Comparison of three fracture toughness testing techniques using a dental glass and a dental ceramic.

OBJECTIVES: Various methods aimed at determining the fracture toughness of ceramics in mode I (KIc) have been described in the literature. The accuracy, scatter and the interexaminer reproducibility of KIc depend strongly on the procedural approach, the test parameters used and the conditioning of the specimens. The purpose of the present study was to compare fracture toughness values obtained using two indentation methods as well as a newly established fracture mechanics test. METHODS: The following methods for KIc determination were applied: (1) indentation fracture (IF), (2) indentation strength (IS) and (3) the single-edge-V-notched-beam test (SEVNB). The materials tested were a low-fusing dental glass (Duceram LFC) and a feldspar-based porcelain (IPS classic). Data were compared by ANOVA and Tukey's multiple comparison test (p < or = 0.05). RESULTS: For both materials, KIc coefficients of variation ranged between 10 and 14% for IF and 7 and 10% for IS. The IS technique demonstrated a load dependency for the IPS porcelain which was not observed when using the IF method. The SEVNB test provided consistent results with coefficients of variation between 1 and 3%. SEVNB toughness values for the IPS porcelain were in agreement with the IS technique. However, halfpenny shaped cracks were observed at the tip of the notch of all LFC specimens thus leading to underestimated KIc values. SIGNIFICANCE: The overall aim of this type of study is to select testing procedures that are as expedient and reliable as possible. This study has shown that all three methods agreed within 10%. However none of the procedures proved absolutely straightforward. Decision on which method to use should be based on a sound understanding of the conceptual limitations and technical difficulties inherent to each technique.     

Fracture toughness comparison of three test methods with four dental porcelains.

OBJECTIVE: The aim of this study was to compare three fracture toughness test methods, using four commercial dental porcelains. MATERIALS AND METHODS: The fracture toughness test techniques involved were: the single-edge-notched beam (SENB), the indentation strength method (IS), and a rather convenient ASTM standard for advanced ceramics, which is still rarely used in dental ceramic research, the Chevron-notched beam method (CN). Duceram, Duceram LFC, Sintagon Zx and Carrara Vincent were chosen for study. Data was analyzed by two-way and paired ANOVA. RESULT: No statistical difference was found between the CN and SENB methods with four dental porcelains, but IS was not always in statistical agreement with SENB or CN. Statistical agreement among all three methods occurred only with Duceram LFC. CONCLUSION: The different test methods did not always lead to the same ranking or values of fracture toughness. Yet the toughness results of the SENB method were comparable to those of the CN method for all the four dental porcelains tested in this study.     

Flexural strength and fracture toughness of dental core ceramics

STATEMENT OF PROBLEM: Many different strengthened all-ceramic core materials are available. In vitro study of their mechanical properties, such as flexural strength and fracture toughness, is necessary before they are used clinically. PURPOSE: The purpose of this study was to evaluate and compare the mechanical properties of 6 commonly used all-ceramic core materials using biaxial flexural strength and indentation fracture toughness tests. MATERIAL AND METHODS: Specimens of 6 ceramic core materials (Finesse, Cergo, IPS Empress, In-Ceram Alumina, In-Ceram Zirconia, and Cercon Zirconia) were fabricated (n=25) with a diameter of 15 mm and width of 1.2 +/- 0.2 mm. For each group, the specimens were tested to compare their biaxial flexural strength (piston on 3 balls) (n=15), Weibull modulus, and indentation fracture toughness (n=10) (IF method). The data were analyzed with 1-way ANOVA test (a=.05). The Tamhane multiple comparison test was used for post hoc analysis. RESULTS: Mean (SD) of biaxial flexural strength values (MPa) and Weibull modulus (m) results were: Finesse (F): 88.04 (31.61), m=3.17; Cergo (C): 94.97 (13.62), m=7.94; IPS Empress (E): 101.18 (13.49), m=10.13; In-Ceram Alumina (ICA): 341.80 (61.13), m=6.96; In-Ceram Zirconia (ICZ): 541.80 (61.10), m=10.17; and Cercon Zirconia (CZ): 1140.89 (121.33), m=13.26. The indentation fracture toughness results showed that there were significant differences between the tested ceramics. The highest fracture toughness values (MPa x m(0.5)) were obtained with the zirconia-based ceramic core materials. CONCLUSIONS: Significant differences were found in strength and toughness values of the materials evaluated. Cercon Zirconia core material showed high values of biaxial flexural strength and indentation fracture toughness when compared to the other ceramics studied.     

