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Journal of Applied Oral Science

Print version ISSN 1678-7757On-line version ISSN 1678-7765

J. Appl. Oral Sci. vol.13 no.1 Bauru Jan./Mar. 2005 



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


Caracterização de cerâmicas e adesão à resina: I - Relação entre microestrutura, composição, propriedades e fractografia



Alvaro Della Bona

University of Passo Fundo, Dental School, Passo Fundo, Brazil





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.

Uniterms: Ceramics; Microstructure; Fracture strength; Fracture toughness; Fracture surface.


O apelo das cerâmicas como materiais odontológicos é baseado no seu peso leve, dureza alta, inércia química e características tribológicas únicas. Um dos maiores objetivos atuais das pesquisas com cerâmicas é a produção de materiais resistentes que possam oferecer uma utilização confiável para o uso odontológico. A quantificação dos parâmetros microestruturais é importante para o desenvolvimento da relação entre estrutura e propriedades. A análise quantitative da microestrutura fornece uma associação entre a composição, as propriedades físicas, e as características estruturais dos materiais. A confiabilidade estrutural das cerâmicas odontológicas é um fator importante para o sucesso clínico das restaurações cerâmicas. Distribuições complexas de estresse estão presentes na maioria das situações clínicas e, por isso, os valores isolados de resistência não podem ser diretamente extrapolados para prever a durabilidade estrutural dos materiais.

Uniterms: Cerâmica; Microestrutura; Resistência à fratura; Tenacidade de fratura; Superfície de fratura.




This two-part review is design to demonstrate (1) the relationship of microstructure, composition, ceramics properties and the resulting characteristics of the fracture surfaces, and (2) the relationship between the ceramics characterization, the surface treatment and the bonding interface to resin, which is discussed in the second part of this review.

Microscopic examination is useful for the study and characterization of materials. Examination of microstructures is often related to material properties and the information is used to predict properties and improve the design of new materials17.

Structurally, all materials are either crystalline, partially crystalline, or amorphous. Most of crystalline ceramics, except for single crystals, are actually polycrystalline because they are made up of a large number of small crystals, or grains, separated from one another by grain boundaries. The atoms are bonded less regularly along a grain boundary, and consequently, there is an interfacial or grain boundary energy similar to the surface energy. Therefore, grain boundaries are more chemically reactive than the grains themselves and this concept has been used to enhance the observation of different material phases by lightly etch the ceramic surface before microscopy analysis6.

There are important relationships between chemical composition, atomic structure, fabrication process, microstructure, and properties of polycrystalline ceramics. The role of the fabrication process, for example, is to produce microstructures with desired chemical characteristics and properties. Each processing step has the potential for producing undesirable microstructural flaws in the ceramic body that can limit its properties and reliability. Thus, the microstructure, which refers to the nature, size, shape, quantity, and distribution of the structural elements or phases in the ceramics, has a profound effect on physical properties. In addition, recent ceramic research has concentrated on developing a fundamental understanding of ceramic damage/failure modes as influenced by microstructure17,64.

Fractography has been used to quantitatively relate the stress at failure, the nature of the stress state, and the amount of residual stress relative to the sizes of the initial crack and surrounding topography20,49. Quantitative fractographic analysis of brittle fracture surfaces shows that there are characteristic markings on the surfaces that are self-similar and scale invariant, implying that fractal analysis is a reasonable approach to analyzing these surfaces16,31,79.

The specific aims of this two-paper review are as follows: (1) to demonstrate the importance of characterizing the microstructure, composition, and basic properties of dental ceramics and the relationship with surface topography and the work of adhesion (WA); (2) to point the differences in calculating the flexural strength of monolithic and multi-layered ceramic specimens; and (3) to comment on the use of Weibull modulus, fractal dimensional increment (D*), fracture toughness (KC) and microtensile bond strength test to predict the structural reliability of ceramics and its bond interface to resin.

Characterization of Microstructure, Composition and Properties of Dental Ceramics

There are several physical and mechanical properties that are used to characterize the behavior of ceramics, such as: elastic modulus, Poisson's ratio, hardness, density, fracture strength and toughness.

