Growth Stress in SiO 2 Formed by Oxidation of SiC
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Growth Stress in SiO2 Formed by Oxidation of SiC Randall S. Hay Air Force Research Laboratory, Materials and Manufacturing Directorate 2230 10th St, Bldg 655, WPAFB, OH ABSTRACT Growth stresses in amorphous SiO2 scales formed during SiC fiber oxidation were calculated. A numerical method using Deal-Grove oxidation kinetics and shear-stress dependent SiO2 viscosity was used. Initial compressive stresses in SiO2 of ~25 GPa from the 2.2× oxidation volume expansion rapidly relaxes. At >1200°C, viscous flow of amorphous SiO2 further relaxes stress to negligible levels. At 700° - 900°C, axial and hoop stress at the GPa level persist in SiO2 near the SiC-SiO2 interface. Radial expansion of the scale causes hoop stress to become tensile, and axial stresses are driven to tensile values by the Poisson effect. These tensile stresses can be >1 GPa for thick scales formed at lower temperatures on surfaces with high curvature. Approximate analytical expressions for growth stress are discussed. Effects of viscosity variation as well as other assumptions and limitations of the calculation method are discussed. INTRODUCTION Constraint of the 2.2× volume expansion during oxidation of SiC to SiO2 generates very large growth stresses. Microstructural evidence for these stresses exists for crystalline scales. High dislocation densities in crystalline SiO2 near the SiC-SiO2 interface are diagnostic of high shear stresses during growth of new scale.[1-2] Axial cracks form in the outer scale from tensile hoop growth stress.[1, 3] Tensile growth stresses may decrease fiber strengths, which is significant for structural applications. Growth stress has been extensively modeled for silicon oxidation, which has volume expansion similar to SiC oxidation.[4-5] Recent models recognize that flow at high stress is viscoelastic and use the Eyring model for shear-stress dependent viscosity.[6-8] Radial compressive and tensile hoop growth stresses are predicted for oxidation of silicon fibers.[9-10] Axial stresses are generally ignored.[6, 9, 11] For structural fibers this is an important omission, because axial residual stress affects fiber strength. Tensile stress increases silicon oxidation rates,[12-13] and recently this has also been demonstrated for SiC.[14] A method to calculate all the growth stress components anywhere in a SiO2 scale formed by oxidation of SiC fibers is presented. The method involves scale discretization and separate calculation for each increment. Calculations are done at temperatures from 700° to 1300°C for amorphous scales. Growth stresses for SiC fibers with different radii are examined. Assumptions and limitations of the method are discussed. Complementary results and a more thorough description of the background information and method are given elsewhere.[15] METHOD A schematic of SiC fiber oxidation with discretization of SiO2 scale is in figure 1. There are two sources of stress. The first is the elastic constraint of the 2.2× volume expansion accompanying oxidation. The second is the circumferential expansion of old s
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