Fracture toughness and hardness evaluation of three pressable all-ceramic dental materials

OBJECTIVES: This study evaluates the fracture toughness and hardness of three pressable all-ceramic materials: IPS-Empress, Empress 2 and an experimental ceramic material. METHODS: Fifteen discs and 15 bars per material were prepared. Fracture toughness was measured with two different techniques: indentation fracture and indentation strength. During the indentation fracture tests the hardness of each material was also measured. Statistical significance among groups of population was studied using one-way Anova and Tukey's multiple comparison tests. RESULTS: Fracture toughness results using the indentation strength technique (with three-point bending and biaxial flexure tests) were: IPS-Empress (1.39 (SD 0.3) and 1.32 (SD 0.3)); Empress 2 (3.14 (SD 0.5) and 2.50 (SD 0.3)) MPa x m(1/2); and the experimental ceramic (3.32 (SD 0.6) and 2.43 (SD 0.3)) MPa x m(1/2). The indentation fracture technique generated orthogonal cracks of different lengths for Empress 2 and the experimental ceramic, whether perpendicular or parallel to the lithium disilicate elongated crystals. Thus, two values were reported: Empress 2 (1.5 (SD 0.2) and 1.16 (SD 0.2)) MPa x am(1/2) and the experimental ceramic (1.67 (SD 0.3) and 1.15 (SD 0.15)) MPa x m(1/2). The IPS-Empress indentation fracture result was 1.26 (SD 0.1). The hardness results were: 6.6, 5.3 and 5.5 GPa for IPS-Empress, Empress 2 and the experimental ceramic, respectively. CONCLUSIONS: No significant differences in fracture toughness and hardness results were found between Empress 2 and the experimental ceramic (P>0.05 ANOVA). Both materials exhibited fracture toughness anisotropy following pressing. They demonstrated improved fracture toughness and reduced hardness compared with IPS-Empress P<0.05(ANOVA), which should be beneficial for clinical applications.     

Mechanical properties of In-Ceram Alumina and In-Ceram Zirconia

PURPOSE: This study compared the mechanical properties of In-Ceram Zirconia and In-Ceram Alumina. MATERIALS AND METHODS: Ninety-four disks and six bars were prepared with the slip-casting technique. The disks were used to assess biaxial flexural strength (piston on three ball), Weibull modulus, hardness, and fracture toughness with two methods: indentation fracture and indentation strength. The bars were used to measure elastic moduli (Young's modulus and Poisson's ratio). X-ray diffraction analysis of the specimens was carried out upon every step of the specimen preparation and of the fractured surfaces. RESULTS: Mean biaxial flexure strengths of In-Ceram Alumina and In-Ceram Zirconia were 600 MPa (SD 60) and 620 MPa (SD 61), respectively. Mean fracture toughness measured according to indentation strength was 3.2 MPa.m1/2 (SD 0.34) for in-Ceram Alumina and 4.0 MPa.m1/2 (SD 0.43) for In-Ceram Zirconia. Mean fracture toughnesses of In-Ceram Alumina and In-Ceram Zirconia measured according to indentation fracture were 2.7 MPa.m1/2 (SD 0.34) and 3.0 MPa.m1/2 (SD 0.48), respectively. X-ray diffraction analysis showed that little phase transformation from tetragonal to monoclinic occurred when the specimens were fractured, supporting the existence of a modest difference of fracture toughness between the two ceramics. CONCLUSION: No statistically significant difference was found in strength. In-Ceram Zirconia was tougher (P < .01) than In-Ceram Alumina when tested according to indentation strength. However, no significant difference was found in the fracture toughness when tested with the indentation fracture technique.     

