Grinding with/without additional sandblasting induces residual compressive stress.
Residual compressive stress increases the bending strength of dental zirconia.
Thermal annealing reduces the bending strength of dental zirconia.
To assess the influence of surface treatment and thermal annealing on the four-point bending strength of two ground dental zirconia grades.
Fully-sintered zirconia specimens (4.0 × 3.0 × 45.0 mm 3 ) of Y-TZP zirconia (LAVA Plus, 3M ESPE) and Y-TZP/Al 2 O 3 zirconia (ZirTough, Kuraray Noritake) were subjected to four surface treatments: (1) ‘GROUND’: all surfaces were ground with a diamond-coated grinding wheel on a grinding machine; (2) ‘GROUND + HEAT’: (1) followed by annealing at 1100 °C for 30 min; (3) ‘GROUND + Al 2 O 3 SANDBLASTED’: (1) followed by sandblasting using Al 2 O 3 ; (4) ‘GROUND + CoJet SANDBLASTED’: (1) followed by tribochemical silica (CoJet) sandblasting. Micro-Raman spectroscopy was used to assess the zirconia-phase composition and potentially induced residual stress. The four-point bending strength was measured using a universal material-testing machine.
Weibull analysis revealed a substantially higher Weibull modulus and slightly higher characteristic strength for ZirTough (Kuraray Noritake) than for LAVA Plus (3M ESPE). For both zirconia grades, the ‘GROUND’ zirconia had the lowest Weibull modulus in combination with a high characteristic strength. Sandblasting hardly changed the bending strength but substantially increased the Weibull modulus of the ground zirconia, whereas a thermal treatment increased the Weibull modulus of both zirconia grades but resulted in a significantly lower bending strength. Micro-Raman analysis revealed a higher residual compressive surface stress that correlated with an increased bending strength.
Residual compressive surface stress increased the bending strength of dental zirconia. Thermal annealing substantially reduced the bending strength but increased the consistency (reliability) of ‘GROUND’ zirconia.
Today, dental zirconia most commonly consist of yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) based on the TZ-3YE (Tosoh, Tokyo, Japan) starting powder . Recently, more aging-resistant and stronger zirconia, such as Ce-TZP/Al 2 O 3 zirconia (NANOZR, Panasonic, Osaka, Japan) or Y-TZP/Al 2 O 3 zirconia (ZirTough, Kuraray Noritake, Tokyo, Japan), have been introduced in dentistry .
In many cases, the dental technician needs to additionally grind the inside of the zirconia core to improve the fit of the zirconia restoration onto the tooth preparation . Furthermore, a recent systematic review revealed that different mechanical surface pre-treatments are essential for the composite cement to durably bond to zirconia, this in light of an adhesive luting procedure . Among them, aluminium oxide (Al 2 O 3 ) sandblasting or tribochemical silica sandblasting with 30- and 110-μm silica-coated Al 2 O 3 particles are quite important to achieve durable bonding to zirconia . However, the surface of the abraded zirconia will be transformed, i.e., constrained as well as damaged; this stress-induced transformation may influence the long-term clinical performance of zirconia restorations .
Several papers reported on the influence of different surface treatments on the mechanical properties of dental zirconia. However, no consensus was reached in literature whether surface treatments are detrimental for the mechanical properties of dental zirconia. Some of those studies reported an increased bending strength of zirconia , whereas others reported a decreased strength as result of surface damage produced by the surface treatments . Moreover, the mechanical properties of surface-treated newer zirconia, such as Y-TZP/Al 2 O 3 zirconia (and Ce-TZP/Al 2 O 3 zirconia, however not investigated in this study) have not much been studied yet. The objective of this study was therefore to evaluate the influence of different surface treatments on the bending strength of two different, i.e., Y-TZP and Y-TZP/Al 2 O 3 , dental zirconia. The null hypothesis tested was that different surface pre-treatments do not influence the bending strength and consistency (reliability) of dental zirconia.
Materials and methods
Fully-sintered Y-TZP LAVA Plus (3M ESPE, Seefeld, Germany) zirconia and Y-TZP/Al 2 O 3 ZirTough (Kuraray Noritake) zirconia were provided by the manufacturers in the form of 3.5 × 4.5 × 45.0 mm bending bars. Next, the specimens were ground to the size of 3.0 × 4.0 × 45.0 mm, after which they were assigned to four surface treatments ( Table 1 ): (1) ‘GROUND’: all surfaces were ground using a polymer-bonded diamond-grinding wheel (D46 SW 50—X2, Technodiamant, Almere, The Netherlands) on a grinding machine (JF415DS, Jung, Göppingen, Germany); (2) ‘GROUND + HEAT’: (1) followed by annealing at 1100 °C in air for 30 min (Nabertherm, Germany); (3) ‘GROUND + Al 2 O 3 SANDBLASTED’: (1) followed by sandblasting using 50 μm Al 2 O 3 particles (Danville Engineering, San Ramon, CA, USA); (4) ‘GROUND + CoJet SANDBLASTED’: (1) followed by tribochemical silica sandblasting with 30 μm silica-coated Al 2 O 3 particles using CoJet (3 M ESPE). All surfaces of the bending bar received the same surface treatment.
|‘GROUND’||All surfaces were ground with a polymer-bonded diamond-grinding wheel (D46 SW 50—X2, Technodiamant, Almere, The Netherlands) on a grinding machine (JF415DS, Jung, Göppingen, Germany).|
|‘GROUND + HEAT’||‘GROUND’ followed by annealing at 1100 °C in air for 30 min.|
|‘GROUND + Al 2 O 3 SANDBLASTED’||‘GROUND’ followed by sandblasting with 50 μm Al 2 O 3 particles (Danville, Danville, CA, USA) for 15 s/cm 2 at a distance of 10 mm with an intraoral air-abrasion device (Microetcher, Danville Engineering, Danville, CA, USA).|
|‘GROUND + CoJet SANDBLASTED’||‘GROUND’ followed by sandblasting with 30 μm silica-coated Al 2 O 3 particles (CoJet sand, 3M ESPE, Seefeld, Germany) for 15 s/cm 2 at a distance of 10 mm with an intraoral air-abrasion device (CoJet Prep, 3M ESPE, Seefeld, Germany).|
One specimen from each surface-treated zirconia was used for micro-structural investigation using scanning electron microscopy (SEM, JSM-6610LV, JEOL, Tokyo, Japan) employing the following conditions (gold coated, 10 −5 mbar pressure, 15 kV energy range, 85 μA beam current, 1000× magnification, secondary electron image).
Micro-Raman spectroscopy was used to measure the monoclinic zirconia ( m -ZrO 2 ) surface-volume fraction, and to assess the nature and extent of potential residual stress on the surface . Depth-profile Raman spectra (SENTERRA, BrukerOptik, Ettlingen, Germany) were also collected from the top 20-μm material of surface-treated zirconia specimens using the following conditions: Ar-ion laser with a wavelength of 532 nm, 20 mW power at sample and 100× objective. The spectrum integration time was 20 s with the recorded spectra averaged over three successive measurements. For each specimen, at least 12 measurements were performed using a pinhole aperture of 50 μm. The m -ZrO 2 surface-volume fraction was calculated using the equation proposed by Tabares et al. :
V m = I m 181 + I m 190 0.32 ( I t 147 + I t 265 ) + I m 181 + I m 190