Evaluation of zirconia surfaces and shear bond strength after acid–etching with ultrasonic vibration

To evaluate the effect of surface reaction process after hydrofluoric (HF) acid etching using ultrasound and the shear bond strength (SBS) of resin cement to zirconia polycrystal (Y-TZP) ceramic. Y-TZP ceramic sheets were divided into rinsing group (Group P), ultrasonic cleaning group (Group C), and ultrasonic reaction + rinsing group (Group CP), and all the groups were treated for 5, 10, 20, 30, 40, 50, and 60 min, respectively. The surface morphology, elements distribution, roughness, and wettability of the ceramic sheets in each group were observed. The SBS of ceramic-resin bonding specimens was tested after immersion and after cooling-heating cycles, respectively. Octahedral and spiculate products were observed on the surface of Y-TZP that was etched with HF acid in Group P. The amount of these products increased over time. In contrast, only a few octahedral products remained on the surface of Y-TZP in Groups C and CP. Within the same reaction time, the surface reaction of the CP group was stronger than that of the other two groups, accompanied by a more uniform morphology. The shear force in Group C was the lowest, and the shear force reduction in Group CP was the least after cooling-heating cycling, with statistically significant differences (P< 0.05). After the reaction time exceeded 30 min, the shear force in each group decreased instead of increasing. Octahedral and spiculate acid etching products on the surface of HF acid-etched Y-TZP can enhance the bonding force of zirconia. Ultrasonic cleaning would drive the exfoliation of acid etching products from the sample surface, leading to the decrease of the bonding force. The acid etching with ultrasonic vibration can accelerate the HF acid etching process of Y-TZP ceramics, which is conducive to improving the bond strength to resin and durability.


Introduction
Zirconia ceramics are strong, stable, esthetic, biocompatible, and extremely reliable materials [1], which have been extensively popularized in dental clinics.Compared to other dental ceramics, yttria-tetragonal zirconia polycrystal (Y-TZP) possesses higher strength and fracture toughness [2].Unfortunately, Y-TZP has insufficiently rough surface with low surface energy and wettability due to the superior resistance to acid etching, additionally, silane coupling agents have no effects on zirconia , which affect the clinical success of resin bonding procedures with zirconia [3,4].How to improve the bond strength at the resin-Y-TZP interface is a research hotspot.
Numerous approaches have been proposed to improve surface roughness, such as the use of selective infiltration etching, airborne-particle abrasion (APA) with aluminum oxide (Al 2 O 3 ) particles or tribochemical silica coating (Rocatec Soft, Rocatec Plus), and laser irradiation [5].Surface roughening increases the surface area which facilitate the penetration of the resin to zirconia, making micromechanical interlocking improved.Moreover, surface roughening can also lead an increase in surface energy, wettability and chemical bonding between resin cement and zirconia, which have thus been enhanced [3].Recently, hydrofluoric (HF) acid has been reported to be able to etch Y-TZP to increase its surface roughness [6].The surface micromorphology of zirconia has been changed by means of HF acid, and the reaction conditions can affect the etching effect [7].The majority of studies have reported that the application of 5% or 9.5% HF acid to the zirconia surface does not cause any morphological changes in the structure or increase the surface roughness, but 40% HF acid can significantly induce surface etching [8].Other studies [9,10], employed 40% HF acid at 100 °C for acid etching to remove uneven particles on zirconia surface, and the acid-etched surface inhibited the attachment of bacteria S. sanguinis and P. gingivalis, and exerted a negative effect on biofilm formation.However, such a high temperature has exceeded the boiling point of HF acid, causing the volatilization of HF acid and possible damage to dental clinics or laboratories [11,12].Mun-Hwan Lee et al suggested that zirconia etching by HF acid proceeds slowly at room temperature, even with HF acid solutions of high concentrations [13].
When it comes to ultrasonic cleaning, ultrasonic waves directly and indirectly act on liquid and dirt through their cavitation, acceleration, and direct inflow effects in the liquid, so that the dirt layer is dispersed, emulsified, and peeled off to achieve cleaning [14].In the study of Marlyni Aparecida ZENS, the sound waves would modify the Y-TZP surface, finally improve the adhesion of Y-TZP with resin cement and cause less damage to the Y-TZP surface [15].Ultrasonic cleaning is usually set as a routine step in the preparation of Y-TZP ceramic samples, while its role as a possible experimental influencing factor has been ignored.
In this experiment, whether ultrasonic cleaning would affect the reaction products and reaction rate on the surface of Y-TZP ceramics was investigated.Moreover, the surface morphologies of Y-TZP ceramics were observed with time prolonging, and the etching mechanism was discussed.The null hypnosis testing suggested that ultrasonic cleaning has an influence on the reaction products, and ultrasonic vibration corrosion can accelerate the HF acid etching of Y-TZP ceramics.

