*2.1. ZrO<sup>2</sup> Nanoparticles Surface Treatment*

To enhance the ZrO<sup>2</sup> nanoparticle's wettability with the resin matrix through the formation of a reactive group on their surface, the salinization process was carried out in the laboratory of Inorganic Chemistry, Chemistry Department, Faculty of Science, Suez Canal University. The ZrO<sup>2</sup> nanoparticle powder of 99.9% purity, with 9 <sup>±</sup> 2 m2/g surface area and an average particle size of 40 ± 3 nm, (Nanogate, Cairo, Egypt) was saline-treated using the saline coupling agent 3-trimethoxysilyl propylmethacrylate (TMSPM) (Sigma-Aldrich, Berlin, Germany) [27]. This procedure was accomplished by mixing 30 g of ZrO<sup>2</sup> nanoparticles with 0.3 g of TMSPM dissolved in 100 mL of acetone for 1 h with the help of a magnetic stirrer at 300 rpm (Hot plate with a magnetic stirrer, MSH-A 30A, Seoul, Korea) for 60 min. Subsequently, the solvent was removed using a rotary evaporator under vacuum for 30 min at 60 ◦C and 150 rpm, followed by heating for 2 h at 120 ◦C. Next, the treated powder was allowed to cool to 28 ◦C [28–30].
