A critical evaluation of bond strength tests for the assessment of bonding to Y-TZP

Abstract

Objectives

To compare three different designs for measuring the bond strength between Y-TZP ceramic and a composite material, before and after ceramic surface treatment, evaluating the influence of the size of the adhesive interface for each design.

Methods

‘Macro’ tensile, microtensile, ‘macro’ shear, microshear, ‘macro’ push-out, and micropush-out tests were carried out. Two Y-TZP surface treatments were evaluated: silanization (sil) and tribochemical silica coating (30 μm silica-modified Al 2 O 3 particles + silanization) (TBS). Failure mode analysis of tested samples was also performed.

Results

Both the surface treatment and the size of the bonded interface significantly affected the results ( p = 0.00). Regardless of the type of surface treatment, the microtensile and microshear tests had higher values than their equivalent “macro” tests. However, the push-out test showed the highest values for the “macro” test. The tensile tests showed the greatest variability in results. The tribochemical silica coating method significantly increased bond strength for all tests.

Significance

Different test designs can change the outcome for Y-TZP/cement interfaces, in terms of mean values and reliability (variability). The ‘micro’ tests expressed higher bond strengths than their equivalent ‘macro’ tests, with the exception of the push-out test (macro > micro).

Introduction

Y-TZP ceramics ( yttrium-stabilized tetragonal zirconia polycrystals ) have been used as an alternative to other dental ceramics due to their relatively high fracture toughness values (7.0–9.0 MPa m 1/2 ) , Weibull modulus ( m = 18.4) and fracture strength (900–1200 MPa) . When compared to alumina-based ceramics, the strength of Y-TZP is twice as high ; therefore, this material has been indicated as the main alternative to replace metal in infrastructures for fixed partial dentures .

Despite the excellent mechanical properties, Y-TZP has the disadvantage of presenting weak adhesion to resin cements due to its inertness to acid etching and low chemical reactivity . In order to solve this problem, various techniques for depositing silica on the bonding surface have been suggested, such as blasting alumina particles coated with silicon oxide, followed by application of silane coupling agents (silicatization) . In this type of sandblasting, the impact of the particles on the surface of the ceramic material results in the deposition of a thin silica film. While creating micro-retentions, the surface is also chemically altered, making it more favorable to react with a silane agent .

The development of new techniques for the surface treatment of Y-TZP to improve adhesion to resin cements has resulted in several studies that measured bond strength between these substrates. However, these measurements are difficult to obtain, as the bond strength between Y-TZP and resin cements is relatively low, especially when the ceramic surface is not chemically modified .

The validity of using in vitro bond strength tests to predict the clinical performance of dental adhesives is questionable, since it is difficult to establish a correlation between laboratory data and the clinical performance of the tested adhesives . The extensive range of data for a single adhesive system in different publications makes this type of prediction very dubious . However, strong correlations (0.81) have been found between clinical follow-up data (5 years) of class V restorations and in vitro bond strength data after aging . Furthermore, these tests have been of great value for identifying the effect of variables related to the substrate , as well as to preliminarily evaluate the adhesive potential of new products .

Recent studies have pointed out several problems related to bond strength tests that use relatively large bonding areas, also called “macro-tests” (bonding area varying from 7 to 28 mm 2 ) . One of the most important problems was demonstrated using Finite Element Analysis and was related to the heterogeneity of stress distribution along the adhesive interface, which led to a high rate of cohesive failures in macro-tensile specimens . The popularity of macro-tests can be explained by the fact that they are easy to perform and require minimal equipment and preparation of the specimens .

The need for new testing methods to overcome the limitations found in macro-tests led to methodologies that use specimens with reduced bonding areas (below 2 mm 2 ), namely micro-tensile and micro-shear methods . These tests show a higher percentage of adhesive failures, lower coefficients of variation, offer the possibility of evaluating different regions in the same specimen and also to assess adhesion to dentin at different depths of a tooth cavity . After the introduction of micro-tests, the relationship between the bonding area and bond strength started to receive more attention, since the strength values increased with the decrease in the adhesive area in a logarithmic fashion . According to Griffith , the effect of the area on bond strength is related to the flaw population found at the interface. The larger the bonding area, the higher the probability of finding a strength-limiting flaw; thereby providing a lower bond strength for the specimen.

