Analysis of Residual Stress and Fracture Data For WC-17%Co HVOF Thermal Spray Coatings

1. Analysis of Residual Stress and Fracture Data For WC-17%Co HVOF Thermal Spray Coatings

2. Introduction • The method of evaluation involves XRD analysis of tested and untested specimens. • Baselines were established using “Virgin” powder, coatings, and base metal. • The data correlation is presented for low medium and high stress level fatigue tests. • Joint research with Univ of Florida, Oak Ridge Labs and NASA/KSC X-Ray Path

3. Equipment – Schintag Goniometer With 18 KW X-ray Generator

4. Sample Setup

5. Baseline Coating XRD Optimized on WC peaks With good resolution

6. JCPDS Peak/Plane Identification Selected WC Peaks 211 and 103 Reflecting Planes

7. Residual Stress Scan Procedure • Samples are rotated through psi angles at three fixed phi angles 0o, 45o, 90o to determine the d (Lattice Spacing) vs. sin2 psi relationship. The three axis stress tensor is obtained from the slope and intercepts on the resulting plot at each angle of scan measurement. • This was performed on the following specimens • Base metal peened • Base metal peened and blasted • Base metal peened, blasted and coated • Base Metal peened, blasted, coated and ground • Fatigue tested specimens at low, medium and high stress. • Testing is still on going with Oak Ridge Labs

8. Results to date • Although final analysis of all the data is NOT complete. Initial results show VERY high compressive residual stresses in the tungsten carbide particles along all three dimensions (axial, hoop and radial) • The bulk matrix, tungsten carbide particles, are distorted (stressed) HCP crystal structures with metastable amorphous and cubic cobalt phases. • These metastable phases, shrinkage porosity, splat morphology all contribute to the inability of the coating to form a stable crystallographic structure. • Correlation between residual stress and fracture behavior is also on-going.

9. Stress Values in Mpa Axial Hoop Radial As Sprayed -834 -795 -947 Polished -2261 -1936 -422 Peened -921 -951 -154 Blasted -1060 -611 -21

10. Fractography • SEM analysis indicates that the primary fracture occurs in the ductile cobalt rich phases first because they are easily deformed. • Crack propagation follows rupture through void nucleation, coalescence, and carbide boundary decohesion. • Cleavage across carbides grains results from high shear stress along weak crystallographic facets. W2C grains fracture first as they are far more brittle and cannot deform due to their strained lattice. • At all stress levels observed, fracture occurs parallel to the axis of load as coating shear and therefore will propagate along splat and bond line interfaces, not into the substrate. • Some substrate cracks appear to initiate at imbedded grit sites with residual aluminum oxide detected by XRD, and EDS.

11. WC Hexagonal Close Packed Structure

12. SEM Photo 220 ksi specimen

13. Summary • There were changes in residual stress for blasting the shot-peened surfaces. Characterizing the changes is on-going. • There were changes in the residual stress profile after finish grinding as well. Full characterization of those affects are also ongoing. • Fracture behavior was relatively consistent throughout the applied stress range for the fatigue specimens. Fracture occurred axially by deformation of the ductile cobalt phases followed by cleavage of SOME carbide facets along the crack path. No coating crack showed any correlation to base metal fatigue site initiation,. • Substrate fatigue initiation did correlate to some imbedded aluminum oxide grit locations.