Grouping and surface treatment of Y-TZP ceramic samples
Table 1 listed the used materials, referred to our previous works [16].The sintered Y-TZP ceramic sheets (10 × 10 × 3 mm 3 ) were provided by Upcera (UPCERA ST, Shenzhen, China).Each ceramic sheet surface was polished to the roughness of 400# by a polishing machine (Phoenix 400 Alpha, USA), with its thickness error kept within ± 0.005 mm.Then, the ceramic sheets were randomly divided into 21 groups (n = 22).In rinsing group (Group P), the Y-TZP ceramic sheets were soaked in 40% HF acid solution, and then gently rinsed in absolute ethanol and deionized water successively for 10 times.In ultrasonic cleaning group (Group C), the Y-TZP ceramic sheets were soaked in 40% HF acid solution, and then subjected to ultrasonic cleaning with absolute ethanol and deionized water for 15 min successively.In ultrasonic reaction + rinsing group (Group CP), the Y-TZP sheets were soaked in 40% HF acid solution in a beaker, then reacted in an ultrasonic cleaning machine and rinsed gently using absolute ethanol for 10 times and deionized water for 10 times.The ultrasonic power was 600 W, and the ultrasonic frequency was 40 kHz.Moreover, each group was divided into 7 time periods, including 5, 10, 20, 30, 40, 50, and 60 min based on treatment time.After the processing, all samples were dried in air.

Contact angle measurement
The water contact angle was measured using an optical contact angle measuring instrument (LSA100, Lauda Scientific GmbH, Germany).4 μl of deionized water was dropwise added onto the surface of each Y-TZP sample.After the droplets became stable (about 3 s), the images were recorded via the matched charge-coupled device (CCD) camera.Meanwhile, the measurement was automatically performed by the image processing software (SurfaceMeter TM , v1.2.1.9)for 6 times, and the left and right contact angles were calculated and averaged.Finally, the average contact angle of samples in each group was solved.

Morphology observation via a scanning electron microscope (SEM)
Surface morphologies were characterized by SEM (TESCAN MAIA3, Brno) under the secondary electron mode, with a working distance of 5 mm, an accelerating voltage of 20 kV, and a beam current of 10 nA.And elemental composition was analyzed by an energy dispersive spectrometer (EDS, ULTIM MAX 170, Britain).

Surface element analysis through x-ray photoelectron spectroscopy (XPS)
Surface chemical composition was semi-quantitatively analyzed using a K-Alpha XPS System (Escalab 250Xi, China).Al-target K α ray was selected as x-ray, and the scanning range of the energy spectrum was 0-1,400 eV.Firstly, full spectrum scanning and high-resolution spectrum scanning were performed to mainly detect such elements as C, F, O, Y, and Zr.Secondly, the measured binding energy was corrected by the standard binding energy of C 1 s = 284.8eV.Thirdly, peak splitting and fitting of the spectra were implemented via XPSPEAK software.Finally, the relative content of acid etching products was semi-quantitatively compared based on the measured peak area.

