Statement of problem
A consensus on the benefits of airborne-particle abrasion of zirconia with alumina particles of different sizes is still lacking. Larger particle size may improve micromechanical retention but may generate deep microcracks on the zirconia surface.
The purpose of this in vitro study was to evaluate the effect of different size of Al 2 O 3 particles used for surface abrasion on the mechanical properties of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP).
Material and methods
Sixty Y-TZP specimens were divided into 6 groups according to the treatment: control (without treatment) or airborne-particle abrasion with Al 2 O 3 particles (45 μm or 150 μm). Half the specimens were stored for 24 hours in water while the other half was exposed to 1.5×10 6 mechanical cycles before flexural strength analysis at 1 mm/min crosshead speed. Specimens were also characterized by micro-Raman spectroscopy and X-ray diffraction (XRD) to evaluate the crystalline composition. The data were subjected to 2-way ANOVA and Tukey HSD test (α=.05).
Airborne-particle abrasion with alumina ( P =.030) and mechanical fatigue ( P <.001) had a significant effect on flexural strength. Specimens abraded with 45-μm Al 2 O 3 particles (847 ±204 MPa) presented higher flexural strength than those of the control group (670 ±210 MPa). The size of the alumina particles was not significant for flexural strength. Flexural resistance (664 MPa) significantly decreased after mechanical fatigue. All groups showed only the tetragonal phase on the micro-Raman spectra, which was confirmed by XRD.
Airborne-particle abrasion with smaller Al 2 O 3 particles increased the flexural strength on Y-TZP without causing phase transformation. However, flexural strength was decreased after mechanical fatigue.
Abrasion of zirconia with alumina particles did not affect the mechanical performance of material, indicating that airborne-particle abrasion may be safely used to improve the interlocking between zirconia restorations and the resin cement underneath. In the long-term, zirconia restorations may be affected by mechanical fatigue.
The use of ceramics in restorative dentistry has increased, and yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) has been widely used when high strength is required. Zirconia is a polymorphic material that can exhibit up to 3 crystalline forms, monoclinic (m), tetragonal (t), and cubic (c), depending on temperature and pressure conditions. To stabilize the tetragonal phase at room temperature and keep the material as a cohesive body, yttrium-trioxide is added.
Bonding zirconia restorations to the tooth structure is achieved mainly through micromechanical interlocking. However, the absence of a glass phase makes yttria-tetragonal zirconia polycrystalline (TZP) unreactive to hydrofluoric acid etching, so it is necessary to use other methods to increase surface roughness, including airborne-particle abrasion with aluminum oxide (Al 2 O 3 ) particles. The size of the Al 2 O 3 particles varies from 25 to 250 μm.
The resulting roughness will depend on the particle type and size and the air pressure applied. A larger particle size may improve micromechanical retention, but it also generates deep microcracks in the zirconia surface that have been reported to reduce fatigue strength by approximately 30%. Consequently, these conditions may also result in either a compressive stress superficial layer or immediate t→m transformation, which may affect the long-term performance of the material. The consequences of generated compressive stresses include higher surface hardness and superior mechanical properties, causing a temporary protective effect to the most superficial layer. , To minimize t→m transformation, small alumina particles (below 50 μm) have been recommended to optimize surface roughness, favoring the zirconia-adhesive bonding and causing a slight volume change to avoid surface damage. However, consensus on the benefits of airborne-particle abrasion with smaller or larger alumina particle size is lacking.
Therefore, the purpose of this in vitro study was to evaluate the effect of different Al 2 O 3 particle sizes on the mechanical properties of Y-TZP after mechanical fatigue. The research hypotheses were that alumina particle size affects the flexural strength of Y-TZP, mechanical fatigue affects the flexural strength of Y-TZP, and alumina airborne-particle abrasion causes phase transformation, regardless of alumina particle size.
