Effects of low‐pressure plasma and ytterbium fiber laser on zirconia surface properties and bond strength to gingiva‐colored composite

Abstract

Purpose

To evaluate and compare the effects of surface activation by low‐pressure plasma (LPP) and surface texturing by ytterbium fiber laser (YFL) treatments on zirconia surface properties and shear bond strength (SBS) to a gingiva‐colored indirect composite under aged and non‐aged conditions.

Materials and Methods

Sixty zirconia specimens were prepared, polished, sintered, and air‐abraded under standardized conditions. The specimens were randomly assigned to three groups according to the applied surface treatment: control, LPP, and YFL. Surface roughness and contact angle were measured before and after surface treatments. A gingiva‐colored indirect composite was bonded to the zirconia surfaces using an 10‐Methacryloyloxydecyl dihydrogen phosphate (MDP) containing primer and standardized mold. Each group was further divided into aged and non‐aged subgroups (n = 10). Thermal aging was performed using thermocycling (5000 cycles, 5°C–55°C). SBS was measured, and failure modes were analyzed. Surface morphology, phase composition, and representative failure modes were evaluated using scanning electron microscopy (SEM) and X‐ray diffraction (XRD) analyses.

Results

Surface treatments influenced zirconia wettability, with LPP showing a greater reduction in contact angle compared to YFL. YFL groups exhibited higher SBS values than both LPP and control groups under both aged and non‐aged conditions. Thermal aging showed a significant overall effect on SBS, although no significant differences were observed between aged and non‐aged specimens within each surface treatment group. Failure modes were predominantly adhesive in low‐strength groups, whereas mixed failures were more frequent in high‐strength groups.

Conclusion

YFL enhanced bond strength at the zirconia and gingiva‐colored composite interface compared to LPP, whereas aging showed a small but significant overall effect on bond strength, with no significant interaction between aging and surface treatment.

Zirconia is a widely used ceramic in prosthetic dentistry due to its favorable biological and esthetic properties, strong mechanical performance, high fracture toughness, sufficient radiopacity, and low tendency for bacterial adhesion. It is also resistant to dissolution and corrosion in aqueous environments. ¹ , ²

It is used in various restorative applications, including as a core material for single crowns, as the framework for fixed partial dentures, ³ , ⁴ as a post after root canal treatment, ⁵ and as an implant or abutment component in implant dentistry. ² , ⁶ High‐strength yttria‐stabilized tetragonal zirconia polycrystal (Y‐TZP) restorations further increased its popularity by offering superior mechanical properties compared with alumina or lithium disilicate–based ceramics. ⁷ , ⁸ Improvements in zirconia formulations have also enhanced optical characteristics, and newer generations show greater translucency due to increased Y₂O₃ content, which raises the proportion of cubic‐phase grains. ⁷

Soft‐ and hard‐tissue deficiencies around restorations and implants, often resulting from bone loss after tooth extraction, can negatively affect the overall esthetic outcome. While surgical augmentation can address these issues, it is invasive, technique‐sensitive, expensive, and not always suitable for every patient. ⁹ This clinical challenge has increased interest in gingiva‐colored materials. Among the options for mimicking peri‐implant soft tissue, indirect composite resins, despite certain drawbacks such as low wear resistance, ¹⁰ low fracture resistance, ¹¹ susceptibility to discoloration, ¹² and a tendency to facilitate plaque accumulation, ¹³ , ¹⁴ stand out for their ease of intraoral repair, stability during firing, convenient handling, and reliable esthetic results. ¹⁵ , ¹⁶ They are particularly useful for replacing missing soft tissue in implant‐supported restorations and are also commonly used as veneering materials for zirconia due to their wide range of gingival and dentin shades and simple intraoral repair. ¹⁷ , ¹⁸ For these reasons, maintaining a strong and durable bond between zirconia and composite resin is crucial for the long‐term success and maintenance of these restorations. ¹⁸

Due to its glass‐free polycrystalline structure, zirconia does not respond to conventional etching and silanization protocols used for silica‐based ceramics. ⁷ As a result, achieving reliable adhesion frequently requires mechanical and/or chemical surface modification. Airborne‐particle abrasion, ¹⁹ MDP‐containing primers, ²⁰ laser‐based microtexturing, ²¹ , ²² , ²³ and surface activation using various plasma applications ²³ , ²⁴ , ²⁵ , ²⁶ , ²⁷ have been investigated to improve surface wettability or micromechanical retention in resin bonding. Among these methods, low‐pressure plasma (LPP) treatment, a chemical surface activation approach that increases surface energy and wettability without causing significant alterations to surface morphology, has been reported, whereas laser‐based treatments, including ytterbium fiber lasers (YFLs), have been proposed to create surface texturing that enhances mechanical interlocking. ¹⁸ , ²¹

