Effect of firing temperature and water exposure on crack propagation in unglazed porcelain

Static fatigue of dental ceramics results from the interaction of residual tensile stress and an aqueous environment. This phenomenon is a potential cause of delayed crack formation and propagation in ceramic or metal-ceramic restorations. For dental ceramics, the influence of microstructural defects such as porosity or fissures caused by incomplete sintering is not known. The objectives of this study were to characterize the influence of porosity (produced by underfiring) on the crack propagation resistance of two feldspathic porcelains and to determine whether lower stress corrosion susceptibility or higher fracture toughness accounts for the superior thermal shock resistance of one of these ceramics. We underfired bars of each porcelain, 25 mm x 4 mm x 4 mm, by as much as 84 degrees C below their recommended firing temperatures. After polishing the specimens through 0.05 microns alumina, we induced cracks in their surfaces with a Vickers microhardness indenter. Semicircular cracks, which were produced under an applied indenter load of 19.6 N, grew with time during storage in distilled water at 37 degrees C. Underfiring of both ceramics caused a slight increase in fracture toughness and a relatively small change in pore volume fraction until we underfired the ceramics at 30 degrees C or more. The crack propagation data indicate that the higher thermal shock resistance of one of the ceramics--as measured previously by a water-quench technique-may be due to its greater resistance to stress corrosion at the initial stage of crack propagation.     

氟磷灰石玻璃陶瓷、长石质饰瓷与天然牙摩擦磨损性能的研究

目的：研究牙科氟磷灰石玻璃陶瓷、长石质饰瓷与天然牙在干摩擦和唾液润滑条件下的摩擦磨损学性能。方法：制备涂覆IPS-emaxcream和Vita vm9 2种饰面瓷的氧化锆-饰面瓷双层材料试样,尺寸为20 mm×20 mm×4.5 mm,各20个。利用MMV-1摩擦磨损试验机,通过设置不同大小的垂直载荷和转速,分别在干摩擦和唾液润滑条件下,对2种氧化锆-饰面瓷试样进行摩擦磨损试验。采用SPSS 17.0软件包进行单因素方差分析和t检验,确定各组间摩擦系数和磨损量是否存在差异。结果：氟磷灰石玻璃陶瓷在唾液润滑条件下的动态摩擦系数和磨损量显著大于干摩擦条件下的数值;与氟磷灰石玻璃陶瓷相反,长石质饰瓷在干摩擦时的动态摩擦系数和磨损量显著大于在唾液润滑条件下的数值。在干摩擦和唾液润滑条件下,氟磷灰石玻璃陶瓷的磨损量均小于长石质饰瓷。扫描电镜显示,在唾液润滑条件下,氟磷灰石玻璃陶瓷及对应天然牙表现出比干摩擦更为严重的磨损表面。在干摩擦条件下,长石质饰瓷及对应天然牙较氟磷灰石玻璃陶瓷磨损更为严重,在唾液润滑条件下差异较小。结论：与长石质饰瓷相比,氟磷灰石玻璃陶瓷与天然牙在干摩擦条件下表现出更优越的耐磨损性能。加入唾液后,能减少2种饰瓷摩擦磨损性能之间的差异。     

Analysis of subcritical crack growth in dental ceramics using fracture mechanics and fractography.