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.     

An evaluation of the technique sensitivity of a hydrothermal low-fusing dental ceramic

OBJECTIVES: To investigate the technique sensitivity in the manipulation of Duceram Low-Fusing Ceramic, a hydrothermal dental veneering material by employing bi-axial flexure tests. METHODS: Disc shaped specimens of Duceram-LFC dentine powder were condensed from varying powder contents manipulated with 0.33 ml of liquid. Bi-axial flexure (ball-on-ring) testing was employed to determine the mean bi-axial flexure strength, standard deviations and Weibull modulus (m). Apparent solid density and apparent porosity were also measured. RESULTS: Mean bi-axial flexure strengths and standard deviations of specimens condensed from 0.82 g of powder were 66.47 and 9.62MPa (m=7.23, 1.32) compared with 59.12 and 13.62 MPa (m=4.19, 0.77) and 63.91 and 14.51MPa (m=4.46, 0.81) for powder contents of 0.78 and 0.86 g, respectively. A decrease in apparent solid density and an increase in apparent porosity were also associated with increasing or decreasing the powder content of slurry consistencies. CONCLUSIONS: A slurry consistency was identified for Duceram-LFC specimens where the reliability of fracture strength data increased. The technique sensitivity in the manipulation of Duceram-LFC would appear to be confirmed due to the relatively small amount of powder required to complete the transition from a 'fluid' to a 'thick' slurry consistency compared with a conventional veneering dental porcelain.     

Mechanism of strength increase for a hydrothermal porcelain.

OBJECTIVES: The objectives of this study were to verify the formation of a hydrolyzed surface layer on Duceram LFC, to determine the effects of such a layer on mechanical material properties, and to identify a specific mechanism responsible for any strength increase observed. METHODS: Specimens were fabricated from dentin porcelain by a vibration blotting technique and were prepared to have either blunt or sharp surface flaws. Half of the specimens underwent accelerated aging. Specimens were fractured in three-point flexure to measure their strength, and fractographic analysis was used to determine fracture toughness and residual surface stress. Surface hardness and elastic modulus were measured using a microindentation method. Porcelain surface topography was examined using atomic force microscopy, and Fourier transform infrared spectroscopy was used to determine the composition of the surface layer. RESULTS: The aging treatment modified the porcelain surface topography but did not create a layer with increased hydroxyl ion content. Porcelain strength increased upon aging, and the increase was proportional to initial flaw severity. The apparent fracture toughness of sharp flaw specimens increased to match that for specimens containing blunt flaws upon aging. Surface hardness and elastic modulus decreased upon aging. SIGNIFICANCE: Previous studies on the strength increase of hydrothermal porcelain were contradictory because a variety of specimen preparation procedures were used. This study resolves the apparent contradiction by determining the effect of specimen preparation on material strength.     

Fracture toughness (K(IC) of a hot-pressed core ceramic based on fractographic analysis of fractured ceramic FPDs

