department of Dental Materials and Prosthodontics, São Paulo State University (UNESP), Institute of Science and Technology, 777 Eng. Francisco José Longo Avenue, 12245-000, São José dos Campos, SP, Brazil b Aeronautics Technological Institute (ITA), 50 Praça Marechal Eduardo Gomes, 12228-900, São José dos Campos, SP, Brazil c National Institute for Space Research, Associated Laboratory of Sensors and Materials, 1758 Astronautas Avenue, 12217-010, São José dos Campos, SP, Brazil d University of São Paulo (USP), School of Dentistry, Department of Biomaterials and Oral Biology, 2227 Prof. Lineu Prestes Avenue, 05508-000, São Paulo, SP, Brazil a r t i c l e i n f o Article history: Received 8 June 2015 Received in revised form 23 November 2015 Accepted 22 March 2016 Keywords: Ceramics Dental porcelain Glass ceramics Subcritical crack growth Stress corrosion a b s t r a c t Objectives. The aim of this study was to characterize the microstructure of four dental CAD- CAM ceramics and evaluate their susceptibility to stress corrosion. Methods. SEM and EDS were performed for microstructural characterization. For evaluation of the pattern of crystallization of the ceramics and the molecular composition, XRD and FTIR, respectively, were used. Elastic modulus, Poisson’s ratio, density and fracture tough- ness were also measured. The specimens were subjected to biaxial flexure under five stress rates (0.006, 0.06, 0.6, 6 and 60 MPa/s) to determine the subcritical crack growth parameters (n and D). Twenty-five specimens were further tested in mineral oil for determination of Weibull parameters. Two hundred forty ceramic discs (12 mm diameter and 1.2 mm thick) were made from four ceramics: feldspathic ceramic – FEL (Vita Mark II, Vita Zahnfabrik), ceramic-infiltrated polymer – PIC (Vita Enamic, Vita Zahnfabrik), lithium disilicate – LD (IPS e.max CAD, Ivoclar Vivadent) and zirconia-reinforced lithium silicate – LS (Vita Suprinity, Vita Zahnfabrik). Computer-aided design Flexural strength Young’s modulus Spectroscopy Results. PIC discs presented organic and inorganic phases (n = 29.1 ± 7.7) and Weibull modulus (m) of 8.96. The FEL discs showed n = 36.6 ± 6.8 and m = 8.02. The LD discs showed a structure with needle-like disilicate grains in a glassy matrix and had the lowest value of n (8.4 ± 0.8) and m = 6.19. The ZLS discs showed similar rod-like grains, n = 11.2 ± 1.4 and m = 9.98. Significance. The FEL and PIC discs showed the lowest susceptibility to slow crack growth (SCG), whereas the LD and ZLS discs presented the highest. PIC presented the lowest elastic ∗ Corresponding author at: Av. Engenheiro Francisco José Longo, 777, 12245-200, São Dimas, São José dos Campos, SP, Brazil. Tel.: +55 12 39479032. E-mail addresses: renata.marinho@ict.unesp.br, marquesdemelo@gmail.com (R.M.d. Melo). http://dx.doi.org/10.1016/j.dental.2016.03.018 0109-5641/© 2016 The Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. dx.doi.org/10.1016/j.dental.2016.03.018 http://www.sciencedirect.com/science/journal/01095641 www.intl.elsevierhealth.com/journals/dema http://crossmark.crossref.org/dialog/?doi=10.1016/j.dental.2016.03.018&domain=pdf mailto:renata.marinho@ict.unesp.br mailto:marquesdemelo@gmail.com dx.doi.org/10.1016/j.dental.2016.03.018 d e n t a l m a t e r i a l s 3 2 ( 2 0 1 6 ) 870–878 871 modulus and no crystals in its composition, while ZLS presented tetragonal zirconia. The overall strength and SCG of the new materials did not benefit from the additional phase or microconstituents present in them. © 2016 The Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. 1 N i a i a i t w w p o d l d h [ l p n h i T a d m l m V p m t t g i b a b m v c p r i a m t Table 1 – Cycle of crystallization of LD and ZLS ceramics. ZLS (Vita Suprinity) LD (IPS e.max CAD) Beginning chamber temperature (◦C) 400 403 Time at the initial temperature (min) 8:00 6:00 Temperature rate increase (◦C/min) 55 90 Crystallization temperature (◦C) 840 820 Holding time (min) 8:00 7:00 Ending temperature (◦C) 680 700 . Introduction ew indirect restorative materials have been recently released n the dental market based on different microstructural pproaches in comparison with those of previously available ndirect materials. One example of these innovative materi- ls is a polymer-infiltrated