Highlights
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Grinding increases the ceramic’s resistance towards ageing at the subsurface.
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The activation energies compares well with previous reported values.
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After aging at 37 °C, predicted and experimental values are in accordance.
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The hardness and elastic modulus of the ceramic were significantly reduced by aging.
Abstract
Objectives
The purpose of the study was to assess the hydrothermal resistance of a translucent zirconia with two clinical relevant surface textures by means of accelerated tests (LTD) and to compare predicted monoclinic fractions with experimental values measured after two years aging at 37 °C.
Methods
Polished (P) and ground (G) specimens were subjected to hydrothermal degradation by exposure to water steam at different temperatures and pressures. The t–m phase transformation was quantified by grazing incidence X-ray diffraction (GIXDR). The elastic modulus and hardness before- and after LTD were determined by nanoindentation.
Results
G specimens presented a better resistance to hydrothermal degradation than P samples. Activation energies of 89 and 98 kJ/mol and b coefficients of 2.0 × 10 −5 and 1.8 × 10 −6 were calculated for P and G samples respectively. The coefficients were subsequently used to predict transformed monoclinic fractions at 37 °C. A good correlation was found between the predicted values and the experimental data obtained after aging at 37 °C during 2 years. Hydrothermal degradation led to a significant decrease of the elastic moduli and hardness in both groups.
Significance
The dependency of the t–m phase transformation rate on temperature must be determined to accurately predict the hydrothermal behavior of the zirconia ceramics at oral temperatures. The current prevailing assumption, that 5 h aging at 134 °C corresponds to 15–20 years at 37 °C, will underestimate the transformed fraction of the translucent ceramic at 37 °C. In this case, the mechanical surface treatment influences the ceramic’s transformability. While mild grinding could potentially retard the hydrothermal transformation, polishing after occlusal adjustment is recommended to prevent wear of the antagonist teeth and maintain structural strength.
1
Introduction
Developing high-strength ceramics led to the increased use of yttrium-stabilized zirconia (3Y-TZP) in dental clinical applications. Typically 3Y-TZP is used in single- or multiunit fixed dental prostheses as a structural material, which is subsequently covered with veneering ceramics. Problematically, such structures are prone to ‘adhesive failures’ – between the core and the veneering ceramic (delamination) – or ‘cohesive failures – within the veneer (chipping). This type of complications is the major risk incurred with zirconia-based, multilayer restorations . To reduce the chances of fracture, monolithic restorations, that is, zirconia without veneering ceramic, have been recently introduced to decrease the risk of delamination and chipping .
Concerns that monolithic zirconia might markedly wear the antagonist teeth have been raised. Interestingly less enamel wear was brought about by polished zirconia than by glazed zirconia , veneering ceramics or enamel antagonists . Another issue, though, with 3Y-TZP ceramics is their hydrothermal instability. At low temperatures (<500 °C) and in the presence of water, zirconia spontaneously undergoes a tetragonal to monoclinic phase transformation also known as ‘low temperature degradation’ (LTD) . The t–m transformation proceeds gradually from the surface into the bulk of the ceramic and is associated with surface roughening , micro-cracking and a decline of the surface or bulk mechanical properties .
For esthetic reasons, monolithic zirconia is typically glazed. Yet the glazed layer will be worn by antagonist enamel after an estimated period of 6 months . Since monolithic zirconia restorations are in direct contact with saliva, there is some concern as to their long term stability . Indeed, the t–m phase transformation at oral temperatures has recently been reported . Moreover, recent studies also disclosed a reduced flexural strength of 3Y-TZP ceramics dependent on the extent and the depth propagation of the t–m phase transformation . It follows that the resistance to hydrothermal degradation of these materials needs to be addressed.
One approach to determine the resistance to hydrothermal degradation of zirconia ceramics is to use accelerated tests at 134 °C and 2 bars in steam. Nevertheless, the activation energy of the t–m phase transformation is required to predict the amount of the monoclinic phase at the surface of the aged ceramic at a given time and temperature. In this context, Chevalier et al. evaluated the activation energy at 106 kJ/mol and estimated that five hours of aging at 134 °C in steam roughly equaled 15–20 years of aging at 37 °C .
Still, 3Y-TZP ceramics differ regarding their susceptibility to hydrothermal degradation. Yttrium oxide content , grain size , density , presence of a cubic phase and residual stresses are factors that affect the ceramics’ resistance toward aging. Differences in the activation energies are thus expected, depending on the 3Y-TZP ceramics composition, sintering conditions and surface treatments.
