Elevated Temperature Static Fatigue of a Nicalon Fiber-Reinforced SiC Composite

Abstract

ABSTRACTStatic fatigue tests of a Nicalon fiber-reinforced SiC matrix composite were conducted in four-point bending over a temperature range of 425° to 1150°C in air at selected stress levels. The composite consisted of a Nicalon cloth with a 0.3 μm graphite interfacial coating and a Forced Chemical Vapor Infiltration (FCVI) SiC matrix composite; samples were tested with or without a final protective SiC seal coat. The results indicated that the fatigue life of the Nicalon-SiC composite decreased with an increase in either applied stresses or test temperatures. However, the composite exhibited a fatigue limit of ∼ 100 MPa at temperatures ≤ 950°C which decreased to ∼ 70 MPa at 1150°C. Both electron microscopy and thermogravimetric studies suggested that the lifetime of the composites was dictated by the oxidation of graphite interfacial layer at temperatures ≤700°C and by oxidation of graphite coating accompanied by formation of silicate interfacial layer via oxidation of the Nicalon fiber (and the SiC matrix) at temperatures ≥ 950°C. Use of a SiC seal coat effectively retarded the oxidation reactions and increased the lifetime by at least one order of magnitude at 425°C. On the other hand, the SiC seal coat made little (if any) difference in fatigue life at 950 °C.