Tribological changes of tooth enamel-mullite/3Y-TZP couple in arti�cial saliva

In-situ mullite toughened 3Y-TZP composite ceramic (mullite/3Y-TZP) with excellent mechanical properties was fabricated by gel-casting. The cytotoxicity of mullite/3Y-TZP was determined by both extract and direct contact methods, and the results indicated that mullite/3Y-TZP had no acute cytotoxicity. Furthermore, the tribological properties of the tooth enamel sliding against mullite/3Y-TZP in arti�cial saliva were investigated by using the pin-on-plate friction method. The friction coe�cient (μ) between the two friction samples was about 0.464 with a stable friction process, and both of them showed slight wear. Analysis of the wear surface and debris demonstrated that the tooth enamel mainly suffered from fatigue wear accompanied by mild adhesive wear, while mullite/3Y-TZP showed slight abrasive wear. This result indicated that mullite/3Y-TZP had good wear resistance and showed potential applications in dental material.


Introduction
Advanced biomaterials made of metals and ceramics have received extensive attention and developed rapidly over the last few decades with the improvement of medical level and the development of materials [1][2][3][4][5].It is well known that biomaterials, especially dental materials, must have the characteristics of good biocompatibility, chemical stability and mechanical properties [6,7].Metal alloys, such as titanium alloys, are generally considered to be bio-inert in human biological systems.However, when they are used in the oral environment, the frequent interaction between alloys and physiological environment will release metal ions, which not only limits their long-term stability in vivo, but also is harmful to the patients' health [8][9][10][11].
Zirconia (ZrO 2 ) ceramic, as one of the most important oxide ceramics, not only in possession of biosafety (no cytotoxicity), but also can exist stably in the oral environment without releasing harmful impurities and degradation [12][13][14][15][16].In comparison with alumina (Al 2 O 3 ) ceramic, ZrO 2 ceramic has more excellent mechanical properties, which can meet the requirements of higher compressive strength and hardness of dental crown [17][18][19][20][21].Moreover, previous studies have con rmed that ZrO 2 ceramic shows poor bacterial adhesion.Scarano et al [22].found that the coverage degree of bacteria on ZrO 2 was 12.1% as compared to 19.3% on titanium.Rimondini et al [22].also con rmed these results through invivo studies, and their results indicated that Y-TZP accumulated fewer bacteria than that of titanium in the total number of bacteria.Quinn et al [23].studied the effects of microstructure and chemical composition on the mechanical properties of dental ceramics, and also believed that ZrO 2 ceramic had better mechanical properties than that of dental ceramics.Despite ZrO 2 ceramic has high hardness, strength as well as good biocompatibility, the inherent brittleness limits its application.Mullite/3Y-TZP, as one of the ZrO 2 composite ceramics, not only improves the exural strength and fracture toughness of pure ZrO 2 ceramic without introducing toxic composition, but also retains high hardness, which has been con rmed in our previous research [24,25].
Nevertheless, in order to determine whether it is a better candidate for dental materials than pure ZrO 2 ceramic, especially for dental crowns, further research is needed on the biotoxicity and tribological properties of mullite/3Y-TZP, which will play an important role in the service life and failure behavior of this material when used as a kind of dental ceramic.
Gergo Mitov et al [26].used natural enamel to slide against Y-TZP ceramic treated in four different methods, and found that there was no signi cant linear correlation between the ceramic surface roughness and abrasive wear.Wang et al [27].studied the wear behavior of tooth enamel sliding against three kinds of dental ceramics (smooth and rough zirconia ceramics, glass ceramic, silicate-based veneer porcelain) with gold-palladium alloy and nickel-chromium alloy as control groups, and the results showed that the frictional coe cient of enamel sliding against polished zirconia or porcelain was between that of metal and glass-ceramic.Enamel showed abrasive wear when sliding against rough zirconia or glass ceramic, while fatigue wear was found on the worn surfaces of enamel when sliding against polished zirconia or nickel-chromium alloy, which showed that the friction and wear performance of zirconia can be improved signi cantly by proper surface polishing.Therefore, as a dental material, studying the oral tribological behavior of mullite/3Y-TZP is very important.
In the previous study, high-performance mullite/3Y-TZP has been prepared by gel-casting combined with pressureless sintering.Based on the comprehensive analysis of mechanical properties and microstructure of this ceramic, its biological toxicity will be con rmed by in vitro cytotoxicity tests including extract and direct contact methods in this study.These methods can be standardized to yield the repeatable results as well as e ciently performed at a relatively low cost [28,29].Moreover, the tribological properties of mullite/3Y-TZP will be deeply analyzed in terms of biology, including the study of wear behavior of the tooth enamel sliding against mullite/3Y-TZP in arti cial saliva environment, and revealing their wear mechanism and wear resistance.

