Abstract
Objective
The aim of this study was to evaluate silica infiltration into dental zirconia (VITA In-Ceram 2000 YZ, Vita Zahnfabrik) and its effects on zirconia’s surface characteristics, structural homogeneity and bonding to a resin cement.
Methods
Infiltration was performed by immersion of the pre-sintered zirconia specimens in silica sols for five days (ZIn). Negative (pure zirconia specimens, ZCon-) and positive controls (specimens kept in water for 5 days, ZCon+) were also performed. After sintering, the groups were evaluated by X-ray diffraction (XRD), grazing angle X-ray diffraction (DRXR), scanning electron microscopy (SEM), contact angle measurements, optical profilometry, biaxial flexural test and shear bonding test. Weibull analysis was used to determine the Weibull modulus (m) and characteristic strength (σ 0 ) of all groups.
Results
There were no major changes in strength for the infiltrated group, and homogeneity (m) was also increased. A layer of ZrSiO 4 was formed on the surface. The bond strength to resin cement was improved after zirconia infiltration, acid conditioning and the use of an MDP primer.
Conclusion
The sol-gel method is an efficient and simple method to increase the homogeneity of zirconia. Infiltration also improved bonding to resin cement.
Clinical significance
The performance of a zirconia infiltrated by silica gel improved in at least two ways: structural homogeneity and bonding to resin cement. The infiltration is simple to perform and can be easily managed in a prosthesis laboratory.
1
Introduction
Dental ceramics are similar to the natural tooth in appearance because of their optical properties. This and other qualities, such as hardness and chemical stability , enabled these materials to be rapidly developed for dental use, to meet the increasing demands for esthetics and durability.
The dental ceramics presenting better esthetics are silicates like feldspathic ceramic and lithium disilicate . However, these ceramics have inferior mechanical properties compared with those of zirconia. Thus, zirconia has been widely used as an infrastructure material in crowns and fixed partial prostheses, because of its excellent mechanical properties .
The remarkable performance of zirconia, as explored in various medical and engineering applications, is due mainly to the transformation of the tetragonal into the monoclinic phase. This transformation can be induced by thermomechanical factors, resulting in a volume increase of about 3–4%. This volume increase generates compressive stresses on the tip of a possible crack. In this case, for the crack to propagate, it must overcome the compressive stresses surrounding it. This toughening mechanism explains the high fracture resistance of zirconia when compared with that of other ceramics .
A common problem in zirconia is its difficulty in promoting adhesion compared with silica-based ceramics . For feldspathic ceramics, conditioning the surface with hydrofluoric acid (HF 10%) is effective in creating mechanical imbrication, while silanization provides the chemical bond between the inorganic ceramic and the organic resin cement. However, for alumina- and zirconia-based ceramics HF is not able to roughen the surface sufficiently, and the absence of silica prevents chemical bonding to silanes . Air abrasion with aluminum oxide alone, commonly performed in laboratories, also does not improve adhesion and creates microdefects that compromise the strength of the material in the long term .
Until now, the preferred method for bonding zirconia is the combination of air abrasion with aluminum oxide particles coated with silica (silicatization) and the application of a phosphate-based monomer as an adhesion promoter. However, silicatization also has the potential to damage the surface .
Based on reports in the literature, the most frequent clinical failures of tooth- and implant-supported zirconia crowns are loss of retention and veneering material fracture . To solve the retention problem, several methods were created to infuse silica into zirconia, making it chemically reactive to the bisphenol-A-glycidyl dimethacrylate (Bis-GMA)–based cements such as the PyrosilPen , the “glaze-on technique” , which uses a thin intermediate coating of acid-etchable glasses, and silica nano-coating . Additional methods, such as selective glass infiltration and a hot acidic solution are intricate techniques that roughen zirconia and improve bonding. A new method for infiltrating pure silica in a Y-TZP using a sol-gel process was proposed in this present study to create a layer sensitive to acid etching and silanization.
By coating the Y-TZP we also aimed to produce a graded zirconia. This type of material presents increased load bearing capacity and a gradual increase in the elastic modulus towards the interior layers , which may improve the interfacial adhesion with porcelains .