ceramic (PIC) CAD-CAM block that s claimed to have higher structural reliability as a result of he so-called crack-stop function mechanism, which occurs hen a crack that is propagating through the polymer net- ork halts due to the presence of the ceramic phase. Recent ublications showed that this type of material is composed f a polymer-infiltrated ceramic network containing urethane imethacrylate (UDMA) and triethylene glycol dimethacry- ate (TEGDMA) cross-linked polymers [1]. In comparison with ental porcelains, this new material has been proven to ave lower elastic modulus and higher damage tolerance 2]. Another recently released indirect dental material is a ithium-silicate-based glass-ceramic reinforced with zirconia articles (ZLS). According to the manufacturer, the zirco- ia particles added to this glass-ceramic are small and omogeneously distributed throughout the microstructure, mproving its strength and providing good surface finish. his material is provided only for CAD-CAM technology, nd, to the authors’ knowledge, no laboratory or clinical ata have yet been published regarding this product. The icrostructure of this new glass-ceramic material is simi- ar to that seen in lithium disilicate ceramics, which, for any years, were protected by a patent held by Ivoclar ivadent and became well-known for their excellent optical roperties [3]. These lithium disilicate ceramics are now com- ercially available as pressable ingots or CAD-CAM blocks, he latter having an intermediate stage of crystallization hat, after milling, still requires heat treatment for crystal rowth. Ceramic restorations are constantly subjected to humid- ty and occlusal loads, and their time in service is controlled y slow crack growth (SCG), which is the environmentally ssisted subcritical growth of cracks. SCG can be estimated y a power law relation in which a coefficient, n, expresses the aterial’s susceptibility to stable crack growth. The higher the alue of n, the lower the susceptibility to SCG. The method ommonly used for measuring the subcritical crack growth arameter is the constant-stress-rate test, in which the mate- ial strength is obtained as a function of various stress rates n a certain environment [6]. The development of newly engineered indirect materi- ls has led to a need for new studies to characterize their echanical and fatigue properties, in attempts to predict heir clinical behavior. Therefore, the primary objective of the present study was to characterize the microstructure and the slow crack growth (fatigue) parameters using constant-stress- rate testing of the following indirect restorative materials: polymer-infiltrated (PIC), zirconia-reinforced lithium silicate (ZLS), lithium disilicate (LD) and feldspathic (FEL) ceram- ics, all designed for CAD-CAM processing. Also, fracture toughness, elastic modulus and Weibull parameters were determined. 2. Materials and methods Sixty discs of each of the following materials were pre- pared: feldspathic ceramic (Vita Mark II, Vita Zahnfabrik, Bad Säckingen, Germany), polymer-infiltrated ceramic (Vita Enamic, Vita Zahnfabrik), lithium disilicate ceramic (IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein) and zirconia-reinforced lithium silicate ceramic (Vita Suprinity, Vita Zahnfabrik). 2.1. Specimen preparation A ring device was glued onto the top surfaces of the CAD- CAM blocks to round them until cylinders 12 mm in diameter were obtained. The cylinders were then cut into several discs approximately 12 mm in diameter and 1.35 mm thick, in a lathe (ISOMET 1000, Buehler, Lake Bluff, IL, USA). IPS e.max CAD and Vita Suprinity require a cycle of crystallization that was performed in their respective furnaces (Programat EP5000, Ivoclar Vivadent; and Vita Vacumat 6000MP, Vita Zahnfabrik) according to the temperature recommendations given by the manufacturers (Table 1). The specimens were then polished with SiC#400, 800 and 1200. According to ISO standard CD 6872, specimens attained final dimensions of 12 mm in diameter and 1.2 mm thick [4]. dx.doi.org/10.1016/j.dental.2016.03.018 l s 3 872 d e n t a l m a t e r i a 2.2. Elastic modulus and fracture toughness determination The determination of the elastic modulus of the material was performed by the pulse-echo method with pulse receiver equipment (MOD 5900 PR, Olympus, Center Valley, PA, USA) connected to an oscilloscope (TDS 1002, Tektronix, Beaverton, OR, USA). The density of each material was measured by the Archimedes method in a precision balance by measurement of the dry mass of the specimens and then measurement of the volume of water displaced when the specimens were put into the water. The formula for calculation of the elastic modulus was: E = � ( 3V2 t V2 l − 4V4 t V2 l − V2 t ) where � is density, and Vt and Vl are the longitudinal and transverse wave velocities, respectively. Fracture toughness determination was based on a descrip- tion of surface cracks in disks in flexure experiments conducted by Strobl et al. [5]. First, Knoop indentations were performed on three discs of each material, to create a con- trolled superficial defect. Samples were indented with 19.61 N loading for 12 s. The samples were then polished with SiC #800 for removal of the residual stress (∼3–5 �m). After being polished, the specimens were fractured under biaxial flexure at a speed of 0.5 mm/min in mineral oil. We then measured the defect depth and surface extension on the fracture surfaces, using a stereomicroscope (Discovery V20, Carl Zeiss, Jena, Germany) at a magnification of 100×, and determined fracture toughness (KIc) (MPa m1/2) according to the equation: KIc = �B3BY √ a� where �B3B is the strength with the ball in the three-ball method (an interactive Web-Mathematica tool can be found at http://www.isfk.at/en/960/), Y is a geometric factor as a func- tion of the crack shape and the relative crack depth obtained by calculations from Strobl et al. and a is the crack depth [5]. The measured properties (elastic modulus and fracture toughness) were analyzed statistically by one-way ANOVA and Tukey’s post-hoc test (p < 0.05). 2.3. Constant-stress-rate testing and Weibull analysis The constant-stress-rate testing for determination of SCG parameters, n and D, was performed at five rates in water: 0.006, 0.06, 0.6, 6 and 60 MPa/s. The numbers of specimens per rate were 10 for the extreme values and 5 for the three inter- mediate rates. The parameters n and D were determined by a linear regression analysis, according to the following equation [6]: log �f = 1 n + 1 log �̇ + log D where �f is the mean strength (MPa) and �̇ is the stress rate (MPa/s). The inert strength was obtained in an inert environment (mineral oil) under the highest stress rate (60 MPa/s). These 2 ( 2 0 1 6 ) 870–878 results were subjected to Weibull statistical analysis by the maximum likelihood method. The piston-on-three-ball biaxial flexure experiment for performing constant-stress-rate testing and inert strength measurements was carried out in a universal testing machine with 500 kgf cell loading. The strength calculations were based on the following equation: � = −0.2387P (X − Y) b2 where P is the load in kgf, X and Y are parameters related to the elastic properties of the material (Poisson’s ratio and elastic modulus) and b is the specimen thickness at the fracture origin in mm. 2.4. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Fourier transformed infra-red (FTIR) analyses SEM analyses were performed on polished specimens fol- lowing several surface-conditionings with hydrofluoric acid until the various phases could be distinguished. The acid concentrations and conditioning times varied according to the material. In addition, the reinforced polymer-ceramic was fired to 600 ◦C for 5 h and kept at this temperature for 5 h to ensure complete removal of the polymer. The specimens were examined in a scanning electron microscope with a high- resolution emission field (Magellan 400L, FEI Company, Brno, Czech Republic) so that the shapes and sizes of the grains could be observed. The chemical analysis of the microcon- stituents was performed by EDS (Bruker Nano GmbH, Berlin, Germany). X-ray diffraction (Model X’pert Powder, PANalytical, Almelo, Netherlands) was also performed with the database software X’Pert High Score (PANalytical) for two specimens from each group, to visualize the pattern of crystallization. For the polymer-infiltrated ceramic, vibrational spec- troscopy was also performed by FTIR to determine the molecular composition of the organic and inorganic compo- nents of this material. 