Another bond strength test widely used in many fields is the push-out test. In this method, the bonding area varies as a function of specimen thickness, and the behavior of the adhesive interfaces can be assessed using shear stresses . The push-out test consists of using compressive forces to push a material out of a ring made of another material. This test provides a decrease in bond strength variation when compared to the microtensile test in root dentin and has been proven to be a more accurate and reliable technique for measuring the bond strength of fiber posts to the root dentin when compared to the microtensile test . In Dentistry, the push-out test has been widely applied to assess the bond strength of intracanal retainers to root dentin . However, no studies in the literature have applied push-out or micropush-out tests to evaluate the bond strength of resin cements to Y-TZP.

Many studies can be found in the literature that compare different bond strength methodologies using dentin as the main substrate . However, to the knowledge of the current authors, no studies have compared these methodologies using Y-TZP as the main substrate. In addition to the fact that this ceramic is resistant to etching because of its highly crystalline microstructure , zirconia has a high elastic modulus, which has been proven to significantly affect bond strength . Understanding how the bond strength between Y-TZP and resin cements is affected by the test design is important in determining the most appropriate test for these materials. Additionally, knowing the effect of test design on bond strength may help to compare bond strength results from studies that use different methodologies.

The aims of this present study were (1) to evaluate the influence of the size of the adhesive interface area on the bond strength between Y-TZP and resin composite, using three different bond strength test designs, (2) to verify the effect of two Y-TZP surface conditioning methods (silanization versus tribosilicatization) on the results obtained from the different tests, and (3) to compare the failure modes as a function of test design/surface area and surface treatment. The tested hypotheses were that (1) “micro” tests would result in higher bond strength values than the corresponding “macro” tests, (2) Y-TZP surface treatment would improve adhesion, regardless of the test design used, and (3) the test design/surface area would affect the failure mode of the bond strength specimens.

Materials and methods

Experiment

Blocks ( N = 360) of yttrium-stabilized tetragonal zirconia polycrystals (Y-TZP) (In Ceram YZ Cubes, VITA) were made according to the parameters of each test, with the bonding side polished with Sof-Lex (3 M/ESPE) sandpaper discs of 400, 600, and 1200 grit (Norton Abrasives ® ). After ultrasonic cleaning in alcohol (96° for 10 min), the blocks were sintered (1530° C for 120 min) and divided into 12 groups ( Table 1 , n = 30), based on the adhesion test to be used and the surface treatment.

Table 1
Study design.
N Mechanical test Y-TZP surface conditioning method ( n = 30)
360 Macro-tensile Silanization a (Sil)
Tribochemical silica coating b (SC)
Micro-tensile Silanization
Tribochemical silica coating
Macro-shear Silanization
Tribochemical silica coating
Micro-shear Silanization
Tribochemical silica coating
Macro-push out Silanization
Tribochemical silica coating
Micro-push out Silanization
Tribochemical silica coating

a Application of a silane coupling agent (3-methacryloxypropyltrimethoxysilane) (ESPE-Sil, 3 M ESPE), waiting 5 min.

b Tribochemical sílica coating: air-abrasion with silica coated aluminum oxide particles (30 μm) (Cojet-Sand, 3 M/ESPE): distance of 10 mm and perpendicular to the Y-TZP surface for 10 s, pressure = 2.8 bar, followed by the silanization (ESPE-Sil, 3 M ESPE), waiting 5 min.

Table 2 presents the general composition and batch number of materials used in this study.