OBJECTIVES: The aim of this study was to test the hypothesis that the flexural strengths and critical flaw sizes of dental ceramic specimens will be affected by the testing environment and stressing rate even though their fracture toughness values will remain the same. METHODS: Ceramic specimens were prepared from an aluminous porcelain (Vitadur Alpha; VITA Zahnfabrik, Bad S?ckingen, Germany) and an alumina-zirconia-glass composite (In-Ceram Zirconia; VITA Zahnfabrik). Three hundred uniaxial flexure specimens (150 of each material) were fabricated to dimensions of 25 mmx4 mmx1.2 mm according to the ISO 6872 standard. Each group of 30 specimens was fractured in water using one of four different target stressing rates ranging on a logarithmic scale from 0.1 to 100 MPa/s for Vitadur Alpha and from 0.01 to 10 MPa/s for In-Ceram Zirconia. The fifth group was tested in inert environment (oil) with a target stressing rate of 100 MPa/s for Vitadur Alpha and 1000 MPa/s for In-Ceram Zirconia. The effects of stressing rate and environment on flexural strength, critical flaw size, and fracture toughness were analyzed statistically by Kruskal-Wallis one-way ANOVA on ranks followed by post hoc comparisons using Dunn's test (alpha=0.05). In addition, 20 Vitadur Alpha specimens were fabricated with controlled flaws to simplify fractography. Half of these specimens were fracture tested in water and half in oil at a target stressing rate of 100 MPa/s, and the results were compared using Mann-Whitney rank sum tests (alpha=0.05). A logarithmic regression model was used to determine the fatigue parameters for each material. RESULTS: For each ceramic composition, specimens tested in oil had significantly higher strength (P0.05). Specimens tested at faster stressing rates had significantly higher strength (P0.05). Regarding critical flaw size, stressing rate had a significant effect for In-Ceram Zirconia specimens (P0.05). Fatigue parameters, n and lnB, were 38.4 and -12.7 for Vitadur Alpha and were 13.1 and 10.4 for In-Ceram Zirconia. SIGNIFICANCE: Moisture assisted subcritical crack growth had a more deleterious effect on In-Ceram Zirconia core ceramic than on Vitadur Alpha porcelain. Fracture surface analysis identified fracture surface features that can potentially mislead investigators into misidentifying the critical flaw.     

Strength and transparency of dental porcelain consisting of high refractive germanate-glass and alumina crystal

A translucent aluminous porcelain was developed for dentistry. The effects of refractive indexes and sintering behaviors on transparency and strength of the aluminous porcelains consisting of high refractive germanate-glass (Na2O-TiO2-GeO2) and alumina crystal powders were examined. The various germanate-glass specimens with a high refractive index were made by fusion at about 1,300 degrees C. The refractive indexes of fused Na2O-TiO2-GeO2 glass specimens were 1.64-1.76, heightened with an increasing TiO2 content. The sintered aluminous porcelains were made from the mixed compacts consisting of 80 wt% germanate-glass and 20 wt% alumina at the densification temperature of 580-820 degrees C. Sintered aluminous porcelains prepared with high refractive germante-glass had a high transparency compared with the other aluminous porcelains, with almost the same transparency as a commercial feldspathic porcelain (body). Aluminous porcelains had lower transparency with different refractive index due to generation of crystals following the crystallization of glass matrix than that without crystallizing property. Bending strength value was 120 MPa, which is similar to that for the glass-alumina ceramics with the same content of alumina volume as germanate-glass aluminous porcelains. Non-crystallized aluminous porcelain had a higher strength compared with the crystallized one.     

Mechanical property evaluation of pressable restorative ceramics.

OBJECTIVE: The aim of this study was to evaluate the flexure strength under static and cyclic loading and the fracture toughness under static loading of six restorative ceramic materials. Specifically the intent was to compare four leucite (K2O.Al2O(3).4SiO2) strengthened feldspathic (pressable) porcelains (OA, OI, E, and FP) to a low fusing feldspathic porcelain (F) and an experimental (EC) lithium disilicate containing ceramic. METHODS: All materials were tested as a control in air and distilled water (without aging) and after three months aging in air or distilled water to determine flexure strength and fracture toughness. A staircase approach was used to determine the cyclic flexure strength. EDS and SEM analysis were conducted on the polished and fracture surfaces. RESULTS: The mean flexure strength for the controls in air and water (without aging or cyclic loading) ranged from 67 to 99 MPa, except the experimental ceramic that was twice as strong with a mean flexure strength of 191-205 MPa. For the mean fracture toughness, the range was 1.1-1.9 MPa/m0.5 with the experimental ceramic being 2.7 MPa/m0.5. The effect of testing in water and aging for three months caused a moderate reduction in the mean flexure strength (6-17%), and a moderate to severe reduction in the mean fracture toughness (5-39%). The largest decrease (15-60%) in mean flexure strength was observed when the samples were subjected to cyclic loading. SIGNIFICANCE: The lithium disilicate containing ceramic had a significantly higher flexure strength and fracture toughness when compared to the four pressable leucite strengthened ceramics and the low fusing conventional porcelain. All of the leucite containing pressable ceramics did provide an increase in mean flexure strength (17-19%) and mean fracture toughness (3-64%) over the conventional feldspathic porcelain. Further, the influence of testing environment and loading conditions implies that these ceramic materials in the oral cavity might be susceptible to cyclic fatigue, resulting in a significant decrease in the survival time of all-ceramic restorations.     