The optimal conditioning method for achieving durable zirconia‐indirect composite adhesion remains unclear in the current literature. Several studies in the literature have focused on improving the bonding between veneering composite resins and zirconia frameworks. ¹⁴ , ¹⁸ , ¹⁹ , ²⁰ , ²⁸ Moreover, limited information is available regarding the bond strength of gingiva‐colored composite resins specifically when applied to zirconia substrates. ²⁹ , ³⁰ , ³¹

Shear bond strength (SBS) testing is widely used to evaluate the adhesion between core and veneering materials because the load is applied perpendicular to the bonding interface, providing more standardized and reproducible measurements. Additionally, the relatively small bonding area used in SBS testing helps minimize the influence of internal structural defects on the results. ¹⁹

Due to the limited number of studies evaluating the interaction between gingiva‐colored indirect composite resins and zirconia surfaces, ¹⁷ , ²⁹ , ³⁰ it remains difficult to fully assess the factors influencing their interfacial behavior. Although various surface treatment methods have been proposed to enhance zirconia bonding, the evidence regarding their effectiveness on gingiva‐colored composites is still insufficient. Moreover, the influence of aging procedures on the long‐term stability of zirconia and composite adhesion has not been clearly established.

Therefore, the purpose of this in vitro study was to evaluate and compare the effects of LPP and YFL treatments on the surface wettability of zirconia and on the SBS between zirconia and a gingiva‐colored indirect composite resin, under both thermally aged and non‐aged conditions. The null hypotheses tested were as follows: surface treatment protocols would not exhibit a statistically significant difference in SBS at the zirconia and gingiva‐colored indirect composite interface; and thermal aging would have no interaction effect with surface treatment on SBS.

MATERIALS AND METHODS

The study design is illustrated in Figure 1, and Table 1 summarizes the brands and key characteristics of the materials evaluated in this study. Rectangular zirconia specimens were obtained by sectioning a single presintered zirconia block (KATANA HT, Kuraray Noritake). Sixty specimens were fabricated in total, each standardized to dimensions of 5 × 11 × 17 mm³. The zirconia specimens were polished in a stepwise manner using silicon carbide (SiC) abrasive papers with grit sizes ranging from #180 to #1000 to obtain a uniform and smooth surface. All polishing procedures were performed under continuous water cooling to prevent heat generation. Following surface preparation, the specimens were ultrasonically cleaned in ethanol and distilled water (Bandelin Sonorex) to eliminate residual debris and ensure complete surface cleanliness. ³² The specimens were sintered in a programmed sintering furnace (Protherm PLF 130/10) according to the manufacturer's instructions. The temperature increased from room temperature (25°C) to 1550°C at a heating rate of 10°C/min and held at the peak temperature for 120 min. Subsequently, the furnace was cooled at a rate of 10°C/min. The total sintering cycle was completed in 7 h. After sintering shrinkage, the zirconia specimens were sequentially ground and polished using the same silicon carbide abrasive papers (#180‐#1000 grit) to obtain standardized final dimensions of 4 × 8 × 13 mm³.

FIGURE 1.

Study design.

TABLE 1.

Materials used in this study.

Material	Products and manufacturer	Composition	Lot number
Zirconia	KATANA Zirconia HT (Kuraray Noritake)	94.4% ZrO2, 5.4% Y2O3	EISPX
Indirect composite resin	Gradia Plus Gum Shades Heavy Body‐GHB‐2 (GC Europe NV Leuven)	Resin: UDMA, NGDMA, TMPTMA Filler: trimodal (pre‐polymerized particles, AlBSiO4,SiO2, 75 wt%)	2409111
Universal primer	G‐multi primer (GC Corporation)	γMPTS, 10MDP, MDTP (methacryloyloxydecyl dihydrogen thiophosphate), BisGMA (bisphenol‐A glycidyl methacrylate), TEGDMA (triethylene glycol dimethacrylate), and ethanol	2407167

The specimen surfaces were airborne‐particle abraded using a dental sandblasting device (EasyBlast, BEGO GmbH & Co. KG) with 50‐µm Al₂O₃ particles (Korox; BEGO) at a pressure of 0.2 MPa from a distance of 10 mm for 20 s. Following airborne‐particle abrasion, the specimens were ultrasonically cleaned in ethanol for 10 min to remove any residual debris. ²¹

The specimens were randomly allocated into three groups of 20 specimens each according to the surface treatment protocol: an untreated control group, an LPP treatment group, and a YFL treatment group.

Initial surface roughness measurements were performed for descriptive characterization and standardization of the specimens. Measurements were conducted using a three‐dimensional (3D) optical confocal profilometer (Zeiss Smartproof 5; Carl Zeiss). From the center of each specimen, a 500 × 500 µm² area was scanned in 3D imaging mode at a total magnification of 10× (C Epiplan‐Apochromat 10×/0.4 DIC) using fast scan mode (4 µm). The acquired images were analyzed using automated software (ConfoMap ST 7.4.8076; Carl Zeiss). The Sa (arithmetical mean height of the surface) values were recorded (T ₀).

