Oxidation kinetics strength of Hi‐Nicalon^TM‐S SiC fiber after oxidation in dry and wet air

Abstract

Hi‐Nicalon™‐S SiC fiber strengths and Weibull moduli were measured after oxidation for up to 100 hours between 700°C and 1400°C in wet and dry air. SiO2 scale thickness and crystallization extent were measured by TEM. The effect of furnace environment on trace element levels in the SiO2 scales was characterized by secondary ion mass spectroscopy. Crystallization kinetics and Deal‐Grove oxidation kinetics for glass and crystalline scale, and the transition between them, were modeled and determined. Crystallization retards oxidation kinetics, and scale that formed in the crystalline state was heavily deformed by the growth stress accompanying SiC oxidation volume expansion. Glass scales formed in dry air slightly increased fiber strength. Glass scales formed in wet air did not increase strength, and in some cases significantly decreased strength. Scales more than 200 nm thick were usually partially or completely crystallized, which degraded fiber strength. Contamination of scales by trace impurities such as Al and Ca during heat treatment inhibited crystallization. The oxidation kinetics and the strengths of oxidized Hi‐Nicalon™‐S fibers are compared with previous studies on SiC fibers, bulk SiC, and single‐crystal SiC. Empirical relationships between oxidation temperature, time, scale thickness, and strength are determined and discussed.

Silicon carbide fiber oxidation behavior in the presence of boron nitride

“…The results reported for the parabolic oxidation of Hi‐Nicalon™‐ Type S fibers and CNTD‐SiC have a higher reported E a. Hay and Chater demonstrated the correlation of the activation energy values with the crystallization of thermally grown SiO 2 scale, particularly at high temperatures (≥ 1300°C).…”

“…The Sylramic rate constants calculated from oxide thickness in this study are greater than rate constants reported by others in literature for CVD‐SiC materials by factors of 4.6 for 1200°C exposures and 3.4 for 1300°C exposures, also reported in Table . An extensive comparison parabolic oxidation rate constants determined from oxide thickness measurements for Hi‐Nicalon™ ‐S fibers, nonstoichiometric SiC fibers, bulk SiC oxidized at temperatures greater than exposure temperatures conducted in this study, and hexagonal polytypes of single crystal SiC is provided in a study by Hay and Chater . The activation energies, E a , for linear and parabolic oxidation mechanisms from this study and select literature values are compared in Table .…”

Crystallization kinetics for SiO2 formed during SiC fiber oxidation in steam

“…This is a concern for long‐term CMC environmental durability. Current applications for SiC–SiC CMCs are turbine shrouds for the Boeing 737 Max and the Airbus A320 . These SiC–SiC CMCs use the Hi‐Nicalon™‐S SiC fiber, which has properties that are described in detail elsewhere …”

Damage evolution in SiC/SiC unidirectional composites by X‐ray tomography

“…Composite failure is completely dependent on fiber failure and ceramic fiber strengths are highly stochastics. There have been studies done on the testing of these fibers and characteristic strengths of 2.5‐4.5 have been reported . The increase in stress on the fibers results in fibers breaking near a matrix crack.…”