In the present study, the resistance to aging of a translucent dental zirconia with two clinically relevant surface textures was assessed. The t–m phase transformation apparent activation energy was estimated in both instances and used to predict the monoclinic fraction at oral temperature (37 °C). Numerical predictions were compared to 2-year experimental data. The effect of hydrothermal aging on the surface mechanical properties of the ceramic was also investigated.
2
Materials and methods
30 sintered specimens (10 mm × 10 mm × 4 mm) of the 3Y-TZP translucent ceramic (Lava Plus, 3M-ESPE, Seefeld, Germany) were provided by the manufacturer. The chemical composition of the ceramic was assessed by X-ray fluorescence spectroscopy (XRF) using a spectrometer (Advant XP, Thermo Fischer Scientific, USA) equipped with a Cu X-ray source, which was set to 40 kV and 120 mA. The analysis was conducted with the UNIQUANT software (Thermo Fischer Scientific, USA). Moreover, the specimens’ bulk density was determined using Archimedes’ method.
The original samples were mirror polished (9, 3, 1 μm diamond pastes) using a RotoPol-22 turntable (Struers GmbH, Switzerland) at a 150 rpm speed. They were subsequently divided into two groups. The first (P, n = 15) was used without further treatment while the second group (G, n = 15) was roughened with a 600 grit (∼30 μm) diamond disk under water cooling at 300 rpm for 3 min to simulate shape adjustments as performed intraorally. All specimens were ultrasonically cleaned in ethanol for 10 min (Fisa Compact, FISA, France). Among the 15 specimens, 12 were subjected to accelerated aging at elevated temperatures and pressures while 3 were merely stored in water for two years at 37 °C thus simulating intraoral conditions.
2.1
Samples characterization
XRD diffraction patterns were obtained with a D500 X-ray diffractometer (Bruker AXS, Germany) in the Bragg-Brentano ( θ –2 θ ) configuration using Cu-Kα 1 (1.5406 Å) radiation at 40 kV and 35 mA. Diffraction data were collected from 26° to 64° 2 θ , with a step size of 0.02° and a counting time of 30 s per step. A Rietveld refinement analysis (Topas software, Bruker AXS GmbH, Germany) was performed to quantify the crystalline phases.
Surface roughness was measured using a high-resolution white light non-contact profilomerter (CyberSCAN CT 100, Cyber technologies GmbH, Germany) with a z-resolution of 20 nm and a lateral resolution of 1 μm. R a was calculated using a Gaussian profile filter with the cut-off wavelength set to λ c = 0.8 mm, the sampling length to 4 mm and the scanning length to 5.6 mm (ISO 11562). Five profiles were generated for each sample.
Images of the surface were obtained using a scanning electron microscope (XL 20, Philips, Netherlands).
2.2
Hydrothermal degradation
Three samples of the P and G group were hydrothermally aged for different time intervals in an autoclave at 100°/0.3 bar, 110°/0.8 bar, 120°/1.2 bar and 134°/2 bar respectively. Three specimens were kept for two years at 37 °C in water.
Tetragonal-to-monoclinic surface phase transformations consecutive to aging were measured using the Grazing Incidence X-Ray Diffraction Technique (GIXRD) with a fixed incidence angle of 2 degrees. At this incidence angle, the X-ray penetration depth is restricted to 1.2 μm as calculated with the AbsorbDX software (DIFFRACplus BASIC Evaluation Package, Bruker, Germany). XRD patterns were generated from 26° to 33° 2 θ with a step size of 0.01° and a counting time of 5 s per step. The integrated intensity ratio ( x m ) and the volume fraction of the monoclinic phase ( f m ) were determined from the X-ray diffraction patterns using Garvie and Nicholson’s and Toraya et al.’s equations. On the ground specimens (G), a Rietveld refinement analysis was also conducted. The monoclinic fractions obtained by this method were used for further calculations.
Hydrothermal degradation kinetics was calculated from Mehl–Avrami–Johnson’s (MAJ) equation:
f ( t ) = f min + ( f max − f min ) [ 1 − exp ( − ( b t ) n ) ]
where f ( t ) is the volume fraction of the monoclinic phase at a given time t , f min and f max are the initial and maximum volume fractions, n and b are parameters describing the kinetics of transformation at a given temperature.
Exponent n relates to nucleation and growth. Coefficient b follows the Arrhenius equation:
where b 0 is a constant, Q [kJ/mol] is the apparent activation energy and R is the ideal gas constant taken as 8.31446 [J mol −1 K −1 ].
The apparent activation energy could therefore be estimated by plotting ln( b ) as an inverse function of the temperature.
Diffraction data were also collected by GIXRD after prolonged hydrothermal degradation (100 h). The fixed incidence angles were varied from 1° to 15° to generate depth profiles.