The elastic or Young's modulus (E) is a measure of the stiffness, or the material's resistance to elastic deformation. The greater the modulus, the stiffer the material, or the smaller the elastic strain that results from the application of a given stress. The modulus is an important design parameter used for computing elastic deflections. The best of all methods of measuring E is to measure the velocity of sound in the material.

Poisson's ratio (u) is the ratio of the lateral to axial strain. Theoretically, a typical u value for isotropic materials is 0.25, but the maximum may be as high as 0.50. It is related to the shear modulus (G) and elastic modulus [E = 2G (1 + u)].

Hardness (H) is a measure of material's resistance to plastic deformation. In a hardness test a load is placed on an indenter that is driven into the surface of the specimen. The degree to which the indenter penetrates the sample is a measure of the material's ability to resist plastic deformation. Material's properties such as tensile strength, wear resistance due to friction, and fatigue resistance have been predicted from hardness data6.

The volume of crystalline materials and their volume changes with temperature are closely related to the crystal structures. The density (r) is directly determined by the crystal structure, that is, the efficiency of atomic packing. The density, as usually measured (g/cm3), depends on the number of atoms per cubic centimeter and on the atomic weight of the constituents. The volume of a glass is largely determined by the nature of the vitreous network. The density is a minimum value for the pure network former and increases as modifier ions are added42.

The structure of each phase in dental ceramics depends greatly upon the firing conditions such as pre-heating temperature, heating rate, final firing temperature, hold-time at final temperature, atmosphere in firing oven, and the cooling rate. The coefficient of thermal expansion (CET), strength (s) values, chemical solubility, transparency, and appearance are some of the properties that show some dependency on the degree and manner to which the structure is fired. The test methods to measure ceramic properties are standardized by the ISO 6872 35.

Some studies reported the chemical composition of certain dental ceramics using wavelength dispersive spectroscopy (WDS), electron dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction XRD, and Fourier-transform infrared reflection spectroscopy (FTIR) 3,10,14,17,34,45,70. Others reported the composition of specific crystal phases, e.g. leucite 17,20,23,46,63. Analyses of surface and bulk composition of commercially available feldspathic ceramics using XPS, WDS, and EDS have shown the presence of a silica-rich surface layer due to a reduction in K and Na relative to the bulk composition. However, the surface composition and chemical states of the ceramics were found to be virtually indistinguishable. This suggests that the compositional analysis protocol can use methods that collect the information up to 1 µm from the specimen surface, such as WDS and EDS 17,20,34.

The microstructure of some dental ceramics has also been studied and related to physical properties 7,17,23,24. The high-expanding mineral, leucite (K2O·Al2O3·4SiO 2), is often associated with microcracks that result from a thermal expansion mismatch between leucite and the surrounding glass matrix (Figure 1) 14,17,22,47. This type of microcracking can be minimized by reducing the leucite particle size and by obtaining a homogeneous distribution of these particles throughout the ceramic (Figure 2) 17,47. Previous research has also shown that the fracture energy first increases with increasing grain size because of increased cracking, then decreases because of more pre-existing microcracks 17,80. These microcracks are rarely observed in high crystalline content ceramics and in glass-infiltrated or hot-pressed ceramics (Figure 3).







Quantitative and qualitative analyses of the microstructure are normally performed using scanning electron microscopy (SEM) in back scattered imaging (BSI) mode, followed by EDS or WDS based on Phi-Rho-Z (PRZ) correction. PRZ is a type of matrix correction scheme that uses a set of equations to correct for X-ray absorption, atomic number effect, and fluorescence from different elements in the sample.

To enhance the observation of microstructural features, e.g., grains and grains boundaries, the ceramic specimens are normally light etched. As ceramics are usually not conductive materials, the specimens should be mounted on aluminum stubs using carbon coating paste or tape for better conductivity and sputter-coated with gold-palladium or carbon. Gold coating is recommended for surface topography analyses using the SEI mode and carbon coating should be used on specimens in which the main purpose is compositional analysis using BSI mode and EDS or WDS. The reason for this distinction is that gold-palladium coats produce element peaks that interfere in the compositional analysis.

