journal of the mechanical behavior of biomedical materials 40 (2014) 390 –396
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Research Paper
The effects of repeated heat-pressing on the mechanical properties and microstructure of IPS e.max Press Xuehua Tanga,b,n, Chengzhong Tanga, Han Sua, Huinan Luoa, Takashi Nakamurab, Hirofumi Yatanib a
Department of Stomatology, Jinling Hospital, Nanjing University School of Medicine, Nanjing 210002, China Department of Fixed Prosthodontics, Osaka University Graduate School of Dentistry, Osaka 565-0871, Japan
b
ar t ic l e in f o
abs tra ct
Article history:
Objectives: We evaluated the effects of repeated heat-pressing on the mechanical proper-
Received 10 July 2014
ties and microstructure of IPS e.max Press.
Received in revised form
Methods: A total of 20 specimens were fabricated including 10 heat-pressed once and
12 September 2014
another 10 heat-pressed twice. The density, porosity, and surface roughness (Ra) were
Accepted 15 September 2014
evaluated. Three-point flexural strength following the ISO 6872 and Vickers hardness were
Available online 28 September 2014
measured, and fracture toughness (KIC) was calculated. Specimens were characterized
Keywords:
using X-ray diffraction (XRD) and scanning electron microscopy (SEM).
IPS e.max Press
Results: Compared to a single heat-press treatment, the density decreased and porosity
Repeated heat-pressing
increased after two heat-pressing events. A significant difference in density was observed.
Mechanical properties
The flexural strength, Vickers hardness and fracture toughness significantly decreased
Microstructure
after two heat-pressing events. The XRD patterns show that the intensity of the crystalline phase better corresponds to lithium disilicate (Li2Si2O5) after two heat-presses than only one. The SEM images detailed the interlocking microstructure of rod-shaped Li2Si2O5 crystals after one heat-press. These became oriented after two heat-presses, and the crystal size became larger. Conclusion: This study showed that repeated heat-pressing was detrimental to the density, porosity, strength, hardness, and toughness of IPS e.max Press. & 2014 Elsevier Ltd. All rights reserved.
1.
Introduction
All-ceramic restorations are attracting extensive attention in prosthetic dentistry because of their translucency and beneficial biocompatibility. They have increasingly become an alternative to conventional metal–ceramic restorations.
n
Ceramics used in all-ceramic restorations are primarily glass–ceramics, densely sintered alumina, and zirconia-based ceramics. Glass–ceramics exhibit some compositional and microstructural differences and combine properties that are typical for both ceramics and glasses. For these ceramics, the original glass composition as well as the presence, volume fraction, crystal
Corresponding author at: 305 East Zhongshan Road, Nanjing 210002, China. Tel./fax: þ86 25 84816508. E-mail address: [email protected] (X. Tang).
http://dx.doi.org/10.1016/j.jmbbm.2014.09.016 1751-6161/& 2014 Elsevier Ltd. All rights reserved.
journal of the mechanical behavior of biomedical materials 40 (2014) 390 –396
size, distribution and morphology of the crystalline phases may account for the variations in the ceramics' mechanical properties (Fernandes et al., 2010; Kim et al., 2012; Tang et al., 2012a; Yuan et al., 2013; Zhang et al., 2013; Akar et al., 2014). Currently, leucite-reinforced glass–ceramics and lithium disilicate glass–ceramics are widely used in heat-pressing and subsequently studied further. The pressable leucite-reinforced ceramic IPS Empress has been in the market for more than 15 years and is used primarily to fabricate laminate veneers, inlay, onlay, crown, etc. This material has had a long clinical track record and offers good esthetic outcomes and clinical survival rates (Heintze et al., 2011; Gehrt et al., 2013; Solá-Ruiz et al., 2013). The pressable lithium disilicate ceramic IPS Empress 2 was introduced in 1998 and demonstrated a higher mechanical strength than that of IPS Empress. It was suitable for threeunit fixed dental prostheses (FDPs) in the anterior region (Heintze et al., 2011; Gehrt et al., 2013; Solá-Ruiz et al., 2013). However, it had to be veneered because of its opacity. The survival rate was 96.8% after 5 years and 95.5% up to 10 years in two retrospective studies of IPS Empress 2 crowns. Another 5-year prospective clinical study showed that the survival rate was 100% for crowns and 70% for FPDs (Marquardt and Strub, 2006; Valenti and Valenti, 2009; Steeger, 2010). To combine durability with excellent esthetics, a new pressable lithium-disilicate glass ceramic named IPS e.max Press with enhanced mechanical properties and improved translucency was launched in 2007 (Heintze et al., 2011). This product is not only a core material, but can also be directly fabricated for various restorations. Moreover, the range of indications includes anterior and posterior teeth. The investigation of Gehrt et al. showed a cumulative survival rate of 97.4% after 5 years and 94.8% after 8 years of clinical service for anterior and posterior crowns made with IPS e.max Press. Another prospective study indicated an 8 years survival rate of 93% for three-unit FDPs made with this ceramic (Wolfart et al., 2009; Gehrt et al., 2013; Pieger et al., 2014). IPS e.max Press ceramics are pressed into a mold by an alumina plunger under pressure using a pneumatic press furnace. The button and sprue portions are usually discarded. However, they are considered useful for re-pressing in some dental laboratories. Hence, it is very important to evaluate the mechanical properties and microstructure of re-pressed glass– ceramic material to determine the feasibility of repeated heatpressing treatment. We hypothesize that after repeated heatpressing, two heat-pressing samples will maintain the same mechanical properties and microstructure as those of a single heat-pressing event.
2.
Materials and methods
2.1.
