Impact wear behavior of human tooth enamel under simulated chewing conditions

Impact wear behavior of human tooth enamel under simulated chewing conditions

Author’s Accepted Manuscript Impact wear behavior of human tooth enamel under simulated chewing conditions Jing Zheng, Yangyang Zeng, Jian Wen, Liang ...

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Author’s Accepted Manuscript Impact wear behavior of human tooth enamel under simulated chewing conditions Jing Zheng, Yangyang Zeng, Jian Wen, Liang Zheng, Zhongrong Zhou www.elsevier.com/locate/jmbbm

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S1751-6161(16)30112-6 http://dx.doi.org/10.1016/j.jmbbm.2016.04.039 JMBBM1906

To appear in: Journal of the Mechanical Behavior of Biomedical Materials Received date: 23 November 2015 Revised date: 28 March 2016 Accepted date: 28 April 2016 Cite this article as: Jing Zheng, Yangyang Zeng, Jian Wen, Liang Zheng and Zhongrong Zhou, Impact wear behavior of human tooth enamel under simulated chewing conditions, Journal of the Mechanical Behavior of Biomedical Materials, http://dx.doi.org/10.1016/j.jmbbm.2016.04.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Impact wear behavior of human tooth enamel under simulated chewing conditions Jing Zheng, Yangyang Zeng, Jian Wen, Liang Zheng, Zhongrong Zhou Tribology Research Institute, Key Laboratory of Advanced Technologies of Materials, Ministry of Education, Southwest Jiaotong University, Chengdu 610031, China

Abstract Previous studies mostly focused on the sliding wear behaviour of human teeth, and little effort has been made so far to study the impact wear of human teeth. The objective of this study was to investigate the impact wear process and mechanism of human tooth enamel and the influence of water content within enamel. In this paper, the impact wear behaviours of fresh and dried human tooth enamel against SiC ceramic have been investigated using a specially designed impact test machine. Tests lasting up to 5×103, 5×104, 2.5×105, 5.5×105, 8×105 and 1×106 cycles were conducted, respectively. Results showed that for the fresh enamel, the surface damage was dominated by plastic deformation at the early stage of impact wear. Iridescent rings appeared around the impact mark as a result of the accumulation and spread of plastic deformation. As the impact wear progressed, delamination occurred on the surface of enamel, and thus the iridescent rings gradually disappeared. Wear loss increased rapidly with the increase of impact cycles. When a wear particle layer was formed on the enamel surface, the wear rate decreased. It was found that the surface hardness of enamel increased with the impact cycles, and no cracks appeared on the cross section of wear scar. Compared with the fresh enamel, the fracture toughness of dried enamel decreased, and thus there were microcracks appearing on the cross section of wear scar. More obvious delamination occurred on the worn surface of dried enamel, and no iridescent rings were observed. The wear loss of dried enamel was higher than that of fresh enamel. In summary, the impact wear behavior of sound human tooth enamel was metal-like to some degree, and no subsurface cracking occurred. The water content within enamel could increase its fracture toughness and protect the surface from impact wear. The wear mechanism of human tooth enamel is determined by its microstructure.

Keywords: Human tooth enamel, Impact wear, Wear mechanism, Water content 1.



Introduction

Corresponding author. Tel: +86-28-87634037, Fax: +86-28-87603142. E-mail address: [email protected] (J. Zheng).

