journal of the mechanical behavior of biomedical materials 40 (2014) 23 –32

Available online at

Research Paper

Investigation of the time-dependent wear behavior of veneering ceramic in porcelain fused to metal crowns during chewing simulations Jiawen Guoa, Beimin Tiana, Ran Weib, Weiguo Wanga, Hongyun Zhangc, Xiaohong Wua, Lin Heb,nn, Shaofeng Zhanga,n a

State Key Laboratory of Military Stomatology, Department of Prosthodontics, School of Stomatology, Fourth Military Medical University, Changle Xi Road 145, Xi’an 710032, China b State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China c State Key Laboratory of Military Stomatology, Department of Oral Anatomy and Physiology, School of Stomatology, Fourth Military Medical University, Changle Xi Road 145, Xi’an 710032, China

art i cle i nfo

ab st rac t

Article history:

The excessive abrasion of occlusal surfaces in ceramic crowns limits the service life of

Received 14 April 2014

restorations and their clinical results. However, little is known about the time-dependent wear

Received in revised form

behavior of ceramic restorations during the chewing process. The aim of this in vitro study

28 July 2014

was to investigate the dynamic evolution of the wear behavior of veneering porcelain in PFM

Accepted 10 August 2014

crowns as wear progressed, as tested in a chewing simulator. Twenty anatomical metal–

Available online 19 August 2014

ceramic crowns were prepared using Ceramco III as the veneering porcelain. Stainless steel


balls served as antagonists. The specimens were dynamically loaded in a chewing simulator


with 350 N up to 2.4  106 loading cycles, with additional thermal cycling between 5 and 55 1C.


During the testing, several checkpoints were applied to measure the substance loss of the

Metal ceramic crowns

crowns’ occlusal surfaces and to evaluate the microstructure of the worn areas. After 2.4  106

Dynamic process

cycles, the entire wear process of the veneering porcelain in the PFM crowns revealed three

Chewing simulator

wear stages (running-in, steady and severe wear stages). The occlusal surfaces showed traces of intensive wear on the worn areas during the running-in wear stage, and they exhibited the propagation of cracks in the subsurface during steady wear stage. When the severe wear stage was reached, the cracks penetrated the ceramic layer, causing the separation of porcelain pieces. It also exhibited a good correlation among the microstructure, the wear loss and the wear rate of worn ceramic restorations. The results suggest that under the conditions of simulated masticatory movement, the wear performance of the veneering porcelain in PFM crowns indicates the apparent similarity of the tribological characteristics of the traditional mechanical system. Additionally, the evaluation of the wear behavior of ceramic restorations should be based on these three wear stages. & 2014 Elsevier Ltd. All rights reserved.


Corresponding author. Tel.: þ86 29 82665165/þ86 29 84776468. Corresponding author. E-mail addresses: [email protected] (L. He), [email protected] (S. Zhang).

nn 1751-6161/& 2014 Elsevier Ltd. All rights reserved.



