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Enhanced hydrophilicity and tribological behavior of dental zirconia ceramics based on picosecond laser surface texturing
Journal Pre-proof Enhanced hydrophilicity and tribology behavior of dental zirconia ceramics based on picosecond laser surface texturing Min Ji, Jinyang Xu, Ming Chen, Mohamed El Mansori PII:
S0272-8842(19)33403-0
DOI:
https://doi.org/10.1016/j.ceramint.2019.11.210
Reference:
CERI 23579
To appear in:
Ceramics International
Received Date: 11 November 2019 Revised Date:
17 November 2019
Accepted Date: 22 November 2019
Please cite this article as: M. Ji, J. Xu, M. Chen, M. El Mansori, Enhanced hydrophilicity and tribology behavior of dental zirconia ceramics based on picosecond laser surface texturing, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.11.210. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
Enhanced hydrophilicity and tribology behavior of dental zirconia ceramics based on picosecond laser surface texturing Min Jia, Jinyang Xu a,*, Ming Chena, and Mohamed El Mansorib, c a
State Key Laboratory of Mechanical System and Vibration, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China b
MSMP – EA 7350, Arts et Métiers ParisTech, Châlons-en-Champagne 51006, France
c
Department of Mechanical Engineering, Texas A&M University, College Station, TX 77840, USA *Corresponding author. Tel.: +86-21-34206317; fax: +86-21-34206317. E-mail address: [email protected] (J. Xu)
Abstract: Zirconia ceramics are promising restorative materials that are being extensively used in clinical dental prosthodontics like inlays, implant crowns, and fixed bridges, etc., due to their high strength, high toughness, high resistance to corrosion and excellent esthetical effects. However, in addition to the superior mechanical and physical properties, the biocompatibility is a more important index to evaluate the performance of the ceramic implant dentures. The aim of the current work is to improve the biocompatibility of zirconia ceramic implants by surface modification. To achieve this goal, a bionic design method is used to imitate the natural human perikymata structure and a series of microtextures were fabricated on the zirconia surfaces by picosecond laser processing. The effectiveness of the microtextures on the biocompatibility of zirconia ceramics was quantified in terms of the contact angles, friction coefficients and surface wear signatures. The results indicate that the laser texturing has a significant effect on the wettability and the tribological behavior of zirconia ceramic dental implants. To improve the biocompatibility of the zirconia ceramic implants, a small groove width with an appropriate groove depth is favorable. Keywords: Zirconia ceramics; Dental materials; Surface modification; Microtexture; Wettability; Tribological behavior.
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1. Introduction With the rapid development of medical technology, the options for oral prosthesis are continuously increasing. Due to the adverse reactions of metal prostheses such as poor chemical ability, low biocompatibility and toxic to the gingivae and surrounding tissues, the use of dental restorative materials has shifted from metals and polymers to bioceramics due to the need to improve the clinical performance [1]. Among the family of ceramic materials, zirconia (ZrO2) ceramic is a promising option due to its high fracture strength, good chemical stability, low wear, high corrosion resistance, excellent toughening effect and esthetical outcomes [2-4], which is being extensively used in clinical dental prosthodontics for fabricating the inlays, implant crowns, and fixed bridges [5, 6]. However, in addition to the excellent physical and mechanical properties, biocompatibility is a more important index to evaluate the performance of the ceramic implant denture. On the basis of current material properties, it is of great significance to improve the compatibility between the ceramic dentures and the oral environment and to reproduce the functions of natural teeth. Recent investigations found that the micro/nanoscale surface texturing can alter surface characteristics, causing the transition of wettability and moreover the tribological enhancement [7-10]. Thus, the desired functional surfaces can be achieved by surface structure modification. To date, various types of machining processes including the micromachining [11-13], chemical etching [14], lithography [15] or laser ablation [16, 17] have been proved capable of producing functional microstructures. Among them, the laser ablation process seems to be the most suitable method for surface texturing because of its high machining efficiency and noncontact characteristics [18]. Considerable experimental studies have been undertaken to investigate the laser treatment on the surface of zirconia ceramics. Yilbas [19] had carried out the laser machining of yttria-stabilized zirconia surfaces with the assistance of nitrogen. The author pointed out that the laser-treated surfaces with relatively low roughness and the formation of ZrN helped to improve the surface hydrophobicity. Cai et al. [20] used the nanosecond pulsed laser to generate grid patterns on the zirconia surface, and successfully obtained a super-hydrophobic surface with a maximum contact angle of 155.7°. Yilbas et al. [21] used the laser ablation to produce micro poles and cavities on the zirconia ceramic surfaces, and the results showed the hydrophobic characteristics were achieved on the textured surface. Jing et at. [22] fabricated different microtextures on the zirconia ceramics in order to identify the role of the microtextures on the wettability. The results indicated that different surface patterns can affect the surface topographies and the surface roughness, which subsequently influences the modifications of the surface wettability. However, most of the studies are focused on the improvement of hydrophobicity, for the ceramic implants, a hydrophilic surface is more preferable. Since the tribological relationship between natural and artificial teeth is significantly affected by oral conditions. Among them, saliva is the most important ingredient in human mouth, which has an critical function to reduce the wear and friction by forming a boundary lubrication system at the occlusive surface of opposing teeth [23]. Additionally, saliva is acting as a protective buffer to acid attack and a matrix for the remineralization of human enamel by providing calcium ions to the demineralized enamel [24, 25]. Therefore, the hydrophilic surface is more conducive to the effect of saliva by completely separating the saliva membrane on the occluding surface. Additionally, a major problem for the ceramic restoration is the excessive harness that will abrade the enamel of the opposing natural teeth during the occlusive process. The wear rate of human enamel against ceramic restoration is far higher than that against metals, which is proved by Ref. [26, 27]. A lot of endeavors have been made to study the tribological behavior of restorative materials. Zheng et al. [28] investigated the friction and wear performances of the zirconia ceramics 2 / 18
fabricated by the ultrasonic vibration-assisted grinding (UVAG) and by the diamond grinding (DG) against the natural teeth, the authors found that the frictional behavior of UVAG zirconia was much better than that of DG zirconia. Liu et at. [29] compared the frictional performance of metallic and ceramic materials against human teeth. The results indicated the zirconia ceramics had a better wear resistance than the titanium alloy. Ramalho and Antunes [30] analyzed the frictional behavior between the composite and the enamel, and the results revealed that the pair teeth-composite possessed lower friction coefficient and wear amount at lower value of normal loads, but the friction coefficient and the wear amount were significantly elevated at higher normal load conditions. Yu et at. [31] investigated the tribological behavior of different dental feldspathic porcelains against Si3N4 during the simulated chewing process. The authors found that Vita VMK95 had a superior tribological performance than Cerec Vitablocs MarkII in terms of the friction coefficient and the wear resistance. However, these studies are mostly focused on the inherent properties of different restorative materials, very limited works have made efforts to improve the tribological behavior of dental restorative materials by means of surface texturing. Therefore, the present paper is aimed at identifying the underlying mechanisms of the microtextures controlling the wettability and tribological behavior of zirconia ceramic dental implants. A bionic design method is used in the current work to generate an imitative perikymata structure by laser processing on the zirconia ceramic surface. The effects of various microtextures on the wettability and frictional characteristics of dental zirconia were rigorously investigated. Aspects including the contact angles, friction coefficients, and wear morphologies were studied. The aim of the current work is to design a microstructure dimension that has the ability to maximize the hydrophilicity and to enhance the tribological behavior of zirconia ceramics. The results discussed in this paper reveal the operating mechanisms of the microtextures affecting the wettability and tribological performance of zirconia ceramics and allow several technical guidance for the surface structure modification when fabricating the dental prosthetic restorations. 2. Process of experiments 2.1. Workpiece specimens and pretreatment The materials used in the current work are fully sintered yttria-stabilized zirconia ceramics. The composition and properties of the used zirconia ceramics are shown in Tables 1 and 2. The zirconia ceramic specimens are cut with diamond saw blades into a rectangular plate having a total size of 10 mm (length) × 10 mm (width) × 5 mm (thickness). Prior to the laser texturing, all the samples were inlaid into a resin matrix and then subjected to mechanical polishing for 2 hours. After polishing, the cold mounting resin was dissolved with an appropriative solution, then the specimens were treated by the ultrasonic cleaning for 15 min in acetone and 10 min in distilled water thereafter. The obtained surface roughness (Ra) of the samples after polishing was 0.30 µm. The procedure of the pretreatment is shown in Fig 1. Table 1 Composition of the sintered zirconia ceramics. Element
ZrO2
Y 2O 3
SiO2
Fe2O3
TiO2
wt.%
94.5
≤5.40
≤0.02
≤0.01
≤0.001
Table 2 Properties of the sintered zirconia ceramics. Properties 3
Density/kg/m Poisson’s ratio Young’s modulus/GPa
Value
Properties
Value
6040 0.34 217
Thermal conductivity/W/(m•K) Hardness/GPa Melting point/℃
2-3.3 1200-1300 2700
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Bending strength/MPa
836
Frature strength/MPa
800-1200
Fig.1. The procedures of the pretreatment for the zirconia ceramic samples.