Specimen preparation
The specimens were fabricated by the lost-wax technique. The dental wax was melted and poured into a rectangular silicon mold with internal dimensions of 30 mm 6 mm 4 mm. After solidification, these wax patterns were sprued and attached to a muffle base with surrounding paper cylinders. This was then invested with a special investment material (Ceravety Press &
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Cast, Shofu, Kyoto, Japan). The wax burnout was at 850 1C in a conventional pre-heated furnace. The ceramic ingots of IPS e.max Press were then plastified and pressed under vacuum into the mold of the investment in a press furnace (EP600 Ivoclar Vivadent). The heat-pressing conditions were as follows: stand-by temperature 700 1C; temperature increase 60 1C/min; press temperature 920 1C; and holding time 25 min. After pressing, the investment molds were removed from the furnace and allowed to air cool. The specimens were then carefully devested using an air abrasion unit with 50 μm glass beads at a pressure of 3 bar. The button and sprue portions were cut; 12 specimens were selected at random. For the remaining specimens, the button and sprue portions were adjusted by grinding to allow proper insertion into the refractory molds for repeated heatpressing. With the same heat-pressing conditions an additional 12 specimens were fabricated. Finally all specimens were ground to final dimensions of 4.070.2 mm in width, 1.270.2 mm in thickness and at least 20 mm in length, using diamond discs following the guidelines of ISO 6872 (2008) (International Organization for Standardization (2008), threepoint flexural strength). Then, the top surface (4 mm 20 mm) was mirror-polished using a polishing machine (PFG500DX; Okamoto Machine Tool Works, Yokohama, Japan) with diamond suspensions. Finally all specimens were checked by optical microscopy (LV-100POL; Nikon, Tokyo, Japan), and 10 specimens without any processing defects were selected for the following test. These included samples that had been heatpressed once and twice.
2.2. Measurement and calculation of the mechanical properties 2.2.1.
Density and open porosity
The density and open porosity of all the specimens were determined by Archimedes' method in distilled water at room temperature. First, each sample was weighed while it was completely dry and the weight was recorded as W1. It was then placed in a beaker with distilled water and a vacuum was pulled for several minutes to ensure that the open pores would be filled with distilled water. Next, the beaker was placed on a stand over a balance. A wire was hung so the end of it was below the distilled water's surface but not touching the sides of the beaker in any way. The sample was lifted onto the wire with tweezers and its suspended weight was recorded as W2. Finally the sample was removed from the distilled water, quickly blotted, and weighed to obtain the wet weight, W3. The bulk density of the sample, ρ, was evaluated using ρ ¼ ρw W1 =ðW3 W2 Þ where ρw is the density of water at the testing temperature ( ρw ¼ 0.99799 g/cm3 at 21 1C).The open porosity, P, was evaluated as P ¼ ðW3 W1 Þ=ðW3 W2 Þ 100%
2.2.2.
Surface roughness
The surface roughness of all the specimens, i.e., the average roughness (Ra), was evaluated by laser profilometry using a
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confocal laser scanner profilometer (LEXT OLS4000; Olympus, Tokyo, Japan). The diameter of the laser beam was 0.2 μm. Four separate areas on each specimen were randomly selected and evaluated. The measured area was 256 μm 256 μm, and the distance between the separate scans was over 3 μm. The resolution used in the X and Y directions was 0.02 μm. The Z resolution was 0.01 μm.
2.2.3.
2.3.2.
Flexural strength
Flexural strength was measured using the three-point bending test. Ten specimens of the first and repeated heat-pressings were labeled 1 to 10 and analyzed with a universal testing machine (AUTOGRAPH AG-5000B; Shimadzu, Kyoto, Japan) resting on a self-aligning fixture with a span of 14 mm. The tests were conducted at a crosshead speed of 0.5 mm/min at room temperature (2071 1C) with a relative humidity of 7075% following the ISO 6872 guidelines (2008).
2.2.4.
Hardness
All specimens almost broke into two almost equal pieces after a three-point bending test (n¼ 10 for the first or repeated heat-pressing), and one fractured segment of each specimen was chosen randomly for Vickers micro-hardness testing. Hardness was measured using a Vickers micro-hardness tester (HM-102; Mitutoyo, Kawasaki, Japan) with a load of 19.6 N and a dwell time of 20 s. The test was performed on the polished surface, and cracks generated along the indentation diagonal were measured. This load was chosen to generate long radial cracks with (c/a)42.5, where c is the crack length and a is half of the indentation diagonal.
2.2.5.
a MultiFlex X-ray diffractometer (Rigaku, Tokyo, Japan) using flat plate geometry. The data were collected from 10 to 501 2θ using a step size of 0.021 2θ and a 0.6-s count time. CuKα radiation was used (40 kV, 40 mA). The identification of the crystalline phases was achieved by comparing the diffraction patterns with the JCPDS standard reference.
Fracture toughness
The fracture toughness (KIC) was calculated using the following equation: 0:5 KIC ¼ 0:016 E=H P=c1:5 : Here, P is the indentation load, c is the combined length of the radial crack, and H is the material hardness obtained by the Vickers hardness test. The term E is the elastic modulus obtained by the flexural strength test (Tang et al., 2012a).
2.3.
Observation of microstructure
2.3.1.
X-ray diffraction (XRD)
Fractured segments were chosen randomly from five specimens per test group and ground into a powder for X-ray diffraction analysis. The samples were placed in the holder of
Scanning electron microscopy (SEM)
Several fractured segments were selected at random from the test groups and were self-glazed to remove the effects of the polish traces for SEM imaging ( JSM-6390LA; JEOL, Tokyo, Japan). After self-glazing, the samples were etched with hydrofluoric acid (HF) and coated with gold to increase conductivity. The veneering ceramics were etched with 1% HF for 7 min.
2.4.
Statistical analysis
The mechanical properties of specimens that had undergone heat-pressing once were compared to those that had undergone twice. Values of each group were assessed using a t-test, and the data were considered significant at 5% (SPSS 17.0; SPSS, Inc., Chicago, IL, USA).
3.
Results
3.1.
Mechanical properties
For the IPS e.max Press, the mechanical properties after repeated heat-pressing are presented in Table 1. Density decreased and porosity increased after two heat-pressings, and significant differences were observed in the density between one and two heat-pressings (P¼ 0.018). The surface roughness of the specimens was 0.143 and 0.144 μm after repeated heat-pressing. No significant differences were found. The flexural strength was 354.46 MPa after one heatpressing and 247.37 MPa after two heat-pressings. The strength after one heat-pressing was higher than that after two heat-pressings (P ¼0.000). The Vickers hardness after repeated heat-pressings was 5.07 and 4.86 GPa. The hardness after one heat-pressing was significantly higher (P¼ 0.000). With respect to fracture toughness, the specimen after one heat-pressing (2.42 MPa m1/2) was tougher than that after two heat-pressings (1.97 MPa m1/2); significant differences were found in fracture toughness between one and two heatpressings (P¼ 0.000).
