• The Journal of Korean Academy of Prosthodontics
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• v.40 no.3
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• pp.287-295
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• 2002

Statement of Problem. Endodontically treated teeth frequently required posts and cores to provide retention and resistance form for crowns. In spite of excellent mechanical properties of metal post and core, its metallic color can be detected through all ceramic restorations occasionally. To solve esthetic problems of metal post and core zirconia post system has been introduced recently. Purpose. The purpose of this study was to examine the fracture strength and mode of resin root analogs restored with zirconia, gold and titanium posts with resin, ceramic and metal cores after cementation with metal crowns. Materials and methods. To avoid the morphological variations of natural teeth, 40 root analogs were fabricated with composite resin. Forty resin root analogs were randomly assigned to four groups according to post and core materials: Group A: cast gold post and core and complete cast crowns, as control. Group B: titanium posts (Parapost, Coltent/Whaledent Inc., NJ, USA) and composite resin cores. Group C: zirconia posts (Cosmopost, Ivoclar AG, Schaan/Liechtenstein) and composite resin cores Group D: zirconia posts and heat-pressed ceramic cores (IPS Empress Cosmo Ingots, Ivoclar AG) After thermocycling ($5^{\circ}C{\sim}55^{\circ}C$, 30 sec.), cyclic loading was applied at 3mm below the incisal edge on the palatal surfaces at an angle of 135 degree to the long axis (2Hz, 50N, 50000cycles). Fracture strength was measured by universal testing machine (Instron, High Wycombe, UK) and fracture pattern of restored resin root analogs was also evaluated. Results and conclusion. Within the limitations of this study following results were drawn. 1. Resin root analogs restored with zirconia posts and composite resins demonstrated lowest fracture strength among tested groups. 2. There was no significant difference in the fracture strength between zirconia posts and heat pressed glass ceramic cores and cast gold posts and cores 3. The fracture strength of resin root analogs restored with titanium posts and composite resin cores was lower than that of gold posts and cores. 4. The deep oblique fracture lines were dominantly observed in root analogs restored with cast gold post and core and zirconia post and heat-pressed ceramic core groups.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09b
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• pp.915-915
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• 2006

• Journal of the Korean Ceramic Society
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• v.26 no.5
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• pp.637-644
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• 1989

Beta-sialon powder(Z=1) was synthesized by the simultaeous reduction and nitridation of the mixed powders of Hadong kaolin and silica. Silicon hydroxide was prepared from Si-alkoxide by a hydrolysis method and amorphous silica was obtained from the calcination of the prepared silicon hydroxide. Hadong kaolin was mixed with both the silicon hydroxide and amorphous silica, respectively. The average particle size was 4${\mu}{\textrm}{m}$ and the morphology of particle was rod-like and equiaxed in the case of beta-sialon powder prepared form Hadong kaolin and silicon hydroxide(COMPOSITION A), whereas the average particle size was 3${\mu}{\textrm}{m}$ and the morphology of particle was equiaxed in the case of beta-sialon powder prepared from Hadong kaolin and amorphous silica(COMPOSITION B). The synthesized beta-sialon powders were hot-pressed at 175$0^{\circ}C$ for 2 hours under 30 MPa in a nitrogen atmosphere after YAG composition(8wt%) was added to these powders as a sintering agent. The hot-pressed specimens were annealed a 140$0^{\circ}C$ for 4 hours in a nitrogen atmosphere. The mechanical properties of sintered bodies were investigated in terms of M.O.R., fracture toughness and hardness. The measured values are as follows. COMPOSITION A : M.O.R. 508MPa, KIC 3.5MN/m3/2, hardness 13.6GPa. COMPOSITION B : M.O.R. 653MPa, KIC 5.4MN/m3/2, hardness 13.5GPa.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2001.07a
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• pp.431-434
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• 2001

The mechanical and phase transformation of the cold isostatically pressed $\alpha$-SiC ceramic were investigated as a function of the sintering temperature. The result of phase analysis by XRD revealed 6H, 4H, 3C and phase transformation between 6H and 4H showed a sudden change over 200$0^{\circ}C$. However, the alongrightarrow$\beta$ reverse transformation did not occur to any sintering temperature. The relative density and the mechanical properties of $\alpha$-SiC ceramic was increased with increased sintering temperature. The flexural strength rapidly inclosed below 210$0^{\circ}C$ and showed the highest value of 410 MPa at 220$0^{\circ}C$. This reason is because crack was propagated through surface flaw. The fracture toughness showed the highest value of 3.3 MPa.m$_{1}$2/ at 220$0^{\circ}C$.