2.2. Design basis A bionic design method is used in the current work to create the surface textures on the zirconia ceramics. The patterns of microtextures are based on the morphologies of natural teeth. As shown in Fig.2(a) [32], there is a regularly layered structure in the section of enamel, which is known as the perikymate. It represents the formation process of the human enamel, which would leave a series of micro grooves on the surface of the tooth crown [33]. The depth of perikymata is about 2 to 5 µm and the spatial periodicity ranges from 50 to 100 µm [32]. Nine zirconia ceramic specimens with different surface grooves were designed in this experiment. Each specimen has different geometrical parameters and the schematic diagram of the microtextures is depicted in Fig. 2(b). The depth (D) of the micro-grooves ranges from 2 to 11 µm, and the width (W) varies from 20 to 80 µm. For each specimen, the interval (I) between two grooves is identical in the horizontal direction and its magnitude is 20 µm. The value of D is set as 2, 7 and 11 µm, and the magnitude of W is selected as 20, 50 and 80 µm.
Fig.2. The design basis of the micro-texture: (a) the structure of perikymata [32] and (b) the schematic diagram of the designed textures. 4 / 18
2.3. Experimental setup The microtextures of the zirconia ceramics were fabricated via a TRUMPF TruMicro 5050 picosecond laser machining system, which has a galvanometric scanning system to deliver the laser beam and enhance the efficiency of the laser machining. The galvanometric scanners integrate a mirror with an actuating motor to control the laser beam by turning the scanning mirror. The laser head with a wavelength of 1,030 nm is fixed on the Z-axis. The maximum output power and pulse repetition rates are 50 W and 400 kHz, respectively. An overview of the experimental setup for the laser texturing of zirconia ceramics is shown in Fig.3. During the laser texturing process, the focal length was set as 100 mm to focus the laser beam onto the zirconia surface, which would generate a spot size of approximately 16 µm in diameter. In order to acquire the desired surface structures, a parameter calibration process was carried out before the laser ablation, and the selected laser texturing parameters and corresponding machining depths are summarized in Table 3.
Fig.3. The experimental setup for the laser texturing process. Table 3 The used laser texturing parameters and the corresponding machining depths. Laser parameters
Level 1
Level 2
Level 3
Single pulse energy/µJ Pulse repetition rate/kHz Scanning speed/m/s Scan times Corresponding machining depths/µm
10 4 5 1 2
10 40 5 5 7
10 400 5 5 11
3. Results and discussion 3.1. Analysis of surface topographies To verify whether the designed microstructure has been produced on the zirconia ceramic surface, the surface analysis of typical specimens with different groove widths and depths were conducted. The three-dimensional topographies of typical textured surfaces were measured by a laser confocal microscopy (Rtec LAMBDA-2) as shown in Fig.4. The measuring area is 500 µm × 500 µm, and the heights of the measuring range are indicated above the sectional profile.
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Fig.4. Surface topographies of specimens with different textures: (a) D2W20, (b) D7W20, (c) D11W20, (d) D11W50 and (e) D11W80.
The different groove depths are shown in Fig.4 (a)-(c). It can be found that the obtained depths are basically consistent with the designed structures. However, since the shallowest groove is only about 2 µm, the profile of this micro-texture is not as obvious as other depths, showing as a microgroove accompanied by wear marks which are generated by the polishing process. The microstructures gradually become apparent and are well separated as the increase of the groove depth. The different widths of the laser textured specimens are depicted in Fig.4 (c)-(e), and the values of the width are 20 µm, 55 µm, and 80 µm, respectively, which shows a good similarity with the desired parameters. For all the laser textured specimens, the contour of the groove are neat and complete, the recast layers of the melted work materials and the cracks induced by the thermal damage are not observed on the microgrooves. That is because the picosecond laser has a narrower pulse width and a higher peak power density compared with the nanosecond laser. During the laser texturing process, nearly no heat is transferred to the machining area by free electrons, which can effectively reduce the heat-affected zone and the formation of recast layers. Additionally, the sectional profile of the laser textured microgrooves appears like a Gaussian curve instead of a rectangle shape, which is related to the energy distribution of the laser beam, as shown in Fig. 4. Since the diameter of the spot size is 16 µm, to ablate the required groove shape, there needs an overlap between the adjacent laser beams. Finally, the micro-grooves are formed by machining multi-pass channels, which leaves a scallop-height in the bottom consisting of overlapped Gaussian curves. Based on the analysis of surface topographies, the designed surface structures have been successfully machined on the zirconia ceramic specimens by the picosecond laser. 3.2. Analysis of wettability To investigate the impacts of the microtextures on the wettability of surfaces, the apparent contact angles were measured by a drop-shape analyzer (type DSA 100), and a photograph depicting the in-situ measurement of the contact angle for a zirconia ceramic specimen is given in Fig.5(a). The shape of the liquid was captured by a CCD camera and the contact angle is automatically calculated by an image identification software. Each measurement of the contact 6 / 18
angle was repeated three times to ensure the reliability of the results. In this work, a 5 µL droplet of artificial saliva was dropped on the non-textured and the laser textured specimens at atmospheric conditions, and the chemical composition of the used artificial saliva was summarized in Table 4 [28, 34]. The intrinsic contact angle of the non-textured zirconia ceramic is about 75° as illustrated in Fig.5(b).