The initial contact angle measurements for each specimen were performed at room temperature (25°C) using an optical tensiometer (Theta Lite II, Biolin Scientific) and the sessile drop technique (One Attension, Biolin Scientific) (T ₀). ³³

Plasma treatment of the zirconia specimens was performed using a low‐temperature, low‐pressure benchtop plasma system (Zepto, Diener Electronic). The specimens were processed inside the chamber of the device operating at 40 kHz and 80 W, with the internal pressure maintained at 0.3 mbar. Air was used as the process gas, and each specimen was exposed to plasma for 10 min. ³²

Laser irradiation was performed using a YFL (FL‐NS10W, FiberLAST) operating at 10 W and 30 kHz. A horizontal hatch‐scanning protocol was applied with a 0.1‐mm line spacing at a scanning velocity of 100 mm/s, producing uniform parallel micro‐grooves across the surface. The hatch angle remained at 0°. All specimens received one standardized irradiation cycle. Following the surface treatments, the surface roughness and contact angle of all specimens were re‐evaluated (T ₁).

The bonding procedure for luting the indirect composite resin onto the zirconia substrates was standardized as follows. A double‐sided adhesive tape with a circular opening of 5 mm in diameter was placed on the zirconia surface to delineate the bonding area. ¹⁸ Following cleaning of the zirconia substrates, an MDP‐containing universal primer (G‐Multi Primer; GC Corporation) was applied to the exposed bonding area using a micro brush in strict accordance with the manufacturer's instructions to optimize chemical adhesion. ³⁴ Subsequently, a cylindrical mold (5 mm internal diameter and 2 mm height) was positioned over the defined bonding area and secured with double‐sided tape. The mold cavity was then filled with a gingiva‐colored indirect composite resin (Gradia Plus Gum Shades Heavy Body, GHB‐2; GC Europe NV). The composite was initially light‐cured for 40 s using an LED curing unit (DTE LUX‐E Plus, Guilin Woodpecker Medical Instrument; 1200 mW/cm²), followed by an additional 3‐min polymerization cycle in a laboratory curing unit (Labolight DUO, GC Europe NV). ³⁵

Each surface treatment group was subdivided into thermally aged and non‐aged subgroups (n = 10). Specimens were stored in distilled water at 37°C for 24 h, which was defined as the non‐aged condition. Subsequently, half of the specimens in each group were subjected to thermal aging using a thermocycling device (SD Mechatronic Thermocycler; SD Mechatronic GmbH) for 5000 cycles between 5°C and 55°C, with a dwell time of 30 s in each bath. ²⁶

The SBS test was conducted using a universal testing device (H5KS model, Tinius Olsen Ltd.). A shear load was applied to the bonding interface at a crosshead speed of 0.5 mm/min until failure occurred. The failure load was normalized to the bonded surface area to obtain SBS values in MPa. SBS was determined using the formula: load (N) / bonded area (mm²). ⁷

After the SBS test, the fractured interfaces were examined to determine the mode of failure using a stereomicroscope (Stemi 508; Carl Zeiss Microscopy GmbH) at 10× magnification. The mode of failure was classified as adhesive, cohesive, or mixed failure. Adhesive failure was defined as complete separation at the composite‐zirconia interface. Cohesive failure was defined as fracture occurring within the composite material. Mixed failure was defined as the simultaneous occurrence of interfacial separation and fracture within the composite material. Scanning electron microscopy (SEM) analysis was performed to evaluate the surface morphology of the zirconia specimens after the different surface treatments and to qualitatively assess representative fracture surfaces after SBS testing. For SEM analysis, the samples were sputter‐coated with a conductive layer of gold‐palladium (Au‐Pd) and examined using a field emission scanning electron microscope (Quanta 400 F FE‐SEM, FEI) operating at an acceleration voltage of 20 kV at different magnifications.

One specimen from each group was subjected to X‐ray diffraction (XRD) analysis using an Ultima IV diffractometer (Rigaku, Japan). Diffraction patterns were collected in continuous scan mode over a 2θ range of 20°–80°, at a scan speed of 1°/min and a sampling interval of 0.02°.

The sample size of each experimental condition was determined by power analysis using GPower 3.1.9.7 software, indicating that n = 10 provided a study power of 90% at a significance level of 0.05.

The normality of continuous variables was evaluated using the Kolmogorov–Smirnov test and Skewness–Kurtosis values. Although the Kolmogorov–Smirnov test indicated deviations from normality for some variables, the Skewness and Kurtosis values were within acceptable limits (±1.5), indicating an approximately normal distribution. Therefore, parametric statistical tests were applied. Descriptive statistics were expressed as mean, standard deviation, frequency (n), and percentage (%). Comparisons of surface roughness and contact angle values among groups were performed using one‐way analysis of variance (ANOVA). Within‐group comparisons between initial and posttreatment measurements were analyzed using the paired‐samples t test.

