• 한국해양공학회지
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• 제25권3호
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• pp.28-33
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• 2011

The thermal shock properties of SiC materials were investigated for high temperature applications. In particular, the effect of thermal shock temperature on the flexural strength of SiC materials was evaluated, in conjunction with a detailed analysis of their microstructures. The efficiency of a nondestructive technique using ultrasonic waves was also examined for the characterization of SiC materials suffering from a cyclic thermal shock history. SiC materials were fabricated by a liquid phase sintering process (LPS) associated with hot pressing, using a commercial submicron SiC powder. In the materials, a complex mixture of $Al_2O_3$ and $Y_2O_3$ powders was used as a sintering additive for the densification of the microstructure. Both the microstructure and mechanical properties of the sintered SiC materials were investigated using SEM, XRD, and a three point bending test. The SiC materials had a high density of about 3.12 Mg/m3 and an excellent flexural strength of about 700 MPa, accompanying the creation of a secondary phase in the microstructure. The SiC materials exhibited a rapid propagation of cracks with an increase in the thermal shock temperature. The flexural strength of the SiC materials was greatly decreased at thermal shock temperatures higher than $700^{\circ}C$, due to the creation of microcracks and their propagation. In addition, the SiC materials had a clear tendency for a variation in the attenuation coefficient in ultrasonic waves with an increase in thermal shock cycles.

• 한국분말재료학회지
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• 제12권6호
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• pp.422-427
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• 2005

The $Al_2O_3$ with various phases were prepared by simple ex-situ hydrolysis and spark plasma sintering (SPS) process of Al powder. The nano bayerite $(\beta-Al(OH)_3)$ phase was derived by hydrolysis of commercial powder of Al with micrometer size, whereas the bohemite (AlO(OH)) phase was obtained by hydrolysis of nano Al powder synthesized by pulsed wire evaporation (PWE) method. Compaction as well as dehydration of both nano bayerite and bohemite was carried out simultaneously by SPS method, which is used to fabricate dense powder compacts with a rapid heating rate of $100^{\circ}C$ per min. under the pressure of 50MPa. After compaction treatment in the temperature ranges from $100^{\circ}C\;to\; 1100^{\circ}C$, the bayerite and bohemite phases change into various alumina phases depending on the compaction temperatures. The bayerite shows phase transition of $Al(OH)_3{\to}{\eta}-Al_2O_3{\to}{\theta}-Al_2O_3{\to}\alpha-Al_2O_3$ sequences. On the other hand, the bohemite experiences the phase transition from AlO(OH) to ${\gamma}-Al_2O_3\;at\;350^{\circ}C.$ It shows AlO(OH) ${\gamma}-Al_2O_3{\to}{\delta}-Al_2O_3{\to}{\alpha}-Al_2O_3$ sequences. The ${\gamma}-Al_2O_3$ compacted at $550^{\circ}C$ shows a high surface area $(138m^2/g)$.

• 한국세라믹학회지
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• 제42권8호
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• pp.567-574
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• 2005

Densification of SiC powder with additives of total amount of2, 4, 8 $wt\%$ Al-B-C was carried out by Spark Plasma Sintering (SPS). The unique features of the process are the possibilities of a very fast heating rate and a short holding time to obtain fully dense materials. The heating rate and applied pressure were kept at $100^{\circ}C/min$ and 40 MPa, while the sintering temperature and holding time varied from 1700 - $1800^{\circ}C$ for 10 - 40 min, respectively. The SPS-sintered specimens with different amount of Al-B-C at $1800^{\circ}C$ reached near-theoretical density. The $3C{\rightarrow}6H,\;15R{\rightarrow}4H$ phase transformation of SiC was enhanced by increasing the additive amount. The microstructure of SiC sintered up to $1750^{\circ}C$ consisted of fine equiaxed grains. In contrast, the growth of large elongated grains in small matrix grains was shown in sintered bodies at $1800^{\circ}C$, and the plate-like grains interlocking microstructure had been developed by increasing the holding time at $1800^{\circ}C$. The grain growth rate decreases with increasing amount of Al-B-C in SiC starting powder, however, the both of volume fraction and aspect ratio of large grains in sintered body increased.

