# Mechanical, Rheological, and Tribological Behavior of an In Situ Silica-Reinforced Experimental Dental Composite: A Comparative Study

Rayenne Latoui, Djallel Bouzid, Youcef Mouadji, Damien Montarnal, Olivier Boyron

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Rayenne Latoui, Djallel Bouzid, Youcef Mouadji, Damien Montarnal, Olivier Boyron. Mechanical, Rheological, and Tribological Behavior of an In Situ Silica-Reinforced Experimental Dental Composite: A Comparative Study. Journal of Applied Polymer Science, 2025, 142 (46), pp.e57769. 10.1002/app.57769. hal-05448772

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# Mechanical, Rheological and Tribological Behavior of an In Situ Silica-Reinforced Experimental Dental Composite: A Comparative Study

Rayenne Latoui¹, Djallel Bouzid¹*, Youcef Mouadji², Damien Montarnal³, Olivier Boyron³*

¹LGPDDPS, Ecole Nationale Polytechnique de Constantine, 25000 Constantine, Algérie.

²LMSE, Ecole Nationale Polytechnique de Constantine, 25000 Constantine, Algérie.

³Univ Lyon, CPE Lyon, CNRS, UMR 5128, CP2M, Equipe PCM, 69100 Villeurbanne, France.

Email*: olivier.boyron@univ-lyon1.fr

<figure><img src="image_2.png" /></figure>

## Abstract

In restorative dentistry, dental composites are extensively employed owing to their advantageous mechanical and aesthetic features. However, achieving an optimal balance between handling properties and long-term durability remains a challenge. This study compares the rheological and tribological properties of an experimental dental composite reinforced with in situ silica and a commercial composite prepared by conventional methods. Rheological tests, including viscosity and thixotropy measurements, were conducted to evaluate the handling behavior before polymerization. Tribological tests, such as wear rate and coefficient of friction (COF), were performed to assess the durability of the materials after polymerization. The results showed that the experimental composite exhibited higher viscosity and improved thixotropic behavior, facilitating clinical application.

Keywords: dental composite, in-situ sol-gel process, rheological behavior, nanoindentation, tribology.
# 1. Introduction

The combination of attractive aesthetics and reliable mechanical performance has led to the broad use of dental composites in restorative dentistry, making them essential materials for a wide range of clinical applications.<sup>1,2</sup> These materials must not only mimic the natural appearance of teeth but also exhibit strong mechanical, rheological, and tribological properties to withstand the complex functional stresses encountered in the oral environment.<sup>3-8</sup>

Among the key properties influencing the performance of dental composites, compressive and flexural strength, hardness, and wear resistance are of paramount importance, as they directly impact the longevity and effectiveness of restorations.<sup>9-14</sup> Consequently, optimizing the formulation of dental composites to enhance their mechanical performance is a significant area of research.

In this study, we investigate the properties of an experimental composite formulated via an in-situ sol-gel process and compare them with those of a commercially available composite. The in-situ sol-gel method offers a promising alternative for synthesizing dental composites with enhanced properties by allowing the controlled generation of silica fillers within the resin matrix. The influence of varying silica content (30%, 40%, and 50% by weight) on the composite's performance is assessed, given that filler content plays a crucial role in determining material characteristics.<sup>15,16</sup>

To comprehensively evaluate the impact of silica content and the in-situ sol-gel process, we examine several key properties, including rheological behavior, hardness, elastic modulus, flexural and compressive strength, and wear resistance. By analyzing these parameters, we aim to determine whether the in-situ sol-gel method can produce dental composites with mechanical and functional properties comparable to or superior to those of commercial alternatives.

The successful validation of this approach could contribute to advancements in dental restorative materials, offering improved mechanical properties, enhanced durability, and better clinical performance. Furthermore, understanding the relationship between silica content and composite behavior will provide valuable insights into the design of next-generation dental composites with optimized properties for long-term clinical success.
## 2. Materials and methods

### 2.1. Materials

Two composites were considered for this study: an experimental composite, denoted by Composite A, and a commercial composite, denoted by Composite B. A detailed description of the two composites is provided in Table 1.

<table><caption>Table 1: Composition and description of experimental and commercial dental composites.</caption><thead><tr><td>Sample</td><td>Filler type</td><td>Amount</td><td>Filler size</td><td>Elaboration process</td></tr></thead><tbody><tr><td>Composite A-30</td><td></td><td>30%</td><td></td><td></td></tr><tr><td>Composite A-40</td><td>Silica</td><td>40%</td><td>100-200nm</td><td>In-situ sol gel process</td></tr><tr><td>Composite A-50</td><td></td><td>50%</td><td></td><td></td></tr><tr><td>Composite B<sup>a)</sup></td><td>Silica</td><td>75%</td><td>0.05-0.4µm</td><td>Ex-situ process (blending)</td></tr></tbody></table>

The formulation of Composite A, synthesized using the sol-gel in-situ process, was discussed in supporting information,¹⁷ with the summarized mechanism presented in Figure 1. This composite includes silica fillers at varying weight percentages of 30%, 40%, and 50%, designated as composite A-30, composite A-40, and composite A-50, respectively.
<figure><img src="image_3.png" /><figcaption>**Figure 1:** Scheme of the formation mechanism of silica nanoparticles in the resin matrix (composite A).</figcaption></figure>