Human teeth are closely associated with speech, pronunciation and facial aesthetics of human beings [1], but the most important physiological function of teeth is mastication. Mastication is a complex and compound process [2-5]. During the process of mastication, foods are cut and torn mainly through impact between the teeth of upper and lower jaws firstly, and then food particles are grinded by the relative sliding of the teeth [6]. Thus, occlusal surface wear is a result of the combination of impact wear and sliding wear [7,8]. Moderate tooth wear has a certain physiological significance, which is useful for improving masticatory efficiency and reducing the susceptibility of dentition to disease and malocclusion [9]. But, if not controlled, excessive tooth wear could result in poor masticatory function, even some dental diseases, such as dentine hypersensitivity [10,11]. Human tooth is composed mainly of hard enamel shell, tough dentin interior, and pulp [12,13]. Occlusal surface wear generally occurs on the surface of enamel. As the most highly calcified and hardest tissue in body, the enamel consists of 92–96% inorganic substances, 1–2% organic materials, and 3–4% water by weight [14]. Most of the inorganic substances are hydroxyapatite particles which are organized and glued together by a nanometre-thin layer of enamelin (a kind of protein). The enamel is assembled by hierarchical structure step by step from fibre-like hydroxyapatite nanocrystals (25 nm thick, 100 nm wide and 500-1000 nm long) to keyhole-shaped structures known as enamel rods (4-8 µm in diameter) [15,16]. Enamel rod is the basic structural unit of enamel [14]. The interfacial area between rods is termed as inter-rod enamel which is generally considered to be rich in protein [15,16]. Enamel is one of those unique natural substances which still cannot be substituted effectively by artificial restorative materials [17]. It has been widely accepted that human tooth enamel has excellent mechanical and anti-wear properties [9]. Therefore, it is necessary to investigate the wear behavior and mechanism of human tooth enamel, which would help provide valuable insights into the development of new dental materials, clinical treatment for excessive tooth wear and bionic tribology design. Increasing attention has been paid to tooth wear, and studies have been carried out on the friction and wear behavior of human tooth enamel in the past two decades [3,5,9,18-27]. Most of these studies focused on the sliding wear of enamel. It was reported that the sliding wear behavior of enamel was closely associated with its microstructure, and was affected significantly by normal load and oral environment [9,19-21,27]. However, research on the impact wear of both human teeth and dental materials is very limited. In 1988, Aziz and Harrison investigated the wear behaviors of 6 restorative materials under the external action of impact stress. It was found that the impact stress affected the wear rates of restorative materials, and the wear loss increased with the impact stress [8]. Almost no studies have been conducted on the impact wear behavior of human tooth

enamel. In this paper, the impact wear behavior of human tooth enamel against SiC ceramic has been investigated in vitro using a specially designed impact test machine. Tests lasting up to 5×103, 5×104, 2.5×105, 5.5×105, 8×105 and 1×106 impact cycles were conducted, respectively. The worn surfaces of enamel specimens were studied by means of various microscope examinations. It has been increasingly recognized that the water content within enamel plays an extremely significant role in its mechanical properties and microtribological behavior [28-30]. Thus, the impact wear behavior of dried enamel subjected to 5 h drying treatment was also investigated, aiming to explore the effect of the water content within enamel. The target of this study was to explore the impact wear process and mechanism of enamel.

2.

Materials and methods

2.1. Sample preparation Enamel samples were prepared from freshly extracted human mandibular third permanent molars (M3). Human teeth are a natural material. Both the topography and microstructure of tooth occlusal surface are related to the age, class and type of teeth [31,32], and there exist obvious variations in the occlusal surface topography between different teeth with same wear ranking. In order to minimize the individual difference among samples, enamel sample with a flat test surface were used in this study, and all the teeth were from individuals of either gender aged between 25 and 35 years without caries and obvious wear, which were collected from participating dental clinics in Chengdu, PRC in conformity with ethical standards of Chinese Psychological Society. These teeth were placed in deionized water at 4 ºC to avoid dehydration before sample preparation. Forty teeth were used in this study. Each tooth was cut into two parts was cut into two halves using a diamond saw, with the cut lying perpendicular to the buccolingual division line. Each part was firstly embedded vertically into a steel mold with self-setting plastic, whose occlusal surface was exposed, and then was ground and polished to obtain an enamel sample with a flat test surface. As a result, two enamel samples were prepared from a tooth. For the detailed method of sample preparation, see reference [21]. Fig. 1 demonstrates enamel sample's preparation. Each sample was ground and polished off only 0.2-0.3 mm in height so as to make its test surface be similar to the original occlusal surface of enamel in the mouth. The cutting, grinding, and polishing were conducted under a water-cooling condition to avoid local overheat which can result in dehydration and changes in both the microstructure and chemistry of human teeth. Given that the direction of enamel crystals

