Influence of alumina air-abrasion for highly translucent partially stabilized zirconia on flexural strength, surface properties, and bond strength of resin cement

Abstract Objective This study aims to evaluate the influence of different air-abrasion pressures and subsequent heat treatment on the flexural strength, surface roughness, and crystallographic phases of highly translucent partially stabilized zirconia (Y-PSZ), and on the tensile bond strength of resin cement to Y-PSZ. Methodology Fully sintered zirconia specimens were ground with SiC paper (control) and/or air-abraded with 50 µm particles of alumina at 0.1, 0.15, 0.2, or 0.3 MPa or left as-sintered. After air-abrasion at 0.2 MPa (0.2AB), additional specimens were then heated to 1500°C, and held for one hour at this temperature (0.2AB+HT1h). Flexural strength and surface roughness were evaluated. Crystalline phase identification was also carried out using X-ray diffraction. Bonded zirconia specimens with self-adhesive resin cement were stored in distilled water at 37°C for 24 h, either with or without aging (thermal cycling 4-60°C/20000). Results were analyzed statistically by ANOVA and Tukey-Kramer tests. Results The flexural strength decreased with the increase in air-abrasion pressure, while in contrast, the surface roughness increased. The lowest flexural strength and the highest roughness value were found for the 0.2AB and 0.3AB groups, respectively. All groups contained cubic-, tetragonal ( t )-, and rhombohedral ( r )-ZrO2 phases with the exception of the as-sintered group. Upon increasing the air-abrasion pressure, the relative amount of the r -ZrO2 phase increased, with a significant amount of r -ZrO2 phase being detected for the 0.2AB and 0.3AB groups. The 0.2AB+HT1h group exhibited a similar flexural strength and t -ZrO2 phase content as the as-sintered group. However, the 0.2AB group showed a significantly higher tensile bond strength (p<0.05) than the 0.2AB+HT1h group before and after aging. Conclusion Micromechanical retention by alumina air-abrasion at 0.2 MPa, in combination with chemical bonding of a resin to highly translucent Y-PSZ using a MDP-containing resin cement may enable durable bonding.


Introduction
All-ceramic dental restoration systems have become increasingly popular due to their good esthetics and biocompatibilities compared with metalporcelain restorations. More specifically, zirconia ceramics have become one of the prime alternatives to metal-ceramic restorations due to their low cost compared with gold metal, in addition to the reduced laboratory costs for ceramic fabrication, and the ease of milling zirconia. 1 Conventional zirconia ceramics are predominantly composed of fine tetragonal zirconia crystals containing 3 mol% yttria stabilizer, otherwise known as yttria-stablized tetragonal zirconia polycrystals (conventional Y-TZP). While being exceptionally strong, conventional Y-TZP ceramics tend to have a poor translucency. As zirconia and alumina have different refraction indices, the introduction of alumina can therefore decrease light transmission, due to which the 0.05 wt% alumina-containing Y-TZP is more translucent than its 0.25 wt% equivalent. The most recent strategy to improve the translucency of zirconia is to increase the significant cubic (c) phase in the zirconia structure. 2 This has been achieved using a higher yttria content (4-6 mol%) to produce partially stabilized zirconia (Y-PSZ). 3 This Y-PSZ is isotropic in different crystallographic direction, thereby decreasing the light scattering that occurs at grain boundaries, and rendering this material more translucent. 4,5 The reliable bonding of zirconia ceramics has been reported by the mechanical retention and chemical bonding of resin luting cement to the ceramic substrate. 6 Micromechanical retention can be achieved by increasing the surface area of the substrate with alumina air-abrasion. In addition, chemical and longterm durable bonding to zirconia ceramics has been demonstrated using phosphate monomer-containing resin cements [7][8][9][10][11] or ceramic primers, 12,13 such as 10-methacryloyloxy-decyl-dihydrogenphosphate (MDP). Furthermore, the alumina air-abrasion of conventional Y-TZP produces a protective surface compressive layer due to a tetragonal (t) to monoclinic (m) phase transformation. Air-abrasion can therefore either reduce 14,15 or increase [16][17][18][19]  Denmark) under water-rinsing. The final dimensions of the specimens were 2.0 ± 0.2 × 2.0 ± 0.2 × 25.0 ± 0.5 mm 3 for burs and 10 × 10 × 2 mm 3 for plates. This surface condition was referred to as the control group.
All specimens were randomly assigned to eight

Surface roughness analyses
The surface roughness values of three specimens from each group were measured using a laser scanning microscope (VK-X200, KEYENCE Co., Ltd; Osaka, Japan) equipped with a 50× objective. A laser beam with a spot size of 1 µm was used to scan the specimen surfaces, and this system exhibited a submicron resolution along all axes. Each surface was measured  the 0.2AB+HT0 group exhibited a degree of crystal precipitation, while that of the 0.2AB+HT1h group showed clear crystal grain boundaries.
The tensile bond strengths of the various test groups are shown in Table 2. As indicated, the 0.2AB groups of both TZP and PSZ showed significantly higher tensile bond strengths than the control groups at TCs of 0 and 20,000 (p<0.05). In addition, the 0.2AB+HT1h group exhibited a significantly lower bond strength than the 0.2AB group (p<0.05), in addition to a similar bond strength to the control group (p>0.05).
There is no significant difference in bond strength between before and after TCs only for the 0.2AB/ TZP group (p>0.05). All specimens across the 0.2AB groups for both TZP and PSZ revealed cohesive failure at TCs of 0 and 20,000. In contrast, the other three  In the present study, upon increasing the pressure Standard deviations followed by identical superscript lowercase letters indicate no significant differences in the group at same TCs (p>0.05). Standard deviations followed by identical superscript uppercase letters indicate no significant differences in the TC at same groups (p>0 .05). of alumina air-abrasion, the flexural strength of the highly translucent Y-PSZ decreased, which is a similar result to previous studies. 21,33 These studies also reported that the decrease in the flexural strength of Y-PSZ after alumina air-abrasion was due to the reduced t→m phase transformation in the presence of a high c-ZrO 2 phase content. 21  In this case, this was assumed to originate from the r-ZrO 2 phase because three peaks at around 29.7°, 49.5°, and 58.7° 2θ were observed in agreement with the previous study. 22 The relative amounts of each phase were also calculated by Rietveld refinement.
Thus, it was found that the presence of around 20 wt% When alumina air-abrasion was used to treat the inner surface of zirconia crowns, even with larger particles, the system behaved as a bonded crown, promoting a higher fatigue resistance for the cemented crowns. 45 Conclusions This study has shown that the flexural strength of highly translucent Y-PSZ decreased with the increase in air-abrasion pressure with alumina. The lowest flexural strength and the highest roughness value were found for the 0.2AB and 0.3AB groups, respectively.
Upon increasing the air-abrasion pressure, the relative amount of the rhombohedral (r)-ZrO 2 phase increased, with a significant amount of r-ZrO 2 phase being detected for the 0.2AB and 0.3AB groups.
The 0.2AB+HT1h group exhibited a similar flexural strength and tetragonal-ZrO 2 phase content as the as-sintered group. However, the 0.2AB group showed a significantly higher tensile bond strength than the 0.2AB+HT1h group before and after aging. These findings suggest that micromechanical retention by alumina air-abrasion, in combination with chemical bonding of a resin to highly translucent Y-PSZ using a MDP-containing resin cement may enable durable bonding, which is similar to the conventional and highly translucent Y-TZP.