Home Physical Sciences Abrasive wear behavior of silane treated nanoalumina filled dental composite under food slurry and distilled water condition
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Abrasive wear behavior of silane treated nanoalumina filled dental composite under food slurry and distilled water condition

  • Shiv Ranjan Kumar EMAIL logo , Amar Patnaik and I.K. Bhat
Published/Copyright: October 18, 2016
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Science and Engineering of Composite Materials
From the journal Science and Engineering of Composite Materials Volume 25 Issue 3

Abstract

The aims of the present study were to develop a dental composite filled with silanized nanoalumina and then to investigate the effect of nanoalumina filler on the two-body and three-body wear behavior under distilled water and food slurry medium, respectively. The dental composites were fabricated by adding silane treated nanoalumina filler particle in the weight percentage of (0–3 wt.%) to the matrix of BisGMA, TEGDMA, CQ and EDMAB. Two-body and three-body wear tests were performed in dental wear simulator machine with varying parameters such as normal load, chewing speed and chamber temperature in such a way as to simulate mastication process. Taguchi’s orthogonal array (L16) design, steady state condition and ANOVA were applied to evaluate the optimum parameter for minimum wear and effect of each parameter on the wear performance of dental composites. The finding of the result indicated that mean volumetric wear rate of dental composite in distilled water (i.e. two-body abrasion) was 33.23% more than that of the same composite in food slurry condition (i.e. three-body abrasion).

Keywords: dental composite; nanoalumina; scanning electron microscopy; wear
Nomenclature
BisGMA

Bisphenol-A glycidyl methacrylate

TEGDMA

tri-ethylene glycol dimethacrylate

EDMAB

Ethyl 4 dimethyl amino benzoate

CQ

Camphorquinone

MPS

Methacryloxypropyltrimethoxy silane

Hv

Vickers hardness

NADC-0

Dental composite filled with 0 wt.% of silane treated nanoalumina

1 Introduction

The dental wear phenomena arising from the complex and complicated masticatory movement of dental materials is still not clearly understood by the researcher. Hence, the excessive loss of dental materials raised a critical concern among the dental community such as dentist, material scientist and researcher. In this regard, various materials have been proposed and tested for better wear performance. On the other hand, the dental composite materials are preferred over the other available dental materials due to better aesthetic look and physical and mechanical properties. However, the major disadvantage of dental composite is low service life due to high wear rate. Hence, the recent research on dental composite was mainly focused to study the wear behavior and improve its wear resistance in different oral environments. The major ingredient of dental composite materials includes base monomers such as BisGMA, TEGDMA, UDMA, BisEMA etc., photo-initiators such as CQ, EDMAB and DMAEMA and fiber/filler such as glass fiber, silica particulate filler etc. Inventions in the field of organic monomers, modifications in filler technology and advances in photo initiators, light curing equipment, pigments etc lead to the advancement of dental composite. Despite these developments, a lot of efforts are going on to increase the service life of dental composite by improving its physical, chemical and mechanical properties. In this regard, change in filler content/shape/size has been considered as one of the methods to improve characteristics of dental composite. Incorporating inorganic fillers in the dental composite leads to the increase in mechanical properties such as stiffness, hardness, compressive strength and flexural strength but decrease in mechanical property such as flexural modulus [1], [2], [3], [4]. It has been reported that the nano-sized particle reinforced dental composite exhibited better physical and mechanical properties [5], polishability [6] and esthetics but weak wear performance [7]. Hence, it becomes essential to study wear behavior of nanocomposite dental material. In this regard, Osiewicz et al. [8] proposed that abrasion of opposing resin composite surface can be reduced by reducing the size of harder filler. In addition, the two-body and three-body abrasive wear produced by nano-filled and nano-hybrid composite was low compared to hybrid, micro-hybrid and micro-filled composites [9]. However, the presence of silane (chemical) coupling between filler and resin, as well as filler particle size, enhances physical, mechanical and wear properties of dental composites [10], [11], [12], [13], [14]. Chamochin et al. [15] focused on the silane treatment of filler to increase the binding strength between resin and filler which lead to improved mechanical properties of the resin based composite. The optimization of the wear behavior of this composite may require a change in the chemistry as well as the size distribution of the filler particles [16]. Addition of up to 1% silanized nanosilica to experimental adhesive system increased resin-dentin microtensile bond strength to dentin. Increased levels of nanofiller may cause a decrease in the resin-dentin bonds. Increase in filler content and decrease in filler size can result into better mechanical properties and high wear resistance [17]. Kumar et al. [18] concluded that addition of silane treated nanoalumina filler improved the physical and mechanical properties of dental composite, whereas Tamura et al. [19] proposed that addition of nanoalumina filler particle in terms of shapes, size and content improved the mechanical properties such as hardness and wear properties such as simulated occlusal wear and toothbrush wear. However, detailed research on the effect of silanization on wear behavior in different lubricating medium is lacking in literature.