Composite B, selected as the reference material for this study, is a commercially available dental composite (NT Premium Enamel, Coltene, Brazil). Its formulation includes 75% inorganic fillers, a resin matrix composed of Bis-GMA, Bis-EMA, and TEGDMA, as well as stabilizing agents such as BHT, photoinitiators, and pigments. It was chosen for its compositional similarity to the experimental composites, particularly regarding the resin system and the incorporation of silica nanoparticles. Furthermore, its conventional ex-situ filler incorporation process serves as a valuable reference point for evaluating the potential improvements offered by the in-situ sol-gel synthesis method. The SEM image is represented in Figure 2
<figure><img src="image_4.png" /><figcaption>**Figure 2:** SEM image of composite B.</figcaption></figure>

## 2.2. Experimental techniques

### 2.2.1. Rheological measurements

Rheology tests assess how easily a composite flows and shapes during placement in a cavity. It simulates the handling properties of the material, ensuring it can be easily manipulated and shaped in the mouth without compromising its final performance.

Rheology tests were carried out on a HAAKE MARS 60 apparatus from ThermoScientific instrument using a Peltier oven and a parallel (or planar) plate geometry of 20 mm diameter (Figure 1-SI). The composite specimens were positioned on the rheometer's lower plate, after which the upper plate was adjusted to a 411 µm gap under a constant normal force of 4 N, corresponding to the minimum gap compatible with all materials. Residual composite and peripheral air entrapments were removed prior to data acquisition.

Strain scanning was carried out by subjecting the materials to various mechanical stresses ranging from low (0.01%) to high (10%).

Oscillatory shear tests were also carried out at different frequencies (0.05Hz-20Hz) at a fixed shear strain of 0.1%. Shear and recovery (elastic recovery) tests were carried out with 10 % strain for 120 s and then 0.1% strain for 1200 s.

Material viscosity was measured as a function of strain rate (shear rate). The tests were carried out at two temperatures, 25°C (ambient temperature) and 37°C (human body temperature).
### 2.2.2. Hardness and elastic modulus

Nanoindentation measures the hardness and elasticity of a composite by applying a force to its surface. This simulates the material's response to small-scale impacts and wear during everyday use in the mouth, such as the pressure from chewing or abrasion from food.

Nanoindentation tests were performed using a Hit 300 (ANTON PAAR) nanoindenter equipped with a standard Vickers indenter tip (Figure 2-S1). Composite materials were introduced into a polyamide mold (5 mm × 2 mm) and exposed to LED light for 40 seconds at ambient temperature to initiate polymerization. The cured specimens were carefully polished under water with silicon carbide paper (up to 1200 grit) to ensure a clean indentation.

The tests were executed at room temperature following calibration of the tip area function on a fused silica reference sample. Indents were made under load control to a maximum load of 300mN held for 10 seconds, with 1800 mN min⁻¹ loading and unloading segments. A spacing of 100 µm between indent centers was maintained to prevent interaction between neighboring indents. At each location, 5 repeat indents were performed. The continuous stiffness measurement (CSM) technique was used to allow the determination of elastic modulus throughout the loading segment. Using the Oliver-Pharr approach, hardness was calculated based on the recorded load-displacement data.¹⁸

### 2.2.3. Compressive properties

This test simulates the forces exerted during biting and chewing. The composite material is subjected to compressive stress to measure its ability to resist deformation or fracture under pressure, mimicking the forces that occur when teeth come together during normal mastication.

To measure compressive strength (σc), a polyamide cast with a height of 6 mm and a diameter of 7 mm was charged with the samples. After filling the cast, the upper and base faces were overlapped with glass slides to obtain a flat face, then irradiated for 40 s. Light-curing was carried out through the side surface also after exposure due to the shallow depth of light penetration. The cured sample was demolded and aged for 24 hours at 37°C. After aging, compression tests were carried out using a testing device (LLOYD EZ 20, AMETKAN Company)
at a rate of 0.5 mm min⁻¹ (Figure 3-SI). The σc (in MPa) was obtained using the following formula.¹⁹

$$ \sigma c = \frac{F}{A} \qquad (1) $$

where F is the compressive force (N) and A is the cross-section area of the specimen.

### 2.2.4. Flexural properties

The flexion test simulates bending forces that occur when a material is subjected to pressure, typically from chewing or biting. The composite is tested for its resistance to bending and breaking, which helps evaluate how well it would perform in extended or unsupported fillings.

To determine the bending stress σf, bar-shaped samples (n=3) of (40 x 6 x 2 mm) were made from each sample using a silicone mold. To prepare the mold, a strip of PMMA with dimensions 40 x 6 x 2 mm was placed in the silicone. Once the silicone has cured, the strip is removed. After ensuring that the mold had the required dimensions, the samples were placed inside the mold and light-cured using blue LED light (1500 mW cm⁻²) for 40 s, on each third of the top and bottom surfaces of the sample. All samples were demolded. Specimen dimensions were checked using a caliper. They were immersed in distilled water and kept at 37°C for 24 hours. σf was measured using the 3-point bending test, in which bar-shaped samples were placed on a universal testing machine (LLOYD EZ 20, AMETKAN Company) with a steady rate of 0.5 mm min⁻¹ (Figure 4-SI). After each flexural test, the highest breaking load of each samples was registered and the flexural strength (σf) was calculated using formula 2.²⁰

$$ \sigma f = \frac{3P \cdot l}{2w \cdot t^2} \qquad (2) $$

Where:

* P: the bending force applied to the middle of the specimen

* l: distance between the supporting rollers, equal to 30 mm.