journal of the mechanical behavior of biomedical materials 40 (2014) 23 –32


With their excellent aesthetics and acceptable physical properties, crowns and bridges with a ceramic–metal structure or a full ceramic structure have been widely used in prosthodontic treatment (Conrad et al., 2007; Zarone et al., 2011). Ceramic restorations that replace missing teeth, which aim to rehabilitate the mastication and aesthetic results of natural teeth, undergo repeated load cycles during functional conditions (Esquivel-Upshaw et al., 2012). However, due to antagonistic occlusal contacts, some ceramic particles are bound to be lost gradually from the contact area of the crown’s surface. This phenomenon is known as wear (Heintze et al., 2008). In the oral environment, wear behavior can adversely affect the structural stability and mechanical properties of the ceramic layer, which constitutes the load-bearing portion of the restoration (Rosentritt et al., 2011). Any causes of wear, including chewing or clenching, can induce surface and subsurface defects (Kim et al., 2008; Suputtamongkol et al., 2010). By the action of stress, the nucleation and propagation of cracks can originate from these flaws (Albakry et al., 2003), and these cracks can penetrate the porcelain, eventually resulting in the clinical failure of the restoration (Aboushelib et al., 2009). When choosing ceramic crowns for restorations, the wear behavior should be considered among the most important factor because it is an irreversible and unavoidable process (Mehta et al., 2012). An appropriate wear resistance or a mild wear regime is able to guarantee the long-term stability of the ceramic restorations, when they are subjected to repetitive masticatory force in the mouth. In contrast, the severe wear of ceramic restoration (rapid material loss at contacting surface of ceramic) is regarded as a significant cause, which lead to the failure eventually (Ren and Zhang, 2014). Therefore, the wear properties of ceramic restorations have a great influence on therapeutic outcomes. Furthermore, oral wear is a complex process that is influenced by many internal and external factors, including the different kinds of restorative materials, surface treatments, parafunctional habits, neuromuscular forces and the properties of saliva (Elmaria et al., 2006; Johansson et al., 1993; Kim et al., 2001; Mayworm et al., 2008). However, most of the early studies have evaluated the wear behavior of dental ceramics according to the traditional fixed pattern, comparing different test groups after the same number of predefined wear cycles (Albashaireh et al., 2010; Hahnel et al., 2011; Mormann et al., 2013; Preis et al., 2011, 2012). In this manner, the wear performance of dental ceramics can only be partially revealed. Using a pin-onblock design, Preis et al. (2011) compared the two-body wear resistance of the zirconia substructure and veneering porcelain. After 1.2  105 wear cycles, zirconia showed higher wear resistance than porcelain, while among various glass ceramics, the wear loss of fluorapatite and nano-fluorapatite glass ceramics was significantly greater than that of leucitereinforced and lithium disilicate glass ceramics (Albashaireh et al., 2010). For the same kind of material, different surface treatments also play important roles in wear performance (Preis et al., 2012). After the same duration of wear testing, glazed ceramic specimens exhibit much more loss than those after polishing. It seems that most researchers are accustomed

to estimating oral wear at a fixed point. In particular, researchers have almost entirely neglected that the wear behavior of ceramics over the entire wear process is not invariable (Wang and Hsu, 1996). The original method that directly compared the wear loss of different specimens at a fixed perspective do not fit to the description in the tribology theory. Normally, based on the tribology theory, the entire wear process of materials should be divided into a running-in wear stage, steady wear stage and severe wear stage (Wen and Huang, 2012). A block of material that experiences mechanical interaction between two relatively moving surfaces would undergo these wear stages in sequence before it wore out or failed. During the running-in stage, the wear loss of the material increases rapidly, and the wear rate of the wear components is higher at the beginning. As the wear process continues, the wear rate gradually decreases from the running-in to the steady wear stage, during which the wear loss grows slightly and slowly. The wear rate then begins to increase from the steady wear to the severe wear stage, which also contributes to the dramatic growth of wear loss. According to different wear rates and wear statuses, the wear behavior of a material reaches the corresponding wear stage. The wear properties of materials, represented by a wear curve – demonstrating that wear behavior continuously changes with the increase in service time – are usually considered the typical tribological characteristics of timedependent wear behavior. And the wear rate, the microstructure of the wear facet and the structure stability of wear component are also not constant. Undoubtedly, investigations of the wear behaviors of materials, based on the change rule in the wear curve, will provide more rational and comprehensive results. Many studies have discussed the wear behavior of dental ceramics from different perspectives; however, only limited studies have been available regarding the entire wear process of the veneering porcelain of restorations. And, during the chewing process, whether the wear behavior of ceramic crown has the similar property of timedependent has not been reported yet. Clinical testing is the ideal method for estimating the complex wear performance of ceramic restorations. However, these in vivo studies have often been time consuming and costly. Even great variation among subjects is unavoidable (Hickel et al., 2007). In contrast, laboratory tests of wear behavior with chewing simulators, which allow for the achievement of comparable results with different materials under standardized conditions, seem to constitute effective methods for the pre-clinical evaluation of dental ceramics (DeLong and Douglas, 1983). Specifically, cyclic mechanical loading and thermal cycling, generated by a chewing simulator, represent the ideal in vitro design for studies to reproduce physiological function (Rosentritt et al., 2006). Therefore, in this study, glass-ceramics were fused to a base alloy, forming anatomical porcelain-fused-to-metal (PFM) crowns. All of these specimens were mounted in a chewing simulator to undergo wear cycle testing. The aim of this in vitro study was to investigate the dynamic evolutionary process of the wear behavior of veneering porcelain in PFM crowns under conditions that mimicked the masticatory movement and moisture conditions of the oral cavity. The

journal of the mechanical behavior of biomedical materials 40 (2014) 23 –32

hypothesis of this study was that the entire wear process of veneering porcelain would consist of three wear stages.