Atomic force microscope (AFM)-aided morphology observation and roughness measurement
The three-dimensional (3D) morphology and roughness were measured under the tapping mode via an AFM (Dimension ICON, Bruker, Germany), during which a gold-doped silicon probe (length: 40 μm, resistivity: 0.01-0.025Ωcm) was used.Subsequently, images were recorded at a scanning rate of 0.999 Hz with a resolution of 256 × 256 pixels and a scanning area of 5 × 5 μm.The Ra value (unit: nm) of each sample was measured at three different positions, and the average value was calculated.

Detection of bonding mechanical properties
The samples used for the evaluation of bonding mechanical properties were fabricated as a cylinder with 3 mm in diameter and 2 mm in height.After that, two resin columns were placed on the Y-TZP ceramic sheet with the surface treated after the flowing resin was coated diagonally.Then, a glass plate with a weight of 50 g was placed on the resin columns with its center opposite to the center of porcelain-TZP for 2 min, and then was photocured at a power of 800 mW cm −2 for 20 s.Next, the glass plate was removed, while the resin columns were photocured for 20 s from four directions, respectively, and placed at room temperature for 30 min.The bonding samples in each group were divided into two parts, one half of which was stored in deionized water at 37 °C for 24 h, and the other half was subjected to the cooling-heating cycling experiments in the circle of the cold bath at 5 °C for 30 sand the hot bath at 55 °C for 30 s.In the cooling-heating cycling experiments, 5,000 cycles were conducted.
The shear bond strength (SBS) test was performed with a universal experimental machine (Instron 3365, ElectroPlus, USA).The applied load was parallel to the zirconia-resin column bonding surface and stuck close to the junction of the zirconia-resin substrate, in which the descending speed was set at 1 mm min −1 until the resin column fell off.The load value (N) at this moment was automatically recorded by the universal experimental machine, and the SBS (Mpa) of each sample was equal to the ratio of the peak load (N) to the bonded area (mm 2 ).

Observation of bonding interface
After the shear experiment, the fracture morphologies were observed under a stereomicroscope (C-DSS230, Nikon; Tokyo, Japan).The fracture modes were classified into three types: (1) adhesive failure (the ceramic surface was completely exposed, without residual resin or resin cement), (2) cohesive failure (the fracture occurred entirely inside the resin or resin cement, without any exposed ceramic surface), and (3) mixed failure (the ceramic surface was locally exposed, accompanied with partial residues of resin or resin cement).

Statistical analysis
The K-S normal distribution test was performed on the data from each group using SPSS 23.0 software, followed by grouped Kruskal-Wallis analysis of non-normally distributed data.Multiple comparisons of differences with statistical significance between groups (p < 0.05) were conducted.For normally distributed data, the repeated measures multivariate analysis of the variance of repeated measurements was carried out after the test.

Surface hydrophilicity
Figure 1 displays the results of water contact angle tests.It was revealed by the significance analysis that the significance level of each group was less than 0.05 (in addition to 40 min and 50 min), and the Y-TZP ceramics with different surface treatments exhibited varying surface hydrophilicity.The hydrophilicity of Group CP was the best, while Group C showed the largest contact angle.With the change in time, the surface hydrophilicity of Y-TZP ceramics tended to increase first and then decrease slowly, and the contact angle was the lowest at 30 min.In Group P, the mean value of water contact angle was (7.85 ± 1.53)°at 30 min, (12.33 ±1.77)°at 40 min, about (6.55 ± 0.75)°at 50 min, and (9.6 ± 1.96)°at 60 min.In Group P, there were no statistical differences in the contact angle among 30, 40, 50, and 60 min (p > 0.05).After the reaction time exceeded 30 min, the water contact angle of the sample surface remained basically constant.

