## 4 Discussion
Consequently, the R\({}_{\text{a}}\) 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 ZrO\({}_{2}\)[35]. 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 \(\upmu\)m 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 2d, which may lead to a reduction in the flexural strength of the zirconia specimens. In contrast, in a study reported by [PERSON] et al. [40], APA enhanced the flexural strength of zirconia, regardless of the particle size, air pressure, or blasting time. [PERSON] 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.