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. Author manuscript; available in PMC: 2016 Apr 1.
Published in final edited form as: Dent Mater. 2015 Feb 14;31(4):435–442. doi: 10.1016/j.dental.2015.01.013

Effects of cementation surface modifications on fracture resistance of zirconia

Ramanathan Srikanth a, Tomaz Kosmac b, Alvaro Della Bona c, Ling Yin d, Yu Zhang a,*
PMCID: PMC4374004  NIHMSID: NIHMS664629  PMID: 25687628

Abstract

Objectives

To examine the effects of glass infiltration (GI) and alumina coating (AC) on the indentation flexural load and four-point bending strength of monolithic zirconia.

Methods

Plate-shaped (12 mm × 12 mm × 1.0 mm or 1.5 mm or 2.0 mm) and bar-shaped (4 mm × 3 mm × 25 mm) monolithic zirconia specimens were fabricated. In addition to monolithic zirconia (group Z), zirconia monoliths were glass-infiltrated or alumina-coated on their tensile surfaces to form groups ZGI and ZAC, respectively. They were also glass-infiltrated on their upper surfaces, and glass-infiltrated or alumina-coated on their lower (tensile) surfaces to make groups ZGI2 and ZAC2, respectively. For comparison, porcelain-veneered zirconia (group PVZ) and monolithic lithium disilicate glass-ceramic (group LiDi) specimens were also fabricated. The plate-shaped specimens were cemented onto a restorative composite base for Hertzian indentation using a tungsten carbide spherical indenter with a radius of 3.2 mm. Critical loads for indentation flexural fracture at the zirconia cementation surface were measured. Strengths of bar-shaped specimens were evaluated in four-point bending.

Results

Glass infiltration on zirconia tensile surfaces increased indentation flexural loads by 32% in Hertzian contact and flexural strength by 24% in four-point bending. Alumina coating showed no significant effect on resistance to flexural damage of zirconia. Monolithic zirconia outperformed porcelain-veneered zirconia and monolithic lithium disilicate glass-ceramics in terms of both indentation flexural load and flexural strength.

Significance

While both alumina coating and glass infiltration can be used to effectively modify the cementation surface of zirconia, glass infiltration can further increase the flexural fracture resistance of zirconia.

Keywords: ceramic restorative materials, surface modification, zirconia, graded zirconia, alumina coating, indentation flexural load, flexural strength

1. Introduction

Yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) has high strength of 800 – 1200 MPa and fracture toughness of 4 – 6 MPa.m1/2 [1, 2]. It serves as an excellent material for all-ceramic restorations. However, zirconia is opaque and does not match the natural color of human teeth [3]. Thus, zirconia restorations need to be veneered with esthetic porcelain to improve their esthetics. These bi-layered structures often cause clinical complications in porcelain-veneered zirconia crowns and fixed partial dental prostheses (FDPs). This is because they are prone to chipping and delamination of the overlay porcelain [4–8] by 25% after 31 months [9], and 13% after 38 months [10] in vivo. To overcome chipping complications in veneered zirconia structures, monolithic zirconia structures with improved esthetics have been developed [11]. This was achieved by infiltrating glass into zirconia surfaces to provide shade options and better translucency [12].

Another limitation of zirconia crowns and FDPs is their insensitivity to the etching-silane bonding technique. To establish adequate zirconia-luting agent bonding strength, surface modifications of zirconia are required [13, 14]. Traditionally, airborne particle abrasion is used to modify the zirconia surface for mechanical retention [1, 15]. This process produces a protective layer of compressive stresses, which may impede fracture [15]. It also introduces microcracks, which may accelerate failure [16]. The formation of a compressively stressed layer in zirconia results from work hardening and volume expansion induced by a tetragonal-monoclinic phase transition [16–18]. The microcrack generation during airborne particle abrasion is attributed to the mechanical impact of abrading particles on zirconia surfaces. Thus, airborne particle abrasion can either reduce [19–21] or increase [22, 23] the flexural strength of zirconia, depending on the type and size of abrading particles, the air pressure applied, and the surface conditions of zirconia. Given the high-cycle fatigue nature of mastication, surface cracks in a zirconia restoration can eventually extend beyond the compressive surface layer, diminishing the fatigue strength of the restoration. Strength reduction is a major cause for failures in ceramic restorations, due to the development of tensile stress-induced radial cracks at the cementation surface [15, 24–26].