PURPOSE: The objective of this study was to test the hypothesis that there is no significant difference between the fracture toughness (K(IC)) of an experimental hot-pressed core ceramic measured by fractographic analysis of failed ceramic prostheses and the values determined by other standard methods. MATERIALS AND METHODS: Four groups were subjected to one of four test methods: group 1 = indentation strength technique (standard numerical calculation); group 2 = indentation strength technique (fractographic analysis); group 3 = flexure test of precracked specimens (fractographic analysis); and group 4 = fractographic analysis of failed three-unit fixed partial dentures (FPD). For groups 1 to 3, 20 ceramic bar specimens were subjected to three-point flexure at a cross-head speed of 0.5 mm/min until fracture occurred. For group 4, 10 failed FPDs were collected from a previous study. Stress values at failure were calculated from either a flexure stress equation (groups 1 to 3) or from finite element analyses (group 4). K(IC) values were calculated from an equation and fractographic measurement data. RESULTS: Mean fracture toughness ranged from 3.1 MPa x m1/2 (SD 0.2) (group 1) to 3.4 MPa x m1/2 (SD 0.2) (group 4). The mean K(IC) value for group 1 was significantly different from that of group 4; however, no significant differences were found between groups 1, 2, and 3, or between groups 2, 3, and 4. CONCLUSION: Fracture toughness for an experimental hot-pressed core ceramic measured by fractographic analysis in combination with finite element analysis was comparable with the values determined with other standard fractographic methods.     

Additive manufacturing of lithium disilicate glass-ceramic by vat polymerization for dental appliances

ObjectivesThe objectives of this study were to evaluate the mechanical properties of lithium disilicate components produced by additive manufacturing (AM) and to assess the effect of build orientation on the resistance to fracture.MethodsOversized bars were printed with a glass-filled photoactive resin using a digital light processing technique. After sintering and post-processing, flexure and chevron notch fracture toughness bars were obtained in three principal orientations (0°, 45°, and 90°) with respect to the build direction. Mechanical properties were obtained according to the relevant ASTM standards. The hardness, indentation fracture resistance, and elastic modulus were measured for each orientation, and a Weibull analysis was conducted with the flexure responses. Fractography of the fracture surfaces was performed to identify the failure origins.ResultsThe 0° orientation exhibited characteristic strength, Weibull modulus, and elastic modulus of 313 MPa, 4.42, and 168 ± 3 GPa, respectively, which are comparable to lithium disilicate materials from traditional processes. However, build orientation contributed significantly to the flexure strength, elastic modulus, and Weibull modulus; the characteristic strengths for the 45° and 90° build orientations were 86 MPa and 177 MPa, respectively. The primary contribution to the orientation dependence was the number of residual build layer-related flaws from incomplete union between printed layers. Of note, hardness and the fracture toughness were not dependent on build orientation.SignificanceAM of lithium disilicate materials can achieve the mechanical properties of materials produced by traditionally processing. Thus, while further process development is warranted, the outlook for dentistry is promising.     

Evaluation of low-fusing ceramic systems combined with titanium grades II and V by bending test and scanning electron microscopy

The bond strength by three point bending strength of two metal substrates (commercially pure titanium or grade II, and Ti-6Al-4V alloy or grade V) combined to three distinct low-fusing ceramic systems (LFC) and the nature of porcelain-metal fracture by scanning electron microscopy (SEM) were evaluated. The results were compared to a combination of palladium-silver (Pd-Ag) alloy and conventional porcelain (Duceram VMK68). Sixty metal strips measuring 25x3x0.5mm were made - 30 of titanium grade II and 30 of titanium grade V, with application of the following types of porcelain: Vita Titankeramik, Triceram or Duceratin (10 specimens for each porcelain). The porcelains were bonded to the strips with dimensions limited to 8x3x1mm. The control group consisted of ten specimens Pd-Ag alloy/Duceram VMK68 porcelain. Statistical analyses were made by one-way analysis of variance (ANOVA) and Tukey test at 5% significance level. Results showed that the bond strength in control group (48.0MPa ± 4.0) was significantly higher than the Ti grade II (26.7MPa ± 4.1) and Ti grade V (25.2MPa ± 2.2) combinations. When Duceratin porcelain was applied in both substrates, Ti grade II and Ti grade V, the results were significantly lower than in Ti grade II/Vitatitankeramik. SEM analysis indicated a predominance of adhesive fractures for the groups Ti grade II and Ti grade V, and cohesive fracture for control group Pd-Ag/Duceram. Control group showed the best bond strength compared to the groups that employed LFC. Among LFC, the worst results were obtained when Duceratin porcelain was used in both substrates. SEM confirmed the results of three point bending strength.     

