The Influence of Alumina Airborne-Particle Abrasion with Various Sizes of Alumina Particles on the Phase Transformation and Fracture Resistance of Zirconia-Based Dental Ceramics

The surface of zirconia-based dental ceramic restorations require preparation prior to adhesive cementation. The purpose of this study was to assess the influence of airborne-particle abrasion with different sizes of alumina particles (50 μm, 110 μm, or 250 μm) on the mechanical strength of zirconia-based ceramics’ frameworks and on the extent of phase transformations. A fracture resistance test was performed. The central surface of the frameworks was subjected to a load [N]. The identification and quantitative determination of the crystalline phase present in the zirconia specimens was assessed using X-ray diffraction. The Kruskal–Wallis one-way analysis of variance was used to establish significance (α = 0.05). The fracture resistance of zirconia-based frameworks significantly increases with an increase in the size of alumina particles used for air abrasion: 715.5 N for 250 μm alumina particles, 661.1 N for 110 μm, 608.7 N for 50 μm and the lowest for the untreated specimens (364.2 N). The X-ray diffraction analysis showed an increase in the monoclinic phase content after air abrasion: 50 μm alumina particles—26%, 110 μm—40%, 250 μm—56%, and no treatment—none. Air abrasion of the zirconia-based dental ceramics’ surface with alumina particles increases the fracture resistance of zirconia copings and the monoclinic phase volume. This increase is strongly related to the alumina particle size.


Introduction
Zirconia-based dental ceramics possess superior mechanical parameters to other types of dental ceramics. They are widely used in dentistry, particularly in prosthodontics, as the material for full-ceramic dental crowns and bridge frameworks, monolithic restorations, and implant suprastructures. However, the adhesive bonding of 3Y-TZP restorations (3 mol % yttria stabilized tetragonal zirconia polycrystals) to resin cements is impaired by the structure and inertness of zirconia [1][2][3]. A number of surface treatments have been recommended in the literature to increase the bond strength of zirconia-based ceramics to resin cements. The method of choice is airborne-particle abrasion with alumina particles and the application of primers incorporating organophosphate monomers [2][3][4][5][6][7][8][9][10]. Laboratory tests indicate that this approach significantly improves the bonding capacity [4,[10][11][12][13][14]. In addition, airborne alumina abrasion guarantees micro-mechanical retention, activates the surface, increases wettability, and ensures proper cleaning of organic and inorganic contamination from the 3Y-TZP surface [15].

Materials and Methods
The blanks of commercial pre-sintered 3Y-TZP ceramics were used in this study (Ceramill Zi; Amann Girrbach AG, Koblach, Austria) in accordance with the norm DIN EN ISO 6872:2009 and ISO 13356:1997. They were available in the form of semi-circular blocks of compressed 3Y-TZP powder intended for CAD/CAM technology (computer aided design-computer aided manufacturing). Two tests were conducted: the fracture resistance of zirconia frameworks and the X-ray diffraction assessment of phase transformations (XRD).

