This study evaluated how the flexural strength and fracture behavior of a zirconia-based ceramic (Y-TZP) were affected by pre- and post-sintering mechanical and thermal treatments.
Treatments included sandblasting with different particle size and type (30 μm SiO 2 ; 50 and 110 μm Al 2 O 3 ) and thermal conditioning. Two hundred bar-shaped specimens of pre-sintered Y-TZP ceramic (Lava Frame, 3M) were prepared (specimen dimensions: 25 mm length × 4 mm width × 0.7 mm thickness) and divided into three groups (before sintering, after sintering and after sintering with heating treatment). The before sintering group specimens were airborne-particle abraded prior to dense sintering. Specimens from the after sintering group were airborne-particle abraded after sintering. The after sintering with heating treatment group specimens were submitted to a heating procedure after airborne-particle abrasion. The controls were the specimens that were sintered and not treated with any conditioning procedures. The specimens from all experimental conditions were analyzed by SEM, CLSM and XRD. All specimens were tested in four-point bending. Data were statistically analyzed using one-way ANOVA and Post Hoc tests ( α = 0.05). A Weibull analysis was used to analyze the strength reliability.
Sandblasting pre-sintered zirconia before sintering significantly decreased the flexural strength, except when the smallest blasting particles were used (30 μm SiO 2 ). Phase transformation ( t–m ) was observed after sandblasting and reverse transformation ( m–t ) was observed after heating.
Sandblasting with 30 μm SiO 2 and 50 μm Al 2 O 3 allowed lower phase transformation. However, 30 mm SiO 2 presented better reliability.
All-ceramic restorations have been widely used in dentistry and they have contributed to the development of ceramic materials, such as densely sintered alumina and yttria-tetragonal zirconia polycrystalline (Y-TZP) ceramics. However, the adhesion of resin cements to zirconia is poor, and presents a barrier to higher longevity restorations. Reliable adhesion principally requires surface treatment of ceramics, with either mechanical surface treatment, chemical conditioning or a combination of the two. In the case of zirconia, established chemical roughening procedures (useful for acid susceptible ceramics such as feldspathic ceramics) do not work, and other methods are required, such as airborne-particle abrasion, to produce a rough surface for micromechanical bonding . Air abrasion can alter the ceramic’s ability to withstand fracture . Transformation from the tetragonal to monoclinic structure may occur during airborne-particle abrasion. This transformation is believed to decrease the longevity of zirconia ceramic restorations or in some cases lengthen lifetime by strengthening the material . It is known that abrasion introduces damage in the ceramic during roughening, such as microcracks, lateral cracks and plastic deformation .
In general, sandblasting is performed as a pre-cementation surface treatment after sintering zirconia which is believed to increase the monoclinic phase fraction in the zirconia structure. To decrease the monoclinic phase, an additional heat treatment is required as the phase transformation is thermally induced. For this reason, Moon et al. and Monaco et al. evaluated the bond strength between zirconia and cement carrying out the airborne-particle abrasion of Y-TZP surfaces before sintering. According to their findings, there was no difference on the bond strength values compared to the air abrasion performed after sintering; however, heating after sandblasting may prevent damage to the Y-TZP microstructure . If sandblasting is performed on the pre-sintered milled restoration, the heat treatment would be the sintering process. If sandblasting is performed on the densely sintered restoration, the heat treatment could be a veneering procedure or reglazing after clinical adjustments.
Sandblasting of pre-sintered zirconia followed by sintering could provide higher flexural strength than sandblasting after the sintering procedure. Sandblasting densely sintered zirconia creates sharp depressions and protrusions on the surface which might result in the initiation of stress concentrations, thus leading to lower flexural strength values. Therefore, if pre-sintered zirconia is sandblasted before sintering, depressions and protrusions could become rounded during sintering, and any cracks could heal, increasing fracture strength values. Thus, this study evaluated the effect of surface treatments on the mechanical properties of zirconia-based ceramics (Y-TZP), and in particular, surface treatments before and after sintering. The proposed work aimed to address the null hypotheses that the preparation order of zirconia ceramic surface does not modify the mechanical properties, different blasting particle types do not affect the mechanical properties and thermal conditioning does not affect the mechanical properties.