Therefore, this first study aimed to evaluate the effects of such infiltration with a simple and low-cost sol-gel method on the mechanical properties, microstructure and surface characteristics of a Y-TZP. Additionally, we evaluated the bond strengths between the infiltrated ceramic and resin cement.
2
Materials and methods
2.1
Preparation of specimens
The pre-sintered zirconia blocks (Vita InCeram YZ, Vita Zahnfabrik, Bad Säckingen, Germany) were wet-ground into cylinders (18 mm in diameter) that were then sectioned into discs in a lathe (ISOMET 1000, Buehler, Lake Bluff, IL, USA). The first section (approximately 0.5 mm) was discarded, and the remaining sections (1.65 mm thickness) were cut under coolant irrigation. The discs were wet-ground flat with silicon carbide (SiC) (#1200) and sintered according to the manufacturer’s instructions (Vita Zyrcomat, Vita Zahnfabrik) according to the following schedule: temperature elevation time, 1 h; final temperature, 1530 °C; waiting time, 2 h; and cooling temperature at which the tray can be downloaded inside the oven, 400 °C. The final dimensions were according to ISO 6872 standard, being 12 mm in diameter and 1.2 mm thick.
The pre-sintered zirconia discs were divided into three groups. One group was immersed in water alone (Positive Control Group, ZCon + ) and/or in silica sol (ZIn), both of which remained in these respective media for 5 d. The silica sol (0.5 mol/L) was obtained by passing an aqueous sodium silicate solution through an ion-exchanging resin. The third group was the negative control (ZCon-), which was not immersed in any medium. After the immersion period, the pre-sintered zirconia discs were removed from the solution and put into an oven at 100 °C for two days. After being dried, the specimens were sintered in the same manner as the specimens of the non-infiltrated group.
The sintered specimens were then evaluated by X-rays, Raman spectroscopy, scanning electron microscopy (SEM), and contact angle measurements. X-ray diffraction (XRD) (X’pert Powder model, PANalytical, Westborough, MA, USA) was performed at 10–90°, with a scan step of 10.1600 s, at a 0.0170° step size, with CuKα radiation. The Rietveld refinement of the diffractometries was performed with the General Structure Analysis System (GSAS)/EXPGUI program (a graphical interface for the Los Alamos GSAS package). Raman spectroscopy was performed in a Micro Raman system (Renishaw 2000, Gloucestershire, UK) under Ar laser excitation, at a wavelength of 514.5 nm. The Raman spectra were collected over 3 accumulation periods of 30 s, between 200 and 3500 cm −1 . The contact angle analysis was performed in a tensiometer (Kruss, model EasyDrop DAS 100, Hamburg, Germany) Contact Angle Measuring Instrument using the sessile drop (Young-Laplace method) and distilled water (3 μL).
2.2
Flexural strength, weibull parameters, and surface analyses
Thirty specimens were prepared for each group. Two outliers of the infiltrated zirconia were disregarded in the analysis and analyzed separately.
To determine flexural strength, we placed each disc on a circular base with three metallic beads of 3.2 mm in diameter, equidistant from each other, forming a plane. A blunt tip, 1.6 mm in diameter, was attached to a testing machine (EMIC DL-1000, EMIC, São José dos Pinhais, PR, Brazil), and a load was applied (1000 Kgf) until fracture at a constant speed of 1 mm/min.
The biaxial flexural strength (σ) (MPa) of the discs was obtained according to the ISO 6872 standard:
where: P = load (N); X and Y = are parameters related to a elastic property of the material [Poisson’s ratio (0.3)] and the radii of the specimen and loading device (mm); and b = thickness of the specimen at the fracture origin in mm.
The Weibull parameters, modulus (Shape parameter) and characteristic strength (Scale parameter), were obtained from the Maximum Likelihood estimation (Minitab 16, Minitab Inc, State College, PA, USA) ( Fig. 1 ).
Topographic and fracture analyses were performed by stereomicroscopy (Discovery V20, Carl Zeiss, Jena, Germany) and scanning electron microscopy (SEM) (Inspect S50, FEI Company, Brno, Czech Republic).