Materials and methods


Preparation of PFM crown specimens

An artificial mandibular first molar positioned in a mannequin was prepared for a complete crown. The preparation consisted of 2.0 mm of occlusal reduction, 1.5 mm of axial reduction and a 1.0 mm shoulder finish line with rounded internal angles that extended 360 degrees around the tooth, following the outlined cementoenamel junction. The axial walls had an approximate taper of 6 degrees. Then, the prepared molar received an impression with polyvinylsiloxane material (Express, 3M ESPE, St. Paul, MN, USA), which was filled with layers of resin-based composite (Z100, 3M ESPE, St Paul, MN, USA) and was cured according to the manufacturer’s recommendations and multiplied, resulting in 20 identically prepared tooth replicas (Fig. 1a). These replicas were incubated in water for a minimum of 30 days to assure water hydration and to eliminate any dimensional alterations after crown cementation (Huang et al., 2008). Following incubation, the replicas were embedded in epoxy resin that was poured into PVC tubes 25 mm-diameter, with their long axis aligned with the tube’s long axis. The finish line preparations were located 2 mm above the resin surface. An impression of each replica was obtained using polyether material (Impregum F, 3M ESPE, St. Paul, MN, USA), and working dies of each replica impression were made of class IV dental stone (Die-Stone, Heraeus Kulzer Dental, Shanghai, China). Based on these stone dies, standard design frameworks with an even thickness of 0.5 mm and 20 metal cores


(nickel–chromium, Co 60%, Cr 24%, Heraeus Kulzer Dental, Shanghai, China) were conventionally fabricated and cast, respectively, according to the manufacturer’s directions (lost wax technique) (Fig. 1b). All 20 castings were placed in an ultrasonic cleaner and were inspected under 10x magnification (Olympus BH2; Olympus Corp, Tokyo, Japan) for surface irregularities. After the removal of all of the positive internal irregularities with a round bur (Dura-Green Stones TC4 HP0042, Shofu, Tokyo, Japan), these frameworks were veneered with porcelain (hand-layered technique) (Ceramco III; Dentsply, Burlington, NJ, USA) by the same experienced technician, following the manufacturer’s guidelines. An impression of the desired anatomy of a waxed mandibular first molar was used to guide the porcelain veneering contour. The approximate porcelain thickness was 1.0 mm on the axial walls and 1.5 mm on the occlusal surface (Fig. 1c). All of the crowns were tried in, and no internal adjustments were necessary for an appropriate fit. The crowns were glazed and cemented on the respective composite resin replicas, using a resin cement (PANAVIA F, Kuraray Medical Inc, Tokyo, Japan) according to the manufacturer’s instructions. The crowns were fully seated with finger pressure. The crown/die assembly was placed under a 50 N static load. After 60 s, excess cement was removed. Ten minutes later, the PFM crown specimens were rinsed thoroughly with an air–water spray. Finally, any residual excess cement was removed, and all of the crowns were stored in 37 1C distilled water for at least 24 h for future use (Fig. 1d).


Wear testing in the chewing simulator

To simulate the wear that occurs in occlusal contact, each specimen was mechanically tested with a custom-designed

Fig. 1 – Pictures of specimen: (a) tooth replica; (b) metal core of crown; (c) PFM anatomical single crown; (d) assembled PFM crown specimen.