and prisms is different on the cusps and in the inner part of the teeth, efforts were made to guarantee that the exposed test surface was located in the outer zone of enamel. All the samples were stored in deionized water at 4 ºC before testing. Efforts were made to keep the preparation time approximately the same for each sample. The average roughness Ra of samples was controlled to be no more than 0.20 μm by a high resolution surface profilometer (TALYSURF6, England) over a 1.0 x 1.0 mm area. To investigate the effect of water content within the enamel on its impact wear behavior, about half of enamel samples were selected randomly to be dried for 5 hours in the vacuum environment (37 ºC, -0.1 MPa) created by a vacuum drying chamber (DZF-6050, China) before impact wear testing. The counter-part was a silicon carbide ceramic (SiC) ball with a diameter of 6 mm. Due to good biocompatibility, silicon carbide ceramic is used as dental implant material in recent years [33]. The mechanical properties of samples are shown in Table 1. Both the hardness and elastic modulus of SiC was much higher than those of enamel. Thus, little deformation would occur to the SiC ball, and the interface contact could basically keep to a ball-on-flat configuration during impact wear testing. All samples were cleaned with deionized water before testing.

2.2. Impact wear test In vitro impact wear tests were conducted at a 90 ° inclination in a ball-on-flat configuration at 25 ºC using a specially designed impact tester. A sine curve of normal impact load with a magnitude of 20 N and a frequency of 2 Hz was used for all tests. The max and min load was 20 N and 0 N, respectively. Tests lasting up to 5×103, 5×104, 2.5×105, 5.5×105, 8×105 and 1×106 cycles were conducted without mediums, respectively. In order to lessen tooth dehydration to a large extent, the relative humidity was controlled to be 90% during impact wear testing. It was reported that the magnitude of masticatory force in the oral cavity ranges from 3 to 36 N during the normal chewing process of human beings [34], and the shape of the occlusal force curve is similar to the positive half of a sine curve [35,36]. Six human tooth enamel samples were tested for each condition. Efforts were made to keep the age, class, type, wear ranking, and region of tooth samples the same or similar, which could minimize the individual difference among samples. After testing, wear marks were examined and analyzed by LCSM and scanning electronic microscopy (SEM) (QUANTA200, FEI Corp., England). The profile and depth of wear scar were measured by a 3D surface profilometer (NanoMap-D, AEP Technology, USA). Wear loss was evaluated by the wear volume. The final wear volume of each test was the average of 5 samples.

2.3. Microhardness characterization The surface hardness of enamel samples after different impact cycles was measured at 25 ºC under a load of 50 g using a Vickers diamond indenter in a microhardness tester (MVK–H21, Japan). For each surface, ten indentations were made in the central area of wear scar and other ten indentations on the unworn surface. The lengths of indentation diagonals were measured under optical microscopy immediately, and the values were converted to microhardness automatically.

2.4. Fracture toughness measurement Five enamel samples were used to measure the fracture toughness of enamel. To investigate the effect of water content within enamel, intermittent drying treatment was conducted for each sample. Before drying, the surface fracture toughness of each sample was measured as a control. Then the samples were placed in the vacuum environment (37 ºC, -0.1 MPa). After 2 hours, the samples were taken out to do fracture toughness measurement. Subsequently, the sample was put back. After 3 hours more drying, the samples were taken out to do fracture toughness measurement again. The fracture toughness measurement was conducted using the above-mentioned microhardness tester and a laser confocal scanning microscopy (LCSM) (OLS1100,Olympus Corp., Japan). For each surface, 5 indentations were made under a load of 1 kg. After indentation, crack lengths were measured using LCSM. The value of fracture toughness (KIC) was calculated from the following equation [37]: 𝐾𝐼𝐶 = 𝛼√

𝐸 𝑃 𝐻 3 𝑐2

(1)

Where P is the applied load, E is Young’s modulus, H is the hardness, and c is the length of the surface trace of the half penny crack measured from the center of the indent, α is an empirical constant and is taken to be 0.016 ±0.004 based on a fit to experimental data using independent fracture toughness measurements [37,38].

