Interactions in the SiC powder–polyacrylic acid (PAA, dispersant)–Y3+ ion (sintering additive) system were investigated in the pH range from 2 to 6. The amount of Y3+ ions adsorbed on SiC particles increased with an increase of pH because of the electrostatic attraction between the negatively charged SiC surface and Y3+ ions. On the other hand, the amount of PAA adsorbed on SiC particles decreased with increasing pH because of the electrostatic repulsion between the negatively charged SiC surface and dissociated PAA. The addition of PAA to the SiC suspension with Y3+ ions increased the amount of Y3+ ions fixed to SiC particles through the strong interaction between Y3+ ions and PAA adsorbed on SiC particles. The above‐described interactions in the SiC–PAA–Y3+ ions system were closely related to the coagulation of SiC particles and the rheology of SiC suspensions. The coagulation of SiC particles through the adsorbed Y3+ ions decreased the specific surface area of SiC powder after calcination in an argon atmosphere. The addition of PAA to the SiC suspensions with Y3+ ions kept the SiC particles separate during calcination, i.e., the PAA addition contributed to enhancement of the driving force of sintering (no decrease of specific surface area) and to control of the amount of Y3+ ions uniformly fixed to the SiC surface.
A SiC powder of median size 0.8 mm was mixed with polyacrylic acid PAA, dispersant in a 0.3 moll-RNO 3 3 solution RࢼYb, Y, Gd, Sm, Nd and La at pH 5 to adsorb uniformly the sintering additive R 3ࢪ ion on the SiC surface. The aqueous 30 volಚ SiC suspension with 0.52 massಚ PAA and 1.50 massಚ R 2 O 3 as RNO 3 3 , relative to SiC, was consolidated by filtration through a gypsum mold to form green compacts of 50-52ಚ of theoretical density. The consolidated green compacts were densified with grain growth to 76-99ಚ relative density by hot-pressing under a pressure of 39 MPa at 1950c C for 2 h in an Ar flow. The addition of R 2 O 3 of smaller R 3ࢪion was effective to enhance the sinterability of SiC and also to achieve smaller grain size of SiC. This result was discussed based on the additional experiment result on the chemical interaction between SiC compact and the SiO 2 -R 2 O 3 liquid. The average flexural strength and Weibull modulus of dense SiC were 612 MPa and 5.1, 719 MPa and 6.7, and 731 MPa and 9.8 for Gd 2 O 3 , Y 2 O 3 and Yb 2 O 3 addition, respectively.
Liquid phase sintering based on the dissolution-precipitation mechanism was applied to densify a 0.8 micrometer SiC powder with alumina (1.2 vol%)-yttria (0.9-3.3 vol%) additives. To uniformly distribute the sintering additives around the SiC particles, a heterocoagulated particle network was formed among negatively charged SiC particles, positively charged 0.2 micrometer alumina and yttrium ions in an aqueous suspension at pH 5. Yttrium ions were electrostatically adsorbed on the negatively charged SiC surfaces. The consolidated green compacts were highly sintered to 97-99% of theoretical density by hot-pressing at 1950°C. The mechanical properties (four-point strength, fracture toughness and Weibull modulus) were highly enhanced when a bimodal particle size system of SiC (0.8 micrometer-30 nanometer SiC) was sintered. The maximum strength of 75 vol% 0.8 micrometer SiC-25 vol% 30 nanometer SiC reached 1.1 GPa at room temperature. The fracture toughness was about 6 MPa·m 1/2 and the Weibull modulus was 5.9. When a small amount of SiC precursor polymer was infiltrated in the green compact, the strength and Weibull modulus were further improved.
Liquid phase sintering based on the dissolution-precipitation mechanism was applied to densify a 0.8 μm SiC powder with alumina (1.2 vol%)-yttria (0.9-3.3 vol%) additives. To uniformly distribute the sintering additives around the SiC particles, a heterocoagulated particle network was formed among negatively charged SiC particles, positively charged 0.2 μm alumina and yttrium ions in an aqueous suspension at pH 5. Yttrium ions were electrostatically adsorbed on the negatively charged SiC surfaces. The consolidated green compacts were highly sintered to 97-99 % of theoretical density by hot-pressing at 1950 °C. Four-point strength, fracture toughness and Weibull modulus were highly enhanced when a bimodal particle size system of SiC (75 vol% 0.8 micrometer-25 vol% 30 nanometer SiC) was sintered. The maximum strength reached 1.1 GPa. The fracture toughness was about 6 MPa•m1/2 and the Weibull modulus was 5.9. When a small amount of SiC precursor polymer was infiltrated in the green compact, the strength and Weibull modulus were further improved.
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