Fig.5. The wettability detection of textured zirconia ceramic specimens: (a) the process setup and (b) the intrinsic contact angle. Table 4 Composition of an artificial saliva [28, 34]. Composition
CaCl2·2H2O
NaH2PO4·2H2O
NaCl
KCl
Na2S·9H2O
Urea
Distilled water
Content (g)
0.795
0.78
0.4
0.4
0.005
1
1000
Fig.6 shows the evolution of the obtained contact angles with different surface microstructures. It can be found that the surface texture plays a significant role in altering the wettability of zirconia ceramics. After the laser texturing, seven hydrophilic surfaces and two hydrophobic surfaces are obtained. The most hydrophobic surface is achieved for the sample D11W80, which contains the largest groove width and depth. By contrast, the minimum contact angle is obtained for the sample D2W20, whose scale of microtextures is the smallest. In addition, there are four samples whose contact angle is smaller than the intrinsic contact angle, which indicates that the zirconia ceramics can be transformed into a more hydrophilic direction by changing the surface topological structure.
Fig.6. The evolution of the obtained contact angles with different surface textures.
To further clarify the interaction mechanisms between the microtextures and the solid-liquid interface, Fig. 7(a) shows the evolution of the contact angles with different groove depths under a fixed groove width (W = 20 µm). It is noticeable that the larger the groove depth is, the bigger the apparent contact angle of the zirconia ceramics. Additionally, in the case of W = 20 µm, the contact 7 / 18
angle is always smaller than the intrinsic contact angle even at the deepest groove depth. This means within a defined range of the groove depth, zirconia ceramic will all transform to a more hydrophilic direction. A schematic diagram depicting the contact interface of solid-liquid in this case is shown in Figure 7(b). As seen in Fig. 7(b), the contact angle becomes the lowest when the groove depth is equal to 2 µm, where an optimum hydrophilic surface is obtained. The phenomenon is related to the solid-liquid contact mode operated. Under these textural parameters, the Wenzel contact mode dominates the solid-liquid contact relationship. The Wenzel contact mode is based on the hypothesis that the droplet always fills the microsurface structures, and its apparent contact angle has the following correlation with the intrinsic contact angle of a non-textured surface [35, 36]. cos θ * = γ cos θ i
(1)
where θ * signifies the apparent contact angle, θi is the intrinsic contact angle, and γ represents the ratio between the actual and projected contact area of the solid/liquid. According to the Wenzel contact mode, when D = 2 µm, the micro-texture has a rough surface on the original surface, so the actual contact area of the solid/liquid becomes larger than the projected geometrical contact area. Since the zirconia is a hydrophilic material with an intrinsic contact angle of 75°, therefore, the original hydrophilicity can be strengthened geometrically based on the Wenzel contact mode. When the depth of the groove is further increased, the solid-liquid contact state is changed, and the initial full contact becomes a composite contact state, where the microgroove fails to fill up with saliva and there is trapped air below the saliva droplets. As shown in Fig.7(b), the liquid only wets a certain depth into the groove. Therefore, the actual contact interface can be separated into two parts. One is the interface between the solid and liquid, and another is the interface between the solid and gas. It is an intervenient contact state between the Wenzel contact mode and the Cassie contact mode. As such, a modified Cassie contact mode is established to predict the contact angle of the textured zirconia ceramic surface as follows: x cos θ * = ( f s + ) cos θ i − f s + 1 (2) D where θ * signifies the apparent contact angle, fs represents the area ratio of solids on a textured surface, x is the infiltration depth of the liquid in the groove, D is the depth of groove, and θi is the intrinsic contact angle. According to Eq. (2), it is more difficult for the liquids to completely fill the entire texture as the groove depth increases. In addition, when the groove depth increases from 7 µm to 11 µm, the value of x/D becomes smaller, which leads to a smaller value of cos θ * , resulting in a more hydrophobic surface. It can be concluded that the groove depth has a negative impact on the hydrophilicity. A smaller groove depth is beneficial to improve the hydrophilicity of the zirconia ceramics by stabilizing the solid-liquid interface under the Wenzel contact mode.
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Fig.7. The variation of wettability of laser textured surfaces with different groove depths: (a) the photographs captured by the CCD camera and (b) the schematic diagram of the solid-liquid interface.
Fig.8(a) shows the evolution of the contact angle with different groove widths under a fixed groove depth (D = 11 µm). Results indicate that when the groove width increases, the contact angle increases accordingly, and even a transition from hydrophilicity to hydrophobicity is noted, which is the same for all the groove depths tested, as shown in Fig.6. As illustrated earlier, when the groove depth becomes larger than 2µm, the actual contact interface is also characterized by a composite contact interface consisting of solid-liquid and gas-liquid interfaces. As the groove width increases, the area ratio of solids tends to become smaller and smaller, resulting in a decrease in the value of fs. Additionally, as the width of the groove increases, the bottom profile of the groove contains a series of overlapping Gaussian curves, which generates a number of peaks. The supporting effects of these micro peaks prevent the liquid from wetting the whole groove, leading to a lower x value and thereby reducing the value of x/D. With respect to the micro peaks residing within the bottom of the droplet, they appear to operate in a complete Cassie contact state, or more precisely in a transitional contact condition between the Wenzel mode and the Cassie mode. However, no matter where the contact state is, the effects of the micro peaks on the whole wettability of micro surface textures are minimal compared with the above two factors. It can be concluded that the groove width exhibits a negative impact on the hydrophilicity of zirconia ceramic surface textures. To improve the hydrophilicity as far as possible, a smaller groove width value should be selected being able to enlarge the area ratio of the solid phase.
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Fig.8. The variation of wettability of laser textured surfaces with different groove widths: (a) the photographs captured by the CCD camera and (b) the schematic diagram of the solid-liquid interface.
3.3. Analysis of tribological behavior To evaluate the tribological behaviors of different laser textured surfaces, tribological tests were conducted using a reciprocating tribometer to investigate the effectiveness of surface textures in alleviating the friction at the contact interface. As shown in Fig.9, the flat specimen is a zirconia ceramic workpiece, and the antagonist body is a Si3N4 ball with a diameter of 4 mm. The stroke length and the reciprocating frequency are set as 6 mm and 0.5 Hz, respectively. A total of 1000 cycles of tests were conducted under the dry conditions. The masticatory force of humans during the chewing process ranges from 3 to 36 N as indicated by Ref. [37]. However, the highest force only takes place sporadically during chewing [30]. Additionally, the surfaces interact theoretically by a point contact at the current ball-on-flat configuration, thus it would provide a larger normal pressure to the zirconia ceramic specimen. Therefore, the normal load was set as 4 N, which is close to the minimum reference value of the masticatory force.
Fig.9. The process setup for the tribological tests of different laser textured zirconia ceramic specimens.
