Compressive creep behavior of hot-pressed Si 3N 4 ceramics using alumina and a rare earth solid solution as additives

Abstract

In this work, Si 3N 4 based ceramics were hot-pressed using Al 2O 3 and a solid solution of Y 2O 3 and rare earth oxides (RE 2O 3) produced at DEMAR, FAENQUIL at a cost of approximately 25% of pure Y 2O 3, as additives. Two compositions with 5 and 20 wt.% additive content at a constant ratio of 60 mol.% Al 2O 3 to 40 mol.% RE 2O 3 were investigated. The highly dense Si 3N 4 ceramics were tested for their creep behavior in compression between 1235 and 1300 °C under stresses ranging from 100 to 350 MPa. Scanning electron microscopy and X-ray diffractometry were used to identify the predominantly acting creep mechanism. Higher additive amounts resulted in larger grains of higher aspect ratios and in a decreased anisotropy in the uniaxial hot-pressed ceramic materials. The compressive creep behavior of the materials depends on the composition and amount of the intergranular phase. While higher amounts of additives resulted in higher creep rates, ε ˙ , and higher stress exponents, the activation energy Q ss, has been inferior for samples with lower additive contents, n. This observation is attributed to the higher SiO 2 content due to the higher amount of Si 3N 4 in the starting powders, resulting in liquid phase formation at lower temperatures. Grain sliding has been identified to be the predominant mechanism responsible for creep deformation of these ceramics.