The Fracture Resistance of Zirconia Frameworks
The specimens, in the form of monolithic crowns, were fabricated in the CAD/CAM technology. The cast model was used to prepare the abutment tooth. The second molar was prepared under the control of a paralelometer (Kavo EWL 990; Kavo Polska Sp. z.o.o., Warsaw, Poland) following guidelines: 6 degrees taper, no sharp edges, and 0.5 mmwidth radial shoulder margin. The occlusal surface was prepared with the concavity matching the dimensions and shape of the spherical intender of the Universal Testing Machine.
The cast model with the prepared abutment tooth was scanned (Ceramill Map 300; Amann Girrbach AG, Koblach, Austria). Then, the shape of the framework with uniformly thick walls (of 0.5 mm) was designed in the software (Ceramill Mind 3.11; Amann Girrbach AG, Koblach, Austria), converted into milling strips, loaded into the milling machine, and was finally dry-processed (Ceramill Motion; Amann Girrbach AG, Koblach, Austria). Sixty-eight identical specimens were milled (Ceramill Mind, Ceramill Motion; Amann Girrbach AG, Koblach, Austria). The specimens before the sintering step are shown in Figure 1a. All specimens were sintered for 10 h (Ceramill Therm; Amann Girrbach AG, Koblach, Austria). The temperature growth was 8 °C/min, starting from 200 °C to 1450 °C, with two hours of a constant temperature and then a cooling time. The material shrinkage was approximately 21%. The figure shows milled samples before the sintering step: (a) specimens used to assess the fracture resistance of zirconia-based frameworks; (b) cuboid-shaped specimens used in X-ray diffraction test to assess the t-m transformation.
Specimens were randomly divided into four groups depending on the size of alumina particles used for air abrasion: 50 µm, 110 µm, and 250 µm (Alustral; Omnident Dental HandelsgesellschaftmbH, Rodgau, Germany). One group was left untreated (the control group). The inner surface of the frameworks was air-abraded at the pressure of 3 bar (Basic Classicl Renfert GmbH, Hilzingen, Germany). Prior to air abrasion, the inner surface of each framework was colored with a permanent black marker, and the exterior Figure 1. The zirconia-based dental ceramics specimens fabricated in the CAD/CAM technology. The figure shows milled samples before the sintering step: (a) specimens used to assess the fracture resistance of zirconia-based frameworks; (b) cuboid-shaped specimens used in X-ray diffraction test to assess the t-m transformation.
Specimens were randomly divided into four groups depending on the size of alumina particles used for air abrasion: 50 µm, 110 µm, and 250 µm (Alustral; Omnident Dental HandelsgesellschaftmbH, Rodgau, Germany). One group was left untreated (the control group). The inner surface of the frameworks was air-abraded at the pressure of 3 bar (Basic Classicl Renfert GmbH, Hilzingen, Germany). Prior to air abrasion, the inner surface of each framework was colored with a permanent black marker, and the exterior of the crowns was protected with Teflon tape. The air abrasion was performed in a systematic manner, recommended by Kern, until complete color removal. Such method ensures the homogenous air abrasion of the inner surface, no omission of any surface, and the protection of the exterior surface [33]. The surface of each specimen was then cleaned with an oil-free air stream [34].
The influence of the size of alumina particles used for air abrasion on thefracture resistance of zirconia-based frameworks was examined. The fracture load in each group was measured in Universal Testing Machine (Zwick Z2.5 zwickLine; Zwick GmbH & Co. KG, Ulm, Germany). Specimens were mounted on a steel basis covered with a thin silicone mat. The central occlusal surface of each specimen was subjected to the load with the spherical indenter (diameter 3.5 mm) at speed 1 mm/min. The maximum load was registered [N].

X-ray Diffraction
To assess the presence of a monoclinic phase in the zirconia-based specimens after alumina air abrasion, the X-ray diffraction test (XRD) was conducted. The examined surface of specimens needed to be flat. Cuboid-shaped specimens with the dimensions 5 × 5 × 20 mm were designed in the computer software (Ceramill Mind; Amann Girrbach AG, Koblach, Austria), processed to the milling device, and dry-machined (Ceramill Motion; Amann Girrbach AG, Koblach, Austria). The specimens before the sintering step are shown in Figure 1b. All specimens were sintered for 10 h (Ceramill Therm; Amann Girrbach AG, Koblach, Austria). The temperature growth was 8 • C/min, starting from 200 • C to 1450 • C, with two hours of a constant temperature and then a cooling time. The material shrinkage was approximately 21%. Eventually, specimens were 4 × 4 × 15.8 mm in dimension.
Specimens were randomly divided into four groups depending on the size of alumina particles used for air abrasion, respectively, 50 µm, 110 µm, and 250 µm (Alustral; Omnident Dental HandelsgesellschaftmbH, Rodgau, Germany). Specimens in one group were left untreated (the control group). Other specimens were air-abraded at a 45 • angle, at a 13 mm distance, at a pressure of 3 bar, for 10 s (Basic Classicl Renfert GmbH, Hilzingen, Germany).
The identification and quantitative determination of crystalline phases present in zirconia specimens was conducted using the X-ray diffraction method (PANalytical Empyrean; PANalytical B.V., EA Almelo, The Netherlands) [35]. The qualitative analysis was performed with High Score Plus Programme and the ICCD PDF 4+ database. The percentage of the monoclinic phase volume in each specimen was calculated. The quantitative analysis was performed with the Chung equation method, in accordance with RIR data.

•
Scans were performed using a Bragg-Brentano design in θ-θ mode with the parameters below; • Incident beam: Co Kα radiation at 40 kV and 45 mA, a programmable divergence slit of 1 /2 degree, a Soller slit of 0.04 rad, and a mask measuring 5 mm; • Diffracted beam: semiconductor detector X'Celerator, a Soller slit of 0.04 rad, and a Kβ filter; • A goniometer of five degrees of freedom: X-Y-Z-Phi-Chi; • For the small-angle X-ray diffractograms, a 0.1 degrees/step and counting time of 100 s per step were used.

