The Nanomechanical and Tribological Properties of Restorative Dental Composites after Exposure in Different Types of Media

The aim of this study was to evaluate the effects of various acidic solutions on the surfacemechanical properties of commercial resin composites with different microstructures (Filtek Z350 XT, TPH, Durafill, and Superlux). Specimens were immersed in orange juice, cola, and distilled water for 5 days and the nanohardness, elastic modulus, and wear behavior of the samples were determined via the nanoindentation test and a reciprocating nanoscratch test. The nanoscratch morphology was observed using scanning electron microscopy (SEM) and the wear depth was recorded by scanning probe microscopy (SPM). The results indicate that the nanofilled resin composites had the greatest hardest and highest elastic modulus, whereas the microfilled composites exhibited the lowest nanohardness and elastic modulus values. SEM observations showed that all resin composites underwent erosion and surface degradation after immersion in acidic solutions. Furthermore, the wear resistance was influenced by the composition of the acidic solution and was correlated with the nanohardness and elastic modulus. The dominant wear mechanism changed from plastic deformation to delamination after immersion in acidic solutions.


Introduction
Nowadays, restorative resin composites are widely used for the direct restoration of teeth because of their durability and better aesthetic, biological, and mechanical properties [1].These composites are subjected to a wide range of physical and chemical conditions in the mouth, including temperature variation, masticatory forces, and chemicals from food.These factors greatly influence the in vivo degradation or failure of resin composites.One important chemical factor is exposure to acidic solutions.In vivo studies indicate that the exposure of resin composites to low-pH liquids can negatively affect their mechanical properties [2,3].Several mechanisms have been proposed to explain the degradation and failure of these restorative materials.Acidic molecules may cause surface erosion that weakens the matrix-filler bonding and/or softens the matrix, resulting in bond failures in the outer layer of the filler [4].
The weakening of the matrix-filler bonding or the matrix itself might be reflected by the wear processes that occur during mastication.Many researchers have studied the influence of low pH on the wear behavior of resin composites using microscopic and visual methods to measure wear [5][6][7].Acidic solutions have been shown to reduce the wear resistance of resin composites [8].These studies focused on the microtribology; the actual wear mechanism and its effect on the microstructure are very difficult to determine.Micromechanics research can sufficiently explain the damage caused by acidic solutions and their specific mechanisms of wear.
The nanomechanics of resin composites exposed to acid have not been thoroughly studied, and nanoindentation and nanoscratch methods have not been applied to this field.Nanoindentation technology is ideal for testing the material properties of composites at the micron and submicron scales.This method can be used to determine the local mechanical properties of a resin composite by measuring the indentation load-displacement response.This technology differs from others because there is no need to image the indention area to confirm the mechanical properties [9].Nanoscratch experiments are performed in conjunction with indentation tests, and the nanotribological properties of resin-based composites are tested by scratching the surface, which can reveal damage and further clarify the wear mechanism of resin composites.The objective of this study was to use the nanoindentation and nanoscratch techniques to characterize the effects of acidic solutions on the surface degradation of resin composites.

Samples Preparation.
Four commercially available dental resin composites with different matrix compositions and filler types were used (Table 1), including Filtek Z350 XT, TPH 3 , Durafill, and Superlux.A total of 15 specimens were prepared for each type of resin composite.A silicone rubber mold (a ring with a hole of diameter 15 mm and depth 2 mm) was placed on a piece of transparent film and Vaseline was used to avoid adhesion to the filled composites.The mold was placed on a glass slide, manually filled with a slight excess of resin composite (with care taken to minimize entrapped air), and covered with another glass slide.A light-curing unit (Translux Power Blue, LED-powered lamp, Heraeus Kulzer, Hanau, Germany) was used to harden the composites.The output light intensity was greater than 1100 mW/cm 2 and the spectral range was 410-500 nm.Both the upper and lower surfaces were cured for 40 s.
The hardened samples were removed from the mold and their test surfaces were wet-polished with 400-grit to 1,200grit SiC papers to remove the resin-rich layer and obtain a standard surface finish for the test.