Crystal size and volume fraction of crystal phases (VV) are important parameters in the materials characterization and can be measured using stereology principles. Stereology describes the relationship between measurements made on the two-dimensional plane of polished surfaces and the three-dimensional microstructural features to be sampled. Some studies have use stereology to measure the VV of dental ceramics and related it to their properties 17,20,33.

The microstructural properties, such as VV and crystal size, are also important for the interpretation of fracture processes. Previous research has shown that the grain size and the crystal structure are correlated with the crack phenomena regardless of processing or composition80. When the grain size of the material becomes large with respect to flaw size, the crack does not encompass enough grains for the full polycrystalline toughness to apply. This results in a reduction in both fracture toughness and fracture strength. Yet, the dominant damage mode in any given material is dictated by the microstructure: (1) fine microstructures with minimal internal weakness tend to exhibit macroscopic cracks; and (2) coarse microstructures with enhanced internal weakness tend to exhibit quasi-plastic zones. Both cracks and quasi-plasticity can lead to degradation of properties, and ultimately compromise the useful lifetimes of restorative structures, in different ways. The two modes may be interactive: the quasi-plasticity can enhance or inhibit fracture by redistributing tensile stresses 44,61,62.

The effect of crystallization as a toughening mechanism for glass ceramics has also been studied 20,27. It is known that a morphology that makes crack propagation more difficult, requiring more energy, increases toughness. Combination of toughening techniques can be microstructurally designed to optimize the toughness and strength of glass ceramics 52.

Microstructure and composition are also controlling factors in the development of micromechanical retention produced by ceramic surface primers, such as acids, airborne-particle abrasion methods and electrodeposition technology and, therefore, affecting the bonding mechanisms to resin 13-15,17,20. This subject will be thoroughly discussed in the second part of this two-paper review.

Therefore, quantitative and qualitative microstructural analyses provide an association among the constitution, physical properties, and structural characteristics of materials. In addition, the microstructure characterization is necessary to calculate relevant mechanical properties and to support further arguments on fracture and bonding phenomena. It is difficult to discuss materials behavior without proper material characterization, which should be the first step of any research proposal involving materials.

Flexural Strength and Structural Reliability of Single- and Multilayer Ceramic Structures

Mechanical failure occurs when the applied stress becomes greater than the strength of the material. The strength of a material is dependent on the size of the initiating crack present in a particular sample or component 59. The large number of pre-existing ceramic cracks, coupled with a low fracture toughness, limit the strength of ceramics and cause a large variability in strength and time-dependency. Variability in strength is a consequence of the distribution in crack sizes, and the time dependency of strength results from the slow growth of these flaws to dimensions critical for catastrophic failure 69.

Failure predictions for ceramics depend on the experimental parameters that measure the strength distribution and time dependency of strength. These parameters can be determined by measuring strength as a function of stressing rate in a test environment that simulates the service environment (Figure 4). Thus, well designed experiments coupled with a reliability analysis can optimize rational design decisions that ensure the successful use of ceramics in demanding structural applications 19,68.



Flexural strength is generally considered as a meaningful mechanical property for brittle materials that are much weaker in tension than in compression. However, it is necessary to control the flaw distribution to validate this approach. Although the "strength" is used as a measure of reliability, toughness is a more meaningful property. Biaxial flexural, three- and four-point bending are the most popular test methods to assess the strength distribution found in components35. The four-point flexure test has been used for strength evaluation of single-component brittle materials4,19,28 and bilayered structures such as glass veneer on core ceramic specimens19,74 and metal-ceramic structures11,12.