Therefore, on the basis of the above analyses, the proposed research mainly focused on the fabrication of dental composites filled with silane treated nanoalumina and study the effect of nanofiller on the wear behavior under food slurry condition and distilled water by using dental wear simulator.

2 Materials and methods

2.1 Specimen preparation

The base resin BisGMA was procured from EsstechInc, Essington, PA, USA. The diluent TEGDMA was purchased from TCI, Tokyo, Japan. Both the monomers were taken in a container in the ratio of 50:49 wt.%. The container was covered with aluminum foil to prevent light-initiated polymerization. The mixture was uniformly stirred up to 6 h in a sonicator. Then, the photo-initiator Camphorquinone (Spectrochem, Mumbai, India) and accelerator EDMAB (Sigma Aldrich, Bangalore, India) were added in the ratio of 0.2:0.8 wt.% and mixed up to 2 h to conduct the polymerization reaction. Initially, nanosilica and nanoalumina filler particles were procured from Sarthak sales, Jaipur, India. Nanosilica was used as received and added to the mixture. Nanoalumina was treated with methacryloxypropyltrimethoxy silane (TCI, Japan). For silane treatment of nanoalumina filler, 10 ml acetone and 5 ml water solution was prepared and adjusted to the pH 4.5–5.5 with acetic acid. Then, nanoalumina was added in the treatment solution consisting of 15 wt.% of MPS (relative to filler weight percentage) [20]. The silane treated alumina was mixed to the solution and stirred for approximately 2 h using a magnetic stirrer REMI 5MLH Plus, Mumbai, India. Then the silane treated nanoalumina and was added in different weight percentage to the resin mixture formulation as per Table 1 and stirred uniformly up to 2 h using the magnetic stirrer (REMI 5MLH Plus, Mumbai, India). Then the mixture was poured into the mold and cured as per specific characterization technique. Curing was performed using visible light source (LED Light Curing Unit, 1200 mW/cm2, 450–490 nm, Dentmark, Mumbai, India) up to 40 s on both the upper and the lower surfaces. To ensure complete curing, all samples were stored in distilled water at 37°C for 7 days.

Table 1:

Detailed composition of dental composite filled with nanoalumina and nanosilica filler.

Sample designationComposition
NADC-0Resin matrix+Filler (0% ST-Al2O3+3% SiO2)
NADC-1Resin matrix+Filler (1% ST-Al2O3+3% SiO2)
NADC-2Resin matrix+Filler (2% ST-Al2O3+3% SiO2)
NADC-3Resin matrix+Filler (3% ST-Al2O3+3% SiO2)

  1. Resin matrix includes 50 wt.% BisGMA, 49 wt.% TEGDMA, 0.2 wt.% CQ and 0.8 wt.% EDMAB. NADC, silane treated nanoalumina filled dental composite; ST-Al2O3, silane treated nanoalumina filler. Bold indicates the filler content varying in the four compositions.

2.2 Determination of Vickers microhardness

The fabrication and curing of specimen were performed as mentioned above in specimen preparation section. Four samples of composite filled with different weight percentage (0, 1, 2 and 3 wt.%) of silane treated nanoalumina filler were fabricated in a glass mold (5 mm diameter and 6 mm height). The hardness test was performed with a microhardness tester (Walter UHL VMHT) as per ASTM E384-11e1 standard [21]. In this test 300 g load was applied for 30 s at six different locations on the same surface. The mean diameter value (d) of six indentations on each sample was calculated. The Hv values were calculated using Eq. (1)

(1)Hv=1854.4×Fd2

2.3 Determination of wear

The wear assessment of dental composite was performed as per Taguchi orthogonal design and steady state condition on a dental wear simulator (Ducom, Bangalore, India) (Figure 1). Specimens (Φ 5 mm×10 mm) were made and tested in the wear apparatus against horizontally rotating disc (SS-304 hardened steel) according to ASTM G172 test procedure. Normal load, chewing speed and chamber temperatures were varied in such a way to simulate mastication process as presented in Table 2. The mediums used in this study were simulated food slurry and distilled water (local vendor Sarthak sales, Jaipur, India). The simulated food slurry was prepared by adding 33 wt.% of pre-ground poppy seeds in distilled water as per the method [22]. Before and after wear tests, the specimens were dried in an oven up to 6 h and weighed using high precision weighing machine accurate to ±0.0001 g (Contech Instrument Ltd, Mumbai, India). The wear volume of each specimen was calculated as per Eq. (2) [23].

Figure 1: Schematic diagram of dental wear test rig.
Figure 1:

Schematic diagram of dental wear test rig.

