Investigation of wear behavior of nanoalumina and marble dust-reinforced dental composites

Abstract In the present work, the effects of adding nanoalumina and marble dust on the wear behavior of dental composites were investigated. The hardness of dental composite was determined using Vickers micro-hardness tester. A two-body abrasive wear test was performed on the dental wear simulator under the medium of artificial saliva. The experiments were performed as per the Taguchi orthogonal array and steady state condition by varying parameters such as filler content, normal load, sliding velocity, and number of cycles. The hardness results indicated that the incorporation of 5 wt. % of nanoalumina increased the hardness of the dental composite by 12%, whereas the incorporation of 5 wt. % of marble dust increased the hardness of the dental composite by 7%. Also, for the experiments as per the Taguchi orthogonal array, the mean volumetric wear in the case of nanoalumina-filled dental composite was 9.6% less than that of marble dust-filled dental composite. However, in both the cases, the volumetric wear increased with the increase in normal load, sliding speed, and number of cycles but decreased with the increase in filler content. Analysis of variance (ANOVA) of the results indicated that normal load was less significant compared to filler content, sliding speed, and number of cycles.


Introduction
The study of wear behavior of dental materials is one of the important factors in understanding the deterioration level under clinical conditions. The excessive differential wear of teeth and restorative materials has significant deleterious effects on the biologic, functional, and aes thetic condition of the masticatory system. Hence, in this regard, the controlled investigations of various factors that might influence tooth and restoration wear have been performed to date. These examinations include the study of the effects of thermocycling [1], cyclic loading [2,3], applied load [4], degree of polymerization [5], aging [6], water absorption [7], lubricant pH [8,9], surface coatings [10,11], tooth brushing/dentifrices [12], and silanization [13][14][15] on the wear of dental material. The wear assess ments for these studies were based on twobody and threebody wear simulation methods. However, the major loss in dental material is due to abrasion and erosion phe nomena in the oral environment. It has been documented that the effect of filler volume on wear resistance follows a linear relationship, with high filler volumes decreasing wear rates [16]. In dental implantology, the alumina was initially used by the dental material scientist mainly in screws, anchorage elements, pin implants, and crowns, etc. [17,18]. The application of alumina as a filler in dental materials provided appropriate aesthetic properties, chemical and biological inertness, high mechanical wear resistance, and overall longterm stability [19].
Currently, most commercial dental composite resins contain inorganic nanoparticles to achieve enhanced performance. These improvements are suggested to arise from the high specific surface area and rich surface func tional groups of the nanoparticles [20]. Although these nanoparticles have substantially improved composite properties including abrasion resistance, polymerization shrinkage, and mechanical strength, more progress is still needed to achieve longterm satisfactory restorations for clinical therapy [21,22]. In Rajasthan, India, one of the serious concerns is how to utilize marble dust or slurry. A very large amount of marble is being dumped by marble cutting industries in any nearby pit or vacant spaces. This imposes the biggest threats to nature and human health. Simultaneously, it produces challenges to the environment in terms of the topographic alteration, land occupation, surface and ground water contamination, and air pollu tion [23][24][25]. Therefore, utilization of marble dust in the production of new materials will help to protect the envi ronment. Recently, the use of marble dust as replacement materials was investigated by various researchers [26][27][28][29][30]. Therefore, recycling and reuse of industrial wastes play a vital role in both solving the industrial waste problem and in getting benefit from it. Marble dust is used as one of filler ingredients that can adequately and reliably con tribute to the performance of dental composite materi als. Apart from the performance aspect, marble dust will greatly reduce the cost of artificial teeth materials.
The aim of the current study was to evaluate the wear performance of dental composite materials filled with nanoalumina and marble dust filler in an artificial saliva medium under steady state conditions and the Taguchi experimental design.

Fabrication of dental composite
The matrix material consists of resin matrix, e.g. BisGMA (52 wt. %) and TEGDMA (45 wt. %) and were mixed together with the polymerization initiator and accelerator CQ, DMAEMA, and BHT. Initially, the fillers nanoalumina and marble dust were mixed with silane (MPS) and water solutions and stirred uniformly up to 6 h. Nine different composites were prepared by varying weight percentages of silanetreated nanoalumina and marble dust, respec tively. Composites were prepared by pouring a blend mixture into a mold of the desired dimensions. The com positions of composite resins are presented in Table 4.

