Front. Mater. Frontiers in Materials Front. Mater. 2296-8016 Frontiers Media S.A. 1769619 10.3389/fmats.2026.1769619 Original Research Enhancement of zirconia–veneering porcelain bond strength via thermal phosphoric acid etching combined with lithium Silicate Coating and the influence of multiple sintering cycles Chen et al. 10.3389/fmats.2026.1769619 Chen Beibei 1 Writing – review and editing Writing – original draft Yang Jingyuan 2 Writing – review and editing Writing – original draft Zou Binlong 1 Writing – review and editing Lin Kaixin 1 Writing – review and editing Hu Xiaohui 2 * Writing – review and editing Denture Technology Center, School & Hospital of Stomatology Wenzhou Medical University, Wenzhou, Zhejiang, China Restoration Department, School & Hospital Of Stomatology Wenzhou Medical University, Wenzhou, Zhejiang, China Correspondence: Xiaohui Hu, dentisth_66@163.com 11 02 2026 2026 13 1769619 17 12 2025 21 01 2026 26 01 2026 Copyright © 2026 Chen, Yang, Zou, Lin and Hu. 2026 Chen, Yang, Zou, Lin and Hu https://creativecommons.org/licenses/by/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. Objective

To evaluate the effect of thermal phosphoric acid etching combined with a lithium silicate (SiLi) coating on the interfacial bond strength between zirconia and veneering porcelain, and to investigate the influence of multiple sintering cycles on veneering porcelain surface properties and zirconia core mechanical performance.

 Methods

Zirconia specimens were allocated into four groups: untreated control, acid etching, SiLi coating, and acid etching + SiLi coating. Each group was subjected to 1, 3, 5, or 7 low-temperature sintering cycles. Shear bond strength, three-point flexural strength, water contact angle (WCA), and Vickers hardness were measured. Microstructural and compositional changes were analyzed via scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD).

 Results

XRD confirmed the formation of zirconium hydrogen phosphate after acid etching and a SiLi layer after coating. The acid etching + SiLi group showed the highest shear bond strength and a significant reduction in WCA compared with the untreated (p < 0.05). Multiple sintering cycles produced a parabolic trend in bond strength and veneering porcelain hardness, with peak values after three cycles in the acid etching + SiLi group. No significant differences in three-point flexural strength were observed among groups (p > 0.05).

 Conclusion

Thermal phosphoric acid etching combined with SiLi coating significantly improves the zirconia–veneering porcelain bond without compromising the flexural strength of zirconia. Clinically, the number of sintering cycles should be limited to three or fewer to preserve optimal interfacial adhesion and veneering porcelain hardness.

 lithium silicate coating sintering cycles thermal phosphoric acid etching veneering porcelain zirconia The author(s) declared that financial support was not received for this work and/or its publication. section-at-acceptance Ceramics and Glass Introduction

With the advancement of economic development and the enhancement of public awareness regarding oral health, the paradigm of dental restoration has evolved from a singular focus on functional rehabilitation to a dual emphasis on both functional performance and esthetic excellence. Owing to their superior biocompatibility, favorable mechanical properties, and enhanced optical performance (Raptis et al., 2006; Spear and Holloway, 2008; Vichi et al., 2011), all-ceramic materials have progressively supplanted conventional porcelain-fused-to-metal restorations, becoming the primary modality for contemporary restorative dentistry (Mclaren, 1998; Wall and Cipra, 1992; Wassell et al., 2002; Van Dijken, 1999).

Zirconia ceramics exhibit exceptional mechanical properties, with flexural strength values reaching up to 1,000 MPa, and are applicable across a wide range of clinical indications. However, monolithic zirconia restorations are limited in restoration of anterior teeth due to inherent drawbacks such as insufficient translucency and monochromatic appearance. In contrast, glass ceramics possess favorable optical translucency and can closely replicate the color, texture, and morphology of natural teeth. Consequently, computer-aided design/computer-aided manufacturing (CAD/CAM) and related digital technologies are frequently employed to fabricate high-strength, low-translucency zirconia core frameworks in clinical practice, which are subsequently veneered with high-translucency glass ceramics through sintering to achieve a natural tooth-like stratification and texture. This bilayered design enables the reproduction of the depth and luster characteristic of natural teeth. Veneering porcelains are primarily composed of a glassy matrix interspersed with various crystalline phases, with silica and multiple metal oxides serving as the principal constituents.

Although the adoption of this bilayer configuration has enhanced the esthetic performance of restorations, it has also introduced a prevalent complication—chipping of the veneering porcelains (Lee et al., 2018). This failure mode is multifactorial in origin, influenced by variables such as sintering temperature, core design, veneering porcelain thickness, disparity in elastic moduli between constituent materials, and suboptimal wettability of the veneering porcelain to the zirconia substrate, all of which can induce residual tensile stresses at the zirconia–veneer interface (Fischer et al., 2008; Komine et al., 2012; Anunmana and Wansom, 2017; Larsson and Wennerberg, 2014). Furthermore, during successive firing cycles of the veneering porcelain, mismatch in the coefficients of thermal expansion between the core and veneering porcelain materials promotes the accumulation of residual stresses, thereby facilitating fatigue crack initiation and propagation, ultimately elevating the risk of structural failure. At present, the progressive increase in veneering porcelain chipping rates has emerged as a clinically significant concern that warrants urgent attention.