Sample preparation
In this study, four human molars without obvious wear scar were derived from 18-year-old females, and were stored in physiological saline for sample preparation after being removed from the body.In order to prevent the original scratches on the enamel surface from affecting the observation of the morphology after the tribological tests and test the roughness of the enamel surface more accurately, each tooth was polished on carborundum sandpaper from 400 to 2000 mesh in water, and then polished by polishing cloth.This treatment method was performed within the range of not affecting the structure and performance of enamel.Finally, those teeth were cold-set by resin to obtain columnar pins with the size of φ10mm×15mm, and the enamel surface was exposed.
More details about this experimental process and raw materials used in this study were described in the Ref. 26.The polishing steps of these samples were similar to those for the teeth.

Characterization methods
The optical density (OD) of the medium in porous plate was measured by absorbance microplate reader (ELx 800, USA).The morphologies of cells cultured with extract and direct contact methods were observed by inverted microscopy.Scanning electron microscope (SEM, JSM-6390, JEOL, Tokyo, Japan) was used to observe the surface morphologies of mullite/3Y-TZP and tooth enamel before and after tribological tests as well as wear debris.Using raman spectrum to analyze elemental composition and chemical bond information of mullite/3Y-TZP before and after tribological tests.X-ray diffraction (XRD, Cu Ka radiation, D/max-2550-18kW, Rigaku, Japan) from 15° to 70° at 40kV with a scanning speed of 8°/min, coupled with energy dispersive X-ray spectroscopy (EDS) were used to analyze elemental composition and phase of mullite/3Y-TZP before and after experiments as well as wear debris.The elemental distribution of the wear surface of mullite/3Y-TZP after experiments was analyzed by electron probe microanalysis (EPMA).Using atomic force microscopy (AFM) and laser scanning confocal microscope (LSM700, Zeiss, Germany) to measure surface roughness (Ra) of the polished tooth enamel and mullite/3Y-TZP as well as undulating state of the wear surface.Ultra depth of eld three-dimensional microscopy system (VHX-500, Keyence, Japan) combine with LSM700 were used to characterize 3D morphologies of wear surface to determine width and depth of the wear track.

Cytotoxicity tests
In vitro cytotoxicity assays were carried out on mullite/3Y-TZP according to ISO10993-5 using both extract and direct contact methods [30,31].The extract assays used L929 mouse broblasts as test cells.
Each sample was ground into a cube of 6×6×6mm, and then they were ultrasonically cleaned with alcohol and deionized water, and nally put them into autoclave for disinfection (121℃, 30min).Pure 3Y-ZrO 2 ceramic and mullite ceramic were used as experimental control groups, while the negative and positive control groups were also set.The ow diagram of the extract method is shown in Fig. 1, and the speci c steps are as follows: (1) Three different groups of the samples were placed in the complete medium for 24h after disinfection, and the ratio of the sample surface area to the volume of the culture medium was 1 and 3cm 2 /mL; (2) After the cells resuscitated and adhered to the wall for growth, the original culture medium was removed and the sample extract was added.6 holes were set for each concentration of each sample group, and then put them in the CO 2 incubator for 1, 3 and 5 days, respectively; (3) The viability of cells was assessed by tetrazolium salt (MTT), and ELx 800 was used to detect the OD value of each hole [32].Then, calculating relative growth rate (RGR) value according to the optical density (OD) value, and determining the cytotoxicity of the samples by combining with the cell morphologies.
Direct contact assays were performed using L929 mouse broblast.A cell suspension with the concentration of 5×10 4 cells/ml was dropped into a petri dish containing the sample, and the ratio of the sample surface area to the volume of the culture medium was 1:1.After being cultured in CO 2 incubator for 1, 3 and 5 days, using inverted microscopy to observe the cell morphologies and determine whether there were transparent areas around the samples.