SBS data were analyzed using two‐way ANOVA, with surface treatment (Control, LPP, YFL) and aging condition (aged/non‐aged) as the independent factors. Main effects and interaction effects were evaluated. When significant differences were identified, Tukey's honestly significant difference (HSD) post hoc test was applied for pairwise comparisons. The homogeneity of variances assumption was assessed using Levene's test. The level of statistical significance was set at α = 0.05. All statistical analyses were performed using SPSS statistical software (IBM SPSS for Windows, version 26).

RESULTS

Regarding the surface roughness (Sa) parameter, no statistically significant difference was observed among the groups in the initial measurements (p = 0.408). In the YFL group, the surface roughness value increased considerably, with a final value of 8.08 µm. In contrast, the LPP group showed no statistically significant change after treatment, with a slight decrease in surface roughness to 1.74 µm (Table 2). Figure 2 presents confocal profilometer images of the control, plasma, and laser group specimens, each obtained from a scan area of 500 × 500 µm², illustrating their two‐dimensional surface topography.

TABLE 2.

The descriptive and comparative statistics of the surface roughness (µm).

	Separate control Mean ± SD	LPP Mean ± SD	YFL Mean ± SD	p value
Initial surface roughness	1.85 ± 0.50	1.91 ± 1.16	1.61 ± 0.57	0.408
Posttreatment surface roughness	1.85 ± 0.50 b	1.74 ± 1.17 b	8.08 ±1.30 a	<0.001
p value (within group)	‐	0.177	<0.001

Note: Intergroup comparisons were performed using one‐way ANOVA. Within‐group differences between initial and posttreatment measurements were analyzed using the paired‐samples t test. Different lower‐case letters (a, b) indicate statistically significant differences among groups (p < 0.05). The initial and posttreatment values of the control group are identical, as it did not undergo any treatment.

Abbreviations: ANOVA, one‐way analysis of variance; LPP, low‐pressure plasma; YFL, ytterbium fiber laser.

FIGURE 2.

Representative confocal profilometer images of control, plasma, and laser groups. LPP, low‐pressure plasma; YFL, ytterbium fiber laser.

In terms of contact angle measurements, no statistically significant difference was observed among the groups in the initial values. However, significant differences emerged after surface treatment. Since no surface treatment was applied to the control group, posttreatment measurements were not performed, and baseline values were retained for comparison. Accordingly, the control group maintained the highest contact angle, with a value of 50.31°. LPP group resulted in the lowest contact angle of 10.12°, indicating the greatest increase in surface hydrophilicity, while the YFL group demonstrated an intermediate value of 15.28°. Within‐group comparisons confirmed that both LPP and YFL treatments exhibited significant reductions in contact angle (Table 3).

TABLE 3.

The descriptive and comparative statistics of the contact angle (°).

	Separate control Mean ± SD	LPP Mean ± SD	YFL Mean ± SD	p value
Initial contact angle	50.31 ± 7.24	48.42 ± 8.13	53.98 ± 6.75	0.062
Posttreatment contact angle	50.31 ± 7.24 a	10.12 ± 2.52 c	15.28 ± 5.61 b	<0.001
p value (within group)	‐	<0.001	<0.001

Note: Intergroup comparisons were performed using one‐way ANOVA. Within‐group differences between initial and posttreatment measurements were analyzed using the paired‐samples t test. Different lower‐case letters (a, b, c) indicate statistically significant differences among groups (p < 0.05). The initial and posttreatment values of the control group are identical, as it did not undergo any treatment.

Abbreviations: ANOVA, analysis of variance; LPP, low‐pressure plasma; YFL, ytterbium fiber laser.

The mean SBS values for all groups are presented in Table 4. In the non‐aged condition, the YFL group exhibited the highest SBS value at 10.68 ± 2.11 MPa, followed by the control group at 8.34 ± 1.83 MPa and the LPP group at 5.58 ± 1.52 MPa. In the aged condition, the YFL group again demonstrated the highest SBS value at 13.07 ± 2.35 MPa, whereas the control group at 7.99 ± 2.49 MPa and the LPP group at 6.69 ± 1.55 MPa showed comparable values. According to the Tukey post hoc comparisons, statistically significant differences were observed among groups within each condition, with the YFL group showing significantly higher SBS values than the other groups.

TABLE 4.

Descriptive statistics (mean ± SD) and 95% confidence intervals for SBS (MPa).

