Effect of Airborne Particle Abrasion Protocols on the Shear Bond Strength and Surface Hardness of High-Translucent Zirconia Under Thermocycling: An In-Vitro Study

1. Introduction

Zirconia, which is a crystalline form of zirconium oxide (ZrO2), has become a material of choice in restorative dentistry and prosthodontics because of its superior mechanical strength, favorable optical properties and excellent biocompatibility [1,2]. The increasing demand for metal-free, esthetically pleasing restorations and advances in computer-aided design and computer-aided manufacturing (CAD-CAM) technologies have driven the development of zirconia systems with tailored translucency, optimized strength and improved machinability [3,4].

Structurally, zirconia exists in three crystallographic phases with distinct clinical implications. The monoclinic phase is stable at room temperature upto 1170 °C but brittle; the tetragonal phase (stable between 1170 °C and 2370 °C) confers transformation toughening and high fracture toughness; and the cubic phase (stable above 2370 °C) yields high translucency but reduced mechanical strength [5,6,7,8]. Stabilization with yttria (yttrium oxide Y₂O₃) retains tetragonal or cubic phases at room temperature and thereby governs the trade-off between optical and mechanical behavior critical for dental applications [6,7].

The generational evolution of dental zirconia reflects efforts to balance strength and esthetics. First-generation 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP), introduced as “white metal”, contained approximately 0.25 wt% alumina (aluminium oxide Al₂O₃) and was composed almost entirely of the tetragonal phase. While offering high strength and fracture toughness, its opacity which was driven by light scattering at grain boundaries, inclusions and porosities, limited its use to frameworks and porcelain-layered crowns [8,9]. Second-generation 3Y-TZP reduced alumina content and employed higher sintering temperatures to lower porosity and improve translucency, yet esthetics remained insufficient for monolithic anterior restorations [8,9]. Subsequent fourth-generation 4 mol% yttria-partially stabilized zirconia (4Y-PSZ) and third-generation 5 mol% yttria-partially stabilized zirconia (5Y-PSZ) materials increased cubic-phase content to deliver enhanced translucency and optical mimicry. However, this improvement was accompanied by diminished transformation toughening and reduced fracture resistance [10,11]. Ban further classified yttria-stabilized zirconia into 12 categories based on yttria concentration, chromatic uniformity, compositional homogeneity and layering, underscoring the persistent challenge of balancing translucency with mechanical reliability [12].

High-translucent 5Y-PSZ (vernacular name “cubic” zirconia) markedly improves optical properties but does so at the expense of mechanical strength [12,13,14,15,16,17,18]. These materials typically exhibit a cubic-tetragonal microstructure with greater than 50% cubic phase and are more translucent because their isotropic crystallography reduces light scattering at grain boundaries [19,20]. Increased yttria reduces tetragonality ratio of remaining tetragonal grains, and some grains become “non-transformable,” which diminishes transformation toughening and contributes to lower fracture resistance [21,22,23]. Although high-translucent yttria 5Y-PSZ is less susceptible to low temperature degradation (LTD), when it occurs, can still reduce shear bond strength and surface hardness and increase surface roughness [24,25,26,27].

