Grain-boundary viscosity of polycrystalline silicon carbides

Internal friction experiments were conducted on three SiC polycrystalline m aterials with different microstructural characteristics. Characterizations of grain-boundary structures were performed by high-resolution electron mic roscopy (HREM), Observations revealed a common glass-film structure at grai n boundaries of two SIC materials, which contained different amounts of SiO 2 glass. Additional segregation of residual graphite and SiO2 glass was fou nd at triple pockets, whose size was strongly dependent on the amount of Si O2 in the material. The grain boundaries of a third material, processed wit h B and C addition, were typically directly bonded without any residual gla ss phase. Internal friction data of the three MC materials were collected u p to approximate to 2200 degrees C. The damping curves as a function of tem perature of the SiO2-bonded materials revealed the presence of a relaxation peak, arising from grain-boundary sliding, superimposed on an exponential- like background, In the directly bonded SiC material, only the exponential background could be detected. The absence of a relaxation peak was related to the glass-free grain-boundary structure of this polycrystal, which inhib ited sliding. Frequency-shift analysis of the internal friction peak in the SiO2-containing materials enabled the determination of the intergranular f ilm viscosity as a function of temperature.