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J Dent Res 83(12):930-935, 2004
© 2004 International and American Associations for Dental Research

RESEARCH REPORTS
Biomaterials & Bioengineering

Wear and Mechanical Properties of Nano-silica-fused Whisker Composites

H.H. K. Xu*, J.B. Quinn, and A.A. Giuseppetti

Paffenbarger Research Center, American Dental Association Foundation, National Institute of Standards and Technology, Building 224, Room A-153, Stop 8546, Gaithersburg, MD 20899-8546, USA;

* corresponding author, hockin.xu{at}nist.gov


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Resin composites must be improved if they are to overcome the high failure rates in large stress-bearing posterior restorations. This study aimed to improve wear resistance via nano-silica-fused whiskers. It was hypothesized that nano-silica-fused whiskers would significantly improve composite mechanical properties and wear resistance. Nano-silicas were fused onto whiskers and incorporated into a resin at mass fractions of 0%-74%. Fracture toughness (mean ± SD; n = 6) was 2.92 ± 0.14 MPa•m1/2 for whisker composite with 74% fillers, higher than 1.13 ± 0.19 MPa•m1/2 for a prosthetic control, and 0.95 ± 0.11 MPa•m1/2 for an inlay/onlay control (Tukey’s at 0.95). A whisker composite with 74% fillers had a wear depth of 77.7 ± 6.9 µm, less than 118.0 ± 23.8 µm of an inlay/onlay control, and 172.5 ± 15.4 µm of a prosthetic control (p < 0.05). Linear correlations were established between wear and hardness, modulus, strength, and toughness, with R = 0.95–0.97. Novel nano-silica-fused whisker composites possessed high toughness and wear resistance with smooth worn surfaces, and may be useful in large stress-bearing restorations.

KEY WORDS: dental resin composite • nano-silica-fused whiskers • wear • mechanical properties • mechanisms


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The performance of a dental composite depends on filler type, resin composition, filler-matrix bonding, and cure conditions (Wendt, 1987; Pallav et al., 1989; Ferracane et al., 1998; Watts and Hindi, 1999; Lim et al., 2002). Short fibers (Krause et al., 1989) and networked fibers (Ruddell et al., 2002) have been used to reinforce composites, resulting in modest strength increases. The degree of polymerization conversion has been increased via heat-treatment, leading to moderate strength increases (Loza-Herrero et al., 1998). Further improvements are needed for composites to overcome brittle fracture and high failure rates in large stress-bearing restorations (Tyas, 1990; Christensen, 1999; Donly et al., 1999), especially those that involve the replacement of tooth cusps.

Recently, nano-silica-fused whiskers were incorporated into resins, resulting in strength increases (Xu, 2000; Xu et al., 2000). Silica was fused onto whiskers to enhance silanization and retention in the resin by roughening the whisker surfaces. Besides strength, occlusal wear resistance is also a major requirement for the longevity of restorations (Pallav et al., 1989; Bayne et al., 1992; Peutzfeldt and Asmussen, 1992; Wassell et al., 1994; Leinfelder and Suzuki, 1999; Manhart et al., 2000). Wear tests have been developed to simulate in vivo wear (Delong et al., 1985; Sakaguchi et al., 1986; Suzuki et al., 1996; Lim et al., 2002). Three-body wear with artificial food slurries produced data that corresponded well with clinical results (de Gee et al., 1996; Condon and Ferracane, 1997; Leinfelder and Suzuki, 1999; Xu et al., 1999). Filler levels, filler treatments, and degrees of cure have been shown to influence wear (Condon and Ferracane, 1997; Lim et al., 2002). Fine fillers have been used to reduce inter-particle spacing to improve wear resistance (Pallav et al., 1989; Bayne et al., 1992).

The aim of this study was to investigate the effects of nano-silica-fused whisker filler level on composite wear, and to examine the relationships between wear and mechanical properties. It was hypothesized that increasing nano-silica-fused whisker fillers would increase the composite wear resistance, and that wear rate would decrease when composite mechanical properties (hardness, elastic modulus, flexural strength, and fracture toughness) were increased.