The absence of silica and a glass phase in zirconia renders hydrofluoric acid etching and silane coupling ineffective, necessitating alternative surface treatment strategies to achieve durable bonding with resin cements [28,29]. Surface treatments for zirconia can be broadly classified into mechanical, chemical, or hybrid approaches. Mechanical methods include airborne particle abrasion (APA) using Al₂O₃ particles [30], tribo-chemical silica coating [31], grinding [32], polishing [33], and various laser irradiations [34,35,36]. Other mechanical techniques include selective infiltration etching [37], plasma spraying [38] or non-thermal plasma etching, including argon plasma with or without surface abrasion [39], ceramic or glaze coatings using low-fusion porcelain [40], zirconia or alumina particle coatings, including nano-structured alumina [41] and alumina via aluminum nitride suspension [42], and hot chemical etching solutions [43,44,45]. Experimental approaches, such as bioglass particle abrasion [46] and slurry silica coating [47] have also been reported. Specialized techniques include zirconia surface architecturing technique (ZSAT), which employs a mixed nitric and hydrofluoric acid solution [48] and commercial etching solutions such as zircos-E, hydrofluoric acid, hydrochloric acid, and Ferric chloride [20]. Chemical methods include primers containing functional monomers such as 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) [49,50] and silane coupling agents [28], immersion in chemical solutions including silicon nitride [51], silicon nitride treated with sodium hydroxide [51], 37% phosphoric acid [52], piranha solution (sulfuric acid and hydrogen peroxide in 3:1 ratio) [53], and hot hydrochloric acid etching [54], plasma treatments [38,39] and silica-based epitaxial transition film formation [55]. Hybrid approaches involve integrating mechanical and chemical methods have also been proposed in literature [56,57]. Despite the diversity of methods, no single surface treatment has been shown to ensure consistently optimal bonding for zirconia restorations [28,29]. However, these treatments aim to optimize micromechanical interlocking and/or chemical adhesion while preserving mechanical integrity [58]. Many surface treatments alter not only alter microroughness and chemical reactivity but also shear bond strength (SBS) and surface hardness (SH)- these properties influence the resistance to surface damage, antagonist wear, fatigue performance and the stability of the adhesive interface [59,60].

The “APC Zirconia Bonding Concept” described by Blatz et al. [61], is the widely adopted surface treatment method, consisting of three steps: APA (step A) to create a rough surface, applying a ceramic primer containing adhesive phosphate monomers (step P), and bonding with dual- or self-cure composites (step C). Some studies also report that APA with 50-µm Al₂O₃ combined with 10-MDP chemistry (MDP-containing primer or cement) maximizes SBS while better preserving surface integrity [62,63]. Alternative, less-aggressive blasting media such as glass beads have been proposed for high-translucent zirconia to reduce iatrogenic damage. Glass-bead abrasion is commonly used on enamel, dentin and nickel-chromium (Ni-Cr) alloys, but typically yields lower bond strengths for Ni-Cr compared with alumina and can reduce adhesion to enamel and dentin [63,64]. Because glass beads are softer than alumina, they may preserve SH in high-translucent zirconia [65]. Few research reported that APA with alumina abrasion impaired flexural strength of zirconia more than glass-bead abrasion, supporting material-specific choices of blasting media [65,66]. The variable parameters in APA with alumina are grain size (25 to 250 µm), propulsion pressure (0.05 to 0.45 MPa), distance (5 to 20 mm) from the nozzle to the specimen, and time of APA (5 to 30 seconds) [67]. Particle size can critically influence outcomes: controlled roughening (50 µm) is generally favorable, whereas larger particles (110- 250 µm) may induce microcracks and surface flaws that reduce fracture resistance and can also decrease SH [67,68,69,70]. However, the evidence about the effects of low-abrasive particles on SH and SBS for high-translucent zirconia remains limited.

Aging simulations, such as thermocycling, can reduce zirconia’s SBS and SH, making initial measurements alone unreliable for predicting long-term performance. Combining SBS and SH assessments under artificial aging better evaluates a surface treatment’s clinical suitability, reflecting adhesive integrity, hardness retention, and failure mode patterns [58].

To address these gaps, this in-vitro study was designed to evaluate the effect of APA protocols on the SBS and SH of high-translucent zirconia under thermocycling. The null hypotheses (H₀) as follows: (1) There is no significant difference in the SBS of high-translucent zirconia treated with 50-µm Al₂O₃ or 100-µm glass microbead APA under thermocycling; (2) There is no significant difference in the SH of high-translucent zirconia treated with 50-µm Al₂O₃ or 100-µm glass microbead APA under thermocycling. The alternative hypotheses (Ha) as follows: (1) Air abrasion with 50-µm Al₂O₃ or 100-µm glass microbeads significantly alters the SBS of high-translucent zirconia under thermocycling; (2) Air abrasion with 50-µm Al₂O₃ or 100-µm glass microbeads significantly alters the SH of high-translucent zirconia under thermocycling.

