Han-Cheol Choe

Electrochemical behavior of Co-Cr and Ni-Cr dental cast alloys

Abstract The cast structures influencing the electrochemical corrosion behavior of Co-Cr and Ni-Cr dental alloys were studied using potentiodynamic polarization and AC impedance in 0.9% (mass fraction) NaCl solution at (37±1) °C. The phase and microstructure of the alloys that were fabricated using two different casting methods viz. centrifugal casting and high frequency induction casting, were examined using X-ray diffraction analysis, scanning electron microscopy and energy dispersive spectroscopy. The roles of alloying elements and the passive film homogeneity on the corrosion resistance of Co-Cr-Mo and Ni-Cr-Mo dental cast alloys were reviewed. The results of electrochemical study show that the dependence of corrosion resistance on the microstructure associated with the casting methods is marginal. The Co-Cr alloy exhibits more desirable corrosion resistance properties than the Ni-Cr alloy. There is severe preferential dissolution of Ni-rich, Cr and Mo depleted zones in the Ni-Cr alloy.

Nanostructure and corrosion behaviors of nanotube formed Ti-Zr alloy

In order to investigate the nanostructures and corrosion behaviors of Ti-Zr alloys, nanotube formed Ti-Zr(10%, 20%, 30% and 40% in mass fraction) alloys were prepared by arc melting and the condition of controlling nanostructure was at 1 000 �� for 24 h in argon atmosphere; formation of nanotubes was conducted by anodizing a Ti-Zr alloy in H3PO4 electrolyte with a small amount of fluoride ions at room temperature. The corrosion properties of specimens were examined through potentiodynamic test (potential range of I1 500I2 000 mV) in 0.9% NaCl solution by using potentiostat. Microstructures of the alloys were observed by optical microscope(OM), field emission scanning electron microscope(FE-SEM) and X-ray diffractometer(XRD). Diameter of nanotube does not depend on Zr content, but interspace of nanotube predominantly depends on Zr content, which confirms that ZrO2 oxides play a role to increase the interspace of nanotube formed on the surface.

Surface characteristics of HA coated Ti-Hf binary alloys after nanotube formation

Abstract Ti-Hf binary alloys contained 10%, 20%, 30% and 40% (mass fraction)Hf were manufactured in the vacuum furnace system. And then, specimens were homogenized for 24 h at 1 000 °C in argon atmosphere. The formation of oxide nanotubes was conducted by anodic oxidation on the Ti-Hf alloy in 1 mol/L H 3 PO 4 electrolytes containing small amounts of NaF at room temperature. The hydroxyapatite (HA) coating made of tooth ash prepared by electron-beam physical vapor deposition (EB-PVD) method. The corrosion behaviors of the specimens were examined through potentiodynamic test in 0.9% NaCl solution by potentiostat. The microstructures of the alloys were examined by field emission scanning electron microscopy (FE-SEM) and x-ray diffractometer (XRD). It was observed that the lamellar structure translated to needle-like structure with Hf contents. Nanotube formed and HA coated Ti- x Hf alloys had a good corrosion resistance.

Effect of various intraoral repair systems on the shear bond strength of composite resin to zirconia

