AFOSR Structural Mechanics Annual Grantee Review

Transcription

1 AFOSR Structural Mechanics Annual Grantee Review Investigation of Fundamental Failure Mechanisms for Lifetime Management of Ceramic Matrix Composites July 25, 2012 Integrity Service Excellence Randall S. Hay, AFRL/RXCC Materials & Manufacturing Directorate WPAFB, OH G. Fair, R. Bouffioux, E. Urban, J. Morrow, M. Wilson, A. Hart, S. Potticary 1

2 Background Fundamental Degradation Mechanisms for Ceramic Matrix Composite (CMC) 1. SiC-SiC CMCs: SiC Fiber Oxidation Strength Degradation 2. Oxide-Oxide CMCs: Environmentally Assisted Subcritical Crack Growth Effects of AlOCl Decomposition Products SiC fiber oxidation Lower fiber strength? But silica scale is in compression, and silica seals surface flaws Fiber strength should be higher Objective: Determine SiC fiber strength as a function of SiO 2 scale thickness, temperature, scale crystallization, time, environment (ph 2 O, po 2 ), stress (s), Generate database Determine degradation and strengthening mechanisms Develop predictive physics based models 2

3 Experiments and Models Experiments: 1. Desize Hi-Nicalon TM -S SiC fiber in boiling water 2. a) Heat-treat in dry air (<10 ppm H 2 O), clean environment, at C, or b) Heat-treat in low po Pa for 1h at C c) Heat-treat in wet air by bubbling dry air through distilled H 2 O, C 3. Measure scale thickness, crystallization, by OM, SEM, and TEM 4. Measure SiC filament strengths with 30 tensile tests 5. Analyze, model data Oxidation Kinetics (T, env) Residual Stress Thermal, Phase Transformation, and Growth Scale Thickness, Microstructure, Flaw Size Fiber Strength Crystallization Kinetics (T, env) 3

4 Hi-Nicalon-S SiC Fiber Microstructure b-sic with 2 3 wt% excess C and O C and O present as intergranular SiOC glass and turbostratic graphite flakes Mild consensus in community that extreme-value clumps of turbostratic graphite flakes are the strengthlimiting flaw (K IC ~ 3 MPa m, several hundred nm in length) Sealing flaws at surface by SiO 2 Reduced effective flaw size, higher strength Expansion or extension of flaws through SiO 2 scale through scale cracking, tensile residual stress Larger effective flaw size, lower strength 4

5 Weibull Characteristic Strength (GPa) (Corrected for SiC Area Reduction) Hi-Nicalon-S SiC Fiber Strength as A Function of SiO 2 Scale Thickness After Oxidation in Dry and Wet Air Same strength trends for wet and dry air oxidation All weak fibers (< 2.75 GPa) have crystallized scales 5

6 TEM of SiO 2 Scales on Oxidized Hi-Nicalon-S 1300 C 1050 C 1300 C 1050 C 1300 C TEM images of SiC oxidation scale after 100 h at a) 700 C, b) 800 C, c) 900 C, d) 1000 C. TEM images of SiC oxidation scale formed after 100 h at 1050 C. The scale in (a) is amorphous; the scale in (b) is crystalline. Dislocation debris from growth stresses driving plastic deformation is present at the SiC-SiO 2 interface in (b). TEM images showing oxide scale formed after 1 h at 1300 C. Images (a) and (b) show porosity in the middle of the scale, extensive twinning in the outer cristobalite scale, and evidence of intense dislocation plasticity in the inner scale. Images in (c) and (d) show healed growth cracks, with relict porosity. 6

7 Tridymite nucleates at SiO 2 surface and grows much faster laterally than through the scale thickness Tridymite Scale initially forms with some carbon in the silica, not pure SiO 2. SiOC has lower thermal residual stress, higher growth stress (higher viscosity). 7

8 Oxidation and Crystallization Kinetics Comparison for Wet and Dry Air Very sensitive to impurities & environment Dry Air Wet Air (x=.03) A o m m Q A 110 kj/mol 108 kj/mol B o m 2 /s m 2 /s Q B 248 kj/mol 249 kj/mol Q B/A 138 kj/mol 141 kj/mol dx/dt = B/(A+x) x 2 = B t (Parabolic) Dry Air Wet Air (x=.03) K o Q 514 kj/mol 487 kj/mol n D growth from site-saturated nucleation f = 1 exp[-kt n ] K = K o exp[-q/rt] 8

9 Thermal residual stress differences between crystalline and amorphous SiO 2 and SiC ~300 MPa compressive thermal stress for amorphous SiO 2 on SiC with ΔT of 1000 C. Scale area% is small and modulus much lower than SiC requires much larger SiC tensile stress (De fixed by large SiC volume) to make SiO 2 stress tensile. Tensile thermal stress >>300 MPa after crystallization. SiO 2 CTE: C -1 SiC CTE: C -1 SiO x C y CTE: 1.5, 3.1, 3.2, C -1 Growth stresses are potentially much larger than thermal stresses Oxidation rate and scale viscosity change growth stress for wet air oxidation 9

