• Journal of the Korean Ceramic Society
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• v.42 no.8 s.279
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• pp.537-541
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• 2005

Silicon nitride is one of the most successful engineering ceramics, owing to a favorable combination of properties, including high strength, high hardness, low thermal expansion coefficient, and high fracture toughness. However, the impact damage behavior of $Si_3N_4$ ceramics has not been widely characterized. In this study, sphere and explosive indentations were used to characterize the static and dynamic damage behavior of $Si_3N_4$ ceramics with different microstructures. Three grades of $Si_3N_4$ with different grain size and shape, fine-equiaxed, medium, and coarse-elongated, were prepared. In order to observe the subsurface damaged zone, a bonded-interface technique was adopted. Subsurface damage evolution of the specimens was then characterized extensively using optical and electron microscopy. It was found that the damage response depends strongly on the microstructure of the ceramics, particularly on the glassy grain boundary phase. In the case of static indentation, examination of subsurface damage revealed competition between brittle and ductile damage modes. In contrast to static indentation results, dynamic indentation induces a massive subsurface yield zone that contains severe micro-failures. In this study, it is suggested that the weak glassy grain boundary phase plays an important role in the resistance to dynamic fracture.

• Journal of the Korean Ceramic Society
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• v.32 no.6
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• pp.726-734
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• 1995

Pb(Zr0.53Ti0.47)O3 (PZT) ceramics having different microstructures were fabricated at low temperatures using calcined PZT powders with addition of excess PbO powder and/or ball milling. The effects of excess PbO and ball milling time on the microstructure, the sintering characteristic, and the mechanical properties of these ceramics were studied. Fine powders with average particle size of 0.38㎛ could be obtained by ball milling with 2.5 mm Ф zirconia balls for 120 hours. By the addition of 2mol% of excess PbO to these powders, it was possible to obtain well-densitified PZT ceramics at low sintering temperature of 980℃. Densification behavior of PZT was affected by the addition of excess PbO powder, while, grain growth was hardly affected by PbO addition. It was observed that Vicker's hardness decreased and fracture toughness increased with the increasing amount of PbO. At 1mol% excess PbO, it was shown that the minimum values of hardness and maximum fracture toughness were achieved. In addition, with increasing sintering time, the fracture toughness decreased and the hardness increased.

• Journal of the Korean Ceramic Society
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• v.27 no.3
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• pp.329-336
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• 1990

In this investigation, bar-shaped specimens which consisted of three layers are prepared to study the effects of residual compressive stress on the mechanical properties in ZTA. The outer layers contained Al2O3 and unstabilized ZrO2 and the central layer contained Al2O3 and stabilized ZrO2(with 5.10wt% Y2O3). When cooled from the sintering temperature, some of zirconia in the outer layers transformed to the monoclinic form while zirconia in the central layer was retained in the tetragonal form. The transformation which induces to dilatational expansion led to the estabilishmenet of compressive stress in the outer layers and balances tensile stress in the central layer. Decrease of outer layer thickness(for a fixed total thickness)increases residual compressive stress. Because of residual compressive stress in the outer layers, the fracture toughness of outer layers of 3-layers composite is 10.21 Mpam1/2, which is increased to 25% above in comparison with 1-layer specimens in ZTA. Also, the 3-layers composite is believed to exhibite greater fracture resistance in contact damage environment from thermal shock test.

• Journal of the Korean Ceramic Society
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• v.33 no.5
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• pp.602-615
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• 1996

The purpose of the present study is to investigate the microstructural effect on the R-curve behavior in three aluminas with different grain size distributions by analyzing the bridging stress distribution. The crack opening displacement (COD) according to the distance behind the stationary crack tip was measured using an in situ SEM fracture method. The measured COD values in the fine-grained alumina agreed well with Wiederhorn's sollution while they deviated from Wiederhorn's solution in the two coarse-grained aluminas because of the increase of the crack closure due to the grain interface bridging in the crack wake. A numerical fitting procedure was conducted by the introduction of the power-law relation and the current theoretical model together with the measured COD's in order to obtain the bridging stress distribution. The results indicated that the bridging stress function and the R-curve computed by the current model were consistent with those computed by the power-law relation providing a reliable evidence for the bridging stress analysis of the current model. The strain-softening exponent in the power-law relation n, was calculated to be in the range from 2 to 3 and was closely related to the grain size distribution. Thus it was concluded from the current theoretical model that the grain size distribution affected greatly the bridging stress distribution thereby resulting in the quantitative analysis of microfracture of polycrystalline aluminas through correlating the local-fracture-cont-rolling microstructure.

• Journal of the Korean Ceramic Society
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• v.42 no.2 s.273
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• pp.140-144
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• 2005

The microstructures and mechanical properties of Tetragonal Zirconia Polycrystals (TZP) sintered bodies, which made by pressureless and spark plasma sintering techniques, were investigated using XRD, SEM, and TEM techniques. In the spark plasma sintered samples, the TZP grains were equiaxed type including many sub-grain boundaries regardless of sintering conditions. The biaxial strength of TZP having an average of 80 nm grains in diameter was high in value with 1025 MPa, but fracture toughness showed a low value due to the absence of a fracture toughening mechanism such as transformation toughening. In the Pressureless Sintered (PLSed) samples, the grain size of TZP was strongly dependent on the sintering temperature; i.e., it gradually increased as the sintering temperature increased. The value of fracture toughness increased as the grain size increased by the stress-induced phase transformation and Borne crack deflection.