Material and methods
One presintered block of Y-TZP (Ceramill Zolid; Amann Girrbach AG) was cut by using a diamond blade (15LC; Buehler) and a high-speed precision saw (Isomet 1000 Precision saw; Buehler Ltd) to generate 60 bars. After sintering (Vita zyrcomat 6000 MS; Vita Zahnfabrik) according to the manufacturer’s instructions, the final dimensions of the specimens were: 16×4×2 mm. One surface of each specimen was ground with 320-, 400-, and 600-grit silicon carbide (SiC) abrasive papers under water cooling by using a rotary grinder/polisher (Lightweight model 900; South Bay Technology Inc). Then, sequential polishing was performed with polishing cloths and diamond suspensions (6 μm, 3 μm, and a mix of 1-μm diamond and 0.6-μm colloidal silica; Buehler) for 4 minutes each. The specimens were ultrasonically cleaned (Bransonic 1800 Ultrasonic Cleaner; Branson) in distilled water for 10 minutes between each application of abrasive paper and polishing cloths and at the end of the polishing procedure.
The specimens were annealed to eliminate compressive strains added to the surface during the finishing and polishing procedures (CWF 13/5; Carbolite Gero Ltd) with a heating rate of 5 °C/min, a dwell time of 2 hours at 1200 °C, and a cooling rate of 1 °C/min. The specimens were stored dry and at room temperature.
The specimens were divided into 6 groups (n=10). Control specimens did not receive any surface treatment. Treatment specimens received alumina airborne-particle abrasion on 1 surface with different particle sizes (45 μm or 150 μm) (Polidental; Polidental LTDA) according to the following protocol: alumina airborne-particle abrasion at 10 mm at 90 degrees, 0.25 MPa pressure for 15 seconds. Half the specimens were tested after 24 hours, while the other half was aged by mechanical fatigue before being evaluated for flexural strength ( Table 1 ).
|Groups||Al 2 O 3 -Particle Abrasion (Size of Particles)||Mechanical Fatigue|
|45 μm NF||45 μm||No|
|150 μm NF||150 μm||No|
|45 μmF||45 μm||Yes|
|150 μmF||150 μm||Yes|
For the mechanical aging, 1.5×10 6 cycles (37 °C, artificial saliva, 6-mm-diameter stainless steel indenter) were applied in a mastication simulator (Chewing Simulator CS4.4; SD Mechatronik Ltd), with a loading force of 0 to 80 N, vertical movement of 0.8 mm, and a downward speed of 60 mm/s. Four specimens were cycled at the same time in individual chambers. The specimens were loaded with the alumina airborne-particle abrasion surface under tensile stresses. After fatiguing, the specimens were stored in a dry container at room temperature (approximately 23 °C) for up to 48 hours before being further characterized.
Flexural strength was evaluated (n=10) according to the 3-point bend test design (ISO6872:2015) in a universal testing machine (Instron Model 8501; Instron Inc) with a 5-kN load cell and a 12-mm span. The load was applied to the center of the specimen with the alumina airborne-particle abrasion surface under tension. The test was performed at a crosshead speed of 1.0 mm/min until the specimen fractured. Load at failure was recorded, and flexural strength was calculated considering the dimensions of the specimen by using the equation:
P is the load at failure in Newtons,
I is the distance between support rollers in millimeters, and
b are the width and thickness of the specimen, respectively, in millimeters.
One specimen of each group was collected, cleaned in an ultrasonic bath for 10 minutes with acetone, and stored in sealed plastic containers at room temperature (approximately 23 °C). These fractured pieces were evaluated by micro-Raman spectroscopy, fractography, and X-ray diffraction (XRD) analysis.
The treated surface of 1 specimen of each group was examined to determine tetragonal and monoclinic crystalline phases by using a micro-Raman spectroscope (Senterra II; Bruker) with a laser wavelength of 532 nm under ×50 magnification. The laser power was kept at 25 mW, and spectra were obtained over the range of 3 to 5 cm -1 .