Group	n	Mean ± SD (MPa)	95% CI	p value
Control/non‐aged	10	8.34 ± 1.83	[7.03, 9.65]	0.488
Control/aged	10	7.99 ± 2.49	[6.21, 9.77]
LPP/non‐aged	10	5.58 ± 1.52	[4.49, 6.67]
LPP/aged	10	6.69 ± 1.55	[5.58, 7.79]
YFL/non‐aged	10	10.68 ± 2.11	[9.17, 12.19]
YFL/aged	10	13.07 ± 2.35	[11.38, 14.75]

Levene's test was calculated across all six cells: F(5,54) = 0.901, p = 0.488. The homogeneity of variances assumption is met.

Abbreviations: 95% CI, 95% confidence interval for the group mean; LPP, low‐pressure plasma; SBS, shear bond strength; YFL, ytterbium fiber laser.

Surface treatment had a statistically significant effect on SBS, accounting for approximately 61% of the total variance, a large effect by conventional benchmarks. Aging also exerted a significant main effect, though its magnitude was considerably smaller. The surface treatment × aging interaction was not statistically significant, indicating that the influence of aging on SBS did not differ significantly across surface treatment groups (Table 5).

TABLE 5.

Two‐way ANOVA for SBS (MPa)—surface treatment × aging.

Source of variation	df	F	p	η 2 p
Surface treatment	2	41.965	0.001	0.608
Aging	1	4.067	0.049	0.070
Surface treatment × aging	2	2.312	0.109	0.079
Error (within)	54	‐	‐	‐
Total	59	‐	‐	‐

Note: Bold p‐values indicate statistical significance at α = 0.05.

Abbreviations: ANOVA, analysis of variance; df, degrees of freedom; η²p, partial eta‐squared; SBS, shear bond strength.

In the surface treatment main effect comparisons, all three pairwise contrasts were statistically significant, with YFL producing the highest SBS and LPP the lowest (Table 6). In the within‐group post hoc comparisons, no significant differences were observed between aged and non‐aged specimens in the control, LPP, or YFL groups. These findings are consistent with the non‐significant interaction between surface treatment and aging (Table 5).

TABLE 6.

Tukey HSD post‐hoc comparisons for SBS (MPa).

Comparison	Mean Diff. (MPa)	p (Tukey)
Control versus LPP (main effect)	2.030	0.007
Control versus YFL (main effect)	3.712	0.001
LPP versus YFL (main effect)	5.742	0.001
Post hoc: within‐group comparisons (aged vs. non‐aged)
Control/non‐aged versus control/aged	0.348	0.999
LPP/non‐aged versus LPP/aged	1.105	0.820
YFL/non‐aged versus YFL/aged	2.383	0.103

Note: Bold values indicate statistically significant differences (p values > 0.05).

Upper panel: comparisons based on marginal group means (n = 20 per group, pooled across aging levels); SE = 0.449. Lower panel: pairwise comparisons between aged and non‐aged specimens within each surface treatment group (n = 10 per cell); SE = 0.636.

Abbreviations: HSD, honestly significant difference; LPP, low‐pressure plasma; SBS, shear bond strength; YFL, ytterbium fiber laser.

The distribution of failure modes is presented in Table 7. According to the failure mode analysis, in the control group, 70% of the non‐aged specimens exhibited adhesive failure and 30% showed mixed failure. In the aged control group, 80% of the failures were adhesive and 20% were mixed. In the LPP group, 80% of the non‐aged specimens demonstrated adhesive failure, whereas 20% presented mixed failure. In contrast, all aged LPP specimens (100%) exhibited adhesive failure. In the YFL group, 30% of the non‐aged specimens showed adhesive failure, 60% mixed failure, and 10% cohesive failure. However, in the aged YFL group, 30% of the specimens exhibited adhesive failure, while 70% demonstrated mixed failure.

TABLE 7.

Distribution (%) of adhesive, mixed, and cohesive failure modes.

	Non‐aged			Aged
	Adhesive %	Mixed %	Cohesive %	Adhesive %	Mixed %	Cohesive %
Control	70	30	0	80	20	0
LPP	80	20	0	100	0	0
YFL	30	60	10	30	70	0

Abbreviations: LPP, low‐pressure plasma; YFL, ytterbium fiber laser.

Representative SEM images of the treated zirconia surfaces are presented in Figure 3, whereas representative images of adhesive, mixed, and cohesive failure modes are shown in Figure 4. SEM examination of the zirconia surfaces revealed distinct morphological differences among the groups. The control group (Figure 3a) displayed a smooth surface topography characterized by regular parallel grinding marks, consistent with the machined zirconia surface and corroborating the low surface roughness values obtained by confocal profilometry. SEM examination of the LPP group surface (Figure 3b) revealed no changes in surface morphology, with an appearance similar to that of the control group. The YFL group surface (Figure 3c) exhibited a markedly different morphology compared with the other groups, characterized by distinct parallel laser tracks ²¹ and microcrack formation. Adhesive failure (Figure 4a,d) was characterized by a zirconia surface devoid of composite residue. Mixed failure (Figure 4b,e) presented as a fracture surface exhibiting both exposed zirconia substrate and retained composite remnants. Cohesive failure (Figure 4c,f) was identified by complete coverage of the zirconia surface with composite resin.

