• Journal of the Korean Ceramic Society
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• v.32 no.12
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• pp.1337-1348
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• 1995

TZP/SUS- and ZT/SUS-functionally gradient materials (FGM) were fabricated by pressureless sintering in Ar-atmosphere. The sintering defects such as warping, frustrum formation, splitting and cracking which originated from shrinkage and sintering behaviors of metal and ceramics different from each other could be controlled by the adjustment with respect to the particle size and phase type of zirconia. The residual stresses generated on the metal and ceramic regions in FGM were characterized with X-ray diffraction method, and relaxed as the thickness and number of compositional gradient layer were increased. The residual stress states in TZP/SUS-FGM have irregular patterns by means of the different sintering behavior and cracking at ceramic-monolith. While in ZT/SUS-FGM, compressive stress is induced on ceramic-monolith by the volume expansion of monoclinic ZrO2 at phase transformation. Also, compressive stress is induced on metal-monolith by the constraint of warping which may be created to the metal direction by the difference of coefficient of thermal expansions. As a consequence, it has been verified that the residual stress generated on FGM is dominantly influenced by the thickness and number of compositional gradient layer, and the sintering defects and residual stress can be controlled by the constraint of the difference of shrinkage and sintering behaviors of each component.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.20 no.4
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• pp.1194-1201
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• 1996

For the case of creep-fatigue interaction, the damage zone developed in front of the growing crack-tip during creep regime is important because it can affect the damage mechanism to be occured by the following fatigue load. These are studied in theis paper through proper consideration of the cavitiy-size dependent sintering stress which is approximated by polynomials. It is shown that the inclination of reversed damage zone size with respect to the applied load parameter can be explained by considering realistic sintering stress distribution. However, the resultant stress field has $r^{1/2+\theta}$ singularity, regardliss of the profile of variable sintering stress, which is the same to that case solved for constant sintering stress.

• Journal of the Korean Ceramic Society
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• v.36 no.1
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• pp.61-68
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• 1999

To analyze several defects and residual stress in sintering of layer structure materials, multiayer materials with TZP/SUS and ZT/SUS, and bilayer materials with porcelain/alumina and porcelain/Y-TZP were fabricated by sintering method. Multilayer materials prepared by pressureless sintering show the sintering defect such as warping, splitting, cracking originated from the difference of sintering shrinkage between each layer, which could be controlled by the adjustment of number and thickness in interlayer. In tape casting, a certain pressure given during sintering relaxed the sintering defects, specially warping. The residual stress in bilayer was examined with Vickers indentation method. A small tensile stress in porcelain/alumina and a large compressive stress in porcelain/Y-TZP were generated on the porcelain interface due to the thermal expansion mismatch, which affected the strength of bilayer materials. As a consequence, the sintering defects of multilayer materials and the residual stresses of bilayer materials were dominantly influenced on material design and starting material constants.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2004.11a
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• pp.408-411
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• 2004

The electrical stability against surge stress of ZPCCY-based varistors were investigated at different sintering times. Sintering time decreased the varistor voltage and nonlinear exponent from 279.6 to 179.1 and from 52.5 to 24.9, respectively. On the contrary, the leakage current and dielectric dissipation factor increased from 1.2 to 9.8 ${\mu}A$ and from 0.0461 to 0.0651 with increase of sintering time. For all varistors, the variation rates of V-I characteristic parameters against surge stress were affected in order of varistor voltage$\rightarrow$nonlnear exponent$\rightarrow$leakage current. On the whole, the electrical stability against surge stress increased with increasing sintering time. Conclusively, it is assumed that the varistor sintered for 2 h exhibited comparatively good characteristics, in view of overall characteristics.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.18 no.7
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• pp.615-621
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• 2005

The electrical stability against surge stress of varistors, which are composed of Zn-Pr-Co-Cr-Y oxide system, were investigated at different sintering times. As sintering time increases, the varistor voltage and nonlinear exponent decreased in the range of $279.6\~179.1$ and $52.5\~24.9$, respectively. On the contrary, the leakage current and dielectric dissipation factor increased in the range of $1.2\~9.8\;{\mu}A$ and 0.0461\~0.0651 with increase of sintering time. For all varistors, the variation rates of V-I characteristic parameters against surge stress were more strongly affected in order of varistor voltage ${\rightarrow}nonlnear$ $exponent{\rightarrow}leakage$ current. On the whole, the electrical stability against surge stress increased with increasing sintering time. Conclusively, it is assumed that the varistors sintered for 2 h exhibited comparatively good characteristics, in view of overall characteristics.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.10
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• pp.1571-1579
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• 1997