PURPOSE This study compared the effect of three intraoral repair systems on the bond strength between composite resin and zirconia core. MATERIALS AND METHODS Thirty zirconia specimens were divided into three groups according to the repair method: Group I- CoJet™ Repair System (3M ESPE) [chairside silica coating with 30 µm SiO2 + silanization + adhesive]; Group II- Ceramic Repair System (Ivoclar Vivadent) [etching with 37% phosphoric acid + Zirconia primer + adhesive]; Group III- Signum Zirconia Bond (Heraus) [Signum Zirconia Bond I + Signum Zirconia Bond II]. Composite resin was polymerized on each conditioned specimen. The shear bond strength was tested using a universal testing machine, and fracture sites were examined with FE-SEM. Surface morphology and wettability after surface treatments were examined additionally. The data of bond strengths were statistically analyzed with one-way ANOVA and Tamhane post hoc test (α=.05). RESULTS Increased surface roughness and the highest wettability value were observed in the CoJet sand treated specimens. The specimens treated with 37% phosphoric acid and Signum Zirconia Bond I did not show any improvement of surface irregularity, and the lowest wettability value were found in 37% phosphoric acid treated specimens. There was no significant difference in the bond strengths between Group I (7.80 ± 0.76 MPa) and III (8.98 ± 1.39 MPa). Group II (3.21 ± 0.78 MPa) showed a significant difference from other groups (P<.05). CONCLUSION The use of Intraoral silica coating system and the application of Signum Zirconia Bond are effective for increasing the bond strength of composite resin to zirconia.

Correlation of immunohistochemical characteristics of the craniomandibular joint with the degree of mandibular lengthening in rabbits

PURPOSE This study examined immunohistochemical changes in the craniomandibular joints of rabbits after distraction osteogenesis following mandibular corticotomy. MATERIALS AND METHODS The experimental animals (n = 8) were divided into 3 groups that underwent 2, 3.5, and 5 mm of unilateral distraction osteogenesis (groups 1, 2, and 3, respectively). After corticotomy of the left mandibular body and a 7-day healing period, a second operation was performed to expose the device. Distraction was then performed at the rate of 0.5 mm/d. A 14-day consolidation period was allowed after the distraction was complete. Changes in cartilage, osteoblast activity, and osteoclast activity were then examined. RESULTS The differentiation and proliferation of cartilage increased in groups 1 and 2, were highest in group 2, and decreased in group 3. Group 2 also showed the greatest increase in the width of the hypertrophic chondrocyte layer. Relative to the control group, osteoclast activity was only somewhat higher in groups 1 and 2 but was significantly higher in group 3. Osteoblast activity was significantly higher in groups 1 and 2 than in the control group. However, the osteoblast activity in group 3 was slightly lower than that in group 2. At the time of unilateral mandibular distraction, no degenerative changes of the temporomandibular joint were observed in groups 1 or 2, but bone resorption was observed in group 3. CONCLUSIONS The unilateral mandibular distraction of 2 or 3.5 mm was acceptable in that no degenerative changes of the temporomandibular joint were observed on either the distraction or the nondistraction sides. Five millimeters of distraction might be beyond physiologic limits.

Mechanical properties and corrosion resistance of low rigidity quaternary titanium alloy for biomedical applications

Abstract Electrochemical corrosion of Ti-35Nb-5Ta-7Zr alloy fabricated by arc melting and heat treatment process was studied in 0.9% NaCl at (37±1) °C. Phase and microstructure of the fabricated alloy were investigated using X-ray diffractometer and scanning electron microscope. Mechanical properties such as yield strength and elastic modulus of the alloy were determined by tensile test. Potentiodynamic polarization technique and impedance spectroscopy were employed to study the corrosion behavior. The results of the study were compared with those obtained for Ti-6Al-4V commercial alloy. The result of the study supports feasibility of Ti-35Nb-5Ta-7Zr alloy for implant applications.

Effects of TiN film coating on electrochemical behaviors of nanotube formed Ti-xHf alloys

Abstract Ti- x Hf ( x =10%, 20%, 30% and 40%, mass fraction) alloys were prepared by arc melting, and the microstructure was controlled for 24 h at 1 000 °C in argon atmosphere. The formation of nanotube was conducted by anodizing on Ti-Hf alloys in 1.0 mol/L H 3 PO 4 electrolytes with small amounts of NaF at room temperature. And then TiN coatings were coated by DC-sputtering on the anodized surface. Microstructures and nanotube morphology of the alloys were examined by field emission scanning electron microscopy(FE-SEM) and X-ray diffractometry(XRD). The corrosion properties of the specimens were examined through potentiodynamic test (potential range from −1 500 to 2 000 mV) in 0.9 % NaCl solution by potentiostat. The microstructure shows the acicular phase and α′ phase with Hf content. The amorphous oxide surface is transformed to crystalline anatase phase. TiN coated nanotube surface has a good corrosion resistance.