Vickers micro-hardness test
The nanoalumina and marble dustfilled dental compos ite specimens of dimensions 8mm diameter and 6mm length were fabricated in the glass mold. After curing was done, the specimens were placed in distilled water for 24 h to ensure complete curing. Then, the specimens were sub jected to 500g load up to 30s test on the machine micro hardness tester (UHL VMHT, Walter, UK). Six indentations were made on the surface of the sample. The mean value (d) of the six indentations on each sample was consid ered. The Hv values were calculated in accordance to the ASTM E38411e1 standard using Eq. (1).

Wear characterization
Ducom dental wear simulator (Ducom, Bangalore, India) was used to evaluate the tribological characterization in the presence of an artificial saliva environment. Dental wear simulator is compatible to conduct tests according to the ASTM F732 test procedure. It was used to find the influ ence of various parameters such as normal load, sliding speed, and number of cycles on the tribological behavior of dental materials. The dental wear simulator is a single system that can characterize materials simultaneously at four multiple stations. All stations have the same identi cal complex movement pattern in two directions (X and Y, mutually orthogonal to the loading direction) over the antagonist disc made of SS302 ( Figure 1).

Experimental design
The Taguchi method is used to reduce the large number of iterations to a few steps. Instead of having to test all the possible combinations, the Taguchi method tests pairs of combinations and gives the optimum result. This reduces the number of experiments, time, and money. The volu metric wear of nanoalumina and micromarble dustfilled dental composites were determined using a combination of four factors, each having five levels, and are presented in Table 5. The number of experiments involved was large in number as the effect of each factor on each variable was to be determined. Using the experimental approach, the impact of the four factors and five variables can be studied using the L25 orthogonal array. Using the L25 orthogonal array, the number of iterations evolved was reduced to 25. The S/N ratio for the four factors and five variables was determined by the selected smaller is the better charac teristic, as our aim was to reduce the specific wear rate to the minimum. Smaller is the better characteristic was used as Eq. (2).
where, n = the number of observations and y = the observed data, and S/N = the signaltonoise ratio.

Effect of hardness on nanoalumina and marble dust-filled dental composite
The hardness of the dental composite filled with different wt. % of nanoalumina and marble dustfilled dental com posites are shown in the Figures 2 and 3, respectively. In Figure 2, it was observed that the unfilled dental compos ite DC0 indicated a hardness value of 83 Hv, whereas the dental composite filled with 5 wt.  Nanoalumina filled dental composite and 20 wt. % marble dust was found to be 89 Hv, 102 Hv, 119 Hv, and 134 Hv, respectively. In both the figures, it can be revealed that hardness increased with an increase in filler content. The increase in hardness with the increase in filler content was attributed to the fact that an increase in the hard and brittle ceramic filler such as nanoalumina and marble dust stiffened the elasticity of the matrix mate rial and improved the material resistance to indentations on the surface of the materials. In both Figures 2 and 3, it was also observed that the addition of 5 wt. % of nanoalu mina increased the hardness of dental composite by 12%, whereas an addition of 5 wt. % of marble dust increased the hardness of the dental composite by 5 wt. %. More hardness of the nanoaluminafilled composite compared to the marble dustfilled composite was attributed to a stronger binding of nanoalumina with silane compared to marble with silane. It can be also attributed to the fact that a decrease in filler size led to the improvement of the dis persion of the filler in the resin, which resulted in stronger interfacial bonding between the filler and the resin. This result was in agreement with the work by Borges et al. [31] in which it was proposed that nanocomposite and nano hybrid composite resins possess better Vickers micro hardness than hybrid and microhybrid resins. They further concluded that the incorporation of nanosized filler particles enhanced the mechanical properties of the dental composites. The increase in surface hardness with the increase in filler loading was in agreement with Kumar et al. [32]. However, the major factor affecting the hardness of the dental composite is the depth of cure of the resins, which depends upon various factors such as source of light polymerization, light intensity, exposure period, and irra diation distance [33,34].