Table 2:

Levels of variables used in wear test.

Control factorsLevelUnit
IIIIIIIV
A: Filler content0123wt.%
B: Normal load406080100N
C: Chewing speed2.557.510%
D: Chamber temperature5152535°C
(2)ΔV=Δmρ

where, ΔV=volumetric wear loss (mm3), Δm=mass loss (g) and ρ=density of specimen (g/cm3), measured according to the Archimedes’ principle.

In this study, Taguchi L16 orthogonal array design was planned to find wear rate using four factors and four levels. Hence, a total of 16 experiments were conducted to find the optimum condition for minimum wear rate. The experimental outputs are further converted into signal to noise (S/N) ratio. Here in this study, the objective was to formulate a composite material with the criteria of minimization in wear rate. Therefore, smaller is the better characteristic for S/N ratio which was used as Eq. (3) [24].

(3)SN=−10 log1n(∑y2)

where n represents the number of observations and y the observed data.

2.4 Statistical analyses

Statistical significance of various factors like filler contents, chewing load, profile speed and chamber temperature on volumetric wear rates of dental composite was described by using general linear model in analysis of variance (ANOVA). Comparisons between three or more treatments were made using one-way ANOVA with Tukey’s post hoc test. All data were analyzed using software MINITAB 16. Statistical differences were considered to be significant at p<0.05.

2.5 Field emission scanning electron microscope investigation of worn surfaces

The worn surfaces after 20,000 cycles of movement were studied by field emission scanning electron microscope (FE-SEM) (Nova Nano FE-SEM 450) at 5 kV acceleration voltages. Before SEM examination, the samples were first sputter-coated with platinum to a thickness of 5 nm.

3 Results and discussion

3.1 Microhardness characterization

Vickers microhardness of silane treated nanoalumina filled dental composite is shown in Figure 2. The value of hardness of fabricated dental composite with 0, 1, 2, and 3 wt.% of nanoalumina was 22 Hv, 26.6 Hv, 28 Hv, and 31.2 Hv, respectively. It can be seen that the hardness of dental composite increased with the increase in filler content. The increase in hardness with the increase in filler content was due to increase in inorganic hard phase filler oxide compared to soft flexible resin matrix. During microhardness testing, the tested load was applied on hard filler and transferred though the ceramic filler. Also, addition of nanoparticles, i.e. high specific surface area provides stronger and more interfacial interactions between the filler and resin, resulting into better properties [25], [26]. Increase in hardness with content was also in agreement with the Boyer et al. [27]. It is reported that there exists direct correlation between the hardness of dental composites and inorganic filler content. However, Manhart et al. [28] observed that the hardness of dental composite material depends upon various factors other than filler content such as resin chemistry, filler size and interfacial strength between the organic monomers and inorganic fillers. Another factor responsible for variation of hardness is polymerization shrinkage of resin matrix. Increase in polymerization shrinkage increased the hardness. Increase in degree of conversion of monomer increased its physical properties as well as Vickers hardness number (VHN) of dental composite [29]. However, it was also reported that during microhardness testing, the contact surface between the diamond indenter and specimen surface plays a major role in judging the effectiveness of the indentation test [30]. In this regard, irregularities on the surface can lead to inaccurate hardness value [31].

Figure 2: Effect of Vickers hardness of silane treated nanoalumina filled dental composites.
Figure 2:

Effect of Vickers hardness of silane treated nanoalumina filled dental composites.

3.2 Wear characterization

3.2.1 Steady state condition for wear analysis: variation of normal load, speed and temperature

In this section, wear experiments were performed as per steady state condition. The effect of variation of each of the factors such as normal load, chewing speed and chamber temperature on the wear behavior of dental composite was studied in both the medium of food slurry and distilled water keeping other factors constant. Figure 3 indicates the effect of variation of normal load on the volumetric wear rate of dental composite keeping other parameters constant such as chewing speed of 5 mm/s and chamber temperature of 35°C under food slurry condition, i.e. three-body abrasion. Figure 4 indicates the effect of variation of normal load on the volumetric wear rate of dental composite keeping other parameter constant such as chewing speed of 5 mm/s and chamber temperature of 35°C under distilled water medium, i.e. two-body abrasion. The variation of load from 40 N to 100 N was within the range (30–120 N) suggested for the mastication process [32]. From both Figures 3 and 4, it can be seen that as the load was increased from 40 N to 100 N in steps of 20 N, the volumetric wear rate was increased significantly. The increase in wear rate with the increase in normal load was obvious, as increase in load increased the true contact area. The increase in true contact area led to increase in shear force and removal of more materials. Suresha et al. [33] concluded that increase in either sliding speed or sliding velocity increases the wear rate of the composite material. It can be also revealed that the three-body wear rate in Figure 3 was less than the two-body wear rate in Figure 4. It was due to the fact that in two-body abrasion, shear stress was more as load was directly applied on the specimen. However, in three-body abrasion, the load was shared by the specimen and third-body abrasive particles. Most of the three-body abrasion was only due to moving abrasive particles. Also, the thin film formed in the case of three-body abrasion by food slurry was thicker than that formed during two-body wear by distilled water.