The bonding mechanisms in porcelain-fused-to-metal (PFM) restorations have been extensively investigated, and are generally recognized to involve a combination of chemical adhesion, mechanical interlocking, compressive bonding, and van der Waals interactions (Zhu et al., 2008; Zhan et al., 2008). In contrast, the interfacial bonding mechanism between zirconia and veneering porcelains remains insufficiently elucidated (Nishigori et al., 2014). Current strategies to improve the bond strength can be broadly categorized into mechanical and chemical surface treatments. Mechanical approaches, such as airborne-particle abrasion (sandblasting), may increase surface roughness but are associated with potential drawbacks, including surface flaws and tetragonal-to-monoclinic phase transformation, which can compromise interfacial strength (Rekow et al., 2011; Korkmaz et al., 2015).

Compared with conventional surface treatments, such as airborne particle abrasion and liner application, the proposed combination of thermal phosphoric acid etching and lithium silicate (SiLi) coating offers a unique advantage by simultaneously improving micromechanical interlocking, chemical reactivity, and surface wettability. Unlike traditional liners, which show inconsistent effects due to thermal expansion mismatch, the SiLi coating forms a stable interfacial layer that integrates with the etched nanostructure, leading to a more reliable bond. Furthermore, in contrast to plasma-enhanced deposition or laser-based methods that often require specialized equipment and cost, this protocol utilizes readily available materials and simple procedures, making it more clinically feasible. However, potential limitations such as the long-term durability of the coating still require further validation.

Laser technology was introduced into the domain of dental restoration more than a decade ago, offering the potential to enhance interfacial bond strength through the augmentation of surface roughness, improvement of hydrophilicity, and modulation of surface energy states. For example, micro-textures generated by picosecond laser irradiation can markedly alter hydrophilicity and thereby influence interfacial adhesion (Ji et al., 2020); rubidium and erbium laser treatments have been reported to optimize the surface topography of zirconia (Liu et al., 2015; Cavalcanti et al., 2009); and carbon dioxide laser application has been employed to promote implant osseointegration and to refine the surface architecture of ceramic substrates (Ural et al., 2010).

Chemical surface modification techniques primarily encompass acid etching and coating treatments. Owing to the inherent chemical stability, high crystallinity, and absence of silicon dioxide in zirconia, conventional hydrofluoric acid etching at 4–10 wt% concentrations is proved ineffective. A commonly employed thermal acid etching protocol utilizes a mixture of concentrated hydrofluoric acid and concentrated sulfuric acid, which increases the surface wettability of zirconia and exposes zirconia grains, thereby enhancing micromechanical interlocking (Ferrari et al., 1989). Alternative methods, such as hydrochloric acid combined with ferric chloride, sulfuric acid with ammonium sulfate, or hydrofluoric acid with nitric acid under thermal conditions, have been reported to produce nanoscale network structures (Xie et al., 2013). These modifications have been demonstrated to improve the adhesive strength between zirconia and resin cements; however, their influence on the bond strength to veneering porcelains remains unclear. Preliminary studies have indicated that phosphoric acid alone, under specific conditions, can generate a robustly adherent honeycomb-like nanostructure without compromising the mechanical properties of the zirconia substrate.

Coating-based surface modification techniques encompass zirconia coating, lining layers, plasma-assisted chemical vapor deposition, and nanoscale self-limiting atomic layer deposition of silica. For example, one study applied a zirconia slurry with particle sizes <30 μm to the inner crown surface of zirconia, followed by sintering to form a porous coating that enhanced the bond strength with veneering porcelains (Teng et al., 2012). Liner layers have been reported to mitigate the mismatch in coefficients of thermal expansion (CTE) between the core and veneering porcelain; however, their influence on bond strength remains inconsistent across studies (Yoon et al., 2015; Wang et al., 2014; Harding et al., 2012; Kim et al., 2018). D.S.H. Silva-Herzog Rivera et al. (2020) employed the sol–gel method and commercial coating materials to fabricate glass coatings, observing that the shear bond strength of the sol–gel-derived coating exceeded that of the commercial counterpart. Emerging technologies such as melt sputtering and microcrystalline glass jet deposition have been utilized to form dense lithium disilicate microcrystalline glass coatings on zirconia surfaces, which promote micromechanical interlocking and enhance surface wettability, thereby strengthening the zirconia–veneering porcelain bond (Peng et al., 2022; Ali et al., 2019; Aboushelib, 2012).