FIGURE 3.

Representative SEM images of zirconia surfaces after different surface treatments. (a) Control, (b) LPP, and (c) YFL (1000×). LPP, low‐pressure plasma; SEM, scanning electron microscopy; YFL, ytterbium fiber laser.

FIGURE 4.

Representative SEM images of the failure modes observed after SBS testing. Adhesive (a), mixed (b), and cohesive (c) failure modes at 100× magnification; adhesive (d), mixed (e), and cohesive (f) failure modes at 2500× magnification. SBS, shear bond strength; SEM, scanning electron microscopy.

The XRD patterns of the specimens together with the reference Bragg positions are shown in Figure 5. The control and LPP groups showed highly similar diffraction profiles. The main XRD peak was located at approximately 2θ of 30.24° in all groups, and no peak shift was observed. YFL sample exhibited differences in peak intensity, peak width, and peak splitting, particularly around 2θ of 50°, 60°, and 74°. Weak peaks around 28.2° and 31.4°, which were present in control and LPP groups, were not resolved after YFL treatment. The main patterns were consistent with tetragonal and cubic‐like zirconia structures, with a possible minor monoclinic contribution in control and LPP groups.

FIGURE 5.

X‐ray diffraction patterns of the YFL, LPP, and Control groups. Vertical lines indicate the reference Bragg positions of monoclinic ZrO₂ (m‐ZrO₂; ICDD PDF 01‐089‐9066), tetragonal yttria‐stabilized zirconia (t‐YSZ; ICDD PDF 01‐080‐2187), and cubic yttria‐stabilized zirconia (c‐YSZ; ICDD PDF 01‐082‐1246). LPP, low‐pressure plasma; YFL, ytterbium fiber laser.

DISCUSSION

Based on the findings, the first null hypothesis was rejected, as surface treatment protocols exhibited statistically significant differences in SBS, with the YFL group demonstrating the highest values. These findings suggest that bond strength is more strongly influenced by surface roughness and the resulting micromechanical retention than by chemical surface activation alone. The novelty of the present study lies in the direct comparison of YFL and LPP surface treatments for bonding a gingiva‐colored indirect composite to zirconia. All experimental groups exhibited SBS values exceeding the minimum requirement of 5 MPa established by ISO Standard 10477. ²⁴ , ³⁶

In this study, plasma treatment operating directly with ambient air and without the need for external process gases was employed, as it enables surface modification without inducing thermal damage. ³² Compared with gas‐assisted plasma systems requiring carrier gases such as argon, oxygen, or helium, this approach offers a simpler, more cost‐effective, and more clinically adaptable alternative. ³⁷ Plasma treatment increases the reactivity of the zirconia surface through the formation of active oxygen‐ and nitrogen‐derived radicals. In addition, by breaking C─H and C─C bonds, plasma effectively reduces surface hydrocarbons and cleans the surface. ³²

Unlike airborne‐particle abrasion or tribochemical conditioning, plasma application does not contaminate the substrate or alter the inherent structure of zirconia, which may enhance both bond durability and long‐term stability. This process also facilitates the development of polar functional groups, improving wettability and leading to a reduction in contact angle. ³⁸ Nonthermal argon plasma has been reported to yield bond strength values comparable to those obtained with conventional airborne‐particle abrasion, while maintaining the surface topography essentially unchanged and consequently reducing the likelihood of fissure or crack formation. ³⁸ Previous studies on plasma‐based zirconia conditioning have primarily used single‐gas nonthermal plasma systems such as argon, oxygen, or helium. ²³ , ²⁴ , ²⁵ , ²⁷ , ³⁸ , ³⁹ , ⁴⁰ This study employed LPP, which operates at room temperature and low pressure and generates a broader spectrum of reactive oxygen and nitrogen‐based species. LPP treatment substantially increased surface hydrophilicity, as evidenced by the lowest posttreatment contact angle (10.12°); however, this did not translate into improved SBS, with the LPP group exhibiting the lowest SBS values in both aged and non‐aged conditions. The absence of a significant change in surface roughness suggests that LPP induces primarily chemical activation without micromechanical modification. Although LPP markedly improved surface wettability, zirconia bonding is influenced by multiple factors, including surface morphology, chemical interactions, and micromechanical retention. ⁴¹ Despite the immediate application of an MDP‐containing primer after plasma treatment, the improved surface wettability alone appeared insufficient to enhance bonding. This finding suggests that the chemical interaction provided by the primer may require concomitant micromechanical retention to achieve a measurable increase in bond strength. Therefore, enhanced wettability alone may not necessarily result in improved bond strength. It should also be noted that plasma‐induced surface activation may diminish over time depending on the plasma parameters and storage conditions. ³² However, in the present study, primer application was performed immediately after plasma treatment to minimize any potential loss of surface activation, and all specimens were maintained under closed storage conditions. Bunz et al. ⁴² evaluated the effect of cold plasma on the bond strength between zirconia and tooth‐colored veneering composite by subjecting specimens to both short‐term (5000 cycles) and long‐term (37,500 cycles) artificial aging. Consistent with the present findings, in which a comparable short‐term aging protocol (5000 cycles) was applied, no statistically significant improvement in bond strength was observed following plasma treatment.