• Journal of the Korean Ceramic Society
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• v.22 no.4
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• pp.33-39
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• 1985

A composition containing a-$Si_3N_4$ with 5w/0 $Y_2O_3$ and 4w/0 $Al_2O_3$ was hot-pressed at 1, $650^{\circ}C$ and 350kg/$cm^2$ for 1.5hrs and specimens of the same composition were pressureless-sintered at 1, 75$0^{\circ}C$ for 1.5 and 5hrs. By X-ray diffraction it was found that hot-pressed specimens were consisted of $\alpha$-and $\beta$-$Si_3N_4$ and sintered specimens were consisted of $\beta$-$Si_3N_4$ and $Si_3N-4Y_2O_3$ which was crystallized out from the grainboundary phase. The 5-hr sintered specimens had higher degree of crystallization than the 1.5 hr sintered specimens. Among these three different specimens the 5-hr sintered specimens showed the highest strength by hot MOR test at 1, 00$0^{\circ}C$. The SPT diagram for the 5-hr sintered $Si_3N_4$ was constructed by measurements of the stress rate dependence of fracture strength.

• Journal of the Korean Ceramic Society
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• v.36 no.9
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• pp.887-892
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• 1999

The inner crack-like pore with controlled amount of Ca impurity in the high purity alumina single crystal sapphire had been created by micro-fabrication technique which includes ion implanation photo-lithography Ar ion milling and hot press technique. The crack-like pores in two-hour hot pressed specimen were extremely stable even after heat treating at 1,80$0^{\circ}C$ for 5 hours almost no healing was observed. But the crack-like pores in one-hour hot pressed specimen at 1,30$0^{\circ}C$ were healed by heat treatment and the amount of healing was increased with the heat treatment time and temperature and the amount of Ca addition. The edges of crack-like pore parallel to <1100> direction in (001) basal plane were stable but the edges normal to this direction in (00101) plane <1120> direction were unstable to facetting This means that the surface energy of alumina along the <1100> direction in (0001) basal plane in much lower than <1120> direction.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.6 no.2
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• pp.194-202
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• 1996

Transmission electron microscopy has been used to characterize the dislocation structure in hot-pressed titanium diboride. The thin foil samples were prepared by the conventional ion beam thinning technique and reveal the main features associated with the dislocations ; low-angle grain boundaries with dislocation arrays, high-angle grain boundaries with ledges/steps on the boundary planes. The ledges/steps on the grain boundaries were characterized as the origin of defect structures such as dislocation formation or crack propagation near grain boundaries. A fraction of the high angle grain boundaries contained periodic arrays of grain boundary dislocations. The Burger's vectors of the dislocations in the $TiB_{2}$specimens were determined.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.6 no.3
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• pp.431-438
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• 1996

Transmission electron microscopy has been used to investigate the microstructure of hot-pressed $TiB_{2}$. Thin foil specimens, prepared by conventional ion beam thinning, revealed many features which originated from the crystallographic anisotropy of hexagonal $TiB_{2}$. It was observed that in these specimens Fe-impurities are precipitated to form secondary Fe-rich phases at grain triple edges, in grain boundaries and sometimes in-grain. These Fe-rich precipitates were characterised by their coherence or semi-coherence to a favourably oriented grain at a grain triple edge or grain boundaries or to the matrix $TiB_{2}$ phase.

• Transactions of Materials Processing
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• v.8 no.3
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• pp.259-259
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• 1999

TiC ceramic particulate-reinforced titanium matrix composites were fabricated and the resultant densification, microstructure, and static and dynamic mechanical properties were studied. Comparing Ti with TiH₂powders as host materials for TiC ceramic reinforcement by pressureless vacuum sintering, TiH₂-started composites showed better sinterability and resistance to both elastic and plastic deformation than Ti-started ones. When TiH₂and TiH₂-45 vol.%TiC samples were hot pressed, TiH₂matrices transformed to alpha prime Ti and alpha Ti phase, respectively. It is interpreted that the diffusion of an alpha stabilizer carbon from TiC into the matrix is one of the plausible reasons far such a microstructural difference. The 0.2% offset yield strengths of the hot pressed TiH₂and TiH₂-45 vol.%TiC samples were 1008 and 1446 MPa, respectively, in a static compressive mode (strain rate of 1×$10^{-3}$/s). Dynamic compressive strengths of the samples were 1600 and 2060 MPa, respectively, at a strain rate of 4×10³/s.

• Journal of the Korean Ceramic Society
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• v.28 no.5
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• pp.422-428
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• 1991

The reaction sintering of Al2O3-ZrO2-Nb composite has been investigated using Al2O3, and ZrAl2 powders. Two kinds of specimens, 78.3Al2O3-14.0Nb2O5-7.7ZrAl2 in wt.% (AZN-5) and 72.3Al2O3-13.8Nb2O5-7.5ZrAl2-6.4ZrO2(AZN-10), were prepared. Powder compacts were sintered at various temperatures between 1$600^{\circ}C$ and 1$700^{\circ}C$ for 30 min in Ar. DTA and X-ray analysis have showen that a reaction between Nb2O5 and ZrAl2 started at 149$0^{\circ}C$ to form Al2O3, ZrO2, and Nb. The sintered density increased with the sintering temperature. AZN-10 specimen showed higher density than AZN-5 specimen for almost all the experimental conditions. Al2O3-ZrO2-Nb composite hot pressed after reaction sintering showed higher toughness and lower hardness than hot pressed Al2O3-ZrO2. The crack propagated through many metallic Nb particles which showed plastic deformation, and this is the cause of the increase in toughness of Al2O3-ZrO2-Nb composite over Al2O3-ZrO2.