To determine by nanoindentation the hardness and elastic modulus of resin-composites, including a series with systematically varied filler loading, plus other representative materials that fall into the categories of flowable, bulk-fill and conventional nano-hybrid types.
Ten dental resin-composites: three flowable, three bulk-fill and four conventional were investigated using nanoindentation. Disc specimens (15 mm × 2 mm) were prepared from each material using a metallic mold. Specimens were irradiated in the mold at top and bottom surfaces in multiple overlapping points (40 s each) with light curing unit at 650 mW/cm 2 . Specimens were then mounted in 3 cm diameter phenolic ring forms and embedded in a self-curing polystyrene resin. After grinding and polishing, specimens were stored in distilled water at 37 °C for 7 days. Specimens were investigated using an Agilent Technologies XP nanoindenter equipped with a Berkovich diamond tip (100 nm radius). Each specimen was loaded at one loading rate and three different unloading rates (at room temperature) with thirty indentations, per unloading rate. The maximum load applied by the nanoindenter to examine the specimens was 10 mN.
Dependent on the type of the resin-composite material, the mean values ranged from 0.73 GPa to 1.60 GPa for nanohardness and from 14.44 GPa to 24.07 GPa for elastic modulus. There was a significant positive non-linear correlation between elastic modulus and nanohardness ( r 2 = 0.88). Nonlinear regression revealed a significant positive correlation ( r 2 = 0.62) between elastic moduli and filler loading and a non-significant correlation ( r 2 = 0.50) between nanohardness and filler loading of the studied materials. Varying the unloading rates showed no consistent effect on the elastic modulus and nanohardness of the studied materials.
For a specific resin matrix, both elastic moduli and nanohardness correlated positively with filler loading. For the resin-composites investigated, the group-average elastic moduli and nanohardnesses for bulk-fill and flowable materials were lower than those for conventional nano-hybrid composites.
Indentation is a versatile measurement procedure because of its relative ease of use. Advances in instrumentation have made indentation a useful research tool for many different systems across size scales (macro to nano) and numerous scientific disciplines . Reasons for its popularity are the relative simplicity of specimen preparation and that a single specimen can be indented several times. Different volumes of materials can be examined by a suitable choice of tip geometry and load. Also instrument developments have made possible: (a) the application of forces from kilo-Newtons down to pico-Newtons and (b) measurement of displacements down to nanometers. Any solid can be indented, including bulk materials, biological entities and nanostructures .
Nanoindentation for characterising mechanical properties of tooth tissues and materials has been reported in the dental literature . Instrumented nanoindentation is an accepted technique for determining the local mechanical properties of a material from its measured indentation load–displacement response . This involves indenting a specimen with a small load and recording the load and displacement continuously . Nanoindentation allows accurate determination and control of the indentation force as well as accurate measurement of the indentation depth . Nanoindentation is different from other conventional methods as there is no need to image the indentation area to determine the mechanical properties (e.g., hardness). Instead, these can be obtained directly from the indentation load and displacement measurements .
However, some problems may arise with nanoindentation investigations. Time-dependent responses may lead to inaccuracy in interpretation of load–displacement traces, which is one of the major obstacles to the use of depth-sensing indentation (DSI). This can be obvious in the case of soft, polymeric or biological materials . Also, at micro and nano scales, indentation size effects (ISE) have a major influence on test results; that is, different sizes of indenter may lead to different results .
Moreover, although nanoindentation techniques may be useful for characterising homogenous materials, they are inappropriate for determining bulk properties of biphasic materials such as dental resin-composites. The reinforcing filler particles used in these materials may be larger than the nanoindenter tip. In this situation, the measurements obtained may reflect only the mechanical properties of the fillers or matrix not the combined material . Masouras et al. reported that the small load applied and the limited depth of penetration provides little information about the bulk of the resin-composites. However, they suggested that the deeper the indentation, the more representative are the elastic moduli. Increasing the indenter’s projected area, with depth, ensures the inclusion of a wider material area potentially containing both fillers and resin matrix.
Most current nanoindentation instruments function as load and displacement sensing systems in which information is derived from the penetration of the indenter on loading and also from the elastic recovery of the specimen on unloading . A diamond indenter with an equilateral triangular base (Berkovich indenter) is the most common indenter used. The indentation is normally carried out in a continuous load/unload cycle .