   MATERIALS & METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Specimen Fabrication
Nano-silica (Degussa, Ridgefield, NJ, USA) having particles of 60 nm to 120 nm (mean = 80 nm) was used. Silicon nitride whiskers (UBE, New York, NY, USA), with diameters ranging from 0.1 µm to 2 µm (mean = 0.4 µm) and lengths from 2 µm to 30 µm (mean = 5 µm), were used. Whiskers were mixed with silica at a whisker:silica mass ratio = 2:1 by being stirred in ethyl alcohol on a hot plate until dry. The mixed powder was heated in a furnace at 800°C for 30 min to fuse the nano-silica onto the whiskers (Xu, 2000). The powder was silanized with mass fractions of 4% 3-methacryloxypropyltrimethoxysilane and 2% n-propylamine in cyclohexane in a rotary evaporator. The silanized powder was mixed with a resin monomer of mass fractions of 48.965% of an oligomeric urethane derivative of Bis-GMA (Caulk/Dentsply, Milford, DE, USA), 48.965% triethylene glycol dimethacrylate (TEGDMA), 2% benzoyl peroxide, and 0.07% 4-methoxylphenol. Filler mass fractions (%) of 0, 20, 40, 60, 70, and 74 were used following recommendations in previous studies (Xu, 2000; Xu et al., 2000). A previous study showed that composite with 70% and 74% fillers possessed high strengths, while the paste with 79% fillers was dry and the specimens had a lower strength (Xu, 2000). For fracture toughness, the paste was placed into steel molds of 2 x 2 x 25 mm3 and heat-cured at 120°C for 30 min for indirect applications, because the specimens were too opaque to be light-cured. For wear-testing, the paste was placed in molds of 4 mm diameter and 3 mm depth and cured in the same manner.

An indirect inlay/onlay composite (Concept, Ivoclar, Amherst, NY, USA), referred to as inlay/onlay control, was cured in the Concept Heat-Integrated Processor at 120°C for 10 min under a pressure of 0.6 MPa. Concept consisted of 53–56% mass fraction of 40 nm silica and 20% radiopaque fillers, for a total of 73–76% in a urethanedimethacrylate resin. An indirect prosthetic composite (Artglass, Heraeus Kulzer GmbH, Wehrheim, Germany), referred to as prosthetic control, was cured in a Dentacolor-XS photo-curing unit for 90 sec. Artglass contained 70% barium-aluminum-silicate (mean particle size = 1 µm) in a resin with tetra- and hexa-functional groups and conventional bi-functional methacrylates.

Testing
Fracture toughness was measured by the use of a single-edge-V-notched beam method that has been extensively studied in a ‘round robin’ commissioned by the Versailles Project on Advanced Materials and Standards (VAMAS) and is currently an ISO draft (Kübler, 1999). This method has been used to measure the fracture toughness of dental materials (Scherrer et al., 1998; Quinn et al., 2003). A notch depth of approximately 500 µm was machined into a specimen by means of a 150-µm-thick diamond blade. Diamond paste of 3 µm was placed into the notch tip, and a sharp blade was used to cut the notch further to a total depth of 700–800 µm with a relatively sharp notch tip. We cut 5 specimens simultaneously by mounting bars side by side, sandwiched between 2 bars of the same material (dummy bars). We used the dummy bars to avoid chipping at groove entry and exit points, and to aid in maintaining an even notch depth (Kübler, 1999). It took several hours of alternating cutting and checking under an optical microscope for each group of specimens. A new blade was used for each specimen group. The sharpness of the notch was deemed sufficient when the tip was less than 20 µm in diameter (Kübler, 1999). For each specimen, the notch length was measured on both sides and averaged. The notched specimen was fractured on a computer-controlled Universal Testing Machine (model 5500R, Instron, Canton, MA, USA) in three-point flexure with a 10-mm span at 1 mm/min cross-head speed (Xu, 2000). Forty-eight specimens were tested for the 6 filler levels and 2 controls with 6 repeats each.