2. Materials and Methods

The detailed specifications of the materials used in this study are summarized in Table 1.

2.1. Test Material (Manufacturer Data) [71]

Shofu ZR Lucent (Shofu Dental Corporation, Kyoto, Japan) is a polychromic, multilayered 5Y-PSZ disk marketed to synchronize strength and esthetics. According to the manufacturer, it is produced from high-quality Tosoh zirconia powder and exhibits a five-layer graded design (approximate enamel/dentin/cervical distribution reported as 30%/35%/35% respectively) with a uniform flexural strength of approximately 1019 MPa across the gradient and translucency values ranging from approximately 31–37% (comparable to lithium disilicate) with reported improved masking of discolored substrates. Manufacturer data also report Vickers hardness 45 HV (unsintered), coefficient of thermal expansion 10.2 × 10⁻⁶ K⁻¹ and sintering temperature 1450 °C; the material is intended for anterior and posterior crowns, short-span bridges, veneers, inlays and onlays with options for staining/micro-layering. The data presented are manufacturer-reported specifications [71]; independent, peer-reviewed thermocycling data on SH and SBS for Shofu ZR Lucent are currently limited or unavailable, which motivates the present investigation.

2.2. Experimental Workflow

The experimental workflow of the study is summarized in Figure 1.

2.3. Sample Fabrication

Thirty samples were fabricated from pre-sintered high-translucent zirconia blocks (Shofu ZR-Lucent, Japan) using subtractive CAD-CAM technology (Imes icore 350i, Germany). Fifteen square samples with dimensions 8 × 8 × 3 mm were allocated for SBS testing (Figure 2a), and fifteen disc-shaped samples with dimensions 15 × 1.2 mm were used for SH evaluation (Figure 2b).

Samples were sintered in a furnace (S-600; Add-in Co., Ltd., Goyang, Korea) at 1450 °C for 120 minutes with a heating and cooling rate of 5–10 °C/min following the manufacturer’s instructions. Surfaces were sequentially polished to minimize surface defects under wet conditions with 600-, 1000- and 1200-grit silicon carbide paper (15 seconds per grit) The samples were then ultrasonicated (GT Ultrasonic Co. Ltd., China) in distilled water for 10 minutes, and air-dried.

2.4. Grouping of Samples based on the APA Protocol

Samples were randomly divided into three groups (n = 10) based on the APA protocol:

Group I (Control): No air abrasion.

Group II: Air abrasion with 50 µm Al₂O₃ (Korox 50, Bego, Germany) [72] for 20 seconds at 2 bar pressure in a sandblaster (BEGO Easy Blast sandblaster, Germany), with the nozzle positioned at 90º to the center of the sample at 10 mm distance.

Group III: Air abrasion with 100 µm glass microbeads (Rolloblast, Renfert, Germany) [73] under identical conditions.

After air abrasion, all samples were cleaned with an air syringe and ultrasonicated in distilled water for 10 minutes.