• The Journal of Advanced Prosthodontics
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• 제7권4호
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• pp.312-316
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• 2015

PURPOSE. This study aimed to present the clinical applicability of restorations fabricated by a new method, by comparing the bond strength of between ceramic powder with different coefficient of thermal expansion and alloys fabricated by Selective laser sintering (SLS). MATERIALS AND METHODS. Fifty Co-Cr alloy specimens ($25.0{\times}3.0{\times}0.5mm$) were prepared by SLS and fired with the ceramic ($8.0{\times}3.0{\times}0.5mm$) (ISO 9693:1999). For comparison, ceramics with different coefficient of thermal expansion were used. The bond strength was measured by three-point bending testing and surfaces were observed with FE-SEM. Results were analyzed with a one-way ANOVA (${\alpha}$=.05). RESULTS. The mean values of Duceram Kiss ($61.18{\pm}6.86MPa$), Vita VM13 ($60.30{\pm}7.14MPa$), Ceramco 3 ($58.87{\pm}5.33MPa$), Noritake EX-3 ($55.86{\pm}7.53MPa$), and Vintage MP ($55.15{\pm}7.53MPa$) were found. No significant difference was observed between the bond strengths of the various metal-ceramics. The surfaces of the specimens possessed minute gaps between the additive manufactured layers. CONCLUSION. All the five powders have bond strengths higher than the required 25 MPa minimum (ISO 9693); therefore, various powders can be applied to metal structures fabricated by SLS.

• 한국재료학회지
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• 제23권7호
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• pp.353-358
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• 2013

To improve coating ability and the life of the coating, Ti based composite materials with hydroxyapatite(HA) should be developed. The raw materials of Ti-26wt%, Nb-1wt%, and Si with 10wt% HA were mixed for 24 h by a mixing machine and milled for 1 h to 6 h by planetary mechanical ball milling. Ti-26%Nb-1%Si-(10%HA) composites, composed of nontoxic elements, were fabricated successfully by spark plasma sintering(SPS) at $1000^{\circ}C$ under 70MPa. The relative density of the sintered Ti-Nb-Si-HA composites using the 24 h mixed powder, and the 6 h milled powder, was 91% and 97 %, respectively. The effects of HA contents and milling time on microstructure and mechanical properties were investigated by SEM and hardness tester, respectively. The Vickers hardness of the composites increased with increasing milling time and higher HA content. The Young's modulus of the sintered Ti-26%Nb-1%Si-10%HA composite using the 6 h-milled powder was 55.6 GPa, as obtained by compression test. Corrosion resistance of the Ti-26wt%Nb-1wt%Si composite was increased by milling and by the addition of 10wt%HA. Wear resistance was improved with increasing milling time. Biocompatibility of the Ti-Nb-Si alloys was improved by the addition of HA.

• 한국산학기술학회논문지
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• 제9권6호
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• pp.1548-1554
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• 2008

은을 이용한 PMC (precious metal clay; 은점토)는 최근 신속한 조형이 가능하여 공예품의 제작용도로 시장이 확대되고 있다. 손으로 직접 조형되는 특성 때문에 PMC자체가 친환경적이어야 하며 소결온도에 따라 적정한 강도와 수축율이 보장되어야 한다. 이미 알려진 선진사의 PMC의 바인더와 달리, 팜오일, 글리콜릭산, 물을 주성분으로 한 새로운 바인더를 제안하여 새로운 PMC를 제안하고, 이를 이용하여 디스크형태의 시편을 성형하여 $700{\sim}900^{\circ}C$ ($50^{\circ}C$간격)구간에서 15분간 소결하여 이들의 경도, 무게변화, 수축률, 소결밀도, 미세구조변화 특성을 선진사의 PMC의 특성과 비교하였다. 제안된 PMC는 선진사의 것과 물성을 비교하여 조형용으로 사용 가능하다고 판단되었으며 $850^{\circ}C$-15분 조건이 조형물 제작에 유리한 소결 조건임을 확인하였다.

