Static fatigue of Hi‐Nicalon™‐S fiber at elevated temperature in air, steam, and silicic acid‐saturated steam

Abstract

A facility for testing SiC fiber tows in static fatigue and creep at elevated temperatures in air and steam was developed. Static fatigue of Hi‐Nicalon™‐S fibers was investigated at 800°C‐1100°C at applied stresses between 115 and 1250 MPa in air, in Si(OH)4(g)‐saturated steam, and in unsaturated steam. Fibers tested in Si(OH)4(g)‐saturated steam and in air had silica scales throughout the test sections, but those tested in unsaturated steam did not develop scales near the steam injection point. Fiber lifetimes in static fatigue were shortest in unsaturated steam, intermediate in Si(OH)4(g)‐saturated steam, and longest in air. Failure strains did not exceed 0.3%. Steady‐state strain rates and static fatigue lifetimes are modelled empirically by the Monkman‐Grant relationship. Failure mechanisms are discussed.

Subcritical crack growth models for static fatigue of Hi‐NicalonTM‐S SiC fiber in air and steam

“…Similar to strain-rate stress exponents (n), the m values are larger and the Q f values lower than those calculated previously from data that did not include 700°C measurements. 85 The scatter for data at 700°C and 800°C is large for both air and steam experiments (Figure 9). In steam the trends for 700°C, 800°C, and 900°C data are hard to discern.…”

“…However, there was no evidence that Hi-Nicalon TM -S failure times (t f ) or steady-state strain rates (ἑ) were independent of the applied stress (σ A ). 85 It is also unlikely that 12-µm diameter fibers contain cracks long enough to be growth rate limited by species transport. For these reasons, stage 2 growth is not considered.…”

“…There was some crystallization of SiO 2 scales during static fatigue at higher temperatures, but most scales were either glass or spent the vast majority of their oxidation time growing as glass. 85 The Hi-Nicalon TM -S fiber oxidation rates have been measured in both air and Si(OH) 4 saturated steam. 63,78 The Deal-Grove expression for SiO 2 thickness for glass scale (x) is 63,78,85,117 B o is the Deal-Grove pre-exponential factor, Q ox is an oxidation activation energy, t is oxidation time and t o is an initial time for pre-existing scale, and RT has the usual meaning.…”

“…Tow failure in unsaturated steam always occurred opposite to the steam injection port where active oxidation caused high rates of SiC recession with no formation of passivating SiO 2 scale. 85 For experiments in Si(OH) 4 saturated steam and air, passivating scales were always present and tow failure points were randomly distributed through the tow hotsections. Empirical relationships between failure times (t f ), steady-state strain rates (ἑ), and initial applied stress (σ A ) were determined.…”

Fatigue of a SiC/SiC ceramic composite with an ytterbium‐disilicate environmental barrier coating at elevated temperature*

“…Oxidation of SiC in steam has been studied extensively 7‐14 . Numerous research efforts were concerned with the effects of oxidation on properties of silicon carbide constituents in CMCs 7,8,12‐16 . At elevated temperature SiC/SiC composites develop dense SiO 2 scale that offers protection from oxidation.…”