10 Ln(Ln(1/(1-x))) Weibull Plots for SiC Fiber Strength Weibull modulus is lowest for partially crystallized fiber Ln (Failure Stress) 10

11 Growth Stress Effects during SiC Oxidation High dislocation concentration High shear stress during growth of new cristobalite. Cracks form in scale during oxidation non-passivating. Cristobalite is stretched and cracks when it becomes part of outer scale as outer radius increases. Externally applied stress affects oxidation rates of silicon; recently confirmed for SiC fibers Geometry affects growth stress Growth stress affects residual stress, oxidation/crystallization kinetics and scale morphology Oxidation/crystallization kinetics, residual stress, and scale morphology affect fiber strength 11

12 Growth Stress Calculation Calculate thickness of new oxide at SiC-SiO 2 interface in time increment Δt from Deal- Grove kinetics s r (i) = volume expansion Δe = 0.3 Calculate principal elastic stresses in 3 layers: i th layer, i-1 th to 1 st layer, and SiC substrate by modified method of Tsui and Clyne (force balance and compatibility equations) Calculate shear stress from elastic stresses in all layers Calculate relaxation of shear stress in all layers using Eyring flow law and Maxwell viscoelastic model Biggest source of Error? Calculate new stresses and strains in all layers after shear stress relaxation Use i = GPa Calculate radial expansion of all layers after relaxation, and calculate added hoop stress from radial expansion Recalculate stresses and strains in all layers from expansion induced hoop stress 12

13 Growth Stress Calculations 13

14 Axial and Hoop Growth Stress at Surface vs. T and Scale Thickness for Flat, 6 mm and 3 mm Radius SiC fibers Constant Stress-State Analytical Approximations for Thick Scales > 1 GPa Tensile s q in < 10 days > 300 MPa Tensile s z in < 10 days High Stress Low Stress 14

15 Growth Stress Calculations for 0.1 & 1 mm Crystalline SiO 2 Scale formed at 1100 C when Scale Crystallized at 10%, 50%, and 90% of Final Thickness Assumptions: B o (crystalline) = 0.1B o (amorphous) h (crystalline) = 10 4 h (amorphous) 15

16 From (T, t, env) can predict SiO 2 scale thickness, crystallization, and residual stress, flaw size, and fiber strength for Hi- Nicalon TM -S, BUT. Axial Growth Stress Oxidation and especially crystallization are VERY sensitive to environment and impurities Silica viscosity, and therefore growth stress, is very sensitive to impurities Study of fundamental mechanisms yields new insight e.g. Amorphous scales have positive effects on strength; crystallized scales do not. Fiber Tensile Strength (GPa) For robust predictive capability, need oxidation and Avrami crystallization kinetics high T viscosity data for other environments & compositions 16

17 Future Work Steam Steam / N 2, O 2 Mixtures Other Combustion Environment Components BN Coated Fibers Effects of Stress on Oxidation / Strength Degradation Tensile stress increases fiber oxidation rate No SiO 2 Scales Form in Steam at 800 C! Fiber strength reduced by ~a factor of 10 17

18 Publications/Accomplishments PUBLICATIONS 1. R. S. Hay, G. E. Fair, R. Bouffioux, E. Urban, J. Morrow, A. Hart, M. Wilson, Relationships between Fiber Strength, Passive oxidation and Scale Crystallization Kinetics of Hi-Nicalon TM -S SiC Fibers, Ceram. Eng. Sci. Proc (2011). 2. R. S. Hay, G. E. Fair, R. Bouffioux, E. Urban, J. Morrow, J. Somerson, A. Hart, M. Wilson, Hi-Nicalon TM -S SiC Fiber Oxidation and Scale Crystallization Kinetics, J. Am. Ceram. Soc. 94 [11] (2011). 3. R. S. Hay, Growth Stress in SiO 2 during Oxidation of SiC Fibers, J. Appl. Phys (2012). 4. R. S. Hay, G. E. Fair, A. Hart, S. Potticary, R. Bouffioux, Kinetics of Passive Oxidation of Hi-Nicalon-S SiC Fibers In Wet Air: Relationships Between SiO 2 Scale Thickness, Crystallization, And Fiber Strength, in press, Ceram. Eng. Sci. Proc. 33 (2012). 5. R. S. Hay, Calculation of Growth Stress Formed by Oxidation of SiC Fibers, in press, Ceram. Eng. Sci. Proc. 33 (2012). ACCOMPLISHMENTS 1. Identified strength governing flaw in oxidized SiC fibers Growth stress, phase transformation, and thermal stress induced cracks formed during and after SiO 2 crystallization. 2. Determined SiC Deal-Grove oxidation and Avrami scale crystallization kinetics in dry air, wet air, and low po 2 for T of C and t up to 100 hours. 3. Measured fiber strength statistics for SiC fibers oxidized in dry air, wet air, and low po 2 for T of C and t up to 100 hours. 4. Developed a rigorous method for calculation of residual (growth + thermal + phase transformation) stress in SiO 2 scales formed during SiC oxidation of cylindrical substrates. 5. Developed physics-based relationships between SiC oxide scale growth and crystallization kinetics residual stress flaw size - fiber strength. 6. R. S. Hay, Growth Stress in SiO 2 Formed by Oxidation of SiC, MRS Symp. Proc. in press (2012). 18