2.5. Sample Preparation for Zirconia-Resin Cement Bonding Procedure

The samples were ultrasonically cleaned with 99% isopropanol for 180 seconds, and air-dried. Each square sample was embedded in one mm self-curing acrylic resin (Takilon, Rodent s.r.l., Milan, Italy) block. To define the bonding area, silicon molds with 2 mm diameter and 2 mm length were fabricated and positioned on the center of the zirconia samples [63] to create a standardized bonding area of 12.25 mm2. A manufacturer-recommended zirconia primer (AZ Primer, Shofu Inc., Kyoto, Japan) [74] was applied to the designated bonding surface, followed by dual-cure resin cement (ResiCem, Shofu, Kyoto, Japan) [75]. The primer was applied to the designated zirconia bonding surface using a disposable microbrush applicator and left undisturbed for 10 seconds. The resin cement was applied directly on the fabricated silicon molds and excess cement was removed using a microbrush. Chemical curing was initiated 30 seconds after primer application and allowed to proceed for 3 minutes. Light cure polymerization was performed for 40 seconds (10 seconds per side) using a light-emitting diode unit (Bluephase, Ivoclar Vivadent, Schaan, Liechtenstein) with an intensity of 1400 mW/cm2. The bonded samples (Figure 3) were stored in distilled water for 7 days at 37 °C prior to thermocycling to allow post-cure polymerization and initial stabilization of the mechanical properties of the resin cement. A 7-day baseline period, consistent with prior research [76], was employed in this study.

2.6. Thermocycling

Artificial aging was simulated by 25,000 thermocycles in two thermostatic water baths (water-to-water transfer) ranging from 5- 55°C using a thermocycler (Haake Circulating Bath DC-15 with Haake DC 10 Thermocontroller, Thermo Haake, USA) in all the samples. Each cycle lasted for 30 seconds and involved the following steps: 10 seconds in a 5°C bath, 5 seconds for transfer, 10 seconds in a 55°C bath, and 5 seconds for returning to the 5°C bath. The chosen temperature range and dwell times follow common laboratory practice (ISO 10477, 1996; ISO 4049:2009; ISO 29022:2013) and the 25,000-cycle regimen was selected to simulate extended clinical aging (approximately 2.5 years of intraoral use based on Gale & Darvell [77] and to exceed the frequently used 10,000-cycle (approximately 1 year) threshold discussed in the literature [58,77]. After thermocycling, the samples were rinsed and dried.

2.7. Shear Bond Strength Testing and Analysis of Bond Failure

SBS testing was performed on the bonded square samples (n= 5 per group) using a universal testing machine (Instron 3345, UK) according to ISO TR11405. Each bonded specimen was mounted in the testing jig of the universal testing machine. A knife-edge shear test was performed using a with a 5 kN load cell and a crosshead speed of 0.5 mm/min, and the shear load at failure (N) was recorded. The knife edge was positioned at the interface between the zirconia and the resin cement to induce bond failure. SBS (MPa) values were converted from Newtons to megapascals (MPa) (1 N/mm² = 1 MPa) and then calculated by dividing the load at failure (N) by the standardized interfacial area (12.25 mm²) [63].

Shear bond strength (MPa)= Failure load (N)/ Surface area (mm²)

Debonded samples were examined under a stereomicroscope (Nikon SMZ25 stereo microscope, Japan) at 40X magnification. Failure modes were recorded as classified by Serra-Prat et al. [78] as follows:

Type I- cohesive (failure within the cement, when approximately two-thirds or more of the luting agent remained on the zirconia surface).

Type II- adhesive (failure at the cement–zirconia interface, when less than one-third of the cement remained).

Type III- mixed (combined adhesive and cohesive features, when approximately one-third to two-thirds of the cement remained or both interfacial and bulk fractures were observed).

2.8. Surface Microhardness Testing

Microhardness testing was performed on disc-shaped samples (n= 5 per group) using a Vickers microhardness tester (Buehler, Lake Bluff, IL, USA) according to ISO 6507-1. A square-based pyramidal diamond indenter was applied at a constant load of 10 N for 10 s. Three indentations were made randomly on each specimen, and the mean value was calculated. The two diagonals (D1, D2) of each indentation were measured using digital microscopy (KH-7700, Hirox, Tokyo, Japan), and their average length (L) was used for calculation [21]. The Vickers hardness number (VHN) was determined using the formula [79]:

Surface Hardness (VHN) =1.854 P/ L2

where P is the applied load (kgf) and L=(D1+D2)/2 is the mean diagonal length in mm.