Figure 3: Effect of normal load on volumetric wear rate of the dental composites in food slurry condition (at constant chewing speed of 5 mm/s and chamber temperature of 25°C).
Figure 3:

Effect of normal load on volumetric wear rate of the dental composites in food slurry condition (at constant chewing speed of 5 mm/s and chamber temperature of 25°C).

Figure 4: Effect of normal load on volumetric wear rate of the dental composites in distilled water condition (at constant chewing speed of 5 mm/s and chamber temperature of 25°C).
Figure 4:

Effect of normal load on volumetric wear rate of the dental composites in distilled water condition (at constant chewing speed of 5 mm/s and chamber temperature of 25°C).

Figures 5 and 6 indicates the effect of variation of chewing speeds on the volumetric wear rate of dental composite under food slurry and distilled water condition. Chewing speeds were varied in steps of 2.5 mm/s from a minimum value of 2.5 mm/s to a maximum value of 10 mm/s keeping other parameters constant, i.e. normal load of 40 N and chamber temperature of 35°C. From both figures, it can be observed that increase in chewing speed increased the wear rate of dental composite. The increase in the wear rate with the increase in chewing speed was attributed to the fact that increase in speed increased the thrust force and shear force resulting in increase in wear rate. Again, it was revealed that the three-body abrasive wear in food slurry (Figure 5) was much less than two-body abrasive wear in distilled water (Figure 6). It can be concluded that speed also played a significant role in the wear of dental composite. At higher speed, the food particles moved away from the wear zone towards the corner side due to centrifugal force. Hence, fewer particles participated in the wear phenomena at high speed. Hence at high speed, the volumetric wear rate of dental composite was lower under three-body wear conditions than under two-body wear conditions.

Figure 5: Effect of chewing speed on volumetric wear rate of the dental composites in food slurry condition (at constant normal load of 40 N and chamber temperature of 25°C).
Figure 5:

Effect of chewing speed on volumetric wear rate of the dental composites in food slurry condition (at constant normal load of 40 N and chamber temperature of 25°C).

Figure 6: Effect of chewing speed on volumetric wear rate of the dental composites in distilled water condition (at constant normal load of 40 N and chamber temperature of 25°C).
Figure 6:

Effect of chewing speed on volumetric wear rate of the dental composites in distilled water condition (at constant normal load of 40 N and chamber temperature of 25°C).

Figures 7 and 8 showed the effect of variation of chamber temperature on the volumetric wear rate of dental composite in food slurry and distilled water medium, respectively. Chamber temperatures were varied in the range of 5–35°C in steps of 10°C keeping other factors constant such as normal load of 40 N and chewing speed of 5 mm/s. In both figures, it was observed that as the temperature increased, wear rate of dental composite increased. However, after reaching a maximum value at 25°C, the wear rate started to decrease. The increase in wear rate with the increase in chamber temperature was due to the fact that as the temperature increased, the molecular mobility inside the polymeric matrix increased resulting into low modulus of the material. Hence, this led to increase in the wear rate. Further increase in temperature led to decrease in wear rate due to the fact that after a certain level decrease in modulus and softening of polymeric matrix provide more lubricating effect during wear test. Hence, wear rate was decreased with the increase in temperature. Further, in Figures 3–8, it was also observed that the wear rate of dental composite decreased with increase in filler content. This decrease in wear rate with the increase in filler content was in agreement with Figure 2, which shows that the hardness was increased with the increase in filler content. It is well known that increase in hardness decreased the wear rate of dental composites [34], [35]. Culhaoglu and Park [36] reported that the wear rate of dental composite varies with different size and amount of fillers, types of matrix and the interfacial bond between resin and filler. However, apart from the temperature variation, the presence of moisture also affects the mechanical properties of composite materials [37].

Figure 7: Effect of chamber temperature on volumetric wear rate of the dental composites in food slurry condition (at constant normal load of 40 N and chewing speed of 5 mm/s).
Figure 7:

Effect of chamber temperature on volumetric wear rate of the dental composites in food slurry condition (at constant normal load of 40 N and chewing speed of 5 mm/s).