3.

Results LCSM micrographs of typical wear scars on the fresh enamel surface (without any drying treatments) for

different impact cycles are shown in Fig. 2. Typical wear profiles (near the centre of wear scar) are shown in Fig. 3. Only a round mark with looming boundary appeared on the surface of enamel after 5×103 impact cycles, and no obvious wear occurred (Fig.2.a). An apparent round wear scar came in to being on the surface of enamel

after 5×104 cycles (Fig.2.b). Both annular uplifts and iridescent rings were observed at the edge of wear scar, but the profile of wear scar was smooth. When the number of impact cycles increased to 2.5×105 cycles, the area of wear scar significantly increased, and the uplifts and iridescent rings at the edge of wear scar became increasingly obvious (Fig.2.c). Additionally, slight delamination occurred in the central region of wear scar, and thus a small amount of wear debris appeared around the wear scar. The profile of wear scar was still relatively smooth. As the impact cycles continued to increase, the delamination on the worn surface was aggravated, the uplifts at the edge of wear scar were broken gradually, and then a great amount of wear debris appeared on the worn surface of enamel (Fig.2.d). The profile of wear scar became rugged. Also found was that the iridescent rings faded away. After 8×105 cycles, an incontinuous wear particle layer was observed on the worn surface, and the iridescent rings at the edge of wear scar disappeared totally, as shown in Fig.2.e. The profile of wear scar was quite rugged. With the impact cycles further increasing, the area of wear scar was observed to increase slowly (Fig.2.f). The profile of wear scar became smooth again. Also found was that the wear depth increased slowly before 2.5×105 cycles, but rapidly from 2.5×105 cycles to 8×105 cycles, and then slowly, as shown in Fig. 3. In addition, the wear of counter-part was examined by optical microscope (OM) after testing. Almost no wear was observed on the surface of SiC ball. Fig. 4 gives the microhardness of the enamel surfaces after different impact cycles. It was found that the hardness of the worn surface was higher than that of the unworn surface. And the increase in the hardness of worn surface became increasingly obvious with the increase of impact cycles. Compared with the fresh enamel without drying, the worn surface of dried enamel subjected to 5 h drying treatment exhibited more significant delamination, as shown in Fig. 5. Only after 5×104 impact cycles, obvious delamination occurred on the worn surface of dried enamel (Fig.5.a). A large number of wear debris appeared on the worn surface after 2.5×105 impact cycles (Fig.5.b). No iridescent rings were observed around the wear scar. Further SEM examinations showed that wear particle layer was formed earlier on the worn surface of dried enamel, as shown in Fig. 6. For the fresh enamel, a disconnected wear particle layer was observed on the worn surface after 8×105 cycles, and some lacunae and much small debris appeared around the rods, while only after 5.5×105 cycles, obvious wear particle layer appeared on the worn surface of dried enamel. Fig. 7 gives the variation of the wear volume of enamel as a function of the number of impact cycles. The wear volume increased non-linearly with the impact cycles, but there existed some differences in wear rate between the fresh and dried enamel. For the fresh enamel, the wear volume increased slowly before 2.5×105 cycles, rapidly from 2.5×105 cycles to 8×105 cycles, and then slowly after 8×105 cycles. For the dried enamel,

however, the wear volume almost increased linearly with the number of impact cycles before 5.5×105 cycles, and then the wear rate decreased a little. It should be noted that under the same number of impact cycles, the wear volume of dried enamel was higher than that of fresh enamel. Obviously, the dried enamel was more easily to be worn out. To give an insight into the effect of water content within enamel, SEM was used to examine and compare the wear scar cross-sections of the fresh and dried enamel. Typical photos of the cross-sections of wear scar after 1×106 impact cycles are shown in Fig. 8. To observe the cross-sections, the samples were ground and polished under a water-cooling condition in cross sections from one side to the center of wear scar. No cracks appeared on the wear scar cross-section of fresh enamel, while some microcracks were observed on the wear scar cross-section of dried enamel. In addition, it was found that the fracture toughness of enamel surface decreased with drying time. Obviously, the decrease of water content within enamel resulted in the reduction of its fracture toughness.