Immersion Solution Preparation.
Three different immersion media, which are common beverages consumed by people worldwide, in which children and adolescents are the majority group consuming the drinks were used in this study (Table 2): (1) distilled water, (2) orange juice, and (3) Coca-Cola.Prior to the experiment, specimens were cleaned in distilled water using an ultrasonic bath and then placed in the immersion solutions.

Characterization of Mechanical Properties and Wear
Behavior.The hardness and elastic modulus were determined by the nanoindentation test using a Hysitron Triboscope nanoindenter with a Berkovich diamond tip.The functional area of the tip was calibrated using a silicon standard.The angle of the tip was 142 ∘ and the radius was approximately 100-200 nm.Nanoindentation was performed in air under ambient conditions and temperature was in the range of 28-32 ∘ C. Loading was performed by following a triangular profile with a maximum load of 6 mN.The loading and unloading rates were all 0.6 mN/s.Five indentations were made on the top of each specimen with a 5-m spacing between indentations.
The reciprocating nanoscratch tests were performed on the sample surface with a constant load of 5 mN and scratch length of 15 m [10].The tip scratch velocity was 30 m/s.The wear scars were observed by scanning probe microscopy (SPM).

SEM Observations.
Representative samples from each group were selected for qualitative scanning electron microscopy (INSPECTF, Czech Republic) observations.The samples were immersed in different media and the wear damage caused by the nanoscratch test was observed by SEM.

Statistical Analysis.
All data were statistically analyzed (SPSS Statistics 17) by using one-way ANOVA test, followed by a LSD -test ( ≤ 0.05) for multiple comparisons between means to determine significant differences, which was used at a significance level set at  ≤ 0.05.

