Zirconia is highly popular in clinical use for dental restorations, due to its exceptional flexural strength, chemical resistance, and good aesthetics [1
]. Depending on whether a glass-matrix phase is present or absent, or whether the material contains an organic matrix highly filled with ceramic particles, all-ceramic materials can be classified into three families: (1) glass-matrix ceramics, (2) polycrystalline ceramics, and (3) resin-matrix ceramics [2
]. However, unlike other dental ceramic materials, zirconia has a polycrystalline phase composition without a glass phase composition, as well as good chemical stability and acid corrosion resistance. Owing to this, conventional bonding technology suitable for silica-based ceramics is not effective for zirconia. Thus, an important scientific problem in the use of zirconia materials is the effective treatment of zirconia to improve its surface roughness and mechanical properties [3
Contemporary surface treatment technologies used for zirconia ceramics include airborne particle abrasion (APA), acid etching, laser etching, silicon coating, pre-treatment agents, and other technologies and combinations thereof. APA with 50 µm Al2
under 0.2 MPa pressure at a distance of 10 mm from the zirconia surface has been found to be an effective method for improving bond strength [4
]. However, this approach may lead to sub-surface damage to the zirconia, resulting in microcracks and debris, which may reduce the mechanical properties of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP), thus affecting its longevity. Studies have shown that APA may cause fractures in zirconia restorations and influence the long-term durability of the bond strength between zirconia ceramics and resin [5
]. The reduced retention rate after APA may be related to microcracks in the zirconia, and APA may, therefore, have an adverse effect on flexural strength and other long-term mechanical properties of restorations [3
Other surface modification methods are therefore required to replace aluminum oxide APA and to avoid adverse effects on the mechanical properties and long-term durability of zirconia [4
]. Recently, scholars worldwide have trialed advanced technologies, such as thermal acid etching solutions [7
], plasma technology [6
], tribochemical silica coating [10
], ultrashort-pulse lasers [12
], and fusion sputtering technology [14
], to treat zirconia surfaces.
Thermal chemical etching solutions have been employed for pre-treating zirconia ceramics. It has been found that this method improves roughness and, therefore, increases the zirconia–resin cement bond strength [7
]; however, the configuration of a safe and effective thermal acid etching solution remains unclear, as thermal acid etching may also affect the physical properties of zirconia [8
The application of plasma technology for the surface modification of zirconia ceramics has also been studied. Fernandes et al. [6
] reported that non-thermal plasma modification without significant damage promoted adequate adhesion, but the bond strength was not found to be significantly different from that under aluminum oxide APA. Plasma modification resulted in a significant increase in the surface free energy of the zirconia ceramic, but no significant changes in surface roughness were observed. The application of plasma treatment in zirconia bonding, therefore, cannot replace APA [9
Tribochemical silica coating is a commonly used silicon coating technology at present, which uses 30 μm alumina particles covered with silica for sandblasting at 0.23 MPa on the zirconia surface [10
]. By increasing the silicon content on the surface of the zirconia, the silica layer can react with the cement-containing silane [11
]. As a result, through copolymerization between the silane and the resin cement, the bond strength between the zirconia resin can be improved.
In addition, it has been reported that application of the fusion sputtering technique promotes a rough surface and significantly enhances the zirconia–resin microshear bond strength [14
]. However, the influence of fusion sputtering technology on the crystal phase change and mechanical properties of zirconia, as well as the long-term bond strength remains unclear.
Holthaus et al. [16
] found that the application of laser treatment could potentially replace traditional surface treatment by APA, due to the high speed and precise control of the laser. A more regular micro-texture and a reduction in contamination were obtained through the application of a femtosecond laser (FSL), compared with CO2 and Nd-YAG lasers, for zirconia surface micromachining [12
]. Ruja et al. [13
] evaluated the use of an ultrashort-pulse laser to irradiate the zirconia ceramic surface, so as to improve adhesive properties in the resin–zirconia interface. The results showed that the topography of the zirconia ceramic surface was regularly roughened and wettability was increased, while an improvement in microtensile bond strength was promoted without a significant tetragonal–monoclinic phase transformation [13
]. This may be attributed to the fact that the energy of the laser was absorbed by the surface of the zirconia, and the thermal induction process produced shell-like ruptures on the surface [17
Through the use of FSLs, the surfaces of zirconia ceramics could be effectively modified without inducing thermal or mechanical damage [17
]. While limited research has been conducted on the surface pre-treatment of zirconia ceramics using laser technology, the ideal effect of zirconia modification by laser, in order to improve the bond strength, has not yet been achieved [19
]. Therefore, the changes in surface morphology and flexural strength upon applying the FSL technique require further exploration.