* w: width of the sample, equal to 6 mm.

* t: thickness of the sample, equal to 2 mm.

### 2.2.5. Tribological tests
This test simulates the wear and friction between dental materials, such as when a composite comes into contact with opposing teeth during chewing. It helps assess how the material withstands friction and abrasion over time, indicating its long-term durability and resistance to wear.

Tribological tests were conducted using a Tribology Trainer Basic Module TM 260 from Gunt (Figure 5-SI). Tests were conducted using a 'pin-on-disc' setup, with composite samples shaped as discs measuring 30 mm in diameter and 3 mm in thickness (Figure 5-SI).

Samples were cured under a LED lamp with an intensity f 1500 mW cm⁻¹ for 40 seconds.²¹ After polymerization, the samples were polished to ensure a smooth surface.

The counter-specimen material used was a natural tooth, selected to ensure consistency, reproducibility, and clinical relevance. The first tribological tests were carried out in dry conditions in order to study the friction and wear behavior of the materials. Tests were performed under 3 distinct normal loads of 5, 10 and 20 N, at a speed of 0.15 m s⁻¹ with a friction track radius of 12 mm.

The total friction distance for each test was 540 meters. This load range was chosen to simulate the varying contact pressures that dental composites can experience under functional conditions. The normal force (N) is transmitted to the sample support through weights placed on the load arm. A force sensor holds the load arm horizontally and measures the friction force generated at the contact interface, and the coefficient of friction (COF) is typically calculated using the relation:

$$COF = \frac{F_f}{N} \qquad (3)$$

Where:

* Ff is the frictional force, measured during the tribological test.

* N is the normal force, the force applied perpendicular to the contact surface.

Defined as the ratio between the frictional force and the applied force, the coefficient of friction is a unitless quantity.

Following the dry tests, additional tribological tests were carried out in the presence of artificial saliva to assess its effect on COF. For these tests, a single load of 20 N was applied,
corresponding to the highest load used in the dry tests. The choice of 20 N allowed a direct comparison of results between dry and saliva-lubricated conditions in the most demanding loading scenario. A volume of 15 ml of artificial saliva was used for each test to ensure even lubrication. Changes in the friction force, and temperature were recorded throughout the test.

A comparative measure of wear was calculated by considering the total friction distance and the applied loads (5, 10 and 20 N) during the tests. The degree of wear was quantified by measuring the mass loss (m) in the samples before and after the tribological tests. The wear rate (w) was then calculated using the following formula (4): ²²

$$w = \frac{m}{F.d} \qquad (4)$$

Where,

* m: wear mass (g),

* F: loading force (N),

* d: distance of friction (m).

Optical microscopy was used to examine the wear surfaces of the composites before and after the tests. This analysis provided interesting information on superficial damage. The combination of different loads, mediums (dry and saliva), and detailed wear analysis allowed for a comprehensive understanding of the tribological behavior of the tested materials under clinically relevant conditions.

### 2.3. Repeatability and error analysis

In order to ensure the reliability and reproducibility of the experimental results, each test was performed in triplicate. This approach minimizes the effect of potential outliers or experimental errors, thereby increasing the validity of the conclusions drawn from the data. For each experimental condition, the values obtained from three separate repetitions were recorded and averaged to provide a robust representation of the behavior of the material under the specified testing conditions.

To quantify the variation and assess the reliability of the data, error bars were included in the figures representing the experimental results. Error bars were calculated based on the standard deviation (SD) of the three repetitions, providing a visual indication of the spread or
variability of the data. The SD is a measure of the dispersion of the data points from the mean, and the error bars are designed to show the range within which most of the data points lie.

The formula for calculating the standard deviation (SD) for a set of measurements x₁, x₂, x₃, ..., xₙ is as follows:

$$SD = \sqrt{\frac{1}{n-1} \sum_{i=1}^{n} (x_i - \bar{x})^2} \qquad (5)$$

where:

* xᵢ represents each individual data point,

* x⁻ is the average value of the dataset,

* n is the total number of repetitions (in this case, n=3).

# 3. Results and discussion

## 3.1. Rheological Properties (before polymerization)

The flow behavior of the materials before polymerization under shear stress, their viscoelastic response, and their ability to recover after deformation are essential parameters for assessing their practicality in restorative dentistry.²³ The following rheological tests were performed to evaluate these properties for both the experimental composite reinforced with in situ silica and the commercial composite prepared by conventional methods.

### 3.1.1. Dynamic Strain sweep test

The strain sweep test was conducted to investigate the effect of shear strain on the viscoelastic properties and to determine the linear viscoelastic region (LVER) for both composites. This region represents the strain range within which the material exhibits an unchanged structure, maintaining its internal integrity. Strain sweep tests were conducted at a fixed frequency of 1 Hz, with shear strains ranging from 0.01% to 10% (Figure 3).
<figure><img src="image_5.png" /><figcaption>**Figure 3:** Storage modulus and loss modulus (G', G'') versus strain (%) (a), and stress (Pa) as a function of strain (%) (b) for composite A-50 and composite B. (G' reflects the material's elastic, solid-like characteristics, whereas G'' denotes its viscous, liquid-like nature).</figcaption></figure>

Figure 3-a shows that G' and G'' do not depend on strain up to a critical strain value of 0.12%, after which they decrease. Table 2 shows the data for the linearity range of the composite A-50, and the results are further compared with those for the commercial Nt premium composite (Composite B).

