Fracture toughness measurements of some dental core ceramics: a methodologic study

Abstract— Fracture toughness is regarded as an important property of dental ceramics. The most widely used methods for fracture toughness (K_Ic) determination are based on assessment of cracks created by hardness indentations. Different formulas have been developed for K_Ic calculations and all these methods and formulas include empirical factors based on pure ceramics, i.e. non-composite ceramics. These factors may, however, vary for a specified method for materials with different and complex structure. An important question is whether the various proposed methods and formulas lead to approximately the same numerical K_Ic values or at least to the same ranking of materials. The aim of this work was to compare two indentation methods and various formulas for calculation of K_Ic values when used on four commercial composite dental ceramics. The two applied methods and the different formulas showed substantial differences in the obtained values for one and the same material and a different ranking of various materials. It is unknown which method gives the most correct K_Ic values for these ceramic materials.     

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.     

Influence of microstructure and chemistry on the fracture toughness of dental ceramics.

OBJECTIVES: the primary aim of this research was to measure fracture toughness for several groups of dental ceramics, and determine how this property is affected by chemistry and microstructure. METHODS: Fracture toughness (KIc) values were obtained using Single Edge Precracked Beam (SEPB) and Single Edge V-Notch Beam (SEVNB) methods. Dynamic Young's modulus, which often scales with strength and has been used in explaining the microstructure/toughness relationship on a theoretical basis, was also obtained for the three groups of materials comprising this study. The first group, consisting of micaceous glass ceramics, included model materials that varied systematically in microstructure but not in chemistry. The second group, the feldspathic porcelains, varied significantly in microstructure, but little in chemistry. The ceramics comprising the third group were significantly different in both chemistry and microstructure. RESULTS: Upper toughness limits for the micaceous glass-ceramics and feldspathic porcelains were significantly raised compared to the base glasses, but remained under 2 MPa m(1/2). The highest toughnesses were associated with high percent crystallinity, large grains and high aspect ratios. The third group KIc values were 2.8 MPa m(1/2) for a lithium disilicate glass-ceramic, 3.1 MPa m(1/2) for a glass-infused alumina, and 4.9 MPa m(1/2) for zirconia. SIGNIFICANCE: the correlations between microstructural characteristics and measured properties supports theoretical predictions in the literature. From a practical standpoint, microstructural effects were found to be important, but only within a limited range; the chemistry apparently defined a band of achievable property values. This suggests very large increases in fracture toughness are unlikely to be attained by changes in microstructure alone. A functional relationship determined for the micaceous glass-ceramics enables quantitative predictions of fracture toughness based on the microstructure.     

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.     

Influence of preparation of ceramic SEVNB specimens on fracture toughness testing results.

OBJECTIVES: Fracture toughness KIc is an important mechanical property of dental ceramics. One of the most reliable methods to determine the fracture toughness of a ceramic material is the single-edge V-notched-beam (SEVNB) method. The objective of the study is to reveal and quantify the influence of the notch root radius on the fracture toughness value proved on the high strength ceramic material zirconia. METHODS: Bar specimens (n=16) made of high strength zirconia ceramics (3Y-TZP) were notched by a diamond charged cutting wheel. The notch roots were sharpened using the razor blade method with systematically varying notch root radii. The critical stress intensities as a function of the respective notch root radii of all specimens were determined in flexural strength test. RESULTS: The notch root radii showed a pronounced effect on the determined fracture toughness values. Notch root radii between 18 and 167 microm were produced. The respective determined fracture toughness values varied between 5.9 and 13.6 MPam0.5. SIGNIFICANCE: It is of decisive importance to sharpen the notch root radius of a SEVNB specimen to determine the true fracture toughness value. If the notch root radius is above a critical value, the measured fracture toughness value will be overestimated.     