Table 2
General composition and batch number of used materials.
Material Composition a Batch number
VITA In Ceram YZ CUBES for CEREC, VITA Zahnfabrik, Bad Säckingen, Germany Zirconium dioxide (ZrO 2 ), yttrium oxide (Y 2 O 3 ) 5%, hafnium oxide (HfO 2 ) < 3%, aluminum oxide (Al 2 O 3 ) and silicon dioxide (SiO2) <1% (weight percentage) 30030
Opallis, FGM, Joinville, Brazil Bis-GMA, Bis-EMA, TEGDMA, Diurethandimethacrylat, silanized ceramic, silanized silicon dioxide, camphorquinone, ethil 4-dimetal aminobenzoate 260111
RelyX U100, 3 M/ESPE, Seefeld, Germany Methacrylate monomers containing phosphoric acid groups, Silanated fillers, Methacrylate monomers, Alkaline (basic) fillers 397924
Cojet-Sand, 3 M/ESPE Dental Products, St. Paul, U.S.A. Silicatized sand (particle size 30 μm) 418152
ESPE-Sil, 3 M ESPE Dental Products, St. Paul, U.S.A. Silane, ethanol 0132

a Indicated by manufacturer.

Microtensile test

Composite resin blocks (Opallis, FGM, Joinville, Brazil) were prepared with the same dimensions of the ceramic blocks (5.5 mm × 5.0 mm × 5.0 mm), and were cemented to the ceramic blocks using a resin cement (RelyX U100, 3 M/ESPE, Seefeld, Germany). Thirty zirconia blocks (half of the specimens) were submitted to the appropriate surface treatment prior to cementation, as described in Table 1 . The resin/cement/ceramic assembly was embedded in self-cured colorless acrylic resin (Vipi Flash ® , Barueri, Brazil).

Each block was sliced in a cutting machine (IsoMet 1000, Buehler) with the interfacial surface positioned perpendicularly to the diamond disk (ref. 15LC, Buehler). After three to four slices, the set was rotated by 90°, a polyvinyl siloxane impression material (Elite HD+, Light Body, Zhermack, Badia Polesine, Italy) was applied inside the slices (to facilitate sectioning), and 3–4 additional slices were obtained, totaling approximately 9–12 specimens per set (adhesive interface of approximately 1 mm 2 ; the sides of each specimen at the interfacial zone were measured with a digital caliper before testing).

Both tips of the specimen were fixed in a tensile test device using cyanoacrylate adhesive gel (Superbond, Loctite, Barueri, Brazil), maintaining unbound the adhesive interface. The device was then attached to a universal testing machine (DL 2000, Emic, São Jose dos Pinhais, Brazil), in which a tensile load was applied parallel to the long axis of the specimen until fracture (speed of 1 mm/min).

Tensile test

The Y-TZP blocks (5.5 mm × 5.0 mm × 3.0 mm) were embedded in acrylic resin and half had the exposed surface treated as described above ( n = 30). Trunk-conical samples of the composite were prepared from a split teflon matrix ( h = 3 mm, bottom = 3.2 mm and top = 4 mm). The bottom surface ( = 3.2 mm) of the composite cone was cemented to the treated Y-TZP.

For the tensile test, the acrylic resin base of the set was fixed to the universal testing machine, and the cemented cone composite was fixed in a traction holder, which had the tapered shape equal to the composite cone. The upper holder was connected to the universal testing machine (Emic DL 2000) and a tensile load was applied until the specimen fractured (speed of 1 mm/min). The adhesion area ( A ) was calculated according to A = π * r 2 , where π = 3.14 and r = 1.6 mm (bonding area = 8.04 mm 2 ).

Microshear test

The Y-TZP blocks (5.5 mm × 5 mm × 3 mm) were embedded in acrylic resin, keeping the bonding surface free, and half of the blocks were treated as described above. The composite samples were prepared using a split teflon template ( = 0.85, h = 3 mm). For the luting procedure, the matrix was positioned and fixed onto the ceramic surface to have the composite sample bonded parallel to the Y-TZP surface. The resulting adhesive interface was approximately 0.57 mm 2 ( A = π * r 2 , where π = 3.14, r = 0.425 mm).

For the microshear test, the specimens were placed on a universal testing machine (Emic DL 2000) and a load was applied (1 mm/min) perpendicularly to the adhesive interface using a metal wire ( Wire Loop ) until failure.

