To evaluate nanoindentation as an experimental tool for characterizing the viscoelastic time-dependent creep of resin-composites and to compare the resulting parameters with those obtained by bulk compressive creep.
Ten dental resin-composites: five conventional, three bulk-fill and two flowable were investigated using both nanoindentation creep and bulk compressive creep methods. For nano creep, disc specimens (15 mm × 2 mm) were prepared from each material by first injecting the resin-composite paste into metallic molds. Specimens were irradiated from top and bottom surfaces in multiple overlapping points to ensure optimal polymerization using a visible light curing unit with output irradiance of 650 mW/cm 2 . Specimens then were mounted in 3 cm diameter phenolic ring forms and embedded in a self-curing polystyrene resin. Following grinding and polishing, specimens were stored in distilled water at 37 °C for 24 h. Using an Agilent Technologies XP nanoindenter equipped with a Berkovich diamond tip (100 nm radius), the nano creep was measured at a maximum load of 10 mN and the creep recovery was determined when each specimen was unloaded to 1 mN. For bulk compressive creep, stainless steel split molds (4 mm × 6 mm) were used to prepare cylindrical specimens which were thoroughly irradiated at 650 mW/cm 2 from multiple directions and stored in distilled water at 37 °C for 24 h. Specimens were loaded (20 MPa) for 2 h and unloaded for 2 h. One-way ANOVA, Levene’s test for homogeneity of variance and the Bonferroni post hoc test (all at p ≤ 0.05), plus regression plots, were used for statistical analysis.
Dependent on the type of resin-composite material and the loading/unloading parameters, nanoindentation creep ranged from 29.58 nm to 90.99 nm and permanent set ranged from 8.96 nm to 30.65 nm. Bulk compressive creep ranged from 0.47% to 1.24% and permanent set ranged from 0.09% to 0.38%. There was a significant ( p = 0.001) strong positive non-linear correlation ( r 2 = 0.97) between bulk creep and nano creep that could also be expressed via a simple fractional-power function. A significant ( p = 0.003) positive linear correlation ( r 2 = 0.69) existed between nano creep recovery and bulk creep recovery. With both methods of examination, except for Venus Bulk Fill™ (VB), the flowable and bulk-fill resin-composites exhibited creep within the range exhibited by the conventional resin-composites.
Despite the differences in loading and unloading conditions, in both methods of examination the correlation observed between the creep and recovery responses for a set of resin-composites was high. Both nano creep and recovery positively correlated with loading and unloading rates, respectively.
Successful application of dental materials as load-bearing structural components of restored teeth requires adequate mechanical properties. Thus general mechanical characterization of candidate materials is essential. The most useful starting point is to measure the stress–strain (or load–deformation) properties . Creep may result from occlusal stresses during clinical service. Such stresses can affect bond-integrity at tooth/restoration interfaces. However, when there is direct transfer of stress to the bulk restorative material, the resin-polymer phase of the resin-composite will often respond in a time-dependent manner, typical of most polymeric biomaterials . Such a response can involve segmental movement of the polymer chains within the constraints of the network connectivity. The magnitude of the response depends on the molecular structural details and external factors, particularly temperature .
Nanoindentation has been developed for material mechanical characterization at mN load scales and length scales below 1 μm. This is particularly useful with heterogeneous multi-phase materials such as resin-composites as the different regions may be investigated separately. Many recent dental resin-composites have nanohybrid filler systems, so it is desirable to probe the response of local structural regions in an attempt at further understanding of higher-level characteristics .
Nanoindentation is a sophisticated, and still rather expensive, instrumented technique that has been adapted for mechanical studies of a range of materials . Nanoindentation, as the term implies, involves probing of far smaller surface regions than with microindentation, as with microhardness . Nano-creep can be measured as an aspect of nanoindentation and is thus especially useful for materials incorporating a polymeric phase .
Four types of nanoindentation creep investigation were identified by Lucas and Oliver : indentation load relaxation method, constant rate of loading method, constant load indentation method, and impression creep method. The most common procedure is the constant load indentation method which records the change of depth with time. The indentation of polymeric materials is usually associated with creep-like behavior even at room temperature, i.e., the indenter tip continues to penetrate into the material surface during holding at a constant load. The extent of this creep depends on: (i) the material type, (ii) the loading level, (iii) the loading rate, (iv) the loading time and (v) the loading temperature .
Current nanoindentation instruments function by generating load/displacement plots as the probe (normally a Berkovich indenter geometry) is driven into the material surface at a constant rate and subsequently unloaded, again at a specific rate . Indentation into specimens normally proceeds with a gradually increasing force load until a pre-set maximum force is reached, and then it is unloaded in a similar manner . Indentations can be done using a pyramidal pointed indenter or a spherical indenter. The indentation is normally carried out in a continuous load/unload cycle .
Though nanoindentation has been used to characterize viscoelastic properties of dental materials and tissues, no previous studies have been found to compare it with bulk creep characterization of resin-composites. Therefore, this study will be focusing on the comparison between “nanoindentation creep” and “bulk compressive creep”. Both methods will be used to investigate creep and recovery of some representatives of different categories of dental resin-composites. The test hypotheses were: (i) there will be correlation between the nanoindentation creep and the bulk compressive creep of the studied materials; (ii) the nanoindentation creep and the bulk compressive creep will vary with different types of resin-composites; and (iii) with nanoindentation investigation, varying the loading/unloading rates will have an effect on the magnitude of nanoindentation creep and recovery.
Materials and methods
Ten dental resin-composites: five conventional, three bulk-fill and two flowable were investigated in this study. Materials description and manufacturers’ information are listed in Table 1 .
|Product||Code||Type||Manufacturer||Lot no.||Resin system||Filler (wt.%)|
|GrandioSo||GS||Conventional||Voco, Cuxhaven, Germany||1048014||Bis-GMA, Bis-EMA, TEGDMA||89|
|Filtek Supreme XTE||FS||Conventional||3 M ESPE, St. Paul, MN, USA||N214152||Bis-GMA, UDMA, TEGDMA, PEGDMA, Bis-EMA||79|
|Venus Diamond||VD||Conventional||Heraeus Kulzer GmbH, Hanau, Germany||010037||TCD, di-HEA, UDMA||80|
|N’Durance||ND||Conventional||Septodont Company, Louisville, USA||J9100-4||Bis-GMA, UDMA, DMA dimer acid||80|
|Premise||PR||Conventional||Kerr Italia S.p.A., Scafati, Italy||3571836||Ethoxylated Bis-GMA||84|
|Estelite Flow Quick||ES||Flowable||Tokuyama Dental Corporation, Japan||E646B||Bis-MPEPP, TEGDMA, UDMA||71|
|SureFil SDR Flow||SF||Flowable||Dentsply Caulk, Milford, Delaware, USA||1003011||EBPADMA, TEGDMA||68|
|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|
|Venus Bulk Fill||VB||Bulk-fill||Heraeus Kulzer GmbH, Hanau, Germany||10028||UDMA, EBADMA||65|