白榴石含量对K2O-Al2O3-SiO2系统牙科玻璃陶瓷的断裂韧性影响的实验研究

目的：确定白榴石含量与K2O-Al2O3-SiO2系统牙科玻璃陶瓷材料断裂韧性的关系,探讨白榴石含量影响K2O-Al2O3-SiO2系统牙科玻璃陶瓷断裂韧性的机理。方法：选择5组相同组分、不同比例的玻璃陶瓷原材料,经＂白榴石微晶化＂热处理工艺加工,制成含有不同白榴石含量的K2O-Al2O3-SiO2系统牙科玻璃陶瓷试件;用单边切口弯曲梁法测试样品的断裂韧性,计算断裂韧性值;分析样品的显微结构和测定白榴石含量;对5组样品的白榴石含量和断裂韧性值数据进行相关回归统计学分析。结果：白榴石晶粒约1.0μm、在玻璃基质中分布均匀;K2O-Al2O3-SiO2系统牙科玻璃陶瓷的断裂韧性与白榴石含量有相关关系。结论：白榴石含量对K2O-Al2O3-SiO2系统牙科玻璃陶瓷的断裂韧性有明显影响,在一定范围内,该陶瓷的断裂韧性随白榴石含量增高而增大。     

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.     

Correlation between fracture toughness and leucite content in dental porcelains

OBJECTIVES: To determine the correlation between fracture toughness and leucite content in dental porcelains. The mechanisms by which leucite influences the fracture toughness of dental porcelains were also investigated. METHODS: Six porcelains were tested: A (Ceramco I/Dentsply), B (Ceramco II/Dentsitply), C (Finesse/Dentsply), D (d.Sign/Ivoclar), Cb (Cerabien/Noritake) and V (Vitadur Alpha/Vita). Bar-shaped specimens were produced, and their fracture toughness was determined by means of the single-edge precracked beam (SEPB) method. The test consisted of fracturing the specimen after a precrack was generated by a bridge-anvil device. KIc was calculated based on fracture force and size of the precrack. Microstructural analysis and determination of the leucite volume fraction were performed on polished specimens etched with 2% HF for 15s by means of scanning electron microscopy. Fractographic analysis was performed on fracture surfaces. RESULTS: Porcelains A and B presented the highest leucite contents (22%) and similar KIc values (1.23 and 1.22 MPa m1/2, respectively), significantly higher than the other materials. Porcelains C and D presented similar K(Ic) values (0.81 and 0.93 MPa m1/2, respectively), but different leucite contents (6 and 15%, respectively). Porcelain D presented higher KIc compared to porcelains Cb and V (0.71 and 0.75 MPa m1/2, respectively), which presented similar values and the lowest leucite contents (0%). Fractographic analysis showed that porcelains with higher leucite content presented higher incidence of crack deflection. CONCLUSIONS: For the materials evaluated in this study, the leucite content was directly related to KIc. The main toughening mechanism observed was crack deflection around leucite particles and clusters.     

Fracture toughness measurement of dental ceramics using the indentation fracture method with different formulas

This study examined fracture toughness (KIC) measurements obtained using the indentation fracture (IF) method with a view to improving their reliability. The KIC values of five dental ceramics were measured using the IF method with five different formulas, and the single-edge precracked beam (SEPB) method was used as a control. The elastic moduli of the dental ceramics were evaluated by dynamic hardness test. Load conditions of the dental ceramics that produced a median/radial crack for the IF method formulas were investigated. Based on the resultant c/a and P/c1.5 values, the indentation load (P) required for median/radial crack occurrence varied greatly from 29.4 to 196 N depending on the ceramic used. Among the five formulas, none of the KIC values obtained by the IF method with Miyoshi's formula differed significantly (p > 0.05) from the values obtained using SEPB method. These results suggested that, after an appropriate indentation load is determined, reliable KIC values for small dental ceramic specimens can be easily obtained using the IF method if Miyoshi's formula is used in combination with the dynamic hardness test.     

Studies on fracture toughness of dental ceramics

One important mechanical property of dental ceramics is fracture toughness, KIC, which represents the serviceability in the oral cavity, such as the resistance to marginal fracture. KIC values of several dental ceramics, natural tooth enamel and industrial ceramics were examined by use of the indentation microfracture (IM) method. This technique was based on the series of radial cracks emanating from the corners of the Vickers indentation. It was observed that appropriate load levels should be selected on each specimen to induce radial/median cracks. For feldspatic dental porcelains, larger loads of 10, 20 and 30 kg were needed to determine their KIC values in the range 1.5-2.1 MN.m-3/2. For natural tooth enamel and a new apatite-based castable glass-ceramic, a smaller force of 1 kg was sufficient to decide the respective KIC values of about 0.9 and 1.8 MN.m-3/2. The KIC values of most dental ceramics examined were slightly higher than that of soda lime glass, but less than one-third that of zirconia. It was confirmed that the IM method is simple and cost-effective for evaluation of KIC of dental ceramics.     

