Model for SiC fiber strength after oxidation in dry and wet air

Abstract

The strengths of oxidized SiC fibers were modeled from the effects of SiO 2 scale residual stress on fracture. Surface tractions from scale residual stress were determined for SiC surface flaws. The residual stress was the sum of the growth stress from oxidation volume expansion, thermal stress from SiO 2 -SiC thermal expansion mismatch, and stress from phase transformations in crystallized scale. The partial relaxation of tensile residual stress from scale cracking was also calculated. Scale thicknesses were determined using Deal-Grove oxidation kinetics for glass and crystalline scales. Kolmogorov-Johnson-Mehl-Avrami (KJMA) kinetics was used to determine scale crystallization rates. Strengths of fibers with glass and with crystalline scales formed by oxidation in dry and wet air between 600°and 1400°C were modeled. The effects of partially crystallized scales were calculated using Weibull statistical methods. Modeled strengths were compared with measurements. Slight strength increases after glass scale formation, large decreases that accompany scale crystallization, and some differences between dry and wet air oxidation were accurately modeled. This suggests that under some conditions the scale residual stress dominates the changes in strength after SiC fiber oxidation. However, modeled strengths were significantly higher than those measured for some fibers oxidized in wet air, which suggests another degradation mechanism is active for these conditions. Modeling assumptions and implications for SiC fiber strength after oxidation for long times are discussed. K E Y W O R D S fibers, modeling/model, oxidation, silicon carbide, strength Abbreviations: A, Deal-Grove oxidation parameter; a, Radius of a penny-shaped surface crack; minor axis of ellipse shaped flaw; B, Deal-Grove parabolic oxidation parameter (parabolic); B C Deal-Grove parabolic oxidation parameter for crystalline SiO 2 scale; c, Major axis of ellipse shaped flaw; c 1 , , SiC fiber fracture toughness (Critical stress intensity factor); K R , Residual stress intensity factor; k V , Pre-exponential factor for Newtonian viscosity; κ, Cooling rate; Λ, Fraction of original thermal strain energy remaining in a cracked scale; λ, Crack spacing; m, Weibull modulus; n Time growth exponent for scale crystallization; P Pressure across SiC-SiO 2 interface imposed by residual stress; Q V Activation energy for Newtonian SiO 2 viscosity; Q f , Activation energy for KJMA SiO 2 crystallization kinetics; Θ A collection of terms used in the elasticity solution for calculating thermal residual stress ; R, Gas constant; r f , Final SiC fiber radius after oxidation; r i , Initial SiC fiber radius before oxidation; r, Risk of failure; σ AV , Average fiber strength; σ GΖ SiO2 , Axial growth stress in SiO 2 scale; σ Gθ SiO2 Hoop growth stress in SiO 2 scale; σ o , Weibull characteristic fiber strength; σ TZ SiO2 , Axial thermal stress in SiO 2 scale; σ Z SiC Axial stress in SiC fiber; σ Z SiO2 Axial stress in SiO 2 scale ; T, Temperature; T L , Lock-in temperature for thermal...

Crystallization kinetics for SiO2 formed during SiC fiber oxidation in steam

“…Crystallization of the glass to cristobalite and/or tridymite is usually undesirable . The 4.9% volume decrease for the b→α cristobalite phase transformation at ~250‐270°C degrades strength and other material properties . For example, SiO 2 glass crystallization causes spallation of silica‐based thermal protection systems for aerospace vehicles .…”

“…In SiC–SiC ceramic matrix composites (CMCs), the silicon bond coat for rare‐earth disilicate environmental barrier coatings (EBCs) oxidizes to cristobalite instead of SiO 2 glass at 1316°C in water vapor, causing delamination and EBC spallation . Crystallization of glass SiO 2 scales reduces SiC oxidation rates, but also causes tensile thermal stresses that degrade SiC fiber strength . This is a concern for long‐term CMC environmental durability.…”

“…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 …”

In situ Y2Si2O7 coatings on Hi‐Nicalon‐S SiC fibers: Phase formation and fiber strength

“…The strength of the control fiber was far lower, ~1 GPa. This may be due to crystallization of silica after the 1200°C/20 hours of heat treatment …”

“…Hi‐Nicalon‐S (Nippon Carbon Co. Ltd., Tokyo, Japan) SiC fibers were used. The fiber microstructure and properties are described in other publications . Reagent grade yttrium nitrate hexahydrate, phosphoric acid, and citric acid were obtained from Aldrich Chemical Co. (Milwaukee, WI) and used as‐received.…”