Figure 8: Effect of chamber temperature on volumetric wear rate of the dental composites in distilled water condition (at constant normal load of 40 N and chewing speed of 5 mm/s).
Figure 8:

Effect of chamber temperature on volumetric wear rate of the dental composites in distilled water condition (at constant normal load of 40 N and chewing speed of 5 mm/s).

3.2.2 Taguchi orthogonal array experimental for wear analysis

In this section, experiments were performed as per Taguchi orthogonal array (L16) experimental design to find out the optimum combination of parameters for minimization of wear rate. Taguchi method was applied to improve the performance output in both the food slurry and distilled water medium. In Table 3, the sixth and eighth columns represent the wear rate of dental composite for all experiments as per Taguchi experimental design in food slurry and distilled water medium, respectively. The mean volumetric wear rate in food slurry and distilled water was 0.002657 and 0.00354 mm3/20,000 cycles, respectively. Hence, the three-body abrasive wear rate was 33.23% lower than the two-body wear rate. From Table 3, it can be revealed that the silane treated nanoalumina filled dental composites have indicated more wear in the distilled water medium (two-body abrasion) than in the food slurry (three-body abrasion). Hence, the wear of dental material depends upon size, shape and other physical characteristics of the third body. Proper dissolution of third body into liquid may lead to increase in the viscosity of the medium, which in turn reduces wear of the dental composite. In Table 3, the seventh and ninth columns represent the S/N ratio of the volumetric wear rate of the composites in food slurry and distilled water medium. The overall mean for the S/N ratio of the specific wear rate was found to be 51.24 db and 53.67 db for the dental composite filled with different weight percentage of nanoalumina particulates in food slurry and distilled water medium, respectively. The analysis was performed using the software MINITAB 16 and is presented in Figure 9 to analyze the wear rate of the nanoalumina particulate filled dental composites. From this analysis (Figure 9) it was observed that the factor combination of A4, B1, C1 and D2 gives minimum volumetric wear rate for dental composite filled with different weight percentage of nanoalumina. It means that the dental composite filled with 3 wt.% nanoalumina at normal load of 40 N, with chewing speed of 2.5 mm/s and at chamber temperature of 15°C exhibited minimum wear rate.

Table 3:

Taguchi experimental results for NADC series composite.

S. NoFiller content (%)Normal load (N)Chewing speed (mm/s)Chamber temperature (°C)Volumetric wear rate in food slurry (mm3/20,000 cycles)S/N ratio (db)Volumetric wear rate in distilled water (mm3/20,000 cycles)S/N ratio (db)
10402.550.002895550.770.0033351.88
20605150.004661746.630.0052847.45
30807.5250.00579144.740.00644345.86
4010010350.006920443.200.00813544.52
51405250.001996254.000.0022855.36
61602.5350.001715455.310.00206361.38
71801050.00399347.970.00479249.34
811007.5150.002432852.280.00285161.38
92407.5350.001842654.690.00205155.36
1026010250.00276451.170.00315651.84
112802.5150.002406752.370.00282155.36
122100550.003967348.030.00462249.34
1334010150.001741155.180.00198855.29
143607.550.001321657.580.001561.31
153805350.002837250.940.00335451.77
1631002.5250.001772555.030.00198261.31
Mean volumetric wear rate (mm3/20,000 cycles)0.0026570.00354
Figure 9: Effect of control factors on the wear rate of dental composites.
Figure 9:

Effect of control factors on the wear rate of dental composites.

3.3 Surface morphology

The morphology of the worn surface of the nanoalumina filled dental composites studied under steady state conditions and Taguchi orthogonal (L16) array design using FE-SEM is presented in Figures 10–13. All the wear analysis was conducted under limited number of cycles, i.e. 20,000 cycles per test sample.

Figure 10: FE-SEM images of worn surfaces of dental composite without nanoalumina under the chewing speed of 5 mm/s, temperature of 25°C and varying normal load and medium of (a) 40 N, food slurry; (b) 100 N, food slurry; (c) 40 N, distilled water; and (d) 100 N, distilled water.
Figure 10:

FE-SEM images of worn surfaces of dental composite without nanoalumina under the chewing speed of 5 mm/s, temperature of 25°C and varying normal load and medium of (a) 40 N, food slurry; (b) 100 N, food slurry; (c) 40 N, distilled water; and (d) 100 N, distilled water.

Figure 11: FE-SEM images of worn surfaces of composite filled with 1 wt.% nanoalumina under the normal load of 40 N and temperature of 25°C, and the varying chewing speed of (a) 2.5 mm/s, food slurry; (b) 10 mm/s, food slurry; (c) 2.5 mm/s, distilled water; and (d) 10 mm/s, distilled water.
Figure 11:

FE-SEM images of worn surfaces of composite filled with 1 wt.% nanoalumina under the normal load of 40 N and temperature of 25°C, and the varying chewing speed of (a) 2.5 mm/s, food slurry; (b) 10 mm/s, food slurry; (c) 2.5 mm/s, distilled water; and (d) 10 mm/s, distilled water.