Tribological tests
All tribological tests were carried out using pin-on-plate reciprocating friction method on the UMT-3 multifunctional friction and wear tester in arti cial saliva conditions.The device and schematic diagram of the tribological tests are shown in Fig. 2. The tests used teeth as bolts, while polished mullite/3Y-TZP were used as counterparts.During the chewing process of human beings, the chewing force between the upper and lower teeth in the mouth ranges from 3 to 36N and the sliding distance between the bite contacted teeth is about 0.9-1.2mm.[33][34][35].Therefore, A normal force of 20 N, cyclic reciprocating displacement of 1mm, and frequency of 2 Hz were used in all tribological tests, as shown in Table 1.During the tests, the wear surfaces of these two friction components were always immersed in arti cial saliva, and the contents of various components of arti cial saliva are shown in Table 2 [27,36].The experiment was repeated three times in each condition.After tribological tests, the teeth and counterparts were ultrasonically cleaned and dried, and the wear rate was replaced by mass loss.(The mass loss of this friction couple was very low after tribological tests, and it was found that their wear rate was less than 1/10000 after rough calculation.In addition, non-regular structure of the tooth enamel selected in the experiment made it di cult to accurately express the wear rate by height loss.Therefore, it was more intuitive to demonstrate wear results by the mass loss.)

Cytotoxicity
The results of extract assays are shown in Fig. 3 and Fig. 4. The data have been analysed by one-way ANOVA, and the minimum signi cant difference at p<0.05 has been calculated and displayed on the histograms.There were no signi cant differences between any extracts of sample groups and the negative control group except the mullite group.A signi cant decrease of OD value happened in the positive control group, indicating that the tests were valid and L929 mouse broblasts cells were susceptible to the degrees of cytotoxicity.
The OD values (490nm) of each set of samples are shown in Fig. 3. Low OD values of all experimental groups were found after the cells had been cultured by extracts for 1 day, which was mainly because the cells did not fully grow and divide, and the cell concentration was low.Three days later, the OD values increased signi cantly except for the pure mullite group with a concentration of 3cm 2 /mL and the positive control group.After being incubated for ve days, the OD values of each experimental group were almost twice that of the cells cultured for 1 day, and each of them was not lower than that of the negative control group except the pure mullite group and positive control group.
Combined with the comparison between RGR value (>100%) and determination standard of cytotoxicity level, ZrO 2 ceramic and mullite/3Y-TZP had no cytotoxicity, and morphologies of the cells of these groups also con rmed this result, as shown in Fig. 4(a-b) [32].Although RGR value of mullite group 100%, the result of morphologies of the cells cultured by extract of mullite for 5 days indicated that mullite had no cytotoxicity or slight cytotoxicity compared with that of the positive and negative control groups, as shown in Fig. 4(c-e).
No adverse reaction was observed in petri dishes by inverted microscopy in the direct contact experiments, and morphologies of the cells are shown in Fig. 5.As depicted in Fig. 5(a-c), no abnormalities or dead cells were found around the samples, and no cell-free transparent regions were observed.Density and morphologies of the cells of these sample groups were similar to those of the negative control group (Fig. 5(d)), while there were signi cant death cells in the positive control group (Fig. 5(e)).The results showed that those three kinds of samples were no-cytotoxicity, which was a further proof of the previous experimental results.