The development of a new material which should be continuously use under severe environment of very high temperature has been urgently requested. For the development of such super-heat resistant materials, the main problem is not only to make the superior thermal barrier properties but also to actively release thermal stress. So, a new concept of functionally graded material(FGM) has been proposed to overcome this problem. A composition and microstructure of FGM are varied continuously from place to place in ways designed to provide it with the maximum function of mitigating the induced thermal stress. So, FGM can be applied in the aerospace, the electronic and the medical field, etc.. In this study, thermal stress analysis of sintering PSZ/NiCrAlY graded material was conducted theoretically using a finite-element program. The temperature condition was sintering temperature assuming a cooling-down process up to room temperature. Fracture damage mechanism was anlayzed by the parameters of residual stress. It could be known that FGM provided with the function of mitigating the induced thermal stress.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.15 no.3
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• pp.244-252
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• 2002

The electrical and dielectric behavior fort DC accelerated aging stress of P $r_{6}$ $O_{11}$-based Zno varistors cnsisting of ZnO-P $r_{6}$ $O_{11}$-CoO-C $r_2$ $O_3$-E $r_2$ $O_3$ were investigated with sintering temperature in the range of 1325~1345$^{\circ}C$. The varistor ceramics with increasing sintering temperature were more densified. A more densified varistors leaded to high stability for DC accelerated aging stress. Furthermore, the stability for DC accelerated aging stress was increased with the leakage current and dtan $\delta$/dV decreasing in order of 1325longrightarrow1335longrightarrow1345longrightarrow134$0^{\circ}C$ in sintering temperature. It was found that the stability for DC stress is affected more greatly by the leakage current and dtan $\delta$/dV than the densification. It is considered that the stability of varistors for DC stress can be estimated by considering the factors, such as the densification, leakage current, and dtan $\delta$/dV. As a result, the varistor sintered at 134$0^{\circ}C$ exhibited the highest stability, with %$\Delta$ $V_{lmA}$=-1.54%, %$\Delta$$\alpha$=-2.49%, %$\Delta$ $I_{\ell}$=+240.68%, 5%$\Delta$tan$\delta$=+29.96%.96%.96%.%.

• Proceedings of the Materials Research Society of Korea Conference
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• 1992.05b
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• pp.126-126
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• 1992

Solid and liquid sintering behaviors of mechanically alloyed 75W-25Cu powders have been studied by using a dilatometry technique. The sintering was performed under hydrogen atmosphere of 1 atm with a heating rate of 3 $^{\circ}C$/min. The mechanically alloyed 75W-25Cu powders were prepared by high energy ball milling process under argon atmosphere of 1 atm with alloying times of 0 to 400 h. To compare with the sintering behaviors of mechanically alloyed powders, pure Cu and W powders were also sintered under the above conditions, As the mechanical alloying time increased from 0 to 400 h, the shrinkage behavior of the alloyed powders was enhanced during the sintering, and staring temperature of liquid sintering decreased from 1083 to 1068 $^{\circ}C$. The saturation temperature, above which the shrinkage was completed, of liquid phase sintering decreased from 1248 to 1148 $^{\circ}C$ with increasing mechanical alloying time from 200 to 400 h. The residual stress of the mechanically alloyed powder was measured by X-raydiffractometer. The microstructure of sintered spcimen was observed by optical and scanning electron microscope. From these results, variations of solid and liquid sintering behaviors with mechanical alloying time were discussed in terms of the amount of residual stress and the distribution of W and Cu powders in the mechanically alloyed powder.

• Journal of the Korean Ceramic Society
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• v.31 no.3
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• pp.321-329
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• 1994

Functionally gradient materials(FGM), which have the continuous or stepwise variation in a composition and microstructure, are being noticed as the material that solves problems caused by heterogeneous interface of coating or joining. And these materials also expect new functions occured by gradient composition itself. Therefore, to examine possibility of thermal barrier materials, TZP/Mo·FGM and TZP/Ni·FGM were fabricated by sintering method. As to the sintered specimens, sintering shrinkage, relative density and Vicker's hardness in each composition were examined. The phenomena due to the difference of sintering shrinkage velocity during sintering process and the thermal stress induced through differences of thermal expansion coefficient in FGM were discussed. And the structure changes at interface and microsturcture of FGM were investigated. As a results, the difference of shrinkage between ceramic and metal was about 14% in TZP/Mo and 7% in TZP/Ni. The relative density and hardness were considerably influenced by metal content changes. Owing to unbalance of sintering shrinkage velocity between ceramic and metal, various sintering defects were occured. To control these sintering defects and thermal stress, gradient composition of FGM should be narrow. The microstructure changes of FGM depended on the ceramic or metal volume percents and were analogous to the theoretical design.

• Journal of the Korean Society of Safety
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• v.9 no.3
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• pp.28-33
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• 1994

It is very important to have exact informations on the properties and characteristics of the sintering material as a new material of machine elements. To study the sintering spur gear and the sintering specimen to be consisted of Fe-Cu-C, the constant stress amplitude fatigue test is performed by using an electrohydrolic survo-controlled pulsating tester. Consequently, the S-N curves are obtained and the fatigue strength is compared with flaw depth. Accordingly, this study presents the fatigue strength of sintering spur gears, the critical notch depth of sintering materials and the effects of flaw depth on the bending fatigue strength.