Deuk Yong Lee

Mechanical properties, phase stability, and biocompatibility of (Y, Nb)-TZP/Al2O3 composite abutments for dental implant

ZrO(2)/Al(2)O(3) composites were prepared by mixing a tetragonal ZrO(2), stabilized by 5.31 mol% Y(2)O(3) and 4.45 mol% Nb(2)O(5), and various amounts of Al(2)O(3). Influence of the amount of Al(2)O(3) on strength and toughness and tetragonal phase stability in the composites under autoclave conditions was investigated. In addition, in vitro and in vivo biocompatibility of the composites was examined. The composite, prepared with addition of 20 vol% Al(2)O(3), exhibited the highest strength of 700 MPa and toughness of 8.1 MPa. m(1/2) and showed no hydrothermal degradation while aging in an autoclave. The biocompatibility of the composite exhibited no cytotoxicity and no significant adverse soft-tissue response for up to 3 months implant period in guinea pigs.

Effect of post-oxidizing time on corrosion properties of plasma nitrocarburized AISI 1020 steel

Abstract Plasma nitrocarburized AISI 1020 steels are oxidized for 15, 30 and 60 min to evaluate the effect of post-oxidizing time on corrosion behavior. After plasma nitrocarburizing for 3 h at 570 °C, the compound layer composed of e-Fe2–3(N,C) and γ′-Fe4(N,C) phases and the diffusion layer above the matrix are observed. The oxide top layer, mainly consisting of magnetite (Fe3O4) and hematite (Fe2O3) phases, is formed after post-oxidation treatment at 500 °C. However, the oxide layer is degraded severely by spallation as a result of the prolonged oxygen plasma bombardment as the post-oxidizing time increases. The difference in corrosion resistance is mainly attributed to the thickness of the oxide top layer, which is governed by the post-oxidizing time.

Influence of sputtering parameters on microstructure and morphology of TiO2 thin films

Abstract Titanium dioxide thin films were sputtered on Si substrate by systematically varying the process conditions including O 2 /(Ar+O 2 ) ratio, working pressure, sputtering time and dc power, respectively, without heating the substrate, to evaluate the microstructure and the morphology of the films. The optimum condition for the films is 23% of O 2 /(Ar+O 2 ), 2×10 −3 Torr of working pressure, sputtering time longer than 10 min and dc power higher than 150 W, respectively. During sputtering, amorphous α-SiO 2 phase was firstly grown on Si substrate, amorphous α-TiO 2 structure close to α-SiO 2 phase was then formed to reduce the surface energy and columnar TiO 2 films normal to amorphous α-TiO 2 were finally developed. The surface morphology of the films was a typical island structure consisting of fine particles of 10–30 nm.

High-temperature properties of plasma-sprayed coatings of YSZ/NiCrAlY on Inconel substrate

Abstract Eleven-layered functionally gradient thermal barrier coatings (TBCs) were sprayed on Inconel substrate by varying the feeding ratio of YSZ/NiCrAlY in 0.1 intervals in the range of 0–1 linearly as well as nonlinearly (concave and convex type). The performance of the coatings was evaluated in terms of high-temperature fatigue, oxidation resistance, and bond strength. The fatigue behavior and the oxidation resistance were governed by the oxidation of Al in the NiCrAlY bond coating. The bond strength was determined by continuity of the composition and the microstructure between the layers. The linearly sprayed TBCs showed the highest endurance at high-temperatures among the coatings investigated, indicating that the linear feeding technique can be used for the high-temperature applications.

Size effect of trivalent oxides on low temperature phase stability of 2Y-TZP

Abstract2 mol% Y2O3stabilized-TZPs (2Y-TZPs) doped with oversized trivalent cations (Sc3+< Yb3+< Y3+< Sm3+< Nd3+< La3+) whose ionic radius is larger than Zr4+was sintered for 1 h at 1500°C over the range containing trivalent oxides from 0 to 2 mol% with 0.5 mol% interval to evaluate the effect of trivalent cation alloying on low temperature phase stability of 2Y-TZP by investigating the variation of Raman spectra and lattice parameters. For a given concentration of dopant, tetragonality (c/aaxial ratio) increases with raising the dopant size. However, monoclinic (m)-ZrO2content for the specimens annealed for 500 h at 220°C in air firstly decreases with increasing dopant size and then increases as dopant size is greater than Y3+ion. Raman modes of Zr-OII(260 cm−1) and Zr-OI(640 cm−1) shift to higher wavenumbers only when Sm2O3, Nd2O3, and La2O3are added. Although full width at half maximum (FWHM) of 640 cm−1is constant, FWHM of 260 cm−1mode decreases with increasing dopant size, indicating that an ordered structure (pyrochlore phase) may be formed. Therefore, dopant size is dependent on phase stability of 2Y-TZP in this system.

