Abstract
Objective
The purpose of this study was to determine the crack resistance behavior ( K R ) of human enamel in relation to its microstructure.
Methods
Human molar teeth were precision cut, polished and tested using Vickers micro-indentation at different loads ranging from 0.98 to 9.8 N. Five indentation load levels were considered, 20 indentation cracks for each load level were introduced on the surface of the test specimen (10 indentations per tooth) and their variability was evaluated using Weibull statistics and an empirical model. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to analyze the crack morphology and propagation mechanisms involved.
Results
The results showed that enamel exhibited increasing cracking resistance ( K R ) with increasing load. It was found that the crack propagation mainly depended on the location and the microstructure it encountered. SEM showed the formation of crack bridges and crack deflection near the indentation crack tip. The crack mode was of Palmqvist type even at larger loads of 9.8 N. This was mainly attributed to the large process zone created by the interwoven lamellar rod like microstructure exhibited by the enamel surface.
Significance
This study shows that there are still considerable prospects for improving dental ceramics and for mimicking the enamel structure developed by nature.
1
Introduction
Human enamel (HE) is one of the unique natural substances which still cannot be substituted effectively by artificial restorative materials . It is the hardest part of the body but still has good toughness, despite fairly poor working conditions, such as widely ranging loads, reciprocating movements, temperature shocks and possible acid attacks . HE is composed of 96% hydroxyapatite (HA) by weight and the remaining 4% consists of water and organic material (mainly protein) . Hence from a materials science point of view, HE could be considered as a functionally graded composite material with a mineralized HA matrix and organic reinforcements. Although many biomechanical studies have focused on understanding the fracture mechanism in HE, it has still not been fully characterized. There are 2 main reasons for this (i) HE possesses complex microstructural components such as enamel rods and dental tubules, which are involved in fracture mechanisms and (ii) structurally HE is very small and the specimens used for testing are even smaller. Although studies have reported the fracture toughness of the entire human tooth using flexural tests (three point bend tests) , notched compact-tension test and chevron-notched short-bar test; the values obtained for the whole teeth may not be representative of the HE surface. Moreover fracture toughness obtained using the above techniques could be related only to crack length and not to the crack location. For instance, micro-indentation studies have shown that fracture toughness values of HE range between 0.7 and 1.27 MPa m 1/2 , dependent on the site where indentation is performed. This meant that that the crack resistance of HE is dependent not only on crack length but also on crack location. Hence it is important to understand such mechanisms as it would help the clinical treatments of teeth and guide the design and development of “tooth-like” restoratives that imitate the structure and properties of HE. The main objective of the present study was to understand the mechanism underlying the crack resistance of HE. The data analyzed using Weibull statistics show that although the interaction of the crack with the microstructure decreases with the increasing load, there is an increase in the crack resistance ( K R ) which is attributed to crack bridging phenomena.
2
Methods
The test specimens of teeth (molar/wisdom teeth) were obtained from Dental College, Daejeon Health Science Center, Korea. The specimens were soaked in boiling water for approximately 1 min to remove the presence of any tartar/calculus (yellow stains) formation and disinfect bacteria which are commonly observed on the surface of HE and rinsed in 0.5 M KOH to remove the presence of any organic substances. The specimens were ultrasonically rinsed in acetone and dried. The top surface of the teeth was precision cut using a modified water-cooled diamond saw and was hot mounted in a resin. The mounted specimens were subsequently polished carefully using a cloth bonded diamond wheel with successively finer diamond pastes (up to 3 μm) to yield a mirror-like surface suitable for indentation. Vickers indentations were made at 0.98, 1.96, 2.94, 4.9 and 9.8 N loads on the polished surfaces of the test specimens using a commercial hardness tester (Matsuzawa Co., Ltd., Japan (DVK-25)). For each of these loads four samples were used and on each sample five indents were made, i.e., a total of 20 indentations for each load were studied on the micropolished surfaces of the test specimens. The average radial-median crack lengths produced at the indentation corners were measured immediately after unloading. The crack resistance was calculated using the following equation :
where K R is the crack resistance, P is the applied indentation load, C is the indentation crack length, E and H V are the elastic modulus and the hardness values, respectively, δ is a dimensionless constant dependent only on the indenter geometry and has an empirical value of 0.016 ± 0.004. The K R data were further analyzed with the conventional ranking method to yield the Weibull parameters, knowledge of which would lead to complete characterization of the statistical properties of the measured K R data. This was done by arranging the data in ascending order. The i th result in the set of n = 20 data was assigned a cumulative probability of occurrence, P i , which was calculated with:
The measured K R and P i , were then analyzed, by using a simple, least-square regression method, according to the alternative form of the well-known two-parameter Weibull distribution equation.
where m and K 0 are the Weibull modulus and the scale parameter, respectively. Microstructural studies of the indentation cracks on specimens were carried out with a scanning electron microscope (SEM) and atomic force microscope (AFM).
2
Methods
The test specimens of teeth (molar/wisdom teeth) were obtained from Dental College, Daejeon Health Science Center, Korea. The specimens were soaked in boiling water for approximately 1 min to remove the presence of any tartar/calculus (yellow stains) formation and disinfect bacteria which are commonly observed on the surface of HE and rinsed in 0.5 M KOH to remove the presence of any organic substances. The specimens were ultrasonically rinsed in acetone and dried. The top surface of the teeth was precision cut using a modified water-cooled diamond saw and was hot mounted in a resin. The mounted specimens were subsequently polished carefully using a cloth bonded diamond wheel with successively finer diamond pastes (up to 3 μm) to yield a mirror-like surface suitable for indentation. Vickers indentations were made at 0.98, 1.96, 2.94, 4.9 and 9.8 N loads on the polished surfaces of the test specimens using a commercial hardness tester (Matsuzawa Co., Ltd., Japan (DVK-25)). For each of these loads four samples were used and on each sample five indents were made, i.e., a total of 20 indentations for each load were studied on the micropolished surfaces of the test specimens. The average radial-median crack lengths produced at the indentation corners were measured immediately after unloading. The crack resistance was calculated using the following equation :
where K R is the crack resistance, P is the applied indentation load, C is the indentation crack length, E and H V are the elastic modulus and the hardness values, respectively, δ is a dimensionless constant dependent only on the indenter geometry and has an empirical value of 0.016 ± 0.004. The K R data were further analyzed with the conventional ranking method to yield the Weibull parameters, knowledge of which would lead to complete characterization of the statistical properties of the measured K R data. This was done by arranging the data in ascending order. The i th result in the set of n = 20 data was assigned a cumulative probability of occurrence, P i , which was calculated with:
The measured K R and P i , were then analyzed, by using a simple, least-square regression method, according to the alternative form of the well-known two-parameter Weibull distribution equation.
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