Effect of a normal load on the volumetric wear of nanoalumina and marble dustfilled dental composites
The effect of variation of the normal load on the volu metric wear of nanoalumina and marble dust dental composite is depicted in Figures 4 and 5, respectively. The steady state experiments were performed by varying the normal load from 20 N to 100 N in the step of 20 N under constant sliding speed of 5 mm/s and number of cycles of 8000. From both the figures, it can be revealed that an increase in load increased the volumetric wear of dental composites. Simultaneously, the increase in filler content led to a decrease in the volumetric wear of the dental composite. Figure 4 indicated that for the unfilled composite, an increase in load from 20 N to 100 N increased the volu metric wear by 18.97%, whereas in the case of a 20 wt. % nanoaluminafilled dental composite, an increase in load from 20 N to 100 N increased the volumetric wear by 52.38%. Therefore, an increase in volumetric wear despite an increase in hardness in the 20 wt. % + nanoalumina filled composite may be attributed to the fact that due to the increase in load, the removed materials acted as a threebody abrasion along with a twobody abrasion. Figure 5 indicates that in comparison to the 18.97% increase in volumetric wear for the unfilled composite, the 20wt. % marble dustfilled dental composite indicated an increase in volumetric wear by 76.26% as a result of a load increase from 20 N to 100 N. Again, the increase in wear rate of the composite despite more hardness may be attributed to the weaker interfacial binding force between the matrix and the resin. Later, these removed materials acted as a part of a threebody abrasion. Turssi et al. [35] DC found that when a normal load was applied, adhesively bonded contact junctions might have been ruptured and detached, generating larger friction coefficients and mate rial loss after many repeated passes of the load.
In this study, less volumetric wear of a nanoalumina filled dental composite compared to a marble dustfilled composite was in agreement with the work of Say et al. [36]. They suggested that wear resistance was improved with a smaller filler particle size. A more closely packed structure was formed with a smaller particle size, which helped to protect the resin matrix for further erosion [37].

Effect of sliding speed on volumetric wear of nanoalumina and marble dust-filled dental composites
The effect of variation of sliding speed on the volumetric wear of nanoalumina and marble dust dental composite is depicted in Figures 6 and 7, respectively. The steady state experiments were performed by varying the sliding speed from 2.5 mm/s to 12.5 mm/s in the step of 2.5 mm/s under constant normal load of 40 N and number of cycles of 8000. In both the figures, it can be revealed that an increase in sliding speed increased the volumetric wear of dental composites. It was attributed to the fact that an increase in sliding speed increased the tangential thrust force. Hence, the volumetric wear of the dental compos ite increased. Increase in sliding speed also increased the friction between two surfaces and their surface rough ness. Apart from the normal load and speed, the presence of the medium also affected the wear performance of the dental composite. It has been documented that the pH of the working medium affected the degradation rates of the material through catalysis, being more unfavorable to hydrophilic resins [38]. Degradation of resinbased dental materials seems to progress at similar rates in water, arti ficial saliva, and in neutral to slightly lowpH media [39]. Figures 6 and 7 also indicated that for an unfilled compos ite, an increase in sliding speed from 2.5 mm/s to 12.5 mm/s increased the volumetric wear by 43.75%, whereas in the case of a 20wt. % nanoaluminafilled dental compos ite, it increased the volumetric wear by 23.52%, and the 20wt. % marble dustfilled dental composite increased the volumetric wear by 29.03%. Hence, it can be seen that apart from the applied load, an increase in sliding speed increased less volumetric wear of the nanoaluminafilled dental composite compared to the marble dustfilled dental composite.