• Journal of the Korean Ceramic Society
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• v.30 no.1
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• pp.62-68
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• 1993

Densification of alumina powder, grain size and fracture toughness of sintered body by hot pressing after cold compaction were investigated and compared to traditional hot pressing process (without cold cyclic compaction). To achieve a higher densification and to reduce the hot pressing time, hot pressing after cold cyclic compaction was more efficient compared to traditional hot pressing. This phenomenon resulted from the increment of packing densityby the acceleration fo rearrangement of powders under cold cyclic compaction. The grain size of sintered body was only dependent on relative density, and densification during hot pressing was governed by thelattice diffusion. Comprisons of grain size, densification mechanism and fracture toughness resulted from hot pressing after/without cold cyclic compaction showed that a low cyclic pressure may not effect on the fragmentation of alumina powders.

• Journal of the Korean Ceramic Society
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• v.32 no.10
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• pp.1194-1202
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• 1995

The influences of TiC additions to the α-SiC on microstructural, mechanical, and electrical properties were investigated. Electrical discharge machinability of SiC-TiC composites was also studied. Samples were prepared by adding 30, 45, 60 wt.% TiC particles as a second phase to a SiC matrix. Sintering of SiC-TiC composites was done by hot pressing under a vacuum atmospehre from 1000 to 2000℃ with a pressure of 32 MPa and held for 90 minutes at 2000℃. Samples obtained by hot pressing were fully dense with the relative densities over 99% except 60wt.% TiC samples. Flexural strength and fracture toughness of the samples were increased with the TiC content. In case of SiC samples containing 45 wt.% TiC, the fracture toughness showed 90% increase compared to that of monolithic SiC sample. The crack propagation and crack deflection were observed with a SEM for etched samples after Vicker's indentation. The electrical resistivities of SiC-TiC composites were measured utilizing the four-point probe. The electrical dischage machining of composites was also conducted to evaluate the machinability.

• Journal of the Korean Ceramic Society
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• v.34 no.8
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• pp.877-885
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• 1997

The dependence of strength, crack growth, fracture mode and degree of domain rearrangement of PZT ceramics on poling strength were studied. The PZT [(Pb0.94Sr0.06)(Zr0.46Ti0.54)O3+Nb(trace)] specimens were poled at 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 kv/mm, and the strength of the specimens was measured by 3 point flexure system. The bending strength of the specimen decreased in different modes according to the bending directions; xz, zx and yz plane direction with x axis of the poling direction in Cartesian coordinate system. The strength differences between the directions increased as the poling strength increased. The fracture mode transferred to intergranular fracture mode from transgranular one as the poling strength increased. The mechanical breakdown occurred when the poling strength higher than 3 kV/mm was applied to the specimen. It was observed that the crack length increased in the normal direction to the poling direction, however, decreased in the parallel direction to the poling direction when the poled PZT specimen was indented by the Vickers indenter. However, the crack produced by indentation continuously was continuously increased little by little after indentation on the specimen. The domain rearrangement occurred as the poling strength increased and the domains were rearranged more effectively when the electric field was continuously increased little by little.

• Journal of the Korean Ceramic Society
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• v.35 no.1
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• pp.66-78
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• 1998

the microstructure and mechanical properties of seeded and ball-milled dry gels prepared by boehmite sol-gel method were discussed. The densification of seeded gels was improved with increasing seed content namely the number of seed The number of seed was 1.09, 3.35, 5.72${\times}$1012/cm3 boehmite when seeded with 1, 3, 5wt% respectively The ball milled gel contained about 0.5wt% seed and the number of seed was 4.72${\times}$1012/cm3 boehmite. The sintered density of 5wt% seeded gel was below 80% when sintered at 1300$^{\circ}C$ for 1h. On the other hand that of ball milled gel was very improved and reached to 97% In the case of 3wt% seeded gels the density over 97% was attained when sintered at 1500$^{\circ}C$for 1h. and the grain of the sintered body was several micrometers in size. However the sintered body of ball milled gel showed grain size of submicrometer when sintered at 1300$^{\circ}C$ for 1h. And this specimen showed highest harness value of 1900kg/mm2 The fracture toughness increased with increasing sintering temperature. The sintered body of ball milled gel showed the largest grain size and the highest fracture toughness without regard to sint-ering temperature.

• Journal of the Korean Ceramic Society
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• v.36 no.1
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• pp.15-20
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• 1999

Elevated temperature static fatigue behavior of silicon has been investigated by stress intensity/life test method. Static fatigue crack growth rate increase with the increase of temperature. Such tendency is found to be mainly related to the decrease of fracture toughness with the increase of temperature. That is, when static fatigue crack growth rate, da/dt is expressed by da/dt=AK1m, a constant A is a function of fracture toughness Kc and the exponent m is a constant which is independent of temperature or Kc. However, in the case of high temperature that glass phase is softened, the crack growth rate is found to be deviated from the above relation. This reason is, thus, discussed.