Morphological observation
The SEM images of Y-TZP ceramics with different treatments are exhibited in figure 2. After 10 min of acid etching, octahedrons were generated on the sample surface in Group P through 10,000 × morphological observation.With the acid etching prolonging to 20 min, such octahedrons could be clearly observed.It could be inferred that the number of octahedrons was increased with the prolonged reaction time.Besides, after acid etching for 20 min, the octahedrons were covered by some 'amorphous flaky and blocky substances', and increased continuously with time.After 60 min of acid etching, some octahedrons on the surface of samples in Group P were reduced since they were covered.By adjusting the magnification of the SEM to 50,000 ×, it was discovered that these 'amorphous substances' were composed of several spicules (figure 3).In Groups C and CP, there were only a very small number of octahedrons on the surface of the samples, and no spicules were observed, indicating that octahedrons and spicules might fall off due to ultrasonic cleaning.As the etching time increased, the grain size of zirconia gradually decreased and the grain spacing increased, demonstrating a substantial evolution of the morphology, while no significant evolution of the granular texture at high magnification.In addition, beyond a certain etching time, the presence of randomly dispersed pits could be detected on the sample surface.The grain gaps and these pits appeared more uniform on the surface of Group CP, and the reaction of Group CP was also more intense at the same etching time.We chose Group C and Group CP as a representative, as shown in figure 4.

Surface chemical changes of zirconia after HF acid etching
According to EDS analysis (figure 5), octahedrons were mainly composed of F, Y and Zr, and spicules consisted of F and Zr elements.It could be intuitively found that F and Y elements in octahedrons were higher in the group with 30 min of acid etching, as shown in figure 6.In Group P, the content of F and Y elements was raised constantly with the extension of reaction time, accompanied by the continuous decrease in the content of O and Zr elements.On the surface of samples in Group CP, the content of F fluctuated within 1.5-10 at%.In Group C, the content of F was the lowest and kept basically constant at 0.4-2 at% (figure 7).The XPS full spectrum of the Y-TZP ceramics after HF acid etching (figure 8(b)) illustrated that the peak positions of the spectra in Groups C, P, and CP were basically the same, indicating that the types of surface etching products cannot be altered by changing the etching time and adopting ultrasonic acid etching.Notably, among the characteristic peaks, the binding energy of the F 1 s peak was about 864.9 eV, and that of the Y 3d peak and Zr 3d peak was about 160.0 and 187.6 eV, respectively.It was inferred that ZrF 4 and YF 3 appeared on the surface of Y-TZP ceramics as the products of HF acid etching.Moreover, the intensity of F 1 s characteristic peak revealed that the area with the content changes of acid etching products was consistent with the EDS results (figure 8(a)).Based on element analysis, octahedrons were Y, F-rich compounds.Y, F-rich compounds in P group were more than that in C group, and larger than CP group.Owing to more and larger Y, F-rich compounds in P group, Y and F elements were more than others and Zr and O elements were less than others.With the increase in acid etching time, Y, F-rich compounds increased so that more Y and F elements were found.However, there were fewer changes in chemical element in other groups owing to rinsing treatment.

3D surface morphology and roughness of Y-TZP ceramics after HF acid etching
The 3D surface morphologies of Y-TZP ceramics were detected by the AFM, as shown in figure 9. Pits appeared on the ceramic surface from 10 min in Groups P and C and from 5 min in Group CP.As the reaction time increased, the pits increased in number and in size.The roughness profile curves of the two diagonal sections were also deepened and then widened and shallowed with time prolonging.The protrusions and depressions formed in Group CP were more uniform and regular than those in the other two groups.The measured Ra values also showed a basically consistent trend, that is, the Ra value was first increased and then decreased with time.The roughness was the highest in Group C [(188.84 ± 69.31) nm] at 40 min, in Group P [(239.2 ± 44.63) nm] at 30 min, and in Group CP [(159.96± 52.71 nm)] at 40 min (figure 10).The surface roughness of Y-TZP ceramics in Group P was statistically significantly different from that in Groups C and CP (p < 0.05), while the difference in surface roughness between Groups C and CP was of no statistical significance (p > 0.05).