Statistical Analysis
The statistical analysis was performed with the Kruskal-Wallis one-way analysis-ofvariance-by-ranks test, with a level of significance set at 0.05.

Results
The mean fracture resistance of zirconia-based frameworks values in N, and the standard deviations of all tested groups are shown in Figure 2. The fracture resistance increased with the increase of the alumina particle size used for airborne-particle abrasion, reaching the highest values for 250 µm alumina particles (715.5 N), followed by 110 µm alumina particles (661.1 N), and 50 µm (608.7 N). For the untreated specimens, the fracture resistance was 364.2 N. Air-abrading of the inner surface of the zirconia frameworks resulted in about twice higher values of fracture resistance. The Kruskal-Wallis one-way analysis-of-variance-by-rank test revealed that the use of air abrasion significantly affected the fracture resistance of zirconia frameworks (p < 0.001). Also, the size of alumina particles significantly affected the fracture resistance of the zirconia frameworks (p < 0.001).
reaching the highest values for 250 µm alumina particles (715.5 N), followed by 110 µm alumina particles (661.1 N), and 50 µm (608.7 N). For the untreated specimens, the fracture resistance was 364.2 N. Air-abrading of the inner surface of the zirconia frameworks resulted in about twice higher values of fracture resistance. The Kruskal-Wallis one-way analysis-of-variance-by-rank test revealed that the use of air abrasion significantly affected the fracture resistance of zirconia frameworks (p < 0.001). Also, the size of alumina particles significantly affected the fracture resistance of the zirconia frameworks (p < 0.001). The XRD results are demonstrated in representative plots in Figures 3-6. X-ray diffractograms are the graphic record of measurements presented as a rectangular coordinate system, in which the X-axis refers to the magnitude of the diffraction angle and the Y-axis refers to relative intensity of peaks. In the control group, in which the specimen surface was untreated, the monoclinic phase was not detected. Meanwhile, in groups in which the air abrasion was used, the monoclinic phase was detected, and its content increased with the increase of the alumina particle size. The mean monoclinic volume fraction reached the following respective values: for air abrasion with 50 µm alumina particles, the m content was 26%; for 110 µm, it was 40%; and for 250 µm, it was 44%. The XRD results are demonstrated in representative plots in Figures 3-6. X-ray diffractograms are the graphic record of measurements presented as a rectangular coordinate system, in which the X-axis refers to the magnitude of the diffraction angle and the Y-axis refers to relative intensity of peaks. In the control group, in which the specimen surface was untreated, the monoclinic phase was not detected. Meanwhile, in groups in which the air abrasion was used, the monoclinic phase was detected, and its content increased with the increase of the alumina particle size. The mean monoclinic volume fraction reached the following respective values: for air abrasion with 50 µm alumina particles, the m content was 26%; for 110 µm, it was 40%; and for 250 µm, it was 44%. Materials 2023, 16, x FOR PEER REVIEW 6 of 13         Representative plot of X-ray diffraction pattern for specimens that were air-abraded with 250 µm alumina particles. Figure 6. Representative plot of X-ray diffraction pattern for specimens that were air-abraded with 250 µm alumina particles.