This study aims to evaluate the effect of zirconia surface modification using the FSL method. The null hypothesis was that the application of the FSL would not affect the surface morphology and flexural strength of translucent monolithic zirconia ceramic material.
The purpose of this study was to evaluate the effect of femtosecond laser treatment on the surface roughness and flexural strength of translucent zirconia. Compared with the no treatment group, the surface roughness of translucent zirconia was significantly increased in the femtosecond laser group, while the flexural strength measured via three-point bending of zirconia modified by the femtosecond laser was found not to be significantly different from that of samples in the no treatment group. Thus, the null hypothesis of the study was partly rejected.
The bending strength and fatigue resistance of translucent zirconia are higher than those of glass ceramics [22
]. Thus, translucent zirconia is a material that offers both mechanical strength and aesthetic performance, and its clinical applications are becoming more and more extensive [22
]. However, translucent zirconia may be adversely affected by the use of high-pressure airborne abrasion, as microcracks and defects are generated, and flexural strength is reduced [23
]. To date, limited studies have evaluated the effect of FSL modification on Y-TZP surfaces and, to the best of our knowledge, no existing study has evaluated the effect of FSL for treating the surface of translucent zirconia.
Studies have reported that the application of FSL forms groove- or pit-like structures on the surface of zirconia specimens, thereby increasing their surface roughness; however, this was accompanied by a reduction in flexural strength [24
]. This approach may lead to sub-surface damage to the zirconia due to 50 μm alumina blasting, which may cause microcracks that limit the life of zirconia restorations [25
]. In this study, we explored a specific parameter of FSL in the surface modification of translucent zirconia, in order to improve surface roughness without producing significant grooves or pits. Even though no statistical significance was found, the results in this study revealed that surface roughness was improved. As studies incorporating the FSL method continue to improve, FSL may become more suitable as a surface treatment for zirconia materials.
Although the use of APA with Al2
particles is popular for the treatment of zirconia surfaces in order to increase the bond strength, studies have shown that the increased roughness imparted by this technique is accompanied by an increased fracture risk, thereby weakening the structure through the introduction of microcracks [26
]. The results of our study demonstrated that FSL-treated translucent zirconia showed significantly higher mean Ra
values than those in the APA and NT groups, and there was no significant difference in the Ra
value between the APA and NT groups. These findings are consistent with those of the study of Inokoshi et al. [29
], where the surface roughness of highly translucent Y-PSZ modified using Al2
APA was not significantly enhanced, except for that of specimens comprising KATANA UTML (Kuraray Noritake, Japan). In contrast, FSL ablation of zirconia ceramic significantly enhanced surface roughness and improved the zirconia ceramic–resin bond strength, due to the presence of groove-like structures [30
]. In our study, the translucent zirconia surface obtained following the FSL treatment presented uniform irregular structures without groove-like structures or pits, as confirmed in the SEM image shown in Figure 2
e. Surface topography can be modified by femtosecond laser surface treatment and surface roughness of the zirconia can be increased; thus, the bond strength can be improved [31
The main height parameters for evaluating surface roughness are average roughness (Ra
), the root mean square of the height of each point of the contour (Rq), and the ten-point height of microscopic unevenness (Rz
). The Ra value can represent the arithmetic mean deviation of the surface roughness profile amplitude parameter. Consequently, the Ra
value was used in this study to evaluate the surface roughness [32
]. Compared with the control group, roughness in the APA group did not increase significantly, which may have been due to the particular cutting texture produced during the processing of the untreated translucent zirconia specimens, which yielded a certain roughness after sintering. The surfaces of the specimens were not polished in this experiment, in order to maintain the original surface morphologies of the final sintered translucent zirconia. On one hand, this allowed for simulation of the surface of a final sintered zirconia crown without polishing; on the other hand, when the surface of a zirconia specimen is highly polished, FSL irradiates a smooth surface, producing reflections, which may affect the treatment of the zirconia surface. In this study, the surfaces of the zirconia specimens were not polished, allowing the FSL to fully exert its plasma effect at a lower energy density. This may also contribute to the discrepancy in the roughness results between APA and FSL treatments reported in this study when compared with those in other studies. Various surface analysis systems, such as SEM and Atomic Force Microscopy (AFM), are useful for qualitative analysis, but three-dimensional microscopy was used for quantitative evaluation of the surface roughness variation.