To improve the luting cement bonding strength, alumina coating [27, 28] and glass infiltration on zirconia cementation surfaces [12] have been used. The non-invasive nano-structured alumina coating on zirconia surfaces can survive the thermal cycling, significantly improving the resin-bond strength to zirconia ceramics [27]. The glass infiltration into zirconia surfaces has enabled the usage of the standard etching-silane technique to selectively remove the glass phase in the graded glass structure and create a three-dimensional surface morphology, increasing the resin bond strength [12]. However, effects of these surface modifications on the fracture resistance of zirconia are not known.

Accordingly, the present study aims to investigate effects of zirconia surface modifications on its fracture resistance using Herzian indentation and four-point bending techniques. Our research hypotheses are (1) glass infiltration and alumina coating on the cementation surface of zirconia monolith significantly increase its load bearing capacity; and (2) the load bearing capacity of monolithic zirconia is significantly greater than commercial porcelain-veneered zirconia and monolithic lithium disilicate glass-ceramic structures.

2. Materials and Methods

2.1. Specimen preparation

2.1.1. Fabrication of ceramic specimens

Five experimental zirconia groups and commercial ceramic compositions were utilized in this study, which are summarized in Table 1.

Table 1.

Test groups and their abbreviations

Materials Group abbreviations
Monolithic Y-TZP Z
Monolithic Y-TZP with glass infiltration (GI) on the cementation surface ZGI
Monolithic Y-TZP with glass infiltration on the cementation and occlusal surfaces ZGI2
Monolithic Y-TZP with alumina coating (AC) on the cementation surface ZAC
Monolithic Y-TZP with alumina coating on the cementation surface and glass infiltration on the occlusal surface ZAC2
Y-TZP veneered with overlay porcelain* PVZ
Monolithic lithium disilicate glass-ceramic† LiDi
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*

VM9, Vita Zahnfabrik, Bad Sackingen, Germany

†

IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein

Group Z were monolithic Y-TZP specimens. The material was made from 28 nm diameter powders of Y-TZP containing 5.18 wt.% Y2O3 (TZ-3Y-E grade, Tosoh, Tokyo, Japan) for green compacts of 80 mm × 64 mm × 15 mm using cold isostatic press at 200 MPa. The green compacts were cut into blocks of 16 mm × 16 mm × 3 mm using a low speed saw (Isomet Low Speed Saw, Buehler, Lake Bluff, Illinois, USA) with a diamond blade. After cutting, the green blocks were polished with 600 grit silicon carbide grinding paper (CarbiMet™ 2, Buehler, Lake Bluff, Illinois, USA) to remove machining groves. The polished green blocks were then sintered at 1450°C for 2 hours in a box furnace (Lindberg/Blue M, Thermo Scientific Corp., Waltham, Massachusetts, USA) with a heating/cooling rate of 10°C/min. During sintering at 1350°C to 1450°C, the Y-TZP material underwent a volumetric shrinkage of approximately 24%.

Group ZGI were the specimens of monolithic Y-TZP with glass infiltration (GI) on cementation surfaces. The polished Y-TZP green blocks were pre-sintered at 1350°C for 1 hour and cooled to room temperature at a heating/cooling rate of 10°C/min to produce templates for glass infiltration. A uniform, powdered glass slurry was coated on cementation surfaces of the pre-sintered Y-TZP specimens and then dried in air. The glass consisted of 65.5 wt. % SiO2, 11.7 wt. % Al2O3, 10.0 wt.% K2O, 7.3 wt. % Na2O, 3.0 wt.% CaO and 2.5 wt. % Tb4O7 [12]. Glass infiltration and densification of the coated specimens were carried out simultaneously by heat treatment at 1450°C for 2 hours in the box furnace with a heating/cooling rate of 14°C/min.