The curves of the friction coefficients versus different textured surfaces are depicted in Fig.10. It is noted that the initial friction stage is very short and only about 50 cycles are noted for the non-textured zirconia ceramic surfaces. Then the non-textured surface falls into a stable friction 10 / 18
stage and its friction coefficient remains about 0.6. However, for the laser textured zirconia ceramic surfaces, the micro-texture is confirmed to have a great impact on the frictional behavior of zirconia ceramics. When the groove depth is 2 µm, the friction state of the textured zirconia ceramic surfaces appears unstable and the friction coefficient fluctuates greatly within the test period, as shown in Fig.10 (a). As the number of cycles reaches over 300, the friction coefficient is found to be higher than that of the non-textured zirconia ceramic surfaces and to continue to progress after the 300 cycles. When the zirconia ceramic surfaces are textured by a groove depth of 7 µm, the friction coefficient at the initial friction stage remains the lowest, and after 480 cycles it tends to exceed the frictional value of the non-textured surfaces even though the difference is not very apparent. For the highest groove depth of 11 µm, the friction coefficient of the textured surface initially rises faster than the other two groove depths (D = 2 and 7 µm) and then it undergoes a slower increasing rate. In general, the laser textured zirconia ceramic surface with 11 µm groove depth exhibits the best frictional behavior in comparison with the other types of textured surfaces within the 1000 cycles. The phenomena are attributed to the two effects of the microtextures exerting on the reduction of the friction coefficient. The first effect is the presence of the micro-texture that can decrease the actual contact area between the frictional pairs [38, 39]. The other effect lies in the ability of the micro-texture to absorb the abrasive debris. The absorption of hard particles prevents the surface from further sliding at the contact interface [40], which inhibits the formation of the three-body abrasion and yields a reduction in the friction coefficient. For micro textures being featured by the identical groove width, since the change in the groove depths does not leave an impact on the actual contact area, the effect of the groove depths on the friction coefficients of different textured surfaces depends on their capability of absorbing silicon nitride abrasives generated during the friction testing.
Fig.10. The curves of the friction coefficients versus different surface textures: (a) different groove depths and (b) different groove widths.
It can be seen in Fig.10 (b) that the friction coefficient of the textured surface increases gradually with the elevated groove width. When the groove width is 50 µm, the friction coefficient is higher than the non-textured surface after about 190 cycles and then it maintains at a higher increasing level. For the zirconia ceramic surface textured by the largest groove width of 80 µm, the friction characteristic becomes the worst one, being inferior to the non-textured surfaces only after 80 cycles. The phenomenon is due to the fact that although increasing the groove width tends to reduce the effective contact area between the friction pairs, the edge shape of the groove is more obvious as the width increases, which poses a more crucial impact on the friction testing against the Si3N4 ball. 11 / 18
This makes the original moderate sliding friction change into a unstable friction with a periodic impact, resulting in the deterioration of the friction characteristics. In addition, an excessively large width may weaken the mechanical strength of the microgrooves, which may cause the fracture of the microtextures, leading to the loss of their original functions. Sample types
Morphologies before the friction tests
Non-textur ed surface
D2W20
D7W20
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Morphologies after the friction tests
D11W20
D11W50
D11W80
Fig.11. The surface morphologies of different laser textured surfaces before and after the friction tests.
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Fig.12. Chemical composition of adhesion existing on the non-textured zirconia ceramic surface.
To further identify the influence of microstructures on the friction and wear behaviors of the laser textured zirconia ceramic specimens, both the SEM and EDS analyses were carried out on the wear marks of the zirconia ceramic surface. The original morphologies and the eventual wear signatures after the sliding friction of the non-textured and textured zirconia ceramic surfaces are shown in Fig.11. Through the SEM analysis, large amounts of lamellar abrasive debris adhere onto the worn surface of the non-textured zirconia ceramics. The analysis of chemical composition of points A and B confirms the presence of quantities of adhesion of Si element detached from the counter Si3N4 ball, as shown in Fig.12. Due to the uniform distribution of adhesion on the zirconia ceramic surfaces, the friction coefficient is relatively stable for the non-textured sample. The texture depth is low and the microstructure is not very obvious for the sample D2W20 as depicted in the morphologies before the friction tests. After the sliding friction, it can be found that several sliding marks exist on the laser textured surfaces. Additionally, many micro-grooves have been smoothened during the sliding process which have lost their original structures. This indicates that a shallow groove depth is not only detrimental to the chip conservation, but also accelerates the wearing of the microstructures during the sliding friction tests. The wear signatures of the laser textured zirconia ceramics D7W20 and D11W20 are very similar. Since the depth of the wear marks is shallow, the original structures of the microgroove are completely preserved. It is worth noting that the grooves in the D5W20 sample are filled up with Si3N4 debris, while the grooves in D11W20 sample still have the ability to absorb the abrasive debris. This confirms the above analysis of the effect of the groove depth on the friction characteristics such that the different friction characteristics between groove depths are determined by their capacity to absorb abrasive debris. By comparing the eventual wear morphologies of D11W20, D11W50 and D11W80, it is noted that when the groove width increases to 50 µm, the micro-texture can retain its original structure, but with a further increase of the groove width (e.g. W = 80 µm), the mechanical strength of the grooves tends to be weakened, which is evidenced by the large particles appearing onto the surface of the D11W80 sample. The large particles originate from the broken microtextures and tend to insert into the micro-grooves, resulting in the deterioration of the frictional behavior of the zirconia ceramic surface. Moreover, the fracture of the peaks occurs at the bottom of the grooves. The SEM observations agree well with the findings obtained in terms of the friction coefficient such that when the groove width increases, its impact on the mechanical strength of micro-grooves becomes larger and larger during the sliding friction tests, resulting in a higher friction coefficient. By 14 / 18
evaluating the friction characteristics of different surface textures, it can be concluded that to enhance the friction and wear characteristics of zirconia ceramics, a combination of a larger groove depth and a smaller groove width is suggested.
4. Conclusions The present paper concerns a comparative investigation into the surface modification of the dental zirconia ceramics in order to improve their biocompatibility. A series of microtextures were fabricated by the laser ablation onto the zirconia ceramic surfaces and the effects of different surface structures were studied in terms of the wettability and the tribological behavior. The significance of the current work is to provide several technical guidance for the surface structure modification when fabricating the dental prosthetic restorations. Based on the results acquired, the following conclusions can be drawn. A bionic design method was adopted in the current work, and the structures of the microtextures were fabricated according to the surface morphologies of natural teeth. Based on the results of the topographical analysis, the morphologies of the perikymata were successfully replicated on the zirconia ceramic surfaces by the picosecond laser texturing. The micro textures have a great impact on the wettability of zirconia ceramics due to the change of the contact mode at the solid/liquid interfaces. The most hydrophilic surface was obtained on a microstructured surface of D2W20, which would promote the saliva to fill in the grooves, resulting in a Wenzel contact mode. With a further increase in the groove depth, an intermediate condition between the Wenzel mode and the Cassie mode would make the surface change towards hydrophobicity. Additionally, the width of the grooves shows a negative effect on the hydrophilic performance of textured zirconia ceramics. The tribological behaviors of different textured zirconia ceramic surfaces are affected by both the groove depth and width. The best frictional performance was obtained for the sample D11W20 since it has the deepest groove that possesses the largest conservation space for the abrasive debris and prevents the wear debris from being sandwiched between the friction pairs and from worsening the friction process. In contrast, a larger groove width would make the edge of the micro-texture appear more visible, which exacerbates its impact on the abrasives. Moreover, a larger groove width means that the texture distribution is much sparser and the mechanical strength of the groove can be degraded easily. Under such circumstances, the microstructural grooves would be detached easily. To achieve a balance between the wettability and tribological behavior of zirconia ceramic implants, it is recommended to use a minimum groove width combined with a suitable groove depth. When the groove width is the smallest, it is beneficial for both the wettability and tribological performance. With an appropriate increase in the groove depth, the change in the contact angle is insignificant, but the tribological behavior can be greatly improved.
