Enamel: From brittle to ductile like tribological response
Introduction
Enamel is the highest mineralized tissue in the human body. It is composed of approximately 97% by weight mineral, essentially non-stoichiometric hydroxyapatite (HAp), 1% organic material, mostly protein, which is not collagen, and 2% water.1 As with most biological materials, it has a hierarchical microstructure, which imparts order from the nm to mm scale.
Crystallites of enamel are roughly rectangular in cross-section with mean width of 45 nm and mean thickness of 25 nm,2 and are “glued” together by a thin protein layer of not more than 2.5 nm. Besides the “glueing” of the microstructure with proteins, White et al.3 highlighted that the remaining proteins spread the damage laterally over a much larger volume and may allow limited differential movement between adjacent rods, preventing thus catastrophic damage.
At the micrometer scale of the hierarchical organization of enamel, apatite crystals are bound together in bundles called prisms or rods that form in a radial manner from the dentine enamel junction to the external surface. Each rod is 3–6 μm in diameter. There is little protein within the prisms, while it is mainly concentrated at the interprism regions.1 This region is often called enamel sheath, and consists also of apatite crystals that are not part of the prisms and have a certain degree of mis-orientation with the axis of the rods.3 The enamel prisms themselves are arranged in a crossed, plywood-like structure1, 4 which is thought to act as a crack arrester. However, the presence of this minor protein component has been suggested as the basis for why enamel behaves in a “metallic-like” rather than “brittle” ceramic manner during nano-indentation stress–strain characterisation.5
The amount of enamel is limited in thickness to some millimetres per tooth sample. This latter situation, plus the complex hierarchical structure described above, make nanoindentation6, 7, 8, 9, 10, 11, 12, 13 and nanoscratching14, 15, 16, 17 suitable techniques for the characterisation of mechanical and tribological properties of this system. Regarding tribological behaviour of enamel, the majority of wear studies in enamel have been made at the micrometer scale18, 19 and macroscopic scale.20, 21, 22, 23, 24 However, there is some incipient literature on wear on enamel at the nanometre scale.14, 15, 16, 25 Jandt25 used AFM and nanoindentation to study the influence of erosion on enamel by measuring the change in mechanical properties after immersing enamel under different acidic soft drinks. Habelitz et al.17 studied the width of the dentine–enamel junction (DEJ) by nano-scratching. Guidoni et al. described the abrasion mechanism of enamel using the same indenter tip and different environments14, 15 and also compared the wear mechanisms generated after using two different indenter tips: conical rounded and sharp cube corner tips.16
Thus, based on previous studies16 with a sharp cube corner indenter tip, the tribological response of enamel is compared with the abrasion response of an amorphous glass and two highly ductile materials, copper and silver mono-crystals. Since the mechanical properties of enamel are known to be viscoelastic,8, 10, 11, 26, 27 the chosen materials for comparison are considered to be at the two extremes of the expected mechanical behaviour of enamel, plus having the advantage of both being well studied and homogenous systems. The emphasis of the study is to try and deduce wear mechanisms of enamel at the nano-scale rather than determine the wear rate of enamel.
Section snippets
Materials
For the investigation reported here one incisor of a 1.5-year-old steer was used. The tooth was cut with a diamond saw into two sections along the crown–root direction, carefully avoiding any overheating. A second permanent premolar from a 12-year-old male individual and a wisdom tooth of a 26-year-old male individual extracted for orthodontic reasons were prepared following the same procedure and also tested.
In all cases, the transversal section of the samples, consisting of enamel and
Tribological tests
Wear tests were carried out using an add-on nanoindentation device (Hysitron Triboscope, Hysitron Inc., Minneapolis, MN, USA) mounted on the scanner head of an AFM stage (Veeco-Digital Instruments, Santa Barbara, CA, USA).
The same sharp cube corner indenter tip (20–50 nm radius) was used for all the tests.