Overall, strategies to enhance the interfacial bond strength between the zirconia core and veneering porcelains can be broadly categorized as follows: (1) increasing surface roughness to promote mechanical interlocking; (2) activating the surface to augment chemical reactivity; and (3) introducing an intermediate layer to optimize surface architecture and interfacial performance (Silva-Herzog Rivera et al., 2020; Yan et al., 2021; Li et al., 2021; Abdullah et al., 2019). Most existing approaches have focused on a single mechanism, yielding only limited improvements. The present study seeks to integrate these three strategies to develop and evaluate a novel zirconia surface treatment protocol aimed at significantly enhancing the bond strength with veneering porcelains.

 Materials and methods Materials and equipment Materials

3D Shining Multicolor Zirconia Ceramic Blocks (Aidite, China); VITA porcelain powder (VITA Zahnfabrik, Germany); Lithium ethanol solution (Aladdin Biochemical Technology Co., Ltd., China); Tetraethyl orthosilicate (TEOS) (Xilong Scientific Co., Ltd., China); Absolute ethanol (Ante Food Co., Ltd., China); Phosphoric acid solution (Aladdin Biochemical Technology Co., Ltd., China); Light-body silicone impression material (DMG, Germany); Waterproof abrasive papers, grit sizes 400, 1,000, and 2000 (Matador, Germany).

 Equipment

Diamond wire cutting machine, Model ST202A (Shenyang Kejing Automation Equipment Co., Ltd., China); Ultrapure water purification system (Millipore, United States); Ultrasonic cleaner, Model PS30A (Guangdong Fuheng Instrument Co., Ltd., China); Universal testing machine (Instron, United States); Field-emission scanning electron microscope (FE-SEM), Model SU8010 (Hitachi, Japan); X-ray diffractometer (XRD), Model D8 Advance (Bruker, Germany); Touchscreen digital Vickers hardness tester, Model HVST-1000C (Shanghai Zhongyan Instrument Manufacturing Co., Ltd., China); Contact angle goniometer (Shengding Precision Instruments Co., Ltd., China); Zirconia sintering furnace, Model Programat P310 (Ivoclar Vivadent, Liechtenstein); Vacuum porcelain furnace (Ivoclar Vivadent, Liechtenstein); Constant-temperature drying oven, Model DHG-9035A (Shanghai Yiheng Scientific Instrument Co., Ltd., China); Stereomicroscope (Nikon, Japan); Laser marking machine (Huagong Technology Industry Co., Ltd., China); Digital vernier caliper (Lvlin Technology Co., Ltd., China).

All instruments used in surface treatments, including the Programat P310 sintering furnace and the pH meter used to check the solution concentration, were calibrated before use. The sintering furnace was calibrated according to the manufacturer’s instructions to ensure accurate temperature control. The pH meter was calibrated using standard pH buffers (pH 4.00, 7.00, and 10.00) before each experiment.

 Methods Preparation of zirconia specimens and surface treatments

Preparation of Zirconia Specimens: Edite 3D colorful zirconia ceramic blocks were selected as the raw material. The blocks were sectioned using a diamond wire cutting machine operating at 300 r/min under continuous water cooling. Ceramic plates with dimensions of 14 mm × 10 mm × 1.5 mm and ceramic bars with dimensions of 40 mm × 3.75 mm × 5 mm were prepared. Following sectioning, the specimens were ultrasonically cleaned in deionized water for 10 min, dried in a vacuum oven, and subsequently sintered in a zirconia sintering furnace according to the manufacturer’s standard sintering protocol.

Phosphoric Acid Etching Treatment: The zirconia specimens were placed in a high-pressure reaction vessel containing a 2.5 wt% phosphoric acid solution and treated in a constant-temperature drying oven at 160 °C for 24 h. Post-treatment, the specimens underwent sequential ultrasonic cleaning as follows: deionized water for 10 min → anhydrous ethanol for 5 min → deionized water for 10 min. The samples were then dried in a vacuum oven prior to subsequent experimental procedures.

Preparation of Lithium Silicate (SiLi) Sol: Ethyl silicate was mixed with a 1 mol/L lithium ethoxide solution in a volume ratio of 1:4.5 (with a molar ratio of Si to Li of 1:1). The mixture was stirred at 750 r/min using a magnetic stirrer at room temperature (25 °C) for 3 h, resulting in a homogeneous sol precursor.

Preparation of SiLi Coating:

Ceramic Sheet Coating: A volume of 500 µL of the SiLi sol was applied to the hole plate containing the ceramic sheet. The assembly was subjected to a negative pressure of −0.1 kPa for 5 min at RT to facilitate the penetration of the sol into the honeycomb structure created by thermal acid etching. This vacuum-assisted step was intended to promote physical infiltration of the SiLi sol into the etching-induced honeycomb pores/grooves, rather than chemical diffusion into the dense zirconia lattice. Following this, 500 µL of deionized water was added, and the sol underwent sol-gel transformation within 1 min to form a white, thin gel layer. After drying at 90 °C, the sample was sintered in a zirconia sintering furnace according to specific procedures.