Microstructure characterization
Fig. 6 shows the microstructure of mullite/3Y-TZP, and there are two obviously different phases in the sample.The preliminary experimental results proved that the black phase was mullite generated by the reaction of Al 2 O 3 and SiO 2 during the sintering process, while the gray area was ZrO 2 [24,25].In the ternary eutectic system of Y 2 O 3 -SiO 2 -Al 2 O 3 formed by Y 2 O 3 with SiO 2 and Al 2 O 3 , local liquid phase appeared in the sample at high temperature, then the contact reaction and nucleation occurred between SiO 2 and Al 2 O 3 .After that, the core of mullite crystal grew into the columnar crystal through mutual diffusion, as shown in Fig. 6(a) [37][38][39][40][41].The size of the columnar mullite was about 10μm, as shown in Fig. 6(b), and there were ZrO 2 particles uniformly distributed in the interior of mullite, which can improve the strength of columnar mullite by the pinning effect.Meanwhile, columnar mullite will also signi cantly enhance and toughen the composite ceramic for this reason.In addition, previous studies have shown that Y 2 O 3 can enter the crystal lattice of ZrO 2 to stabilize the tetragonal/cubic phase [25].
The surface morphologies of the tooth enamel before the tribological tests are shown in Fig. 6(c-d).It retained complete character with no protrusions or microcracks on the surface, providing a reliable premise for subsequent friction experiments.EDS analysis indicated that the main components of the tooth enamel surface were calcium and phosphorus, and the atomic ratio of the two elements was about 1.6:1, con rming that tooth enamel was indeed made of hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ), as shown in Fig. 6(d), which provided a theoretical basis for subsequent analysis of wear debris.7. Their Ra values were 33.6nm and 148.22nm respectively, indicating that the surfaces of these two kinds of materials using for the experiments were smooth, which can signi cantly reduce the frictional resistance (F x ) and μ [42].As is known to us, there should be an appropriate μ when the dental ceramics, especially ceramic crowns, sliding against natural teeth, so as not to cause excessive wear on one side or affect the chewing of food.The results were obtained by repeating the friction experiments for three times, and one of them was shown in Fig. 8.It can be seen that after a brief run-in period, the μ nally stabilized at 0.464.Combined with the results of Wang et al. (as shown in Fig. 9), this value was between the μ of glass ceramics and Au-Pd alloy when rubbing against tooth enamel, and was particularly similar to the result of polished zirconia and porcelain [27].Meanwhile, we also understood the in uence of the surface treatment on the results of friction experiments of dental ceramics from their research, and this is why the frictional couple were polished before the tests.The reason for getting this result was that even though the surfaces of this couple were very smooth before the tribological tests, they would be destroyed after the initial contact under the action of applied load, resulting in unstable μ and F x until new wear surfaces were generated.In the process of stable friction, the existence of arti cial saliva played an important role in lubrication and cooling, but also could wash away the debris generated during the friction process and clean the wear surface.Therefore, the μ and F x were decreased and extremely stable, and they were the factors that determined the mass loss of the teeth and mullite/3Y-TZP [34,42].This frictional behavior indicated that there was a good match between the tooth enamel and mullite/3Y-TZP.
The mass loss of the teeth and mullite/3Y-TZP was very small, even though the former value was slightly higher than that of the later, both of them lost a few milligrams, only were 0.5±0.1mgand 0.3±0.1mg,indicating that the wear resistance of the friction couple was well in this environment.Stable friction process and low mass loss proved that mullite/3Y-TZP had application potential in the eld of oral cavity.
Moreover, Lee et al [43].found that ZrO 2 would undergo phase transition under the action of load during the friction process, and the low mass loss of mullite/3Y-TZP may partly attributed to transformation toughening induced by ash-temperature.In this study, the diffraction peaks of different ZrO 2 phases of mullite/3Y-TZP did not show signi cant changes before and after the tribological tests, as shown in the XRD patterns of Fig. 10.The diffraction peaks intensity of m-ZrO 2 , t-ZrO 2 and c-ZrO 2 were almost unchanged, which may be caused by stress dispersion and cooling effects of arti cial saliva.In addition, the internal structure of mullite/3Y-TZP did not change before and after the tribological tests, as shown in the Raman spectrum of Fig. 11 (black and red curves).The intensity of the diffraction peak and Raman shift of m-ZrO 2 and t-ZrO 2 did not change signi cantly [44].The -OH peak (3625cm -1 ) was observed in the Raman spectrum of mullite/3Y-TZP after the tribological tests, and Lang et al. [45,46] mentioned that Y 2 O 3 could react with water to form α-Y(OH) 3 due to the action of water and pressure during the friction process.However, compared with the polished sample whose -OH peak most likely generated during the pretreatment process before the tribological tests, no new -OH peak formed during the friction process.This hypothesis was demonstrated by immersing mullite/3Y-TZP in arti cial saliva for 5 days and then performing a Raman test, and the spectrum was shown in the blue curve.No signi cant changes happened in the intensity and Raman shift of the -OH peak, which was similar to those of the previous two results.These results indicated that mullite/3Y-TZP had good stability during friction and arti cial saliva environments.