Effect of number of cycles on the volumetric wear of nanoalumina and marble dust-filled dental composites
The effect of variation of the number of cycles on the volu metric wear of nanoalumina and the marble dust dental composite are depicted in Figures 8 and 9, respectively. The steady state experiments were performed by varying the number of cycles from 4000 to 20,000 in the step of 4000 under a constant sliding speed of 5 mm/s and a normal load of 40 N. In both the figures, it can be revealed that an increase in the number of cycles increased the vol umetric wear of the dental composites. The increase in volumetric wear with the increase in the number of cycles was attributed to the fact that increas ing the number of cycles increased the sliding distance and time period of the contact between the specimen and the antagonist disc. These results are in agreement with those reported by other investigators, which found that the volumetric wear in a dental composite increased lin early with the number of cycles, which was proportional to the sliding distance or sliding time [40]. Here, the wear resistant of the nanoaluminafilled composite was more than the marble dustfilled dental composite. It can be attributed to the fact that nanosized particles occupy the interspacing between the resin matrix, which caused the reduction in erosion of the material. The more fine the par ticles are in the resin matrix, the more will be the wear resistance. The wear of large particles was more compared to the fine particles as they easily detach.
A study by Manhart et al. [41] showed that the wear of dental composites increased with an increase in the hard ness, which is very different from the wellknown relation ship between wear and hardness. Other studies by Kon et al. [42] and Wonglamsam et al. [43] indicated that a sim ulated occlusal wear of a composite consisting of porous spherical filler was lower than that of the conventional composites despite the lower flexural strength of the former. These studies also indicated that the mechanical properties and wear of dental composites were not only the effect of filler content but also of filler morphology. Hence, in this regard, the wear of composites consisting of porous spherical filler would be differently influenced compared with that of conventional composites consist ing of an irregularshaped filler.

Analysis of experimental results by the Taguchi experimental design
The Taguchi experimental approach was applied to determine the dominant factors responsible for the overall reduction in volumetric wear of nanoalumina and marble dustfilled dental composites. The seventh and ninth columns in Table 6 indicate the S/N ratio of the nanoalumina and marble dustfilled dental com posites. The overall mean values for the S/N ratio of the volumetric wear for the nanoalumina and marble dust filled dental composites were found to be 50.45 db and 48.61 db, respectively. Also, the sixth and eighth columns in Table 6 indicated the volumetric wear of nanoalu mina and marble dustfilled dental composites as per the Taguchi orthogonal experimental design. It can be seen that the mean volumetric wear of the nanoalumina filled dental composite (0.00372 mm 3 ) exhibited 9.6% less wear compared to the marble dustfilled dental com posite (0.00408 mm 3 ). The results of the Taguchi experi mental data for the nanoalumina and marble dustfilled  dental composites were analyzed using MINITAB 17 and are presented in Figures 10 and 11, respectively. Figure 10 indicated that the combination of factors should be in the form of A4, B5, C1, D1, and it would give a minimum volumetric wear for the dental composite filled with dif ferent wt. % of nanoalumina. It means that the dental composite filled with 15 wt. % nanoalumina at a normal load of 100 N, with sliding speed of 2.5 mm/s and number of cycles of 4000, exhibited a minimum wear rate. On the other hand, Figure 11 indicates that the combination of factors should be A4, B4, C1, D1, and it would give the dental composite filled with different wt. % of marble dust a minimum volumetric wear. It means that the dental composite filled with 20 wt. % of marble dust at a normal load of load of 80 N, with sliding speed of 2.5 mm/s and number of cycles of 4000, exhibited minimum wear rate.

ANOVA and the effects of factors
A general linear model analysis of variance (ANOVA) was performed to find the significance of factors such as filler content, normal load, sliding speed, and number of cycles on the volumetric wear of dental composites. ANOVA was performed based on the Taguchi experimental results. Tables 7 and 8 shows the results of the ANOVA with the volumetric wear of nanoalumina and marble dustfilled dental composites taken from the Taguchi experimental design. This analysis was undertaken for a level of confi dence of significance of 5%. The last columns in Tables 7  and 8 indicate the percentage contribution of each factor. The percentage contribution indicates that the main effects are highly significant if the corresponding value of the percentage contribution is low. In Table 7, it can be observed that for the nanoaluminafilled dental com posites, the level of significance follows the order filler content (p = 0.055), sliding speed (p = 0.138), number of cycles (p = 0.402), and normal load (p = 0.798). On the other hand, in Table 8, it can be observed that for the marble dustfilled dental composites, the level of signifi cance follows the order number of cycles (p = 0.037), filler content (p = 0.043), sliding speed (p = 0.066), and normal load (p = 0.453). Hence, it can be revealed that in the case of a nanoaluminafilled dental composite, the filler content  is the most significant factor, whereas in the case of the marble dustfilled dental composite, the number of cycles is the most significant factor influencing volumetric wear of the dental composite. Hence, it can be concluded that as far as mechanical and wear properties are concerned, nanoalumina should be preferred over marble dust as a filler for the dental composite. Among all the factors, a normal load is the least significant factor.  Signal-to-noise: Smaller is better Figure 11: Effect of control factors on the wear rate of marble dust-filled dental composites.