Ceramic Strip Coating: For the ceramic strip coating, 1 mL of the SiLi sol was added to the hole plate containing the ceramic strip. The system was subjected to negative pressure for 5 min, followed by the addition of 1 mL of deionized water. The subsequent drying and sintering procedures were identical to those used for the plate specimens.

 Preparation of zirconia–veneering porcelain bonding specimens

Following sintering, the zirconia ceramic sheets were randomly assigned to four experimental groups based on the surface treatment method: group A (untreated), group B (acid etching), group C (SiLi coating), and group D (acid etching + SiLi coating).

VITA veneering porcelain powder was mixed and packed into cylindrical silicone rubber molds with an internal diameter of 6 mm and a height of 3 mm. Subsequently, 80 zirconia sheets from each group were selected, and an identical ceramic coating layer was applied to the surface of each sheet (Figures 1a,b). Each group of specimens was further subdivided into four subgroups, each consisting of 20 specimens. These subgroups were subjected to one, three, five, or seven low-temperature sintering cycles.

(a, b) Zirconia–veneering porcelain bonding specimens; (c) Sintering procedure.

Panel a shows a circular mesh tray holding multiple small, cylindrical beige samples. Panel b depicts a single round, semi-transparent specimen on a square base. Panel c displays a dental furnace control panel with temperature settings, a digital timer, and start and stop buttons.

The zirconia specimens were sintered in a Programat P310 zirconia sintering furnace according to the following conditions: starting temperature was set at 600 °C, with a peak sintering temperature of 935 °C. The heating rate was 6 min to reach the peak temperature, where the specimens were held for 1 min. After the peak temperature, the specimens were cooled from 935 °C to 600 °C over a period of 12.5 min (shown in Figure 1c). After sintering, excess material was removed, and the specimens were dried at 37 °C for storage.

 Preparation of three-point bending test specimens

Sintered the zirconia ceramic strips were randomly assigned to four experimental groups, each consisting of 10 strips: group E (untreated), group F (acid etching, 2.5 wt% phosphoric acid solution, 160 °C, 24 h), group G (SiLi coating), and group H (acid etching + SiLi coating). All zirconia strips were subsequently used to assess three-point bending strength.

The length (L), width (w), and thickness (b) of each strip were measured using digital vernier calipers. The dimensions were verified to comply with the ISO 6872 standard for “Dental Ceramic Materials” (w = 4.0 ± 0.2 mm; b = 1.2–3.0 ± 0.2 mm; the length was at least 2 mm greater than the span; b/L ≤ 0.1). All specimens were then cleaned, dried, and prepared for testing.

 Preparation of vickers hardness specimens and vickers hardness testing on the porcelain surface

A total of 12 ceramic sheet specimens were prepared using the previously described molding method and randomly assigned to 4 groups based on the number of sintering cycles (1, 3, 5, and 7 times, with 3 samples per group). The specimens were successively polished with 400-, 1,000-, and 2,000-grit sandpaper for 5 min each, followed by mechanical polishing with polishing liquid and cloth for an additional 5 min until the surface exhibited a mirror-like finish. The specimens were then cleaned, dried, and stored.

The Vickers hardness test was performed using a digital Vickers hardness tester (load: 9.807 N, loading time: 20 s, objective lens: ×40). Five measurement points (the center and four corners) were selected for each specimen. The effective indentation was a regular rhombic shape, free from chipping or side peeling. The Vickers hardness value (HV) was calculated using the following formula: H V = 1.8544 × P × d − 2 where HV is the Vickers hardness value (kgf/mm2), P is the applied experimental force (N), and d is the average length of the two diagonals of the rhombic indentation (mm).

 Test of WCA between zirconia and veneering porcelain surfaces

This test evaluated the WCA of veneering porcelain surfaces subjected to multiple sintering cycles and zirconia surfaces treated with different surface modification techniques. The veneering porcelain samples were subjected to sintering cycles of 1, 3, 5, and 7 times, respectively, while the zirconia samples were selected from groups A-D. An equal number (n = 20) of zirconia sheets were taken from each group.

The sessile drop method was employed, with a droplet volume of 5 µL of ultra-pure water applied each time. The water droplets were deposited vertically onto the surface of the samples and allowed to remain for 10 s before images were captured using a high-speed camera. The liquid droplet shape analysis software was utilized to fit the images and calculate the WCA for each sample.

 Veneering porcelain shear strength test

The bonding strength between zirconia and veneering porcelain was evaluated using a universal testing machine with a shear speed of 1 mm/min. The specimens were securely mounted in a dedicated fixture to ensure that the interface was parallel to and in direct contact with the shear head (Figure 2). The shear strength was then calculated using the following formula: P = F / S where P represents the shear strength (MPa), F is the fracture force (N), and S is the bonding area of the veneering porcelain (mm2).

Schematic diagram of veneering porcelain shear strength test.