<table><caption>**Table 2:** Linearity domain of Composite A-50 and Composite B.</caption><thead><tr><th colspan="4">Composite A-50</th><th colspan="4">Composite B</th></tr><tr><th>γ (%)</th><th>τ(Pa)</th><th>G'(Pa)</th><th>G'' (Pa)</th><th>γ (%)</th><th>τ(Pa)</th><th>G'(Pa)</th><th>G'' (Pa)</th></tr></thead><tbody><tr><td>0.01</td><td>14.3</td><td>142700</td><td>15380</td><td>0.01</td><td>9.6</td><td>92360</td><td>26990</td></tr><tr><td>0.02</td><td>24.3</td><td>148500</td><td>13910</td><td>0.02</td><td>17.2</td><td>101200</td><td>27890</td></tr><tr><td>0.04</td><td>40.8</td><td>151600</td><td>13010</td><td>0.04</td><td>29.3</td><td>105900</td><td>28000</td></tr><tr><td>0.06</td><td>67.4</td><td>152900</td><td>12800</td><td>0.06</td><td>48.7</td><td>107300</td><td>28120</td></tr><tr><td>0.08</td><td>110.7</td><td>153200</td><td>13110</td><td>0.08</td><td>77.2</td><td>103800</td><td>27680</td></tr><tr><td>0.12</td><td>174.6</td><td>151800</td><td>13430</td><td>0.12</td><td>110.5</td><td>100500</td><td>26520</td></tr><tr><td>0.19</td><td>179.7</td><td>149560</td><td>13800</td><td>0.19</td><td>114.5</td><td>998000</td><td>25300</td></tr></tbody></table>

From the Figure 3-b and the table it is possible to deduce the yield stress, which is 114.5 Pa for the commercial composite and 179.7 Pa for the experimental composite.
The variation in yield stress observed between the two materials could be attributed to differences in the distribution and dispersion of fillers within the matrix. A greater percolation network formed by the fillers increases the yield stress, enhancing the material's mechanical resistance. This means a higher force must be applied to initiate flow, reflecting the material's robustness.

Stress (τ) and strain (γ) describe a material's response to external forces. In linear elastic materials, they are proportional, following Hooke's Law:²⁴

$$τ = G' * γ \qquad (6)$$

As observed in Figure 4.8, for small deformations (below 0.1%), the stress-strain ratio remains constant, indicating that G' does not change and the material behaves elastically. However, as deformation increases, the material exits the linear region, and G' begins to decrease. This decrease in the stress-strain ratio reflects the material's progressive softening under higher strains.

### 3.1.2. Dynamic frequency sweep test

The frequency sweep test was conducted in the linear strain region at a fixed strain of 0.1%, varying the frequency from 0.05 Hz to 20 Hz. Figure 4 shows the variation of the elastic (G') and viscous (G'') components as a function of frequency.

<figure><img src="image_6.png" /></figure>
**Figure 4:** Frequency sweep experiment (G', G'' (Pa) vs. frequency (Hz)) of Composite A-50 and composite B in the linear strain region.

Figure 4 showed that G' is greater than G'' and frequency-independent behavior (very little variation with frequency) for both materials, indicating the formation of a percolation network formed by the silica fillers.

It is also observed that composite A exhibited higher G' compared to Composite B across all frequency regions. This could be attributed to the homogeneous and well-dispersed particle system in Composite A. The uniform distribution of silica particles within the resin matrix promotes stronger interactions between the filler and the resin, enhancing the material's stiffness and resistance to deformation.²³

### 3.1.3. Shear and recovery test

The shear recovery test is a crucial method for understanding the viscoelastic behavior of materials, particularly their ability to recover after deformation.

The test begins with oscillations at 10% strain, which is well above the yield point, applied for 120 seconds to impose significant deformation and disrupt the material's internal structure. This step helps evaluate the material's resistance to stress and its structural breakdown under shear. Following this, the strain is reduced to 0.1% (within the Linear Viscoelastic Region, LVER) and monitored for 1200 seconds to assess the recovery of the network across the silica fillers. Figure 5 represents the shear and recovery test for both composites.

<figure><img src="image_7.png" /></figure>
**Figure 5:** Shear (10%) and recovery (0.1%) test for composite A-50 (a) and composite B (b) at 25°C and 37°C.

Figure 5 illustrates the evolution of the storage modulus (G') over time during the shear and recovery phases. In the recovery phase, G' increases as the internal structure gradually rebuilds.²⁶ At 1200 s, G' continues to rise, indicating that a steady state has not yet been reached. The figure also highlights the effect of temperature on G' for both composites, showing a noticeable decrease as the temperature rises from 25°C to 37°C. This decline indicates a reduction in the material's elastic properties, primarily due to the increased mobility of polymer chains at higher temperatures.

The importance of measuring the G' recovery time lies in evaluating material's ability to regain structural integrity after deformation. Following the decreasing strain step change, a clear increase in G' over time was observed, indicating the thixotropic recovery of the composite material.²⁵ A first order model (Equation (4.7), line in figure 5) was used to fit the data (R² vary between 0.996 and 0.999). The fitting parameters are shown in Figure 2.

$$G'(t) = G'0 - Ae^{-t/\tau} \quad (7)$$

Where:

* τ: is the time constant which characterizes the recovery time of the material.

* G'₀: the modulus when the material has returned to its initial state (after recovery).

* A: A is the amplitude of the maximum elastic deformation.

