Fracture Strength Properties of Sintered-Si₃N₄ at High Temperatures and Unified Estimation Method with Considering Temperature-Dependence

Abstract

For the purpose to clarify the temperature dependence above 1000°C of fracture strength properties of silicon nitride ceramics, which are expected to apply to a gas turbine from the viewpoint of high resistances to heat, corrosion and wear as well as light weight, fast-fracture, cyclic fatigue and static fatigue tests were carried out in 3-point bending at 1000°C, 1100°C, 1200°C and 1300°C.
On the basis of the unified estimation method, which is proposed here for the evaluation of ceramic strength property, the temperature dependence of fracture strength of sintered Si₃N₄ was investigated in the temperature range from 1000°C to 1300°C. The results obtained are summarized as follows;
(1) The normalized fracture strength data, which were measured by three kinds of high temperature tests, can be commonly expressed by a formula with two statistical parameters (m, σ₀), the crack growth property parameter, n, and the activation energy, Q^*.
(2) The value of n, which is the index in the formula of crack growth rate, decreases with elevating temperature. It becomes more than twice compared with the value below 1000°C.
(3) The temperature-dependence of fracture strength can be expressed well in Arrhenius plots.
(4) The strength follows the Weibull distribution with two parameters. So, both fracture strength and fracture life at an arbitrary temperature, T_θ, within 1000°C-1300°C, can be probabilistically estimated, if the effective volume, V_eff, and the effective hold time, t_eff, are given.