Acknowledgments The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant No.51705319) and the Shanghai Pujiang Program (Grant No.17PJ1403800).
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Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.
S0272-8842(19)33403-0
DOI:
https://doi.org/10.1016/j.ceramint.2019.11.210
Reference:
CERI 23579
To appear in:
Ceramics International
Received Date: 11 November 2019 Revised Date:
17 November 2019
Accepted Date: 22 November 2019
Please cite this article as: M. Ji, J. Xu, M. Chen, M. El Mansori, Enhanced hydrophilicity and tribology behavior of dental zirconia ceramics based on picosecond laser surface texturing, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.11.210. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
Enhanced hydrophilicity and tribology behavior of dental zirconia ceramics based on picosecond laser surface texturing Min Jia, Jinyang Xu a,*, Ming Chena, and Mohamed El Mansorib, c a
State Key Laboratory of Mechanical System and Vibration, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China b
MSMP – EA 7350, Arts et Métiers ParisTech, Châlons-en-Champagne 51006, France
c
Department of Mechanical Engineering, Texas A&M University, College Station, TX 77840, USA *Corresponding author. Tel.: +86-21-34206317; fax: +86-21-34206317. E-mail address: [email protected] (J. Xu)
Abstract: Zirconia ceramics are promising restorative materials that are being extensively used in clinical dental prosthodontics like inlays, implant crowns, and fixed bridges, etc., due to their high strength, high toughness, high resistance to corrosion and excellent esthetical effects. However, in addition to the superior mechanical and physical properties, the biocompatibility is a more important index to evaluate the performance of the ceramic implant dentures. The aim of the current work is to improve the biocompatibility of zirconia ceramic implants by surface modification. To achieve this goal, a bionic design method is used to imitate the natural human perikymata structure and a series of microtextures were fabricated on the zirconia surfaces by picosecond laser processing. The effectiveness of the microtextures on the biocompatibility of zirconia ceramics was quantified in terms of the contact angles, friction coefficients and surface wear signatures. The results indicate that the laser texturing has a significant effect on the wettability and the tribological behavior of zirconia ceramic dental implants. To improve the biocompatibility of the zirconia ceramic implants, a small groove width with an appropriate groove depth is favorable. Keywords: Zirconia ceramics; Dental materials; Surface modification; Microtexture; Wettability; Tribological behavior.
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1. Introduction With the rapid development of medical technology, the options for oral prosthesis are continuously increasing. Due to the adverse reactions of metal prostheses such as poor chemical ability, low biocompatibility and toxic to the gingivae and surrounding tissues, the use of dental restorative materials has shifted from metals and polymers to bioceramics due to the need to improve the clinical performance [1]. Among the family of ceramic materials, zirconia (ZrO2) ceramic is a promising option due to its high fracture strength, good chemical stability, low wear, high corrosion resistance, excellent toughening effect and esthetical outcomes [2-4], which is being extensively used in clinical dental prosthodontics for fabricating the inlays, implant crowns, and fixed bridges [5, 6]. However, in addition to the excellent physical and mechanical properties, biocompatibility is a more important index to evaluate the performance of the ceramic implant denture. On the basis of current material properties, it is of great significance to improve the compatibility between the ceramic dentures and the oral environment and to reproduce the functions of natural teeth. Recent investigations found that the micro/nanoscale surface texturing can alter surface characteristics, causing the transition of wettability and moreover the tribological enhancement [7-10]. Thus, the desired functional surfaces can be achieved by surface structure modification. To date, various types of machining processes including the micromachining [11-13], chemical etching [14], lithography [15] or laser ablation [16, 17] have been proved capable of producing functional microstructures. Among them, the laser ablation process seems to be the most suitable method for surface texturing because of its high machining efficiency and noncontact characteristics [18]. Considerable experimental studies have been undertaken to investigate the laser treatment on the surface of zirconia ceramics. Yilbas [19] had carried out the laser machining of yttria-stabilized zirconia surfaces with the assistance of nitrogen. The author pointed out that the laser-treated surfaces with relatively low roughness and the formation of ZrN helped to improve the surface hydrophobicity. Cai et al. [20] used the nanosecond pulsed laser to generate grid patterns on the zirconia surface, and successfully obtained a super-hydrophobic surface with a maximum contact angle of 155.7°. Yilbas et al. [21] used the laser ablation to produce micro poles and cavities on the zirconia ceramic surfaces, and the results showed the hydrophobic characteristics were achieved on the textured surface. Jing et at. [22] fabricated different microtextures on the zirconia ceramics in order to identify the role of the microtextures on the wettability. The results indicated that different surface patterns can affect the surface topographies and the surface roughness, which subsequently influences the modifications of the surface wettability. However, most of the studies are focused on the improvement of hydrophobicity, for the ceramic implants, a hydrophilic surface is more preferable. Since the tribological relationship between natural and artificial teeth is significantly affected by oral conditions. Among them, saliva is the most important ingredient in human mouth, which has an critical function to reduce the wear and friction by forming a boundary lubrication system at the occlusive surface of opposing teeth [23]. Additionally, saliva is acting as a protective buffer to acid attack and a matrix for the remineralization of human enamel by providing calcium ions to the demineralized enamel [24, 25]. Therefore, the hydrophilic surface is more conducive to the effect of saliva by completely separating the saliva membrane on the occluding surface. Additionally, a major problem for the ceramic restoration is the excessive harness that will abrade the enamel of the opposing natural teeth during the occlusive process. The wear rate of human enamel against ceramic restoration is far higher than that against metals, which is proved by Ref. [26, 27]. A lot of endeavors have been made to study the tribological behavior of restorative materials. Zheng et al. [28] investigated the friction and wear performances of the zirconia ceramics 2 / 18
fabricated by the ultrasonic vibration-assisted grinding (UVAG) and by the diamond grinding (DG) against the natural teeth, the authors found that the frictional behavior of UVAG zirconia was much better than that of DG zirconia. Liu et at. [29] compared the frictional performance of metallic and ceramic materials against human teeth. The results indicated the zirconia ceramics had a better wear resistance than the titanium alloy. Ramalho and Antunes [30] analyzed the frictional behavior between the composite and the enamel, and the results revealed that the pair teeth-composite possessed lower friction coefficient and wear amount at lower value of normal loads, but the friction coefficient and the wear amount were significantly elevated at higher normal load conditions. Yu et at. [31] investigated the tribological behavior of different dental feldspathic porcelains against Si3N4 during the simulated chewing process. The authors found that Vita VMK95 had a superior tribological performance than Cerec Vitablocs MarkII in terms of the friction coefficient and the wear resistance. However, these studies are mostly focused on the inherent properties of different restorative materials, very limited works have made efforts to improve the tribological behavior of dental restorative materials by means of surface texturing. Therefore, the present paper is aimed at identifying the underlying mechanisms of the microtextures controlling the wettability and tribological behavior of zirconia ceramic dental implants. A bionic design method is used in the current work to generate an imitative perikymata structure by laser processing on the zirconia ceramic surface. The effects of various microtextures on the wettability and frictional characteristics of dental zirconia were rigorously investigated. Aspects including the contact angles, friction coefficients, and wear morphologies were studied. The aim of the current work is to design a microstructure dimension that has the ability to maximize the hydrophilicity and to enhance the tribological behavior of zirconia ceramics. The results discussed in this paper reveal the operating mechanisms of the microtextures affecting the wettability and tribological performance of zirconia ceramics and allow several technical guidance for the surface structure modification when fabricating the dental prosthetic restorations. 2. Process of experiments 2.1. Workpiece specimens and pretreatment The materials used in the current work are fully sintered yttria-stabilized zirconia ceramics. The composition and properties of the used zirconia ceramics are shown in Tables 1 and 2. The zirconia ceramic specimens are cut with diamond saw blades into a rectangular plate having a total size of 10 mm (length) × 10 mm (width) × 5 mm (thickness). Prior to the laser texturing, all the samples were inlaid into a resin matrix and then subjected to mechanical polishing for 2 hours. After polishing, the cold mounting resin was dissolved with an appropriative solution, then the specimens were treated by the ultrasonic cleaning for 15 min in acetone and 10 min in distilled water thereafter. The obtained surface roughness (Ra) of the samples after polishing was 0.30 µm. The procedure of the pretreatment is shown in Fig 1. Table 1 Composition of the sintered zirconia ceramics. Element
ZrO2
Y 2O 3
SiO2
Fe2O3
TiO2
wt.%
94.5
≤5.40
≤0.02
≤0.01
≤0.001
Table 2 Properties of the sintered zirconia ceramics. Properties 3
Density/kg/m Poisson’s ratio Young’s modulus/GPa
Value
Properties
Value
6040 0.34 217
Thermal conductivity/W/(m•K) Hardness/GPa Melting point/℃
2-3.3 1200-1300 2700
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Bending strength/MPa
836
Frature strength/MPa
800-1200
Fig.1. The procedures of the pretreatment for the zirconia ceramic samples.