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chewing simulator. A schematic diagram of the testing machine used in this research appears in Fig. 2. In principle, the load was produced by the compressed spring (7), which was confined in a cylindrical container (8). Each container was mounted on a separate vertical bar (6), and each test chamber (4) had a vertical bar and a cylindrical container. The chewing simulator contained 12 test chambers. Every 6 vertical bars were linked by a transverse bar (5), driven by a step motor (1). All 12 test chambers were divided into two columns (2) arranged parallel to each other on a flat plane (3). The rotation of the transverse bar induced axial movement of the vertical bar and its cylindrical container. When the indenter (9), which had a rigid connection with the lower surface of the cylindrical container, came into contact with the specimen (11), the spring became pressed and the desired loading force was released. In this manner, the apparatus provided reciprocating movement and cyclic loading of the testing sample. Some similarity to the masticatory pattern had been achieved. In addition, the simulator included a thermocycling system, using magnetic valves in conjunction with a heating and cooling system controlled by computer programs. Through a metal pipe (10), hot or cool water was sprayed on the specimen at regular intervals. For wear loading, metal sphere indenters were used as antagonists to substitute for cusps of teeth. These metallic balls with a diameter of 10 mm were made of stainless steel (ASTM 403), and had a Vickers hardness value of about 350 HV after a quenching and tempering treatment. The hardness is close to that of natural enamel. The full crown specimens were set up in brass sample holders (12) immediately beneath the indenters. With the antagonist declined, it first contacted the lingual inclined surface of the disto-buccal cusp 1 mm from its cuspal tip and then passively slid into the central fossa, accompanied by approximately 1 mm of lateral movement. When it reached the final cusp-fossa relationship, three occlusal point contacts (distobuccal, mesiolingual and distolingual) were created. Both the beginning and the final contact positions between the antagonists and crowns were assessed with black articulating paper (thickness of 20 μm) and were adjusted if necessary. Then, the specimens were fastened by pouring PMMA resin (GC Unifast Trad, GCCorporation, Tokyo, Japan) into brass holders. In the current

Fig. 2 – Schematic drawing of the chewing simulator: (1) step motor; (2) two columns of the test chamber; (3) flat plane; (4) test chamber; (5) transverse bar; (6) vertical bar; (7) compression spring; (8) cylindrical container; (9) indenter; (10) metal pipe; (11) specimen; (12) sample holder.

wear test, the loading force was 350 N, which was in an appropriate load range measured during mastication and swallowing (Kelly, 1999). According to the literature, the wear produced by 240,000–250,000 masticatory cycles in a chewing simulator corresponded to the wear measured after 1 year of clinical service (Sakaguchi et al., 1986). Therefore, to simulate 10 years of clinical service, a total of 2.4  106 loading cycles were performed. During wear simulation, the specimens were subjected to thermal cycles in distilled water at alternating temperatures of 5 and 55 1C, with a duration of 2 min for each cycle.


Wear measurements and date analysis

To observe the dynamic evolution process of the wear behavior of veneering porcelain in PFM crowns, seven checkpoints were employed over 2.4  106 cycles. After 1.25  105, 2.5  105, 5  105, 1  106, 1.5  106, 2  106 and 2.4  106 loading cycles each, impressions of the specimens’ occlusal surfaces (n ¼12) were obtained using a polyvinylsiloxane impression material. Replicas were made with epoxy resin, and the substance loss of the replicas’ surfaces was measured with a non-contact 3D white light profilometer (PS50, Nanovea, Irvine, CA, USA) and its dedicated software (Nanovea 3D Software, Nanovea, Irvine, CA, USA). The profilometer consisted of a three-dimensional digitizer and a PC with a color monitor. The digitizer had a motordriven XY measurement table and a vertically adjustable optical sensor with an auto-focus system. The sensor had a measurement range of 12 mm in the vertical direction. It also had vertical resolution of 200 nm and lateral resolution of 500 nm. When the wear test reached a checkpoint, each replica of twelve crown specimens was embedded in PMMA resin with its occlusal surfaces facing vertically upward, using the same split mold (stainless steel, hollow cylinder shaped: 20 mm inner diameter  14.5 mm height). Three reference marks were made on the top surface of the resin stub around the replicate crown, by pressing a wax tool tip against the uncured resin. After curing, the entire resin replica was removed from the mold and placed under the optical sensor. The scanned area was 7 mm  7 mm and contained the entire replicate crown and three reference marks. All of the scans were obtained at a 30 Hz frequency using 50 μm step sizes in both the x and y directions. In the resultant digitized images, three reference marks, which acted as reference points, could determine a plane, making all of the specimens comparable. In the present study, the surface area of the wear facets was chosen as the parameter representing the substance loss of the veneering porcelain. Defining the wear area on each crown, the surface area of the wear facets was calculated using the dedicated software. As the wear test proceeded, the ceramic loss at all seven checkpoints was evaluated in sequence. After collecting all of the separate data at different checkpoints, a fitted wear curve was generated, which reflected the dynamic evolution process of the wear behavior of the crown’s occlusal surface. The wear rate was calculated from the derivative of the wear loss data relative to time, using numerical differentiation.