Group ZGI2 were the specimens of monolithic Y-TZP with glass infiltration on both cementation and occlusal surfaces using the same method for Group ZGI.

Group ZAC were the specimens of monolithic Y-TZP with alumina coating on cementation surfaces. This coating was applied according to the technique described by Kocjan et al. [28]. The sintered zirconia specimens were immersed, with the cementation surface up, into deionized water at 90°C for 30 seconds. Then, AlN powders of particle size of approximately 1.2 µm (Grade C, H.C. Starck, Berlin, Germany) were added to make a diluted solution with 3 wt. % AlN, in which the hydrolysis reaction occurred for 7 minutes. The specimens were then removed from the solution, rinsed with deionized water, dried in air and heat treated at 900°C to convert the precipitated aluminum hydroxide (AlOOH) to transient δ-Al2O3.

Group ZAC2 were the specimens of monolithic Y-TZP with alumina coating on cementation surfaces and glass infiltration on occlusal surfaces using the same glass infiltration and alumina coating described above.

Plate-shaped specimens from groups Z, ZGI, ZGI2, ZAC, and ZAC2 were made to a final dimension of 12 mm × 12 mm × 1.5 mm (n = 10). Additional plates with the same lateral dimension of 12 mm × 12 mm but 1.0 mm and 2.0 mm thicknesses were also made for groups Z and ZGI (n = 10).

Group PVZ were the specimens of monolithic zirconia veneered with porcelain. 18 plate-shaped specimens of monolithic zirconia of 12 mm × 12 mm × 0.5 mm were fabricated by a professional dental laboratory (Marotta Dental Studio, Farmingdale, New York, USA). Then, they were veneered with porcelain (VM9, Vita Zahnfabrik, Bad Sackingen, Germany) of 0.5 mm, 1 mm and 1.5 mm thicknesses. Thus, three porcelain/zirconia bilayer groups with three thicknesses of 1 mm, 1.5 mm, or 2 mm (n = 6) were obtained.

Group LiDi were the CAD/CAM-milled lithium disilicate glass-ceramic specimens. 18 plate-shaped specimens of 12 mm × 12 mm were divided into 3 groups (n = 6) according to their final thicknesses of 1 mm, 1.5 mm, and 2 mm.

Finally, bar-shaped specimens of 3 mm × 4 mm × 25 mm from groups Z, ZGI, ZAC and LiDi were fabricated (n = 10).

2.1.2. Fabrication of ceramic/cement/composite multilayer structures

For the Hertzian indentation test, ceramic specimens were cemented onto composite blocks of 12 mm × 12 mm × 4 mm (Z100, 3M/ESPE Dental Products, St. Paul, Minnesota, USA). Prior to bonding, these blocks were stored in deionized water for 30 days to stabilize the water sorption-induced dimension change [29, 30]. The bonding surfaces of the composite blocks were roughened with 600 grit silicon carbide abrasive papers, then silane-treated (Multlink Automix, Ivoclar Vivadent AG, Schaan, Liechtenstein), and finally coated with a layer of cement before placing the ceramic specimens under a constant load of 750 g. The average cement thickness was controlled to be approximately 0.07 mm for all ceramic systems.

Specimens from groups Z, ZAC, ZAC2 and PVZ were sonically cleaned in 70% ethanol and air-dried prior to cementation. Once removing the excess cement, the cement was light cured (UltraLume 5LED, Ultradent Products, South Jordan, Utah, USA) for 40 s per surface. After curing, specimens were kept in deionized water at 37°C for 7 days for cement hydration [29].

Specimens from groups ZGI and ZGI2, and group LiDi were etched with 4% hydrofluoric acid (Porcelain Etchant, Bisco Inc., Schaumburg, Illinois, USA) for 5 min and 20 s, [31, 32], respectively. Then, they were water-rinsed for 15 s and air-dried before the silane treatment and cement application as described above.