Wear specimens were tested in a four-station apparatus (Caulk/Dentsply, Milford, DE, USA) (Suzuki et al., 1996; Xu et al., 1999). Each specimen was surrounded by a brass ring filled with a water slurry, 63% of which was comprised of polymethyl methacrylate (PMMA) beads (mean particle size = 44 µm). A carbide steel pin with 3-mm tip diameter was loaded onto the specimen submerged in the PMMA slurry. The pin was pressed down against the PMMA particles on the specimen and rotated 30°. Upon reaching a maximum load of 76 N, the pin was counter-rotated during unloading and moved upward back to its original position. Each specimen was subjected to 400,000 wear cycles. Forty-eight specimens were tested for the 6 filler levels and 2 controls with 6 repeats. The sizes and depths of the wear scars were measured with the use of a computer-controlled profilometer (Mahr, Cincinnati, OH, USA) with a 5-µm diamond stylus. For each wear scar, profilometric tracings were made at intervals of 50 µm in 2 directions perpendicular to each other, with the unworn surface as baseline. The maximum values in the 2 perpendicular directions were averaged to yield the maximum depth and diameter for each wear scar (Xu et al., 1999).

The worn specimens were gold-coated and observed in a scanning electron microscope (SEM, JSM-5300, JEOL, Peabody, MA, USA). The data were analyzed by one-way ANOVA and Tukey’s Multiple Comparison procedures ({alpha} = 0.05).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Nano-sized silica particles were fused onto the whiskers at 800°C (Fig. 1AGo). Fracture toughness increased with filler level (Fig. 1BGo). Fracture toughness (mean ± SD; n = 6) at 74% silica-fused whiskers was (2.92 ± 0.14) MPa•m1/2, significantly higher than those from 0% to 40%, 1.13 ± 0.19 MPa•m1/2 for the prosthetic control, and 0.95 ± 0.11 MPa•m1/2 for the inlay/onlay control (Tukey’s at 0.95).



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  		Figure 1. Nano-silica-fused whisker, fracture toughness, and a notch. In (A), the nano-silica particles (S) are indicated by the arrow, and ‘W’ designates the whisker. In (B), each value is the mean of 6 measurements, with the error bar showing 1 standard deviation (SD) (mean ± SD; n = 6). Dissimilar letters indicate values that are significantly different (Tukey’s multiple-comparison test; family confidence coefficient = 0.95). In (C), optical micrograph shows an example of a notch in unfilled resin showing the sharpness of the notch tip produced by the single-edge-V-notched beam method.

 
Wear scar depth, diameter, and volume decreased significantly (one-way ANOVA; p < 0.001) with increasing filler level (Fig. 2Go). In (A), whisker composite with 74% fillers had a wear depth of 77.7 ± 6.9 µm, significantly less than the depth of 118.0 ± 23.8 µm for the inlay/onlay control and 172.5 ± 15.4 µm for the prosthetic control (Tukey’s at 0.95). In (B), wear scar diameter of a whisker composite with 74% fillers was 742 ± 46 µm, not significantly different from that of 878 ± 165 µm for the inlay/onlay control; both were less than that of 1184 ± 34 µm for the prosthetic control (p < 0.05). We estimated wear volume in (C) by assuming a parabolic shape for the wear scar (Xu et al., 2004). Wear volume for the composite with 74% whiskers was 12.1 ± 3.2 (106 µm3), not significantly different from that of 29.3 ± 11.9 (106 µm3) for the inlay/onlay control; both were less than that of 91.2 ± 11.8 (106 µm3) for the prosthetic control (p < 0.05).