Shear test

This procedure was similar to that described for the microshear test; however, the matrix for the composite sample preparation had the dimensions of = 3.25 mm, h = 3 mm. The adhesive area under stress was 8.30 mm 2 ( A = π * r 2 , where π = 3.14, r = 1.625 mm).

Micropush-out test

A trunk conical-shaped hole was made in the Y-TZP blocks before sintering using a diamond drill (end = 1.92 mm; base = 2.43 mm, and h = 4 mm; # 3131, KG Sorensen, Barueri, Brazil). The blocks (5.5 mm × 5.0 mm × 1.0 mm) were used as templates for the preparation of the composite trunk conical samples, which were later cemented to their respective Y-TZP block. Half of the ceramic blocks were previously treated as described above.

For the micropush-out test, each specimen was placed over a metallic device with a central aperture ( = 3 mm), and the load was applied from the small to the large diameter of the composite resin specimen using a metallic cylinder ( = 0.85 mm) until fracture.

The test was carried out on a universal testing machine (Emic DL 2000) with speed of 1 mm/min. To calculate the adhesive interface, the following formula for the lateral area of a trunk-conical solid was used, as described by Valandro et al. : A = π * g *( R 1 + R 2 ), where π is 3.14, R 1 and R 2 are the radii of the smaller and larger bases, respectively, and g is the slant height – g 2 = h 2 + ( R 2 R 1 ) 2 ; where h is the specimen height, measured with a digital caliper.

Push-out test

The procedures for this test were similar to those described for the micropush-out test, although the blocks of Y-TZP had dimensions of 5.5 mm × 5 mm × 4 mm. Since the blocks were thicker, the adhesive interface was increased.

Failure mode analysis

All specimens were analyzed under stereomicroscopy (Discovery V20, Carl-Zeiss, Gottingen, Germany) with 10–60× magnification to identify the type of failure. The failures were classified as: adhesive (fracture in the adhesive interface, with no residue of cement bonded to zirconia), cohesive (fracture into the cement or the resin composite bulk) or mixed (adhesive and cohesive failures).

Data analysis

The bond strength values (in MPa) for each specimen were calculated by dividing the force to failure ( F , in N ) by the bonding area ( A , in mm 2 ) ( R = F / A ). For the micro tensile test, the means for the samples (sticks) from each block (30 blocks per group) were calculated. Specimens showing cohesive failures in the resin cement or in the resin composite were excluded from the statistical analysis. Bond strength data from the “macro” and “micro” versions, for each mechanical test method, were compared, taking into account each type of Y-TZP surface treatment. The results obtained from the Push-out and Micropush-out tests were submitted to Student’s t -test, and the other methodologies were compared using the Mann Whitney’s test ( α ≤ 0.05).

Additionally, the Weibull analysis was performed to determine the Weibull modulus ( m ) and characteristic strength ( σ 0 ) for each condition. The Weibull modulus represents the variation of bond strength data and expresses the size distribution of the flaw population in a structure , while the characteristic strength indicates the strength value at which 63.2% of the specimens survive.

Materials and methods

Experiment

Blocks ( N = 360) of yttrium-stabilized tetragonal zirconia polycrystals (Y-TZP) (In Ceram YZ Cubes, VITA) were made according to the parameters of each test, with the bonding side polished with Sof-Lex (3 M/ESPE) sandpaper discs of 400, 600, and 1200 grit (Norton Abrasives ® ). After ultrasonic cleaning in alcohol (96° for 10 min), the blocks were sintered (1530° C for 120 min) and divided into 12 groups ( Table 1 , n = 30), based on the adhesion test to be used and the surface treatment.