Two photographs of a metal testing apparatus show a close-up of grips, springs, and a notched sample held in place, with a horizontal ruler for precise measurement below the clamping area. Fracture mode analysis

After the shear test, the zirconia specimens were examined under a stereomicroscope (25×) to assess their fracture modes. The fracture modes of the ceramic layer delamination were categorized based on the coverage rate of the residual ceramic layer on the zirconia sheet surface, using a five-point scale: 1: 0%–20%; 2: 21%–40%; 3: 41%–60%; 4: 61%–80%; 5: 81%–100%. A score of 1 indicates destruction of the ceramic bond, while scores of 2–4 represent mixed destruction of the ceramic bond and cohesion, and a score of 5 indicates destruction of the ceramic cohesion. The images of the three fracture modes under the stereomicroscope are shown in Figure 3.

Schematic diagrams of three fracture modes (a) Cohesive failure; (b) Mixed failure; (c) Bond failure.

Three micrographs show types of failure at a microscopic scale, with scale bars indicating five micrometers. Panel a shows cohesive failure with uniform texture, panel b displays mixed failure with partial texture disruption, and panel c illustrates bond failure with separation at the material interface. Microscopic morphology observation

Samples from groups A-D were subjected to gold sputter-coating for 120 s and then analyzed for surface morphology using a scanning electron microscope (SEM). Additionally, samples from each group were frozen in liquid nitrogen for 2 min, subsequently fractured, gold-sputter-coated, and analyzed under the SEM to observe the fracture surface morphology. The elemental distribution of the fracture surface was also analyzed using an energy-dispersive X-ray spectrometer (EDS).

 Crystal phase structure analysis

Samples from groups A-D were selected and analyzed using a copper-target wide-angle diffraction scan on an X-ray diffraction (XRD) instrument. The scanning angle ranged from 5° to 90°. The diffraction peak positions and intensities were examined to determine the crystal phase composition and assess any phase transformations.

 Three-point bending strength test

Zirconia strip specimens from groups E − H were tested for three-point bending strength using a universal testing machine. The specimens were positioned on a testing fixture with a span of 30 mm, a width of 4.0 mm, and a thickness of 3.0 mm. The upper testing head was applied vertically at a rate of 1 mm/min until fracture occurred, and the maximum load at the point of fracture was recorded. The three-point bending strength was calculated as following: R = 3 F L / 2 w b 2 where R is the three-point bending strength (MPa), F is the maximum load at fracture (N), L is the span of the testing fixture (mm), w is the width of the specimen (mm), and b is the thickness of the specimen (mm). Figure 4 shows the test schematic diagram.

Schematic diagram of three-point bending strength test.

Four photographs show different views of a mechanical test setup involving a metal fixture with parallel jaws, holding a rectangular sample, likely for material strength testing. Each panel highlights various angles and configurations of the testing device and sample placement. Statistical analysis

Data processing was carried out using GraphPad Prism 8 software, while statistical analyses were performed using SPSS 25.0 software. A significance level of α = 0.05 was set, with p ≤ 0.05 indicating statistically significant differences. Inter-group comparisons were conducted using one-way analysis of variance (ANOVA) and t-tests.

 Results Surface characteristics SEM observation results

Figure 5 presents the SEM images of zirconia specimens subjected to different surface treatment conditions. In group A, a well-defined grain structure of zirconia was observed, with grain diameters ranging from 3 μm to 10 μm. The grains were closely bonded, arranged in a regular pattern, and no discernible intergranular gaps were detected. Group B exhibited a uniform honeycomb-like porous structure on the zirconia surface, with evenly distributed pore openings measuring 1 μm–5 μm in diameter. Cross-sectional observations in Figure 6 revealed that the thickness of this honeycomb-like layer was approximately 5 μm. In groups C and D, the zirconia surfaces were uniformly covered with a crystalline-like layer, interspersed with scattered protruding rounded features.

SEM images of the zirconia ceramic sheet surfaces of each group (a) untreated group (group A); (b) acid etching group (group B); (c) SiLi coating group (group C); (d) acid etching + SiLi coating group (group D).

Panel a shows a scanning electron microscope image of a rough surface with compact, irregular particles. Panel b displays a highly porous, reticulated microstructure. Panel c presents scattered, elongated fragments on a textured background. Panel d features rounded, dispersed particles of varying sizes on a flatter surface. Each image includes a one micrometer scale bar for reference.

SEM images of the zirconia ceramic sheet fracture surface in each group (a) untreated group (group A); (b) acid etching group (group B); (c) SiLi coating group (group C); (d) acid etching + SiLi coating group (group D).

Scanning electron micrographs labeled a, b, c, and d show cross sections of membrane structures at one micrometer scale, with images b and d displaying a noticeably more porous internal morphology compared to the denser appearances in a and c.