Effects of surface conditioning on bond strength of metal brackets to all-ceramic surfaces

The aim of this study was to determine the effectiveness of bonding brackets to ceramic restorations. Sixty feldspathic and 60 lithium disilicate ceramic specimens were randomly divided into six groups. Shear bond strength (SBS) and bond failure types were examined with six surface-conditioning methods: silane application to glazed surface, air particle abrasion (APA) with 25- and 50-microm aluminium trioxide (Al(2)O(3)), etching with 9.6 per cent hydrofluoric acid (HFA), and roughening with 40- and 63-microm diamond burs. Silane was applied to all roughened surfaces. Metal brackets were bonded with light cure composite, then stored in distilled water for 1 week and thermocycled (x500 at 5-55 degrees C for 30 seconds). The ceramic surfaces were examined with a stereomicroscope at a magnification of x10 to determine the amount of composite resin remaining using the adhesive remnant index. The lowest SBS values were obtained with HFA for feldspathic (5.39 MPa) and lithium disilicate (11.11 MPa) ceramics; these values were significantly different from those of the other groups. The highest SBS values were found with 63-microm diamond burs for feldspathic (26.38 MPa) and lithium disilicate (28.20 MPa) ceramics, and were not significantly different from 40-microm diamond burs for feldspathic and lithium disilicate ceramics (26.04 and 24.26 MPa, respectively). Roughening with 25- and 50-microm Al(2)O(3) particles showed modest SBS for lithium disilicate (22.60 and 26.15 MPa, respectively) and for feldspathic ceramics (17.90 and 14.66 MPa, respectively). Adhesive failures between the ceramic and composite resin were noted in all groups. Damage to the porcelain surfaces was not observed. The SBS values were above the optimal range, except for feldspathic ceramic treated with HFA and silane. With all surface-conditioning methods, lithium disilicate ceramic displayed higher SBS than feldspathic ceramic.     

Shear stress relaxation of dental ceramics determined from creep behavior.

OBJECTIVE: To test the hypothesis that shear stress relaxation functions of dental ceramics can be determined from creep functions measured in a beam-bending viscometer. METHODS: Stress relaxation behavior was determined from creep data for the following materials: (1) a veneering ceramic-IPS Empress2 body ceramic (E2V); (2) an experimental veneering ceramic (EXV); (3) a low expansion body porcelain-Vita VMK 68 feldspathic body porcelain (VB); (4) a high expansion body porcelain-Will Ceram feldspathic body porcelain (WCB); (5) a medium expansion opaque porcelain-Vita feldspathic opaque porcelain (VO); and (6) a high expansion opaque porcelain-Will Ceram feldspathic opaque porcelain (WCO). Laplace transform techniques were used to relate shear stress relaxation functions to creep functions for an eight-parameter, discrete viscoelastic model. Nonlinear regression analysis was performed to fit a four-term exponential relaxation function for each material at each temperature. The relaxation functions were utilized in the ANSYS finite element program to simulate creep behavior in three-point bending for each material at each temperature. RESULTS: Shear stress relaxation times at 575 degrees C ranged from 0.03 s for EXV to 195 s for WCO. SIGNIFICANCE: Knowledge of the shear relaxation functions for dental ceramics at high temperatures is required input for the viscoelastic element in the ANSYS finite element program, which can used to determine transient and residual stresses in dental prostheses during fabrication.     

Creep functions of dental ceramics measured in a beam-bending viscometer.