Table 1 – Density, porosity, surface toughness (Ra), flexural strength, Vickers hardness and fracture toughness after repeated heat-pressing. Heat-pressing times
Density (g cm 3)
Porosity (%)
Surface toughness (mm)
Flexural strength (MPa)
1 2
2.45(0.03)n 2.39(0.04)n
1.55(1.78) 2.52(1.86)
0.143(0.004) 0.144(0.003)
354.46(30.27) 247.37(42.67)
( ): standard deviation. n nn , Significant differences (n: Po0.05;
nn
nn nn
: Po0.001) between 1 heat-pressing and 2 heat-pressing.
Vickers hardness (GPa)
5.07(0.06) 4.86(0.08)
nn nn
Fracture toughness (MPa m1/2) 2.42(0.08) 1.97(0.13)
nn nn
journal of the mechanical behavior of biomedical materials 40 (2014) 390 –396
3.2.
Microstructural evaluation
3.2.1.
XRD analysis
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The XRD patterns showed that the lithium disilicate (Li2Si2O5) phase, lithium metasilicate (Li2SiO3) phase and lithium orthophosphate (Li3PO4) phase are present in both the one heatpressing and two heat-pressing samples. The main crystalline phase was lithium disilicate (Fig. 1). The major peaks of the lithium disilicate phase were observed at the 2θ values of 23.781, 24.341 and 24.841. This is as predicted by the XRD standard files for lithium disilicate (JCPDS# 40-0376). The peaks were at 2θ values of 23.781, 24.341 and 24.841 after two heat-pressings. These were higher than those seen for the one heat-pressing sample. The major peaks of the lithium metasilicate phase were found at 2θ values of 18.881, 26.621 and 33.01 (JCPDS# 29-0829). The peaks for the heat-pressing sample were lower than those after one heat-pressing. The lithium orthophosphate peaks were at 2θ values of 22.261, 23.041 and 36.281 (JCPDS# 15-0760).
3.2.2.
SEM analysis
Many needle-like crystals are present in the glass matrix for all specimens (Fig. 2). One heat-pressing showed densely packed and multi-directionally interlocking microstructure of numerous needle-like crystals protruding from the glass matrix. However, after two heat-pressing events, the crystals appeared oriented, wider and larger with a sparser distribution. The sharp needle shaped ends of crystals disappeared. Moreover, increased porosity and cracks were found in the crystals after two heat-pressings.
4.
Discussion
The purpose of this study is to evaluate the effects of repeated heat-pressing on the mechanical properties and microstructure of IPS e.max Press. The results do not confirm the hypothesis that after repeated heat-pressings, the two heat-pressings maintain the same mechanical properties and microstructures as those of one heat-pressing. In fact, versus one heat-pressing, the density decreased and the porosity increased. The strength, hardness, and toughness significantly decreased after two heat-pressings. Moreover, their microstructures were changed to different degrees in both the XRD and SEM analysis. Thus, the hypothesis of this study is rejected. The XRD analysis revealed that the major peaks of the lithium disilicate phase were higher after two heat-pressings than after one heat-pressing. The major peaks of the lithium metasilicate phase were lower after two heat-pressings than after one heat-pressing. This may indicate there is some chemical reaction of the phase transition between Li2SiO3 and Li2Si2O5 during the heat treatment (Li2SiO3þSiO2-Li2Si2O5) (Zheng et al., 2008; Tulyaganov et al., 2009; Cheng et al., 2010; Yuan et al., 2013). The results of the XRD analysis also confirmed that a small amount of Li3PO4 presented in IPS e.max Press. The Li3PO4 crystals are sites for the nucleation of stable lithium disilicate (Zheng et al., 2008; Chung et al., 2009; Tulyaganov et al., 2009).
Fig. 1 – XRD pattern of one heat-pressing and two heatpressing for IPS e.max Press specimens.
Some studies have shown that repeated heat-pressing significantly influenced the microstructure of lithium disilicatereinforced glass–ceramic materials and produced the blunt rodlike shape, larger grains, and orientation of the crystals (Albakry et al., 2004a, 2004b; Zheng et al., 2008; Chung et al., 2009; Tulyaganov et al., 2009; Cheng et al., 2010; Yuan et al., 2013). These results are consistent with this study. The orientation of the lithium disilicate crystals was probably a result of plastic deformation of the glass matrix phase and occurred during sprue extrusion (Albakry et al., 2004a, 2004b; Chung et al., 2009). The blunt rod-like shape and the larger grains of the lithium disilicate crystals after the repeated heat-pressing may be considered as Ostwald ripening and is common across all precipitated materials. This occurs when the microstructure coarsens and liberates free surface energy due to the solubility of small particles (Albakry et al., 2004a, 2004b). Consequently, larger grains are expected to grow at the expense of small particles. This may be the result of the phase transition between lithium metasilicate and lithium disilicate (Zheng et al., 2008; Tulyaganov et al., 2009; Cheng et al., 2010; Yuan et al., 2013). Li2SiO3 is needle-like and Li2Si2O5 is columnar, and these findings are consistent with the XRD results as well.
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Fig. 2 – SEM images of one heat-pressing and two heat-pressing for IPS e.max Press specimens.