Sevilla et al. ⁴⁰ conducted a study in which argon and oxygen plasma were used as surface treatments to evaluate and compare their effects on contact angle, SBS of zirconia and hybrid composite, and their failure mode. Their findings demonstrated that both plasma treatments reduced the contact angle, a result consistent with the observations of the present study. However, in contrast to our findings, Sevilla et al. reported that plasma exposure led to an increase in SBS in both plasma groups compared with the control, noting a strong correlation between reduced contact angle and improved adhesion to zirconia. This discrepancy may be attributed to methodological differences, particularly the use of argon and oxygen gases and the bonding of the composite specimens to zirconia with resin cement, which may have enhanced the chemical interaction at the interface. ⁴⁰ This difference may be primarily attributed to differences in plasma type (gas‐assisted systems vs. ambient air LPP) and bonding protocols.

Gökşen et al. ²³ reported that plasma treatment can enhance zirconia‐resin bonding primarily through chemical surface activation rather than mechanical modification. In particular, the use of argon‐based plasma jet systems was shown to significantly increase surface energy and promote the formation of oxygen‐containing functional groups, thereby improving wettability and interfacial adhesion. However, in this study, LPP did not result in a comparable increase in bond strength. This discrepancy may be attributed to differences in plasma application parameters, particularly the use of ambient conditions and the absence of a gas‐assisted plasma jet. It can be suggested that argon‐based plasma jet systems provide more effective surface activation, leading to improved chemical interaction at the zirconia‐resin interface, whereas the LPP approach used in this study may have been less efficient in enhancing bond strength despite improving wettability.

Previous studies have investigated the influence of different laser systems on zirconia‐composite bonding; ¹⁸ , ⁴³ , ⁴⁴ , ⁴⁵ however, studies evaluating the effect of YFL irradiation on the bond strength between zirconia and indirect composite resins remain limited.

In this study, YFL was preferred for its ability to achieve precise and controlled surface modification of zirconia. ²¹ The data obtained in this study demonstrated that YFL irradiation resulted in a statistically significant increase in the SBS of gingiva‐colored indirect composites to zirconia in both aged and non‐aged groups, and also increased the surface roughness and wettability of the zirconia surface. The increased surface roughness observed in the YFL group was supported by SEM observations, which revealed pronounced micro‐ and macro‐retentive surface features, including distinct parallel laser tracks ²¹ and microcrack formation. These morphological alterations may have facilitated resin penetration and micromechanical interlocking, thereby contributing to the higher bond strength observed in the present study. Although microcracks were observed on the representative specimen from the YFL group, the YFL group exhibited the highest bond strength values, suggesting that these microcracks were not associated with reduced bond strength under the conditions of the present study. Toyoda et al. ²¹ evaluated the effect of YFL surface treatment on the bond strength of zirconia using two different resin cements and reported a significant enhancement in SBS. They demonstrated that YFL application improved bond strength compared with untreated surfaces, with the effect varying depending on the type of resin cement. In addition, laser treatment was associated with increased surface roughness, which may have contributed to enhanced micromechanical interlocking at the interface. These findings support the present results, where YFL treatment yielded the highest bond strength values. ²¹

Consistent with this mechanism, Unal et al. ⁴⁶ demonstrated that YFL irradiation significantly increased both surface roughness and bond strength, supporting the concept that laser‐induced topographical modification enhances micromechanical retention. Despite differences in bonding substrates and protocols, their findings reinforce the present results by highlighting the dominant role of surface roughness in improving adhesion.

Similarly, Demetoğlu et al. ⁴⁷ reported that the SBS of the YFL group lithium disilicate ceramics was comparable to that achieved with conventional hydrofluoric acid etching. These findings suggest that YFL treatment may provide favorable bonding performance in different ceramic systems, although the underlying mechanisms may vary depending on the intrinsic microstructure and bonding characteristics of the substrate.

However, Fornaini et al. ²² observed an increase in bond strength following fiber laser treatment; however, the lack of statistical significance in their results suggests that laser‐induced improvements may depend on the specific laser parameters and application protocols. These variations underline the importance of optimizing irradiation conditions to achieve consistent and clinically relevant outcomes.