To confirm the data from micro-Raman spectroscopy and quantify the m-phase, the analysis was also performed by XRD (Ultima IV; Rigaku Corp) at 20 mA and 40 kV. Spectra were collected at a 2θ of between 20 and 80 degrees at a step interval of 1 second and step size of 0.03 degree. The amount of m-phase (X M ) was calculated by using the equation:
M corresponds to 2θ=28 degrees, (111)
M corresponds to 2θ=31.2 degrees, and (101)
T corresponds to (2θ=30 degrees). Both, (111)
M and (-111)
M represent monoclinic peaks, while (101)
T represents the intensity of the tetragonal peak. The volume fraction (F
m ) of the m-phase was calculated according to the following equation:
For qualitative analysis of the fractured interface, 1 fractured specimen of each group was gold-coated by using a sputter-coating machine (POLARON E5100 II; Polaron Instruments Inc) and evaluated under a SEM (JSM-6610LV; JEOL Ltd) operated at 15 to 20 kV under high-vacuum power. The images were captured at ×30 and ×850 magnification to identify the crack initiation site.
The data were analyzed by the Shapiro-Wilk test, and the normal distribution was confirmed. Then, the data from flexural strength were analyzed by 2-way ANOVA, with the independent variables being alumina airborne-particle abrasion and fatigue and the Tukey HSD test. A statistical software program (SAS Estat 9.3; SAS Institute Inc) was used for the analysis (α=.05).
The 2-way ANOVA showed that alumina airborne-particle abrasion (F=3.72; P =.030) and fatigue (F=13.89; P <.001) had a significant effect on flexural strength. However, the interaction between the factors was not significant (F=0.29; P =.750). The results of the Tukey HSD test are presented in Tables 2 and 3 .
|Airborne-Particle Abrasion||Mean ±Standard Deviation|
|45 μm||847 ±204 A|
|150 μm||772 ±258 AB|
|Control||670 ±210 B|
|Fatigue||Mean ±Standard Deviation|
|No||862 ±230 A|
|Yes||664 ±193 B|
Airborne-particle abrasion with 45-μm aluminum oxide particles resulted in flexural strength values that were significantly higher than those of the control group ( P =.030). Specimens treated with 150-μm alumina particles presented intermediate results ( Table 2 ). For specimens submitted to mechanical fatigue, fatigue negatively affected flexural strength values ( Table 3 ).
Table 4 shows the prominent peaks found for each group analyzed under Raman spectroscopy. According to Hallman et al, all the groups showed typical bands of the tetragonal phase. The nonaged groups showed bands typical of alumina peaks. The XRD analysis showed that all the specimens presented high-intensity spectra compatible with the tetragonal phase after mechanical fatigue ( Fig. 1 ) and that no peaks compatible with the monoclinic phase were detected.
|CNF||45 μm NF||150 μm NF||CF||45 μm F||150 μm F|
|Tetragonal||˜ 648||˜ 648||˜ 647||˜ 647||˜ 647||˜ 647|
|Tetragonal||˜ 610||˜ 610||˜ 611||˜ 611||˜ 611||˜ 611|
|Tetragonal||˜ 469||˜ 470||˜ 467||˜ 467||˜ 467||˜ 467|
|Tetragonal||˜ 323||˜ 328||˜ 325||˜ 325||˜ 325||˜ 325|
|Tetragonal||˜ 268||˜ 266||˜ 267||˜ 267||˜ 267||˜ 267|
|Tetragonal||˜ 153||˜ 153||˜ 154||˜ 154||˜ 154||˜ 154|
|˜ 441||˜ 443||˜ 442|
|˜ 430||˜ 430||˜ 430|
The failure origins of fractured specimens were always located on the most superficial region under tensile stress, the alumina airborne-particle-abraded surface ( Fig. 2 ). Concentration during the test and a compression curl were observed on the side opposite to the failure origin on flexural strength testing.