The dental restoration matrix consisted of 70 wt.% Bis-GMA and 30 wt.% viscosity-modifying TEGDMA obtained from BOC sciences, USA. The photoini- tiator camphorquinone (CQ) (0.2 wt.%) and the amine reducing agent ethyl 4-dimethylaminoben-zoate (EDAB) (0.8 wt.%) were added to help trigger the photo-polymerization process when exposed to radiation. The silanized nano- SiO₂ particles were then incorporated into the matrix with an average diameter of 20 nm and varying SiO₂ contents: 0 wt.%, 2 wt.%, 4 wt.%, 6 wt.%, 8 wt.%, and 10 wt.%. All the samples were stirred for about 3 minutes using the high speed centrifuge to ensure the spherical nanoparticles were dispersed homogenously in the composite matrix. After that, they were irradiated for 180 s under the visible blue light with an irradiation flux of 1000 mV/cm³. According to the content of the nanoparticle filler, the samples were designated as S0, S2, S4, S6, S8, and S10, respectively.

The mechanical properties of the dental resin composites were investigated by micro/nano-scale indentation tests. All the tests were performed using the commercially available Triboindenter system (Hysitron USA T-950) with a diamond equilateral triangular indenter (Berkovich). This instrument provides accurate automated testing for the mechanical properties at micro/nano-scale and high resolution real-time imaging of surfaces. The load resolutionis 1 nN and displacement resolution is 0.0002 nm.

The indentation tests were carried out in quasi-static force-control mode, in which the applied force is controlled in a pre-programmed manner and the displacement continuously monitored. The loading function of all the indentations in this work consisted of a 5^−s linear loading and 5^−s unloading segment together with a 10^−s holding at the peak load of 8 mN. This kind of loading could reduce the influence of the creeping effect [15] and its schematics is presented in Figure 1. The hardness and modulus could be obtained according to the load-dis- placement curve based on the Oliver and Pharr’s method [14]. The final values of hardness and modulus data were calculated as the average of 6 distinct regions on each sample surface. Each region was again marked by and calculated as a mean of 12 spots with a spacing of 40 μm either in parallel or perpendicular direction to eliminate mutual influences.

Figure 2 shows the typical load-displacement curves of the S0, S2, S8, and S10, from which we can clearly see that the hmax (maximum indentation depth) decreases gradually with the increasing filler content. At the peak load, the hmax of S0 is about 300 nm more than that of S10, which implies that the dental resins reinforced with nano-silica particles exhibits much larger hardness than the free

one. However, the loading and unloading segments of the image possess steeper slopes with the increasing filler content. During the unloading process, the samples underwent elastic recovery and presented a decreasing hir (irreversible indentation depth) as the concentration of the nanoparticles increases. Especially for each increment (2 wt.%) in the filler content, hirhas the largest decrease of 71 nm from S0 to S2, which shows that the nano-SiO₂ particles can greatly improve the mechanical properties of dental resin composites even at low filler contents.

Figure 3 shows the results of hardness H and modulus E for different filler content. It can be seen that both H and E increase monotonously with the nanoparticle content. Hardness and modulusrise from 0.126 and 3.15 GPa of S0 to 0.208 and 4.09 GPa of S10, with an increment of 65% and 30%, respectively. The apparent increase in H and E is mainly due to two reasons. Firstly, the nano-SiO₂ particles have much higher H and E, and the resulting mixing effect of the dental resin matrix and the particles can lead to improvement of H and E of dental resin/SiO₂ composites. Secondly, the incorporation of nanoparticles can enhance the load-carrying capacity and inhibit the formation of internal micro-cracks based on the strong particle-matrix bonding, thereby improving the mechanical properties [3].

The plasticity index, which describes the relative plastic/elastic behavior of the material effectively when it undergoes external stresses and strains. The results of the plasticity index φ as a function of the filler particle content is shown in Figure 4(a). Figure 4(b) shows the schematics of the calculation of plasticity index and it is defined as [16]

φ = W i r / ( W i r + W r ) (1)

where W i r and W r represent the irreversible work done during indentation and the reversible work recovered by viscoelastic processes during unloading,

respectively. From Figure 4(a), we can find that the plasticity index decrease initiallywith the nano-SiO₂ particle content from 0 wt.% to 4 wt.%, and then increases afterward when the filler mass ratio increases from 4 wt.% to 10 wt.%. A larger plasticity index meansmore plastic deformation proportion, therefore the dental resin composite with 4 wt.% nano-SiO₂ particles exhibits the best elastic recovery in the indentation process.

The Fourier transform infrared spectroscopy (FTIR) profiles of the samples S0, S2 and S6 are shown in Figure 5. Through the spectrogram, we can see that the characteristicabsorption peak for the carbonyl C=O is located at the wavenumber of 1726 cm⁻¹, and those for the aliphatic C=C and aromatic C=C

are at around 1638 cm⁻¹ and 1582 cm⁻¹, and the stretch vibration absorption for the esteryl C-O is found at around 1182 cm⁻¹, 1249 cm⁻¹ and 1297 cm⁻¹, respectively. It is also shown that the transmittance of the S6 sample is much lower than S0 at the whole frequency range, indicating that the dental resin composite with nano-SiO₂ particles possesses a higher crystallinity than the standalone one. More importantly, the crystallinity increases with the filler content. For polymer-based composites, the incorporated nanoparticles play roles as the nucleating agents during crystallization. With the nuclei, compact and fine spherulite particles can be formed in the polymeric matrix, and the hybrid composite exhibits microcrystalline structure, causing increased crystal- linity [17]. For the dental resin composites, the nano-SiO₂ particles are used as the crystal nuclei and have a positive effect on the crystallization process of the resin matrix. This leads to improved filler-matrix coupling and thus contributes to increased mechanical performance, i.e., the hardness and modulus, of the resin/nano-SiO₂ composites as the filler content increases.