The SEM also found porosity and cracks in the lithium disilicate crystals after two heat-pressing events. This may be from dissolution of the lithium orthophosphate crystals located in the glass matrix and at the lithium disilicate crystal grain boundaries because the lithium orthophosphate phase had a higher etching rate than that of the lithium disilicate phase. There is also a possibility that the increase in the porosity and cracking is due to multiple nucleation sites during crystallization (Zheng et al., 2008; Chung et al., 2009; Tulyaganov et al., 2009; Cheng et al., 2010; Yuan et al., 2013). The opposing faces of all specimens were flat and parallel to within 0.05 mm according to the guidelines of ISO 6872: 2008 (for three-point flexural strength). After one or two rounds of heat-pressing, specimens were ground, polished, and cleaned. This study revealed that the surface roughness of all the specimens ranged from 0.143 to 0.144 μm. No significant differences were found in the surface roughness of any specimens. This could be because identical grinding methods and procedures were applied to all the specimens. This meets the guidelines of ISO 6872: 2008. This ISO states that the test specimens must have a smooth surface with a roughness below 0.5 μm Ra. Compared with one heat-pressing, the density decreased and the porosity increased after two heat-pressings. This result is likely explained by microstructure evaluation.
The Vickers hardness after two heat-pressings was significantly lower than that after one heat-pressing. This result was probably linked to the result of the density and porosity evaluation. Some ceramic-related studies have shown that the porosity is lower and the density is higher for the ceramic bulk—its hardness is higher (Yang et al., 2009; Chavan et al., 2011; Zhong et al., 2011; Tang et al., 2012a). In this study, the flexural strength was 354.46 MPa after one heat-pressing and 247.37 MPa after two heat-pressings. The strength after one heat-pressing was significantly higher than that after two heat-pressings. Recently, two investigations showed that repeated heat-pressing events modified the strength of lithium disilicate-reinforced glass–ceramic materials (Empress 2) (Albakry et al., 2004a; Chung et al., 2009). Chung et al. reported that the strength of Empress 2 became significantly higher after repeated heat-pressings (Chung et al., 2009). However, Albakry et al. reported that the strength of Empress 2 became lower after repeated heatpressings (Albakry et al., 2004a). Although the effects of repeated heat-pressings on the mechanical properties and microstructure of IPS e.max Press have been studied previously, our result is similar to that reported of Albakry et al. This higher strength was possibly attributed to the homogeneous crystallization of interlocked needle-like crystals with appropriate aspect ratio (Cattell et al., 2002; Wen et al.,
journal of the mechanical behavior of biomedical materials 40 (2014) 390 –396
2007; Wang et al., 2010; Tulyaganov et al., 2009; Kang et al., 2013; Yuan et al., 2013). On the other hand, the microstructure of a wider crystal size and sparser crystal distribution after repeated heat-pressing suggests that the intergranular crack can propagate more easily through the residual glass matrix and decrease the specimen strength. The microstructure of the porosity and crack seen in the lithium disilicate crystals after two heat-pressings may act as fracture-initiating flaws and severely limit the strength of the specimens (Albakry et al., 2004a, 2004b; Tang et al., 2012a). Fracture toughness after one heat-pressing was significantly larger versus that after two heat-pressings seen here. The fracture toughness is the material's resistance to crack propagation. The toughening mechanism of ceramics is mainly due to crack deflection, particle (fiber) bridging, stress dispersion in the crack tip, etc. (Tang et al., 2012a; Yoshimura et al., 2012). For lithium disilicate glass–ceramics, the toughening mechanism may be primarily attributed to crack deflection and particle bridging. Their fracture toughness depends on the size, aspect ratio, and orientation of the crystals (Albakry et al., 2003, 2004b; Guazzato et al., 2004; Pollington and van Noort 2012; Yoshimura et al., 2012). Apel et al. thought that the crack propagation was an intergranular process in lithium disilicate glass–ceramics because the crack propagated only through the residual glass matrix and did not crystallize (Apel et al., 2008). The high strength crystals of the interlocking microstructure are more resistant. Consequently, crack development takes place at the weaker interfaces between the crystals (Albakry et al., 2003; Apel et al., 2008; Kang et al., 2013; Wang et al., 2013; Yuan et al., 2013). The microstructure of orientation and sparser distribution of lithium disilicate crystals after repeated heat-pressing may cause the intergranular crack to propagate more easily through the residual glass matrix, causing a lower fracture toughness of the specimens (Albakry et al., 2003; Apel et al., 2008; Wang et al., 2013; Yuan et al., 2013). The oral environment is very harsh because of temperature changes, shifts in pH, wear, and mechanical load (Tang et al., 2012b). Esquivel-Upshaw et al. reported that lithium disilicate crowns showed more wear and surface roughness than metal– ceramic crowns (Esquivel-Upshaw et al., 2006). The rough ceramic surface may cause excessive wear of the opposing teeth, surface discoloration, and inflammation of soft oral tissue (Suputtamongkol et al., 2010; Kim et al., 2012; Tang et al., 2012b; Theocharopoulos et al., 2012). Other investigations have revealed that common complications include cracking, chipping and fracture for lithium disilicate materials. These complications have been reported to be higher in the posterior region. Moreover, fixed dental prostheses (FDPs) exhibit higher rates of fracture of the core framework than single crown restorations (Pjetursson et al., 2007; Della Bona and Kelly 2008; Sax et al., 2011; Raigrodski et al., 2012; Schmitter et al., 2012; Pieger et al., 2014; Wang et al., 2014). The lithium disilicate restorations must have very high hardness, strength, and fracture toughness to reduce these complications. However, this study showed that the density, hardness, flexural strength, and fracture toughness significantly decreased after repeated heat-pressing. This process may significantly increase the complications mentioned above for the dental prostheses, especially molars, and reduce the
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service life of the inlay, onlay, crown, and bridge. Consequently, it is not favorable in clinical situations for the repeated heat-pressing to change the mechanical properties and microstructure of IPS e.max Press specimens.
5.
Conclusions
In this study, the effect of repeated heat-pressing on the mechanical properties and microstructure of IPS e.max Press was investigated. After repeated heat-pressings, the microstructure changed and the porosity increased. The density, hardness, flexural strength, and fracture toughness significantly decreased.
Acknowledgments The authors would like to thank Mr. Yoshimoto, Mr. Takahashi, and Mr. Watanabe of Shofu Inc., and Mr. Ishida of Dental Studio Laboratory for their technical assistance in this study.