2.9. Statistical Analysis

Statistical analyses were performed using IBM SPSS Statistics, version 30 (IBM Corp., Armonk, NY, USA). Data normality was assessed using the Shapiro–Wilk test (p > 0.05). One-way ANOVA evaluated the effect of surface treatment on SBS and SH, followed by Tukey’s post-hoc test for pairwise comparisons. Homogeneity of variances was confirmed using Levene’s test. Statistical significance was set at p ≤ 0.05, and results are presented as mean ± standard deviation.

3. Results

3.1. Shear Bond Strength

The mean SBS values for control, 50 µm Al₂O₃, and 100 µm glass microbead groups were 5.64 ± 1.49 MPa, 6.49 ± 1.59 MPa, and 6.42 ± 4.05 MPa, respectively (Table 2).

3.2. Surface Hardness

Vickers microhardness values differed significantly among groups (F (2,12) = 903.83, p < 0.001). The mean SH was lowest in the control group (876.34 ± 25.10 VHN), followed by 100 µm glass microbeads (1246.94 ± 33.81 VHN) and 50 µm Al₂O₃ (1747.26 ± 37.37 VHN) (Table 3).

Post-hoc Tukey tests revealed all pairwise differences were statistically significant (p < 0.001), with mean differences of 370.60 VHN (100 µm glass microbeads vs control), 870.92 VHN (50 µm Al₂O₃ vs control), and −500.32 VHN (50 µm Al₂O₃ vs 100 µm glass microbeads). The digital microscopy images reveal the brittle nature of the tested material (Figure 5).

4. Discussion

APA protocols exert distinct and sometimes opposing effects on bond strength and surface hardness in high-translucent zirconia. The null hypothesis (H₀) was accepted in the study as there was no significant difference in the SBS of high-translucent zirconia treated with 50-µm Al₂O₃ or 100-µm glass microbead APA under thermocycling. These results align with prior reports showing that increased surface roughness from APA does not inevitably produce durable bond gains and that thermal ageing often unmasks interfacial weakness [80,81,82]. Similarly, some studies report minimal or no SBS gain after APA for certain ultra-translucent or high-cubic zirconia [27,60,63,66,83]. In fact, a few studies have documented reduced bulk strength after aggressive APA. These effects may result from altered tetragonal→monoclinic (t→m) transformation dynamics, increased cubic content, or surface flaws introduced during blasting [83,84].

Mechanistically, these divergent outcomes can be explained by the impact physics of different abrasives and the chemistry of contemporary high-translucent zirconia. Angular, hard Al₂O₃ particles induce localized plastic deformation and a peening effect, referring to the localized hammering action of abrasive particles, which compacts the surface. This densifies the outermost microns and can trigger a thin t→m surface transformation [30,68,85]. By contrast, spherical glass microbeads are less aggressive, generating milder topographical changes while preserving subsurface integrity [63,86].

Importantly, increased topographical roughness alone does not guarantee improved micromechanical retention. Resin penetration into micro-retentions, wetting behavior, and the integrity of the chemically bonded interface ultimately determine durable SBS [70,87]. Tzanakakis and colleagues emphasized that grain size, pressure, and exposure time produce variable roughness but do not reliably predict long-term SBS [82]. Aggressive blasting can increase monoclinic content and microcracking, undermining mechanical performance [82].