Surface morphology
The worn surface morphology of the nanoalumina and marble dustfilled dental composites studied under steady state conditions using a field emission scanning electron microscope (FESEM) is presented in Figures 12 and 13, respectively. FESEM was used to characterize the morphol ogy of unworn and worn surfaces of dental composites. Figure 12 indicates the unworn and worn surface morphol ogy of the nanoaluminafilled dental composite under varying conditions of filler content, load, sliding speed, and number of cycles. Figure 12A shows the unworn surface of a 20wt. % nanoaluminafilled dental composite (DCAL 20). It can be seen that there were a number of uneven and rough faces, voids, and cracks on the surface of the dental composite. Figure 12B and C indicates the effect of varying normal loads on the volumetric wear of the dental compos ite (DCAL20). Figure 12B indicates the micrograph of the surface at 20N normal load, whereas Figure 12C indicates the micrograph of a surface at 100N normal load. It can be observed in both figures that at initial loading, wear track was not visible, but still, there was presence of wear scars and microploughing, which was due to the presence of an uneven surface before the wear test. However, when the load was maximum at 100 N, wear tracks were visible, and the surface was smooth. Therefore, an increase in load increased the volumetric wear, but wear rate was not signifi cant, which is in agreement with our Taguchi result with the least normal load compared to other factors ( Figure 10) and the steady state result (Figure 4). Figure 12D and E indicates the effect of varying sliding speeds on the volumetric wear of a dental composite (DCAL20). Figure 12D indicates a micrograph of a surface at 2.5mm/s sliding speed, whereas Figure 12E indicates a micrograph of a surface at a sliding speed of 12.5 mm/s keeping the other factors constant. At a low speed ( Figure 12D), the filler particles were exposed and loosened on the surface, and delamination of the upper surface can be seen, but at maximum speed ( Figure 12E), the removed particles participated in the threebody abrasion. More wear can be seen at high speed. Figure 12F and G indicates the effect of varying numbers of cycles on the volumetric wear of a dental composite (DCAL20). Figure 12F indi cates a micrograph of a surface at 4000 number of cycles, whereas Figure 12G indicates a micrograph of a surface at 20,000 numbers of cycles. At 4000 cycles of the run, the worn surfaces show pores and wear scars on the surface. However, after 20,000 cycles of the run, the surfaces become damaged, and a twobody abrasion can be seen on the surface. Figure 12H and I indicates the effect of varying filler contents on the volumetric wear of a dental com posite. Figure 12H indicates a micrograph of an unfilled dental composite (DC0), whereas Figure 12I indicates a micrograph of a 10wt. % nanoaluminafilled dental com posites (DCAL10). As per Figure 12H, the unfilled compos ites indicate an interlayer debonding of the matrix creating a fracture on the surface. As per Figure 12I, there was less wear compared to the unfilled dental composite, but still wear scars, small pits, holes were visible. Figure 13 indicates the unworn and worn surface mor phology of a marble dustfilled dental composite (DCMD20) under different varying conditions of filler content, load, sliding speed, and number of cycles. Figure 13A shows the unworn surface of a 20wt. % marble dustfilled dental composite (DCMD20). The fabricated surfaces show a large number of voids, holes, and cracks on the surface. Figure 13B and C indicates the effect of varying normal loads on the volumetric wear of a 20wt. % marble dust filled dental composite (DCMD20). Figure 13B indicates a micrograph of a surface at 20N normal load, whereas Figure 13C indicates a micrograph of a surface at 100N normal load. It can be observed in both the figures that at 20N load, wear tracks were visible on the surface, whereas in the same working condition, wear track was  not visible in nanoaluminafilled dental composites (DCAL20) in Figure 12A. Hence, it can verify our result that the nanoaluminafilled dental composite exhibited better hardness compared to the marble dustfilled dental com posite (DCMD20). However, when the load was increased up to 100 N ( Figure 13C), the worn surface was full of deep grooves, damaged surfaces, wear scars, etc. The delami nation of the matrix material was also seen on the surface at a high loading of 100 N. Figure 13D and E indicates the effect of varying sliding speeds on the volumetric wear of a dental composite (DCMD20). Figure 13D indicates a micrograph of a surface at a 2.5mm/s sliding speed, whereas Figure 13E indicates a micrograph of a surface at a sliding speed of 12.5mm/s. At a low speed at 2.5 mm/s ( Figure 13D), the worn surface was smooth, interrupted by numerous cracks extending through the matrix. However, still, the removed materials were exposed on the surfaces along with some voids, which were left after exfoliation of the filler particles. As the speed was increased up to 12.5 mm/s ( Figure 13E), matrix filled debonding can be observed along with some longitudinal cracks perpendicular to the sliding direction. More wear can be seen at high speeds. Figure 13F and G indicates the effect of varying numbers of cycles on the volumetric wear of dental composites (DCMD20). Figure 13F indicates a micrograph of a surface at 4000 number of cycles, whereas Figure 13G indicates a micrograph of a surface at 20,000 numbers of cycles. At 4000 cycles of run, the worn surfaces were smooth, but existence of voids, pores, and filler material can be seen on the surface. However, after 20,000 cycles of the run, the surfaces become damaged due to microploughing by the twobody abrasion phenomena ( Figure 13G). Figure 13H and I indicates the effect of varying filler contents on the volumetric wear of dental composites. Figure 13H indicates a micrograph of an unfilled dental composite (DC0), whereas Figure 13I indicates a micro graph of 10wt. % marble dustfilled dental composites (DCMD10). As per Figure 13H, the unfilled composites indicated more wear compared to the 10wt. % marble dustfilled dental composites in Figure 13H and the 20wt. % marble dustfilled composites in Figure 13B. The unfilled dental composite ( Figure 13H) shows the pres ence of longitudinal crack and interlayer debonding of the matrix that led to create a fracture on the surface. As per Figure 13I, wear tracks were visible along with the pres ence of some wear scars and removed material.