OBJECTIVE: To characterize the high temperature viscoelastic properties of several dental ceramics by the determination of creep functions based on mid-span deflections measured in a beam-bending viscometer (BBV). METHODS: Six groups of beam specimens (58 x 5.5 x 2.5 mm) were made from the following materials: (1) IPS Empress2 body--a glass veneer ceramic (E2V); (2) an experimental glass veneer (EXV); (3) Vita VMK 68 feldspathic body porcelain--a low-expansion body porcelain (VB); (4) Will-Ceram feldspathic body porcelain--a high-expansion body porcelain (WCB); (5) Vita feldspathic opaque porcelain--a medium-expansion opaque porcelain (VO); and (6) Will-Ceram feldspathic opaque porcelain--a high-expansion opaque porcelain (WCO). Midpoint deflections for each specimen were measured in a BBV under isothermal conditions at furnace temperatures ranging from 450 to 675 degrees C. Non-linear regression and linear regression analyses were used to determine creep functions and shear viscosities, respectively, for each material at each temperature. RESULTS: The shear viscosities of each group of dental ceramics exhibited bilinear Arrhenius behavior with the slope ratios (x) ranging from 0.19 for WCB to 0.71 for EXV. At the higher temperature ranges, activation energies ranged from 363 kJ/mol for VO to 386 kJ/mol for E2V. SIGNIFICANCE: The viscoelastic properties of dental ceramics at high temperatures are important factors in understanding how residual stresses develop in all-ceramic and metal-ceramic dental restorations.     

Wear evaluation of the human enamel opposing different Y-TZP dental ceramics and other porcelains

PurposeThis study examined the wear resistance of human enamel and feldspathic porcelain after simulated mastication against 3 zirconia ceramics, heat-pressed ceramic and conventional feldspathic porcelain.Materials and methodsHuman teeth and feldspathic porcelain cusp were tested against ceramic discs. 5 brands were tested – 3 monolithic zirconia, Prettau, Lava, and Rainbow, one lithium disilicate, IPS e.max Press, and one feldspathic porcelain, Vita-Omega 900. The surface was polished using a 600 grit and 1200 grit SiC paper. Each group was loaded for 300,000 cycles in a chewing simulator. The wear resistance was analyzed by measuring the volume of substance lost. The wear surfaces were observed by scanning electron microscopy to determine the wear characteristics.ResultsVita-Omega 900 led to the greatest amount of enamel wears followed by IPS e.max Press, Prettau, Lava and Rainbow. There was a significant difference between Vita-Omega 900 and IPS e.max Press (p < 0.05). The wear values for human enamel were significantly greater than those for feldspathic porcelain, regardless of the surface roughness of the ceramic specimens (p < 0.05).ConclusionThe wear behaviour of human enamel and feldspathic porcelain varies according to the type of substrate materials. On the other hand, 3 zirconia ceramics caused less wear in the abrader than the conventional ceramic.Clinical significanceDental professionals should be aware of the wear effect of dental restorations on the opposing teeth or restorations. The amount of enamel wear was highest in feldspathic porcelains whereas zirconia ceramics caused less wear on the opposing teeth.     

Mechanical fatigue degradation of ceramics versus resin composites for dental restorations

Objectives

For posterior partial restorations an overlap of indication exists where either ceramic or resin-based composite materials can be successfully applied. The aim of this study was to compare the fatigue resistance of modern dental ceramic materials versus dental resin composites in order to address such conflicts.

Methods

Bar specimens of five ceramic materials and resin composites were produced according to ISO 4049 and stored for 14 days in distilled water at 37 °C. The following ceramic materials were selected for testing: a high-strength zirconium dioxide (e.max ZirCAD, Ivoclar), a machinable lithium disilicate (e.max CAD, Ivoclar), a pressable lithium disilicate ceramic (e-max Press, Ivoclar), a fluorapatite-based glass-ceramic (e.max Ceram, Ivoclar), and a machinable color-graded feldspathic porcelain (Trilux Forte, Vita). The composite materials selected were: an indirect machinable composite (Lava Ultimate, 3M ESPE) and four direct composites with varying filler nature (Clearfil Majesty Posterior, Kuraray; GrandioSO, Voco; Tetric EvoCeram, Ivoclar-Vivadent; and CeramX Duo, Dentsply). Fifteen specimens were tested in water for initial strength (σ_in) in 4-point bending. Using the same test set-up, the residual flexural fatigue strength (σ_ff) was determined using the staircase approach after 10⁴ cycles at 0.5 Hz (n = 25). Weibull parameters σ₀ and m were calculated for the σ_in specimens, whereas the σ_ff and strength loss in percentage were obtained from the fatigue experiment.