Adhesive bond failures in the control groups align with expectations, as untreated zirconia is chemically inert, has low surface energy, and limited micromechanical retention. Consequently, unconditioned zirconia often debond at the cement–ceramic interface, particularly after aging [88,89]. In the present study, APA-treated groups also exhibited exclusively adhesive failures. This contrasts with previous literature reporting predominantly mixed or cohesive failures [49,86,90]. Non-adhesive failures are indirect markers of stronger interfacial adhesion because surface roughening increases contact area and promotes resin penetration. Mehari et al. and others reported a shift toward mixed/cohesive failures with Al₂O₃ APA compared with glass-bead or untreated surfaces, which disagrees with the present observations [86]. The adhesive failures observed here could be attributed to the thermocycling procedure. Cyclic thermal shocks and associated water sorption accelerate hydrolytic degradation of the adhesive interface, reliably reducing resin-zirconia bond strength compared with immediate values [81,91]. Thermocycling thus provides a more stringent evaluation of APA protocols and adhesive strategies. Other studies similarly report adhesive bond failures after ageing, despite initial increases in immediate SBS [92,93].

These findings align with multiple laboratory reports showing that Al₂O₃ APA increases surface roughness and mechanical interlocking, producing higher immediate SBS values compared with glass beads or untreated surfaces [86,94]. The benefit of APA is often amplified when combined with chemical coupling. Clinically, the adhesive failures and non-significant SBS differences observed here underscore the need to pair mechanical conditioning with robust chemical bonding protocols to optimize outcomes. Previous literature suggests that APA benefits are magnified when combined with MDP-containing primers or cement chemistry that promotes durable chemical bonding, resulting in higher SBS and often mixed/cohesive failure modes [50,58,81,87,95,96,97].

From a practical perspective, modest APA using 50-µm Al₂O₃ at controlled pressure, followed by an MDP-containing primer or cement, provides a reliable pathway to durable bonding for many zirconia types [58,70,96]. Where conservation of core strength is critical (e.g., thin sections, highly cubic 5Y-PSZ), less aggressive protocols-using finer particles, lower pressure or alternative surface treatments such as tribochemical silica coating and optimized primers may offer a better balance between bondability and structural integrity [51,86,94].

Hardness represents resistance to plastic indentation and is a key surface property influencing finishing, scratch resistance, and potential antagonist wear [97]. The alternative hypothesis (Hₐ) was accepted as air abrasion with 50-µm Al₂O₃ or 100-µm glass microbeads significantly altered the SH of high-translucent zirconia under thermocycling. APA-treated surfaces exceeded untreated zirconia and were within ranges reported for contemporary 5Y-PSZ/ultra-translucent zirconia [27,83,98,99]. Several studies report similarly high absolute hardness for zirconia and relative stability with aging [27,100]. De Araújo-Júnior et al. found 5Y-PSZ hardness to be essentially unchanged after autoclave/hydrothermal aging, suggesting intrinsic hardness is robust for modern yttria-stabilized zirconia [27]. Dejak et al. highlighted that conventional and transparent zirconia ceramics are substantially harder than enamel and often exhibit favorable, low-friction surfaces after polishing [100].

However, the near doubling of surface hardness after 50 µm Al₂O₃ APA contrasts with Yoo et al., who reported no significant change [30]. Differences may arise from APA parameters (particle type/size, pressure, time, distance, incidence angle), manufacturing route (milled vs additively made), indentation load, dwell time, or measurement of surface-localized versus subsurface hardness. Thermocycling also affects surface hardness variably. Mavriqi and Traini reported that some aging procedures can increase surface hardness, while others show little change [26]. This highlights the influence of LTD/phase transformation, surface flaws, and measurement protocols. Definitive conclusions regarding thermocycling effects on surface hardness cannot be drawn because this parameter was not explored in this study.

Clinically, zirconia hardness alone does not dictate antagonist tooth wear. Highly polished zirconia produces less enamel wear than glazed or rough surfaces. While APA increased surface hardness, it also increased surface roughness, as confirmed by stereomicroscopy and digital microscopy (Figure 4 and Figure 5). Roughness, not hardness, is the principal driver of antagonist abrasion [87,101,102]. Effective polishing post-APA can restore a mirror finish, enabling higher hardness to confer better scratch and wear resistance. If APA leaves a rough, unpolished surface, the risk to antagonist enamel increases despite higher hardness [12,26].

