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Int Chin J Dent 2009; 9: 23-30.

Effect of surface abrasion and silica coating on tensile bondstrength of a resin cement to zirconia ceramics
Lei Zhu, DDS,a Toru Nikaido, DDS, PhD,a Shuzo Kitayama, DDS, PhD,a Masaomi Ikeda,
BSc, RDT, PhD,b Richard M. Foxton, BDS, PhD, MFDS RCS (Ed), FHEA,c and Junji
Tagami, DDS, PhDa,da

Cariology and Operative Dentistry, Department of Restorative Science, Graduate School of Medicalband Dental Sciences, Tokyo Medical and Dental University, Faculty of Dentistry, School for DentalcTechnology, Tokyo Medical and Dental University, Division of Conservative Dentistry, King's College
London Dental Institute at Guy's, King's College and St. Thomas' Hospitals, King’s College London,dLondon, UK , and Global Center of Excellence (GCOE) Program; International Research Center for
Molecular Science in Tooth and Bone Diseases at Tokyo Medical and Dental University, Tokyo, Japan
Purpose: To evaluate the effects of different surface treatments on the tensile bond strengths of a resin cement tozirconia ceramics.
Materials and Methods: Zirconia discs (Lava) were fabricated and the surfaces of the specimens were groundwith #600-grit SiC paper. Then the specimens were divided into four groups according to the surface treatment:(1) no pretreatment (control); (2) air-abrasion with 110 µm Al2O3; (3) laboratory tribochemical silica coating(Rocatec) with 110 µm Al2O3 and 30 µm silica modified Al2O3; (4) coating the surface (INT coating) withsilica-based porcelain (Vintage ZR). Each group was subsequently divided into two subgroups assigned to besilanated using a ceramic primer (RelyX Ceramic Primer) or not. The bonding procedure was then performed
using a dual-cure resin cement (RelyX ARC). After 24 h storage in water, the specimens were thermocycled(5-55˚C, 5,000 cycles). The bonding performance was evaluated by the tensile bond strength test. Statisticalanalyses were performed at the level of p=0.05.
Results: The control groups showed the lowest bond strengths. The silanated groups showed significantly
higher bond strengths than the non-silanated groups for each surface condition (p>0.05). The highest bondstrength (27.2±6.2 MPa) was provided by the INT coating group with silanization. However, no significant
difference was obtained between the INT coating and Rocatec groups after silanization (p>0.05).
Conclusion: With silanization, the INT and tribochemical silica coatings were the most effective in improving
the bond strengths of the resin cement to zirconia ceramics. (Int Chin J Dent 2009; 9: 23-30.)
Key Words: INT coating, resin cement, silica coating, tensile bond strength, zirconia.

Introduction
Since zirconia is tough, has high strength, is a metal-free material, and its color is sufficiently white, it is used
today in many dental ceramic systems,1 and also in biomedical applications.2 Due to their high resistance tofracture, full coverage zirconium oxide-based crowns can be cemented conventionally, as recommended bysome manufacturers.3
restoration.

However, the cementation technique is important for the clinical success of a

4-6

Bonding between the tooth substrate and the restoration is advocated for improving the retention, marginal
adaptation and inhibition of secondary caries.4-7 Acid etching and silanization are not expected to improve theadhesion of resin cement to high-strength ceramics, such as alumina and zirconia-based materials, because they
have little or no silica content.8-13
For zirconia ceramics, airborne-particle abrasion is an alternative method for roughening the ceramic
surface.12,14,15 A tribochemical silica coating, such as Rocatec system and Cojet system, is another surface
abrasion technique, which improves bonding to alumina and zirconia surfaces.8,10,16 With the tribochemicalsilica coating, the surface of the restoration is air-abraded with the silica-coated alumina particles.8,10 The
blasting pressure results in silica particles becoming embedded in the ceramic surface, rendering thesilica-modified surface chemically more reactive to resin through a silane coupling agent.