The failure strength of a brittle material is statistically distributed as a function of the homogeneity of the material. One commonly used statistic for the description of this distribution parameter is the Weibull distribution. The Weibull modulus (m) is a measure of the distribution of critical flaws. Higher values of "m" correspond to a higher level of structural integrity of the material. Most ceramics are reported to have "m" values in the range of 5 to 15, whereas metals, which produce ductile failures, have "m" values in the range of 30 to 100 37. This analytical method based on statistical concepts is easily applied when a reasonable number of samples are examined, and it enables fracture probability to be calculated as a function of applied stress 19,50. Yet, Weibull analysis has some limitations that challenge its ability to predict failure of components having complex geometries, especially when they are subjected to a multi-axial stress state. This may play an important role when dental restorations are analyzed 4. Therefore, the failure probability of monolithic and laminated ceramic structures can be calculated from the results of a flexural test 19.

Surface cracks can be induced by machining or grinding. Usually, failure of the ceramic originates from the most severe flaw. The size and spatial distribution of flaws justify the necessity of a statistical approach to failure analysis 78. Thus, the reliability of ceramics under flexural loading can be based on Weibull analysis.

Evaluation of the damage modes in bilayer ceramic structures using an Hertzian contact test has shown that the substrate has a profound influence on the evolution damage from initiation to ultimate failure in the bilayer systems38. Nevertheless, the crack initiation tends to occur at the top surface in systems having a strong bonded interface and a small elastic-plastic mismatch (glass/glass-ceramic); whereas in systems with a large mismatch, crack initiation tends to occur at the internal interface 81. Yet, the core/veneer thickness ratio (tC/tV) appears to be the dominant factor that controls the failure initiation site in bilayer ceramic structures 74,77. The crack initiation site shifts from veneer to core as the tC/tV ratio increases, but the increase in the elastic modulus of the supporting substrate did not affect the crack initiation site 77. Therefore, the load to fracture initiation is primarily influenced by the thickness of the restoration and, to a lesser extent, the E of the supporting substrate.

Investigations of clinically failed all-ceramic restorations have shown that the fracture origin is typically located at the internal (tensile) surface of the crowns 39,40,75. These results suggest that the ceramic core surface should be placed as the tensile side for flexural testing of multi-layer structures 19.

Mean flexural strength values also vary according to the test method and test environment. Same ceramic material can show up to 30% higher values if tested in three-point bending at room atmosphere than in four-point bending under water 19,33.

Fracture surface analysis (fractography) is well-established as a means of failure analysis in the field of glasses and ceramics. It has been recognized as a powerful analytical tool in dentistry 20,39,75. The application of fractography is based on the principle that the entire history of the fracture process is encoded on the fracture surface of brittle materials 26,58.

Fracture in glass occurs when preexisting cracks propagate under excessive tensile stresses. These cracks can be induced by mechanical means (e.g., grinding or polishing), by processing, or by intrinsic defects (e.g., imperfections in the structure). Most evidence shows that crack propagation is determined by varying levels of stress intensity or energy and, because of these relationships, much information is contained within the fracture surface 57. Fractography principles have been used for qualitative analysis of fracture dental restorations confirming the presence of characteristic markings of the fracture process (Figures 5 and 6) 19,20. Note that these markings are more evident on the fracture surface of amorphous glasses (Figure 5) than on crystalline ceramics (Figure 6). As mentioned, fractography principles can be applied to analyze any fractured ceramic surface, however, the more complex the microstructure the more difficult to identify the characteristic fracture markings (Figures 5 and 6) and expert knowledge is mandatory.





Ceramic specimens tested in bending are very sensitive to edge or surface machining damage. Fractographic analysis has been shown that most failures start from either a surface (Figure 5) or a corner flaw (Figure 6) located along the tensile surface of the specimens 19,20,68,74. These observations may suggest rounding specimen edges as a revision of the specimen preparation standard ISO 6872 35, as proposed by Della Bona, et al.20,2004.

In cases where the Weibull moduli are similar among experimental structures, a crack difference cannot explain the strength differences. Thus, the differences in strength can be explained by the differences in toughness that, in turn, are related to the way the materials are processed. For instance, if the mean flexural strength and Weibull modulus of a core monolithic ceramic are similar to those of the same core ceramic veneered by a glass (bylayered specimen) and the fracture analysis also shows similar results for these two structures, it can be said that the structural reliability of the veneered core ceramic structure is controlled primarily by that of the core ceramic. Yet, the investigator has to determine the critical core/veneer thickness ratio (tC/tV) below which strength and structural reliability become significantly reduced. This information will improve our ability to design ceramic-based prostheses with a sufficiently high margin of safety 19,20.