Effects of specimen geometry on the measurement of fracture toughness.

The objective of this study was to investigate the use of compact test specimen geometries of reduced size for the measurement of fracture toughness, KIc, of dental resin composites. Provisional fracture toughness values were determined for four dental composites by use of a reduced-size compact test specimen. The specimens were volumetrically 8% of the size of compact test specimens used in previous studies on dental composites. The test results and conditions met validity tests. These results were compared with those from previous studies that used large compact test specimens. It is concluded that the reduced-size compact test specimen is an acceptable alternative for measurement of fracture toughness.     

Relevance of in-vitro tests of adhesive and composite dental materials. A review in 3 parts. Part 2: non-standardized tests of composite materials

Summary The first part of this review examined ISO approval requirements and in vitro testing. In the second part, non-standardized test methods for composite materials are presented and discussed. Physical tests are primarily described. Analyses of surface gloss and alterations, as well as aging simulations of dental materials are presented. Again, the importance of laboratory tests in determining clinical outcomes is evaluated. Differences in the measurement protocols of the various testing institutes and how these differences can influence the results are also discussed. Because there is no standardization of test protocols, the values determined by different institutes cannot be directly compared. However, the ranking of the tested materials should be the same if a valid protocol is applied by different institutes. The modulus of elasticity, the expansion after water sorption, and the polishability of the material are all clinically relevant, whereas factors measured by other test protocols may have no clinical correlation. The handling properties of the materials are highly dependent on operators' preferences. Therefore, no standard values can be given.     

Characterizing ceramics and the interfacial adhesion to resin: I - The relationship of microstructure, composition, properties and fractography

The appeal of ceramics as structural dental materials is based on their light weight, high hardness values, chemical inertness, and anticipated unique tribological characteristics. A major goal of current ceramic research and development is to produce tough, strong ceramics that can provide reliable performance in dental applications. Quantifying microstructural parameters is important to develop structure/property relationships. Quantitative microstructural analysis provides an association among the constitution, physical properties, and structural characteristics of materials. Structural reliability of dental ceramics is a major factor in the clinical success of ceramic restorations. Complex stress distributions are present in most practical conditions and strength data alone cannot be directly extrapolated to predict structural performance.     

Effect of novel filler particles on the mechanical and wear properties of dental composites.

OBJECTIVES: The purpose of this study was to investigate the method of producing pre-polymerized fused-fiber filler modified composite (PP-FFMC) particles and the effectiveness of incorporating these novel filler particles into dental composites. METHODS: Fused-fiber filler (FFF) blocks were impregnated with composite by two different methods. Three-point flexure tests were utilized to determine which was more effective. In order to assess the effect of the addition of PP-FFMC particles, two Bis-GMA/TEGDMA based conventional composite compositions were utilized as baselines, to which the novel particles were added. Mechanical and wear tests were performed to determine the fracture toughness, biaxial flexure strength, and in vitro wear of the materials. RESULTS: Mechanical testing showed that the addition of PP-FFMC particles decreased the strength and toughness of the conventional composites. Wear tests indicated that addition of the same particles improved the wear behavior of the conventional composites. SEM analysis of the fracture surfaces indicated that the PP-FFMC particles were incorporated without creating porosity, and that fracture was transgranular through the reinforcing particles. Microscopic flaws observed in the novel particles are the likely explanation for the observed strength and toughness values. SIGNIFICANCE:The results indicate that PP-FFMC particles have the potential to improve the wear properties of dental composites, however, they adversely affect the fracture behavior. Existing processing techniques for these particles, which introduce imperfections, limit their current usefulness.     

Heat-pressed ionomer glass-ceramics. Part II. Mechanical property evaluation.