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2.4. Morphological observations of the wear surfaces and subsurfaces To reveal the different statuses of the microstructure over the entire wear test, four observation points were selected, which represented the period before the test and the early, middle and later wear periods respectively. After 0, 2.5  105, 1  106 and 2.4  106 loading cycles each, two samples were gold sputtered (MC 1000, Hitachi High-Technologies Corporation, Tokyo, Japan), followed by morphological observation of the worn surface using a scanning electron microscope (SEM) (S-4800, Hitachi HighTechnologies Corporation, Tokyo, Japan). Then, these samples were embedded in PMMA resin and were cross-sectioned along the central area of the wear surface, parallel to the sagittal plane of the crown. After being serially polished with increasingly finer grit silicon carbide paper and diamond paste, the subsurface of the worn area was also gold sputtered and was inspected by means of the SEM.


Statistical analysis

All of the wear rate data were analyzed using repeated measures ANOVA to determine whether the differences between the mean wear rate values of veneering porcelain observed at each checkpoint were statistically significant (α¼0.05). A repeated measures design was used because repeated measurements of the same sample were obtained at different time intervals. Tukey’s post-hoc test was used for multiple pair-wise comparisons of the means. By comparing the mean wear rate of each pair of neighboring checkpoints, three wear stages were distinguished.



All of the crowns survived 2.4  106 cycles, and none of them exhibited any signs of defects after dynamic loading. Only typical worn surfaces were observed on the veneering porcelain of the PFM crowns. The mean wear loss and wear rates of Table 1 – Summary of wear loss and wear rate after various chewing cycles (n ¼12). Cycles (1  104 times)

Mean wear loss and standard deviations of tested porcelain (in mm2)



12.5 25 50 100 150 200 240

2.4170.47 3.7270.57 5.8970.65 7.2370.84 7.5370.85 10.4371.54 12.8571.26


the crowns after different chewing cycles are summarized in Table 1. Setting the chewing cycles as the x-axis, a wear curve is presented below (Fig. 3), showing the tendency of three distinct wear stages over the whole test. Overall, the boundary between the running-in wear stage and the steady wear stage was situated at approximately 1  106 cycles because the mean wear rate at 1  106 cycles (0.02770.017 mm2/1  104 cycles) was significantly lower than that at 5  105 cycles (0.08770.034 mm2/1  104 cycles) (po0.05), while no significant difference in wear rate was observed between 1  106 and 1.5  106 cycles. Thus, under the conditions of the current study, the running-in wear stage of the veneering porcelain ranged from about 0 to 1  106 cycles. During this period, the mean surface area of the wear facets increased continuously, while the wear rate decreased from 0.19370.038 mm2/1  104 cycles to 0.02770.017 mm2/1  104 cycles. Furthermore, the boundary between the steady wear stage and the severe wear stage was situated at approximately 1.5  106 cycles, because the wear rate between 1  106 and 1.5  106 cycles was always sustained at a relatively stable low value, while the wear rate at 2.0  106 cycles (0.05870.017 mm2/1  104 cycles) obviously increased, compared with that at 1.5  106 cycles (0.0067 0.003 mm2/1  104 cycles) (po0.05). Therefore from about 1  106 to 1.5  106 cycles, the porcelain of the PFM crowns underwent a steady wear stage such that the mean surface area of the wear facets increased slowly and slightly. After approximately 1.5  106 cycles, the severe wear stage was reached during which both the mean surface area of the wear facets and the mean wear rate showed upward trends. Although the reduction in the wear rate between 2.5  105 and 5  105 cycles did not showed a significant difference (p40.05), the mean wear rate at 1  106 cycles resumed showing an obvious decrease, compared with that at 5  105 cycles (po0.05). Additionally, the period between 0 to 1  106 cycles demonstrated an overall downward trend. Therefore, the wear process between 2.5  105 and 5  105 cycles belonged to the running-in wear stage and could not be considered a boundary that distinguished different wear stages. Based on the wear curve determined above, morphological observations at 0, 2.5  105, 1  106 and 2.4  106 cycles demonstrated the veneering porcelain of PFM crowns situated before the test period and during running-in, steady and severe wear

Mean wear rate and standard deviations of tested porcelain(in mm2/ 1  104 cycles)

0.19370.038 0.10570.045 0.08770.034 0.02770.017 0.00670.003 0.05870.017 0.06170.017

Tukey test A B B CD D BC BC

The means are not statistically different at P¼ 0.05 in groups with the same letter (Tukey test).