Elastic modulus, E , and hardness, H , are the two mechanical properties most commonly measured using load and depth-sensing indentation techniques . These are amongst the several mechanical properties that can express the resistance of a material to occlusal forces. Hardness is the resistance of a material to indentation or penetration. It has been used to predict the wear resistance of a material and its ability to abrade or be abraded by the opposing tooth structure and materials . Because of the relationship between hardness and other physical properties, hardness is important in characterising and ranking dental restorative materials .
Elastic modulus describes the relative stiffness of a material. Together with adhesive properties, elastic modulus is an important factor in microleakage, secondary caries and filling dislodgement . In stress-bearing occlusal areas, materials with low modulus deform more under masticatory stresses and may cause catastrophic failure. High elastic modulus is required to withstand deformation and cuspal fracture . Elastic modulus thus has a relationship to the reliability of restorative dental materials . Particularly when placed in posterior regions of the dentition, restorative materials should have an adequate elastic modulus to resist deformation under masticatory stresses. The most appropriate elastic modulus could be one comparable to that of dentin . Hence, knowledge of elastic moduli is important in understanding the clinical behavior of many biomaterials .
Flowable resin-composites, developed from the mid-1990s , can be easily injected through small-gauge dispensers, rendering the placement of a filling resin-composite into the tooth cavity easier and widening the range of possible clinical applications . Flowable resin-composites, initially developed for Class V cavities, are now being incorporated into small carious lesions, pit and fissure sealants, liners, crown cementation, and other adhesive restorative procedures . Moreover, flowable resin-composites may be used for resurfacing resin-composite or glass ionomer restorations or for rebuilding worn resin-composite contact areas . However, concern has been raised regarding their relatively inferior mechanical properties compared with conventional materials and their use in high-stress applications has been discouraged . The strength and modulus of several commercial flowable composites can be about 60–90% of those of conventional resin-composites .
Bulk-fill resin-composites are available that may be applied in thicknesses of 4 mm with enhanced curing and controlled shrinkage thus reducing the time of the treatment session.
The aim of this study was to evaluate the nanohardness and elastic modulus of several flowable and bulk-fill resin-composites and to compare them with other conventional resin-composites using nanoindentation. The main test hypothesis was: for a specific dimethacrylate resin matrix, the elastic modulus and nanohardness increase with filler loading. The null hypotheses were: (i) there will be no difference in the elastic moduli and nanohardness between all the investigated resin-composites; (ii) there will be no correlation between the elastic moduli and the nanohardness; and (iii) varying the unloading rates will have no effect on the indentation moduli.
Materials and methods
Ten dental resin-composites: three flowable, three bulk-fill and four conventional were investigated in this study. Three of these materials (GF, GH and GS) have the same resin matrix but increasing filler fraction (80–89%, w/w). The material’s descriptions and manufacturers’ information are listed in Table 1 .
|Product||Code||Type||Manufacturer||Lot No.||Resin system||Filler (wt%)|
|GrandioSo Flow||GF||Flowable||Voco, Cuxhaven, Germany||1104372||Bis-GMA, Bis-EMA, TEGDMA||80%|
|GrandioSo Heavy Flow||GH||Flowable||Voco, Cuxhaven, Germany||1051279||Bis-GMA, Bis-EMA, TEGDMA||83%|
|GrandioSo||GS||Conventional||Voco, Cuxhaven, Germany||1048014||Bis-GMA, Bis-EMA, TEGDMA||89%|
|Venus Diamond||VD||Conventional||Heraeus Kulzer GmbH, Hanau, Germany||010037||TCD, di-HEA, UDMA||80%|
|Filtek Supreme XTE||FS||Conventional||3M ESPE, St. Paul, MN, USA||N214152||Bis-GMA, UDMA, TEGDMA, PEGDMA, Bis-EMA||79%|
|Spectrum TPH3||SP||Conventional||Dentsply Detrey GmbH, Konstanz, Germany||1007002359||Bis-GMA, Bis-EMA, TEGDMA||–|
|X-tra Base||EX||Bulk-fill||Voco, Cuxhaven, Germany||V 45252||MMA, Bis-EMA||75%|
|Tetric EvoCeram Bulk Fill||TE||Bulk-fill||Ivoclar Vivadent, Schaan, Liechtenstein||PM0213||Dimethacrylate co-monomers||80%|
|SureFil SDR||SF||Bulk-fill||Dentsply Caulk, Milford, Delaware, USA||1003011||EBPADMA, TEGDMA||68%|
|Estelite Flow Quick||ES||Flowable||Tokuyama Dental Corporation, Japan||E646B||Bis-MPEPP, TEGDMA, UDMA||71%|
Disc specimens (15 mm × 2 mm) were prepared (one specimen per material) by first injecting the resin-composite material into a metallic mold. Two microscope glass slides, covered with transparent polystyrene matrix strips, were used to compress the material in the mold. The specimens were irradiated in multiple overlapping points (40 s each) to ensure optimal polymerisation with a halogen light curing unit (Optilux 501, Kerr, USA), with light irradiance at 650 mW/cm 2 , as measured with a calibrated radiometer. After irradiation, the specimens were mounted in 3 cm diameter phenolic ring molds (Buehler, Coventry, UK) and embedded in a self-curing polystyrene resin (Castoglas Resin, Buehler, Coventry, UK).