4.

Discussion Under oral conditions, masticatory forces range from 10 N to more than 150 N and the contact area is

restricted to a few square millimeters [39]. The maximal contact stress that is generated in enamel due to direct contact with opposite teeth or external objects can even reach 2.5 GPa [40,41]. According to Hertzian contact theory [42], when a normal impact load of 20 N was applied, the associated mean contact pressure between the SiC ceramic ball with a diameter of 6 mm and human tooth enamel flat sample was about 1.12 GPa, which was smaller than the compressive strength of enamel (about 1.85 GPa) [43]. The Young’s modulus and Poisson ratio used in this paper were 94 GPa and 0.28 for human enamel [15,44], and 410 GPa and 0.16 for SiC ball, respectively. Additionally, the diameter of wear scar on the surface of enamel was no more than 500 µm after 1×106 impact cycles, that is, the maximum of wear area was almost 0.2 mm2. Thus, a diameter of 6 mm seems to be appropriate for the ball counter-part. Under impact loading, compression deformation was much more apt to occur to the organic matter in the enamel. Meanwhile, the nano-fibers assembled by the hydroxyapatite particles glued together were crushed into hydroxyapatite particles. Hence, the surface damage of enamel was characterized mainly with slight plastic deformation during the initial process of impact wear, and no obvious material removal happened, as shown in Fig.2.a. The plastic deformation in the central area of impact mark grew further with the increase of impact cycles, and annular uplift appeared at the edge of impact mark (Fig.2.b). Due to the fact that the enamel contains 92–96 wt% hydroxyapatite, its resistance against plastic

deformation is limited. As the impact wear proceeded, therefore, delamination occurred as a result of microplastic extrusion on the enamel surface (Fig.2.c). That is, wear particles appeared at the contact interface of enamel and counter-part and acted as wear medium to bear normal loading. Obviously, with the occurrence of delamination, the number of microcontacts diminished, meaning that those wear particles will shoulder higher individual loads [45]. This could greatly accelerate enamel abrasion rate by effecting a transition from mild (microplasticity) to severe wear (microfracture) [45]. As a consequence, obvious material removal occurred, and the wear loss increased rapidly with the impact cycles (Fig. 7). Much debris appeared on the worn surface of enamel, as shown in Fig.2.d. Under impact loading, the size of the wear debris became smaller and smaller, and finally the worn surface was covered with a wear particle layer (Fig.2.e). The existence of wear particle layer could help increase the real contact area and then decrease the contact stress. Additionally, the particle layer suffered from repeated compaction and delamination under impact loading, which could absorb part of the impact energy. Thus, the wear area increased slowly with the further increase of impact cycles (Fig.2.f). And the wear rate decreased a little (Fig. 7). Our finding about the influence of wear particle on material removal was consistent with the results of Oscar Borrero-Lopez et al [45,46]. Their results indicated that enamel wear was sensitive to specific test conditions, and the intrusion of microscopic third-body particulates can accelerate the enamel removal rate by orders of magnitude [46]. Larger particles in a wear medium will make fewer microcontacts, and thus greatly accelerate enamel abrasion rate [45]. Obviously, with the particle size decreasing, the material removal rate would decrease. There existed some lacunae around enamel rods on the worn surface of enamel (Fig.6.a), suggesting that the inter-rod enamel was worn out easier than the enamel rods during the process of impact wear. Compared with the inter-rod enamel, the enamel rods contain more inorganic substances and have higher hardness and elastic modulus [15,16]. Thus, most of the impact loading applied to enamel surface was born by the enamel rods, while large plastic deformation and even delamination occurred to the protein-rich inter-rod enamel. Both the plastic deformation and delamination of the inter-rod enamel could absorb some of the impact energy, and then to a certain extent, the inter-rod enamel acted as a stress buffer to protect the enamel rods, the main structure and load bearing unit of enamel, from severe damage. Furthermore, the impact wear behaviour of enamel was observed to be similar to that of metallic materials. Firstly, iridescent rings appeared around the wear scar during the process of impact wear. Secondly, the surface hardness of enamel increased after impact loading, and the increase of hardness became more and more obvious with the increase of impact number. The above observations suggested that the impact wear behavior