Fig. 3 – The wear curve of dental porcelain over the entire wear process.


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Fig. 4 – SEM images of the morphology of the wear facets of veneering porcelain on PFM crowns at different stages: (a) before the test; (b) dotted circle on (a) at greater magnification; (c) running-in wear stage; (d) dotted circle on (c) at greater magnification; (e) steady wear stage; (f) dotted circle on (e) at greater magnification; (g) severe wear stage; (h) dotted circle on (g) at greater magnification.

stages, respectively. Regarding these morphological observations of the wear surface, SEM images of the non-dynamic loading specimens showed intact and smooth surfaces (Fig. 4a and b), while during the running-in wear stage, roughening of the ceramic surfaces was observed over the occlusal contact area. The mass of small wear traces was densely distributed over these roughening surfaces (Fig. 4c and d). During the steady wear stage, these wear traces grew larger and coalesced with each other, forming several cracks (Fig. 4e), while some relatively smooth surfaces emerged among the roughening surfaces (Fig. 4f). When the severe wear stage was reached, the size of cracks increased, while the area of relatively smooth surfaces decreased (Fig. 4g and h). Regarding morphological observations of the subsurface of the worn area, SEM images of the non-dynamic loading specimens showed an intact inner structure of porcelain and a continuously smooth occlusal surface (Fig. 5a and b).

During the running-in wear stage, SEM fractographic analysis revealed an uneven occlusal surface with many tiny defects (Fig. 5c and d), while the inner structure of porcelain remained intact. During the steady wear stage, the initiation of cracks occurred at the shallow subsurface of the porcelain, closing onto the occlusal surface (Fig. 5e and f). The path of propagation of the cracks was oblique. When the severe wear stage was reached, the bulk of the porcelain was fractured, leaving hollow defects on the occlusal surface (Fig. 5g and h).



In this study, the time-dependent wear behavior of veneering porcelain in PFM crowns was investigated in a custom-designed chewing simulator. The research hypothesis that the wear process of the veneering porcelain would consist of three

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Fig. 5 – The sectional SEM morphology of worn veneering porcelain on PFM crowns at different stages: (a) before the test; (b) dotted circle on (a) at greater magnification; (c) running-in wear stage; (d) dotted circle on (c) at greater magnification; (e) steady wear stage; (f) dotted circle on (e) at greater magnification; (g) severe wear stage; (h) dotted circle on (g) at greater magnification.

distinct wear stages was confirmed. According to the wear curve, the tendency of the three wear stages could be identified. From about the beginning to 1  106 cycles, the porcelain of the PFM crowns underwent a running-in wear stage; from about 1  106 to 1.5  106 cycles, the samples were sustained in a steady wear stage; and after approximately 1.5  106 cycles, they reached a severe wear stage. Although the wear loss of the PFM crowns’ occlusal surfaces showed an obvious increasing trend during the running-in and severe wear stages, it increased only slightly during the steady wear stage. The wear rate first decreased from the running-in to the steady wear stage and then increased from the steady to the severe wear stage. These changing features in wear behavior were similar to the typical tribological characteristics of wear components, which usually showed the changing trend of the three wear stages during the entire wear process (Wen and Huang, 2012). Among different wear assays, the simulation of masticatory movement by chewing simulators has been widely used and has

been perceived as a valid model for in vitro testing, to provide a clinically relevant loading pattern and test environment (Heintze, 2006; Mehl et al., 2007; Mormann et al., 2013; Preis et al., 2012). When the blunt indenter contacted with the occlusal surface of the ceramic crown, several different types of stress zones generated in the bi-layer structures. Apart from the inevitable phenomenon of oral wear, these stress fields were also responsible for the generation of cone cracking, plastic deformation and radial cracking respectively (Lawn et al., 2004). And these failure (cracking) modes were closely related to the clinical failed ceramic restorations (Zhang et al., 2013). In addition, there were two main reasons for choosing the veneering porcelain of PFM crowns as the representatives: metal–ceramic crowns remain widely used clinically (Reitemeier et al., 2006; Walton, 1999), and compared with lithium disilicate ceramic and zirconia ceramic, veneering porcelain, which is a feldspathic ceramic, usually has moderate wear resistance (Kim et al., 2012; Preis et al., 2011; Wang et al., 2012), which would render the