Load-displacement data of single point indentations (0D) were used to ascertain the maximum penetration depth over the load range investigated. The average penetration depths for all the
Glass sample
The single scratches (1D) show a linear increment of lateral force with lateral distance. A constant normal force of 100 μN leads to a 60 μN lateral force at the beginning of the displacement to 90 μN at the end of the 10 μm single scratches. Two different arbitrary orientations were tested, no significant differences were found.
The area scans show a flat and homogenous abraded area. Fig. 1, in the left upper corner, shows an AFM image of one abraded area with its corresponding line profiles
Discussion
Although most of the literature dealing with mechanical properties of teeth studied human samples,7, 12, 13, 16, 27, 32, 33, 34 there are investigations using bovine samples.35, 36, 37 In addition, Reeh et al.38 clearly showed that bovine enamel may be used as a model for human enamel in salivary lubrication studies. Furthermore, within the scope of the present study, they were found to behave comparatively similar.
Conclusions
From the above observations the following conclusions can be drawn:
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Even though the single scratch behaviour was similar at low loads, glass deformed plastically while enamel did not. While the incrementing lateral load with lateral displacement was related to plastically deformed and densified areas in glass, it corresponds to disaggregation and pressing and re-distribution of the removed material of enamel.
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On the other hand, the observed frictional behaviour of the single scratches on enamel
Acknowledgments
Financial aid through EC Contract No. MEST-CT-2004-504465, “Marie Curie Host Fellowships for Early Stage Research Training”, is gratefully acknowledged. Dipl.-Ing. Andreas Jaeger is kindly thanked for his help with the single scratch tests. People in charge of the Triboindenter (including Dipl.-Ing. Andreas Jaeger) from Institute for Mechanics of Materials and Structures from Vienna University of Technology are also acknowledged for allowing us to carry out the single scratch tests using their
References (44)
- et al.
Enamel microstructure—a truly three-dimensional structure
Journal of Human Evolution
(2003) - et al.
Enamel—a “metallic-like” deformable biocomposite
Journal of Dentistry
(2007) - et al.
Nanoindentation mapping of the mechanical properties of human molar tooth enamel
Archives of Oral Biology
(2002) - et al.
Elastic modulus and stress–strain response of human enamel by nano-indentation
Biomaterials
(2006) - et al.
Nanoindentation derived stress–strain properties of dental materials
Dental Materials
(2007) - et al.
Nanoindentation and storage of teeth
Journal of Biomechanics
(2002) - et al.
Property variations in the prism and the organic sheath within enamel by nanoindentation
Biomaterials
(2005) - et al.
The functional width of the dentino-enamel junction determined by AFM-based nanoscratching
Journal of Structural Biology
(2001) - et al.
On the friction and wear behaviour of human tooth enamel and dentin
Wear
(2003) - et al.
Development and evaluation of a low erosive blackcurrant juice drink. 2. Comparison with a conventional blackcurrant juice drink and orange juice
Journal of Dentistry
(1999)
On evaluation of wear resistance of tooth enamel and dental materials
Wear
Intra-oral restorative materials wear—rethinking the current approaches: how to measure wear
Dental Materials
Variation in tooth wear in young adults over a two-year period
The Journal of Prosthetic Dentistry
Wear of enamel and veneering ceramics after laboratory and chairside finishing procedures
The Journal of Prosthetic Dentistry
Probing the future in functional soft drinks on the nanometre scale—towards tooth friendly soft drinks
Trends in Food Science & Technology
Stacking fault energy and indentation size effect: do they interact?
Scripta Materialia
Microstructural investigation of the volume beneath nanoindentations in copper
Acta Materialia
Effects of polar solvents on the fracture resistance of dentin: role of water hydration
Acta Biomaterialia
Fracture-toughening mechanisms responsible for differences in work to fracture of hydrated and dehydrated dentine
Journal of Biomechanics
Comparison of the number and diameter of dentinal tubules in human and bovine dentine by scanning electron microscopic investigation
Archives of Oral Biology
Lubrication of human and bovine enamel compared in an artificial mouth
Archives of Oral Biology
Ceramics in dentistry: historical roots and current perspectives
The Journal of Prosthetic Dentistry
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