SBS analysis
The ceramic-resin SBS analysis results before and after cooling-heating cycles were shown in figure 11.It was demonstrated by the significance analysis that except for the treatment at 50 min, the significance level between Groups C and CP was p > 0.05, indicating no statistically significant differences.Under the other time conditions, the significance levels among the three groups were all less than 0.05, and the difference in SBS among the three groups was statistically significant.With the increase in reaction time, the SBS in each group was increased first and then decreased slowly.In group C, the SBS [(11.34 ± 3.74) MPa] reached the maximum at 40 min, followed by that [(10.33 3.69)] MPa at 30 min, and there was no statistical difference (p > 0.05).The SBS peaked at 30 min in Group P [(15.59 5.13) MPa], and at 30 min in Group CP [(13.37 4.34) MPa], with no significant difference (p > 0.05) (a).After cooling-heating cycles, the SBS in each group decreased to various degrees, but still presented a trend of increasing first and then slowly decreasing with time.The maximum SBS was maintained at 30 min or 40 min in all groups.Following cooling-heating cycles, the reduction of SBS in Group CP was smaller than that in Groups C and P (figure 11(b)).

Observation of bonding sections
All debonded specimens showed adhesive failure at 10 × magnification, regardless of the experimental groups or thermocycling conditions.However, such an adhesive failure did not indicate poor resin-zirconia bonding because adhesive failures occurred even in the groups that produced high bond-strength values.