Wear appearances
Different observation methods were selected based on the different characteristics of the wear surface.
3D morphologies of the wear surface of the tooth enamel were measured by LSM700, and they showed a at wear surface with almost no grooves or undulations, which can be obtained from Fig. 12(a).Surface roughness of the wear surface was measured to be only 4.166μm, even though it was not as smooth as that of the original surface of tooth enamel, and the curve at the bottom of Fig. 12(a) also illustrated no signi cant uctuation.This phenomenon may be caused by the formation of smooth lm on the wear surface due to the deformation of the debris falling from tooth enamel under action of arti cial saliva and load during the friction process, which indicated that no signi cant abrasive wear had occurred on the surface of enamel, providing a basis for the following wear mechanism.Mullite/3Y-TZP, as a counterpart, did not produce severe wear on its wear surface compared to enamel, as shown in Fig. 12(b).The 3D morphologies measured by VHX-500 demonstrated that the worn area of mullite/3Y-TZP was very shallow, and the vertical height difference between the centre of the pit and the unworn surface was only 8.37μm, as shown by the curve below Fig. 12(b).This was mainly because both mullite/3Y-TZP and enamel have high hardness, it was di cult to destroy the surface structure of mullite/3Y-TZP and cause serious wear during the friction process with the lubricating of arti cial saliva,.Fig. 13(a) represents overall appearance of the wear surface of enamel at low magni cation, and it showed a at surface without signi cant scratches, which was consistent with its 3D morphologies.Fig. 13(b) shows the morphology obtained by local magni cation of Fig. 13(a).A small amount of abrasive debris adhered to the at surface, the analysis of EDS showed that they were calcium-phosphorus compounds (hydroxyapatite) derived from the surface of enamel, and contain a small amount of elements of mullite/3Y-TZP and arti cial saliva, as shown in the upper right corner of Fig. 13(b).Fig. 13(c) shows a portion of the edge of the worn region, and it was apparent that delamination of the layered debris had occurred in this region, and the size was about 25μm, while cracks presented around it, which was closely related to stress-induced fatigue fracture.Fig. 13(d) represents overall appearance of the wear surface of mullite/3Y-TZP at a low magni cation, and only a very small amount of scratches presented on the at surface.A small amount of abrasive debris adhered to the surface of the counterpart, and EDS analysis showed that their compositions were the same as that of the debris on enamel surface, as shown in Fig. 13(e).Unlike the wear surface of enamel, the surface of mullite/3Y-TZP did not exhibit large-scale peeling, even though some scratches and microcracks appeared, as shown in Fig. 13(f), which was closely related to the mechanical properties of mullite/3Y-TZP.In addition, the pinning effect of mullite and good combination between the ZrO 2 particles reduced the likelihood of particle aking.
Similar study of Wang et al [27].(as shown in Fig. 14) can be compared with the study in the text.Fig. 14(a-c) represents the wear surface morphology of enamel after sliding against polished zirconia ceramic, and the enamel surface generated micro cracks and a layer of wear debris, which was almost the same as the results of this study, indicating that the tooth enamel had appeared fatigue wear; What was different from the results of this experiment was that the wear surface of zirconia ceramic had obvious particle shedding phenomenon (as shown in Fig. 14(d)), that is, abrasive wear had occurred on the surface of zirconia ceramic, which was closely related to the brittleness of zirconia ceramic.Because mullite/3Y-TZP used in this experiment had better fracture toughness, the nailing effect of mullite and alumina particles reduced the possibility of zirconia particles falling off [24].Fewer particles such as hard zirconia would reduce the damage to the friction couple during the friction process.Therefore, mullite/3Y-TZP is more suitable as dental materials than pure zirconia ceramic.
In order to further analyze element distribution of the wear surface of mullite/3Y-TZP, EPMA analysis was performed, as shown in Fig. 15, where Al and Si elements were not indicated.As shown in Fig. 15(a-d), internal components Zr and Y of mullite/3Y-TZP were detected, and it clearly proved that Y element distributed uniformly in ZrO 2 , which played a role in stabilizing t-ZrO 2 .In addition to the components of ceramic matrix, there were also Ca and P elements on the wear surface, as shown in Fig. 15(e-f), indicating that there was wear debris from enamel presented on the wear surface, which can be found in the grooves in combination with Fig. 15(a).The representative elements of arti cial saliva, such as Na, Cl, etc. also existed on the wear surface, and their distribution was consistent with that of Ca and P elements besides a small amount of them evenly distribute in other areas, which was mainly because of the adsorption of wear debris to arti cial saliva.It was apparent that the lling of grooves and cavities with wear debris and arti cial saliva can signi cantly lubricate the wear surface and reduce friction resistance.