2.2. Mechanical testing

2.2.1. Hertzian indentation test

Figure 1 is a schematic diagram of the Hertzian indentation test, which was designed to simulate a ceramic restoration (ceramic plate) cemented to a compliant tooth dentin support (resin-based composite) under occlusal loading. Each specimen was loaded on a universal testing machine (Instron 5566, Norwood, MA). A spherical tungsten carbide (WC) indenter of 3.18 mm radius was used to deliver the applied load. Each specimen was loaded monotonically at a 1 mm/min rate to avoid slow crack growth. Continuous data points were recorded during the test for load and extension at a time interval of 0.02 s until failure [33]. Failure was defined as an initiation of cementation radial cracking accompanied by a small load drop from the force-displacement curve [15]. All zirconia plates were cut by hand and underwent sintering-induced volume reductions of approximately 24%, inevitably causing some small variations in thickness amongst specimens. Additionally, the critical load is proportional to the square of a plate thickness [15]. Thus, it was necessary to normalize the thickness to a nominal thickness for each specimen group (i.e. 1.0 mm, 1.5 mm, or 2.0 mm) using the following equation:

Pc=tnominal2×Pmeasured/tactual2

where PC = critical indentation flexural load (N), Pmeasured = measured load (N), tnominal = nominal specimen thickness (mm), and tactual = actual specimen thickness (mm).

Figure 1.

Figure 1

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Schematic of experimental setup for Hertzian indentation using a tungsten carbide spherical indenter on a ceramic plate resin cemented to a dental composite. Indenter radius r at load P. Ceramic tensile surface radial cracks (R), resulting from the yield of the composite support.

2.2.2. Four-point flexural strength test

Groups Z, ZGI, ZAC and LiDi were also subject to four-point bending. For groups ZGI and ZAC, the glass-infiltrated and alumina-coated surfaces were loaded in tension. The loading rate was 1 mm/min and the fracture load was measured. The flexural strength, σ, was calculated using the following equation [34]:

σ=3FL/4wb2

where σ = maximum tensile stress (MPa), F = load at failure (N), L = center-to-center distance between the outer support rollers (mm), w = width of the specimen (mm), b = thickness of the specimen (mm)

2.3. Fracture analysis

Fracture surfaces of the specimens tested in four-point bending were carbon coated (Emitech K650 sputter coater, Quorum Technologies, UK), and examined under a scanning electron microscope (SEM) (S3500N, Hitachi, Tokyo, Japan). Fractographic principles were used to determine the fracture origins. SEM images were also taken for microstructural analyses.

2.4. Statistical analyses

The differences between each material system and among various specimen thicknesses were statistically analyzed using a nonparametric one-sample Kolmogorov-Smirnov test (p = 0.05).

3. Results

3.1. Surface modifications

Figure 2 shows the SEM micrographs of the surface features of the alumina-coated (ZAC) and glass-infiltrated Y-TZP (ZGI). Figure 2a shows the surface (top) and the cross-section (bottom) views of the alumina coating layer on zirconia. The surface view in this figure was produced by masking the zirconia surface prior to the alumina coating process. The chemically deposited coating presents uniformly distributed plate-shaped alumina nanocrystals across the zirconia surface. The nano alumina crystals emanated from the zirconia surface and grew outward in the form of thin plates. The thickness of this porous alumina coating is approximately 240 nm.

Figure 2.

Figure 2

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Ceramic microstructures. (a) Nano-structured alumina coating on the monolithic zirconia substrate: surface view (top) and section view (bottom). The surface view in 2a was produced by masking the zirconia surface prior to the alumina coating process. (b) Glass infiltrated zirconia: surface view (top) and section view (bottom), showing the outer surface glass layer and the graded glass zirconia layer. The surface view in 2b was created by dipping a portion of the glass-infiltrated zirconia plate in hydrofluoric acid (10%) for 1 min.