Discussion
This study rejected the null hypothesis that the airborne-particle abrasion with alumina particles of different sizes does not affect the fracture resistance of zirconia frameworks and the phase transformation induction in the surface layers of zirconia-based ceramics. The obtained results indicate that performing alumina air abrasion of the inner surface of zirconia frameworks results in a double increase of their fracture resistance. Moreover, the increase in the size of alumina particles entails an increase in the fracture resistance of zirconia frameworks. It is strongly related to the phase transformation toughening phenomenon. The XRD revealed that the air abrasion induced the tetragonal-to-monoclinic phase formation.
In this study, the fracture resistance of zirconia-based ceramics frameworks was assessed by subjecting the specimens to a load until they fractured. There are different shapes of zirconia specimens recommended for this test in the literature on f.e. cylindrical specimens, specimens in the shape of FPRs (fixed prosthetic restorations), such as single crowns or bridges, cemented on abutments made of metal or other materials, with or without the imitation of periodontal ligaments, where the load is applied along the long axis or at different angles [36][37][38][39][40][41]. In this study, the specimens were designed as frameworks (with walls of uniform thickness of 0.5 mm) without porcelain veneering, so that they would resemble the features of zirconia restorations used in vivo. In the literature, the mechanical strength of 3Y-TZP after airborne-particle abrasion was assessed also with the biaxial flexural strength test ( Meanwhile, air abrasion with 250 µm alumina particles decreased the mechanical strength of zirconia-based ceramics. Air abrasion was performed at a 90 • angle in this case, while the air abrasion at a 30 • angle had only a mild influence on the mechanical strength of zirconia, irrespective of the alumina particle size [47]. On the other hand, Karakoca et al. did not find any significant difference between alumina that were air-abraded zirconia specimens (110 µm Al 2 O 3 , at a pressure of 4 bar) and the untreated specimens [48]. The aforementioned researchers explain the increase of the mechanical strength of zirconiabased ceramics after alumina air abrasion with the increase of the monoclinic phase volume and the PTT phenomenon.
The results of the XRD test obtained in the current investigation demonstrate that alumina air abrasion of the zirconia-based ceramics' surface induces the phase transformation and the increase of the monoclinic phase volume. Such results are similar to those obtained by the other research studies, in which the monoclinic phase was also detected after performing the alumina air abrasion. Karakoca et al. detected a 9.44-14.5% monoclinic phase volume in the surface layers of zirconia after air abrasion with 110 µm alumina particles [48]. Moqbel et al. indicate a 7.5-10.4 vol % monoclinic phase ratio after air abrasion with 50 µm alumina (at a pressure of 0.1 MPa and 0.25 MPa) [44]. Kosmač et al. found a 13.9-15.2% monoclinic phase volume increase [43]. Guazzato et al. found a 9.5% [16] and a 13.9% monoclinic phase volume increase [42]. Sato et al. found a 4.5% monoclinic phase volume increase after air abrading of the zirconia surface with 70 µm alumina particles [20], while Moon et al. found an 11.4% monoclinic phase volume increase [49]. In other research studies, a monoclinic phase volume increase after alumina air abrasion was also confirmed: 6% (50 µm alumina particles) [50], 8.68% (150 µm alumina particles) [51], and 4% [18].
In the present study, we compared three different sizes of alumina particles used for air abrasion and their influence on the phase content in the zirconia surface-it is clear that the alumina particle size used for air abrasion significantly affects the monoclinic phase volume detected in the topmost layer of 3Y-TZP. Such observations are confirmed by Hallmann et al., Monaco et al.,and Kelch et al. [19,45,52,53]. Hallmann et al. compared the influence of air abrasion of 3Y-TZP with 50 µm and 110 µm alumina particles on the monoclinic phase volume, and it was 5.16% and 7.92%, respectively [19]. In other studies conducted by these researchers, they demonstrated a 3.99% monoclinic phase volume increase when zirconia specimens were air-abraded with 50 µm alumina particles and a 6.46% increase for 110 µm alumina particles [53]. Monaco et al. detected a 10% monoclinic phase volume increase for zirconia that were air-abraded with 50 µm alumina particles and a 14% increase for 110 µm alumina particles [52]. Chen et al. detected 7.3% of a monoclinic fraction and 14.5% depending on the 3Y-TZP manufacturer [54]. On the other hand, Chintapalli et al. found that the size of alumina particles used for the air abrasion of zirconia is insignificant for the monoclinic phase volume value [17]. They showed that air abrasion with 110 µm and 250 µm alumina particles of the zirconia surface resulted in a 12-15% monoclinic phase volume increase, irrespective of the particle size [17]. Kim et al. found that air abrasion produced a small monoclinic phase fraction (0-2.3 wt%) with the highest value of 2.3 wt% observed for specimens that were air-abraded with 125 µm alumina (they compared five sizes of alumina particles: 25 µm, 50 µm, 90 µm, 110 µm, and 125 µm at a 0.2 MPa pressure) [55].