Discussion
Appropriate roughness and wettability of zirconia surface are essential for obtaining satisfactory adhesion with resins [17].In general, increased surface roughness and enhanced wettability imply a higher bond strength [18].For cementation, HF acid etching is commonly used on silica-based ceramics to generate surface roughness, thus achieving good bonding.This process also enhances the wettability and surface energy of the ceramic surface, thereby increasing the bond strength between the ceramic and cement.In contrast, zirconia is a silicafree ceramic, and has resistant to conventional etching techniques [8].The acid etching possesses the advantage of homogenous roughening of the material regardless of its size and shape.In addition, this method exhibits no risk of material delamination as it does not exert stress on the material [19,20].Therefore, more studies should be performed to create microretention to improve the efficacy of surface treatment methods for zirconia, particularly with the use of stronger acids.The chemical bond can improve the bond strength between resin cement and zirconia due to the existing micromechanical retention [21].
Although it has been well-known that the chemical surface treatment with HF acid is inadequate for zirconia, previous studies have been conducted on the HF acid etching mechanism to explain the zirconia topography remodeling.Ju-Hyoung Lee considered that the low-temperature degradation (LTD) phenomenon occurring in the wet state induces the phase change of zirconia [22].On the contrary, Xie et al [23] believed that chemical degradation rather than LTD, plays an important role in changing the physical properties and surface roughness of Y-TZP.According to the findings of Owaleker et al [24], HF acid makes zirconia and yttria dissolve to form fluoride, oxide, and hydroxide.Mun-Hwan Lee et al [13], indicated that the small amount of zirconium oxyfluoride generated on the HF acid-etched zirconia surface can modify the surface to become more reactive than the non-etched surface.It has been demonstrated that etching is faster at the particle border, but it may also  happen inside the particles.Furthermore, it has been revealed that HF acid induces the formation of etching pits over time, thus raising the microroughness [25].
The concentration of 40% leads to the fastest and most uniform etching, so it is considered the most appropriate for the treatment of zirconia [8].The surface roughness of zirconia can be remarkably enhanced by 40% HF acid within the shortest time, reaching the maximum in 1 h, but after that, the roughness value increases slowly and the material strength will be obviously affected [8,26].Therefore, in our study, we used 40% HF acid to induce a rough zirconia surface for a maximum of 1 h.It was observed under the SEM that the surface of zirconia exhibited a morphological evolution with increasing HF acid etching time (figures 2, 4).As the etching time prolonging, the spicules appeared and continued to increase, while the octahedrons seemingly to be gradually cover-ed so that the number of octahedrons on the surface was evidently reduced at 60 min (figure 2).Heidarpour Akbar et al indicate that the stoichiometry of reactants and the kinetic roughing mechanism with increasing the HF etching time leading to the occurrence of shape evolution [20,27].According to the EDS analysis, these spicules were composed of F and Zr, whose atomic percentage was approximately to 1:4.Combining with the XPS analysis, the reaction product was ZrF 4 .The results of the EDS analysis also revealed that the octahedrons consisted of Y, F, and a small amount of Zr, which were confirmed as YF 3 crystals combined with the XPS analysis.This conclusion coincided with the products discovered on the ceramic fluorination surface in Literature [28].The results of image analysis uncovered that the small amount of Zr on the surface of octahedrons might come from the spicules attached to their surface.Xie et al found in a study that ZrF 4 was detected in the HF acid etching solution of Y-TZP ceramics [23], suggesting that the substance is partially soluble.This could also be confirmed by the change in the contact angle during the experiment.After 30 min, the contact angle was increased to some extent in Groups C and CP, while the samples in Group P were completely wet due to the presence of the 'adhesive layer'.The XPS full spectrogram analysis reflected that the chemical reaction of HF acid-etched Y-TZP ceramics was relatively stable (figure 8(b)).The spicules kept gathering to form an 'adhesive layer', and the delayed development of the layer could be attributed to the saturation of ZrF 4 in solution.
After ultrasonic cleaning, only a small number of octahedrons remained on the surface of samples in Group C, proving the effectiveness of ultrasonic cleaning.Meanwhile, SEM and AFM in this study manifested that the etching effect of HF acid on Y-TZP surface was accelerated by ultrasonic cleaning.Therefore, both of the initially proposed null hypotheses were accepted.Ultrasonic waves propagate into the liquid in the form of longitudinal waves, accompanied by high-frequency changes in the density of sound waves.In case that the tensile stress is generated, a group of vacuum nuclear bubbles (cavitation effect) will be formed in the liquid [29].These bubbles break under pressure, thus producing a strong impact to accelerate the etching at the crystal grain edge and promote the penetration of the HF acid solution.Due to the enhanced penetration, the Y-TZP ceramic particles below the surface grains contact with the HF acid solution, leading to the formation of deeper cracks along the crystal grain edge.Given the possible instability of crystals under some conditions, however, grain pull-out may serve as the HF acid etching mechanism of Y-TZP ceramics under ultrasonic waves [30].The SEM results showed that the shallow Y-TZP ceramics were etched in a time-dependent fashion.Specifically, the longer the etching time was, the more severe the crystal grain dissolution would be.As shallow Y-TZP ceramic grains were etched, the degree of separation between adjacent grains was enhanced and pores were enlarged.In Group CP, ultrasound not only elevated the reaction rate but also cleaned off the surface products.Meanwhile, ceramic dissolution was also accelerated as the etching time was lengthened, and the three influencing factors presented dynamic changes.Therefore, EDS and XPS showed that the content of element F on the surface fluctuated now and then yet within a certain range.A small amount of element F with basically constant content existed on the surface of samples in Group C, which was in line with the conclusion obtained by Lee et al [13].However, in the study of Aifang Han et al no element F was detected by EDX, presumably ascribed to different ultrasonic cleaning time and frequencies [9].
The AFM 3D topography displayed that the surface of zirconium underwent a process from flat to rough and then from rough to flat again (figure 9).In each group, the trend of roughness was basically consistent with its morphological changes (figure 10).In addition, instead of enhancing the shear strength and surface roughness, the extension of acid etching time might reduce the material strength, forming a potential hazard [29].Compared with the other two groups, the protrusions and depressions on the sample surface in Group CP were more regular and uniform.Despite the lower Ra value on the surface, the SBS in Group CP was higher, with smaller reduction, than that in the other two groups after cooling-heating cycles.These findings coincide with our early-stage study results, that is, the SBS is not only influenced by the roughness height and width but also by its frequency and regularity.Moreover, the SBS is consistent with the pore area rather than the pore size [31].Therefore, the size and distribution of pores on the sample surface cannot be fully explained through the simple roughness measurement, so the porosity and total pore area, should be evaluated in future research, not just the roughness [32].In addition, due to the surface adsorption of fluoride ions, the surface energy has decreased by increasing the immersion time, which may also be responsible for the decrease in bond strength after longer etching [20].The shear stress of samples in Group P was higher than that in Group C, remaining higher after cooling-heating cycles, indicating that the octahedrons formed enhanced the surface roughness and improved the zirconium-resin bonding force.Long-term and stable bonding is an important indicator to evaluate the bonding performance of Y-TZP ceramics and plays an important role in reducing the shedding of Y-TZP allceramic restorations in clinics [16].Thermal cycling and water storage are commonly applied methods for artificially aging bonding samples to test the durability of bonding [13].Combined with the hydrolysis effect and thermal stress, thermal cycling can be used to simulate the natural aging process of the bonding interface.In this study, the durability of resin-Y-TZP ceramic bonding was evaluated through 5,000 cooling-heating cycles in accordance with ISO 10477 [33].To sum up, 30 min was considered as a suitable HF acid etching time for Y-TZP ceramics, considering the initial bond strength and bonding durability in this study.In addition, the long-term success of Y-TZP ceramic prosthesis should be achieved through mechanical and chemical bonding [21].Due to the most extensive application and closer clinical relevance of Shear bond strength testing, we used it for bond strength evaluation.However, it has some drawbacks, such as high dispersion and uneven stress at the interface, causing relatively large standard deviations of the data [34].Although we strictly followed the experimental process, it might be difficult to ensure complete consistency in the thickness and area of the adhesive due to the unsatisfactory surface wettability of zirconia.In this in vitro study, it aimed to detect the bond strength between the traditional Bis-GMA-based resin and HF acid-etched Y-TZP ceramics.To observe the micromechanical locking effect after etching, no primer coating containing functional groups was applied to the surface of zirconium before the resin was bonded to the Y-TZP ceramics.Hence, further research with bonding primers and HF acid etching is necessary to improve clinical relevance and SBS.Moreover, the firmness of new octahedron-zirconia bonding remains to be further evaluated.