Ceramic Fracture Toughness Determined by Fractography and Fractal Analyses

Brittle fracture has been shown to be a complex process 26,29,79. The fracture process creates at least two new surfaces with distinct topography and texture that can be characterized using principles of fractography.

Most fracture surface observations yield substantial information about the fracture process and enable the calculation of the fracture toughness of the material. In addition, the roughness of the fracture surface gives qualitative information on the extent of crack deflection25,67, or other toughening mechanisms32.

It is difficult to directly measure the flaw-initiating site, especially in very high-strength, fine-grained glass ceramics, and in cases where failure is caused by poor machining practices. However, where the flaw itself cannot be measured, the region from which the failure occurred can be determined by observing the patterns on the fracture surface 66. This is the case of the majority of multi-layer structures.

Quantitative fractographic analysis applies the principles of fracture mechanics to the topography observed on the fracture surface of brittle materials. There is specific, quantitative information to be obtained from the fracture surface including: (1) the identification of the size and location of the fracture initiating crack or defect, (2) the stress state at failure, (3) the existence, or not, of stress corrosion, (4) a knowledge of local processing anomalies that affect the fracture process, and (5) the calculation of the fracture toughness20,58.

Fracture toughness values are used extensively to characterize the fracture resistance of brittle materials 2,8,20,41,72,73. The fracture toughness of brittle ceramics is usually controlled by the fracture in Mode I (opening mode, tensile load). Irwin (1957)36 defined failure at the point when the Mode I stress intensity (KI) reaches a critical value (KI > KIC). The critical stress intensity factor (KIC) is in many cases a material constant and is one measure of the toughness of the material, i.e. the resistance to crack propagation. Therefore, the fracture toughness or critical stress intensity factor (KIC) can often be determined using the Griffith-Irwin equation:

where Y is a geometrical factor that accounts for the location and geometry of the crack and loading20,65, sf is the stress at fracture, and c is the radius of an equivalent semi-circular crack for a semi-elliptical crack of semiminor axis "a" and semimajor axis "b" (Figure 7) 20,51,59.



Therefore, the fractography approach to determine fracture toughness involves the identification and measurement of the initial (starting) defect or critical crack (c) using fractographic principles (Figure 7). In case of corner cracks (Figure 6), the critical size (c) is calculated using the same equation as for the equivalent semi-circular surface crack [c = (ab)½] (Figure 7)20. However, "a" is the length of one side of the corner crack and "b" is the length of the other side of the corner crack. So, in this case, (c) corresponds to the distance from the crack corner to the critical flaw-mirror region limit, which corresponds to 84 µm in Figure 6. For internal flaws (Figure 8), "c" is also calculated by [c = (ab)½]. However, "a" is half of the crack major axis and "b" is half of the crack minor axis.



Under certain service and/or environmental conditions, stable crack extension or slow crack growth can occur at stress intensities that are less than the critical value, KIC. Under such conditions, KI becomes dependent on the crack growth rate (crack velocity, V) and, hence, the characteristics of the system. The calculation of KIC in environmental conditions that promote slow crack growth can lead to erroneous values of KIC because of an incorrect assumption of (initial versus final) crack size. Nevertheless, the actual value of KIC should not change due to loading rate or test geometry. If the environment degrades the entire material, then the degraded material is, essentially, a different material. If the environment degrades the local crack, then the crack usually grows and the component is weaker, i.e., lower strength, but also with a larger final crack size, so the toughness of the unaffected region is still the same.

Quantitative fractographic analysis of brittle fracture surfaces shows that there are characteristic markings on the surfaces that are self-similar and scale invariant, implying that fractal analysis is a reasonable approach to analyzing these surfaces16,31,79. The fracture surfaces of both monolithic and granular materials have been shown to have fractal characteristics 54,76. The study of fracture surfaces using fractal geometry has led to the observation that tougher materials tend to form more complex, irregular fracture surfaces. In many cases, fractal geometry allows for the complexity of the surface features to be quantified by a single value, the fractal dimension 49,54.