Influence of alumina particle size on fracture toughness of (Y,Nb)-TZP/Al2O3 composites

Abstract ZrO 2 /Al 2 O 3 composites were prepared by mixing a tetragonal ZrO 2 , stabilized with 5.31 mol% Y 2 O 3 and 4.45 mol% Nb 2 O 5 , and Al 2 O 3 having different particle size (0.2 μm and 2.8 μm) to investigate the influence of the Al 2 O 3 particle size on flaw tolerance of the composites. The composites exhibited rising R-curve behavior and plateau fracture toughness of 7.9 and 8.8 MPa√m for the additions of 20 vol.% of 0.2 and 2.8 μm Al 2 O 3 particles, respectively. The difference in the fracture resistance was attributed mainly to the grain size of tetragonal phase in the composites, which scaled with the Al 2 O 3 particle diameter, and partially to the dispersion toughening.

Phase stability of (Y,NB)-TZP/Al2O3 composites under low temperature hydrothermal conditions

Abstract Y 2 O 3 and Nb 2 O 5 co-doped tetragonal zirconia polycrystals (TZP) containing 10 to 30 vol.% Al 2 O 3 ((Y,Nb)-TZP/Al 2 O 3 ) were sintered for 5 h at 1550°C in air and hydrothermal stability of the composites was evaluated after aging for 5 h at 180°C in 0.3 MPa H 2 O vapor pressure. (Y,Nb)-TZP/Al 2 O 3 composites showed superior phase stability under the hydrothermal condition as compared with 3Y-TZP/Al 2 O 3 composites due to the effects of Y–Nb ordering in t-ZrO 2 and the Al 2 O 3 addition. The optimized strength and fracture toughness were 670 MPa and 7.3 MPa m 1/2 , respectively, when 20 vol.% of 2.8 μm Al 2 O 3 particles were added.

Tissue response to implants of hyaluronic acid hydrogel prepared by microbeads

Hyaluronic acid hydrogels (HAHs) were synthesized by immersing the HA microbeads crosslinked with divinyl sulfone in phosphate buffered saline solution to assess the responses of tissues to the implant by means of the subchronic systemic toxicity and the intradermal implant test. The HAHs, prepared by the microbeads with an average diameter of 140 μm and a swelling rate of 800 to 1200%, exhibited a porous network channeled with 10 μm pores. The HAHs retained their space and structure by maintaining over 95% of the initial volume 12 weeks after injection to a rabbit. The histological analysis indicated that an acute inflammation, occurred in the rabbit 4 weeks after injection, was alleviated dramatically after 8 weeks. No capsule was formed. The HAHs had no subchronic systemic toxicity under the condition of this study and were considered non-irritant. The implants were excellent in biological synthesis and transplantation as evidenced by non-capsule reaction and disappearance of inflammatory cells. It can be concluded that the implants of HAHs are clinically safe and effective.

Effect of organic vehicle addition on service lifetime of Ti/IrO2–RuO2 electrodes

Abstract Ti/IrO 2 –RuO 2 electrodes, prepared by mixing the conductive IrO 2 –RuO 2 powders and organic vehicle (ethyl cellulose) followed by dissolution in isopropanol-α-terpineol solution, were evaluated with an aid of the Taguchi method and orthogonal arrays to elucidate the effect of the experimental parameters, such as the ratio of powders to ethyl cellulose, the ratio of isopropanol to α-terpineol and heat treatment temperature, on the service lifetime of the electrodes. Although the current density of the as-deposited IrO 2 –RuO 2 films was almost insensitive regardless of the processing conditions, it was found that the mixing ratio of IrO 2 –RuO 2 powders to ethyl cellulose was a critical factor to the service lifetime of the Ti/IrO 2 –RuO 2 anodes.

Effect of Nb2O5 and Y2O3 alloying on the mechanical properties of TZP ceramics

Abstract In the ZrO 2 Y 2 O 3 Nb 2 O 5 ternary system, Nb 2 O 5 and Y 2 O 3 co-doped tetragonal ZrO 2 polycrystals (TZPs) were synthesised by sintering and hot-pressing (HPing) to evaluate the effect of alloying on the mechanical properties and phase stability of TZPs. Monolith containing the composition of 90.24 mol% ZrO 2 -5.31 mol% Y 2 O 3 -4.75 mol% Nb 2 O 5 , which was prepared by both sintering and hot-pressing, exhibited a good relation between strength and toughness. Also, no t → m phase transformation after ageing at temperatures in the range of 100 to 400 °C for up to 1000 h was observed. TZPs sintered at 1550 °C for 10 h in air showed the flexural strength of above 500 MPa and the fracture toughness of 9 MPa m 1 2 . The flexural strength increased greatly by HPing at 1400 °C for 1 h under an Ar gas atmosphere from 500 MPa to above 1 GPa, while the fracture toughness decreased slightly from 9 MPa m 1 2 to 7.5 MPa m 1 2 .