of enamel was “metallic-like” to some degree. The appearance of iridescent rings was very interesting. In general, when metallic materials suffer from impact loading, iridescent rings are commonly seen around impact marks as a result of high temperature oxidization [47] or plastic deformation [48]. Given that the primary composition of enamel is hydroxyapatite, the iridescent rings on the enamel surface should mainly result from plastic deformation. Due to the high hardness of enamel, only slight plastic deformation occurred in the central area of impact mark at the early stage of impact wear (Fig.2.a). With the number of impact cycles increasing, the plastic deformation was aggravated, and its area spread out from the center of impact mark. Thus, iridescent rings were formed around the impact marks (Fig.2.b). As the impact wear proceeded further, delamination occurred, and then the iridescent rings gradually disappeared (Fig.2.e). It has been widely accepted that the surface hardness of most metallic materials increases under impact loading, which is generally called work hardening. As for the enamel, the “surface hardening” should be related to its unique composition and microstructure. Under impact loading, the protein-rich inter-rod enamel suffered from compression deformation, while the nano-fibers within enamel rods were crushed into hydroxyapatite particles. The more the impact cycles was, the more compacted the hydroxyapatite particles were. Hence, the hardness of the worn enamel surface increased with the number of impact cycles (Fig. 4). Our observations were consistent with the results of He and Swain [49]. Their nanoindentation test results indicated that enamel had similar stress–strain response to that of cast alloy and gold alloy, all of which showed work-hardening effect. They pointed out that the mechanical properties of enamel were metallic-like, which was mainly resulted from the small remnant volume fraction of protein fragments within the enamel. It should be noted that for the dried enamel, no obvious iridescent rings were observed around the wear scars (Fig. 5). The result demonstrated that with the water content within enamel being removed, the “metallic-like” impact wear behavior of enamel became inconspicuous. Due to 92–96 wt% hydroxyapatite, enamel is often considered as a biological ceramic material. However, our recent studies showed that human tooth enamel exhibited not only a different microtribological behavior from artificial hydroxyapatite [17] but also a different sliding wear behavior from brittle ceramic material of Al2O3 [50]. Guidoni et al found that the wear behavior of enamel at the nano-level resembled that of glass at low loads (50 µN) and that of metal mono-crystals at high load (100 µN) [51]. Additionally, the indentation stress–strain curves and creep behavior of enamel were reported to be totally different to pure hydroxyapatite [49]. It was suggested that the matrix proteins and water within the enamel play an important role in regulating the mechanical behavior of enamel as a biocomposite [30,52]. Most free water within enamel is located within the protein matrix and has an influence

on the enamel’s compressibility, permeability and ionic conductivity [30]. Once the water was removed, the degree of plastic deformation that the enamel can sustain without delamination reduced. Thus, almost no iridescent rings appeared on the worn surfaces of dried enamel. In this study, the fracture toughness of enamel surface was found to decrease with the increase of drying time. Generally, the impairment of materials’ fracture toughness means a high risk of the initiation and propagation of cracks on subsurface. As shown in Fig. 8, no cracks appeared on the wear scar cross-section of fresh enamel, but some microcracks were observed on the wear scar cross-section of dried enamel. Hence, delamination occurred not only earlier but also more significantly on the worn surface after drying treatment (Fig. 5). As a result, a wear particle layer was formed earlier on the worn surface of the dried enamel than the fresh enamel (Fig. 6). The existence of wear particle layer normally is helpful to decrease the wear rate during the next process of impact wear [53]. So, the wear rate decreased after 5.5×105 cycles for the dried enamel, but after 8×105 cycles for the fresh enamel. It seems that the water content within enamel could prevent the surface from impact wear. Tooth wear in the mouth is a complex multifactorial phenomenon involving chemical, physical, and mechanical processes. This study investigated the impact wear of human tooth enamel in order to extend the understanding of the wear mechanism of human teeth. It would be helpful in deepening the scientific knowledge of natural biological materials and developing new dental materials and bionic design. Our future studies will explore the interaction of other factors, such as tooth-to-tooth variation, different conditions, and “the third body”, for example, food slurry, on impact wear behavior of human teeth.