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  		Figure 2. Composite wear rates vs. nano-silica-fused whisker filler level. (A) Wear scar depth WD, (B) wear scar diameter WL, and (C) wear scar volume WV. Data for the prosthetic and inlay/onlay composite controls were included in each plot near the right axis. Each value is the mean of 6 measurements, with the error bar showing 1 standard deviation (mean ± SD; n = 6). The line through the data for the whisker composites was the linear best fit with the equation and the correlation coefficient R shown in each plot.

 
We examined SEM micrographs of the worn surfaces inside the wear scars for unfilled resin and for whisker composites with 60% and 74% fillers (Figs. 3AGo, 3BGo, 3CGo). Whisker composites (Figs. 3BGo, 3CGo) had smooth surfaces free of the cracks seen in unfilled resin. Such cracks were also absent in composite with 20% and 40% whiskers. Long protruding whiskers, observed in fracture surfaces (Xu, 2000), were not seen in the worn surfaces. Instead, the whiskers were worn down by microfracture even with the composite surface, with the whisker tip showing signs of wear by microfracture (lower arrows in Fig. 3DGo).



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  		Figure 3. Worn surfaces. SEM micrographs inside wear scars after 400,000 cycles for (A) unfilled resin (0 %), and (B) and (C) whisker composites at filler levels of 60% and 74%, respectively. Arrows in (A) point to microcracks in the worn surface of the unfilled resin. The whisker composites in (B) and (C) had relatively smooth surfaces inside the wear scars, free of cracks like those in the unfilled resin. A higher magnification in (D) for whisker composite at the 70% filler level shows a worn-down whisker with the tip exhibiting signs of wear by microfracture (lower arrows). The left arrow in (D) indicates that the whisker was firmly embedded in the resin matrix.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Increasing the nano-silica-fused whisker filler level improved the composite wear resistance. This is consistent with previous studies on wear and filler level (Bayne et al., 1992; Condon and Ferracane, 1997; Lim et al., 2002). The cracks in the wear scars of unfilled resin were also consistent with previous observations (Baran et al., 1998). Whisker composites with fillers from 20% to 74% had relatively smooth worn surfaces, free of cracks like those in unfilled resin. The transition from cracking to non-cracking likely occurred between filler levels of 0% and 20%. A similar transition from brittle behavior to a more tough behavior was observed in a previous study (Xu, 1999). The unfilled resin cracked more readily, creating a flat surface from fast fracture (Fig. 2BGo in Xu, 1999). In contrast, the whisker composite exhibited toughening and crack deflection, with accompanying fracture steps on the fractured surfaces at filler levels starting from 10% (Fig. 2CGo in Xu, 1999). In addition to increased fracture resistance, the whisker composite with 70% and 74% fillers exhibited wear depths about half those of the unfilled resin. The whisker composite was also more wear-resistant than commercial glass-filled controls at similar filler levels.

Comparison can also be made with dental amalgam, which is known for its resistance to occlusal wear and is taken as the standard by which newer restorative materials are judged. A previous study, in which the same operator used the same equipment, subjected amalgam (Dispersalloy, Dentsply, Milford, DE, USA) to 400,000 cycles of three-body wear, and measured a wear scar depth of 134 ± 54 µm and a diameter of 778 ± 270 µm (Xu et al., 1999). The whisker composite with 74% fillers had a wear scar depth of 77.7 ± 6.9 µm and a wear diameter of 742 ± 46 µm. Regarding the correlation between these in vitro wear values and clinical wear, a previous study reported on results with use of the same type of wear machine, compared with in vivo data (Leinfelder and Suzuki, 1999). These investigators found that the 400,000-cycle in vitro wear values agreed with the in vivo wear values over a three-year period. This is consistent with results from another study showing that a wear depth of 100–160 µm occurred for amalgam in 2–3 yrs (DeLong et al., 1985).