3. Results 3.1. Microstructural characterization The results obtained from the X-ray diffraction analysis are displayed in Fig. 1. IPS e.max CAD (LD) showed crystalliza- tion peaks corresponding to lithium disilicate (ds – Li2O5Si2). Vita Suprinity (ZLS) showed peaks corresponding to lithium monosilicate (ms – Li2O3Si, and ls – Li8O6Si), aluminum silicate (als – Al0.5O2.25Si0.75) and tetragonal zirconia (zr – O2Zr0.952). Vita Mark II (FEL) showed two crystallization pat- terns, with peaks identified as “a” and “b”, with “a” being a sodium potassium aluminum silicate peak (Al8K2Na6O34Si9) and “b” being a potassium sodium aluminum silicate peak (AlK0.29Na0.71O8Si3). The polymer-infiltrated ceramic was found to be amorphous (both phases), with no evidence of crystallization. Table 2 shows the chemical constituents of each studied material. As expected, lithium could not be identified in any dx.doi.org/10.1016/j.dental.2016.03.018 http://www.isfk.at/en/960/ d e n t a l m a t e r i a l s 3 2 ( 2 0 1 6 ) 870–878 873 0 20 40 60 80 10 0 2θ R el at iv e in te ns ity (a .u ) LD ZLS FEL PIC Fig. 1 – XRD spectra: (LD) lithium disilicate, (ZLS) lithium silicate reinforced by zirconia, (FEL) feldspathic ceramic and ( o p a o t a r b i s a s l b 40080012001600200024002800320036004000 R el at iv e tra ns m itt an ce (a .u ) Wavenumber (cm-1) A B CDE F G H I of the polymer-reinforced ceramic after being etched with hydrofluoric acid (Fig. 3C) and after the polymeric network was removed when the material was heated (Fig. 3D). PIC) polymer-infiltrated ceramic. f the analyzed materials. Oxygen, silicon, aluminum, and otassium were detected in all materials, whereas sodium ppeared only in PIC and FEL. Noteworthy are the percentages f carbon (∼15%) and zirconia (∼21%) in PIC and ZLS, respec- ively, and also the fact that carbon was not totally eliminated fter PIC was fired. Fig. 2 shows the vibration spectra (FTIR) of the polymer- einforced ceramic. The letters at the top indicate the different ands, which are further explained in Table 3. These bands ndicate the presence of tetra-coordinated aluminum and ilicon in this composite. The XRD analysis indicated that lthough the composition of the amorphous ceramic phase is imilar to that of feldspathic ceramics, only the latter contain eucite as a crystalline phase. Still in Fig. 2, one can see that ands F, G, H, and I refer to the inorganic component, with Table 2 – EDS analysis of bulk ceramic composition (in percentage). Ceramics FEL PIC PIC fired ZLS LD Oxygen 46.3 41.6 46.4 51.2 54.3 Silicon 20.1 18.5 26.3 29.6 39.3 Carbon – 21.2 4.4 – – Zirconia – – – 15.5 – Aluminum 13.3 9.3 11.4 1.3 2.3 Sodium 6.6 4.7 6.0 – - Potassium 6.4 4.5 5.2 2.3 4.0 Fig. 2 – FTIR spectrum of the polymer-infiltrated ceramic. F and G bands corresponding to tetrahedral aluminum and tetrahedral silica. This material did not show bands related to octahedral aluminum. The organic fraction of this polymer- reinforced ceramic is represented by bands A, B, C, D and E. “A” is related to the stretching of the N–H bonds, and “B” refers to C–H bonds, symmetric and asymmetric stretches of carboxylic groups. “C” and “D” are carbonyl bands corre- sponding to the polymeric network. Bands “C” and “D” are most likely related to the vibrations of urethane dimethacry- late (UDMA) and triethylene glycol dimethacrylate (TEGDMA) [7]. Specimens of the polymer-reinforced ceramic were observed under Scanning Electron Microscope with Field Emission Gun (SEM-FEG, JEOL 7500-F, Peabody, MA, USA) for microstructural characterization (Fig. 3). These micrographs showed the presence of two phases (Fig. 3A). Mapping with EDS was carried out to characterize regions corresponding to ceramic and polymer (Fig. 3B). Fig. 3 also shows micrographs Table 3 – Assignment of the FTIR bands of the polymer-infiltrated ceramic. Wavenumber (cm−1) Assignment Band 3432 Stretching (–NH) nto (–OH) A 2912 Stretching (CH) B 1730 Stretching (C O) C 1617 Stretching (C O) D 1530 Amide E 1070 Asymmetric stretching (Si–O–Si and Si–O–Al) F 1058 Asymmetric stretching (Si–O–Si and Si–O–Al) G 792 Tetracoordinated Al–O stretching H 469 Bending (Si–O) I dx.doi.org/10.1016/j.dental.2016.03.018 874 d e n t a l m a t e r i a l s 3 2 ( 2 0 1 6 ) 870–878 Fig. 3 – Micrographs of PIC and FEL. (A) PIC without acid etching. (B) EDS mapping, where the red line is the carbon (polymer) content. (C) PIC etched with 5% HF for 20 s, where arrows show the polymer net. (D) PIC after firing, with arrows indicating the empty spaces after the polymer removal. (E) FEL etched with 5% HF for 20 s, with arrows showing the holes caused by the dissolution of the glassy matrix.