19 Highlight Slide From (T, t, env) can predict SiO 2 scale thickness, crystallization, and residual stress, flaw size, and fiber strength for Hi-Nicalon TM -S, BUT. Oxidation and especially crystallization are VERY sensitive to environment and impurities Silica viscosity, and therefore growth stress, is very sensitive to impurities Study of fundamental mechanisms yields new insight amorphous scales have positive effects on strength; crystallized scales do not. For robust predictive capability, need oxidation and Avrami crystallization kinetics high T viscosity data for other environments & compositions 19

20 Possible Reasons for Outliers Rogue Fibers some crystallization regardless of T and t? Most look like this clean scale, no crystallization But some, maybe 5 10%, look like this. Limited crystallization? Location of filament in tow bundle has some effect on oxidation and extent of crystallization Side 1 of flat tow Side 2 of flat tow 20

21 Growth Stress Calculations Axial and Hoop Growth Stress.01, 0.1 and 1.0 mm scales Flat plates, 6 mm and 3 mm radius SiC fibers 21

22 Assumptions: 1. Oxidation volume expansion is initially dilational. 2. Stresses resulting from constraint of oxidation expansion are relaxed by radial flow of SiO 2 with shear stress-dependent viscosity (Rapidly relaxes to nearly uniaxial expansion). 3. Discretization of oxidation to small increments and rapid relaxation of high shear stresses allows use of linear elasticity. 4. Growth stress effects on oxidation kinetics are not explicitly considered. 5. Oxidation kinetics are described by Deal-Grove model for flat plate. w 2 = B t (Parabolic) Scale thickness t i = (w i 2 + Aw i )/B Dry Air A o m Q A 110 kj/mol B o m 2 /s Q B 248 kj/mol 138 kj/mol Q B/A 22

23 Calculation of Elastic Stress in Each Layer Compatibility Solve 3 compatibility equations for 3 unknowns f, p is, and p i using force balance Force Balance E SiO 2 Δε = σ z SiO 2 i 1 σz SiO 2 i + υsio 2 σ θ SiO 2 i σθ SiO 2 i 1 + σr SiO 2 i σr SiO 2 i 1 [7] E SiO 2 Δε = σ Θ SiO 2 i 1 σθ SiO 2 i + υsio 2 σ z SiO 2 i σz SiO 2 i 1 + σr SiO 2 i σr SiO 2 i 1 [8] 1 E SiO 2 where σ θ SiO 2 (i) υ SiO 2 σ z SiO 2 (i) + σ r SiO 2 (i) = SiC σ z i = Δε = 3 Ω SiO 2 Ω SiC E SiC f π E SiC b 2 i + E SiO 2 b i + w i w i 1 2 b 2 i Si O σ 2 z i = 1 E SiC σ θ SiC (i) υ SiC σ z SiC (i) + σ r SiC (i) [9] 1 [10] SiC + σ z i 1 [11] σ SiC r (i) = σ SiC r (i 1) p is [12] σ SiC θ (i) = σ SiC θ (i 1) p is [13] E SiO 2 f Δe = 0.3 π E SiC b i 2 + E SiO 2 b i + w i 2 b i 2 [14] Si O σ 2 r (i) = p i [15] Si O σ 2 z i 1 = Si O σ 2 p j b i + w i θ i = w i w i 1 π b i 1 + w i 1 w i 2 b 2 i 1 f [16] [17] Tsui, Y. C. & Clyne, T. W. An Analytical Model for Predicting Residual Stresses in Progressively Deposited Coatings Part 2: Cylindrical Geometry. Thin Solid Films 306, (1997). Si O σ 2 r i 1 = 0 [18] 2p Si O is b i 1 2 p i b i 1 + w i b i 1 2 σ 2 θ i 1 = b i 1 + w i 1 2 [19] b i

24 T C Eyring Model for Shear Stress Dependence of Glass Viscosity 1400 Maxwell Viscoelastic Solid Pa s Pa s dτ(t) dt = G τ(t) η(τ) Pa s Pa s t (GPa) Gt τ t = 4kT Coth 1 eη o V c Solution for Shear Stress Tanh V cτ o 4kT Gt = 2τ 2 ccoth 1 eη o Tanh τ o 2τ c 2 24

25 Fiber Strength (GPa) Hi-Nicalon TM -S Strength After Active Oxidation (t = 1 h) 1400 po 2 (Pa) Residual SiO 2 in intergranular SiOC removed as SiO(g), leaves graphite flakes in pores and GBs. 25