23

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Int Chin J Dent 2009; 9: 23-30.

technique cannot coat and modify the entire surface of a zirconia restoration.17,18
The internal (INT) coating technique has been introduced as a new coating method in a laboratory work.19
With the INT coating, the internal surface of the zirconia restoration can be fully or partially coated by fusing
silica-based ceramics. The goal of the INT coating is to improve the bond strength of resin cement to zirconia
ceramics. Additionally, this technique may optimize the marginal fitness of the restorations.
Therefore, the aim of this study was to evaluate the effects of different surface abrasion and silica coating
techniques on the tensile bond strengths of a resin cement to zirconia ceramics.

Materials and Methods
Tensile bond strength
The materials used in this study are listed in Table 1. Seventy yttria-stabilized tetragonal zirconia discs (Lava,
3M ESPE, St. Paul, MN, USA) were fabricated according to the manufacturer’s instructions. The specimens had
a diameter of 15 mm and a thickness of 2 mm. The surfaces of the specimens were ground with #600-grit SiC
paper under running water and cleaned ultrasonically in distilled water.
Table 1.

Materials used in this study.

Material
Zirconia

Brand name
Lava

Resin cement

RelyX ARC

Silane coupling
agent
Air-abrasion

RelyX Ceramic
PrimerRocatec-Pre
Rocatec-Soft

INT coating

Vintage ZR(Shade B4B)

Manufacturer
3M ESPE, St. Paul,
MN, USA
3M ESPE

Batch no.
282361

Composition(ZrO2, HfO2)<95%, Y2O3<5%, Al2O3<0.25%

FRGM12-08

3M ESPE

6XR

Bis-GMA, TEGDMA, Functional DMA, Silica
filler, Zirconia filler, Peroxide, Initiator,
Amine, Pigment
A silane, Ethanol, Water

3M ESPE, Seefeld,
Germany

246433

Al2O3 particles (110 µm)

257956

Silica-coated aluminum trioxide particles(30 µm)Shofu, Kyoto,Japan

070704

SiO2, Al2O3, K2O, Na2O, CaO, B2O3, Pigment,
Fluorescence

Bis-GMA, Bisphenol-A-diglycidylmethacrylate; TEGDMA, Triethylene glycol dimethacrylate; DMA, Dimethacrylate

They were then randomly assigned to one of thefollowing four groups: (1) no pretreatment (negative
control); (2) air-abrasion with Al2O3 only; (3) laboratory
tribochemical silica coating (Rocatec system, 3M ESPE,
Seefeld, Germany); (4) INT coating. The details of eachsurface treatment are described in Fig. 1. For the Al2O3
sandblasting group, the surfaces of the specimens were
air-abraded with 110 µm Al2O3 (Rocatec-Pre, 3M ESPE)
at 0.28 MPa for 15 s.

For the Rocatec group, thesurfaces were firstly air-abraded with 110 µm Al2O3(Rocatec-Pre) in the same manner as described for the
Al2O3 sandblasting group.
air-abraded

with

30

µm

The surfaces were thensilica-modified

Al2O3(Rocatec-Soft, 3M ESPE) for 13 s at 0.28 MPa. Air-

24

Fig. 1.

Flowchart of sample preparation fortensile bond strength test.

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Int Chin J Dent 2009; 9: 23-30.

abrasion with Rocatec Sand (Rocatec-Pre and Rocatec-Soft) was performed using a laboratory type of air
abrasion device (Rocatec Junior, 3M ESPE). During blasting, the tip of the nozzle was rotated perpendicularly to
the zirconia surface at a distance of 10 mm from the surface.

Fig. 2.

Illustration of procedure for the INT coating.