The cross-sectional SEM observations of zirconia specimens in each group are shown in Figure 6. In group B, compared with group A, alterations in the surface grain structure were observed, accompanied by the presence of distinct intergranular gaps. In group C, a surface coating on the zirconia was visible; however, the boundary between the coating and the etched grooves was relatively well-defined. In group D, the surface coating on zirconia was also evident, but partial interlocking between the coating and the zirconia substrate was observed. Consistent with the results in Figure 7, cross-sectional EDS mapping of the acid etching + SiLi group showed a Si-enriched zone within the near-surface region. Importantly, the apparent ‘penetration depth’ of Si in the map reflects the combined thickness of the external SiLi coating and the SiLi phase physically retained within the etching-induced porous network (honeycomb pores/grooves), rather than long-range chemical diffusion of Si into the dense ZrO2 lattice.

EDS images of the fracture surface of SiLi coating Cross-sectional EDS elemental maps of the fracture surface of the acid-etched + SiLi specimen: (a) O, (b) Si, and (c) Zr.

Electron microscopy elemental maps display oxygen in red (panel a), silicon in green (panel b), and zirconium in cyan (panel c) with distinct spatial distributions at micrometer scale, highlighting compositional heterogeneity. Water contact angle (WCA)

The statistical analysis of the WCAs on the veneering porcelain surfaces of specimens subjected to different numbers of sintering cycles is presented in Figure 8a. The WCA exhibited a trend of initial decrease followed by an increase with successive sintering cycles: it was significantly reduced from one to five cycles (p < 0.05), indicating enhanced hydrophilicity, but increased significantly from five to seven cycles (p < 0.05). These findings indicate that the WCA of veneering porcelain decreased initially and then increased as the number of sintering cycles increased.

WCA analysis under different processing conditions (a) WCA of veneering porcelain surfaces for different sintering cycles, n = 20; (b) WCA of the zirconia ceramic sheet surfaces with different surface treatments, untreated group (group A), acid etching group (group B), SiLi coating group (group C), acid etching + SiLi coating group (group D), n = 20.

Bar graph (a) compares WCA values across one, three, five, and seven cycles, showing significant decreases with more cycles, except a rise at seven cycles. Bar graph (b) compares WCA for untreated, acid etching, SiLi coating, and acid etching plus SiLi, revealing significant reductions with treatments, especially the combined treatment. Statistical significance is indicated by asterisks and "ns" for not significant.

Following different surface treatments, the WCA of zirconia in the acid etched group was slightly higher than that of the untreated group, but the difference was not statistically significant (p > 0.05). In contrast, both the SiLi group and the acid etching + SiLi group exhibited significantly lower WCAs compared with the untreated (p < 0.05), as shown in Figure 8b.

 Vickers hardness

The variations in the surface hardness of veneering porcelain under different numbers of sintering cycles are presented in Figure 9. The surface hardness was significantly higher after three sintering cycles compared with a single sintering cycle (p < 0.05). However, when the number of sintering cycles increased to five and seven, the hardness values were significantly lower than those observed after one sintering cycle (p < 0.05). These findings indicate that three sintering cycles could enhance the hardness of veneering porcelain, whereas excessive sintering may result in a reduction in hardness.

Statistical analysis of the Vickers hardness of the veneering porcelain surface in different sintering cycles n = 3.

Bar chart comparing Vickers hardness in kilograms-force per square millimeter for one, three, five, and seven cycles, showing highest hardness at three cycles and significant differences indicated by asterisks. Three-point flexural strength

Thermal phosphoric acid etching may potentially exert an adverse effect on the mechanical strength of specimens, and three-point flexural strength is commonly employed as a critical parameter for evaluating the mechanical performance of materials. As shown in Figure 10, the three-point flexural strength values for the untreated, acid etched, SiLi, and acid etching + SiLi groups were 485.82 ± 113.78 MPa, 546.75 ± 146.10 MPa, 566.81 ± 107.77 MPa, and 580.89 ± 108.24 MPa, respectively. One-way ANOVA revealed no statistically significant differences among the four groups (p > 0.05), indicating that thermal phosphoric acid etching, either alone or in combination with a SiLi coating, did not produce a significant impact on the three-point flexural mechanical performance of zirconia.

Three-point bending strength of zirconia ceramic strips with different surface treatments untreated group (group E), acid etching group (group F), SiLi coating group (group G), acid etching + SiLi coating group (group H), n = 10.

Bar graph illustrating three-point flexural strength in megapascals for four surface treatment groups: Untreated, Acid etching, SiLi coating, and Acid etching plus SiLi. All comparisons are labeled as not significant (ns). Shear bond strength of veneering porcelain

As shown in Figure 11, under the single-sintering condition, the untreated group exhibited the lowest shear bond strength (3.66 ± 3.27 MPa), whereas the acid etched group, the SiLi group, and the acid etching + SiLi group all demonstrated significantly higher values compared with the untreated group (p < 0.05). Among these, the acid etching + SiLi group achieved the highest shear bond strength (11.05 ± 4.85 MPa), with a statistically significant difference between the two groups (p < 0.05).

