Esthetic demands and biocompatibility have prompted the development of all-ceramic dental crowns. Yttria tetragonal zirconia polycrystalline (Y-TZP) framework material has the best mechanical properties compared to other all-ceramic systems, but the interface is the weakest component of core veneered restorations. Confocal Raman microscopy possibilities are used to ensure the understanding of the zirconia-feldspathic ceramic relationship, which is not well known.
Bilayered zirconia (Vita In-Ceram ® YZ) veneer (Vita VM ® 9) blocks were manufactured. Raman analyses were performed using two protocols: (1) single spectra, line scans and images on a sectioned and polished specimen and (2) in depth line scans on unprepared specimen. Single spectra, images and line scans provide information about the crystalline phases, their distribution and the existence of a possible diffusion at the Y-TZP/VM ® 9 interface, respectively. The elemental distribution of zirconium (Zr) and silicon (Si) around this interface were investigated using energy dispersive X-ray spectroscopy (EDS).
Raman single spectra embodied a unique spectrum (crystalline) on Y-TZP and two spectra (crystalline and amorphous) on VM ® 9; these spectra were clearly distinguished. Raman line scans showed a series of transition spectra across the interface from VM ® 9 to Y-TZP. It emphasized an interdiffusion zone, which was estimated at a maximum of 2 microns, found on 2d Raman images and confirmed by EDS. The elemental distribution with EDS showed a mutual diffusion of Zr and Si and was mainly dominated by Si diffusion in Y-TZP.
Confocal Raman microscopy highlights an interdiffusion zone at the zirconia-feldspathic ceramic interface. The elemental transition layer is estimated and is supported by EDS analysis as a coupling technique.
The alloys of zirconium dioxide (ZrO 2 ) have become one of the essential materials in both industrial and medical fields over the last 40 years because of their low thermal diffusion and excellent mechanical and biological properties . Improvements of its properties, such as the stabilization of the tetragonal phase at room temperature , combined with computer-aided design/manufacturing (CAD/CAM) technology, have made zirconia an alternative to traditionally used metal substructures for prosthetic dentistry. The rising interests in esthetics and questionable biocompatibility of competitive restorative systems have accelerated development of zirconia-based restorations .
Despite the high stability of the zirconia framework, the long-term success of these restorative materials is determined by the veneering process. The in vivo fracture rate of layering ceramic is 15% after 24 months, 25% after 31 months and 8% after 36 months . Yet, no fractures of the zirconia core have been reported. Failure in the layering ceramic is in the form of cone cracking either cohesively within the veneering ceramic (chipping) or adhesively at the core veneer interface (delamination) . The location of the interface as a failure origin has been reported previously , which suggests that the bond strength between the veneering ceramic and the zirconia framework is a weak point in layered all-ceramic restorations . That is none of the zirconia core and veneering ceramics attained the high bond strength values of the metal ceramic combination .
The causes of fracture of veneering ceramic are multifactorial, both clinically and technically. Clinically, inadequate framework design with respect to a lack of tooth preparation or inadequate margin taper and volume seems to be decisive issues . Technically, the zirconia-veneer bond strength depends on the materials that are used . Well-known factors, such as the surface finish of the core (toughness and roughness) can affect the mechanical retention of layering ceramic. Undesired tensile stress that is generated by a mismatch in the thermal expansion coefficient (TEC) or volumetric shrinkage of the veneer after firing and its wetting properties over the core , have been reported. Impurities content, grain size, density, and temperature lead to the transformation of the tetragonal into the monoclinic phase of the zirconia core during various manufacturing methods .
The usual methods for assessment of the interface between layering ceramic and core material are shear bond strength or microtensile bond strength testing . These methods are usually coupled with scanning electron microscopy (SEM) to identify the kind of failure (cohesive or interfacial) or to understand its mechanism . Morphological information is also obtained using optical interferometry (OI) and atomic force microscopy (AFM) . Energy dispersive X-ray spectroscopy (EDS) provides elemental composition, and X-ray diffraction (XRD) or infrared spectroscopy (IR) measures phase fractions quantitatively. However, each of these techniques has advantages and limitations . SEM, even if it can observe the impact of laboratory procedures (temperature factor, humidity) or aging of yttria-stabilized tetragonal zirconia phase (Y-TZP) surface grain pull out , cannot detect the first stages of aging and suffers from poor depth resolution . The dynamic and mechanism of zirconia transformation evaluated by traditional XRD suffers from accuracy during the early stages of the aging process because of the absence of local information . This analysis is also limited to the first hundredth of a micron below the surface . AFM is a unique and extremely powerful tool for the investigation of martensitic transformation in zirconia because of its unique lateral and depth resolution and the possible observation of bulk samples . AFM can provide valuable insights on the nucleation and growth processes during the very first stages of zirconia aging, but it remains a surface analysis .
The effects of sample preparation on material properties must be taken into account. For example, sectioning may change surface observations by inducing microcrack zones that can lead to extensive grain pull out , which interferes with measurements . To provide a wide range of material characterization the coupling of experimental techniques is required, such as SEM evaluation with XRD , OI with XRD or OI with AFM .