An illustration of the procedure for the INT coating group is shown in Fig. 2. The surfaces of specimens were
air-abraded with 70 µm Al2O3 (Hi Aluminas, Shofu, Kyoto, Japan) at 0.5 MPa for 5 s and then coated with amedium fusing porcelain (Vintage ZR, shade B4B, Shofu), which is a ceramic veneering material designed
exclusively for use with the zirconia framework. The porcelain powder was stirred in an excessive amount ofwater and immediately painted on the ceramic surfaces and fired at 925˚C for 1 min with a vacuum to make acoating with a thickness of 200 µm. Following this, the specimens were cleaned ultrasonically in distilled waterfor 10 min and air-dried, except the Rocatec group, which was just air-blown strongly according to the
manufacturer’s recommendations. Then they were further divided into two subgroups according to whether theywould be silanized or not. For the silanization groups, two coats of a silane coupling agent (Ceramic Primer, 3M
ESPE) was applied with a fine brush onto the specimen surface, and gently air-blown according to the
manufacturer’s instructions. A piece of polyethylene tape with a circular hole of 4.0 mm in diameter was
positioned on the specimen surface to demarcate the area of bonding.
A dual-cured resin cement (RelyX ARC, 3M ESPE) was then applied to the treated surface with a stainlesssteel rod as a handle for the subsequent tensile bond testing. After the specimens were kept in water and left atroom temperature for 40 min without light curing, they were stored in water at 37˚C for 24 h. Following this,they were subjected to thermal cycling of 5,000 times in water (5-55˚C, dwell time of 30 s each). The tensile
bond strengths were then measured using a universal testing machine (Autograph AGS-J, Shimadzu, Kyoto,
Japan) at a crosshead speed of 1 mm/min.
Failure mode analysis
After tensile bond testing, the failure mode of each specimen was examined under a light microscope(Olympus OCS 912042, Olympus, Tokyo, Japan) at ×40 magnification. The failure modes were classified into
the following five categories: a - adhesive failure between zirconia and resin cement; b - mixed failure, including
adhesive failure between zirconia and resin cement and cohesive failure in resin cement; c - cohesive failure in
resin cement; d - adhesive failure between porcelain and resin cement; e - mixed failure, including adhesive
failure between porcelain and resin cement, cohesive failure in resin cement and cohesive failure in porcelain.
Scanning electron microscopic (SEM) observation
The treated surfaces of each group and typical fracture surfaces after tensile testing were examined using a
SEM (JSM-5310LV, JOEL, Tokyo, Japan) after the specimens were desiccated and gold-sputter coated.
Statistical analysis
Number of the specimens was seven for each group. The data for the tensile bond strengths were analyzed

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Int Chin J Dent 2009; 9: 23-30.

using two-way ANOVA except for the no pretreatment groups. The two factors analyzed were surface treatment
and silanization. Bonferroni and student’s t-test were used as post-hoc test for multiple comparisons. The
failure mode data were analyzed using Fisher’s exact test (Extended). P values less than 0.05 were considered to
be statistically significant for the above-mentioned tests.

Results
Tensile bond strengths
The results of the tensile bond strengths of resin cement to zirconia ceramics after 5,000 thermocycles are
summarized in Table 2. Since the control specimens (no pretreatment group) without silanization debonded
spontaneously during thermal cycling, the bond strength values could not be detected. Therefore the statisticalanalyses were performed for all groups, except for the no pretreatment groups. Two-way ANOVA revealed thatthe tensile bond strengths were influenced by surface treatment (F=38.159, p<0.001) and silanization(F=182.663, p<0.001). There was a significant interaction between surface treatment and silanization (p<0.001).
The no pretreatment groups showed the lowest bond strengths among the groups regardless of silanization.
Silanization significantly improved the bond strengths for each surface condition (p<0.05). With silanization,
the Rocatec and INT coating groups showed significantly higher bond strength than the Al2O3 sandblastinggroup (p<0.05). The highest bond strength (27.2±6.2 MPa) was obtained in the INT coating group withsilanization. However, there were no differences between the INT coating and Rocatec groups when they weresilanized (p>0.05).
Table 2.

Tensile bond strengths of a resin cement to zirconia after 5,000 thermocycles.

Silanization
Without
With

No pretreatmentn. d.
2.9±0.9

Surface treatment
Al2O3 sandblasting
Rocatec system
4.1±1.3a
10.0±2.3bA
8.1±2.2
24.1±4.7B

INT coating
1.9±0.5a
27.2±6.2B

n=7, Mean±standard deviation.
Statistical analysis was performed in all groups, except for the no pretreatment groups.
Within the same row, means with the same superscript letter demonstrate no significant differences (p<0.05).

