Abstract
Objective
This study evaluated the effect of a chromium interlayer deposited by electroplating on the shear bond strength between titanium and porcelain.
Methods
Seventy specimens of machined commercially pure titanium (CP Ti) plates grade II (10 mm × 10 mm × 1 mm) were prepared. The specimens were divided into three groups according to the concentration of electroplating solution, Gr I (control without electroplating, n = 10), Gr II (5%, w/v, of chromium nitrate solution, n = 30) and Gr III (10%, w/v, of chromium nitrate solution, n = 30). Groups II and III were further divided into three subgroups ( n = 10) according to the electroplating time (0.5, 1 and 2 h). Two titanium-porcelains (Vita Titankeramik and Triceram) were applied to each subgroup ( n = 5). The titanium–porcelain interfaces were loaded under shear in a universal testing machine (crosshead speed: 0.5 mm/min) until failure occurred. Failure types were examined with stereomicroscope and the titanium–porcelain interface examined by SEM. Data were analyzed using ANOVA and Tukey’s test.
Results
Bond strength values were significantly affected by the type of electroplating treatment ( P < 0.05), but not by the type of porcelain ( P > 0.05). The CP Ti/Vita Titankeramik (0.5 h, 10%, w/v) and the CP Ti/Triceram (0.5 h 5%, w/v) groups showed the highest bond strength (MPa) (26.72 ± 5.78 and 25.48 ± 4.14) respectively among the groups. Stereomicroscope and SEM images showed that chromium interlayer enhanced the bond strength between porcelain and titanium.
Significance
Bond strength between porcelain and CP Ti can be improved by the use of chromium interlayer prior to porcelain firing.
1
Introduction
Metal–ceramic restorations have been used successfully for several years . Recently, noble alloys have gradually been replaced by base metal alloys due to high cost and low sag resistant of the former. Although base metal alloys have many superior mechanical properties, most have disadvantages, such as poor biocompatibility, low corrosion resistance and discoloration of porcelain .
In recent years, titanium has become a material of great interest in prosthetic dentistry . It has been used in metal–ceramic restorations because of its several advantages, such as good corrosion resistance, excellent biocompatibility, low density, low thermal conductivity and reasonable price. However, many practical problems with titanium for prosthodontic applications remain to be solved. High melting temperature and exceptional chemical reactivity at high temperatures create difficulties for both casting and porcelain bonding .
The metal–ceramic bond formed at firing temperatures should be strong enough to resist both transient and residual thermal stresses and mechanical forces in function . The application of porcelain on titanium requires high temperatures typically greater than 800 °C. Factors that affect the titanium–porcelain bond should be considered, such as growth of an oxide layer on the titanium at elevated temperatures, adherence of the self-formed oxide to the titanium substrate, bonding by fusion of the self-formed oxide with the porcelain, and stress generated during cooling resulting from a thermal expansion mismatch at the porcelain-oxide–titanium interface .
From this, it is evident that the principle problem in the fusing of dental porcelain to titanium is the excessive dissolution of oxygen into the titanium lattice, resulting in thick oxygen-rich titanium layers . The formation of such excessive, nonprotective, and nonadherent titanium oxide during the porcelain application is a major cause of critical failures in titanium–porcelain restoration .
To overcome this problem with titanium–porcelain bonding, a strategy to control titanium oxide formation at elevated temperatures and to generate an adherent oxide scale was devised. The procedure involved modifying the surface of titanium by coating it with an element that not only bonded to titanium but also served as an oxygen diffusion barrier while forming an oxide to which porcelain would bond. A number of surface modification methods to improve oxidation properties of titanium to overcome this problem have been proposed such as sandblasting, gold coating, acid treatment, silicon nitride coating, and pre-oxidation .
It was hypothesized that chromium can be used as an intermediate layer to meet the criteria described above to improve the bonding of porcelain to titanium . Accordingly, the present study aimed to evaluate the effect of chromium coating by electroplating on the shear bond strength between porcelain and CP Ti.
2
Materials and methods
Seventy specimens of commercially pure titanium plates (ASTM, Grade II, Modern Techniques and Materials Engineering Center, Egypt) (10 mm × 10 mm × 1 mm) were prepared, sandblasted with 110 μm aluminum oxide (Korax, Bego, Germany) at a pressure of 2.5 bar and then cleaned in an ultrasonic (Bandelin, Sonorex, Germany) bath filled with distilled water for 5 min. Two types of titanium–ceramic were used: Vita Titankeramik (Vita Zahnfabrick, Bad Säckingen, Germany) and Triceram (Dentaurum, Esprident, Germany).
2.1
Electroplating process
2.1.1
Specimen preparation for electroplating
All of the titanium specimens were soaked for 2 min in a pickling solution that contained 50 mL of distilled water, 40 mL of conc. nitric acid and 10 mL of hydrogen fluoride 40% solution. The specimens were then rinsed in distilled water and immediately electroplated.