Results

The zirconium oxide ceramic showed the highest σ_in and σ_ff (768 and 440 MPa, respectively). Although both lithium disilicate ceramics were similar in the static test, the pressable version showed a significantly higher fatigue resistance after cyclic loading. Both the fluorapatite-based and the feldspathic porcelain showed equivalent initial and cyclic fatigue properties. From the composites, the highest filled direct material Clearfil Majesty Posterior showed superior fatigue performance. From all materials, e.max Press and Clearfil Majesty Posterior showed the lowest strength loss (29.6% and 32%, respectively), whereas the other materials lost between 41% and 62% of their flexural strength after cyclic loading.

Conclusions

Dental ceramics and resin composite materials show equivalent fatigue strength degradation at loads around 0.5σ_in values. Apart from the zirconium oxide and the lithium disilicate ceramics, resin composites generally showed better σ_ff after 10,000 cycles than the fluorapatite glass-ceramic and the feldspathic porcelain. Resin composite restorations may be used as an equivalent alternative to glass-rich-ceramic inlays regarding mechanical performance.     

Fracture toughness (KIc) of a dental porcelain determined by fractographic analysis.

OBJECTIVES: Fractographic analysis of indentation cracks is performed following flexure testing as part of the ASTM (1999) standard for fracture toughness, KIc, determination in advanced ceramics. This method depends on the conduciveness of the material towards fractographic interpretation. The purpose of this study was to evaluate the use of fractography in fracture toughness methods with a feldspathic dental porcelain, in which KIc was measured fractographically as well as numerically using two controlled-flaw beam bending techniques. METHODS: The following methods for KIc determination were applied to a dental porcelain containing a leucite volume fraction of 15-20%: (1) surface crack in flexure (SCF) (dependent upon fractographic analysis); and (2) indentation strength (IS) at indentation loads of 9.8 and 19.6 N (applying both standard numeric calculations and fractographic analysis). The testing environments were (1) ambient air (IS and SCF) and (2) flowing dry nitrogen (SCF). RESULTS: No significant differences were found between numeric and fractographic KIc values for the IS technique at both indentation loads (9.8 and 19.6 N) in ambient air, although KIc values were sensitive to indentation load. Due to the presence of residual stresses, stable crack extension was observed fractographically in all IS specimens, as evidenced by differences between initial (ainitial) and critical (acritical) crack dimensions. For the SCF method, there was a significant difference in toughness between specimens tested in air versus dry nitrogen, however no fractographic evidence for chemically assisted slow crack growth (SCG) was observed. SIGNIFICANCE: The SCF method as described by the ASTM standard was applicable to the feldspathic porcelain and produced very comparable results with the numeric toughness calculations of the IS procedure. However, fractographic analysis of the surface crack was somewhat difficult for this glassy ceramic compared with polycrystalline ceramics. Knowledge about stable crack extension or slow crack growth and its fractographic appearance is essential when estimating the toughness from examination of flaw dimensions on fractured surfaces since large calculation errors may occur if these effects are not taken into account.     

Cyclic fatigue in water of veneer-framework composites for all-ceramic dental bridges.

OBJECTIVES: Ceramic materials applied in dentistry may exhibit significant subcritical crack growth due to the severe cyclic loading in the aqueous environment encountered in the mouth during mastication. The authors report on the subcritical crack growth behavior of three dental restoration systems (Empress 2/IPS Eris, TZP/Cercon S and Inceram-Zirconia/Vita VM7) under cyclic loading in water, in order to establish guidelines for the use and design of long-lifetime all-ceramic posterior bridges. METHODS: Inert strength and lifetime tests under cyclic loading in an aqueous environment were performed in a mechanical bending apparatus and evaluated with Weibull statistics. RESULTS: Subcritical crack growth occurred predominantly in the outer veneer layer of the veneer-framework composites. The apatite-based veneer (IPS Eris) was more susceptible to subcritical crack propagation compared to the feldspathic glass veneers (Cercon S and Vita VM7). SIGNIFICANCE: Dental restoration systems containing apatite-based veneers and weak frameworks (Empress 2/IPS Eris) are not recommended for the fabrication of all-ceramic bridges in the molar region. Conversely, veneer-framework systems consisting of feldspathic glass veneers and tough zirconia-based frameworks (TZP/Cercon S and Inceram-Zirconia/Vita VM7) may exhibit lifetimes longer than 20 years if the bridge connector is properly designed.     