A bond strength analysis was performed on square-shaped zirconia specimens. Specimens were divided in infiltrated and non-infiltrated groups (control). After sintering, the infiltrated groups were conditioned with 2% HF acid for 10 s. This acid concentration delivered the most favorable topography based on profilometry images of 2%, 5% and 10% of infiltrated specimens, after several exposition times (10 s and 15s) (not shown here for brevity).
Specimens from both groups were then silanated with Monobond Plus or Monobond S (Ivoclar Vivadent, Schaan, Liechtenstein) and a resin cement cylinder (3 mm diameter and 6 mm height) (Variolink II, Ivoclar Vivadent, Schaan, Liechtenstein) was built up on each zirconia specimen. After photopolymerization for 40 s, specimens were stored in distilled water for 24 h at 37 °C. Half of them were then tested in shear in a universal testing machine, while the other half was further subjected to thermalcycling (5°–55 °C). After 6 × 103 cycles, the specimens were tested in shear and all fractured surfaces were analyzed in a stereomicroscope to determine the failure modes.
2
Materials and methods
2.1
Preparation of specimens
The pre-sintered zirconia blocks (Vita InCeram YZ, Vita Zahnfabrik, Bad Säckingen, Germany) were wet-ground into cylinders (18 mm in diameter) that were then sectioned into discs in a lathe (ISOMET 1000, Buehler, Lake Bluff, IL, USA). The first section (approximately 0.5 mm) was discarded, and the remaining sections (1.65 mm thickness) were cut under coolant irrigation. The discs were wet-ground flat with silicon carbide (SiC) (#1200) and sintered according to the manufacturer’s instructions (Vita Zyrcomat, Vita Zahnfabrik) according to the following schedule: temperature elevation time, 1 h; final temperature, 1530 °C; waiting time, 2 h; and cooling temperature at which the tray can be downloaded inside the oven, 400 °C. The final dimensions were according to ISO 6872 standard, being 12 mm in diameter and 1.2 mm thick.
The pre-sintered zirconia discs were divided into three groups. One group was immersed in water alone (Positive Control Group, ZCon + ) and/or in silica sol (ZIn), both of which remained in these respective media for 5 d. The silica sol (0.5 mol/L) was obtained by passing an aqueous sodium silicate solution through an ion-exchanging resin. The third group was the negative control (ZCon-), which was not immersed in any medium. After the immersion period, the pre-sintered zirconia discs were removed from the solution and put into an oven at 100 °C for two days. After being dried, the specimens were sintered in the same manner as the specimens of the non-infiltrated group.
The sintered specimens were then evaluated by X-rays, Raman spectroscopy, scanning electron microscopy (SEM), and contact angle measurements. X-ray diffraction (XRD) (X’pert Powder model, PANalytical, Westborough, MA, USA) was performed at 10–90°, with a scan step of 10.1600 s, at a 0.0170° step size, with CuKα radiation. The Rietveld refinement of the diffractometries was performed with the General Structure Analysis System (GSAS)/EXPGUI program (a graphical interface for the Los Alamos GSAS package). Raman spectroscopy was performed in a Micro Raman system (Renishaw 2000, Gloucestershire, UK) under Ar laser excitation, at a wavelength of 514.5 nm. The Raman spectra were collected over 3 accumulation periods of 30 s, between 200 and 3500 cm −1 . The contact angle analysis was performed in a tensiometer (Kruss, model EasyDrop DAS 100, Hamburg, Germany) Contact Angle Measuring Instrument using the sessile drop (Young-Laplace method) and distilled water (3 μL).
2.2
Flexural strength, weibull parameters, and surface analyses
Thirty specimens were prepared for each group. Two outliers of the infiltrated zirconia were disregarded in the analysis and analyzed separately.
To determine flexural strength, we placed each disc on a circular base with three metallic beads of 3.2 mm in diameter, equidistant from each other, forming a plane. A blunt tip, 1.6 mm in diameter, was attached to a testing machine (EMIC DL-1000, EMIC, São José dos Pinhais, PR, Brazil), and a load was applied (1000 Kgf) until fracture at a constant speed of 1 mm/min.
The biaxial flexural strength (σ) (MPa) of the discs was obtained according to the ISO 6872 standard:
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