Table 1
Study design.
N Mechanical test Y-TZP surface conditioning method ( n = 30)
360 Macro-tensile Silanization a (Sil)
Tribochemical silica coating b (SC)
Micro-tensile Silanization
Tribochemical silica coating
Macro-shear Silanization
Tribochemical silica coating
Micro-shear Silanization
Tribochemical silica coating
Macro-push out Silanization
Tribochemical silica coating
Micro-push out Silanization
Tribochemical silica coating

a Application of a silane coupling agent (3-methacryloxypropyltrimethoxysilane) (ESPE-Sil, 3 M ESPE), waiting 5 min.

b Tribochemical sílica coating: air-abrasion with silica coated aluminum oxide particles (30 μm) (Cojet-Sand, 3 M/ESPE): distance of 10 mm and perpendicular to the Y-TZP surface for 10 s, pressure = 2.8 bar, followed by the silanization (ESPE-Sil, 3 M ESPE), waiting 5 min.

Table 2 presents the general composition and batch number of materials used in this study.

Table 2
General composition and batch number of used materials.
Material Composition a Batch number
VITA In Ceram YZ CUBES for CEREC, VITA Zahnfabrik, Bad Säckingen, Germany Zirconium dioxide (ZrO 2 ), yttrium oxide (Y 2 O 3 ) 5%, hafnium oxide (HfO 2 ) < 3%, aluminum oxide (Al 2 O 3 ) and silicon dioxide (SiO2) <1% (weight percentage) 30030
Opallis, FGM, Joinville, Brazil Bis-GMA, Bis-EMA, TEGDMA, Diurethandimethacrylat, silanized ceramic, silanized silicon dioxide, camphorquinone, ethil 4-dimetal aminobenzoate 260111
RelyX U100, 3 M/ESPE, Seefeld, Germany Methacrylate monomers containing phosphoric acid groups, Silanated fillers, Methacrylate monomers, Alkaline (basic) fillers 397924
Cojet-Sand, 3 M/ESPE Dental Products, St. Paul, U.S.A. Silicatized sand (particle size 30 μm) 418152
ESPE-Sil, 3 M ESPE Dental Products, St. Paul, U.S.A. Silane, ethanol 0132

a Indicated by manufacturer.

Microtensile test

Composite resin blocks (Opallis, FGM, Joinville, Brazil) were prepared with the same dimensions of the ceramic blocks (5.5 mm × 5.0 mm × 5.0 mm), and were cemented to the ceramic blocks using a resin cement (RelyX U100, 3 M/ESPE, Seefeld, Germany). Thirty zirconia blocks (half of the specimens) were submitted to the appropriate surface treatment prior to cementation, as described in Table 1 . The resin/cement/ceramic assembly was embedded in self-cured colorless acrylic resin (Vipi Flash ® , Barueri, Brazil).

Each block was sliced in a cutting machine (IsoMet 1000, Buehler) with the interfacial surface positioned perpendicularly to the diamond disk (ref. 15LC, Buehler). After three to four slices, the set was rotated by 90°, a polyvinyl siloxane impression material (Elite HD+, Light Body, Zhermack, Badia Polesine, Italy) was applied inside the slices (to facilitate sectioning), and 3–4 additional slices were obtained, totaling approximately 9–12 specimens per set (adhesive interface of approximately 1 mm 2 ; the sides of each specimen at the interfacial zone were measured with a digital caliper before testing).

Both tips of the specimen were fixed in a tensile test device using cyanoacrylate adhesive gel (Superbond, Loctite, Barueri, Brazil), maintaining unbound the adhesive interface. The device was then attached to a universal testing machine (DL 2000, Emic, São Jose dos Pinhais, Brazil), in which a tensile load was applied parallel to the long axis of the specimen until fracture (speed of 1 mm/min).

Tensile test

The Y-TZP blocks (5.5 mm × 5.0 mm × 3.0 mm) were embedded in acrylic resin and half had the exposed surface treated as described above ( n = 30). Trunk-conical samples of the composite were prepared from a split teflon matrix ( h = 3 mm, bottom = 3.2 mm and top = 4 mm). The bottom surface ( = 3.2 mm) of the composite cone was cemented to the treated Y-TZP.

For the tensile test, the acrylic resin base of the set was fixed to the universal testing machine, and the cemented cone composite was fixed in a traction holder, which had the tapered shape equal to the composite cone. The upper holder was connected to the universal testing machine (Emic DL 2000) and a tensile load was applied until the specimen fractured (speed of 1 mm/min). The adhesion area ( A ) was calculated according to A = π * r 2 , where π = 3.14 and r = 1.6 mm (bonding area = 8.04 mm 2 ).