The in-vitro nature of this study limits direct clinical extrapolation. Therefore, several methodological limitations and future research directions warrant careful consideration.

The manufacturer-recommended AZ zirconia primer was used. It may enhance SBS but requires strict adherence to the application protocol [74,82]. Dual-cure resin cement ResiCem, containing MDP-like phosphate monomers and fillers, can provide higher bond strength, translucency, and durable adhesion to zirconia [75]. Independent studies report superior bond strengths for ResiCem compared with self-cure cements alone, justifying its selection [96]. Using a single cement/primer and only two APA protocols limits conclusions regarding adhesive chemistry or the full APA parameter space (particle size, pressure, time, distance, incidence angle) on SBS, SH, and surface roughness [20,63,86]. Future work could expand the experimental matrix to include multiple cements and primers and systematically explore APA variables to define safe and effective protocols for cubic containing zirconia.

Although thermocycling mimics oral thermal stress, it cannot reproduce complex mechanical fatigue and multi-directional masticatory forces, which may produce different degradation patterns in adhesive interfaces [81,90]. The specific influence of thermocycling on SBS and SH under different APA protocols was not assessed in this study and warrants further study.

A macro-shear design was used for SBS testing because it is simpler and widely reported [58,103]. While tensile tests better reflect ‘true’ adhesive strength [82], the specimen geometry, bonding area, and loading protocols were standardized in this study. However, macro-shear bond strength testing concentrates load at a point and generates non-uniform stress fields, potentially overestimating bond performance compared with micro-shear or micro-tensile methods [82].

For SH estimation, Vickers microhardness was chosen for its ability to detect subtle surface changes from APA, offering superior spatial resolution compared with conventional microhardness [68,82]. APA produced statistically significant increases in surface hardness after thermocycling, demonstrating the method’s sensitivity to particle type and size. Limitations include sensitivity to surface roughness, operator variability, and focus on surface rather than bulk properties, with potential microcrack induction in brittle ceramics [80,104]. Multiple indents were averaged, but complementary bulk and surface analyses remain necessary for clinical interpretation. Micro-mechanical testing and finite-element modelling could further evaluate stress distributions under multi-directional loading [82].

Comprehensive characterization of surface damage or phase transformation (e.g., X-ray diffraction, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, micro-computed tomography, nanoindentation mapping) was not performed. This limits interpretation of whether SH changes reflect beneficial surface hardening or deleterious subsurface flaws [81,105]. Future studies should integrate these analyses alongside bulk mechanical testing (flexural strength, fracture toughness) and antagonist wear measurements.

Small subgroup sizes (n = 5) reduce statistical power and make reliability analyses such as Weibull modulus imprecise. While Weibull parameters can be computed, estimates have high uncertainty and should be interpreted cautiously. Increasing sample sizes (≥20–30 per subgroup) permits robust analyses and uncertainty reporting via bootstrap or Bayesian intervals [106].

Material specificity further limits generalizability. Zirconia formulations vary in grain structure, phase composition, and mechanical properties; surface treatment effects observed here may not apply to other 5Y-PSZ or graded zirconia [27,60,63,66,82,83,98,99]. Translation of laboratory findings into prospective clinical trials is necessary to verify long-term retention and structural durability before issuing definitive clinical recommendations.

5. Conclusions

Within the limitations of this study, the following conclusions were obtained:

• Airborne particle abrasion did not improve the shear bond strength, and all failure modes between the dual-cure resin cement and high-translucent zirconia were adhesive, indicating a limited benefit for resin bonding.
• Airborne particle abrasion improves the surface hardness of high-translucent zirconia, particularly with 50 µm Al₂O₃ producing the greatest effect.

Despite existing challenges, high-translucent zirconia, with its continually evolving esthetic and mechanical properties, remains a highly versatile material in restorative dentistry and prosthodontics. Advances in surface treatments are expected to further improve its surface hardness and bond durability, thereby expanding its clinical applications.