Conclusion
The dental composites filled with different weight per centages of nanoalumina and marble dust were success fully fabricated. The Vickers hardness test of the dental composites was performed along with the twobody wear test in artificial saliva medium to assess their mechanical and wear properties. The hardness results indicated that the incorporation of 5 wt. % of nanoalumina increased the hardness of the dental composite by 12%, whereas incor poration of 5 wt. % of marble dust increased the hardness of the dental composite by 7%. The wear result indicated that the experimental composites based on nanoalumina and marble dust have been shown to behave differently in the artificial saliva medium. The nanoaluminafilled dental composites exhibited less volumetric wear com pared to the marble dustfilled dental composite. In addi tion, other factors were found to play an important role in determining the wear performance of dental composites such as the filler content, applied normal load, sliding speed, and number of cycles.
The steady state condition tests indicated that the volumetric wear of dental composites increased with the increase in normal load and further increased with increasing number of cycles and sliding speed. The Taguchi orthogonal experiment suggested that for the nanoaluminafilled dental composite, the best combi nation of factors for minimum volumetric wear was the percentage of filler content of 15%, normal load 100 N, sliding speed 2.5 mm/s, and number of cycles 4000. On the other hand, for the marble dustfilled dental com posite, the best combination of factors for minimum volumetric wear was percentage of filler content at 20%, normal load 80 N, sliding speed 2.5 mm/s, and number of cycles 4000. Further, it was also concluded that in the Taguchi orthogonal experiments, the mean volumetric wear of the nanoaluminafilled dental composite was 9.6% less than that of the marble dustfilled dental com posite in artificial saliva medium.