2.2. Design basis A bionic design method is used in the current work to create the surface textures on the zirconia ceramics. The patterns of microtextures are based on the morphologies of natural teeth. As shown in Fig.2(a) [32], there is a regularly layered structure in the section of enamel, which is known as the perikymate. It represents the formation process of the human enamel, which would leave a series of micro grooves on the surface of the tooth crown [33]. The depth of perikymata is about 2 to 5 µm and the spatial periodicity ranges from 50 to 100 µm [32]. Nine zirconia ceramic specimens with different surface grooves were designed in this experiment. Each specimen has different geometrical parameters and the schematic diagram of the microtextures is depicted in Fig. 2(b). The depth (D) of the micro-grooves ranges from 2 to 11 µm, and the width (W) varies from 20 to 80 µm. For each specimen, the interval (I) between two grooves is identical in the horizontal direction and its magnitude is 20 µm. The value of D is set as 2, 7 and 11 µm, and the magnitude of W is selected as 20, 50 and 80 µm.
Fig.2. The design basis of the micro-texture: (a) the structure of perikymata [32] and (b) the schematic diagram of the designed textures. 4 / 18
2.3. Experimental setup The microtextures of the zirconia ceramics were fabricated via a TRUMPF TruMicro 5050 picosecond laser machining system, which has a galvanometric scanning system to deliver the laser beam and enhance the efficiency of the laser machining. The galvanometric scanners integrate a mirror with an actuating motor to control the laser beam by turning the scanning mirror. The laser head with a wavelength of 1,030 nm is fixed on the Z-axis. The maximum output power and pulse repetition rates are 50 W and 400 kHz, respectively. An overview of the experimental setup for the laser texturing of zirconia ceramics is shown in Fig.3. During the laser texturing process, the focal length was set as 100 mm to focus the laser beam onto the zirconia surface, which would generate a spot size of approximately 16 µm in diameter. In order to acquire the desired surface structures, a parameter calibration process was carried out before the laser ablation, and the selected laser texturing parameters and corresponding machining depths are summarized in Table 3.
Fig.3. The experimental setup for the laser texturing process. Table 3 The used laser texturing parameters and the corresponding machining depths. Laser parameters
Level 1
Level 2
Level 3
Single pulse energy/µJ Pulse repetition rate/kHz Scanning speed/m/s Scan times Corresponding machining depths/µm
10 4 5 1 2
10 40 5 5 7
10 400 5 5 11
3. Results and discussion 3.1. Analysis of surface topographies To verify whether the designed microstructure has been produced on the zirconia ceramic surface, the surface analysis of typical specimens with different groove widths and depths were conducted. The three-dimensional topographies of typical textured surfaces were measured by a laser confocal microscopy (Rtec LAMBDA-2) as shown in Fig.4. The measuring area is 500 µm × 500 µm, and the heights of the measuring range are indicated above the sectional profile.
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Fig.4. Surface topographies of specimens with different textures: (a) D2W20, (b) D7W20, (c) D11W20, (d) D11W50 and (e) D11W80.
The different groove depths are shown in Fig.4 (a)-(c). It can be found that the obtained depths are basically consistent with the designed structures. However, since the shallowest groove is only about 2 µm, the profile of this micro-texture is not as obvious as other depths, showing as a microgroove accompanied by wear marks which are generated by the polishing process. The microstructures gradually become apparent and are well separated as the increase of the groove depth. The different widths of the laser textured specimens are depicted in Fig.4 (c)-(e), and the values of the width are 20 µm, 55 µm, and 80 µm, respectively, which shows a good similarity with the desired parameters. For all the laser textured specimens, the contour of the groove are neat and complete, the recast layers of the melted work materials and the cracks induced by the thermal damage are not observed on the microgrooves. That is because the picosecond laser has a narrower pulse width and a higher peak power density compared with the nanosecond laser. During the laser texturing process, nearly no heat is transferred to the machining area by free electrons, which can effectively reduce the heat-affected zone and the formation of recast layers. Additionally, the sectional profile of the laser textured microgrooves appears like a Gaussian curve instead of a rectangle shape, which is related to the energy distribution of the laser beam, as shown in Fig. 4. Since the diameter of the spot size is 16 µm, to ablate the required groove shape, there needs an overlap between the adjacent laser beams. Finally, the micro-grooves are formed by machining multi-pass channels, which leaves a scallop-height in the bottom consisting of overlapped Gaussian curves. Based on the analysis of surface topographies, the designed surface structures have been successfully machined on the zirconia ceramic specimens by the picosecond laser. 3.2. Analysis of wettability To investigate the impacts of the microtextures on the wettability of surfaces, the apparent contact angles were measured by a drop-shape analyzer (type DSA 100), and a photograph depicting the in-situ measurement of the contact angle for a zirconia ceramic specimen is given in Fig.5(a). The shape of the liquid was captured by a CCD camera and the contact angle is automatically calculated by an image identification software. Each measurement of the contact 6 / 18
angle was repeated three times to ensure the reliability of the results. In this work, a 5 µL droplet of artificial saliva was dropped on the non-textured and the laser textured specimens at atmospheric conditions, and the chemical composition of the used artificial saliva was summarized in Table 4 [28, 34]. The intrinsic contact angle of the non-textured zirconia ceramic is about 75° as illustrated in Fig.5(b).