<table><caption>**Table 3:** Recovery behavior of composite A and B.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Composite A</th><th colspan="2">Composite B</th></tr><tr><th>25°C</th><th>37°C</th><th>25°C</th><th>37°C</th></tr></thead><tbody><tr><td>G' before shear</td><td>151800</td><td>136050</td><td>100500</td><td>75962</td></tr><tr><td>G' after recovery</td><td>144980</td><td>127072</td><td>92553</td><td>68581</td></tr><tr><td>Recovery (%)<sup>a)</sup></td><td>95.5</td><td>93.4</td><td>92.1</td><td>90.2</td></tr><tr><td>τ (s)</td><td><strong>30</strong></td><td><strong>48</strong></td><td><strong>40</strong></td><td><strong>69</strong></td></tr></tbody></table>

<footnote>a) Recovery (%) = $\frac{G'_{after recovery}}{G'_{before shear}} \times 100$</footnote>
The table shows that the characteristic recovery times (τ) for both composites, are within a few dozen seconds, with the material reaching more than 90% of its initial state within approximately 2 minutes after manipulation and placement. This indicates that after being positioned in the cavity, the material rapidly regains its solidity, minimizing flow or deformation and ensuring good dimensional stability.

We observe from the table that the experimental composite exhibits higher recovery properties than the commercial one. This suggests that its filler distribution and interactions create a stronger percolation network, which allows stress redistribution and helps the material recover its structure quickly after deformation.

The superior recovery behavior indicates improved thixotropic properties, which are essential for clinical use. The faster recovery time means that the material can regain its elastic structure quickly after manipulation, reducing the risk of slumping or displacement during placement. This property is particularly important in achieving precise and accurate restorations in dental procedures.

### 3.1.4. Steady rate sweep test

The viscosity of both composites was measured across a range of shear rates to assess their shear-thinning behavior (Figure 6). The tests were conducted at two different temperatures, 25°C and 37°C, to simulate room temperature and the oral environment.

<figure><img src="image_8.png" /><figcaption>**Figure 6:** Flow curves of composite A-50 (a) and composite B (b) at 25°C and 37°C.</figcaption></figure>
Viscosity in both materials decreased with higher shear rates, reflecting typical shear-thinning behavior. The underlying cause is the breakdown of intermolecular interactions such as hydrogen bonds and π-π interactions in the resin matrix, which weaken under elevated shear and temperature conditions.²⁴

The viscosity curves at 25°C and 37°C revealed that both composites exhibited lower viscosity at 37°C, reflecting the temperature-dependent flow behavior of the materials. This behavior aligns with previous studies that suggest an increase in temperature enhances the flowability of composites.⁶,²³ Such temperature-sensitive changes in viscosity are a common phenomenon in composite rheology, where higher temperatures reduce the material's resistance to flow.

The influence of shear rate on the viscosity of experimental composites containing varying silica contents (0% [pure resin], 30%, 40%, and 50%) was also examined (Figure 6).

<figure><img src="image_9.png" /><figcaption>**Figure 7:** Viscosity of experimental composites with different silica content at 25°C.</figcaption></figure>

The pure resin exhibited Newtonian behavior, meaning its viscosity remained constant regardless of the shear rate. As the filler content increased, the viscosity also increased, which is typical in composite materials. This rise in viscosity with higher filler content can be attributed to the increased resistance to flow caused by the solid silica particles interacting with the resin matrix. At higher filler contents (30%, 40%, and 50%), the material likely
becomes more structured, with more interactions between the filler particles and the resin, resulting in a more complex rheological behavior.

This increase in viscosity is consistent with what is expected in filled systems, where the filler particles disrupt the flow of the resin and create more resistance as their concentration rises.

## 3.2. Properties after polymerization

### 3.2.1. Hardness and elastic modulus

The nanoindentation test was performed to evaluate the hardness and elastic modulus of the dental composites, providing insights into their mechanical properties at the nanoscale. This technique involves applying a controlled force to a sharp indenter to assess the material's resistance to indentation. Figures 8 and 9 present the results obtained from this test, highlighting the loading-unloading curve for Composite A with 50% silica content, as well as the hardness and elastic modulus values for all samples tested.

Figure 8 presents the loading-unloading curve for Composite A-50, where the load is applied up to a maximum force of 300 mN.

<figure><img src="image_10.png" /><figcaption>**Figure 8:** Loading and unloading curve of composite A-50 with Fmax = 300 mN.</figcaption></figure>

Figure 9 shows the hardness and modulus of elasticity values for all samples.
<figure><img src="image_11.png" /><figcaption>**Figure 9:** Hardness in MPa HV (a) and elastic modulus in GPa (b) for All Samples.</figcaption></figure>

As anticipated, the filler content plays a significant role in the mechanical performance of the composites. The results demonstrated an increase in both hardness (H) and elastic modulus (E) with rising silica content. This finding aligns with the work of Zha et al., who investigated dental resin composites using nanoindentation tests and observed that the incorporation of silica nanoparticles enhances both hardness and elastic modulus.²⁶

To evaluate the impact of the in-situ sol-gel process on mechanical properties, the experimental values were compared to those of a conventional composite containing 75 % of silica. Notably, the in-situ produced composite with only 50 % of silica showed mechanical performance comparable to the commercial reference composite, achieving a hardness of 72 HV and an elastic modulus of 10 GPa. The literature provides extensive data on the mechanical behavior of conventional composites.¹⁹,²⁷ For example, Anupama et al. found hardness values from 49 to 84 HV for five dental composites with filler volumes between 60 and 70%.²⁷

