High-temperature creep response of a commercial grade siliconized silicon carbide

Abstract

Creep studies conducted in four-point flexure of a commercial siliconized silicon carbide (Si-SiC, designated as Norton NT230) have been carried out at temperatures of 1300, 1370, and 1410°C in air under selected stress levels. The Si-SiC material investigated contained ∼90% α-SiC, 8% discontinuous free Si, and 2% porosity. In general, the Si-SiC material exhibited very low creep rates (2 to 10 × 10⁻¹⁰ s⁻¹) at temperatures ≤1370°C under applied stress levels of up to 300 MPa. At 1410°C, the melting point of Si, the Si-SiC material still showed relative low creep rates (∼0.8 to 3 × 10⁻⁹ s⁻¹) at stresses below a threshold value of ∼190 MPa. At stresses >190 MPa the Si-SiC material exhibited high creep rates plus a high stress exponent (n = 17) as a result of slow crack growth assisted process that initiated within Si-rich regions. The Si-SiC material, tested at temperature ≤1370°C and below the threshold of 190 MPa at 1410°C, exhibited a stress exponent of one, suggestive of diffusional creep processes. Scanning electron microscopy observations showed very limited creep cavitation at free Si pockets, suggesting the discontinuous Si phase played no or little role in controlling the creep response of the Si-SiC material when it was tested in the creep-controlled regime.