Results and Discussion
The representative images of initial microstructures of the four composites before immersion in solutions are shown in the SEM micrograph in Figure 1.Resin composites are exposed to complex and varying intraoral conditions.Some substances with low pH values, such as beverages, may influence the chemical degradation of the composites.Degradation over time is inevitable and can be predicted by observing the decrease in wear resistance and loss of substance that occur under acidic conditions [11][12][13].In this study, changes in the nanohardness, elastic modulus, and wear depth were measured to evaluate the degradation of dental materials exposed to different pH solutions.
The use of nanoindentation to measure the elastic modulus and nanohardness has several advantages.The tests are simple, reproducible, and relatively nondestructive.Nanohardness is obtained from the plastic depth rather than the final depth, similar to the permanent deformation used in microhardness calculations.
Figure 2 shows the means and standard deviations of nanohardness of nanoindentation between the diamond tip and the specimens that were immersed in distilled water, cola, or orange juice.The highest nanohardness was exhibited by specimen Z350 immersed in cola (0.58 ± 0.08 GPa) and the lowest value was found for Durafill immersed in orange juice (0.23 ± 0.02 GPa).
For resin Z350 treated with orange juice, the surface hardness was significantly higher ( < 0.05) compared to the distilled water treatment, although there was no significant difference in the nanohardness between the cola and orange juice treatments ( = 0.213).
The TPH 3 composite resin showed a pattern similar to Z350; that is, it had significantly higher surface hardness after immersion in orange juice or cola compared to distilled water ( < 0.05).In addition, the nanohardness values were significantly different between the cola and orange juice treatments ( < 0.05).
For the Superlux material, there was a significant decrease in the surface hardness after treatment with cola.The hardness was lower than that obtained with distilled water, although it was greater than the value obtained for the orange  juice treatment.For Durafill, the nanohardness was greater when treated with distilled water and the hardness values for orange juice and cola treatment were different from those obtained for the Superlux resin composite.The elastic modulus describes the relative stiffness of a material.In stress-bearing occlusal contact areas, materials with a low modulus deform more under masticatory stresses and may cause a catastrophic failure (Figure 3).A high elastic modulus is required to withstand deformation and cuspal fracture [14,15].
Durafill exhibited the lowest elastic modulus values of the tested composites regardless of the treatment solution.The Durafill elastic modulus values were different for the various treatments ( < 0.05).For Z350, the elastic modulus was higher when treated with orange juice or cola than when treated with distilled water.However, for TPH 3 , the value for orange juice was higher than with distilled water but lower than with cola immersion.For the two resin composites, the modulus with orange juice immersion was significantly higher than that with the cola treatment ( < 0.05).
Conditioning of the Superlux with TEGDMA, Bis-GMA, and urethane dimethacrylate (UDMA) resulted in greater nanohardness deterioration in orange juice or cola than in water.The Z350 and TPH 3 composites are BisGMAbased.The nanohardness deterioration in distilled water was significantly greater than in acid solution.In acid, an increase in the nanohardness was observed.The resin matrix of composites is known to absorb a small amount of water, which alters certain physical properties.For example, the surface hardness has been reported to be affected by water sorption and the resin matrices of Z350 and TPH 3 are more susceptible to the softening effects of water.In addition to the effect of the matrix, the particulate filler also plays an important role in determining mechanical properties.
An increase in the filler content is expected to increase the hardness to some extent.The filler content of any two materials is similar, although the distribution of the particles in nanohybrid resins is less homogeneous.Additionally, the range of fillers for nanohybrid resins is larger than for nanofillers.However, a less homogeneous distribution of filler and matrix is less susceptible to acid attack.An increased nanoparticle content might increase the hardness to some extent [16,17].The filler contents are shown in Table 1.The percentages of filler in the microfilled and universal hybrid composites are lower than those of the other tested materials.Thus, our results are in agreement with these previous findings.
The ingredients of acids also influence the surface nanohardness of specimens [18][19][20].Orange juice contains citric acid, whereas Coca-Cola contains phosphoric acid [21].Citric acid is a carboxylic acid that is capable of chelating ions such as calcium to form complexes with a moderate solubility in water.Phosphoric acid is capable of chelating calcium but forms an essentially insoluble complex [22].Phosphate ions could slow the dissolution of calcium phosphate; that is, the organic acids were found to induce softening of bis-GMA polymers [8,23] and the softening was strongly affected by the acidity.Specific carboxylic acids are responsible for the damage caused by orange juice, and a low pH is not the sole factor, since an acidic solution should decrease the rate of such hydrolysis [24].SEM analysis showed that all composite resins experienced surface changes after the nanoscratch test.The changes could be considered to be part of a process of degradation and erosion of the matrix.The surface features of the specimens aged in orange juice were more affected than those of the specimens aged in distilled water or cola.Several protruding particles, voids, and cracks were observed in all of the analyzed specimens.These findings are consistent with those of a previous study [25].As shown in Figures 4(b), 5(b), and 6(b), the morphology of Superlux between each other are quite different.The composites also displayed a poor bond at the interfacial region between the matrix and the reinforcement particles.Superlux presented some gaps in the interface where particles and fibers contact the matrix.More obviously, pieces of the mixture, components, and matrix were disrupted and moved from the basement of the matrix under the scratch mark under the orange juice and cola treatments (Figure 5(b)).The fractures between the matrix and fillers were still present, but the desquamation of the mixture components has been slightly reduced.There were many visible porous cracks in or on the surface of the specimens (Figure 4(b)).
In terms of the nanoscratch mark morphology, Durafill presented a uniform thin tribolayer, and there were some cracking and segmentation of the fillers.There was also consistent wear of the matrix and particles.However, there were also some fragments distributed around the scratch.Figures 5(c), 6(c) illustrates the poor wear resistance in samples treated with orange juice or cola.There were few particles that broke or were removed from the matrix.The layer detached from the matrix and some particles remained in the scratch on the specimen surface (Figure 5(c)).
The acid-treated Z350 samples that only showed a small amount of debris had a tribolayer with a smaller portion of contact on the resin composite surface.
TPH 3 presented a tribolayer, although the layer was broken by detached particles and fibers from the matrix that remained in contact with the specimen surface after According to previously published results and our experimental observations, the wear behavior of both composites could be attributed to the delamination/pull-out or cracking/fracture mechanisms.Delamination is generally defined as the desquamation of reinforcement particles in the form of sheets or flakes caused by subsurface crack propagation along the sliding direction [26].Such cracks occur in deformed areas, thus, it is probable that plastic flow occurred in the resin matrix after filler displacement during the wear process.
Generally, TPH 3 and Z350 experience less degradation than Superlux and Durafill.This may be attributed to the lower polymer matrix content in the composites.Because TPH 3 and Z350 have less polymer matrix than Superlux and Durafill, the nanofiller composites showed a lower reduction in nanohardness after immersion in acidic drinks compared to a universal hybrid and microfilled composite.This difference can be attributed to the lower organic matrix content of Superlux relative to TPH 3 .For example, the microfilled composite showed a greater reduction in hardness when compared to a universal hybrid composite.The reasoning presented in a previous study that the nanosize of the hard phase in biological materials will enhance the interface strength that might be used to explain this result [4,27].
It is suggested that as the diamond tip slides on the composite surface, differences in the stress concentration between the filler and matrix interfaces may exist due to differences in the size of fillers.This implies that the stress and degradation associated with larger filler particles were greater than those with smaller particles which is consistent with the present study [28].
As shown in Table 3, the One-way ANOVA analysis and LSD -test indicated a significant difference for the tested immersion media and resin composites.The nanoscratch depths for all materials indicate that a higher inorganic content corresponded to a lower the scratch depth.The depth of nanoscratch of among composites followed by the decreasing order Durafill > Superlux > TPH 3 > Z350.The orange juice significantly increased the depth of nanoscratch more than the distilled water and cola.There was also significant difference in depth changes between distilled water, orange juice, and cola in all specimens under the same test condition.( < 0.05).As can be observed, increase in inorganic content can decrease scratch depth, in scratching process compact and hard inorganic network can resist penetration of indenter and in this manner restricts deformation of the surface.
It is generally understood that highly filled composites are more resistant to degradation [29,30] because of the limited spaces and pathways available for acid molecules to  diffuse within the polymer network.Thus, when a composite is subjected to scratching, the compact and hard inorganic network can resist the penetration of the indenter, which limits the deformation.
In this study, it has been confirmed that all of the tested factors can affect the resin performance.The behavior of each material was influenced by its components but was not entirely determined by one or more factors.Manufacturers should consider these factors when producing materials with a homogeneous body.