### 3.2.2. Compressive Properties

The compression tests were performed to evaluate the mechanical performance of the composites under compressive loading. Figure 10 (a) illustrates the force-deformation behavior of the experimental composite A-50 as an example, highlighting its response to applied compressive forces. Additionally, Figure 10 (b) presents the compressive strength ($\sigma_c$) values in MPa for all tested samples, enabling a comparative analysis of their compressive performance.
<figure><img src="image_12.png" /><figcaption>**Figure 10:** Load-Deformation Curve for the Compression Test of Composite A-50 (a) and Compressive Strength ($\sigma_c$) in MPa for All Samples (b).</figcaption></figure>

The force-deformation curve shown in Figure 10 (a) represents the behavior of Composite A-50 under compressive loading. Initially, the curve exhibits a linear region, indicating elastic deformation where the material deforms proportionally to the applied force. This linear response corresponds to the material's stiffness and reflects its ability to resist deformation.

As the applied force increases, the curve deviates from linearity, marking the transition to a region where the material begins to undergo fissuring or damage. Finally, the curve reaches its peak, representing the maximum compressive force the composite can withstand before failure. Beyond this point, the material experiences a significant drop in force, indicating crack propagation and ultimate failure under compression.

Figure (b) illustrates the compressive strength ($\sigma_c$) values in MPa for all tested composites, highlighting the influence of silica content on the mechanical performance of the materials. A clear trend is observed, with the $\sigma_c$ values increasing as the silica content in the composite increases. The observed improvement results from silica's reinforcing effect, which boosts the composite matrix's stiffness and strength through effective load distribution and deformation resistance.

### 3.2.3. Flexural Properties
The flexion test was conducted to evaluate the mechanical behavior of the composites under bending loads. Figure 11 (a) illustrates the flexural strain vs. load curve for Composite A, highlighting its response to applied bending forces and providing insights into its stiffness, deformation behavior, and flexural strength. Figure (b) presents the flexural strength ($\sigma_f$) values in MPa for all tested samples, enabling a comparative analysis of their performance under flexural stress.

<figure><img src="image_13.png" /><figcaption>**Figure 11:** Flexural Strain vs. Load Curve for the Flexion Test of Composite A (a) and flexural Strength ($\sigma_f$) in MPa for All Samples (b).</figcaption></figure>

The flexural strain vs. load curve for Composite A-50 demonstrates the material's behavior under bending stress during the flexion test. Initially, the curve shows a linear relationship, indicating elastic deformation where the material bends proportionally to the applied load and retains its original shape upon load removal. This region reflects the material's stiffness or flexural modulus. As the load increases, the curve deviates from linearity. The peak of the curve represents the maximum load the composite can withstand before failure, corresponding to its flexural strength. Beyond this point, a sharp drop in the curve indicates crack propagation and eventual failure. This analysis highlights the composite's ability to resist bending forces and its potential suitability for dental applications requiring materials with high flexural performance.
Figure 11 (b) displays the flexural strength ($\sigma_f$) values in MPa for all the tested composites, illustrating how the flexural strength of the materials vary based on their composition. The data reveals a clear trend: as the silica content increases, the flexural strength improves. Composite A, with the highest silica content, exhibits the highest flexural strength, indicating superior resistance to bending forces. This can be attributed to the reinforcing effect of the silica particles, which enhance the material's rigidity and load-bearing capacity.

In contrast, composites with lower silica content or pure resin show comparatively lower flexural strength, demonstrating that the addition of silica significantly contributes to the composite's ability to withstand bending stresses. These results suggest that filler content, particularly silica, plays a crucial role in improving the flexural performance of dental composites. The higher the silica content, the better the composite's potential for use in applications requiring resistance to bending, such as dental restorations exposed to occlusal forces.

Sonwane et al. explored five dental composites containing a high silica content ranging from 78 to 83 wt.%.¹⁹ Their findings showed compressive strengths between 54 and 176 MPa, along with flexural strengths spanning 88 to 171 MPa. Despite employing lower silica levels in this study through the in-situ sol-gel approach, the mechanical properties observed are on par with those of conventional composites reported in the literature. This is likely because physical blends of organic polymers and pre-formed inorganic particles often separate into discrete phases, particularly when silica particles cluster, resulting in uneven stress distribution and reduced mechanical performance. Conversely, enhanced interfacial bonding and improved compatibility between the phases contribute to significantly better mechanical behavior.

### 3.2.4. Tribological properties

The wear behavior, coefficient of friction (COF) and temperature profiles under dry and saliva-lubricated conditions over a sliding distance of 540 meters are analyzed and compared. The influence of applied loads (5 N, 10 N and 20 N) and silica content (30%, 40% and 50%) on wear and friction is examined. In addition, a comparison between the experimental composites and the commercial composite is provided.

#### 3.2.4.1. Friction coefficient behavior
The recorded values of the coefficient of friction for both composites during the tribological tests on the distance of 540m are presented in Figure 12.

<figure><img src="image_14.png" /><figcaption>**Figure 12:** Evolution of the coefficient of friction with sliding distance for all samples at applied loads of 5 N (a), 10 N (b), and 20 N (c).</figcaption></figure>

The friction coefficient (COF) curves for both composites exhibited two distinct phases during the tribological test. In the initial phase, the COF increased rapidly as the composite surfaces adjusted to contact with the counter-surface. In the steady-state phase, the COF stabilized at a constant value, suggesting equilibrium between the surfaces in contact.