Figure 12: FE-SEM images of worn surfaces of composite filled with 2 wt.% nanoalumina under normal load of 40 N, chewing speed of 5 mm/s and varying chamber temperature of (a) 5°C, food slurry; (b) 35°C, food slurry; (c) 5°C, distilled water; and (d) 35°C, distilled water.
Figure 12:

FE-SEM images of worn surfaces of composite filled with 2 wt.% nanoalumina under normal load of 40 N, chewing speed of 5 mm/s and varying chamber temperature of (a) 5°C, food slurry; (b) 35°C, food slurry; (c) 5°C, distilled water; and (d) 35°C, distilled water.

Figure 13: FE-SEM images of worn surfaces of composite filled with 2 wt.% nanoalumina under Taguchi experimental design: (a) Expt. 15, food slurry; (b) Expt. 16, food slurry; (c) Expt. 15, distilled water; and (d) Expt. 16, distilled water.
Figure 13:

FE-SEM images of worn surfaces of composite filled with 2 wt.% nanoalumina under Taguchi experimental design: (a) Expt. 15, food slurry; (b) Expt. 16, food slurry; (c) Expt. 15, distilled water; and (d) Expt. 16, distilled water.

3.3.1 Micrograph for varying normal load

The effect of variation in the normal load on the wear behavior of unfilled dental composite was studied in Figure 10. Figure 10 consists of four micrographs and indicates the effect of variation of load from 40 N to 100 N in food slurry and distilled water medium under constant chewing speed of 5 mm/s and chamber temperature of 35°C. Figure 10a and c represent the worn surfaces morphology of unfilled dental composite under variable normal load of 40 N and 100 N, respectively, at constant chewing speed of 5 mm/s and chamber temperature of 35°C under food slurry condition. Figure 10c and d represent the worn surfaces morphology of unfilled dental composite under variable normal load of 40 N and 100 N, respectively, at constant chewing speed of 5 mm/s and chamber temperature of 35°C under distilled water condition. A micrograph for unfilled dental composite under 40 N normal load is shown in Figure 10a. Here microploughing can be seen due to three-body abrasion; a crack was generated but still wear was less and the surface was rough. When the load increased and reached the maximum value of 100 N (Figure 10b), a fracture on the surface started to be visible. A micrograph for unfilled dental composite under 40 N normal load in distilled water is shown in Figure 10c. It clearly indicates larger cracks and wear scars. It reveals that the same composition in distilled water medium has shown more wear than in food slurry. Further increase in load to 100 N (Figure 10d) led to catastrophic failure.

3.3.2 Micrograph for varying chewing speed

Figure 11 shows worn surfaces of the composite filled with 1 wt.% nanoalumina (NADC-1) under the variable chewing speed of 2.5 mm/s and 10 mm/s under food slurry and distilled water medium while normal load at 40 N and chamber temperature at 35°C were kept constant. It can be seen that this NADC-1 composite showed less wear rate than the unfilled composite but still has blow holes, scratches, wear scars and cracks in the filled composite.

Figure 11a shows the micrograph of 1 wt.% nanoalumina filled dental composite studied under chewing speed of 2.5 mm/s in food slurry. At 2.5 mm/s chewing speed, wear was less, but wear track direction was visible and a few voids and pores were also present. However, with the increase in chewing speed to 10 mm/s while keeping other factors constant, the wear rate was found to be slightly higher as shown in Figure 11b. This may be due to the fact that at higher chewing speed, three-body abrasion by food slurry may play a more important role in wear mechanism. The formation of voids and cracks may be due to improper binding between the matrix material and nanoalumina particulates at higher chewing speed. Figure 11c indicates that for the same composite (NADC-1) in different medium like distilled water, the wear rate was higher than that in food slurry at similar conditions. Lot of grooves and wear track were observed on the surface of the dental composites. With further increase in chewing speed to 10 mm/s (Figure 11d), there was delamination of matrix and filler interface. The surface was completely deformed with higher wear rate and was in agreement with Figure 6 (steady state condition graph). Hence, from both the steady state condition and worn surface morphology, it can be concluded that the chewing speed also played a major role to increase the wear rate of the dental composite. It will be verified in the ANOVA analysis. This micrograph can also illustrate the wear phenomenon. After running 20,000 cycles, the matrix becomes worn and fillers are getting exposed, extended from resin and can be observed to be present on the surface. The new layers come into picture and the process repeats [38].