Figure 2b shows the surface (top) and the cross-section (bottom) views of the glass-infiltrated layer on zirconia substrate. The surface view in this figure was created to reveal the infiltrated glass structure in the zirconia surface by dipping a portion of the glass-infiltrated zirconia plate in hydrofluoric acid (10%) for 1 min. The amorphous glass layer, approximately 10 µm thick, is the outer surface followed by a graded glass-zirconia layer under which lies the monolithic zirconia substrate. The gradation of glass infiltration is approximately 100 µm. At the interface, the diameters of individual Y-TZP particles are less than 0.5 µm.

3.2. Load bearing capacity determined by Hertzian indentation

Figure 3a shows the critical indentation flexural loads of each ceramic system with 1.5 mm nominal thickness cemented to a dental composite base and loaded on the occlusal surface with a WC indenter. The statistical analysis showed a significant difference (p < 0.001) in the critical fracture loads among groups with and without glass infiltration at the ceramic cementation surface. The failure loads for groups with glass infiltration were 32% higher than the other groups, indicating that glass infiltration at the tensile surface increased the resistance to flexural fracture. However, no significant difference was observed between groups ZGI and ZGI2 (p > 0.05), suggesting that glass infiltration at the compressive surface had little influence on the flexural damage resistance of ceramic restorations. Furthermore, the critical fracture loads for groups ZAC, ZAC2 and Z were also found to be insignificantly different (p > 0.05), indicating that alumina coating did not alter the flexural damage resistance of these systems. In addition, comparing to groups PVZ and LiDi, monolithic zirconia materials (Z) showed a significantly higher load bearing capacity than the bi-layered zirconia structures (PVZ) and monolithic lithium disilicate glass-ceramic (LiDi) system (p < 0.001).

Figure 3.

Figure 3

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Bar chart showing mean loads at failures in Hertzian indentation test. The error bars represent standard errors for each system. (a) Bar chart compares critical indentation flexural loads for ceramic plates with a nominal thickness of 1.5 mm resin-cemented to dental composites. Ceramic specimens include glass-infiltrated zirconia (ZGI, ZGI2), alumina coated zirconia (ZAC, ZAC2), monolithic zirconia (Z), porcelain-veneered zirconia (PVZ), and monolithic lithium disilicate (LiDi). (b) Bar chart compares critical indentation flexural loads for ceramic plates of 1 mm, 1.5 mm and 2.0 mm in thickness, respectively, resin cemented to composites. Ceramic specimens include glass-infiltrated zirconia (ZGI), monolithic zirconia (Z), porcelain-veneered zirconia (PVZ), and monolithic lithium disilicate (LiDi).

Figure 3b shows the indentation flexural loads for groups ZGI2, Z, PVZ and LiDi of thicknesses of 1 mm, 1.5 mm and 2 mm, respectively. ZGI2 of any thickness demonstrated superior fracture resistances, while PVZ and LiDi presented the lowest fracture loads among all groups. Comparing the fracture loads for various thicknesses, the groups with 1.5 mm thickness exhibited the fracture loads nearly two times higher than the groups with 1 mm thickness. The 2 mm groups exhibited higher fracture loads than the 1.5 mm groups, but the increases of the fracture load were not as great as those between 1 mm and 1.5 mm thickness groups.

3.3. Flexural strength measured by four-point bending

Since the critical indentation flexural loads in Hertzian indentations were not significantly different between groups ZGI and ZGI2, and between groups ZAC and ZAC2, only groups ZGI and ZAC were selected for four-point bending testing in comparison with groups Z and LiDi. Figure 4 demonstrates the flexural strength values for groups ZGI, ZAC, Z and LiDi from the four-point bending. Statistically, there were significant differences in strength among groups ZGI, Z/ZAC, and LiDi (p < 0.001). However, the strength difference between groups Z and ZAC was insignificant (p > 0.05). Similar to the Hertzian indentation results in Figure 3a, group ZGI had the highest flexural strength, approximately 32% higher than groups Z and ZAC, and over 2.5 times stronger than LiDi.