The average coefficient of COF for all samples is shown in Figure 13
<figure><img src="image_15.png" /><figcaption>**Figure 13:** Average coefficient of friction for all sample at loads of 5 N, 10N and 20N.</figcaption></figure>

Figure 13-a shows that the COF increased with the silica content from 0% to 30%, which is likely due to the roughness and introduced by the silica particles, which increased surface friction. However, as the silica content increased further from 30% to 50%, the COF decreased. This suggests that the higher silica content improved the material's surface hardness (Figure 9) and wear resistance, leading to reduced friction. Additionally, better distribution of the filler particles within the resin matrix likely contributed to improved load-bearing capacity and less interfacial friction, ultimately lowering the COF.

Figure 13-b illustrates the effect of load on the coefficient of friction.

At lower loads (5 N), the COF showed minimal variation between composites. In this case, the asperities of the rubbing surfaces are unlikely to tear, since a small number of peaks are in contact.

As the load increased to 10 N and 20 N, a noticeable reduction in the COF was observed for experimental composites with higher silica content (50%), decreasing from 0.35 to 0.34 and then to 0.33. Similarly, the commercial composite showed a decrease in COF, from 0.36 to 0.35 and 0.34. However, for the experimental composite with 40% silica (A-40), only a small decrease was observed, while for A-30, the COF increased with increasing load (Figure 13-b).

This can be explained by the fact that harder materials, like the higher silica composites and the commercial composite, resist deformation under load. As the load increases, these
materials compress but maintain their structure, which reduces friction and results in a lower COF. In contrast, softer materials, such as A-30, are more prone to deformation under the same load. This deformation creates surface irregularities and roughness, which increases the friction and leads to a higher COF for these materials under higher loads.

The experimental composite (50% silica) consistently displayed the lowest COF across all load conditions due to the in-situ silica generation. The in-situ process ensures a more uniform distribution of silica particles within the resin matrix, which enhances the filler-matrix interaction and contributes to a more effective reduction in friction.

The effect of saliva on the COF is also illustrated in Figure 14-a.

<figure><img src="image_16.png" /><figcaption>**Figure 14:** (a): Composites: A-30 (light blue), A-40 (medium blue), A-50 (dark blue) and composite B (green), under dry (solid lines) and saliva-lubricated conditions (dash lines) at a load of 20 N, (b): Temperature Profile Near the Friction Surface for composite A-50 at a load of 20 N, under dry and Saliva-lubricated conditions.</figcaption></figure>

For all composites, COF was reduced by approximately half when saliva was introduced. This significant reduction highlights the role of saliva as a natural lubricant, which minimizes adhesion and asperity interactions by forming a lubricating film at the interface.²²

The temperature near the friction surface was measured continuously during both dry and saliva-lubricated tests to evaluate the thermal effects of friction under different conditions (Figure 14-b).
As shown in the figure, the temperature near the friction surface was significantly lower in the presence of artificial saliva compared to dry conditions. While the temperature reached 60°C under dry conditions, it was considerably reduced to 28°C when saliva was present. Moreover, the temperature increase was more gradual in saliva-lubricated conditions, in contrast to the rapid rise observed in dry conditions. This reduction can be attributed to the lubricating effect of saliva, which reduces frictional heat generation and likely functions as a cooling agent, dissipating heat away from the contact interface.

The lower temperatures observed in saliva-lubricated conditions can alleviate thermal stresses, minimizing the potential for material degradation and enhancing the long-term performance of dental composites in clinical settings.

This temperature reduction aligns with the observed decrease in the COF under saliva-lubricated conditions (Figure 14-a), indicating that the thermal and frictional behaviors are closely interlinked and influenced by the presence of a lubricating medium.

### 3.2.4.2. Wear resistance

Figure 15 illustrates the wear mass (in grams) for all samples subjected to applied loads of 5, 10 and 20 N. This analysis provides insight into the wear behavior of the composites under different loading conditions, highlighting the impact of applied force on the material's ability to resist wear. The results enable a comparison of the wear resistance of the experimental composites and the commercial composite, shedding light on how different silica contents influence the wear characteristics of the materials.
<figure><img src="image_17.png" /><figcaption>**Figure 15:** Wear mass (g) for all samples at loads of 5, 10 and 20 N.</figcaption></figure>

At 5 N, all composites exhibited low mass loss, with minor differences between them.

At 10 N and 20 N, the mass loss increased significantly for all composites. The evolution of wear mass follows the trend observed in the coefficient of friction (COF). For harder materials like the high silica composites and commercial composite, the COF decreases as the load increases, indicating less friction and slower wear. In contrast, for softer materials such as A-30, the COF increases with higher loads, resulting in more friction and faster wear, leading to a quicker increase in wear mass. Thus, the wear mass evolution is closely tied to the friction behavior, with harder materials experiencing less wear and softer materials undergoing more rapid wear under increased load.

Figure 16 presents the wear rate (g/m·N) for all samples under applied loads of 5, 10 and 20 N. This analysis provides a detailed comparison of the wear performance of the experimental and commercial composites, highlighting the influence of silica content and distribution on the wear resistance.

Normalizing the wear mass with the sliding distance and applied load allows the wear rate to account for the actual testing conditions, providing a standardized measure of material wear. This normalization ensures that the wear rate is independent of the specific test parameters,
such as the load or sliding distance, enabling direct comparisons between different materials
and conditions.