Wear of dental materials is a complex process involving fatigue, erosive, adhesive, abrasive, and corrosive components. Nevertheless, wear occurs via microfracture and material removal; hence it is inherently related to mechanical properties. McKinney et al.(1987) suggested that "wear does not necessarily vary in a manner consistent with the hardness." Pallav et al.(1989) found "the absence of a relationship between wear" and hardness or diametral tensile strength. Hardness of the nano-silica-fused whisker composites has been measured by nano-indentation (Xu et al., 2000). Contrary to previous reports, we found a good correlation between wear depth and hardness for whisker composites, with a correlation coefficient of R = 0.97 (Fig. 4AGo).



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  		Figure 4. Relationships between wear and mechanical properties. To understand the mechanisms of three-body wear of dental composites, we established linear best-fits between wear depth WD and (A) hardness H, (B) elastic modulus E, (C) flexural strength S, and (D) fracture toughness KIC of the nano-silica-fused whisker composites at filler level mass fractions ranging from 0% to 74%. Each value is the mean of 6 measurements, with the error bar showing 1 standard deviation (mean ± SD; n = 6).

 
Tyas (1990) found some correlation, but not significant correlation, between wear and elastic modulus. Peutzfeldt and Asmussen (1992) concluded that no correlation was established between wear and modulus of elasticity. For the whisker composites, with the wear depth measured here and elastic modulus from a previous study (Xu et al., 2000), a linear correlation was found between wear and elastic modulus, with R = 0.97 (Fig. 4BGo).

A linear correlation between wear and flexural strength was found previously (Peutzfeldt and Asmussen, 1992). A similar relationship was found for whisker composites (Fig. 4CGo), with strength from an earlier study (Xu, 2000).

Wear involves microfracture; hence, it is also expected to depend on fracture toughness (arrows in Figs. 3AGo, 3DGo). However, besides correlating fracture toughness with surface chipping and bulk fracture, little evidence was available on the relationship between fracture toughness and wear of dental composites (Tyas, 1990). Using the fracture toughness and wear depth data measured here, we obtained a correlation with R = 0.95 (Fig. 4DGo).

When the inlay/onlay and prosthetic controls were included in the fitting, the correlation coefficients between wear depth and hardness, elastic modulus, flexural strength, and fracture toughness fell to 0.86, 0.87, 0.74, and 0.78, respectively. This indicates that the resin composition and degree of cure may also have influenced wear. Further study should examine whether such relationships are unique to a single class of composites or can be generalized across classes of composites. The measured wear and mechanical properties depend on the measurement methods. For example, flexural strength may depend on the bending span and the loading rate. Therefore, while fundamental relationships may exist, one should not expect to produce relationships similar to those illustrated in Fig. 4Go by simply taking data in the literature of different materials measured in different laboratories.

In conclusion, novel nano-silica-fused whisker composites were developed with in vitro wear resistance higher than that of conventional glass-particle-filled composites and similar to that of dental amalgam. The wear surfaces of whisker composites were smooth and free of cracks. Linear correlations were established between composite wear and hardness, elastic modulus, flexural strength, and fracture toughness. Nano-silica-fused whisker composites with superior strength, fracture toughness, and wear resistance are relatively opaque and may be useful in large stress-bearing posterior restorations involving cusps and indirect applications. Further studies should match the refractive index of whiskers to that of the resin to improve the esthetics for anterior applications.


   ACKNOWLEDGMENTS
 
The authors thank Dr. Frederick C. Eichmiller for discussions, and Dr. Joseph M. Antonucci for providing the resin monomer. This study was supported by NIDCR grant R29 DE12476 (Xu), by NIST, and by the ADAF.

DISCLAIMER
Certain commercial materials and equipment are identified to specify the experimental procedure. This does not imply recommendation or endorsement by NIST or ADAF or that the material or equipment identified is necessarily the best available for the purpose. One standard deviation was given in this paper for comparative purposes as the estimated standard uncertainty of the measurements. These values should not be compared with data obtained in other laboratories under different conditions.

Received July 28, 2003; Last revision September 24, 2004; Accepted September 28, 2004


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 DISCUSSION
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