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Research Paper
The effects of repeated heat-pressing on the mechanical properties and microstructure of IPS e.max Press Xuehua Tanga,b,n, Chengzhong Tanga, Han Sua, Huinan Luoa, Takashi Nakamurab, Hirofumi Yatanib a
Department of Stomatology, Jinling Hospital, Nanjing University School of Medicine, Nanjing 210002, China Department of Fixed Prosthodontics, Osaka University Graduate School of Dentistry, Osaka 565-0871, Japan
b
ar t ic l e in f o
abs tra ct
Article history:
Objectives: We evaluated the effects of repeated heat-pressing on the mechanical proper-
Received 10 July 2014
ties and microstructure of IPS e.max Press.
Received in revised form
Methods: A total of 20 specimens were fabricated including 10 heat-pressed once and
12 September 2014
another 10 heat-pressed twice. The density, porosity, and surface roughness (Ra) were
Accepted 15 September 2014
evaluated. Three-point flexural strength following the ISO 6872 and Vickers hardness were
Available online 28 September 2014
measured, and fracture toughness (KIC) was calculated. Specimens were characterized
Keywords:
using X-ray diffraction (XRD) and scanning electron microscopy (SEM).
IPS e.max Press
Results: Compared to a single heat-press treatment, the density decreased and porosity
Repeated heat-pressing
increased after two heat-pressing events. A significant difference in density was observed.
Mechanical properties
The flexural strength, Vickers hardness and fracture toughness significantly decreased
Microstructure
after two heat-pressing events. The XRD patterns show that the intensity of the crystalline phase better corresponds to lithium disilicate (Li2Si2O5) after two heat-presses than only one. The SEM images detailed the interlocking microstructure of rod-shaped Li2Si2O5 crystals after one heat-press. These became oriented after two heat-presses, and the crystal size became larger. Conclusion: This study showed that repeated heat-pressing was detrimental to the density, porosity, strength, hardness, and toughness of IPS e.max Press. & 2014 Elsevier Ltd. All rights reserved.
1.
Introduction
All-ceramic restorations are attracting extensive attention in prosthetic dentistry because of their translucency and beneficial biocompatibility. They have increasingly become an alternative to conventional metal–ceramic restorations.
n
Ceramics used in all-ceramic restorations are primarily glass–ceramics, densely sintered alumina, and zirconia-based ceramics. Glass–ceramics exhibit some compositional and microstructural differences and combine properties that are typical for both ceramics and glasses. For these ceramics, the original glass composition as well as the presence, volume fraction, crystal
Corresponding author at: 305 East Zhongshan Road, Nanjing 210002, China. Tel./fax: þ86 25 84816508. E-mail address: [email protected] (X. Tang).
http://dx.doi.org/10.1016/j.jmbbm.2014.09.016 1751-6161/& 2014 Elsevier Ltd. All rights reserved.
journal of the mechanical behavior of biomedical materials 40 (2014) 390 –396
size, distribution and morphology of the crystalline phases may account for the variations in the ceramics' mechanical properties (Fernandes et al., 2010; Kim et al., 2012; Tang et al., 2012a; Yuan et al., 2013; Zhang et al., 2013; Akar et al., 2014). Currently, leucite-reinforced glass–ceramics and lithium disilicate glass–ceramics are widely used in heat-pressing and subsequently studied further. The pressable leucite-reinforced ceramic IPS Empress has been in the market for more than 15 years and is used primarily to fabricate laminate veneers, inlay, onlay, crown, etc. This material has had a long clinical track record and offers good esthetic outcomes and clinical survival rates (Heintze et al., 2011; Gehrt et al., 2013; Solá-Ruiz et al., 2013). The pressable lithium disilicate ceramic IPS Empress 2 was introduced in 1998 and demonstrated a higher mechanical strength than that of IPS Empress. It was suitable for threeunit fixed dental prostheses (FDPs) in the anterior region (Heintze et al., 2011; Gehrt et al., 2013; Solá-Ruiz et al., 2013). However, it had to be veneered because of its opacity. The survival rate was 96.8% after 5 years and 95.5% up to 10 years in two retrospective studies of IPS Empress 2 crowns. Another 5-year prospective clinical study showed that the survival rate was 100% for crowns and 70% for FPDs (Marquardt and Strub, 2006; Valenti and Valenti, 2009; Steeger, 2010). To combine durability with excellent esthetics, a new pressable lithium-disilicate glass ceramic named IPS e.max Press with enhanced mechanical properties and improved translucency was launched in 2007 (Heintze et al., 2011). This product is not only a core material, but can also be directly fabricated for various restorations. Moreover, the range of indications includes anterior and posterior teeth. The investigation of Gehrt et al. showed a cumulative survival rate of 97.4% after 5 years and 94.8% after 8 years of clinical service for anterior and posterior crowns made with IPS e.max Press. Another prospective study indicated an 8 years survival rate of 93% for three-unit FDPs made with this ceramic (Wolfart et al., 2009; Gehrt et al., 2013; Pieger et al., 2014). IPS e.max Press ceramics are pressed into a mold by an alumina plunger under pressure using a pneumatic press furnace. The button and sprue portions are usually discarded. However, they are considered useful for re-pressing in some dental laboratories. Hence, it is very important to evaluate the mechanical properties and microstructure of re-pressed glass– ceramic material to determine the feasibility of repeated heatpressing treatment. We hypothesize that after repeated heatpressing, two heat-pressing samples will maintain the same mechanical properties and microstructure as those of a single heat-pressing event.
2.
Materials and methods
2.1.