5.

Conclusions The impact wear process of the fresh and dried human tooth enamel was investigated by specially

designed ball-on-flat impact tester. Based on the given testing conditions, the conclusions could be summarized as follows: (1) The surface damage of fresh enamel was dominated by plastic deformation at the early stage of impact wear, and iridescent rings appeared around the impact mark as a result of the accumulation and spread of plastic deformation. As the impact wear proceeded, delamination occurred on the surface of enamel, and then the iridescent rings gradually disappeared. The wear loss increased rapidly with the increase of impact cycles. With a wear particle layer being formed on the surface of enamel, the wear rate decreased. The surface hardness of enamel increased with the increase of impact cycles, and no cracks appeared on the cross section of wear scar.

(2) The fracture toughness of enamel decreased with the increase of drying time. For the dried enamel, microcracks appeared on the cross section of wear scar. More obvious delamination and higher wear loss occurred on the worn surface in comparison with the fresh enamel. No iridescent rings appeared on the surface of dried enamel during the whole process of impact wear. (3) The impact wear behavior of sound human tooth enamel was metal-like to some degree, and no subsurface cracking occurred. The water content within enamel could protect the surface from impact wear. The wear mechanism of human tooth enamel is closely associated with its microstructure.

Acknowledgements This work was supported by National Natural Science Foundation of China (51222511, 51305366 and 51535010), and Sichuan Youth Science & Technology Foundation (2012JQ0016).

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Figure captions

Fig.1: Schematic representation of enamel samples’ preparation Fig.2: LCSM micrographs of wear scars on the surfaces of fresh enamel subjected to different impact cycles, (a) 5×103 cycles; (b) 5×104 cycles; (c) 2.5×105 cycles; (d) 5.5×105 cycles; (e) 8×105 cycles; (f) 1×106 cycles Fig.3: Profile of wear scars on the surfaces of fresh enamel Fig.4: A comparison of surface hardness of the unworn region and worn region of fresh enamel subjected to different impact cycles Fig.5: LCSM micrographs of wear scars on the surfaces of dried enamel, (a) 5×104 cycles; (b) 2.5×105 cycles Fig.6: SEM micrographs of the worn surfaces of the fresh and dried enamel, (a) fresh enamel, 8.0×105 cycles; (b) dried enamel, 5.5×105 cycles Fig.7: Wear volumes of fresh enamel and dried enamel subjected to different impact cycles Fig.8: SEM micrographs of the wear scar cross-section after 1×106 cycles, (a) fresh enamel; (b) dried enamel

Fig. 1

Mesial-distal division line Lingual side Buccolingual division Buccal side

Fig. 2

Plastic

Enamel

Fig. 3

(a)

(b)

(c)

(d)

(e)

(f)

2 3

5×10 cycles 4 5×10 cycles 5 2.5×10 cycles 5 5.5×10 cycles

Depth/μm

0 -2

5

8.0×10 cycles

-4

6

1.0×10 cycles

-6 -8 -10 -12 -350

-150

50

250

Width/μm

Fig. 4

450

500 Unworn region

Worn region

Hardness/HV50g

400

300

200

100

0 5.0x103

5.5x105

Impact number

Fig. 5

1.0x106

(a)

Fig. 6

(b)

(a)

Fig. 7

(b)

Wear Volume/×104μm3

40 Dried enamel Fresh enamel

30

20

10

0 0

20

40

Impact

Fig. 8

60

80

number/×104

100

120

(a)

(b)

Table caption

Table1: The mechanical parameters of enamel and SiC

Sample

Hardness (GPa)

Elastic modulus (GPa)

Poisson ration

Enamel

3.60

94

0.28

SiC

24.50

410

0.16