サブマイクロメーターサイズ粉体を用いた SiC のプロセッシングと力学特性へ及ぼすナノメーターサイズ SiC 粉体の混合効果

Abstract

An 800 nm-SiC powder (75 vol%) was mixed with a 30 nm-SiC powder (25 vol%) in a 0.05-0.3 M-Y(NO3)3 solution containing an Al2O3 powder of median size 0.2 μm at pH 5 to distribute homogeneously the sintering additives (Al2O3+ yttrium ions) around the SiC particles. In a 20 vol% SiC suspension, the network structure of negatively charged SiC particles was formed by the heterocoagulation through the positively charged Al2O3 and yttrium ions. Addition of polyacrylic acid increased the dispersibility of colloidal SiC particles and the amount of yttrium ion fixed to the SiC surfaces owing to the electrostatic interaction among negatively charged polymer, yttrium ions and SiC surfaces. The aqueous suspension of the SiC (800 nm SiC 75%-30 nm SiC 25%)-Al2O3 (1.2 vol%)-Y3+ ions (1.2-3.2 vol% Y2O3) system was consolidated by casting in a gypsum mold and formed green compacts of 36-38% theoretical density. These SiC compacts were hot-pressed to relative density 96.8-100% at 1950°C in an Ar atmosphere. The addition of 30 nm-SiC to 800 nm-SiC decreased the grain size and flaw size of the hot-pressed SiC and improved the flexural strength and fracture toughness. The dense SiC with 2.0 μm of average grain size, prepared from the mixed powder system, showed the following excellent mechanical properties: average four-point flexural strength of 812 MPa, fracture toughness of 6.0 MPa•m1/2 and Vickers hardness of 18-20 GPa.