Figure 4.

Figure 4

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Bar chart showing mean flexural strength values in four-point bending test. The error bars represent standard errors. Ceramic specimens include glass-infiltrated zirconia (ZGI), alumina coated zirconia (ZAC), monolithic zirconia (Z), and monolithic lithium disilicate (LiDi).

3.4. Fractographic failure analysis

The fractured four-point bending bars from groups Z, ZGI, ZAC and LiDi were analysed using SEM to determine the facture origins. It was found that specimens from groups Z, ZAC, and LiDi had similar facture patterns in which failures originated from their surface flaws and defects, as shown in Figures 5a and 5b. In comparison, specimens from group ZGI had failures initiating from internal flaws, typically located 50–100 µm beneath the tensile surface, as shown in Figure 6.

Figure 5.

Figure 5

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SEM micrographs showing fracture origins, pointed out by the arrows, in typical failures under four-point bending of alumina coated zirconia (ZAC) and monolithic zirconia (Z) groups. Note: fracture originated from the surface. Images were taken under secondary electron mode with an operating voltage of 15 kV. (a) Fracture surfaces of an alumina coated zirconia (ZAC) bar. (b) A close up of the fracture origin seen in 5a in which the mirror and mist regions are clearly observed.

Figure 6.

Figure 6

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SEM micrographs indicating an internal flaw from which the fracture originated in a glass-infiltrated zirconia (ZGI) specimen. Image was taken under secondary electron mode with an operating voltage of 15kV.

4. Discussion

This study demonstrates that glass infiltration at cementation (tensile) surfaces can improve load bearing capacities of ceramic systems. Group ZGI had a mean indentation flexural load of 3463 ± 172 N and flexural strength of 972 ± 45 MPa which were much higher than group Z of 2624 ± 115 N indentation flexural load and 781 ± 53 MPa flexural strength, respectively. The increase in load bearing capacity by glass infiltration can be attributed to the change in flexural tensile stress distribution in the graded zirconia structure under bending, which was detailed by Dorthé and Zhang [33], and Zhang and Ma [35]. This change in stress distribution is a result of the transference of the maximum tensile stress into the ceramic core via the graded lower modulus surface which can play a cushioning effect. This phenomenon was explained by the composite beam theory [35, 36] and the finite element analysis [37]. The infiltrated glass penetrated into the surface flaws induced by CAD-CAM milling or other handling processes, and enabled the shifting maximum tensile stresses from ceramic surfaces to interior structures. Thus, the graded zirconia structure became more tolerant to surface flaws [37]. As a result, glass-infiltrated zirconia exhibited a better resistance to fracture, achieving a substantially higher load capacity than the non-infiltrated zirconia.

In addition, the existence of a glass phase in the graded zirconia layer offers great potential for adhesive bonding using etching-silane techniques to replace the current airborne particle abrasion procedure for cementation of homogeneous zirconia. The glass phase in the graded layer can be selectively removed by hydrofluoric acid to create a three-dimensional surface morphology. A strong resin bond can be established by the application of a silane coupling agent followed by a resin-based adhesive system. Other non-air-abrasion approaches to obtain chemical bond to zirconia using silane plus resin adhesives have been reported, including low fusion glass coating [38], silica-based chemical coating [39], and vapor phase deposition [40]. However, the stability of the interfacial bond among zirconia, coating, and cement and the influence of the coating on the fitting of the restoration are still questionable.