Fig.5. The wettability detection of textured zirconia ceramic specimens: (a) the process setup and (b) the intrinsic contact angle. Table 4 Composition of an artificial saliva [28, 34]. Composition
CaCl2·2H2O
NaH2PO4·2H2O
NaCl
KCl
Na2S·9H2O
Urea
Distilled water
Content (g)
0.795
0.78
0.4
0.4
0.005
1
1000
Fig.6 shows the evolution of the obtained contact angles with different surface microstructures. It can be found that the surface texture plays a significant role in altering the wettability of zirconia ceramics. After the laser texturing, seven hydrophilic surfaces and two hydrophobic surfaces are obtained. The most hydrophobic surface is achieved for the sample D11W80, which contains the largest groove width and depth. By contrast, the minimum contact angle is obtained for the sample D2W20, whose scale of microtextures is the smallest. In addition, there are four samples whose contact angle is smaller than the intrinsic contact angle, which indicates that the zirconia ceramics can be transformed into a more hydrophilic direction by changing the surface topological structure.
Fig.6. The evolution of the obtained contact angles with different surface textures.
To further clarify the interaction mechanisms between the microtextures and the solid-liquid interface, Fig. 7(a) shows the evolution of the contact angles with different groove depths under a fixed groove width (W = 20 µm). It is noticeable that the larger the groove depth is, the bigger the apparent contact angle of the zirconia ceramics. Additionally, in the case of W = 20 µm, the contact 7 / 18
angle is always smaller than the intrinsic contact angle even at the deepest groove depth. This means within a defined range of the groove depth, zirconia ceramic will all transform to a more hydrophilic direction. A schematic diagram depicting the contact interface of solid-liquid in this case is shown in Figure 7(b). As seen in Fig. 7(b), the contact angle becomes the lowest when the groove depth is equal to 2 µm, where an optimum hydrophilic surface is obtained. The phenomenon is related to the solid-liquid contact mode operated. Under these textural parameters, the Wenzel contact mode dominates the solid-liquid contact relationship. The Wenzel contact mode is based on the hypothesis that the droplet always fills the microsurface structures, and its apparent contact angle has the following correlation with the intrinsic contact angle of a non-textured surface [35, 36]. cos θ * = γ cos θ i
(1)
where θ * signifies the apparent contact angle, θi is the intrinsic contact angle, and γ represents the ratio between the actual and projected contact area of the solid/liquid. According to the Wenzel contact mode, when D = 2 µm, the micro-texture has a rough surface on the original surface, so the actual contact area of the solid/liquid becomes larger than the projected geometrical contact area. Since the zirconia is a hydrophilic material with an intrinsic contact angle of 75°, therefore, the original hydrophilicity can be strengthened geometrically based on the Wenzel contact mode. When the depth of the groove is further increased, the solid-liquid contact state is changed, and the initial full contact becomes a composite contact state, where the microgroove fails to fill up with saliva and there is trapped air below the saliva droplets. As shown in Fig.7(b), the liquid only wets a certain depth into the groove. Therefore, the actual contact interface can be separated into two parts. One is the interface between the solid and liquid, and another is the interface between the solid and gas. It is an intervenient contact state between the Wenzel contact mode and the Cassie contact mode. As such, a modified Cassie contact mode is established to predict the contact angle of the textured zirconia ceramic surface as follows: x cos θ * = ( f s + ) cos θ i − f s + 1 (2) D where θ * signifies the apparent contact angle, fs represents the area ratio of solids on a textured surface, x is the infiltration depth of the liquid in the groove, D is the depth of groove, and θi is the intrinsic contact angle. According to Eq. (2), it is more difficult for the liquids to completely fill the entire texture as the groove depth increases. In addition, when the groove depth increases from 7 µm to 11 µm, the value of x/D becomes smaller, which leads to a smaller value of cos θ * , resulting in a more hydrophobic surface. It can be concluded that the groove depth has a negative impact on the hydrophilicity. A smaller groove depth is beneficial to improve the hydrophilicity of the zirconia ceramics by stabilizing the solid-liquid interface under the Wenzel contact mode.
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Fig.7. The variation of wettability of laser textured surfaces with different groove depths: (a) the photographs captured by the CCD camera and (b) the schematic diagram of the solid-liquid interface.
Fig.8(a) shows the evolution of the contact angle with different groove widths under a fixed groove depth (D = 11 µm). Results indicate that when the groove width increases, the contact angle increases accordingly, and even a transition from hydrophilicity to hydrophobicity is noted, which is the same for all the groove depths tested, as shown in Fig.6. As illustrated earlier, when the groove depth becomes larger than 2µm, the actual contact interface is also characterized by a composite contact interface consisting of solid-liquid and gas-liquid interfaces. As the groove width increases, the area ratio of solids tends to become smaller and smaller, resulting in a decrease in the value of fs. Additionally, as the width of the groove increases, the bottom profile of the groove contains a series of overlapping Gaussian curves, which generates a number of peaks. The supporting effects of these micro peaks prevent the liquid from wetting the whole groove, leading to a lower x value and thereby reducing the value of x/D. With respect to the micro peaks residing within the bottom of the droplet, they appear to operate in a complete Cassie contact state, or more precisely in a transitional contact condition between the Wenzel mode and the Cassie mode. However, no matter where the contact state is, the effects of the micro peaks on the whole wettability of micro surface textures are minimal compared with the above two factors. It can be concluded that the groove width exhibits a negative impact on the hydrophilicity of zirconia ceramic surface textures. To improve the hydrophilicity as far as possible, a smaller groove width value should be selected being able to enlarge the area ratio of the solid phase.
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Fig.8. The variation of wettability of laser textured surfaces with different groove widths: (a) the photographs captured by the CCD camera and (b) the schematic diagram of the solid-liquid interface.
3.3. Analysis of tribological behavior To evaluate the tribological behaviors of different laser textured surfaces, tribological tests were conducted using a reciprocating tribometer to investigate the effectiveness of surface textures in alleviating the friction at the contact interface. As shown in Fig.9, the flat specimen is a zirconia ceramic workpiece, and the antagonist body is a Si3N4 ball with a diameter of 4 mm. The stroke length and the reciprocating frequency are set as 6 mm and 0.5 Hz, respectively. A total of 1000 cycles of tests were conducted under the dry conditions. The masticatory force of humans during the chewing process ranges from 3 to 36 N as indicated by Ref. [37]. However, the highest force only takes place sporadically during chewing [30]. Additionally, the surfaces interact theoretically by a point contact at the current ball-on-flat configuration, thus it would provide a larger normal pressure to the zirconia ceramic specimen. Therefore, the normal load was set as 4 N, which is close to the minimum reference value of the masticatory force.
Fig.9. The process setup for the tribological tests of different laser textured zirconia ceramic specimens.