Failure mode analysis
The failure mode distribution is summarized in Fig. 3. Fisher’s exact test (Extended) revealed a relationshipbetween silanization and failure mode for each surface condition (p<0.05). The Al2O3 sandblasting, Rocatec and
INT coating groups with silanization demonstrated significantly higher percentage of failure modes b, c and e,
respectively (p<0.05). The no pretreatment groups with/without silanization and the groups without silanization
except INT coating mainly showed adhesive failures between zirconia and resin cement, whereas the INT
coating groups mainly showed adhesive failure between porcelain and resin cement. Moreover, the failure
patterns were significantly different among the silanated groups and also among the non silanated groups(p<0.05).
SEM observation
Fig. 4 shows	
  the SEM images of the surface textures after each treatment. Roughened surfaces were observedin the Al2O3 sandblasting and Rocatec groups (Figs. 4B and 4C). For the INT coating (Fig. 4D), a smoothsurface was observed since the surface was not air-abraded. Fig. 5 shows the SEM images of the fractured
surfaces of the zirconia after debonding. Failure mode a was shown in A, B, C, and E. Failure modes b, c, d, and26

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Int Chin J Dent 2009; 9: 23-30.

e were observed in D, F, G, and H, respectively.

Fig. 3.

Failure mode distribution of each group. An asterisk (*) indicates significant difference in failure mode between thegroups with (+) and without (-) silanization in each surface condition (p<0.05).

Fig. 4. Upper left
SEM images of the surface textures after pretreatment(x1,500, A-D).
A, #600-grit SiC paper; B, 110 µm Al2O3 sandblasting;C, tribochemical silica coating with Rocatec system; D,
INT coating.
Roughened surfaces were observed in the Al2O3
sandblasting and Rocatec groups (B and C). For the
INT coating (D), the smooth surface was observed
since the surface was not air-abraded.

Fig. 5. SEM images of the debonded surface of each group (x1,500, A-H).
A, no pretreatment without silanization; B, no pretreatment with silanization; C, 110 µm Al2O3 sandblasting withoutsilanization; D, 110 µm Al2O3 sandblasting with silanization; E, Rocatec system without silanization; F, Rocatec
system with silanization; G, INT coating system without silanization; H, INT coating system with silanization.
Failure mode a was shown in A, B, C, and E. Failure modes b, c, d, and e were observed in D, F, G, and H,
respectively.

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Int Chin J Dent 2009; 9: 23-30.

Discussion
Thermocycling is one of the methods most widely used to test the durability of resin bonds and results in thehighest clinically relevant stress.20,21 It was reported to have a much higher impact on the durability of resinbond strength to zirconia than did water storage at a constant temperature alone.21 In the present study, 5,000
thermocycles were performed to represent an aging of the bond of approximately 6 months in vivo.20
Airborne-particle abrasion with alumina particles is the preferred surface treatment method for high-strength
ceramic materials, such as alumina and zirconia ceramics, which creates high surface energy and promotes
micro-retention.8,9,12,15,17,21 Roughening the substrate surface promotes adhesion, since it allows the polymer(resin composite) to flow into the surface and forms irregularities on the substrate surface.22 The present study
also demonstrated that the Al2O3 sandblasting groups showed higher bond strength than the no pretreatmentgroups regardless of silanization.
For the tribochemical silica coating (Rocatec) groups, the surface of the specimens was air-abraded with 110µm Al2O3 followed by air-abrasion with 30 µm silica modified Al2O3 (Rocatec-Soft), according to the Rocatec
protocol by 3M ESPE. The improved bond strengths found in the present study with the Rocatec groups
compared with the Al2O3 sandblating groups may be due to an increased surface area resulting from theadditional treatment with Rocatec-Soft, irrespective of silanization.23
Silane molecules react with water to form three hydroxy-silyl groups (-Si-OH) from the corresponding
methoxy-silyl groups (-Si-O-CH3).24-28 The silanol groups then react further to form a siloxane (-Si-O-Si-O-)
network with the silica deposited on the silica-based ceramics. The methacryloyl groups of the silane molecules
react with the methacryloyl groups of adhesive resins in a free radical polymerization process. In summary, achemical bond is formed via the silica layer on the surface, silane coupling agent, and resin cement. The
pretreatment with RelyX Ceramic Primer significantly increased the bond strength for each surface condition.
Silane coupling primers have been reported to increase the wettability of the surface, which could explain the
improved bond strengths in the control and alumina-abrasion groups.11 However, no chemical reaction would be
expected for these groups, thus the bond strengths were very low and likely to decrease more when subjected tosevere aging conditions. For the Rocatec group, silanization significantly improved the bond strength. Duringtribochemical silica coating, alumina particles modified with silica are blasted, producing high heat, which,
together with pressure, cause the silica-modified alumina particles to be embedded within the ceramic surface.
This allows for an effective reaction of silane agents with zirconia ceramics.17
Regarding the INT coating group, the bond strength without silanization was very low (1.9 MPa) due to theabsence of a chemical bond between the resin cement and the porcelain coated on the surface of the zirconia. On
the other hand, silanization drastically improved the bond strength in the INT coating group, as was the case with
the Rocatec group, because a chemical bond was created via the silane coupling agent between the resin cement
and the silica-based porcelain coated on the surface of the zirconia. We expected that the INT coating is more
effective than the tribochemical silica coating in terms of coating the surface with silica since the surface ofzirconia can be fully coated with silica-based ceramics in the INT coating, suggesting that a greater amount ofsilica is available for silanization in the INT coating group. However, there was no significant difference in thebond strengths between the Rocatec and the INT coating groups with silanization after 5,000 cycles of thermal
cycling. Since the size and the amount of silica as well as the surface roughness is different between the INT
coating and the Rocatec groups, the bonding durability needs to be further evaluated in severe aging conditions.