3.3.3 Micrograph for varying chamber temperature

Figure 12 indicates the worn surfaces of dental composite filled with 2 wt.% nanoalumina (NADC-2) under different chamber temperatures of 5°C and 35°C in the medium of food slurry and distilled water, while other parameters like normal load at 40 N and chewing speed of 5 mm/s were kept constant. NADC-2 has shown less wear than unfilled and 1 wt.% filled dental composite. Scratches were generated and wear debris was also visible. Some partially exposed filler particles can be observed on the worn surface. Figure 12a indicates that at 5°C chamber temperature, wear was very less and some scratches were present. When temperature was increased to 35°C, cracks started to be visible (Figure 12b). Plowing grooves and slight scratching caused by sharp abrasive particles was present on the surface. The micrographs shown in Figure 12c and d indicate the worn surfaces at temperatures of 5°C and 35°C under distilled water medium. It can be seen that more cracks and voids were present in the case of distilled water medium. Figure 12d also indicates that more wear scars were found on the surface.

3.3.4 Micrographs for Taguchi experiments

Figure 13 shows worn surfaces of composites filled with 3 wt.% nanoalumina (NADC-3) under food slurry and distilled water medium as per Taguchi experimental design during experiments number 15 and number 16 in Table 3. In these experiments, load, speeds and temperatures were simultaneously applied and varied. Figure 13A indicates the micrograph for the experiment performed under high load of 80 N, speed of 5 mm/s and chamber temperature of 35°C. Therefore, with the increase in both the load and speed, the composite surface was completely damaged as indicated in Figure 13a. Removal of particle was due to applied frictional and shear thrust force developed during high chewing speed. Figure 13b shows the micrograph for the experiment performed under chewing speed of 2.5 mm/s, load of 100 N and chamber temperature of 25°C. Here load was increased to maximum of 100 N, but the speed was decreased to minimum of 20 N. There were several long cracks extending through the polymer matrix along filler boundaries, microploughing and some damaged surface. It indicates that load played a more significant role than in the case of three-body abrasive wear. Figure 12c indicated the micrograph for the experiment that was performed under high load of 80 N, speed of 5 mm/s and chamber temperature of 35°C in distilled water medium. Severe damages were present. Void and pores were visible. Figure 12d shows the micrograph for the experiment performed under high chewing speed of 2.5 mm/s, normal load of 100 N and chamber temperature of 25°C; interlayer debonding due to fractured surface occurred.

3.4 ANOVA analysis

After the experiments were performed as per Taguchi (L16) orthogonal array experimental design, the statistical significance of various factors like filler contents, normal load, chewing speed, chamber temperature on volumetric wear rates of dental composite was described by analysis of variance (ANOVA). The level of confidence of significance for ANOVA was 5%. ANOVA is presented in Table 4. The last column of Table 4 indicates the percentage contribution (level of significance) of each factor. From Table 4, the statistical significance of factors (in descending order) is observed as follows: filler content (p=58%), normal load (p=24%), chewing speed (p=17%) and chamber temperature (p=0.3%). Hence, filler content, normal load and chewing speed indicated significant effect on volumetric wear rate of the filled composites. However, chamber temperature indicated less significant contribution on volumetric wear rate of the composites. Therefore, it can be concluded that among all factors, filler content has the most significant effect on the wear performance of dental composites. The normal load and chewing speed influence the wear rate significantly. Chamber temperature has least significant influence on the wear performance of dental composite. Further, one-way ANOVA followed by post hoc Tukey’s HSD test showed that filler content significantly differ with each of normal load, chewing speed and chamber temperature. Normal load and chewing speed were statistically similar to each other (p>0.05) but different from chamber temperature (p<0.05).

Table 4:

ANOVA analysis for NADC series composite.

SourceDFSeq SSAdj SSAdj MSFP(%) P
Filler content (wt.%)3149.398149.39849.79974.140.00358
Normal load (N)361.86261.86230.7020.6210.00924
Chewing speed (%)343.84443.84421.7614.6150.01517
Chamber temperature (°C)30.8240.8240.410.2750.7590.3
Error32.0152.0150.6720.7
Total15257.943

  1. DF, degree of freedom; Seq SS, sequential sum of square; Adj SS, adjacent sum of square; Adj MS, adjacent sum of mean square; F, variance; P, percentage contribution of each factor in overall performance.

3.5 Confirmation experiment of proposed composites

The experiments performed as per Taguchi (L16) orthogonal array experimental design was verified by performing another experiment by taking a random set of factors. Here, a new set of factors A2B2C3D1 was taken to predict the wear rate in both the medium of food slurry and distilled water. The estimated S/N ratio for specific wear rates was calculated as Eq. (4).

(4)η=ηm+∑i=1n(η¯i−ηm)

where ηm is the mean S/N ratio at optimal level, n is the number of parameters that affect the quality characteristics and η̅1 is the mean S/N ratio for each parameter at optimal level.