The curves of the friction coefficients versus different textured surfaces are depicted in Fig.10. It is noted that the initial friction stage is very short and only about 50 cycles are noted for the non-textured zirconia ceramic surfaces. Then the non-textured surface falls into a stable friction 10 / 18
stage and its friction coefficient remains about 0.6. However, for the laser textured zirconia ceramic surfaces, the micro-texture is confirmed to have a great impact on the frictional behavior of zirconia ceramics. When the groove depth is 2 µm, the friction state of the textured zirconia ceramic surfaces appears unstable and the friction coefficient fluctuates greatly within the test period, as shown in Fig.10 (a). As the number of cycles reaches over 300, the friction coefficient is found to be higher than that of the non-textured zirconia ceramic surfaces and to continue to progress after the 300 cycles. When the zirconia ceramic surfaces are textured by a groove depth of 7 µm, the friction coefficient at the initial friction stage remains the lowest, and after 480 cycles it tends to exceed the frictional value of the non-textured surfaces even though the difference is not very apparent. For the highest groove depth of 11 µm, the friction coefficient of the textured surface initially rises faster than the other two groove depths (D = 2 and 7 µm) and then it undergoes a slower increasing rate. In general, the laser textured zirconia ceramic surface with 11 µm groove depth exhibits the best frictional behavior in comparison with the other types of textured surfaces within the 1000 cycles. The phenomena are attributed to the two effects of the microtextures exerting on the reduction of the friction coefficient. The first effect is the presence of the micro-texture that can decrease the actual contact area between the frictional pairs [38, 39]. The other effect lies in the ability of the micro-texture to absorb the abrasive debris. The absorption of hard particles prevents the surface from further sliding at the contact interface [40], which inhibits the formation of the three-body abrasion and yields a reduction in the friction coefficient. For micro textures being featured by the identical groove width, since the change in the groove depths does not leave an impact on the actual contact area, the effect of the groove depths on the friction coefficients of different textured surfaces depends on their capability of absorbing silicon nitride abrasives generated during the friction testing.
Fig.10. The curves of the friction coefficients versus different surface textures: (a) different groove depths and (b) different groove widths.
It can be seen in Fig.10 (b) that the friction coefficient of the textured surface increases gradually with the elevated groove width. When the groove width is 50 µm, the friction coefficient is higher than the non-textured surface after about 190 cycles and then it maintains at a higher increasing level. For the zirconia ceramic surface textured by the largest groove width of 80 µm, the friction characteristic becomes the worst one, being inferior to the non-textured surfaces only after 80 cycles. The phenomenon is due to the fact that although increasing the groove width tends to reduce the effective contact area between the friction pairs, the edge shape of the groove is more obvious as the width increases, which poses a more crucial impact on the friction testing against the Si3N4 ball. 11 / 18
This makes the original moderate sliding friction change into a unstable friction with a periodic impact, resulting in the deterioration of the friction characteristics. In addition, an excessively large width may weaken the mechanical strength of the microgrooves, which may cause the fracture of the microtextures, leading to the loss of their original functions. Sample types
Morphologies before the friction tests
Non-textur ed surface
D2W20
D7W20
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Morphologies after the friction tests
D11W20
D11W50
D11W80
Fig.11. The surface morphologies of different laser textured surfaces before and after the friction tests.
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Fig.12. Chemical composition of adhesion existing on the non-textured zirconia ceramic surface.
To further identify the influence of microstructures on the friction and wear behaviors of the laser textured zirconia ceramic specimens, both the SEM and EDS analyses were carried out on the wear marks of the zirconia ceramic surface. The original morphologies and the eventual wear signatures after the sliding friction of the non-textured and textured zirconia ceramic surfaces are shown in Fig.11. Through the SEM analysis, large amounts of lamellar abrasive debris adhere onto the worn surface of the non-textured zirconia ceramics. The analysis of chemical composition of points A and B confirms the presence of quantities of adhesion of Si element detached from the counter Si3N4 ball, as shown in Fig.12. Due to the uniform distribution of adhesion on the zirconia ceramic surfaces, the friction coefficient is relatively stable for the non-textured sample. The texture depth is low and the microstructure is not very obvious for the sample D2W20 as depicted in the morphologies before the friction tests. After the sliding friction, it can be found that several sliding marks exist on the laser textured surfaces. Additionally, many micro-grooves have been smoothened during the sliding process which have lost their original structures. This indicates that a shallow groove depth is not only detrimental to the chip conservation, but also accelerates the wearing of the microstructures during the sliding friction tests. The wear signatures of the laser textured zirconia ceramics D7W20 and D11W20 are very similar. Since the depth of the wear marks is shallow, the original structures of the microgroove are completely preserved. It is worth noting that the grooves in the D5W20 sample are filled up with Si3N4 debris, while the grooves in D11W20 sample still have the ability to absorb the abrasive debris. This confirms the above analysis of the effect of the groove depth on the friction characteristics such that the different friction characteristics between groove depths are determined by their capacity to absorb abrasive debris. By comparing the eventual wear morphologies of D11W20, D11W50 and D11W80, it is noted that when the groove width increases to 50 µm, the micro-texture can retain its original structure, but with a further increase of the groove width (e.g. W = 80 µm), the mechanical strength of the grooves tends to be weakened, which is evidenced by the large particles appearing onto the surface of the D11W80 sample. The large particles originate from the broken microtextures and tend to insert into the micro-grooves, resulting in the deterioration of the frictional behavior of the zirconia ceramic surface. Moreover, the fracture of the peaks occurs at the bottom of the grooves. The SEM observations agree well with the findings obtained in terms of the friction coefficient such that when the groove width increases, its impact on the mechanical strength of micro-grooves becomes larger and larger during the sliding friction tests, resulting in a higher friction coefficient. By 14 / 18
evaluating the friction characteristics of different surface textures, it can be concluded that to enhance the friction and wear characteristics of zirconia ceramics, a combination of a larger groove depth and a smaller groove width is suggested.
4. Conclusions The present paper concerns a comparative investigation into the surface modification of the dental zirconia ceramics in order to improve their biocompatibility. A series of microtextures were fabricated by the laser ablation onto the zirconia ceramic surfaces and the effects of different surface structures were studied in terms of the wettability and the tribological behavior. The significance of the current work is to provide several technical guidance for the surface structure modification when fabricating the dental prosthetic restorations. Based on the results acquired, the following conclusions can be drawn. A bionic design method was adopted in the current work, and the structures of the microtextures were fabricated according to the surface morphologies of natural teeth. Based on the results of the topographical analysis, the morphologies of the perikymata were successfully replicated on the zirconia ceramic surfaces by the picosecond laser texturing. The micro textures have a great impact on the wettability of zirconia ceramics due to the change of the contact mode at the solid/liquid interfaces. The most hydrophilic surface was obtained on a microstructured surface of D2W20, which would promote the saliva to fill in the grooves, resulting in a Wenzel contact mode. With a further increase in the groove depth, an intermediate condition between the Wenzel mode and the Cassie mode would make the surface change towards hydrophobicity. Additionally, the width of the grooves shows a negative effect on the hydrophilic performance of textured zirconia ceramics. The tribological behaviors of different textured zirconia ceramic surfaces are affected by both the groove depth and width. The best frictional performance was obtained for the sample D11W20 since it has the deepest groove that possesses the largest conservation space for the abrasive debris and prevents the wear debris from being sandwiched between the friction pairs and from worsening the friction process. In contrast, a larger groove width would make the edge of the micro-texture appear more visible, which exacerbates its impact on the abrasives. Moreover, a larger groove width means that the texture distribution is much sparser and the mechanical strength of the groove can be degraded easily. Under such circumstances, the microstructural grooves would be detached easily. To achieve a balance between the wettability and tribological behavior of zirconia ceramic implants, it is recommended to use a minimum groove width combined with a suitable groove depth. When the groove width is the smallest, it is beneficial for both the wettability and tribological performance. With an appropriate increase in the groove depth, the change in the contact angle is insignificant, but the tribological behavior can be greatly improved.
Acknowledgments The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant No.51705319) and the Shanghai Pujiang Program (Grant No.17PJ1403800).
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Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.