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.) The microstructure of the feldspathic ceramic after acid- etching is shown in Fig. 3E. It can be seen that this is a single-phase and very porous material. Fig. 4 depicts the microstructure of both the lithium disilicate (LD) and zirconia- reinforced lithium silicate ceramics (ZLS). Fig. 4A shows that the lithium disilicate crystals in LD were acicular; after hydrofluoric acid-etching, the same crystals appeared larger and more elongated (Fig. 4B). The lithium silicate crystals in ZLS could not be observed without etching. After etching, it was noted that these crystals were slightly larger and more rounded compared with the disilicate crystals, with a rod- like appearance (Fig. 4C). Some heavily charged zones (Fig. 4D) were analyzed by EDS, and showed composition similar to that of the non-charged zones, leading to rejection of the hypoth- esis that zirconia particles were concentrated in these areas. Conversely, zirconium oxide and cerium (a stabilizing agent of tetragonal zirconia) were identified throughout the entire surface of ZLS. dx.doi.org/10.1016/j.dental.2016.03.018 d e n t a l m a t e r i a l s 3 2 ( 2 0 1 6 ) 870–878 875 Fig. 4 – LD and ZLS microstructures. (A) LD without etching and (B) etched with 5% HF for 20 s, showing needle-like crystals (C and D), and ZLS etched with 5% HF for 20 s, showing rod-like crystals and charged areas (arrows) with the same composition as that of the non-charged areas. In comparison of the etched surfaces of B and D (same magnifications), one can see that the LD crystals are larger than the ZLS crystals, which are finer structures. 3 T m a r p t c .2. Measured properties, SCG and Weibull parameters able 4 displays the means and standard deviations for the easured properties of all studied materials, including SCG nd Weibull parameters. The elastic modulus and Poisson’s atio of PIC were the highest and the lowest, respectively, com- ared with those of the other materials tested. With respect o KIc (p = 0.054) and Weibull modulus, no statistically signifi- ant differences were detected among materials (Chi-square Table 4 – Poisson’s ratio, density, elastic modulus (E), fracture to strength, Weibull modulus (m), confidence interval of Weibull m materials. Means that do not share a letter in the same line are PIC Poisson’s ratio 0.28 ± 0.009a 0. Density 1.87 1. E (GPa) 34.7 ± 2.2a 48 KIc (MPa m1/2) 0.86 ± 0.27a 0. n 29.1 ± 7.7 36 Inert strength (MPa) 159.0 ± 20.6a 11 m 9.0 8. CI (lower-upper) 6.90–11.63 6. �0 (MPa)/minimum–maximum values 167.5/106.6–206.9 12 test for equal shape parameters, p = 0.074). As to inert strength measurements, i.e., measurements made in the absence of slow crack growth, FEL showed the lowest strength, whereas LD showed the highest value. FEL and PIC were the least sus- ceptible to SCG, since their stress corrosion coefficients (n) were significantly higher than those obtained for LD and ZLS. Fig. 5 consists of graphic representations of the strength val- ues obtained for the different materials as functions of the stress rate (inert strength data are included in the graph). ughness (KIc), coefficient of slow crack growth (n), inert odulus (CI) and characteristic strength (�0) of the significantly different (p < 0.05). FEL LD ZLS 23 ± 0.02b 0.22 ± 0.03b 0.23 ± 0.03b 67 1.65 1.60 .7 ± 1.9b 63.9 ± 4.8c 65.6 ± 4.1c 84 ± 0.06a 1.23 ± 0.26a 1.25 ± 0.79a .6 ± 6.8 8.4 ± 0.8 11.2 ± 1.4 6.8 ± 14.0b 346.1 ± 67.3c 207.3 ± 23.9d 0 6.2 10.0 28–10.25 4.94–7.75 6.92–14.41 3.7/75.8–158.9 371.4/211.7–527.8 217.5/151.84–238.61 dx.doi.org/10.1016/j.dental.2016.03.018 876 d e n t a l m a t e r i a l s 3 2 ( 2 0 1 6 ) 870–878 Fig. 5 – (A) PIC, (B) FEL, (C) LD and (D) ZLS. Graphic representations of the means of strength at each stress rate and the inert strength. Steeper regression lines demonstrate higher susceptibility to SCG. Fig. 6 – SEM images of fractured inert strength specimens. (A) FEL, (B) PIC, (C) LD, (D) ZLS. The