2.1.2
Apparatus and electrodes
The anode is a Pt sheet of area 1 cm 2 with a platinum wire sealed into the bottom of glass tube. The cathode is the titanium plate which is suspended by a suitable platinum hook sealed into the bottom of a glass tube. For electrical connection, in each glass tube, a small amount of mercury is introduced in which a copper wire is immersed. The electroplating solution consisted of chromium (III) nitrate (Cr(NO 3 ) 3 ·9H 2 O, 99%, Sigma–Aldrich) in distilled water. Electroplating was performed using a 12 V DC source and 0.5 A (High Voltage Supply, Model 1030A, PASCO Scientific, Japan).
2.2
Grouping of specimens
The specimens were divided into three groups, as follows:
Group I : Consists of ten titanium plates without electroplating, which were exposed only to pickling as control.
Group II : Consists of thirty titanium plates using 5% (w/v) chromium nitrate solution.
Group III : Consists of thirty titanium plates using 10% (w/v) chromium nitrate solution.
Groups II and III were further divided into three subgroups according to the electroplating time as follows:
Subgroup A : Consists of ten titanium plates electroplated for 30 min.
Subgroup B : Consists of ten titanium plates electroplated for 60 min.
Subgroup C : Consists of ten titanium plates electroplated for 120 min.
Each group was further equally divided into two sub-subgroups according to the type of porcelain used as follows:
Sub-subgroup 1: Consists of five specimens of titanium/Vita Titankeramik porcelain.
Sub-subgroup 2: Consists of five specimens of titanium/Triceram porcelain.
2.3
Porcelain application
A specially designed split Teflon mold (6 mm in diameter and 4 mm in thickness) was used to apply porcelain onto the titanium plate. This mold fabricated with a circular area on one side and the other side has a centralized area for titanium plate to be adapted to create a standardized area for the ceramic application over the center of each titanium plate. The porcelain was applied onto a grit blasted surface of the titanium plate. For each type of porcelain, a thin layer of bonder porcelain was applied, followed by a second opaque layer and two dentin body layers, each of them fired separately according to the manufacturer’s instructions in a dental vacuum porcelain furnace (Programat P500, Ivoclar Vivadent, Germany). Finally, the specimens were submitted to final glaze firing. All specimens were stored in distilled water at 37 °C for 24 h in an incubator before thermal cycling. The specimens were subjected to thermocycling, between 5 °C and 55 °C for 1000 cycles with a 1-min dwell time using a thermocycling machine (Conservative Dentistry Department, Faculty of Dentistry, Tanta University, Egypt).
2.4
Measurements of the shear bond strength
Each specimen was embedded in self-cured acrylic resin (Acrostone, Egypt) inside a plastic ring (25 mm in diameter and 20 mm high). The shear bond strength test was performed in a universal testing machine (Lloyd Model TT-B, Instron Corp., Canton, MA, USA). The shear test specimens were mounted in a V-shaped holding device and sheared with a 30° monobeveled chisel edged blade which was aligned 0.1 mm away from the bonded interface The bonded porcelain specimens were placed under sustained, continuous loading at 0.5 mm/min until fracture occurred. The shear bond strength in megapascals (MPa) was calculated by dividing the fracture load ( F ) in Newton by the surface area ( A ) in mm 2 . After debonding, an optical stereomicroscope (Olympus Zoom Stereomicroscope, Japan) with 20× magnification was used to examine the fractured specimens to determine the nature of failure (cohesive, adhesive, or combination).
Bond strength mean values were compared with an analysis of variance (ANOVA) and a Tukey’s multiple comparison tests, considering two factors (electroplating treatment and type of porcelain) and their interaction. Statistical significance was set at the 0.05 probability level.
2.5
SEM analysis of titanium–porcelain interface
Two representative specimens of each subgroup of titanium–porcelain systems were used to examine the titanium–porcelain interface with a scanning electron microscopy (JEOL, JXA-840A, JEOL Ltd., Tokyo, Japan). The specimens were embedded in clear autopolymerizing acrylic resin (Acrostone, Egypt). After the complete polymerization of the acrylic resin, the specimen/resin assembly was sectioned along the longitudinal axis of the specimens with a diamond disc (DICA 72, Germany) using a low speed hand piece. The sectioned specimens were ultrasonically cleaned for 10 min and polished using silicon-carbide papers through grits of 320, 400, and 600. Final polishing was accomplished with a rotary polisher with aluminum-oxide polishing paste. After that, the specimens were sputtered (Sputter Coater S150A, Japan) with a gold layer and then examined using the scanning electron microscope.
2
Materials and methods
Seventy specimens of commercially pure titanium plates (ASTM, Grade II, Modern Techniques and Materials Engineering Center, Egypt) (10 mm × 10 mm × 1 mm) were prepared, sandblasted with 110 μm aluminum oxide (Korax, Bego, Germany) at a pressure of 2.5 bar and then cleaned in an ultrasonic (Bandelin, Sonorex, Germany) bath filled with distilled water for 5 min. Two types of titanium–ceramic were used: Vita Titankeramik (Vita Zahnfabrick, Bad Säckingen, Germany) and Triceram (Dentaurum, Esprident, Germany).