An evaluation of the mechanical properties of 'hydrothermal' dental glass after water immersion and surface polishing.

OBJECTIVES: To investigate claims that Duceram low-fusing ceramic (Duceram-LFC), a 'hydrothermal' dental veneer will increase in strength after hydrolytic testing and surface polishing.Methods. Using pre-determined powder/liquid slurry constituents, nominally identical Duceram-LFC disc-shaped specimens were fabricated to obtain mean bi-axial flexure strengths, standard deviations, Weibull modulus (m) and Vickers hardness. Thirty specimens were tested 'dry' (control), after 24 and 48h immersion in a water bath maintained at 37+/-1 degrees C and a further 30 samples polished to a 6 micrometer nap finish. RESULTS: Mean bi-axial flexure strengths of specimens tested dry were found to be 67.04+/-13.87MPa (m=7.23+/-1.32) compared with 61.01+/-15.23MPa (m=4.29+/-0.78) and 68.42+/-23.36MPa (m=3.29+/-0.60) for 24 and 48h immersion, respectively. Following surface polishing, the bi-axial flexure strength was 95.91+/-22.4MPa (m=4.37+/-0.8). Vickers hardness numbers showed no significant difference between the dry control group and after 24 and 48h immersion (P>0.05). SIGNIFICANCE: Results obtained do not support claims for surface flaw and/or defect healing through the formation of an SiOH layer at the surface of Duceram-LFC specimens over time. However, the polishing regime interrupted the defect population associated with flaws present in dry (control) samples of Duceram-LFC which can be attributed to an associated reduction of initial surface flaws and/or defects.     

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.     

Fracture toughness determination of two dental porcelains with the indentation strength in bending method.

OBJECTIVE: This study was to investigate the influence of the bending test configurations and the crosshead displacement speeds on the fracture toughness (K(Ic)) of dental porcelains obtained with the indentation strength in bending (ISB) method. METHODS: The strength of the dental veneering porcelains Duceram and Sintagon Zx, Vickers' indented at a load of 2 kg was measured at a crosshead speed of 0.5mm/min with three test configurations, which were 3-point, 4-point, and biaxial bending. Two more groups of Sintagon Zx were tested the same way, but at speeds of 0.1, and 0.05 mm/min, respectively. Both porcelains, the three crosshead speeds, and the three test configurations were compared statistically. RESULTS: Duceram had a higher toughness than Sintagon Zx with all three test configurations and there was no significant difference between three test configurations with either porcelain. Within the crosshead speed groups of Sintagon Zx, a significant difference was found only in the 0.5mm/min group between the 3-point, and 4-point configurations. Within the configuration groups, significant differences were found between all speeds with the 3-point configuration and only between the highest and lowest speed with the 4-point and the biaxial tests. CONCLUSION: The crosshead displacement speed can cause statistically different results of fracture toughness obtained with the ISB method.     

Fracture behavior of lithia disilicate- and leucite-based ceramics

OBJECTIVE: This study was designed to characterize the fracture behavior of ceramics and test the hypothesis that variation in strength is associated with a variation in fracture toughness. METHODS: The following four groups of 20 bar specimens (25 x 4 x 1.2 mm) were fabricated (ISO standard 6872): E1, a hot-pressed leucite-based core ceramic (IPS Empress); E2, a hot-pressed lithia-based core ceramic (IPS Empress 2); ES, a hot-pressed lithia-based core ceramic (Experimental); and GV, a glass veneer (IPS Empress2 body). Specimens were subjected to four-point flexure loading in 37 degrees C distilled water. Fractographic analysis was performed to determine the fracture origin (c) for calculation of fracture toughness (KIC). Weibull analysis of flexure strength (sigma) data was also performed. RESULTS: Differences in mean sigma and KIC were statistically significant for E1 and GV (p<0.05). These differences are associated with processing effects and composition. SIGNIFICANCE: The higher mean sigma and KIC values of E2 and ES core ceramics suggest potentially improved structural performance compared with E1 although the Weibull moduli of E1 and E2 are the same.     