Wear mechanism
The shape of the wear debris provides a reliable clue to the wear mechanism of the specimen.Fig. 16 shows morphologies of the wear debris obtained after tooth enamel sliding against mullite/3Y-TZP in arti cial saliva, from which abrasive grains and layered wear debris with different sizes can be seen.
Abrasive grains showed small sizes with obvious aggregation and mutual adhesion, while the layered debris had two kinds of morphologies: ( ) large layered debris with a rough surface, as shown in Fig. 16(a-b), and ( ) layered debris with a smooth surface, as shown in Fig. 16(c-d).On the surface of the rst kind of lamellar debris, there were aggregated ne particles and obvious microcracks, which was caused by the aggregation of abrasive debris under the combined action of arti cial saliva and load during the friction process, or directly from the lamellar shedding on the surface of the enamel.The second kind of lamellar debris had clear outline with distinctly straight boundaries and microcracks, indicating that they were mainly caused by brittle fracture.The elements of these wear debris were shown in Fig. 17(a-d), it can be seen that the mass ratio of three elements of calcium, phosphorus and oxygen in the wear debris was high, reaching 26%, 18% and 24% respectively, which was consistent with the composition of enamel, and they were distributed almost every debris combined with Fig. 17(a).In addition, the surface of the wear debris was evenly distributed with a small amount of debris from the surface of the mullite/3Y-TZP and elements in the arti cial saliva, as shown in Fig. 17(e-i), which indicated that only slight wear occurred in mullite/3Y-TZP, thus it was di cult to nd large pieces of ZrO 2 particles in the wear debris.The lling and lubrication of the wear surface by arti cial saliva reduced the frictional resistance and mass loss, and result in the adhesion of elements, such as Na and K on the surface of the enamel and counterpart.
The morphologies of the wear debris combined with the character of the wear surface indicated that enamel mainly experienced fatigue wear.Because enamel on the surface of tooth is hard and brittle, repeated friction and load will cause stress concentration in the contact part, resulting in fatigue fracture [34].In addition, the shape of the edge portion of the wear surface indicated that adhesive wear happened locally (Fig. 13(a)), which was due to the abrasive was wetted and pressed to form lm on the wear surface, and then the lm was peeled off due to repeated friction for a long time.Thereby, an adhesive wear zone was formed.However, whether it was from the mass loss and wear surface morphologies of mullite/3Y-TZP or the elemental analysis of the wear debris, mild wear of mullite/3Y-TZP could be obtained.The uniform distribution of Zr, Al and Si elements in the wear debris indicated that mullite/3Y-TZP did have a slight particle aking phenomenon and only occur mild abrasive wear, which was also consistent with a few scratches on its wear surface.The lubrication and cooling effects of the wear debris and arti cial saliva which lled in the pits of the wear surfacec of mullite/3Y-TZP maintained the entire friction process at an extremely low mass loss during the repeated tribological tests.Because the hardness of the enamel was lower than that of mullite/3Y-TZP and arti cial saliva had lubrication and cooling effects, it was di cult to cause extensive fatigue wear on the composite ceramic.In addition, almost no phase transformation of ZrO 2 was obtained by the previous XRD analysis during the friction process, and Raman analysis found no signi cant reaction between Y 2 O 3 with water, indicating that the internal structure of mullite/3Y-TZP was retained, which played an important role in retaining high mechanical properties.This phenomenon showed that mullite/3Y-TZP had good stability and wear resistance in the human oral environment.