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entire test more efficient. All of the specimens were made into anatomical full PFM crowns, aiming to bridge the gap between laboratory tests and clinical cases. Sornsuwan et al. (2011) indicated that occlusal surface geometry should also be considered with regard to the generation of stress during occlusal contact. Therefore, the choice of a standard anatomical crown not only standardized the experimental conditions, but it also provided a clinically relevant stress state. Permanent thermal cycling with water kept the specimens wet during the entire test and caused additional aging of the ceramics. A wet environment should enhance subcritical crack growth due to structural changes in the ceramic (Taskonak et al., 2008). Throughout the entire wear process, the changing trends in the wear curve and the SEM images suggested that there existed a significant correlation among the wear loss, the wear rate and the microstructure of the full crown specimens. At the beginning of the wear testing, high contact stress was generated as a result of point contacts between the antagonist and the occlusal surface of the PFM crown. Under such stress conditions, the veneering porcelain, which consisted of a brittle glass ceramic with little or no plastic deformation, was subject to brittle breakage (Gonzaga et al., 2011; Zhang et al., 2013). Ravikiran and Lim (1999) also indicated that a greater load was more likely to cause ceramic particle fracture and easy separation from the base material. Therefore, during the running-in wear stage, the mass of superficial porcelain of the PFM crown in the contact zone, with a smaller contact area and under higher contact stress, experienced obvious wear damage and material loss. Correspondingly, the wear curve showed rapid growth during this period. SEM images of the wear facets during the running-in wear stage, revealing rough surfaces with grooves, defects and chipping (Fig. 4c and d), suggested active wear behavior at the occlusal surface. While from the cross-sectional view, brittle breakage and chipping were confined to the superficial ceramic layer, with the deeper ceramic structure of the subsurface remaining intact. As wear progressed, the contact area increased gradually, which in turn reduced the contact stress (Bailey et al., 1981). Consequently, the wear rate persistently decreased from the running-in wear stage to the steady wear stage. When it decreased to less than a certain threshold, a balance might be achieved among the stress condition, the wear behavior and the microstructure of the PFM crown. Such a period was called the steady wear stage, during which the surface area of the wear facets increased slowly and the wear rate maintained at a relatively low value. Under these circumstances, the stress intensity factor caused by the lower stress condition did not reach a critical level (Baran et al., 2001; Cesar et al., 2008). Therefore, delaminating and chipping of the superficial ceramic layer of the crown presented slow and stable growth, rather than dramatic fracturing, which predominated during the running-in wear stage. During the steady wear stage, decreasing the density of wear tracks and having smooth surfaces (Fig. 4e and f) were observed in the contact zone, indicating lower wear activity on the crown’s surface. However, from the cross-sectional view, the generated cracks were located in the shallow subsurface of the porcelain and closed onto the occlusal surface