Specimen preparation
The specimens were fabricated by the lost-wax technique. The dental wax was melted and poured into a rectangular silicon mold with internal dimensions of 30 mm 6 mm 4 mm. After solidification, these wax patterns were sprued and attached to a muffle base with surrounding paper cylinders. This was then invested with a special investment material (Ceravety Press &
391
Cast, Shofu, Kyoto, Japan). The wax burnout was at 850 1C in a conventional pre-heated furnace. The ceramic ingots of IPS e.max Press were then plastified and pressed under vacuum into the mold of the investment in a press furnace (EP600 Ivoclar Vivadent). The heat-pressing conditions were as follows: stand-by temperature 700 1C; temperature increase 60 1C/min; press temperature 920 1C; and holding time 25 min. After pressing, the investment molds were removed from the furnace and allowed to air cool. The specimens were then carefully devested using an air abrasion unit with 50 μm glass beads at a pressure of 3 bar. The button and sprue portions were cut; 12 specimens were selected at random. For the remaining specimens, the button and sprue portions were adjusted by grinding to allow proper insertion into the refractory molds for repeated heatpressing. With the same heat-pressing conditions an additional 12 specimens were fabricated. Finally all specimens were ground to final dimensions of 4.070.2 mm in width, 1.270.2 mm in thickness and at least 20 mm in length, using diamond discs following the guidelines of ISO 6872 (2008) (International Organization for Standardization (2008), threepoint flexural strength). Then, the top surface (4 mm 20 mm) was mirror-polished using a polishing machine (PFG500DX; Okamoto Machine Tool Works, Yokohama, Japan) with diamond suspensions. Finally all specimens were checked by optical microscopy (LV-100POL; Nikon, Tokyo, Japan), and 10 specimens without any processing defects were selected for the following test. These included samples that had been heatpressed once and twice.
2.2. Measurement and calculation of the mechanical properties 2.2.1.
Density and open porosity
The density and open porosity of all the specimens were determined by Archimedes' method in distilled water at room temperature. First, each sample was weighed while it was completely dry and the weight was recorded as W1. It was then placed in a beaker with distilled water and a vacuum was pulled for several minutes to ensure that the open pores would be filled with distilled water. Next, the beaker was placed on a stand over a balance. A wire was hung so the end of it was below the distilled water's surface but not touching the sides of the beaker in any way. The sample was lifted onto the wire with tweezers and its suspended weight was recorded as W2. Finally the sample was removed from the distilled water, quickly blotted, and weighed to obtain the wet weight, W3. The bulk density of the sample, ρ, was evaluated using ρ ¼ ρw W1 =ðW3 W2 Þ where ρw is the density of water at the testing temperature ( ρw ¼ 0.99799 g/cm3 at 21 1C).The open porosity, P, was evaluated as P ¼ ðW3 W1 Þ=ðW3 W2 Þ 100%
2.2.2.
Surface roughness
The surface roughness of all the specimens, i.e., the average roughness (Ra), was evaluated by laser profilometry using a
392
journal of the mechanical behavior of biomedical materials 40 (2014) 390 –396
confocal laser scanner profilometer (LEXT OLS4000; Olympus, Tokyo, Japan). The diameter of the laser beam was 0.2 μm. Four separate areas on each specimen were randomly selected and evaluated. The measured area was 256 μm 256 μm, and the distance between the separate scans was over 3 μm. The resolution used in the X and Y directions was 0.02 μm. The Z resolution was 0.01 μm.
2.2.3.
2.3.2.
Flexural strength
Flexural strength was measured using the three-point bending test. Ten specimens of the first and repeated heat-pressings were labeled 1 to 10 and analyzed with a universal testing machine (AUTOGRAPH AG-5000B; Shimadzu, Kyoto, Japan) resting on a self-aligning fixture with a span of 14 mm. The tests were conducted at a crosshead speed of 0.5 mm/min at room temperature (2071 1C) with a relative humidity of 7075% following the ISO 6872 guidelines (2008).
2.2.4.
Hardness
All specimens almost broke into two almost equal pieces after a three-point bending test (n¼ 10 for the first or repeated heat-pressing), and one fractured segment of each specimen was chosen randomly for Vickers micro-hardness testing. Hardness was measured using a Vickers micro-hardness tester (HM-102; Mitutoyo, Kawasaki, Japan) with a load of 19.6 N and a dwell time of 20 s. The test was performed on the polished surface, and cracks generated along the indentation diagonal were measured. This load was chosen to generate long radial cracks with (c/a)42.5, where c is the crack length and a is half of the indentation diagonal.
2.2.5.
a MultiFlex X-ray diffractometer (Rigaku, Tokyo, Japan) using flat plate geometry. The data were collected from 10 to 501 2θ using a step size of 0.021 2θ and a 0.6-s count time. CuKα radiation was used (40 kV, 40 mA). The identification of the crystalline phases was achieved by comparing the diffraction patterns with the JCPDS standard reference.
Fracture toughness
The fracture toughness (KIC) was calculated using the following equation: 0:5 KIC ¼ 0:016 E=H P=c1:5 : Here, P is the indentation load, c is the combined length of the radial crack, and H is the material hardness obtained by the Vickers hardness test. The term E is the elastic modulus obtained by the flexural strength test (Tang et al., 2012a).
2.3.
Observation of microstructure
2.3.1.
X-ray diffraction (XRD)
Fractured segments were chosen randomly from five specimens per test group and ground into a powder for X-ray diffraction analysis. The samples were placed in the holder of
Scanning electron microscopy (SEM)
Several fractured segments were selected at random from the test groups and were self-glazed to remove the effects of the polish traces for SEM imaging ( JSM-6390LA; JEOL, Tokyo, Japan). After self-glazing, the samples were etched with hydrofluoric acid (HF) and coated with gold to increase conductivity. The veneering ceramics were etched with 1% HF for 7 min.
2.4.
Statistical analysis
The mechanical properties of specimens that had undergone heat-pressing once were compared to those that had undergone twice. Values of each group were assessed using a t-test, and the data were considered significant at 5% (SPSS 17.0; SPSS, Inc., Chicago, IL, USA).
3.
Results
3.1.
Mechanical properties
For the IPS e.max Press, the mechanical properties after repeated heat-pressing are presented in Table 1. Density decreased and porosity increased after two heat-pressings, and significant differences were observed in the density between one and two heat-pressings (P¼ 0.018). The surface roughness of the specimens was 0.143 and 0.144 μm after repeated heat-pressing. No significant differences were found. The flexural strength was 354.46 MPa after one heatpressing and 247.37 MPa after two heat-pressings. The strength after one heat-pressing was higher than that after two heat-pressings (P ¼0.000). The Vickers hardness after repeated heat-pressings was 5.07 and 4.86 GPa. The hardness after one heat-pressing was significantly higher (P¼ 0.000). With respect to fracture toughness, the specimen after one heat-pressing (2.42 MPa m1/2) was tougher than that after two heat-pressings (1.97 MPa m1/2); significant differences were found in fracture toughness between one and two heatpressings (P¼ 0.000).