Corrosion behavior between dental implant abutment and cast gold alloy

Two types of HL hexed abutments of a Steri-Oss system, gold/plastic coping and gold coping, were compared in terms of corrosion behavior. The anodic polarization behavior and the galvanic corrosion between abutments and Type III gold alloys, before and after casting, were analyzed. In addition, the crevice corrosion of the casting samples was analyzed with cyclic potentiodynamic polarization tests using the |Er-Ecorr| value and scanning electron microscopy. Before casting, gold/plastic coping and gold coping were shown to have similar corrosion patterns in the anodic polarization test. Type III casting gold alloy was shown to have a lower higher than that of gold coping, but the passive region for the gold/plastic coping was smaller than that of gold coping. The contact current density between the cast gold alloys and gold/plastic before casting was higher than that between gold coping and cast gold alloy. The contact current density of the samples after casting was shown to be similar to that before casting. The crevice corrosion resistance of cast samples using gold coping was lower than that of cast samples using gold/plastic coping, and severe corrosion was observed by SEM at the abutment-casting gold alloy interface.

Effects of nitrogen ion implantation on the surface characteristics of iron aluminides

Abstract Iron aluminides are of considerable interest for low to intermediate temperature structural applications in which low cost, low density and good corrosion or oxidation resistance are required. However, their application is currently limited by room temperature brittleness and low corrosion resistance. In this study, in order to improve the wear and corrosion resistance of iron aluminides in acidic solutions, steel containing Mo and Cr were fabricated and nitrogen ion implanted on the surface of samples with doses of 3.0×10 17 ions/cm 2 at an energy of 150 keV. The effects of nitrogen ion implantation on the electrochemical characteristics of samples were investigated using various electrochemical methods in H 2 SO 4 +KSCN and HCl solutions. AlN, CrN and Fe 4 N were formed in the surface by the nitrogen ion implantion. The grain boundary activation (DOS, degree of sensitization), the active current density (I a ) and reactivation current density (I r ) were significantly decreased by the nitrogen ion implantation in H 2 SO 4 +KSCN solution. Nitrogen implantation promoted the corrosion potential, pitting potential, repassivation potential and E hys value for iron aluminides containing Mo. Whereas, the pitting and repassivation potential for iron aluminides containing B was reduced in comparison with iron aluminides containing Cr and Mo in HCl solution. From current and potential–time curves, pitting resistance and passive film stability of the nitrogen implanted iron aluminides containing Mo was higher than that of the unimplanted iron aluminides. The nitrogen ion implanted iron aluminides containing B and Mo showed rounded pit shapes in comparison with irregular pit shape of ion implanted iron aluminides without B and Mo additions. In conclusion, it is considered that the corrosion problem of iron aluminides could be solved, to some extent, by the addition of Mo and Cr to iron aluminides and nitrogen implantation on the surface of iron aluminides.

Electrochemical behavior of dental implant system before and after clinical use

Abstract Electrochemical behavior of dental implant system before and after clinical use (in vivo and in vitro) was researched by using abutment and titanium fixture. To simulate an oral environment, the samples of clinically used and non-used implant systems as a working electrode were exposed to artificial saliva at (36.5±1) °C. Electrochemical tests were carried out using a potentiostat. After electrochemical test, the corrosion morphology of each sample was investigated by FE-SEM and EDS. The corrosion potential and pitting potential of clinically used implant system are lower than those of non-used implant system, and clinically used implant system exhibits a lower range of passivation, indicating a less degree of inherent resistance against chloride ion. The polarization resistance decreases in the case of clinically used implant system, whereas, R p for clinically non-used implant system increases compared with clinically used implant system.