Raman Analysis of Microscopic Residual Stresses Stored in the Secondary Phase of Sc2O3-Doped Si3N4
Residual stress studies were performed on the intergranular phase of a Sc2O3-doped Si3N4 polycrystal. Sc2O3 additive represents an ideal substance for residual stress analysis because of the intense Raman spectrum of its related silicate (intergranular) phases, which form during sintering. Crystallization of Sc2O3 to Sc2SiO7 during the cooling process after sintering causes a negative volume change at triple pockets of polycrystalline Si3N4 which overlaps mismatches in thermal expansion coefficients between secondary phase and the Si3N4 matrix. Conventional piezo-spectroscopic (PS) stress analyses have usually been limited to measurements in the Si3N4 matrix. In this paper, we show for the first time measurements of residual stresses stored within the secondary phase of Si3N4, using the Raman spectrum of Sc2O3. A change in crack propagation mode from intergranular to transgranular was found after annealing the Sc2O3-doped Si3N4 material, such a difference leading to an embrittlement of the material. Residual stress analysis of the annealed sample revealed that the tensile residual stress stored within the secondary phase was removed, thus explaining why the rising R-curve behavior of the material was substantially suppressed as compared to the as-sintered sample.
Hai-Doo Kim, Hua-Tay Lin and Michael J. Hoffmann
S. Tochino and G. Pezzotti, "Raman Analysis of Microscopic Residual Stresses Stored in the Secondary Phase of Sc2O3-Doped Si3N4 ", Key Engineering Materials, Vol. 287, pp. 427-431, 2005