arrows indicate the structural flaw on the tensile surface where failure initiated. H stands for “hackles”, which were the only noticeable fracture marking surrounding the zone of failure origin. In A, a semi-elliptical flaw led to fracture. dx.doi.org/10.1016/j.dental.2016.03.018 3 2 3 T f o v t 4 T c p m s w t d t i a i o b t ( t m o T w c s F c c m m n m a c t t r e a l i t s r c b h f a b d d e n t a l m a t e r i a l s .3. Fractographic analysis he fracture origins were mostly structural pores on the sur- ace under tensile stress (Fig. 6). The fractographic analysis f specimens of PIC showed coarse fracture surfaces. Con- ersely, ZLS showed glassy surfaces and secondary chipping, he reasons for which are unclear. . Discussion his paper evaluated the microstructure, reliability and sus- eptibility to fatigue by stress corrosion of several ceramics rocessed by CAD-CAM technology. VM II (FEL) showed icrostructure based on an aluminum-, potassium- and odium-based silicate with grains of about 4 �m, in agreement ith findings reported in the literature [8]. The microstruc- ure of PIC is also in agreement with previous studies that escribed this material as a porous feldspathic ceramic con- aining polymers infiltrated by capillary action [1,2]. However, n the present investigation both X-ray diffraction and EDS nalyses did not detect leucite, although Della Bona et al. [9] nferred that leucite was present in the material, based solely n EDS analysis. PIC showed elastic modulus value similar to that reported y the manufacturer (around 30 GPa) [10]; however, the frac- ure toughness values measured in the present investigation 0.86 MPa m1/2) were lower than that reported by the manufac- urer (1.5 MPa m1/2). The method used by the manufacturer to easure KIc was not specified. The fracture toughness value btained for PIC was similar to that of the feldspathic ceramic. herefore, the idea that the presence of a polymer network ould create toughening mechanisms in the microstructure ould not be confirmed. In addition, PIC showed increased usceptibility to SCG (i.e., lower n value) compared with the EL. This raises the question of whether the polymer is sus- eptible to water permeation and degradation. However, the alculated density of PIC was the highest among the four aterials; therefore, the issue of water penetration in this aterial requires further investigation. The glass ceramic containing lithium disilicate (LD) showed eedle-like particles with different orientations. Its elastic odulus (∼64 GPa) and particle size (from 0.5 to 4 �m) also gree with those previously reported in the literature [11]. One ould expect that LD would show lower fracture toughness han the lithium silicate reinforced by zirconia (ZLS), because he latter would have an additional toughening mechanism elated to the presence of zirconia in the microstructure. How- ver, these two materials showed similar fracture toughnesses nd similar Weibull moduli. In fact, the addition of ZrO2 to ithium metasilicate and disilicate did not lead to an increase n strength [12]. Similarly, ZLS did not show higher resistance o crack propagation when compared with LD. The results of this study showed that susceptibility to low crack growth was significantly affected by the mate- ial’s microstructure. No previous studies were found that alculated the SCG parameters of LD for the CAD system, ut two reports have estimated n for this material using the ot-pressing technique (Empress 2). These studies showed dif- erent values of n for this material, ranging from 17.2 [13] to ( 2 0 1 6 ) 870–878 877 28.07 [14]. Thus, one has to take into consideration the varia- tions in the methods used for determining SGC parameters. In the present study, LD and ZLS showed similar stress corrosion coefficients, which were lower than those obtained for the other materials. These results indicate that the microstructural differences observed between LD and ZLS did not affect their slow crack growth behavior. It is interesting to note that when fast fracture was considered (sigma zero), both materials showed better mechanical behavior than did PIC and FEL. This indicates that the mechanisms affecting slow crack growth are not the same as those that govern fast fracture, because the mode of crack propagation