Materials and methods
Zirconia bar production and experimental groups
Zirconia blocks were provided as “partially sintered” by the manufacturer (Lava™ Frame, 3M ESPE, St. Paul, MN, USA, LOT: 480872) and then bar specimens were fabricated. Two hundred bar-shaped specimens were machined and one hundred forty were sintered in advance in a Lava furnace 200 (3M ESPE, St. Paul, MN, USA), according to the manufacturer’s instructions. All specimens were finished subsequently by machine grinding and polishing using diamond lapping film in sequence (30, 15, 9 and 3 microns) under water cooling (Allied High Tech Products Inc., Compton, CA, USA). After polishing, the average surface roughness was determined and recorded, R a = 0.003 (0.03) μm (Dimension Edge, Bruker, Billerica, MA, USA). The remaining specimens were airborne-particle abraded then sintered in a Lava furnace 200, according to the manufacturer’s instructions. The final dimensions of the specimens were: 25 mm × 4 mm × 0.7 mm. The specimens’ edges were chamfered using a holding device according to ISO 14704 recommendations. All specimens were randomly divided into 10 groups ( n = 20), according to particle type and size, preparation order, and heating procedure ( Table 1 ). The control was the untreated specimens ( n = 20) which were sintered and polished. Prior to the preparation procedures, all specimens were cleaned ultrasonically for five minutes in distilled water (Bransonic ® ultrasonic cleaner, Branson Ultrasonic, Danbury, CT, USA).
|Blasting particle type||Sandblasting protocol||Groups a|
|Air abrasion with SiO 2
|After sintering, no heat||G3|
|After sintering + heat treatment||G4|
|Air abrasion with Al 2 O 3
|After sintering, no heat||G6|
|After sintering + heat treatment||G7|
|Air abrasion with Al 2 O 3
|After sintering, no heat||G9|
|After sintering + heat treatment||G10|
Zirconia surface treatment
One hundred and eighty specimens of pre-sintered Y-TZP bars were randomly divided into 4 groups. One test group was the control (sintered and polished). For all the other groups, the specimens’ tensile surface was sandblasted. The second test group (air abrasion before sintering) was prepared with sandblasting in advance and then densely sintered according to the manufacturer’s instructions. The third test group (air abrasion after sintering) was densely sintered in advance under the same conditions, and then sandblasted. Finally, the fourth group (air abrasion after sintering with heating procedure) was prepared as the second group which was followed by a heat treatment. Air-abrasion was performed by making circular movements with the nozzle at a distance of 10 mm with 2.8 bar pressure for 15 s. Air abrasion with Al 2 O 3 was performed using 50 μm and 110 μm diameter particles (Bego Korox 50 and Korox 110, Bremen, Germany; LOT: 1266548, LOT: 1329797, respectively), and the remaining specimens were sandblasted with SiO 2 30-μm (Rocatec™ Soft, 3M ESPE, Seefeld, Germany, LOT: 450384).
The specimens from the heat treated groups were submitted to the firing cycle used to sinter the porcelain veneer VITA VM9 (950 °C), according to the manufacturer’s instructions (Vita Zahnfabrik, Bad Säckingen, Germany).
The specimens were investigated by X-ray diffractometry (Rigaku Ultima IV, Tokyo, Japan) before sintering (no treatment), after sintering (unpolished, no treatment), sintered and polished (no treatment), sandblasted before and after sintering, and sandblasted after sintering with thermal conditioning. The influence of surface treatments was assessed by the phase composition. Data was collected using Co Kα radiation (1.79026 Ǻ) at 38 kV and 38 mA. A diffractogram was obtained between 20° and 90° 2 θ , at a scan speed of 1.5°/min with a step size of 0.02°. The relative amount of transformed monoclinic, m , structure ( F M ) on the zirconia surfaces was determined based on the following equations :
F M = 1.311 X M 1 + 0.311 X M
X M = ( − 111 ) M + ( 111 ) M ( − 111 ) M + ( 111 ) M + ( 101 ) T
where (−111) M and (111) M are the most intense monoclinic peaks (2 θ = 28° and 2 θ = 31.2°, respectively) and (101) T corresponds to the intensity of the respective tetragonal peak (2 θ = 30°).
The transformed zone depth (TZD) was determined on the treated zirconia surface and calculated according to the amount of the monoclinic phase (assuming that the transformation from tetragonal to monoclinic took place symmetrically along the surface). The TZD was obtained based on Eq. (C) :
T Z D = sin θ 2 μ ln 1 1 − F M
where θ = 15° is the angle of reflection; μ (=0.0642) is the absorption coefficient, and F M is the relative amount of monoclinic phase calculated from the Eqs. (A) and (B) .
Flexural strength testing
Specimens were subjected to a four-point flexural strength test. The bar specimens were fixed between four rollers ( ∅ = 2 mm, distance of inner/outer rollers: 10/20 mm) and the load was applied by a universal testing machine (Instron; Instron Corp, Canton, Mass., Norwood, MA, USA) until failure. The maximum load (P, [N]) was recorded at the first sign of fracture and a change in the force versus displacement curve. The flexural strength ( σ f ) was calculated using the following equation :
σ f = 3 P ( L − L i ) 2 W T 2