Je Hun Lee

Improvement of Grain-Boundary Conduction in 15 mol % Calcia-Stabilized Zirconia by Postsintering Heat-Treatment

The grain-boundary conductivity of a 15 mol % calcia-stabilized zirconia specimen was increased about seven times by postsintering heat-treatment at 1300°C for 10 h. From the change in the crystallinity, morphology, and composition of the intergranular phase by heat-treatment, the de-wetting behavior of the intergranular phase due to crystallization was suggested to decrease the current constriction near the grain boundary. The postsintering heat-treatment was also effective at the high sintering temperature where adding Al 2 O 3 could not scavenge the resistive intergranular phase.

Fabrication of Heat-Resistant Silicon Carbide Ceramics by Controlling Intergranular Phase

The effect of glassy-phase, using AlN and Lu2O3 as sintering additives, on the microstructure and mechanical properties of liquid-phase-sintered, and subsequently annealed SiC ceramics was investigated. The microstructure was strongly influenced by the sintering additive composition, which determines the intergranular phase (IGP). The average thickness of SiC grains increased with increasing the Lu2O3 /(AlN + Lu2O3) ratio, whereas the average aspect ratio decreased with increasing the molar ratio. The homophase and heterophase boundaries of the SiC ceramics were completely crystalline in all specimens. The room temperature (RT) strength decreased with increasing the molar ratio whereas the RT toughness showed a minimum at the molar ratio of 0.6. The best results at RT were obtained when the molar ratio was 0.2. The flexural strength and fracture toughness of the ceramics were >700 MPa and ~6 MPa.m1/2 at RT. The high temperature strength was critically affected by the chemistry, especially the content of Al in the IGP. The best strength at temperatures ³ 1500oC was obtained when the molar ratio was 0.5. Flexural strengths of the ceramics at 1500oC and 1600oC were 610 ± 80 MPa and 540 ± 30 MPa, respectively. The beneficial effect of the new additive compositions (Lu2O3-AlN) on high-temperature strength of SiC ceramics was attributed to the crystallization or removal of IGP and introduction of Al into SiC, i.e., removal or reduction of Al content from the IGP, resulting in an improved refractoriness of the IGP.

Liquid-phase redistribution during sintering of 8 mol% yttria-stabilized zirconia

Abstract The distribution of the intergranular liquid phase during the sintering of 8 mol% Y2O3-stabilized ZrO2 containing 0.5 mol% of SiO2 and Al2O3 was investigated using spatially resolved impedance spectroscopy. The variation in the grain-boundary resistivity as a function of the distance from the specimen surface indicated that the liquid phase coagulates at the specimen center in the initial stages of sintering, and spreads outward upon further sintering. The liquid redistribution was also examined by transmission electron microscopy.

Effect of Sintering Additive Composition on Grain Boundary Structure in Liquid-Phase-Sintered Silicon Carbide

Both the presence and absence of an amorphous intergranular film (IGF) between the SiC grains have previously been reported in liquid-phase-sintered SiC ceramics (LPS-SiC). The dominant factor(s) responsible for the grain boundary structure in LPS-SiC has not been clearly revealed. In the present study, LPS-SiC ceramics containing different compositions of sintering additives were fabricated and characterized with respect to their grain boundary structure, using both scanning and transmission electron microscopy. The results suggest that the sintering additive composition plays a dominant role in the evolution of grain boundary structure in LPS-SiC.

Processing of Nanoceramics and Nanoceramic Composites: New Results

This contribution reports the fabrication of WC composites with 6 wt. % zirconia via spark plasma sintering (SPS) route. The sinterability, microstructure and mechanical properties of the SPS processed samples are compared with those fabricated by presseureless sintering. Fully dense WC composites with nano-ZrO2 reinforcement were obtained after SPS at a lower temperature of 1300 o C (5 min). In contrast, high sintering temperature of 1600-1700 o C is necessary to obtain fully dense WC composites via pressureless sintering. Superior hardness (21-24 GPa) is obtained with the newly developed WCZrO2 materials when compared to that of the WC-Co materials (15-17 GPa). Due to extremely high hardness, the WC–ZrO2 nanocomposites seem to be promising materials for wear applications. Introduction In recent times, nanoceramics and nanoceramic composites have received increased attention among the researchers in the materials science community [1–6]. Ceramic nanocomposites were first discovered in Japan, mainly by K. Niihara claiming that ceramic materials with excellent mechanical properties can be obtained through nanocomposite technology [7]. According to Niihara, nanocomposites are fabricated by dispersion of nanosized particles within micron-sized matrix grains or at the grain boundaries of the matrix, whereas nano-nanocomposites are formed when matrix grains also are in the nanosize scale. Nanocomposite ceramics can exhibit improved mechanical properties, wear resistance, chemical inertness, corrosion resistance and thermal insulating properties [3–6]. The unique properties of nanocrystalline ceramics have opened a wide range of applications including durable ceramic parts for automotive engines, cutting tools etc. The processing of nanoceramics demands the adoption of a specific processing route, which will inhibit grain growth while achieving full densification. This has particularly become feasible with the use of field activated sintering technique (FAST). FAST involves the imposition of an electrical field during sintering. SPS is one of the widely used FAST techniques. A considerable part of recent research on the development of several structural nanocomposites using SPS techniques have been reported in the literature [8-11]. In the present research, the major focus is to develop WC-ZrO2 composites via SPS route. Experimental Procedure Commercial high purity WC powder (average particle size 0.2 μm, H.C Starck, Germany), Co powder, Tosoh grade 3 mol % Y-ZrO2 (TZ-3Y, particle size 27 nm according to supplier) are used in the present work. An amount of 50-75 g powder, with a WC: ZrO2 weight ratio 94:6, was mixed for 24 hours in n-propanol using WC balls and subsequently dried using a rotary evaporator. Similarly, WC-6 wt. % Co powder mixtures were also prepared. The dried powder mixture is sintered inside the SPS chamber under a vacuum of 50-60 mtorr at different temperatures in the range of 1200-1400°C with a Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 2293-2296 doi:10.4028/www.scientific.net/KEM.264-268.2293