Conclusion
The cytotoxicity of mullite/3Y-TZP has been preliminarily studied based on its dental application, and the tribological properties of the tooth enamel sliding against mullite/3Y-TZP in arti cial saliva environment have been deeply analyzed with mullite/3Y-TZP as the counterpart.The main conclusions are as follows: 1.The cytotoxicity of mullite/3Y-TZP was tested by both extract and direct contact methods.The results indicated that mullite/3Y-TZP did not show acute cytotoxicity like ZrO 2 ceramic, even though the second phase of mullite was introduced.
2. In arti cial saliva environment, the friction process between the tooth enamel and mullite/3Y-TZP was extremely stable with the μ of 0.464, which was in the range of μ when natural teeth chewing food.The mass loss of the two materials was low due to the lubrication and cooling effects of arti cial saliva.These tribological properties showed that the friction pairs matched well.The curves of the coe cient of friction (μ), friction resistance (Fx) and applied load (Fz) during the friction test.
Page 20/26 The coe cient of friction of natural enamel against different dental materials plotted versus the number of cycles [27].
Figure 10 XRD patterns of mullite/3Y-TZP before and after the friction test.
Raman spectrum of mullite/3Y-TZP with different methods of handling.Figure 16 (a-d) Microscopic morphologies of wear debris generated during the friction test.

3. 3
Wear behavior 3.3.1 Coe cient of friction (μ) and mass loss Ra and 3D topographies of the polished tooth enamel and mullite/3Y-TZP before the tribological tests are shown in Fig.

Figures Figure 1 Flow
Figures

Figure 2 Device
Figure 2

Figure 7 Surface
Figure 7

Figure 13 a
Figure 13

Table 1 .
3.No signi cant phase transitions happened to mullite/3Y-TZP during the tribological tests.The tooth enamel mainly suffered from fatigue wear accompanied by slight adhesive wear due to the lling of arti cial saliva and wear debris.While mullite/3Y-TZP only showed slight abrasive wear in this condition.These results indicated that mullite/3Y-TZP had good stability and wear resistance.Declarations 4 .Lange F F, Dunlop G L, Davis B I. Degradation During Aging of Transformation -Toughened ZrO 2 -Y 2 O 3 Materials at 250°C.J Am Ceram Soc 2010, 69: 237-240.Experimental parameters and results of the friction test