Abstract
Objective
Delamination of porcelain from intact zirconia framework was recently reported as the most common failure mode of these restorations. The aim of this study was to investigate the influence of different laboratory surface treatments on crystals structure and fracture strength of zirconia veneered restorations.
Methods
Zirconia discs received airborne particle abrasion with either 50 or 120 μm alumina particles then half of the specimens were annealed to remove surface pre-stresses, while assintered discs served as a control. Crystal structure of each group was evaluated using X-ray diffraction analysis (XRD). The discs were then veneered with porcelain and biaxially loaded to fracture with the veneer surface in tension ( α = 0.05).
Results
Regarding debonding failure, 50 μm particle abrasion significantly increased biaxial flexure strength compared to as-sintered specimens. On the contrary 120 μm particle abrasion resulted in significant reduction in flexure strength and was associated with higher percent of monoclinic phase (7%). However for both types of particle sizes, annealing reduced initial failure load as it led to complete reverse transformation of the monoclinic phase which was associated with zirconia grain pull-out at the critical crack location.
Significance
Paying attention to the surface treatment of zirconia before veneering could reduce chances of delamination and significantly improve the strength of the bilayered restorations.
1
Introduction
The introduction of zirconia to the dental field allowed fabrication of long span all-ceramic restorations with high success rate. Due to its high mechanical properties and high fracture toughness zirconia became the framework material of choice for fabrication all-ceramic restorations. Combined with state of the art CAD/CAM technology, fabrication of zirconia frameworks requires nothing more than few keyboard clicks while the design, milling, and sintering became fully automated procedures .
On the other hand, zirconia has a brittle nature and is sensitive to handling procedures . In a recent study, the flexure strength of zirconia was significantly affected by types of different surface treatment methods compared to a highly polished surface (used as golden standard). Particle abrasion with large alumina particles (120 μm) resulted not only in significantly lowering the flexure strength of zirconia but in lowering also its survival rate as well. Grinding performed during fit corrections also had a deteriorating effect . A point worth mentioning, is that the CAM procedure (computer assisted milling phase) also influenced the flexure strength of zirconia as it leaves behind milling landmarks in the form of micro-grooves and scratches thus while zirconia has a high theoretical flexure strength (1200 MPa), different forms of surface damage resulted in significant reduction in its flexure strength (up to 50%). With consideration to the influence of fatigue and chemically assisted crack tip propagation, failure could occur at unexpectedly low loads .
Such reduction in flexure strength is directly related to the introduced surface and sub-surface damage as a result of particle abrasion or grinding procedures . Surface damage in the form of scratches, grooves, indentations, and porosity allow concentration of the applied stresses at narrow crack tips which can easily exceed the threshold stress intensity factor of the material. Beside stress concentration, surface damage also introduces mechanical stresses on the surface grains leading to activation of the stress-induced tetragonal–monoclinic transformation of the influenced crystals. This transformation is accompanied by slight expansion (4%) which, in the bulk of the material, compresses propagating crack tips resulting in high fracture toughness .
On the other hand, when this transformation activity occurs on the surface, the accompanying volumetric expansion could lead to grain pull-out, surface lifts, and plastic deformation . Structural changes on the surface of zirconia could directly influence the contact interface with the veneer porcelain leading to weakening zirconia veneer bond strength or even delamination under function. It should be mentioned here that tetragonal–monoclinic transformation is a reversible process, thus veneer porcelain applied on a mono-clinic rich surface, induced by surface damage, could lose adequate contact with the underlying zirconia framework as a result of reversed monoclinic tetragonal transformation.
Low energy particle abrasion using small sized particles at low pressure could produce the required surface modifications without inducing excessive surface damage . The aim of this study was to investigate the influence of different surface treatment methods on the flexure strength of bilayered zirconia restorations. Scanning electron microscopy, X-ray diffraction analysis, and chemical microanalysis were used to complement the data. It was hypothesized that alteration of surface crystal structure would reduce the fracture strength of zirconia veneered restorations.
2
Materials and methods
2.1
Preparation of the specimens
100 as-sintered zirconia discs (Procera Zirconia; Noble Biocare, Goteborg, Sweden) received either of the following surface treatments:
- –
airborne particle abrasion (G 400; Harnisch & Rieth, Winterbach, Germany) with 50 μm aluminum oxide particles (S-U-Alustral; Schuler-Dental, Ulm, Germany) at 0.15 MPa pressure (defined as low energy particle abrasion).
- –
airborne particle abrasion with 120 μm aluminum oxide particles at 0.3 MPa pressure.
- –
as-sintered surface served as a control.
The nozzle was placed at 10 cm from the contact surface and the contact time was set at 10 s/cm 2 . Half of the specimens of each group were subjected to a heat treatment (annealing at 1000 °C in open air for 1 h) to eliminate presence of any monoclinic phase on the surface. 20 specimens were fabricated for each group ( n = 20).
2.2
X-ray diffraction analysis (XRD)
The central surface of the zirconia discs was examined using analysis XRD (XRD; M18XHF-SRA, Mac Co., Japan) to reveal changes in the crystallographic content before and after heat treatment for all groups. As-sintered zirconia surface was used as a base line. XRD data were obtained with a diffractometer using nickel-filtered CuK radiation. The tetragonal/monoclinic zirconia ratio was determined using the integrated intensity (measuring the area under the diffractometer peaks) of the tetragonal (1 0 1) and two monoclinic (1 −1 1) and (1 1 1) peaks. The obtained volume fractions were individually normalized. Although the X-ray penetration is too deep to be considered as a real surface analysis, it allowed precise evaluation of the transformation activity.