(Fig. 5f), due to the dynamic loading (Kim et al., 2008; White et al., 1997). Compared with the intact inner structure of porcelain in the non-dynamically loaded specimens, the presence of cracks indicated that the mechanical properties of the veneering porcelain began to degenerate (Chen et al., 1999; Kelly, 1999). With the propagation of cracks and the penetration of the ceramic layer, the balance that was created during the steady wear stage collapsed. Blocks of ceramic material separated from the occlusal surface, leaving only pit-like defects (Fig. 5h), which increased the area of surface wear. Such a performance implied that the wear behavior of the crown reached the severe wear stage. Even the contact stress consistently decreased, so the subsurface ceramic structure, which was already fragile, might fracture easily under lower stress conditions (Attia et al., 2006; Kelly, 1999; Zhang et al., 2013). Ceramco III, which is generally used for the veneering of metal substructures with the hand-layered technique, is a type of low-fusing feldspathic-based glass-ceramic (Cesar et al., 2008). With its fine-grained leucite (K2O  Al2O3  4SiO2) crystalline phase (Cattell et al., 2001; Ong et al., 2000), the mechanical properties of dental porcelain can be promoted to some extent due to the toughening mechanisms of crack deflection and crack-tip shielding action (Cesar et al., 2005; Morena et al., 1986). However, the inherent brittleness of the ceramic remains unchanged because it continues to exhibit low tensile strength and low ductility and toughness values (Quinn et al., 2003). Both brittle fracturing and fatigue wear are known to be responsible for the wear behavior of ceramic materials (D’Incau et al., 2012; Mair et al., 1996; Suputtamongkol et al., 2010). According to Mair et al. (1996), the compression and tension zones were generated in porcelain when an antagonist contacted and slid over porcelain. With limited capacity for plastic deformation, microcracks can nucleate and propagate as a result of repeated stress (Yu et al., 2006). Eventually, these cracks might connect with one another, resulting in the separation and delamination of material pieces. For the veneering porcelain of the PFM crown investigated in this study, the wear behavior during the running-in wear stage was dominated by brittle fractures of the superficial ceramic, which resulted in a large number of wear traces, while with the ongoing wear process, the wear behavior was converted to fatigue wear during the severe wear stage. When the propagation of cracks reached the occlusal surface, fairly large fragments of material detached. As for the steady wear stage, it was a transitional period before the cracks reached the critical condition. Our investigation of the three featured wear stages of PFM crowns should be a starting point for exploring their underlying wear mechanisms, adding a new dimension to the understanding of the complex wear behavior of the oral cavity and providing a novel method for estimating the wear resistance of all kinds of ceramic restorations. The previous evaluation methods, which mainly focused on the amount of wear loss but ignored the existence of three wear stages, might not be ideal for analyzing the wear behavior of ceramic restorations. For example, after the same number of wear cycles, the wear loss of two different kinds of ceramic crowns are at the same level, but the wear statuses are in different wear stages, and the situation is too arbitrary to decide whether the two kinds of

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crowns have the same wear resistance. Moreover, when under the same test conditions, if the one crown with low wear loss reaches the severe wear stage, while another experiences relatively high wear loss but is still in the steady wear stage, it’s also irrational to conclude that the wear performance of the former is superior. In addition, the duration of each wear stage should also be a primary determinant regarding the evaluation of wear performance. Rather than reaching a certain condition instantaneously, the change in wear behavior between the different stages was likely to be experienced as a gradual transition because the wear rate and the microstructure of the PFM crown maintained constantly evolving processes. Therefore, in the current study, the boundaries of the three wear stages, which were determined by the wear curve and by statistical analysis, were simply approximate demarcations of wear behavior, mainly aiming to reveal the existence of different wear stages during the crown’s mastication. In addition, according to several surveys, the maximum bite force of the population ranges from 446 N to 1221 N (Bonakdarchian et al., 2009; Ferrario et al., 2004; Regalo et al., 2008), and when chewing different kinds of food, the crushing forces are also variable (55.6 to 529.5 N) (Schindler et al., 1998). Although the ceramic prostheses were not always under maximum bite forces in clinical conditions, it was necessary to consider the possibility that the ceramic restorations would work under different complex and rigorous conditions for long duration. Moreover, when opposing different antagonists, the porcelain exhibited corresponding wear resistance (Kadokawa et al., 2006), because the wear components were always paired. Therefore, the influence of different stress levels on the changing trend in wear behavior, as well as the way how veneering porcelain performs in combination with enamel, deserves to be investigated in further studies.



Under the simulation of masticatory movement, the timedependent wear behavior of the veneering porcelain of PFM crowns exhibited three typical wear stages (running-in wear stage, steady wear stage and severe wear stage), similar to the tribological characteristics presented in mechanical systems. Both the wear loss and the wear rate were not linearly dependent. The wear rate decreased firstly and increased subsequently, maintaining the lowest wear rate during the steady wear stage. Meanwhile, no matter the worn surface or the subsurface, the microstructure of ceramic crown degenerated gradually during the entire wear process.

Acknowledgements The authors thank Prof. Zhao Xinyi and Mr. Gong Xu from the Department of Dental Materials of the Fourth Military Medical University for their technological support. This study was financially supported by the Natural Science Foundation of China (no. 81371176).


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Investigation of the time-dependent wear behavior of veneering ceramic in porcelain fused to metal crowns during chewing simulations.

The excessive abrasion of occlusal surfaces in ceramic crowns limits the service life of restorations and their clinical results. However, little is k...
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