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Int Chin J Dent 2009; 9: 23-30.

In the INT coating group, there were no adhesive failures at the interface between zirconia and porcelain,
which indicates that the bonding of porcelain to zirconia exceeded that of resin cement to porcelain. Although
the INT coating and the Rocatec groups showed high bond strengths with silanization, they differed in failuremodes. With silanization, the Rocatec group showed cohesive failure in resin cement, whereas the INT coatinggroup showed mixed failures including cohesive failure in cement/porcelain and adhesive failure between the
cement and the porcelain coated on the surface of zirconia. The bond strength of the INT coating could beenhanced by improving the mechanical properties of the porcelain for coating. For the INT coating, the surfaceof zirconia was coated with a 200 µm thickness of porcelain to standardize the thickness by a dental technician in
the present study. Beuer et al.29 reported that three CAD/CAM systems of zirconia fabrication showed marginal
gaps below 120 µm which were considered clinically acceptable. On the other hand, Reisch et al.30 reported thatthe marginal gaps and internal fitness of zirconia fabricated FPDs varied between 8 µm and 272 µm, andbetween 39 µm and 502 µm, respectively. In clinical situations, the coating should be thin when the gap is small.
On the other hand, after sintering or adjustment with burs, a zirconia frame with a large marginal or internal gap
can be repaired fully or partially with a silica-based ceramic using the INT coating in the laboratory.
It must be considered that in vitro testing is simpler than the in vivo situation. In the present study, thermal
cycling was used as means of aging, however, teeth in the oral environment are continuously subjected todifferent types of stresses that may impair the bonding effectiveness of the cement. Therefore, further studies
should be carried out to evaluate the long-term durability of the resin bond strength to zirconia ceramics. In
addition, the application of the INT coating technique in the laboratory should be improved. Moreover, clinicalevaluations of zirconia restorations are also required to establish reliable application methods.
Acknowledgments
This work was supported by the grant from the Japanese Ministry of Education, Global Center of Excellence (GCOE)
Program; International Research Center for Molecular Science in Tooth and Bone Diseases. The authors gratefully thank Mr.
Norio Ishimatsu at TENSEI systems Co., Ltd for assisting in the technical procedures.

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Correspondence to:
Dr. Lei Zhu
Cariology and Operative Dentistry, Department of Restorative Science,
Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University
1-5-45, Yushima, Bunkyo-ku, Tokyo 113-8549, Japan
Fax: +81-3-5803-0195 E-mail: zhulei7979@hotmail.co.jp
Accepted June 9, 2009.

Copyright ©2009 by the International Chinese Journal of Dentistry.

30