Shear strength of the veneering porcelain specimens in each group untreated group (group A), acid etching group (group B), SiLi coating group (group C), acid etching + SiLi coating group (group D, n = 3.

Bar chart showing shear bond strength in megapascals for four surface treatments—untreated, acid etching, SiLi coating, and acid etching plus SiLi—across one, three, five, and seven cycles. Acid etching plus SiLi yields the highest bond strength, particularly at five cycles. Significant differences are marked with asterisks, and "ns" indicates no significant difference between some groups. Error bars represent variability in measurements.

In the untreated group, shear bond strength increased significantly as the number of sintering cycles rose from one to five (maximum 6.82 ± 3.41 MPa, p < 0.05), followed by a marked decline after seven cycles (p < 0.05). Both the acid etched group and the SiLi group displayed a similar pattern, reaching their respective peak values after five sintering cycles (acid etched group: 12.06 ± 2.08 MPa; SiLi group: 11.61 ± 2.49 MPa) before subsequently decreasing. In contrast, the acid etching + SiLi group reached its maximum value after three sintering cycles (16.14 ± 6.03 MPa, p < 0.05), after which the strength gradually declined, returning to a level comparable to that of the single-sintering condition by the seventh cycle.

 XRD analysis results

The XRD patterns for each group are shown in Figure 12. The untreated group exhibited the characteristic diffraction peaks of zirconia, with the most prominent peak observed at a diffraction angle of 29.7°. In addition to the zirconia characteristic peaks, the phosphoric acid etched group displayed several new peaks. Comparison with the crystallographic database revealed that the earliest emerging peaks closely matched those of zirconium hydrogen phosphate, suggesting the possible formation of zirconium hydrogen phosphate on the surface. In the remaining two groups, zirconia specimens subjected to coating treatment also exhibited multiple diffraction peaks, which corresponded to the standard diffraction peaks of SiLi, indicating that a SiLi coating had been successfully formed on the zirconia surface.

XRD spectra of zirconia after different surface treatments.

X-ray diffraction (XRD) patterns compare untreated, acid-etched, SiLi-coated, and acid-etched plus SiLi-treated samples, showing peak shifts and intensity changes at labeled 2-theta positions with a legend indicating treatment colors. Discussion

This study proposed and validated a zirconia surface treatment protocol involving thermal phosphoric acid etching combined with a SiLi coating, and further investigated the influence of varying sintering cycles on the bond strength between zirconia and veneering porcelain, as well as on the surface properties of the veneering porcelain. The findings revealed that, compared with acid etching alone, the combined treatment significantly increased shear bond strength, thereby more effectively reinforcing the adhesion between zirconia and veneering porcelain, with the potential to reduce the clinical risk of porcelain chipping. Notably, bond strength declined when the number of sintering cycles exceeded a certain threshold, suggesting that repeated sintering of clinical restorations should be avoided to minimize the risk of subsequent veneer fracture; when necessary, remanufacturing may be the preferred option.

It is widely accepted that the bonding between zirconia and veneering porcelain primarily relies on a combination of mechanical interlocking and chemical adhesion, with the latter generally exerting a stronger influence. SEM in the present study revealed that the SiLi coating could infiltrate the honeycomb-like structures generated by acid etching, indicating that acid etching substantially enhances mechanical interlocking. Although earlier research offered limited insights into the chemical bonding mechanism between the two materials, more recent studies have demonstrated that elements such as silicon, aluminum, and sodium may diffuse from the veneering porcelain into the zirconia substrate (Kim et al., 2018; Aboushelib et al., 2006).

The mismatch in coefficients of thermal expansion (CTE) is another critical factor influencing bond strength (Guazzato et al., 2004). The CTE of zirconia is slightly higher than that of veneering porcelain, which can generate beneficial compressive stresses at the interface (Role and Significance of Restorative Dental Materials [M], 2012). Monoclinic-phase zirconia has a markedly lower CTE (7.5 × 10−6/K) compared with tetragonal-phase zirconia (10.8 × 10−6/K) (Fischer and Stawarczyk, 2007). Multiple sintering cycles tend to reduce the monoclinic phase fraction and increase the cubic phase content, thereby altering the overall CTE (Alkurt et al., 2016; Guazzato et al., 2005; Kosmač et al., 2008). At lower sintering frequencies, the reduction of monoclinic phase moderately increases the CTE mismatch, which is conducive to strengthening the interfacial bond; however, excessive sintering leads to an overly large mismatch, which in turn compromises adhesion. This mechanism plausibly accounts for the trend observed in the present study, wherein bond strength initially increased and subsequently declined with the number of sintering cycles.