Variations in the results of different studies may be attributed to differences in laser devices, pulse characteristics, scanning patterns, operator‐dependent variables, and the intrinsic properties of zirconia.  Roughening with YFL is still controversial for zirconia ceramics, and roughening studies with YFL are very limited in the literature. ⁴⁷ Given the scarcity of research and the inconsistency of available findings, further investigations with standardized irradiation parameters are necessary to establish clear clinical guidelines for YFL surface treatment of zirconia to enhance the bond strength of indirect composite materials.

The second null hypothesis was not rejected, as the effect of short‐term thermal aging on SBS was not statistically different among the surface treatment groups. Although aging showed a statistically significant overall effect on SBS (p = 0.049), post hoc comparisons revealed no significant differences between aged and non‐aged specimens within the LPP, YFL, and control groups, consistent with previous reports. ¹⁴ , ¹⁸ , ²⁹ , ³⁹

Komine et al. ⁴⁸ evaluated the bond strength between zirconia and an indirect composite material before and after 5000 thermocycles, representing approximately 6 months of clinical aging, using different functional primers. They reported that thermocycling did not significantly reduce bond strength in the MDP‐, 4‐META‐, and 6‐MHPA‐containing primer groups, whereas a significant reduction was observed in the primer group lacking these functional monomers. The authors suggested that the durable chemical bonding provided by these functional monomers, together with continued polymerization of the indirect composite during thermocycling, may have contributed to maintaining bond strength. These findings support the limited effect of thermal aging observed in the present study. The present findings reflect the effects of short‐term thermocycling under the applied experimental conditions, and longer‐term aging protocols may produce different outcomes.

Failure mode analysis indicated that surface treatment influenced the observed failure patterns. In the YFL group, the higher proportion of mixed failures, particularly in non‐aged specimens, suggests that failure occurred across combined regions rather than exclusively at the interface, reflecting improved bond integrity. In contrast, the predominance of adhesive failures in the LPP and control groups indicates weaker interfacial bonding. The predominance of mixed failures in the YFL group compared with the control and LPP groups further supports the formation of a stronger and more stable interface. Representative SEM images also supported the failure mode analysis by demonstrating the characteristic morphological features of adhesive, mixed, and cohesive failures.

The XRD findings suggested that plasma treatment did not produce a detectable phase transformation, since it generally modifies surface energy, wettability, and surface contamination rather than crystalline structure. In contrast, the YFL group showed changes in peak intensity and profile, while the main peak positions did not change. The reduced peak splitting, particularly around 50°, may indicate a higher contribution from a cubic‐like or metastable tetragonal‐prime (t′) phase, which is difficult to distinguish from the true cubic phase by conventional XRD. ⁴⁹ The laser scanning lines and surface cracks observed in the SEM images suggest that laser treatment caused local heating and thermal stress, which may have affected the XRD patterns. Weak peaks related to a possible minor monoclinic phase were seen in the control and plasma groups but not in the laser group; however, their low intensity does not allow a clear conclusion. Since only one specimen from each group was analyzed and no quantitative refinement was performed, these XRD findings should be confirmed by repeated measurements or Rietveld analysis.

There are several limitations to the present study. First, only a single laser type (YFL) and a low‐pressure, low‐temperature air plasma system were evaluated; different laser systems, parameters, or plasma modalities using various carrier gases may produce different outcomes. The in vitro design represents another limitation, as it does not fully replicate the complex biological and mechanical conditions of the oral environment. In particular, the absence of simulated oral fluids and the lack of temperature or pH fluctuations limit the generalizability of the findings. In addition, mechanical fatigue loading, saliva contamination, and intraoral cleaning/rebonding scenarios were not evaluated. Therefore, the clinical relevance of the present findings should be interpreted with caution, and these factors should be investigated in future studies. Moreover, bond strength was assessed only using standardized specimens and the SBS test, which does not fully reflect the complex loading conditions encountered clinically. Color changes observed after YFL application were not evaluated. The ability of the veneering composite to mask this alteration remains unclear, and further investigations are scheduled to test this situation. Finally, the aging protocol was limited to 5000 thermocycles, representing a short‐term aging condition. Long‐term aging effects should be evaluated in future studies.

CONCLUSIONS

Within the limitations of this study, it was found that the YFL treatment significantly improved the bond strength between zirconia and gingiva‐colored indirect composite compared with LPP treatment and control groups. YFL treatment increased surface roughness and promoted micromechanical retention. In contrast, although plasma treatment remarkably improved surface wettability, this effect alone did not translate into higher bond strength. Thermal aging demonstrated a statistically significant but relatively small main effect on bond strength; however, the interaction between surface treatment and aging was not significant, indicating that the influence of aging was similar across the tested surface treatment groups.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.