Table 1 – Density, porosity, surface toughness (Ra), flexural strength, Vickers hardness and fracture toughness after repeated heat-pressing. Heat-pressing times
Density (g cm 3)
Porosity (%)
Surface toughness (mm)
Flexural strength (MPa)
1 2
2.45(0.03)n 2.39(0.04)n
1.55(1.78) 2.52(1.86)
0.143(0.004) 0.144(0.003)
354.46(30.27) 247.37(42.67)
( ): standard deviation. n nn , Significant differences (n: Po0.05;
nn
nn nn
: Po0.001) between 1 heat-pressing and 2 heat-pressing.
Vickers hardness (GPa)
5.07(0.06) 4.86(0.08)
nn nn
Fracture toughness (MPa m1/2) 2.42(0.08) 1.97(0.13)
nn nn
journal of the mechanical behavior of biomedical materials 40 (2014) 390 –396
3.2.
Microstructural evaluation
3.2.1.
XRD analysis
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The XRD patterns showed that the lithium disilicate (Li2Si2O5) phase, lithium metasilicate (Li2SiO3) phase and lithium orthophosphate (Li3PO4) phase are present in both the one heatpressing and two heat-pressing samples. The main crystalline phase was lithium disilicate (Fig. 1). The major peaks of the lithium disilicate phase were observed at the 2θ values of 23.781, 24.341 and 24.841. This is as predicted by the XRD standard files for lithium disilicate (JCPDS# 40-0376). The peaks were at 2θ values of 23.781, 24.341 and 24.841 after two heat-pressings. These were higher than those seen for the one heat-pressing sample. The major peaks of the lithium metasilicate phase were found at 2θ values of 18.881, 26.621 and 33.01 (JCPDS# 29-0829). The peaks for the heat-pressing sample were lower than those after one heat-pressing. The lithium orthophosphate peaks were at 2θ values of 22.261, 23.041 and 36.281 (JCPDS# 15-0760).
3.2.2.
SEM analysis
Many needle-like crystals are present in the glass matrix for all specimens (Fig. 2). One heat-pressing showed densely packed and multi-directionally interlocking microstructure of numerous needle-like crystals protruding from the glass matrix. However, after two heat-pressing events, the crystals appeared oriented, wider and larger with a sparser distribution. The sharp needle shaped ends of crystals disappeared. Moreover, increased porosity and cracks were found in the crystals after two heat-pressings.
4.
Discussion
The purpose of this study is to evaluate the effects of repeated heat-pressing on the mechanical properties and microstructure of IPS e.max Press. The results do not confirm the hypothesis that after repeated heat-pressings, the two heat-pressings maintain the same mechanical properties and microstructures as those of one heat-pressing. In fact, versus one heat-pressing, the density decreased and the porosity increased. The strength, hardness, and toughness significantly decreased after two heat-pressings. Moreover, their microstructures were changed to different degrees in both the XRD and SEM analysis. Thus, the hypothesis of this study is rejected. The XRD analysis revealed that the major peaks of the lithium disilicate phase were higher after two heat-pressings than after one heat-pressing. The major peaks of the lithium metasilicate phase were lower after two heat-pressings than after one heat-pressing. This may indicate there is some chemical reaction of the phase transition between Li2SiO3 and Li2Si2O5 during the heat treatment (Li2SiO3þSiO2-Li2Si2O5) (Zheng et al., 2008; Tulyaganov et al., 2009; Cheng et al., 2010; Yuan et al., 2013). The results of the XRD analysis also confirmed that a small amount of Li3PO4 presented in IPS e.max Press. The Li3PO4 crystals are sites for the nucleation of stable lithium disilicate (Zheng et al., 2008; Chung et al., 2009; Tulyaganov et al., 2009).
Fig. 1 – XRD pattern of one heat-pressing and two heatpressing for IPS e.max Press specimens.
Some studies have shown that repeated heat-pressing significantly influenced the microstructure of lithium disilicatereinforced glass–ceramic materials and produced the blunt rodlike shape, larger grains, and orientation of the crystals (Albakry et al., 2004a, 2004b; Zheng et al., 2008; Chung et al., 2009; Tulyaganov et al., 2009; Cheng et al., 2010; Yuan et al., 2013). These results are consistent with this study. The orientation of the lithium disilicate crystals was probably a result of plastic deformation of the glass matrix phase and occurred during sprue extrusion (Albakry et al., 2004a, 2004b; Chung et al., 2009). The blunt rod-like shape and the larger grains of the lithium disilicate crystals after the repeated heat-pressing may be considered as Ostwald ripening and is common across all precipitated materials. This occurs when the microstructure coarsens and liberates free surface energy due to the solubility of small particles (Albakry et al., 2004a, 2004b). Consequently, larger grains are expected to grow at the expense of small particles. This may be the result of the phase transition between lithium metasilicate and lithium disilicate (Zheng et al., 2008; Tulyaganov et al., 2009; Cheng et al., 2010; Yuan et al., 2013). Li2SiO3 is needle-like and Li2Si2O5 is columnar, and these findings are consistent with the XRD results as well.
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journal of the mechanical behavior of biomedical materials 40 (2014) 390 –396
Fig. 2 – SEM images of one heat-pressing and two heat-pressing for IPS e.max Press specimens.