In the fabrication of bilayer ceramic restorations, the hydrophilicity of the zirconia surface is critical to the stability of the veneering porcelain–substrate interface (Ji et al., 2020; Bitencourt et al., 2018), and an increase in surface energy can markedly enhance adhesive performance (Valverde et al., 2013). The WCA is a widely adopted parameter for evaluating hydrophilicity and surface energy (Vechiato-Filho et al., 2017). In the present study, the WCAs of zirconia subjected to various surface treatments, as well as those of veneering porcelain after multiple sintering cycles, were measured. The results revealed that, although the acid etched group exhibited a slightly higher WCA than the untreated group, the difference was not statistically significant. By contrast, both the SiLi–only group and the acid etching + SiLi group demonstrated a significant reduction in WCA, indicating enhanced hydrophilicity and surface energy—findings consistent with the shear bond strength results.

Following multiple sintering cycles, the WCA of the veneering porcelain surface decreased significantly between the first and fifth sintering cycles, but increased again after the seventh cycle. Previous studies have suggested that sintering can disrupt surface microstructures and increase surface roughness, thereby lowering the WCA (Moon et al., 2021). A similar mechanism may account for the trends observed in the present study: repeated high-temperature exposure likely leads to the breakdown of the surface oxide crystalline layer, increasing surface roughness and thus enhancing hydrophilicity in the early sintering stages; however, excessive sintering may alter the surface morphology, resulting in an increased WCA. These findings suggest that multiple sintering cycles can, to some extent, improve the hydrophilicity of veneering porcelain, thereby enhancing the compatibility of restorations with the moist oral environment.

During clinical service, the veneering porcelain coating of a dental restoration remains exposed to the oral environment and is directly involved in masticatory function. Surface hardness serves as a critical parameter for assessing its wear resistance. In the present investigation, the hardness of the veneering porcelain exhibited an initial increase followed by a subsequent decline with successive sintering cycles, attaining its peak value after the third sintering cycle. A reduction in ceramic porosity enhances both density and hardness (Zhong et al., 2011), and previous reports have indicated that repeated sintering can diminish porosity, thereby improving these properties (El-Kheshen and Zawrah, 2003). This phenomenon aligns with the findings of the current study. However, with further increases in the number of sintering cycles, hardness values declined, a trend that may be attributable to the formation of surface microfractures or structural degradation induced by recurrent high-temperature exposure. The literature presents divergent perspectives on this matter: certain researchers have reported a sustained increase in hardness with multiple sintering (Alkurt et al., 2016), whereas others have documented brand-dependent variations (Tang et al., 2015), with some ceramic coatings demonstrating increased hardness and others exhibiting a reduction. The underlying mechanism remains to be fully elucidated.

The structural strength of the zirconia crown core is pivotal to the long-term serviceability of the restoration and is typically assessed by flexural strength, defined as the maximum stress a material can withstand before bending and yielding. The effect of repeated sintering cycles on its three-point flexural strength remains inconclusive, with some studies reporting no significant variation (Subasi et al., 2014; Ozdogan and Tosun, 2023), while others have reached differing conclusions. The present investigation did not examine the influence of sintering frequency; instead, it evaluated the impact of distinct surface treatments on the flexural strength of the zirconia crown core. The findings demonstrated that neither acid etching nor SiLi coating significantly compromised its strength, thereby confirming the stability of its mechanical properties.

Although the proposed surface modification technique significantly enhanced the zirconia–veneering porcelain bond strength, the long-term hydrolytic stability of the modified interface remains an important concern. Currently, data regarding the durability of the SiLi coating after long-term water storage is not available. Future studies should investigate the impact of prolonged water exposure on the integrity of the SiLi coating and the interfacial bond strength to further validate the clinical reliability of the surface treatment over extended periods of time. Due to experimental constraints, this study did not employ atomic force microscopy to characterize the surface roughness of the zirconia and veneering porcelain, nor did it conduct phase composition analysis of zirconia following multiple sintering cycles. Consequently, some mechanistic interpretations remain tentative and require further investigation. Thess limitation will be addressed in future experiments.

 Conclusion

In this study, the zirconia surface was modified via thermal phosphoric acid etching in combination with a SiLi coating, and the effects of multiple sintering cycles on its bond strength with veneering porcelain as well as the mechanical properties of the veneering porcelain surface were investigated. The results demonstrated that thermal phosphoric acid etching combined with SiLi coating markedly enhanced the bond strength between zirconia and veneering porcelain, without adversely affecting the flexural strength of the zirconia core. Repeated low-temperature sintering exhibited a trend of initial increase followed by a subsequent decline in both bond strength and porcelain hardness, with optimal performance observed after the third sintering cycle. Taken together, these findings suggest that, in clinical applications of this surface modification technique, the number of repeated sintering cycles should be limited to no more than three to maintain optimal interfacial bonding and mechanical performance.

 Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

 Author contributions

BC: Writing – review and editing, Writing – original draft. JY: Writing – review and editing, Writing – original draft. BZ: Writing – review and editing. KL: Writing – review and editing. XH: Writing – review and editing.

 Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

 Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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Edited by: Joana Mesquita—Guimarães, University of Aveiro, Portugal

Reviewed by: Ita Junkar, Institut Jožef Stefan (IJS), Slovenia

Mi-Kyung Han, Chonnam National University, Republic of Korea

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