Once the polystyrene resin had set, the embedded specimens were hand-ground with a sequence of SiC abrasive papers of decreasing abrasiveness (P240, P400, P800, P1200, P2500-grit) under continuous water cooling. Specimens were further polished (Buehler, Coventry, UK) using a sequence of felt cloths (Whitefelt, Buehler, Coventry, UK) with 6, 1 and 0.25 μm embedded diamond, Al 2 O 3 and SiO 2 grains under periodical lubrication with an oil based lubricant (Metadi, Buehler, Coventry, UK). Each step was carried out for 3 min with light pressure applied to the specimen against the felt cloths of the polishing machine. The specimens were ultrasonically cleaned in distilled water for 3 min using an ultrasonic bath (Nusonic, Transsonic T310, Germany) and then stored in distilled water at 37 °C for 7 days.
Nanoindentation measurements were performed at 23 °C using an Agilent Technologies XP nanoindenter (Agilent Technologies Co., Nano Instruments, Oak Ridge, TN, USA) equipped with a Berkovich (three-sided pyramidal) diamond tip (100 nm radius). The load and displacement resolution of the instrument were 50 nN and 0.01 nm, respectively. Calibration indents were made on fused silica with an elastic modulus of 73 GPa and Poisson’s ratio of 0.17.
Each specimen was loaded at one loading rate and three different unloading rates ( Table 2 ). Thirty indentations for every loading/unloading rate were performed on randomly selected areas on each specimen. The indents were located 30 μm apart to avoid the influence of residual stresses from adjacent impressions. Examination of specimens was undertaken at room temperature and began once thermal drift had been stabilised below 0.05 nm/s. The maximum load applied by the nanoindenter to examine the specimens was 10 mN. Poisson’s ratio for all resin-composites was assumed to be 0.3 . Each testing cycle consisted of four segments ( Fig. 6 ): (a) the loading segment, (b) the peak load holding segment, (c) the unloading segment and (d) the holding segment at 10% of the maximum load. The initial contact of the indenter with the specimen surface was identified by change in stiffness. The data used for the calculation of the elastic modulus and nanohardness were those obtained over a depth range from 100 to 2000 nm.
|Experiments||Time to load (s)||Hold time (s)||Time to unload (s)||Maximum load (mN)||Loading rate (mN/s)||Unloading rate (mN/s)|
Data collection and analysis were conducted using the Oliver and Pharr method . The elastic modulus was calculated based on the slope of the upper portion of the unloading curve from the load–displacement data . The modulus obtained from analysis of the raw data (load–displacement curve, Fig. 6 ) incorporated stiffness of the specimen and the indenter. This is usually referred to as the “reduced or combined” modulus . The relationship of the reduced modulus to the elastic modulus of the material is given by the following formula:
1 E = 1 − v 2 E + 1 − v i n d 2 E i n d
where E * is the reduced modulus, v and E are the Poisson’s ratio and elastic modulus of the material, respectively and v ind and E ind are the Poisson’s ratio (0.07) and elastic modulus (1141 GPa) of the indenter, respectively . The elastic moduli data were presented as the mean of ninety indentations. Also, to investigate the effect of varying the unloading rate on the elastic moduli, the mean data for each unloading rate (30 indentations) were calculated separately.
Nanohardness was obtained from depth-sensing indentation instruments according to the following formula :
H = P A P