The higher critical indentation flexural load of group Z than the commercial porcelain-veneered zirconia structure in Figure 3a suggests that monolithic Y-TZP could meet the structural demand for posterior applications. One concern is that Y-TZP has much higher hardness and elastic modulus compared with those of natural enamel, which could cause excessive wear of the opposing dentition. However, it has been demonstrated that polished zirconia surface causes less enamel wear than veneered zirconia or glazed zirconia [41, 42]. Another concern is that zirconia has an unnaturally white opaque appearance, which compromises the esthetic outcome of monolithic restorations. To improve esthetic demands of zirconia restorations, the presence of a glass layer on the occlusal surface is needed. This can be achieved by glass infiltration into the occlusal surface of zirconia structures. Although our results indicate that glass infiltration on the occlusal surface does not enhance the flexural strength significantly, it can give the desired esthetics [43]. The esthetic glass layer also has the superior resistance to contact fatigue compared to commercial porcelain-veneered zirconia [43]. Therefore, in clinical practice, glass-infiltration is recommended to both occlusal and cementation surfaces of zirconia restorations for improved damage resistance and esthetics.

In short, glass infiltrated zirconia has merged as a suitable material for monolithic restorations than homogeneous Y-TZP because of its increased fracture resistance, absence of airborne particle abrasion damage, coverage of surface flaws, and assistance to adhesive cementation. Additionally, glass infiltration suggested by Zhang and Ma [35] can be used not only for zirconia but also for other alternatives such as alumina [33, 44, 45] and alumina-zirconia composites [45–47].

This study also indicates that alumina coating on zirconia did not increase the load bearing capacity of zirconia. This is supported by the evidence of its similar indentation flexural load values 2536 ± 71 N relative to monolithic zirconia 2624 ± 115 N in Hertzian indentation of ceramic plates with 1.5 mm thickness cemented to composite bases in Figure 3a. The four-point bending results also show the similar flexural strength values of 770 ± 43 MPa for alumina-coated zirconia with 781 ± 53 MPa for monolithic zirconia (Figure 4). In addition, group Z and group ZAC had similar facture patterns with failures originating from surface flaws and defects (Figure 5). This signifies that the maximum tensile stresses were concentrating at the ceramic tensile surfaces for both coated (ZAC) and non-coated (Z) structures.

The non-invasive alumina coating process relies on the application of nano-structured alumina crystals with a high surface area and good wetting ability [27]. Although alumina itself is not suitable for silanation and the subsequent adhesive bonding, an effective micro-mechanical interlocking between resin cement and alumina coating can be achieved (Figure 2a). This unique coating structure can increase the resin-bond strength in zirconia ceramics by a factor of 2–4, and is stable for thermal cycling without reducing the bond strength [27]. In addition, this 240 nm uniform alumina layer on zirconia surfaces does not interfere with the fitting of restorations. However, a stronger cementation bond alone is not enough to increase the load bearing capacity of zirconia.

5. Conclusions

We have conducted Hertzian indentation and four-point bending testing of glass-infiltrated and alumina-coated zirconia structures to examine the surface modification effect on their flexure loading and strength capacities. We have found that glass infiltration can significantly improve the flexural strength of zirconia by redistributing the flexural tensile stress through a graded glass-zirconia surface layer; alumina coating has no significant effect on the flexural strength of zirconia. Our study also indicates that monolithic zirconia mechanically performed better under in vitro conditions relative to its prevalent commercial alternatives, i.e., porcelain-veneered zirconia and lithium disilicate glass-ceramics. This work provides scientific and practical insights into restorative dentistry on how to improve flexural strength capacities, as well as esthetics by surface modifications of zirconia restorative structures.

Highlights.

  • Alumina coating and glass infiltration can be used to effectively modify the intaglio (cementation) surface of zirconia-based restorations.

  • Glass infiltration significantly improves the flexural strength of zirconia.

  • Alumina coating has no substantial effect on the load bearing capacity of zirconia.

  • Monolithic zirconia mechanically outperforms its porcelain-veneered zirconia and lithium disilicate glass-ceramic counterparts.

Acknowledgements

The authors wish to thank Dr. M. Kaizer for valuable discussions. Funding was provided by the United States National Institute of Dental and Craniofacial Research (Grant 2R01 DE017925) and the National Science Foundation (Grant CMMI-0758530).

Footnotes

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Conflict of interest

All authors declare no conflict of interest.

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