The monoclinic phase volume present on the superficial layers of 3Y-TZP is a relatively small fraction; the exact transformation depth is bigger. In theresearch conducted, the examined maximum depth of zirconia was 21 µm, while in Chintapalli et al.'s studies, the penetration depth of the X-rays was 3 µm. This may be the reason for the higher values of the monoclinic phase volume obtained in the current study [17]. Chintapalli et al. proved that the total transformation depth is 12 ± 1 µm for a 110 µm alumina particle abrasion and 13 ± 1 µm for a 250 µm alumina particle air abrasion [17]. Cotič et al. showed that the transformation zone depth is about 10 µm. They observed that the transformation zone is not homogenous. The tetragonal-to-monoclinic ratio and the microstructure observed on the FIB-prepared cross-sections changes with different depths. They demonstrated that within the topmost 1 µm, severe plastic deformation is seen with only a subtle outline of the zirconia grains. At a depth between 1 and 6 µm of the zirconia specimen, deformed zirconia grains were observed with more distinct boundaries, and new domains were seen. At 10 µm, they observed less severe alterations [31]. On the other hand, Wongkamhaeng et al. assessed the depth of the phase transformation as 14.5 ± 1.2 µm (air abrasion with 50 µm alumina) and 47.2 ± 3.0 µm (250 µm alumina) [46]. Kim et al. indicate that the deepest transformation layer did not exceed the 2.9 µm distance (they compared five grades of alumina particles: 25 µm, 50 µm, 90 µm, 110 µm, and 125 µm at a 0.2 MPa pressure) [55].
In the current study, it is demonstrated that the air abrasion with alumina particles of the surface of zirconia results in the increase of the mechanical strength of this material and the increase in the monoclinic phase content. Such observations are confirmed by the other authors. The rise of the mechanical strength of 3Y-TZP is explained as the result of compressive stresses that occur within the topmost layer of zirconia specimens as a result of the 4-5% monoclinic crystal volume increase [20,43,48]. As the airborne alumina particle abrasion used to be considered as the detrimental treatment for 3Y-TZP, although it has also many advantages, recent studies reject those opinions. Alumina air abrasion entails the superficial erosion of 3Y-TZP and induces the tetragonal-to-monoclinic phase transformation, leading to a PTT or an LTD phenomenon [52]. The microstructure alterations may affect the mechanical parameters and reliability of zirconia-based restorations. However, Cotič et al. demonstrated that the LTD annihilates the stresses present at the topmost layer of 3Y-TZP and gradually leads to a decrease of the PTT zone. As a result, the strength of zirconia decreases and finally reaches the values measured prior to alumina particle air abrasion [31]. They found that the speed of LTD equates with the speed of the LTD of as-sintered zirconia specimens (untreated). Also, a prolonged time of in vitro LTD does not affect the strength of air-abraded specimens in comparison with the as-sintered ones [31]. Cotič et al. conclude that alumina particle air abrasion does not affect the zirconia restorations' reliability and stability during the clinical service. If the appropriate air abrasion parameters are applied, then such treatment should not be harmful to the material's properties over time [31]. Moreover, Inokoshi et al. revealed that air abrasion with alumina particles of the zirconia surface increases the zirconia resistance to the LTD phenomenon when compared to the as-sintered zirconia specimens [32]. Kelch et al. also observed that aging of zirconia specimens resulted in higher flexural strength values for air-abraded specimens [45].
Assuming all the results obtained in this study, the alumina airborne-particle abrasion of 3Y-TZP results in an increase of the monoclinic phase volume in the topmost layers of zirconia and in an increase of the fracture resistance of zirconia-based frameworks. An increase of the size of alumina particles entailed an increase of the monoclinic phase volume and an increase of the mechanical strength of 3Y-TZP. According to the aforementioned research studies conducted by Cotič et al. and Inokoshi et al., alumina airborne-particle abrasion does not affect the reliability and stability of 3Y-TZP restorations over time despite the rise in the monoclinic phase content present in the alumina air-abraded specimens [31,32]. Nevertheless, it is still unknown what volume of the monoclinic phase can be still considered as harmless. Also, it must be noted that the tetragonal-to-monoclinic phase transformation is irreversible in the mouth environment. Therefore, we conclude that the most favorable proceeding would be performing the airborne-particle abrasion with alumina particles of such size which causes a phase transformation in the minimal extent and still ensures a high bond strength of 3Y-TZP to resin cements. We conclude that the use of 50 µm alumina particles for the air abrasion of zirconia-based dental ceramics would be the most favorable option to enhance the bond strength of zirconia to resin cements, because such a proceeding has a mild influence on the t-m transformation in comparison with bigger alumina particles and still increases the fracture resistance of zirconia-based frameworks.

1.
Alumina airborne-particle abrasion of the zirconia surface results in an increase in the fracture resistance of zirconia-based dental ceramics frameworks. The fracture resistance increases with an increase of the alumina particle size. 2.
Alumina airborne-particle abrasion of the zirconia surface induces a phase transformation in the surface layers of zirconia. An increase of the alumina particle size entails an increase of the monoclinic phase presence in the surface layers of zirconia.