during sub- critical growth of the crack is different from that seen during fast fracture [15]. SCG occurs before fast fracture which is why we must be aware of how each material behaves under differ- ent stress rates. Yet, from a clinical standpoint, one can say that, for low load zones, PIC and FEL are superior, since their slow crack growth is less affected in the long term; conversely, if the load level is high, LD and ZLS are better candidates for consideration. A previous SCG study on dental ceramics found that lithium disilicate was the material with the lowest n value [13]. In this case, the alignment of crystals during injection was unable to create a favorable crack path during the biaxial flex- ure test. From our observations, the crystals were randomly distributed in the glass matrix, and an alignment pattern was not detected. Therefore, we believe that the glass phase qual- ity and the effects of residual thermal stresses between the glass and crystal phases may be responsible for the increased susceptibility to SCG observed for LD. From the fractographic analysis, it was possible to note that the fracture origins were mostly related to structural flaws on the materials’ surfaces, and that the fractured surface of the ZLS was indeed very glassy, while that of LD showed a rougher aspect, indicat- ing different crack propagation patterns in these materials. Therefore, more detailed analyses are needed to assess crack propagation in particle boundary regions of these materials. 5. Conclusions The conclusions drawn from our findings were as follows: . The four studied ceramics have very different micro- structures. The polymer-infiltrated ceramic has two phases, one organic and one inorganic, forming a network. The lithium silicate has elongated grains that are more rounded than the needle-like disilicate grains. . PIC does not have a crystalline phase, while LS showed tetragonal zirconia in its composition. c. The feldspathic and the infiltrated ceramic, as well as the lithium disilicate and lithium silicate, showed similar frac- ture toughness values. Thus, the polymer present in PIC and the zirconia present in the ZLS did not improve their fracture toughness. . The structural reliability (Weibull modulus) of all materials was the same under the condition of fast fracture. Surface flaws were identified as failure origins in all materials. e. The behavior of the materials in terms of slow crack growth differed significantly. The feldspathic ceramic showed dx.doi.org/10.1016/j.dental.2016.03.018 l s 3 r 878 d e n t a l m a t e r i a the lowest susceptibility to SCG, while lithium silicate reinforced by zirconia and lithium disilicate showed the highest. Acknowledgments The authors acknowledge the São Paulo Research Foundation for financial support (FAPESP #2013/15541-7). e f e r e n c e s [1] Coldea A, Swain MV, Thiel N. Mechanical properties of polymer-infiltrated-ceramic-network materials. Dent Mater 2013;29(4):419–26. [2] Coldea A, Swain MV, Thiel N. In-vitro strength degradation of dental ceramics and novel PICN material by sharp indentation. J Mech Behav Biomed 2013;26:34–42. [3] Guess PC, Zavanelli RA, Silva NR, Bonfante EA, Coelho PG, Thompson VP. 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http://refhub.elsevier.com/S0109-5641(16)30012-4/sbref0160 http://refhub.elsevier.com/S0109-5641(16)30012-4/sbref0160 http://refhub.elsevier.com/S0109-5641(16)30012-4/sbref0160 http://refhub.elsevier.com/S0109-5641(16)30012-4/sbref0160 http://refhub.elsevier.com/S0109-5641(16)30012-4/sbref0160 http://refhub.elsevier.com/S0109-5641(16)30012-4/sbref0160 http://refhub.elsevier.com/S0109-5641(16)30012-4/sbref0160 http://refhub.elsevier.com/S0109-5641(16)30012-4/sbref0160 http://refhub.elsevier.com/S0109-5641(16)30012-4/sbref0160 Microstructure characterization and SCG of newly engineered dental ceramics 1 Introduction 2 Materials and methods 2.1 Specimen preparation 2.2 Elastic modulus and fracture toughness determination 2.3 Constant-stress-rate testing and Weibull analysis 2.4 Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Fourier transfor... 3 Results 3.1 Microstructural characterization 3.2 Measured properties, SCG and Weibull parameters 3.3 Fractographic analysis 4 Discussion 5 Conclusions Acknowledgments References