Evaluation of translucency,Marten’s hardness,biaxial flexural strength and fracture toughness of 3Y-TZP, 4Y-TZP and 5Y-TZP materials

ObjectivesTesting and comparing of different non-shaded zirconia materials (3Y-TZP, 4Y-TZP and 5Y-TZP) on optical and mechanical properties.Materials and methodsZirconia materials (N = 320, Opaque O, Translucent T, Extra Translucent ET, High Translucent HT) were investigated on translucency, Martens parameter, biaxial flexural strength, Chevron-Notch-Beam (CNB) fracture toughness (K_IC) and grain size. The grain size was analyzed using a scanning electron microcopy (SEM). Univariate ANOVA, post-hoc Scheffé, partial eta-squared, Kolmogorov-Smirnov-, Kruskal-Wallis- and Mann-Whitney-U-tests (p < 0.05) were performed. The reliability of flexural strength was calculated with two-parametric Weibull analysis and 95 % confidence level.ResultsThe translucency of ET and HT increased with the thermo-mechanical aging (p < 0.001). The zirconia material and aging had no impact on the Martens hardness and the indentation modulus. ET showed the highest flexural strength values after initial and thermo-mechanical aging (p < 0.001 – 0.683). All four materials showed the highest flexural strength after thermo-mechanical aging after 1.2 Mio cycles. Thermo-mechanically (1.2 Mio cycles) aged HT presented the highest Weibull modulus (m = 15.0) regardless of aging. Within initial groups, T (p ≤ 0.001) showed the highest fracture toughness, followed by O (p ≤ 0.001), ET (p < 0.003) and HT (p ≤ 0.001).SignificanceTranslucency of ET and HT increases with thermo-mechanical aging. Chevron-Notch-Beam (CNB) is a valid alternative to the single-edge-V-notched beam (SEVNB) method for testing fracture toughness.     

Apparent interfacial fracture toughness of resin/ceramic systems

We suggest that the apparent interfacial fracture toughness (K(A)) may be estimated by fracture mechanics and fractography. This study tested the hypothesis that the K(A) of the adhesion zone of resin/ceramic systems is affected by the ceramic microstructure. Lithia disilicate-based (Empress2-E2) and leucite-based (Empress-E1) ceramics were surface-treated with hydrofluoric acid (HF) and/or silane (S), followed by an adhesive resin. Microtensile test specimens (n = 30; area of 1 +/- 0.01 mm(2)) were indented (9.8 N) at the interface and loaded to failure in tension. We used tensile strength (sigma) and the critical crack size (c) to calculate K(A) (K(A) = Ysigmac(1/2)) (Y = 1.65). ANOVA and Weibull analyses were used for statistical analyses. Mean K(A) (MPa.m(1/2)) values were: (E1HF) 0.26 +/- 0.06; (E1S) 0.23 +/- 0.06; (E1HFS) 0.30 +/- 0.06; (E2HF) 0.31 +/- 0.06; (E2S) 0.13 +/- 0.05; and (E2HFS) 0.41 +/- 0.07. All fractures originated from indentation sites. Estimation of interfacial toughness was feasible by fracture mechanics and fractography. The K(A) for the systems tested was affected by the ceramic microstructure and surface treatment.     