Conclusions
We can draw the following conclusions from this study.Modern dental restorative materials have been shown to behave differently when stored in different types of media.The microfilled and universal hybrid resin composites were significantly affected by exposure to cola and orange juice.The nanofilled resin composites showed greater wear resistance.In addition, other factors were found to play an important role in determining the resin surface stability characteristics, such as the filler loading, filler type, and the proportion of filler/matrix.
Currently, the filler particles of resin composites can be classified according to size as microfilled hybrid, nanohybrid, and nanofilled.Hybrids have different incorporation particles sizes (15-20 m and 0.01-0.05m) and fillers offer excellent mechanical properties.Microfill composites have excellent aesthetic effects and include colloidal silica and particles (0.01-0.05 m).As the dental market improves, nanocomposites are increasingly used in clinical settings.These materials combine the good mechanical strength of the hybrids [31] with the aesthetic performance, surface smoothness, and superior polish of the microfills [32].Several types of nanocomposites are available, including nanohybrids, which can contain glass fillers and nanoparticles, as well as nanofills, which contain many different sizes of nanofill particles [33].

Figure 3 :
Figure 3: Mean values and standard deviations (vertical bar) for elastic modulus of four composites tested under different storage media.

Table 1 :
Specification of dental composites investigated in the present study. 3.

Table 2 :
Immersion media used in the present study.

Table 3 :
Mean and standard deviation (S.D.) of depth parameters from nanoscratch test (nm).