In 1977, Ishigame, who studied the different phases of zirconia, indicated that Raman scattering is useful for investigation of phase transformation, because the technique reflects the dynamic properties that are associated with the change in crystal structure . Raman spectroscopy is a nondestructive, “non-radiating” analysis technique that allows for collection of quantitative and qualitative information. This technique is based on detection of inelastically scattered photons through the interaction of the sample with monochromatic light. The frequency difference between incident photon and the scattered photon provide information on the chemical nature of the molecule or crystal geometry, responsible for the diffusion. Atomic motions in molecules and crystals are organized into vibrational modes. In crystals these modes are called phonons. Phonon is a quantum-mechanical description of a special type of vibrational motion, in which a lattice uniformly oscillates at the same frequency . Raman spectroscopy helps identify and distinguish the different crystalline phases of zirconia or feldspathic ceramic , on very small and specific sample surface areas , providing enhanced spatial resolution . Using Raman spectroscopy, it is possible to determine the chemical nature of a species, to study the distribution of a component (profile), or to determine its degree of crystallinity without any sample preparation. The method allows for monitoring of zirconia phase transformation and represents an alternative method for quantification of zirconia transformation . Raman spectroscopy has also been used to model the surfaces of silicate or aluminosilicate glasses as well as in their depth . Using a piezo motor XYZ microscope stage, the Raman imaging mode allows for visualization of the distribution and concentration of various compounds (Raman mapping 2 or 3D) .
The purpose of the present work was to utilize confocal Raman microscopy (CRM) to examine the interface between zirconia and a feldspathic ceramic, an area that is not well known. The allocation of crystalline or amorphous phases was estimated around the zirconia core-veneering ceramic interface. As a coupling technique, the elemental distribution around this interface was estimated using energy dispersive X-ray spectroscopy (EDS).
Materials and methods
The exact procedure for specimen fabrication recommended by the manufacturers was followed. Yttria partially stabilized tetragonal zirconia polycrystalline ceramic (Y-TZP, VITA In-Ceram ® YZ, VITA Zahnfabrik, Germany) was used as the framework material. Porously pre-sintered Y-TZP cube blanks suitable for CAD/CAM machines were milled (Cerec ® InLab Sirona, Germany) to obtain two blocks (12 mm × 7 mm × 1.5 mm) and were then sintered in a furnace (Vita ZYrcomat, VITA Zahnfabrik, Germany) at a 1530 °C for 2 h. A feldspathic veneering ceramic (VITA VM ® 9, color shade 3M3) was mixed with a modeling liquid (VITA VM ® Modeling Liquid) and was built-up on the zirconia surface. A wash-dentin firing procedure was used for the first stage of ceramic build-up (washbake firing). This stage involves firing of a thin aqueous mixture layer (approximately 50 μm thick) of veneering ceramic to 950 °C . Technical information, such as preheating or temperature rise, is given in Table 1 . A furnace was used for baking the layering ceramic (VITA Vacumat ® 4000 T, VITA Zahnfabrik, Germany). The second step in layering application was placement of a thicker layer (approximately 2-mm thick) of veneering ceramic that was sintered to 910 °C (1st dentin firing, Table 1 ). Block No. 1 received only the washbake firing and was preserved in that state. Block No. 2 received two layers of ceramic build-up. To enable an expanded view of the interface, block No. 2 was sectioned perpendicular to the interface using a water-cooled diamond saw (600 μm) (ISOMET ® 2000 Precision Saw, Bueher, Germany) and polished (ESCIL manuelle, Chassieu, France) to remove external irregular scratches and defects. The polishing protocol was as follows: pre-polishing with abrasive papers of decreasing grain size (120, 320, 800, and 1200); polishing with a succession of pen discs (FD3, FD1 and FD1N, chronologically. ESCIL, Chassieu, France) that were used with diamond suspensions to 6, 1, and 1/4 μm, respectively; terminal polishing using colloidal silica (POM8. ESCIL, Chassieu, France). The polished specimen was cleaned by soaking in ethanol at 95°. The powdered form of the feldspathic ceramic (VM ® 9) was also analyzed.
|Start temp. (°C)||Hold for (min)||Heating time (min)||Rate increase (°C/min)||Approx. temp. (°C)||Hold for (min)||Long-term cooling (°C)||Vacuum holding time (min)|
|1st dentin firing||500||6.00||7.27||55||910||1.00||600||7.27|
Confocal Raman microscopy (CRM)
A confocal Raman microscope was used (WITec ® Alpha 300R, Ulm, Germany), which was composed of a laser source (Nd:YAG 532 nm and 40 mW power), a 20× or 100× objective depending on the experiment (which focused the incident laser spot on the sample surface, then directed the scattered photons to the spectrometer), a rejection filter (holographic «notch» used to separate the signal of interest of the Rayleigh signal), and a monochromator (pinhole to spatially locate the light). A prismatic mirror directed the photons on a holographic grating of 600 grooves/mm (BLZ 500 nm to disperse them according to their wavelength) and a 1024 × 127 CCD multichannel detector that simultaneously collected spatial and spectral information about the sample. The laser spot diameter or lateral resolution (LR), which was improved for confocal study , was defined by the relation