Conclusion
(1) Spiculate ZrF 4 and octahedral YF 3 are formed on the surface of Y-TZP ceramics after HF acid, which helps increase the zirconium-resin SBS.Ultrasonic vibration can accelerate the etching effect of HF acid on Y-TZP ceramics, but ultrasonic cleaning can remove the spicules and octahedrons , resulting in the reduction of the zirconium-resin SBS.
(2) In this study, 30 min of HF acid etching of Y-TZP ceramics is a 'critical point', and the shear samples reach the highest ceramic-resin SBS.

Figure 1 .
Figure 1.Boxplot of contact angle on zirconia ceramic surface in different surface treatment groups at different time.C: ultrasonic cleaning group; P: rinsing group; CP: ultrasonic reaction + rinsing group.

Figure 2 .
Figure 2. SEM images of different treatment groups at different reaction time.C: ultrasonic cleaning group; P: rinsing group; CP: ultrasonic reaction + rinsing group.

Figure 3 .
Figure 3. SEM image of Group P at 20 min.

Figure 6 .
Figure 6.EDS surface analysis diagram of acid etching groups at 30 min.C: ultrasonic cleaning group; P: rinsing group; CP: ultrasonic reaction + rinsing group.

Figure 7 .
Figure 7. Time line chart of element content obtained through EDS surface scanning.C: ultrasonic cleaning group; P: rinsing group; CP: ultrasonic reaction + rinsing group.

Figure 11 .
Figure 11.Influence of cooling-heating cycles on bond strength.(a) Shear strength analysis before cooling-heating cycles; (b) Shear strength analysis after cooling-heating cycles.C: ultrasonic cleaning group; P: rinsing group; CP: ultrasonic reaction + rinsing group.

Table 1 .
Materials used in the current study (source: manufacturers).