• 대한금속재료학회지
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• 제50권1호
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• pp.34-38
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• 2012

Titanium has good deformability, high hardness, high biocompatibility, excellent corrosion resistance and low density. Due to these attractive properties, it has been used in many industrial applications. Dense nanostructured Ti was sintered from mechanically activated Ti and $TiH_2$ powders by high frequency induction heating under pressure of 80 MPa. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition of grain growth. $TiH_2$ powder was decomposed to Ti during sintering. The hardness of Ti increased and the average grain size of Ti decreased with increasing milling time. The average grain sizes of Ti samples sintered from Ti and $TiH_2$ powder milled for 5 hrs were about 26 nm, 44 nm, respectively. The hardness of Ti sintered from Ti and $TiH_2$ powder milled for 5 h was $504kg/mm^2$ and $567kg/mm^2$, respectively.

• 한국재료학회지
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• 제28권11호
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• pp.653-658
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• 2018

$ZrO_2$ is a candidate material for hip and knee joint replacements because of its excellent combination of biocompatibility, corrosion resistance and low density. However, the drawback of pure $ZrO_2$ is a low fracture toughness at room temperature. One of the most obvious tactics to cope with this problem is to fabricate a nanostructured composite material. Nanomaterials can be produced with improved mechanical properties(hardness and fracture toughness). The high-frequency induction heated sintering method takes advantage of simultaneously applying induced current and mechanical pressure during sintering. As a result, nanostructured materials can be achieved within very short time. In this study, W and $ZrO_2$ nanopowders are mechanochemically synthesized from $WO_3$ and Zr powders according to the reaction($WO_3+3/2Zr{\rightarrow}W+3/2ZrO_2$). The milled powders are then sintered using high-frequency induction heating within two minutes under the uniaxial pressure of 80MPa. The average fracture toughness and hardness of the nanostructured W-3/2 $ZrO_2$ composite sintered at $1300^{\circ}C$ are $540kg/mm^2$ and $5MPa{\cdot}m^{1/2}$, respectively. The fracture toughness of the composite is higher than that of monolithic $ZrO_2$. The phase and microstructure of the composite is also investigated by XRD and FE-SEM.

• 대한금속재료학회지
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• 제49권8호
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• pp.614-618
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• 2011

Nanopowders of MgO, $Al_2O_3$ and $SiO_2$ were made by high energy ball milling. The rapid sintering of nanostructured $MgAl_2O_4-Mg_2SiO_4$ composites was investigated by a high-frequency induction heating sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition of grain growth. Nanocrystalline materials have received much attention as advanced engineering materials with improved physical and mechanical properties. As nanomaterials possess high strength, high hardness, excellent ductility and toughness, undoubtedly, more attention has been paid for the application of nanomaterials. Highly dense nanostructured $MgAl_2O_4-Mg_2SiO_4$ composites were produced with simultaneous application of 80MPa pressure and induced output current of total power capacity (15 kW) within 2min. The sintering behavior, gain size and mechanical properties of $MgAl_2O_4-Mg_2SiO_4$ composites were investigated.

• Elastomers and Composites
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• 제58권1호
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• pp.44-56
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• 2023

Additive manufacturing (AM) or three-dimensional (3D) printing of metals has been drawing significant attention due to its reliability, usefulness, and low cost with rapid prototyping. Among the various AM technologies, fused deposition modeling (FDM) or fused filament fabrication is receiving much interest because of its simple manufacturing processing, low material waste, and cost-effective equipment. FDM technology uses metal-filled polymer filaments for 3D printing, followed by debinding and sintering to fabricate complex metal parts. An efficient binder is essential for producing polymer filaments and the thermal post-processing of printed objects. This study involved an in-depth investigation of and a fabrication route for a novel multi-component binder system with steel alloy powder (45 vol.%) ranging from filament fabrication and 3D printing to debinding and sintering. The binder system consisted of polyvinyl pyrrolidone (PVP) as a binder and thermoplastic polyurethane (TPU) and polylactic acid (PLA) as a carrier. The PVP binder held the metal components tightly by maintaining their stoichiometry, and the TPU and PLA in the ratio of 9:1 provided flexibility, stiffness, and strength to the filament for 3D printing. The efficacy of the binder system was examined by fabricating 3D-printed cubic structures. The results revealed that the thermal debinding and sintering processes effectively removed the binder/carrier from the cubic structures, resulting in isotropic shrinkage of approximately 15.8% in all directions. The scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) patterns displayed the microstructure behavior, phase transition, and elemental composition of the 3D cubic structure.