## 4 Discussion
### Microhardness  
Microhardness is dental materials' most significant mechanical property, mainly when used in vital pulpal treatment. It is the principal indicator that denotes resistance against deformation and dramatically influences the elasticity or strength of a material [22]. The surface microhardness of Biodentine is equal to that of natural dentine [23]. As per [PERSON] et al., the hardness of the bodentine was around 130 HV and then decreased to 90 HV after etching with 35% phosphoric acid for 1 min, considered the highest value compared with other materials in the study [9]. Similarly, [PERSON] et al., in another study, reported a hardness value of 48.4 HV, which was, again, the highest value compared with materials such as bio-aggregate and IRM when immersed in HBSS for 28 days [6]. As per [PERSON] et al., the hardness of biodentine was 62.35 HV, which was double that of Pro Root MTA, and [PERSON] et al. stated that the microhardness of biodentine is 45.4 HV. As mentioned above, differences in values are mainly due to varying protocols followed in the respective studies [24; 25]. A reduced water powder ratio and the formation of a tag-like structure are the principal reasons that biodentine is a structurally more resistant material against forces, permitting it to be used broadly [22; 26].  
In our study, the hardness of the control group, i.e., BiodentineTM, was 32.39 HV when following the aforementioned methodology. Adding nano zirconia in different percentages to elevate the properties of biodentine was the new approach. Nano zirconia, the 'nano material' with enhanced physio-mechanical properties, was added to PMMA to increase the repair strength of the denture base. Different studies have shown the increased transverse strength, flexural strength and microhardness of nano-zirconia-re-enforced groups compared with un-reinforced groups [16; 17; 27]. The incorporation of nano-silica into MTA in different percentages leads to a reduction in setting time and increases compressive and flexural strength accordingly [28]. Hence, it is clear from the details mentioned above that the addition of nano materials significantly enhances the material's properties.  
In the present study, when nano zirconia was added to different groups, group 3 with 20% nano zirconia showed favorable results with increased microhardness compared to the control group, as mentioned (Tables 3 and 4). In contrast, the values were reduced in group 4. Major factors determining the outcome are the size of the used nanoparticles, the percentage of nanofillers added and the type of material into which nanofillers are incorporated [12]. Nano zirconia can form strong bonds between the fillers and material, which in turn establishes strong molecular bonds that permit increases in desirable material properties [15]. The homogenous distribution of filler particles that fill the gap and that establish inter-atomic solid bonds is the utmost important factor for the desired outcome [16; 17; 29; 30]. A decrease in or an inappropriate percentage of nanofillers does not make any statistical difference. An increased percentage of nanofillers results in poor adhesion and agglomeration. There can be the formation of multiple voids or the complete saturation of nano zirconia with the material [27; 29]. When a higher content of filler is added, it leads to the formation of defects and weakening of the material. When the matrix reaches a saturation point, it cannot accept more filler, and if fillers are added to it, this results in the interruption of the continuity of the matrix and thus results in decreased strength [31]. Henceforth, when the percentage of nanofillers increases or decreases to a certain level, it can cause structural defects in the material and adversely affect its properties. Therefore, in this study, the 20% addition of nano zirconia to biodentine is considered the ideal ratio for enhancing microhardness and obtaining a statistically significant increase in the values compared with the control group.