2.2.1
Application of veneer ceramic
The prepared zirconia discs (18 mm × 0.5 mm) were placed in an adjustable aluminum mold and slurry of properly mixed veneer porcelain (Nobel Rondo Zirconia dentin; A3.5, Nobel Biocare) was applied and condensed in the mold. The slurry was blot dried and the veneered zirconia discs were removed from the mold and sintered in a computer controlled electrical oven (EP 500; Ivoclar Vivadent, Shaan, Liechtenstein) following recommendations of the manufacturer. The outer surface of the veneer porcelain was polished using ascending grit silicon carbide paper and a diamond paste. The final thickness of the veneer porcelain was 1.5 mm. 20 specimens were prepared for every test group ( n = 20). The chemical composition of the materials used is summarized in Table 1 .
| Material | Manufacturer | Chemical composition |
|---|---|---|
| Zirconia discs | Procera Zirconia; Noble Biocare, Goteborg, Sweden | ZrO 2 (90%), Y 2 O 2 (4.5–5.4%), HF 2 (5%), Al 2 O 3 (0.5%) |
| Veneering porcelain | (Nobel Rondo Zirconia dentin; A3.5, Nobel Biocare) | SiO 2 (55–70%), Al 2 O 3 (16–20%), CaO (0.5–5.0%), MgO (0.5–5.0%), Li 2 (O 1.0–5.0%), Na 2 (O 2.0–5.0%), K 2 O (12.5–22.5%), Ce 2 O 3 (0–1.0%) |
2.3
Biaxial flexure strength
The bilayered discs were placed on metallic ring with the veneer porcelain in tension and axial load was applied till fracture using a universal testing machine (Instron 6022, Instron Limited, High Wycombe, UK). Stress–strain curve for every specimen was obtained from a computer generated file. The crosshead speed (0.5 mm/min) was adjusted using a micrometer while the load cell was calibrated using standardized weights. At the first sign of failure indicated by sudden drop in the applied load, the test was stopped and the specimens were examined using scanning electron microscopy (XL30; Philips, Eindhoven, the Netherlands). The stress at failure (MPa) was calculated using the following equation :
where F is the fracture load, υ is the Poisson ratio, D s and D are the diameters of the support ring and of the tested disk, respectively.
The specimens were cleaned, dried, gold sputter coated (S150Z, Edwards sputter coater, England), and examined under different angles at different magnifications. Then the discs were subjected to an additional loading cycle till complete fracture of the zirconia disk. The interface side of the broken veneer was chemically analysis using energy dispersive spectroscopy X-ray microanalysis (EDAX Inc., Mahwah, NJ) to identify pulled out zirconia grains that remained attached to the veneer side.
2.4
Statistical analysis
One way analysis of variance (ANOVA) and Bonferroni post doc tests were used to analyze the data. According to sample size ( n = 20), the test of choice had adequate power to detect large effect size differences which could justify clinical relevance. The estimated Weibull Moduli ( m *) of every test group were calculated as previously described for research studies with small sample size .
2
Materials and methods
2.1
Preparation of the specimens
100 as-sintered zirconia discs (Procera Zirconia; Noble Biocare, Goteborg, Sweden) received either of the following surface treatments:
- –
airborne particle abrasion (G 400; Harnisch & Rieth, Winterbach, Germany) with 50 μm aluminum oxide particles (S-U-Alustral; Schuler-Dental, Ulm, Germany) at 0.15 MPa pressure (defined as low energy particle abrasion).
- –
airborne particle abrasion with 120 μm aluminum oxide particles at 0.3 MPa pressure.
- –
as-sintered surface served as a control.
The nozzle was placed at 10 cm from the contact surface and the contact time was set at 10 s/cm 2 . Half of the specimens of each group were subjected to a heat treatment (annealing at 1000 °C in open air for 1 h) to eliminate presence of any monoclinic phase on the surface. 20 specimens were fabricated for each group ( n = 20).
2.2
X-ray diffraction analysis (XRD)
The central surface of the zirconia discs was examined using analysis XRD (XRD; M18XHF-SRA, Mac Co., Japan) to reveal changes in the crystallographic content before and after heat treatment for all groups. As-sintered zirconia surface was used as a base line. XRD data were obtained with a diffractometer using nickel-filtered CuK radiation. The tetragonal/monoclinic zirconia ratio was determined using the integrated intensity (measuring the area under the diffractometer peaks) of the tetragonal (1 0 1) and two monoclinic (1 −1 1) and (1 1 1) peaks. The obtained volume fractions were individually normalized. Although the X-ray penetration is too deep to be considered as a real surface analysis, it allowed precise evaluation of the transformation activity.
2.2.1
Application of veneer ceramic
The prepared zirconia discs (18 mm × 0.5 mm) were placed in an adjustable aluminum mold and slurry of properly mixed veneer porcelain (Nobel Rondo Zirconia dentin; A3.5, Nobel Biocare) was applied and condensed in the mold. The slurry was blot dried and the veneered zirconia discs were removed from the mold and sintered in a computer controlled electrical oven (EP 500; Ivoclar Vivadent, Shaan, Liechtenstein) following recommendations of the manufacturer. The outer surface of the veneer porcelain was polished using ascending grit silicon carbide paper and a diamond paste. The final thickness of the veneer porcelain was 1.5 mm. 20 specimens were prepared for every test group ( n = 20). The chemical composition of the materials used is summarized in Table 1 .