Fractal geometry is a non-Euclidean geometry that can quantitatively define irregular shapes and surfaces. Fractals are geometrical objects that are self-similar (or self-affine) and scale invariant and are characterized by non-integer dimensions. A scale invariant object is one in which the geometric surface will be statistically the same at any magnification scale 5,16,31.

The use of fractal dimension, D, measurements to characterize rough surfaces, e.g. fracture surfaces, has become popular since Mandelbrot48 (1982) re-introduced the concept of fractal geometry. Several authors have used fractal geometry to quantitatively describe irregular fracture surfaces 1,9,16,31,49,54,56. The larger the value of D, the more tortuous the surface. Thus, a fracture surface may have a fractal dimension of 2.3 where 2 is the topological dimension and 0.3 is the fractal dimensional increment, D*.

Fractography also has been used to relate the flaw/mirror size ratio and the fracture toughness, which, in turn, is related to the elastic modulus. The combination of these relationships show that the D* is directly related to the flaw/mirror size ratio. This implies that there is a linear scaling law between the energy of crack initiation and the energy of microbranching at fracture and this relationship is reflected in the features on the fracture surface 16,31,55,60.

It has been shown that D* is correlated to KIC for many brittle materials using the equation53:

where E is the elastic modulus, and a0 is the characteristic fracture length on the atomic scale. The a0 value is the slope of a graph of fracture energy (g) versus ED* (a0 = 2g / ED*) 54. The a0 can be assumed to be 20-80 Å for glass ceramic materials and 10-20 Å for feldspathic ceramics 16,31,53,79.

Therefore, the fractal approach for fracture toughness determination uses equation 2 and involves the calculation of the fractal dimensional increment (D*), which can be obtained using the slit-island analysis (SIA) along with the Richardson technique9,16,31,49,56,71. Thus, there is a positive correlation between D* and KIC values. Although fractal analysis can be useful in failure analysis as one of many tools, it is not recommended to use fractal analysis alone as a standard technique for measuring toughness. However, the measurement of D* on fracture surfaces of failed crowns or bridges can be potentially used to determine the difference between poor processing and over-load intra-oral failures.



This review demonstrated that quantitative microstructural analysis can provide an association among the constitution, physical properties, and structural characteristics of materials17. It also became evident that structural reliability of dental ceramics is a major factor in the clinical success of ceramic restorations. In addition, it has been shown that complex stress distributions are present in most practical conditions and strength data alone cannot be directly extrapolated to predict structural performance19,20,41.

Therefore, for the strength test to accurately reflect the variability and time-dependency of a ceramic component in service, the test environment must be similar to the service environment, and the strength-controlling flaw population must be the same as that responsible for failure in service. These factors should be the basis for the selection of a research protocol. As the distribution of strength is a measure of the distribution of the effective flaw sizes leading to failure, fractography principles should be applied for the quantitative and qualitative analyses of fractured surfaces, improving the understanding of the fracture phenomenon, which is, at the end, the most common failure cause of ceramic restorations.



This review is based on the knowledge gathered during the graduate course in Restorative Dentistry at the University of Texas Health Science Center at San Antonio, U.S.A., supervised by Dr. James B. Summitt, followed by the MMedSci program in Restorative Dentistry at the University of Sheffield, UK, supervised by Dr Richard van Noort, and, mainly, during the PhD program in Materials Science and Engineering at the University of Florida, USA, supervised by Dr Kenneth J. Anusavice and Dr John J. Mecholsky Jr. I also thank the continuing support of CNPq-Brazil, grant 300659/03-2, for my research work at the University of Passo Fundo.



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Correspondence to
Prof. Dr. Alvaro Della Bona
Dental School, University of Passo Fundo - Campus I
BR 285 - POBox 611
Passo Fundo, RS 99001-970 - Brazil
Phones: 55-54-581-0657 and 55-54-311-5142



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