The SEM also found porosity and cracks in the lithium disilicate crystals after two heat-pressing events. This may be from dissolution of the lithium orthophosphate crystals located in the glass matrix and at the lithium disilicate crystal grain boundaries because the lithium orthophosphate phase had a higher etching rate than that of the lithium disilicate phase. There is also a possibility that the increase in the porosity and cracking is due to multiple nucleation sites during crystallization (Zheng et al., 2008; Chung et al., 2009; Tulyaganov et al., 2009; Cheng et al., 2010; Yuan et al., 2013). The opposing faces of all specimens were flat and parallel to within 0.05 mm according to the guidelines of ISO 6872: 2008 (for three-point flexural strength). After one or two rounds of heat-pressing, specimens were ground, polished, and cleaned. This study revealed that the surface roughness of all the specimens ranged from 0.143 to 0.144 μm. No significant differences were found in the surface roughness of any specimens. This could be because identical grinding methods and procedures were applied to all the specimens. This meets the guidelines of ISO 6872: 2008. This ISO states that the test specimens must have a smooth surface with a roughness below 0.5 μm Ra. Compared with one heat-pressing, the density decreased and the porosity increased after two heat-pressings. This result is likely explained by microstructure evaluation.
The Vickers hardness after two heat-pressings was significantly lower than that after one heat-pressing. This result was probably linked to the result of the density and porosity evaluation. Some ceramic-related studies have shown that the porosity is lower and the density is higher for the ceramic bulk—its hardness is higher (Yang et al., 2009; Chavan et al., 2011; Zhong et al., 2011; Tang et al., 2012a). In this study, the flexural strength was 354.46 MPa after one heat-pressing and 247.37 MPa after two heat-pressings. The strength after one heat-pressing was significantly higher than that after two heat-pressings. Recently, two investigations showed that repeated heat-pressing events modified the strength of lithium disilicate-reinforced glass–ceramic materials (Empress 2) (Albakry et al., 2004a; Chung et al., 2009). Chung et al. reported that the strength of Empress 2 became significantly higher after repeated heat-pressings (Chung et al., 2009). However, Albakry et al. reported that the strength of Empress 2 became lower after repeated heatpressings (Albakry et al., 2004a). Although the effects of repeated heat-pressings on the mechanical properties and microstructure of IPS e.max Press have been studied previously, our result is similar to that reported of Albakry et al. This higher strength was possibly attributed to the homogeneous crystallization of interlocked needle-like crystals with appropriate aspect ratio (Cattell et al., 2002; Wen et al.,
journal of the mechanical behavior of biomedical materials 40 (2014) 390 –396
2007; Wang et al., 2010; Tulyaganov et al., 2009; Kang et al., 2013; Yuan et al., 2013). On the other hand, the microstructure of a wider crystal size and sparser crystal distribution after repeated heat-pressing suggests that the intergranular crack can propagate more easily through the residual glass matrix and decrease the specimen strength. The microstructure of the porosity and crack seen in the lithium disilicate crystals after two heat-pressings may act as fracture-initiating flaws and severely limit the strength of the specimens (Albakry et al., 2004a, 2004b; Tang et al., 2012a). Fracture toughness after one heat-pressing was significantly larger versus that after two heat-pressings seen here. The fracture toughness is the material's resistance to crack propagation. The toughening mechanism of ceramics is mainly due to crack deflection, particle (fiber) bridging, stress dispersion in the crack tip, etc. (Tang et al., 2012a; Yoshimura et al., 2012). For lithium disilicate glass–ceramics, the toughening mechanism may be primarily attributed to crack deflection and particle bridging. Their fracture toughness depends on the size, aspect ratio, and orientation of the crystals (Albakry et al., 2003, 2004b; Guazzato et al., 2004; Pollington and van Noort 2012; Yoshimura et al., 2012). Apel et al. thought that the crack propagation was an intergranular process in lithium disilicate glass–ceramics because the crack propagated only through the residual glass matrix and did not crystallize (Apel et al., 2008). The high strength crystals of the interlocking microstructure are more resistant. Consequently, crack development takes place at the weaker interfaces between the crystals (Albakry et al., 2003; Apel et al., 2008; Kang et al., 2013; Wang et al., 2013; Yuan et al., 2013). The microstructure of orientation and sparser distribution of lithium disilicate crystals after repeated heat-pressing may cause the intergranular crack to propagate more easily through the residual glass matrix, causing a lower fracture toughness of the specimens (Albakry et al., 2003; Apel et al., 2008; Wang et al., 2013; Yuan et al., 2013). The oral environment is very harsh because of temperature changes, shifts in pH, wear, and mechanical load (Tang et al., 2012b). Esquivel-Upshaw et al. reported that lithium disilicate crowns showed more wear and surface roughness than metal– ceramic crowns (Esquivel-Upshaw et al., 2006). The rough ceramic surface may cause excessive wear of the opposing teeth, surface discoloration, and inflammation of soft oral tissue (Suputtamongkol et al., 2010; Kim et al., 2012; Tang et al., 2012b; Theocharopoulos et al., 2012). Other investigations have revealed that common complications include cracking, chipping and fracture for lithium disilicate materials. These complications have been reported to be higher in the posterior region. Moreover, fixed dental prostheses (FDPs) exhibit higher rates of fracture of the core framework than single crown restorations (Pjetursson et al., 2007; Della Bona and Kelly 2008; Sax et al., 2011; Raigrodski et al., 2012; Schmitter et al., 2012; Pieger et al., 2014; Wang et al., 2014). The lithium disilicate restorations must have very high hardness, strength, and fracture toughness to reduce these complications. However, this study showed that the density, hardness, flexural strength, and fracture toughness significantly decreased after repeated heat-pressing. This process may significantly increase the complications mentioned above for the dental prostheses, especially molars, and reduce the
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service life of the inlay, onlay, crown, and bridge. Consequently, it is not favorable in clinical situations for the repeated heat-pressing to change the mechanical properties and microstructure of IPS e.max Press specimens.
5.
Conclusions
In this study, the effect of repeated heat-pressing on the mechanical properties and microstructure of IPS e.max Press was investigated. After repeated heat-pressings, the microstructure changed and the porosity increased. The density, hardness, flexural strength, and fracture toughness significantly decreased.
Acknowledgments The authors would like to thank Mr. Yoshimoto, Mr. Takahashi, and Mr. Watanabe of Shofu Inc., and Mr. Ishida of Dental Studio Laboratory for their technical assistance in this study.
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