<figure><img src="image_18.png" /><figcaption>**Figure 16:** The wear rate (gm⁻¹N⁻¹) for all samples at loads of 5 N, 10 N and 20 N.</figcaption></figure>

The wear rate, calculated as mass loss normalized by load and sliding distance, decreased with increasing silica content in the experimental composites.

At 20 N, the wear rate for the experimental composite with 50% silica approached the performance of the commercial composite, demonstrating the potential of in-situ generated silica for enhancing wear resistance.

Figure 17 shows a view of the surface after friction for the composite A-50.

<figure><img src="image_19.png" /><figcaption>**Figure 17:** The view of composite A-50, area × 50 before (a) and after friction at 5N (b) and 20N (c).</figcaption></figure>
The various wear mechanisms developed depend on the normal load and sliding time.²⁸ At high loads, a change in the wear tracks is observed. The worn surfaces are smoother and the deformation is greater. In the case of the A-50 composite under a load of 20N, severe material removal in the form of plateaus with the presence of a bed of compacted debris (Figure 17-c) is due to strong adhesive interactions operating in the contact and assists in a decrease in the coefficient of friction (Figure 13).

In the case of a low load of 5N (Figure 17-b), a band morphology is observed with a dispersion of some wear debris on the friction track, which leads to the formation of transfer layers on the friction face and more or less pronounced plastic deformation. This is due to the fact that the irregularly shaped particles were detached from the surface, they are again deformed and compacted to take on an elongated shape in the direction of movement. The features observed on the used surfaces and debris support the dominance of an adhesive abrasion process at low load.²⁹

## 4. Results overview

<table><caption>**Table 4:** Overview of Mechanical, Rheological, Tribological, and Shrinkage Data.</caption><thead><tr><th rowspan="2">Composite</th><th rowspan="2">Resin</th><th>Composite</th><th>Composite</th><th>Composite</th><th>Composite</th></tr><tr><th>A-30</th><th>A-40</th><th>A-50</th><th>B</th></tr></thead><tbody><tr><td>Viscosity 25°C (Pas)<sup>a)</sup></td><td>4</td><td>200</td><td>1100</td><td>110000</td><td>130000</td></tr><tr><td>Hardness (HV)</td><td>24</td><td>39</td><td>52</td><td>72</td><td>68</td></tr><tr><td>Elasticity modulus<br>(GPa)</td><td>3.9</td><td>5.9</td><td>8.0</td><td>10.0</td><td>10.9</td></tr><tr><td>Compressive<br>strength (GPa)</td><td>119.7</td><td>166.3</td><td>210.2</td><td>236.3</td><td>223.9</td></tr><tr><td>Flexural strength<br>(GPa)</td><td>90</td><td>100</td><td>105</td><td>110</td><td>95</td></tr><tr><td>COF<sup>b)</sup></td><td>0.10</td><td>0.37</td><td>0.35</td><td>0.33</td><td>0.34</td></tr><tr><td>COF<sup>c)</sup></td><td>0.085</td><td>0.20</td><td>0.18</td><td>0.015</td><td>0.016</td></tr><tr><td>Wear rate<sup>d)</sup> (gm<sup>-1</sup>N<sup>-1</sup>)</td><td>1×10<sup>-6</sup></td><td>0.9×10<sup>-6</sup></td><td>0.8×10<sup>-6</sup></td><td>0.7×10<sup>-6</sup></td><td>0.78×10<sup>-6</sup></td></tr></tbody></table>

<footnote>a) of uncured composite, b) at a load of 20 N in dry conditions, c) at a load of 20 N in the environment of artificial saliva, d) at a load of 20 N, a distance of 540 m and in dry conditions.</footnote>
5.Conclusion

This study highlights the impact of silica content and the in-situ sol-gel synthesis method on the mechanical, rheological, and tribological behavior of dental composites. The results demonstrate that incorporating silica via the sol-gel method significantly enhances the overall performance of the composites, particularly in terms of reduced polymerization shrinkage, improved mechanical strength, and enhanced tribological behavior.

The volumetric shrinkage analysis revealed that increasing silica content significantly reduces polymerization contraction, from 2.3% for the commercial composite (Composite B) to only 0.5% for the experimental composite. This reduction is attributed to the homogeneous dispersion of silica nanoparticles within the matrix, which minimizes the free volume available for polymerization-induced shrinkage.

Rheological tests demonstrated a stable viscoelastic behavior with a notable increase in elastic modulus and viscosity for the silica-reinforced composites. The elastic recovery study showed that the experimental composite reestablishes its structure more rapidly after deformation compared to the commercial composite, which is a crucial advantage for clinical handling.

Mechanical properties, including hardness, elastic modulus, compressive strength, and flexural strength, were significantly improved due to the homogeneous incorporation of in-situ synthesized silica particles. Despite having a lower filler content than the commercial composite (50% vs. 75% silica), the experimental composites achieved comparable or even superior performance, owing to the enhanced matrix-filler interaction.

The tribological study confirmed that the experimental composites exhibit increased wear resistance and a lower coefficient of friction under high loads. Additionally, the lubricating effect of artificial saliva reduced the coefficient of friction by nearly 50%, emphasizing the importance of this parameter under real clinical conditions.

Overall, these findings demonstrate that the sol-gel methodology applied to dental composites enables the development of materials with an optimal combination of mechanical and functional properties. These strategy paves the way for its adoption in the design of next-generation restorative materials, offering improved durability and enhanced clinical performance.
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