Fracture toughness of packable and conventional composite materials

STATEMENT OF PROBLEM: The introduction of packable composite has expanded the choices of materials for the restoration of posterior teeth. Few independent studies are available on the fracture toughness (K(IC)) of the presently available packable composites compared with more conventional composite alternatives. PURPOSE: This investigation evaluated the relative fracture toughness of 3 packable composites, 2 conventional composites, and 1 laboratory-processed composite. MATERIALS AND METHODS: Six composite materials were tested in this study. These included: 3 packable composites (Alert, SureFil and Solitaire), 2 conventional composites (Herculite and Heliomolar), and 1 laboratory-processed composite (Belleglass). K(IC) was determined by preparing 8 mini-compact test specimens (8.2 mm diameter x 2 mm thickness) for each composite in a polytetrafluoroethylene split-mold with introduced precracks created with a razor blade. Specimens were stored in distilled water at 37 degrees +/- 2 degrees C for 7 days. Testing was performed on a universal testing machine at a displacement rate of 0.5 mm/min until fracture. Analysis of variance (P<.0001) and Ryan-Einot-Gabriel-Welsch multiple range tests (P<.05) were performed on all data. RESULTS: The mean fracture toughness of Alert (1.57 Mpa x m(1/2)) was significantly greater than any of the other composites tested. Solitaire, a packable composite, exhibited a mean fracture toughness (0.67 MPa x m(1/2)) that was significantly lower than any of the other materials tested. No significant difference was noted between Belleglass (1.27 MPa x m(1/2)), SureFil (1.25 MPa x m(1/2)) and Herculite (1.16 MPa x m(1/2)). CONCLUSION: Within the limitations of this study, the glass fiber-reinforced packable composite exhibited improved fracture toughness when compared with the other composite materials tested.     

Machinable glass-ceramics forming as a restorative dental material

MgO, SiO(2), Al(2)O(3), MgF(2), CaF(2), CaCO(3), SrCO(3), and P(2)O(5) were used to prepare glass-ceramics for restorative dental materials. Thermal properties, phases, microstructures and hardness were characterized by DTA, XRD, SEM and Vickers microhardness. Three-point bending strength and fracture toughness were applied by UTM according to ISO 6872: 1997(E). XRD showed that the glass crystallized at 892°C (second crystallization temperature+20°C) for 3 hrs consisted mainly of calcium-mica and fluorapatite crystalline phases. Average hardness (3.70 GPa) closely matched human enamel (3.20 GPa). The higher fracture toughness (2.04 MPa√m) combined with the hardness to give a lower brittleness index (1.81 μm(-1/2)) which indicates that they have exceptional machinability. Bending strength results (176.61 MPa) were analyzed by Weibull analysis to determine modulus value (m=17.80). Machinability of the calcium mica-fluorapatite glass-ceramic was demonstrated by fabricating with CAD/CAM.     

Ceramic whisker reinforcement of dental resin composites

Resin composites currently available are not suitable for use as large stress-bearing posterior restorations involving cusps due to their tendencies toward excessive fracture and wear. The glass fillers in composites provide only limited reinforcement because of the brittleness and low strength of glass. The aim of the present study was to reinforce dental resins with ceramic single-crystalline whiskers of elongated shapes that possess extremely high strength. A novel method was developed that consisted of fusing silicate glass particles onto the surfaces of individual whiskers for a two-fold benefit: (1) to facilitate silanization regardless of whisker composition; and (2) to enhance whisker retention in the matrix by providing rougher whisker surfaces. Silicon nitride whiskers, with an average diameter of 0.4 microm and length of 5 microm, were coated by the fusion of silica particles 0.04 microm in size to the whisker surface at temperatures ranging from 650 degrees C to 1000 degrees C. The coated whiskers were silanized and manually blended with resins by spatulation. Flexural, fracture toughness, and indentation tests were carried out for evaluation of the properties of the whisker-reinforced composites in comparison with conventional composites. A two-fold increase in strength and toughness was achieved in the whisker-reinforced composite, together with a substantially enhanced resistance to contact damage and microcracking. The highest flexural strength (195+/-8 MPa) and fracture toughness (2.1+/-0.3 MPa x m(1/2)) occurred in a composite reinforced with a whisker-silica mixture at whisker:silica mass ratio of 2:1 fused at 800 degrees C. To conclude, the strength, toughness, and contact damage resistance of dental resin composites can be substantially improved by reinforcement with fillers of ceramic whiskers fused with silica glass particles.