Contrast ratio of veneering and core ceramics as a function of thickness

PURPOSE: The recent development of several dental ceramic products has raised questions concerning the relative translucency potential of these materials. The objective of this study was to test the hypothesis that the contrast ratio of dental core and veneering ceramics is a linear function of ceramic thickness. MATERIALS AND METHODS: Four groups of disk-shaped core ceramic specimens and four groups of veneering ceramic specimens (15 mm in diameter and 0.70, 1.10, 1.25, or 1.50 mm in thickness) were prepared for analysis. Five disks were randomly assigned to each of the eight groups. Four core ceramics were selected for study: (1) tetrasilicic fluormica glass ceramic, (2) quadruple-chain silicate glass ceramic, (3) barium silicate glass ceramic, and (4) sintered alumina. The four veneering ceramics included two feldspathic body porcelains, one fine-grained veneering porcelain, and one ultralow-fusing porcelain. RESULTS: There were significant differences among the mean contrast ratio values of these materials. The most translucent group of the core materials was tetrasilicic fluormica glass ceramic, and the least translucent material was sintered alumina. The most translucent group among the veneering ceramics was one of the feldspathic ceramics (Ceramco), and the least translucent material for all thicknesses was the ultralow-fusing veneering ceramic (Duceram LFC). The mean contrast ratio values were significantly different at a thickness of 1.50 mm of the four core ceramic groups and among the four veneering ceramic groups. CONCLUSION: The results of this study indicate that the tetrasilicic fluormica glass ceramic is generally the most translucent core ceramic for thicknesses of 0.70, 1.10, and 1.25 mm, and Ceramco porcelain is the most translucent veneering ceramic. However, for a thickness of 1.50 mm, the quadruple-chain silicate glass ceramic was the most translucent core ceramic. The relationship between contrast ratio and thickness was linear for all ceramics except Vita VMK 68.     

粉浆涂塑铝瓷冠的匹配性研究

粉浆涂塑铝瓷冠核冠与面瓷间良好的匹配性是保证铝瓷冠修复成功的重要前提。作者采用扫描电镜，Ｘ射线能谱仪观察和热休克试验三种方法，观察了两种粉浆涂塑铝瓷冠（Ｉｎ－Ｃｅｒａｍ和ＧＩ－Ｉ）的匹配性，结果表明，Ｉｎ－Ｃｅｒａｍ的热休克抗力的１５８℃，ＧＩ－热休克抗力为１７２℃，两种铝瓷冠面瓷与铝瓷核冠间界面均结合紧密，界面两侧有元素扩散现象。提示Ｉｎ－Ｃｅｒａｎ和ＧＩ－Ｉ型粉浆涂塑铝瓷冠的匹配性均良好。     

Shear bond strength of composite resin to porcelain

This study evaluated the effect of porcelain surface treatment on the shear bond strength of composite resin to various porcelains and porcelain combinations. A variety of feldspathic porcelains with low and medium alumina content were tested. Porcelain/composite resin samples were stored in 37 degrees C water, thermocycled 1,000 times, and tested in shear. A 3-minute etching using hydrofluoric acid significantly increased the bond strength of most of the feldspathic porcelains with low and medium alumina content. Silane application to all types of etched porcelain had no significant effect on bond strength.     

Slow crack growth and reliability of dental ceramics

Objective

To determine the slow crack growth (SCG) and Weibull parameters of five dental ceramics: a vitreous porcelain (V), a leucite-based porcelain (D), a leucite-based glass-ceramic (E1), a lithium disilicate glass-ceramic (E2) and a glass-infiltrated alumina composite (IC).

Methods

Eighty disks (Ø 12 mm × 1.1 mm thick) of each material were constructed according to manufacturers’ recommendations and polished. The stress corrosion susceptibility coefficient (n) was obtained by dynamic fatigue test, and specimens were tested in biaxial flexure at five stress rates immersed in artificial saliva at 37 °C. Weibull parameters were calculated for the 30 specimens tested at 1 MPa/s in artificial saliva at 37 °C. The 80 specimens were distributed as follows: 10 for each stress rate (10⁻², 10⁻¹, 10¹, 10² MPa/s), 10 for inert strength (10² MPa/s, silicon oil) and 30 for 10⁰ MPa/s. Fractographic analysis was also performed to investigate the fracture origin.