Microshear test

The Y-TZP blocks (5.5 mm × 5 mm × 3 mm) were embedded in acrylic resin, keeping the bonding surface free, and half of the blocks were treated as described above. The composite samples were prepared using a split teflon template ( = 0.85, h = 3 mm). For the luting procedure, the matrix was positioned and fixed onto the ceramic surface to have the composite sample bonded parallel to the Y-TZP surface. The resulting adhesive interface was approximately 0.57 mm 2 ( A = π * r 2 , where π = 3.14, r = 0.425 mm).

For the microshear test, the specimens were placed on a universal testing machine (Emic DL 2000) and a load was applied (1 mm/min) perpendicularly to the adhesive interface using a metal wire ( Wire Loop ) until failure.

Shear test

This procedure was similar to that described for the microshear test; however, the matrix for the composite sample preparation had the dimensions of = 3.25 mm, h = 3 mm. The adhesive area under stress was 8.30 mm 2 ( A = π * r 2 , where π = 3.14, r = 1.625 mm).

Micropush-out test

A trunk conical-shaped hole was made in the Y-TZP blocks before sintering using a diamond drill (end = 1.92 mm; base = 2.43 mm, and h = 4 mm; # 3131, KG Sorensen, Barueri, Brazil). The blocks (5.5 mm × 5.0 mm × 1.0 mm) were used as templates for the preparation of the composite trunk conical samples, which were later cemented to their respective Y-TZP block. Half of the ceramic blocks were previously treated as described above.

For the micropush-out test, each specimen was placed over a metallic device with a central aperture ( = 3 mm), and the load was applied from the small to the large diameter of the composite resin specimen using a metallic cylinder ( = 0.85 mm) until fracture.

The test was carried out on a universal testing machine (Emic DL 2000) with speed of 1 mm/min. To calculate the adhesive interface, the following formula for the lateral area of a trunk-conical solid was used, as described by Valandro et al. : A = π * g *( R 1 + R 2 ), where π is 3.14, R 1 and R 2 are the radii of the smaller and larger bases, respectively, and g is the slant height – g 2 = h 2 + ( R 2 R 1 ) 2 ; where h is the specimen height, measured with a digital caliper.

Push-out test

The procedures for this test were similar to those described for the micropush-out test, although the blocks of Y-TZP had dimensions of 5.5 mm × 5 mm × 4 mm. Since the blocks were thicker, the adhesive interface was increased.

Failure mode analysis

All specimens were analyzed under stereomicroscopy (Discovery V20, Carl-Zeiss, Gottingen, Germany) with 10–60× magnification to identify the type of failure. The failures were classified as: adhesive (fracture in the adhesive interface, with no residue of cement bonded to zirconia), cohesive (fracture into the cement or the resin composite bulk) or mixed (adhesive and cohesive failures).

Data analysis

The bond strength values (in MPa) for each specimen were calculated by dividing the force to failure ( F , in N ) by the bonding area ( A , in mm 2 ) ( R = F / A ). For the micro tensile test, the means for the samples (sticks) from each block (30 blocks per group) were calculated. Specimens showing cohesive failures in the resin cement or in the resin composite were excluded from the statistical analysis. Bond strength data from the “macro” and “micro” versions, for each mechanical test method, were compared, taking into account each type of Y-TZP surface treatment. The results obtained from the Push-out and Micropush-out tests were submitted to Student’s t -test, and the other methodologies were compared using the Mann Whitney’s test ( α ≤ 0.05).

Additionally, the Weibull analysis was performed to determine the Weibull modulus ( m ) and characteristic strength ( σ 0 ) for each condition. The Weibull modulus represents the variation of bond strength data and expresses the size distribution of the flaw population in a structure , while the characteristic strength indicates the strength value at which 63.2% of the specimens survive.

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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on A critical evaluation of bond strength tests for the assessment of bonding to Y-TZP
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