Hence, for this predictive level of A2B2C3D1, Eq. (4) becomes Eq. (5)

(5)η=T¯+(A2¯−T¯)+(B2¯−T¯)+(C3¯−T¯)+(D1¯−T¯)

Where η is the predicted optimum value for dental composites and T̅=overall experimental average value of S/N ratio.

Table 5 indicates that the errors in both the cases were found to be 4.65% and 3.96%. These errors were within the limit of 5% as per Taguchi methodology.

Table 5:

Results of the confirmation experiments.

Optimum control parametersDental composite filled with silane treated nanoalumina in food slurryDental composite filled with silane treated nanoalumina in distilled water
PredictionExperimentalErrorPredictionExperimentalError
LevelA2B2C3D1A2B2C3D1%A2B2C3D1A2B2C3D1%
S/N ratio for wear rate (db)54.7452.194.6560.2957.903.96

4 Conclusions

Dental composites reinforced with silane treated nanoalumina have been fabricated and characterized. The dental composite filled with 3 wt.% silane treated nanoalumina exhibited the maximum hardness and minimum volumetric wear rate than the other experimental composite. The results also indicated that the wear rate of dental composite in distilled water (i.e. two-body abrasion) was 33.23% more than that of the same composite in food slurry condition (i.e. three-body abrasion). In steady state condition, the wear rate increased with increase in each of the factors of normal load and chewing speed, but with chamber temperature, the wear rate increased up to 25°C and then further decreased. As per Taguchi experimental design, the optimum parametric selection for the minimum wear of dental material was filler content of 3 wt.% nanoalumina, normal load of 40 N, chewing speed of 2.5 mm/s and chamber temperature of 15°C. Finally, ANOVA indicated that filler content has the most significant effect on the wear performance of dental composites. Normal load and chewing speed were statistically similar to each other (p>0.05) but different from chamber temperature (p<0.05).

Corresponding author: Shiv Ranjan Kumar, Associate Professor, Mechanical Engineering Department, JECRC University, Jaipur, India, Tel.: +91-8233684418

Acknowledgments

The authors gratefully acknowledge the financial support for this study by the CDOS, Jaipur (grant number: 0290032/2013), Advanced Material Research Centre and Advanced Research Centre for Tribology for providing experimental support to complete the research work smoothly.

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Received: 2016-6-10
Accepted: 2016-9-13
Published Online: 2016-10-18
Published in Print: 2018-4-25

©2018 Walter de Gruyter GmbH, Berlin/Boston

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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https://doi.org/10.1515/secm-2016-0175
Keywords for this article
dental composite; nanoalumina; scanning electron microscopy; wear
Creative Commons
BY-NC-ND 3.0

Articles in the same Issue

  1. Frontmatter
  2. Original articles
  3. Review of the mechanical performance of variable stiffness design fiber-reinforced composites
  4. Exact solution for bending analysis of functionally graded micro-plates based on strain gradient theory
  5. Synthesis, microstructure, and mechanical properties of in situ TiB2/Al-4.5Cu composites
  6. Microstructure and properties of W-Cu/1Cr18Ni9 steel brazed joint with different Ni-based filler metals
  7. Drilling studies on the prepared aluminum metal matrix composite from wet grinder stone dust particles
  8. Studies on mechanical properties of thermoplastic composites prepared from flax-polypropylene needle punched nonwovens
  9. Design of and with thin-ply non-crimp fabric as building blocks for composites
  10. Effect of coir fiber reinforcement on mechanical properties of vulcanized natural rubber composites
  11. Investigation and analysis of glass fabric/PVC composite laminates processing parameters
  12. Abrasive wear behavior of silane treated nanoalumina filled dental composite under food slurry and distilled water condition
  13. Finite element study into the effects of fiber orientations and stacking sequence on drilling induced delamination in CFRP/Al stack
  14. Preparation of PAA/WO3 composite films with enhanced electrochromism via layer-by-layer method
  15. Effect of alkali treatment on hair fiber as reinforcement of HDPE composites: mechanical properties and water absorption behavior
  16. Integration of nano-Al with one-step synthesis of MoO3 nanobelts to realize high exothermic nanothermite
  17. A time-of-flight revising approach to improve the image quality of Lamb wave tomography for the detection of defects in composite panels
  18. The simulation of the warpage rule of the thin-walled part of polypropylene composite based on the coupling effect of mold deformation and injection molding process
  19. Novel preparation method and the characterization of polyurethane-acrylate/ nano-SiO2 emulsions
  20. Microwave properties of natural rubber based composites containing carbon black-magnetite hybrid fillers
  21. Simulation on impact response of FMLs: effect of fiber stacking sequence, thickness, and incident angle
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