OBJECTIVES: A series of ionomer glasses based on the formula: 4.5SiO2-1.5P2O5-(X)Al2O3-4.5CaO-0.5CaF2, were investigated where X was varied from 3.0 to 1.5 in order to develop heat pressable dental ceramics. METHODS: The glasses were heat-pressed and then subjected to different heat-treatment cycles. The mechanical properties of the glass-ceramics were investigated, specimens were tested for hardness, fracture toughness (indentation method) and flexural strength (biaxial method). RESULTS: Good mechanical properties were obtained for heat-treatments at lower temperatures (i.e. 1150 degrees C). At intermediate heat-treatment temperatures the glass-ceramics were highly crystalline which did not favor the mechanical properties. There appears to be an inverse relationship between fracture toughness and flexural strength. High fracture toughness values of 2.7 (0.4) MPam0.5 were produced for the X = 2.8 glass heat-treated for 8 h at 1150 degrees C, the flexural strength was lowest for this heat-treatment. High flexural strengths of 194.4 (75.0) MPa were obtained by heat-treating the same glass for 1 h at 1150 degrees C. Increasing the hold time increases crystal size thereby increasing the extent of microcracking in the glass-ceramic thus lowering the flexural strength. Microcracks appear to increase the fracture toughness of the glass-ceramics probably by a crack termination mechanism. SIGNIFICANCE: Good flexural strength and high fracture toughness are attainable in this system, but appear to be mutually exclusive in the materials studied. With further investigation this system could provide clinically useful materials.     

Nondestructive estimation of the strength of dental ceramic materials.

OBJECTIVES: The aim of this study was to prove a nondestructive method to evaluate the strength of dental ceramic materials with respect to its potentiality and limitations. METHODS: The Young's moduli of 13 dental ceramic materials were determined by the resonance frequency method. Additionally, the flexural strengths of eight of these materials were evaluated by the four-point bending test. Strength values for the other five ceramic materials were taken from the literature. The Young's moduli were correlated with the strength values by Hook's law, respectively. RESULTS: Young's modulus for ceramics can be determined using a resonance frequency method. Fracture strain values of the ceramic materials tested (with the exception of Empress 2) have fracture strain values between 0.08 and 0.15%. A mean strain can be calculated (0.11%) and used with the value of Young's modulus to estimate fracture strength of ceramics. For the materials evaluated, predicted strength was within 39% of the measured values. SIGNIFICANCE: A non-destructive method to estimate strength of dental ceramic materials is possible, even though the accuracy of the predicted values is not very high. Nevertheless, the method permits new materials proposed for dental ceramic restorations to be screened for probable clinical success. Expensive and time-consuming testing would only need to be done on those materials which pass this initial criterion.     

Slip-cast zirconia dental roots with tunnels drilled by laser process

Zirconia was used in this study for implant dental roots because of its superior properties over other materials used as dental roots in mechanical strength, toughness, workability, and biocompatibility. Zirconia blades were formed by the slip casting method and tunnels were drilled in the laser processing. The results indicate that (1) the slip casting method has made it possible to form a complicated artificial dental root with an accurate shape and size; (2) the zirconia blade has adequate strength in occlusion; (3) the neodymium: yttrium-aluminum-garnet (Nd:YAG) laser machine can drill many tunnels of the same size in diameter by using it repeatedly under the same conditions of power, pulse width, and focus; (4) zirconia is a recommended material for laser processing from the standpoint of toughness and less heat conductivity; and (5) the opacity of zirconia to x-ray penetration presents better observation during and after implantation than other ceramics.     

Determination of elastic properties of metal alloys and dental porcelains

This study aimed to determine Young's modulus, shear modulus and Poisson's ratio of some metal alloys and dental porcelains used in fixed prosthodontics using the technique of impulse excitation of vibration. It also aimed to compare Young's modulus values of these materials with those obtained using the other two methods: the four-point flexural test and the indentation test using the ultra micro-indentation system (UMIS). Five types of metal alloys and four types of dental porcelains were tested. The samples were prepared to a rectangular shape of approximately 8 x 30 x 1.5 mm. Frequency of vibration in a sample was read when a singular elastic strike was made with an impulse tool. The elastic constants were calculated from the frequency of vibration, dimension and mass of each sample. Young's modulus values resulting from the impulse excitation of vibration are not significantly different (P<0.05) from those obtained using the flexural test and the UMIS test in most metal alloys but are different in titanium, titanium alloy and most of the dental porcelains. The technique of impulse excitation of vibration has proven to be an accurate method and is simple to operate. The elastic properties of these alloys and porcelains are essential for determining the other mechanical properties (fracture toughness) and are relevant in clinical application.