2.1
Electroplating process
2.1.1
Specimen preparation for electroplating
All of the titanium specimens were soaked for 2 min in a pickling solution that contained 50 mL of distilled water, 40 mL of conc. nitric acid and 10 mL of hydrogen fluoride 40% solution. The specimens were then rinsed in distilled water and immediately electroplated.
2.1.2
Apparatus and electrodes
The anode is a Pt sheet of area 1 cm 2 with a platinum wire sealed into the bottom of glass tube. The cathode is the titanium plate which is suspended by a suitable platinum hook sealed into the bottom of a glass tube. For electrical connection, in each glass tube, a small amount of mercury is introduced in which a copper wire is immersed. The electroplating solution consisted of chromium (III) nitrate (Cr(NO 3 ) 3 ·9H 2 O, 99%, Sigma–Aldrich) in distilled water. Electroplating was performed using a 12 V DC source and 0.5 A (High Voltage Supply, Model 1030A, PASCO Scientific, Japan).
2.2
Grouping of specimens
The specimens were divided into three groups, as follows:
Group I : Consists of ten titanium plates without electroplating, which were exposed only to pickling as control.
Group II : Consists of thirty titanium plates using 5% (w/v) chromium nitrate solution.
Group III : Consists of thirty titanium plates using 10% (w/v) chromium nitrate solution.
Groups II and III were further divided into three subgroups according to the electroplating time as follows:
Subgroup A : Consists of ten titanium plates electroplated for 30 min.
Subgroup B : Consists of ten titanium plates electroplated for 60 min.
Subgroup C : Consists of ten titanium plates electroplated for 120 min.
Each group was further equally divided into two sub-subgroups according to the type of porcelain used as follows:
Sub-subgroup 1: Consists of five specimens of titanium/Vita Titankeramik porcelain.
Sub-subgroup 2: Consists of five specimens of titanium/Triceram porcelain.
2.3
Porcelain application
A specially designed split Teflon mold (6 mm in diameter and 4 mm in thickness) was used to apply porcelain onto the titanium plate. This mold fabricated with a circular area on one side and the other side has a centralized area for titanium plate to be adapted to create a standardized area for the ceramic application over the center of each titanium plate. The porcelain was applied onto a grit blasted surface of the titanium plate. For each type of porcelain, a thin layer of bonder porcelain was applied, followed by a second opaque layer and two dentin body layers, each of them fired separately according to the manufacturer’s instructions in a dental vacuum porcelain furnace (Programat P500, Ivoclar Vivadent, Germany). Finally, the specimens were submitted to final glaze firing. All specimens were stored in distilled water at 37 °C for 24 h in an incubator before thermal cycling. The specimens were subjected to thermocycling, between 5 °C and 55 °C for 1000 cycles with a 1-min dwell time using a thermocycling machine (Conservative Dentistry Department, Faculty of Dentistry, Tanta University, Egypt).
2.4
Measurements of the shear bond strength
Each specimen was embedded in self-cured acrylic resin (Acrostone, Egypt) inside a plastic ring (25 mm in diameter and 20 mm high). The shear bond strength test was performed in a universal testing machine (Lloyd Model TT-B, Instron Corp., Canton, MA, USA). The shear test specimens were mounted in a V-shaped holding device and sheared with a 30° monobeveled chisel edged blade which was aligned 0.1 mm away from the bonded interface The bonded porcelain specimens were placed under sustained, continuous loading at 0.5 mm/min until fracture occurred. The shear bond strength in megapascals (MPa) was calculated by dividing the fracture load ( F ) in Newton by the surface area ( A ) in mm 2 . After debonding, an optical stereomicroscope (Olympus Zoom Stereomicroscope, Japan) with 20× magnification was used to examine the fractured specimens to determine the nature of failure (cohesive, adhesive, or combination).
Bond strength mean values were compared with an analysis of variance (ANOVA) and a Tukey’s multiple comparison tests, considering two factors (electroplating treatment and type of porcelain) and their interaction. Statistical significance was set at the 0.05 probability level.
2.5
SEM analysis of titanium–porcelain interface
Two representative specimens of each subgroup of titanium–porcelain systems were used to examine the titanium–porcelain interface with a scanning electron microscopy (JEOL, JXA-840A, JEOL Ltd., Tokyo, Japan). The specimens were embedded in clear autopolymerizing acrylic resin (Acrostone, Egypt). After the complete polymerization of the acrylic resin, the specimen/resin assembly was sectioned along the longitudinal axis of the specimens with a diamond disc (DICA 72, Germany) using a low speed hand piece. The sectioned specimens were ultrasonically cleaned for 10 min and polished using silicon-carbide papers through grits of 320, 400, and 600. Final polishing was accomplished with a rotary polisher with aluminum-oxide polishing paste. After that, the specimens were sputtered (Sputter Coater S150A, Japan) with a gold layer and then examined using the scanning electron microscope.
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