## 4 Discussion  
In this experimental study, several mechanical and tribological parameters of Activa(tm) Bio-active Restorative(tm), Filek Supreme(tm) XTE and Ketac(tm) Fil Plus Aplicap(tm) were tested. The oral temperature changes were simulated by undertaking thermocycling for half of the samples included in each study group. Thermocycling results in surface stresses and interface microcracks due to high thermal gradients, so it can assess material stability and filler/matrix interaction [19]. The variability of the composition of restorative materials influences its physical parameters, such as flexural strength, fracture toughness, Vickers hardness, elastic modulus, and curing depth, among others [20]. The diffusion of water molecules that occur in the polymer matrix when dental restorations are in contact with saliva can compromise their physical and mechanical properties [12]. Ketac(tm) exhibited the highest microhardness and a significant decrease of its values after thermocycling was found, which makes it an unstable material. Although Filtek(tm) XTE and Activa(tm) were associated with lower microhardness values before thermocycling, no significant differences were found before and after thermocycling. As such, this material may be considered the most stable in terms of hardness. These results are in accordance with the investigation of [PERSON] et al. [4], who also evaluated the microhardness of Activa(tm) and Ketac(tm) before and after thermocycling. The arithmetic average of the absolute heights (R\({}_{\text{a}}\)) and the mean square root of the height of profiles (R\({}_{\text{q}}\)) were selected since these roughness measurements are widely used and allow the comparison of sample roughness concerning their esthetic aspects [21,22]. Ketac(tm) was the only material that presented statistically significant differences in R\({}_{\text{a}}\) after  
Figure 16: SEM micrograph showing part of the wear track made by scratch tribotesting, for Ketac(tm) after thermocycling. thermocycling, which is in accordance with the results of previous studies [23; 24]. This increase in surface roughness, observed in glass-ionomer specimens, can determine the increase in bacterial adhesion and infiltration, allowing fast microbial colonization [23; 24]. R\({}_{\rm z}\), the average distance between the highest peak and lowest valley in each sampling length, enables an idea of the profile's total height [21; 22]. The differences found between Filtek(tm) and Activa(tm), regarding this parameter, could be due to the fact that nanohybrid composites have nanofillers and micrometer particles (0.4-5 um) in their composition [25]. The only parameter that did not vary significantly with thermocycling was the skewness (R\({}_{\rm sk}\)), which is related to the surface load capability. A negative value of R\({}_{\rm sk}\) indicates that there are valleys in the surface [21; 22]. Therefore, the general observed trend of an increase in the R\({}_{\rm sk}\) values was correlated with a decrease in the superficial pores by swelling induced by the thermocycling. There was a decrease in all groups, except for Filtek Supreme(tm), but the differences were not statistically significant. This fact was corroborated by [PERSON] et al. who reported a decrease in R\({}_{\rm sk}\) values of a nanohybrid composite after thermocycling [12]. According to [PERSON] et al. [26], a negative R\({}_{\rm sk}\) shows the positive load-resistance ability of the surface of dental restorative materials, which would have a relatively stable wear rate during function. Regarding the 2D roughness parameters, it is difficult to draw precise conclusions as the final surface was identical for all specimens and that this difference is due to the type of microstructure that makes up the different specimens. Wear resistance of a dental material is a prerequisite to its acceptance by both dentists and patients. A high level of resistance may contribute to its durable esthetics and longevity.