The results of XRD showed that the monoclinic content of the APA group was increased, which indicated that aluminum airborne abrasion may lead to t–m phase transformation. At present, there is controversy about the influence of sandblasting on mechanical properties [33
]. Some scholars have reported that t–m phase transformation can increase volume and produce protective residual compressive stress, thus preventing the further expansion of microcracks and, consequently, leading to enhanced mechanical strength. This is called the phase transformation toughening mechanism [34
]. However, other scholars have indicated that small grain size (within 200 nm) can have a negative impact on the phase transformation and toughening mechanism, consequently reducing the mechanical strength of translucent ZrO2
]. In this experiment, the XRD results indicated that the sintered translucent zirconia specimen appeared to be amorphized after the FSL treatment, which may have been caused by the phenomenon of “avalanche ionization” during interaction of the high-power and high-repetition FSL with the zirconia specimen surface, resulting in high-speed motion [36
]. The hypothesis is that the plasma carries a certain element, which is deposited on the surface of the specimen to form a coating, resulting in amorphization [37
]. It is also possible to form salt crystals on the surface of the ceramic; however, the interfacial topography between the “amorphized” zirconia layer after FSL and the substrate was not investigated, and further experimental verification is required.
Air particle abrasion may lead to sub-surface damage of the zirconia surface by 50 μm alumina, which may lead to microcracks that limit the life of zirconia restorations [38
]. It has been reported that impact-induced defects were observed on zirconia surfaces modified by APA treatment; thus, the longevity of APA-treated zirconia ceramic prostheses may be shortened [39
]. Consistent with previous studies, in our study, the surface treated with APA presented a number of microcracks and defects as revealed in Figure 2
d, which may lead to a reduction in the flexural strength of the zirconia specimens. In contrast, in a study reported by Wang et al. [40
], APA enhanced the flexural strength of zirconia, regardless of the particle size, air pressure, or blasting time. Song et al. [41
] reported that flexural strength was significantly higher in the group of air-abraded zirconia specimens than that in the group without any treatment. The content of the monoclinic phase of the lower zirconia surface determines the mechanical behavior of the zirconia specimens, as this is where tensile stress is dispersed. Furthermore, this study indicated that APA of the inner surface of zirconia specimens, in order to improve their bonding performance, might also enhance their fracture strength [40
]. In this experiment, we applied 50 µm Al2
particles to treat the surface of the translucent zirconia specimen, in order to reduce tetragonal–monoclinic phase transformation.
There are two main methods for testing the bending strength of ceramics: uniaxial bending and biaxial bending. In uniaxial bending tests, a cuboid specimen is supported by two points and loaded vertically at one point (i.e., a three-point bending test) or two points (i.e., a four-point bending test). In a biaxial bending test, a thin disk is supported by a ring or three balls close to it, and a load is applied through a ball or a piston in its central area, or a smaller ring in its center. The above methods have been recognized in international standards [20
]. Therefore, a three-point bending test was used to evaluate the used methods after surface treatment in this study. New possible methods for the mechanical analysis of materials have been reported as a future perspective for classic dynamometer systems, such as Dynamic Mechanical Analysis (DMA) and Brillouin’s micro-spectroscopy [42
Importantly, the flexural strength results presented herein were consistent with the observed SEM images. However, further investigations are required to evaluate the long-term stability of zirconia treated using different methods. It should also be noted that, in the experimental setup, line-patterning of zirconia surfaces was achieved; thus, tuning the FSL parameters should allow for independent variation of the pattern depth, overall roughness, and surface finish. More specifically, increasing both the fluence and the number of pulses will allow for deeper patterning, with the maximum achievable depth being 1 µm. However, increasing the number of pulses can have a detrimental effect on the quality of the lines produced, and surface damage can occur (e.g., intergranular cracking, open porosity, and nanodroplet formation), depending on the FSL parameters employed [43
]. In future experiments, our research team will try to design an integrated processing device featuring a femtosecond laser. After the zirconia restorations are sintered, the dental technician can hold the working end of the device to modify the tissue surface of dental zirconia restorations, in order to achieve a clean, efficient, and damage-free effect.
One limitation of this study is that the long-term effects of FSL modification technology on the flexural strength of zirconia were not investigated. Therefore, an evaluation of resin–zirconia bond strength and durability using different surface modification methods will be reported in a future article.