Results

E2 showed the lowest slow crack growth susceptibility coefficient (17.2), followed by D (20.4) and V (26.3). E1 and IC presented the highest n values (30.1 and 31.1, respectively). Porcelain V presented the lowest Weibull modulus (5.2). All other materials showed similar Weibull modulus values, ranging from 9.4 to 11.7. Fractographic analysis indicated that for porcelain D, glass-ceramics E1 and E2, and composite IC crack deflection was the main toughening mechanism.

Significance

This study provides a detailed microstructural and slow crack growth characterization of widely used dental ceramics. This is important from a clinical standpoint to assist the clinician in choosing the best ceramic material for each situation as well as predicting its clinical longevity. It also can be helpful in developing new materials for dental prostheses.     

A comparative study of the strength of aluminous porcelain and all-ceramic crowns

This study compared the breaking strengths of all-ceramic crowns constructed with aluminous porcelain, the Cerestore system, and the Dicor system on an anterior tooth preparation. Fifteen crowns from each group were constructed to comparable dimensions on dies from the same master die. Ten crowns from the same group were cemented on the master die and loaded until catastrophic failure. Five crowns from each group were embedded and sectioned to examine internal adaptation. No significant difference was found between the load necessary to produce failure in the aluminous porcelain and Dicor crowns and the load needed to initiate crack formation in the Cerestore crowns. However, seven of the 10 Cerestore crowns showed a two-phase failure pattern; the difference between crack initiation and catastrophic failure was significant. Within the limitation of this study, neither the Cerestore (high alumina core) nor the Dicor (castable glass-ceramic) produced an all-ceramic crown superior to the current aluminous porcelain crown.     

Correlation of topography to bond strength of etched ceramic

PURPOSE: Bonding techniques are commonly used today to ensure the durability of porcelain laminate veneers and ceramic inlay-onlays, which are esthetically pleasing solutions to the problem of dental restoration. Acid etching and silanization of porcelain have been widely used to enhance the retention between bonding resins and ceramic restorations. The purpose of this study was to evaluate the effect of surface modifications with 10% hydrofluoric acid gel and a coupling agent (a dual component silane) on the bond strength of 2 dental feldspathic ceramics (GC and PVS) bonded with an unfilled resin (Super-Bond). MATERIALS AND METHODS: Eighty samples of GC and eighty samples of PVS were built for a mechanical study. Mechanical tests were conducted using a Lloyd T 6000 R tensile machine, which determined the bond strengths of the 2 ceramics after different surface treatments (etching or silanization). In the next part of the study, 10 surfaces of GC and 10 surfaces of PVS were studied with a scanning mechanical microscope to evaluate the action of hydrofluoric acid gel on the roughness of the 2 ceramics. RESULTS: Etching ceramic with hydrofluoric acid gel increased the developed surface of feldspathic ceramics, especially for PVS, but this treatment was not sufficient to obtain the highest shear bond strength. The highly positive influence of silanization was shown, particularly for GC ceramic bonded with an unfilled resin. CONCLUSION: Silane combined with the action of hydrofluoric acid gel is the most effective surface treatment for ceramic.     

牙科陶瓷循环和静态疲劳的对比实验

目的 比较牙科陶瓷循环和静态加载后的强度下降规律以及结构损伤差异。方法应用赫兹触压实验方法对可切削陶瓷标准试件（VitamarkⅡ）进行循环和静态加载，测试试件的残余三点挠曲强度，观测试件表面及表面下损伤，数据进行单因素方差分析。结果试件在两种加载模式下，首次加载后残余强度均降低，同时出现外锥型损伤。循环加载至1000次时，内锥型损伤出现并伴随强度进一步降低，差异有统计学意义（P〈0．05）。不同加载时间下的静态加载试件强度下降的差异无统计学意义，同时不伴随内锥状裂纹的产生。结论静态加载和循环加载都能降低陶瓷试件的强度，但循环加载使损伤更严重。