2.3
Mechanical characterization at the surface
The local mechanical properties at the surface (i.e. hardness and E-modulus) were assessed by nanoindentation using a NHT nanoindenter (CSM Instruments, Switzerland) was used. A load of 300 mN (maximum) was applied at a constant strain rate of 0.05 s −1 . The Berkovich diamond tip was calibrated using a fused silica standard provided by the manufacturer. Hardness ( H ) and E-moduli ( E ) were calculated from the unloading portion of the indentation curves using Oliver and Pharr’s method . To compute the E-moduli, Poisson’s ratio was taken as 0.32 . Ten indentations were placed for each specimen (1) before hydrothermal degradation, (2) after 100 h aging time at 134 °C or (3) after 2 years at 37 °C.
A statistical analysis was conducted on the E-moduli and hardness calculated from the unloading portions of the indentation curves. An analysis of variance followed by Fisher’s LSD multiple comparison test at 95% level of significance was used to detect differences before- and after aging.
2
Materials and methods
30 sintered specimens (10 mm × 10 mm × 4 mm) of the 3Y-TZP translucent ceramic (Lava Plus, 3M-ESPE, Seefeld, Germany) were provided by the manufacturer. The chemical composition of the ceramic was assessed by X-ray fluorescence spectroscopy (XRF) using a spectrometer (Advant XP, Thermo Fischer Scientific, USA) equipped with a Cu X-ray source, which was set to 40 kV and 120 mA. The analysis was conducted with the UNIQUANT software (Thermo Fischer Scientific, USA). Moreover, the specimens’ bulk density was determined using Archimedes’ method.
The original samples were mirror polished (9, 3, 1 μm diamond pastes) using a RotoPol-22 turntable (Struers GmbH, Switzerland) at a 150 rpm speed. They were subsequently divided into two groups. The first (P, n = 15) was used without further treatment while the second group (G, n = 15) was roughened with a 600 grit (∼30 μm) diamond disk under water cooling at 300 rpm for 3 min to simulate shape adjustments as performed intraorally. All specimens were ultrasonically cleaned in ethanol for 10 min (Fisa Compact, FISA, France). Among the 15 specimens, 12 were subjected to accelerated aging at elevated temperatures and pressures while 3 were merely stored in water for two years at 37 °C thus simulating intraoral conditions.
2.1
Samples characterization
XRD diffraction patterns were obtained with a D500 X-ray diffractometer (Bruker AXS, Germany) in the Bragg-Brentano ( θ –2 θ ) configuration using Cu-Kα 1 (1.5406 Å) radiation at 40 kV and 35 mA. Diffraction data were collected from 26° to 64° 2 θ , with a step size of 0.02° and a counting time of 30 s per step. A Rietveld refinement analysis (Topas software, Bruker AXS GmbH, Germany) was performed to quantify the crystalline phases.
Surface roughness was measured using a high-resolution white light non-contact profilomerter (CyberSCAN CT 100, Cyber technologies GmbH, Germany) with a z-resolution of 20 nm and a lateral resolution of 1 μm. R a was calculated using a Gaussian profile filter with the cut-off wavelength set to λ c = 0.8 mm, the sampling length to 4 mm and the scanning length to 5.6 mm (ISO 11562). Five profiles were generated for each sample.
Images of the surface were obtained using a scanning electron microscope (XL 20, Philips, Netherlands).
2.2
Hydrothermal degradation
Three samples of the P and G group were hydrothermally aged for different time intervals in an autoclave at 100°/0.3 bar, 110°/0.8 bar, 120°/1.2 bar and 134°/2 bar respectively. Three specimens were kept for two years at 37 °C in water.
Tetragonal-to-monoclinic surface phase transformations consecutive to aging were measured using the Grazing Incidence X-Ray Diffraction Technique (GIXRD) with a fixed incidence angle of 2 degrees. At this incidence angle, the X-ray penetration depth is restricted to 1.2 μm as calculated with the AbsorbDX software (DIFFRACplus BASIC Evaluation Package, Bruker, Germany). XRD patterns were generated from 26° to 33° 2 θ with a step size of 0.01° and a counting time of 5 s per step. The integrated intensity ratio ( x m ) and the volume fraction of the monoclinic phase ( f m ) were determined from the X-ray diffraction patterns using Garvie and Nicholson’s and Toraya et al.’s equations. On the ground specimens (G), a Rietveld refinement analysis was also conducted. The monoclinic fractions obtained by this method were used for further calculations.
Hydrothermal degradation kinetics was calculated from Mehl–Avrami–Johnson’s (MAJ) equation:
f ( t ) = f min + ( f max − f min ) [ 1 − exp ( − ( b t ) n ) ]
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