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SAE Aerospace Materials
Specifications
 SAE/AMS – 7881 “Tungsten Carbide Cobalt Powder, Agglomerated
and Sintered.”
 SAE/AMS – 7882 “Tungsten Carbide Cobalt Chrome Powder,
Agglomerated and Sintered.”
 SAE/AMS – 2448 “Application of Tungsten Carbide Coatings on
Ultra High Strength Steels, High Velocity Oxygen / Fuel Process.”
 AMS 7881 and 7882 were approved by Aerospace Council for
release. Minor editorial changes to be incorporated by the SAE
editor prior to issuance.
 AMS 2448 has major errors in translation from Committee “B”
approved document, to the SAE formatted version submitted to
Aerospace Council. Currently working with SAE editor to reconcile
the versions for re-submission to Aerospace Council for approval
and release.
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FRACTURE AND RESIDUAL STRESS CHARACTERIZATION OF
TUNGSTEN CARBIDE 17% COBALT THERMAL SPRAY COATINGS
APPLIED TO HIGH STRENGTH STEEL FATIGUE SPECIMENS
ACKNOWLEDGMENTS
University of Florida
Department of Materials Science Engineering
Oak Ridge National Laboratory
X-Ray Diffraction Users Facility
NASA/Kennedy Space Center
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INTRODUCTION
 This Research is intended to
provide an understanding of
the fracture initiation and
propagation characteristics
of the HVOF Sprayed WC17% Co coatings subjected
to axial fatigue stresses on
high strength steel
substrates.
 This research involves
evaluation of coated fatigue
specimens using Scanning
Electron Microscopy and Xray Diffraction, Residual
Stress Analysis.
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Residual stress results from distortion of the
WC Hexagonal Close Packed Structure

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X-RAY DIFFRACTION
 Joint Project with Oak
Ridge National
Laboratory.
 User agreement #2002032 with the High
Temperature Materials
Laboratory.
 “Hands On” work with
three days of training 2
weeks of running
experiments using HTML
equipment, data analysis
software and database,
library.
 Joint publication of
results in technical
journal.
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PROCESS
• X-ray diffraction baselines determined from theta – 2 theta scans.
• WC 17% Co powder – determine peak location of constituents.
• Virgin WC – extract do for strain calculation.
• Sprayed Coating – identify peaks with good resolution.
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X-RAY DIFFRACTION STRESS
MEASUREMENT PROCESS
• Base Metal – Shot-Peened.
• Base Metal – Shot Peened and Grit Blasted.
• Scanned the 117 and 121 degrees 2-theta peaks (211 and
103 reflections) for WC coatings.
• Coated Specimen – As-sprayed.
• Coated Specimen – Sprayed and Finish Ground.
• Fatigue Tested Specimens.
• 110ksi, 150ksi, 220ksi, 1 duplicate each.
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BASE METAL RESULTS
• Baseline measurements indicate that grit blasting the base metal
substrate (shot-peened) increased the compressive stress in the
axial direction, and decreased the compressive stress in the hoop
and radial directions.
Condition
Axial
Hoop
Radial
4340 substrate
Shot-peened
-133 ksi
-137 ksi
-22 ksi
Shot-peened & Al2O3
grit blasted
-154 ksi
-89 ksi
-3.1 ksi
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AFFECT OF GRINDING ON
COATING RESULTS
• Baseline measurements indicate that grinding and polishing the coating
increased the compressive stress in the axial and hoop direction and decreased
the compressive stress in and radial directions.
Condition
Axial
Hoop
Radial
(211) reflection
-85 ksi
-116 ksi
-99 ksi
(103) reflection
-50 ksi
-73 ksi
-76 ksi
(211) reflection
-264 ksi
-224 ksi
-33 ksi
(103) reflection
-193 ksi
-245 ksi
-22 ksi
As-sprayed
WC-17%Co
Ground & Polished
WC-17%Co
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RESIDUAL STRESS FOR THE
FATIGUE SPECIMENS
Condition
Axial
Hoop
Radial
-158 ksi
-146 ksi
-12 ksi
(211) reflection
-280 ksi
-215 ksi
-21 ksi
(103) reflection
-140 ksi
-130 ksi
+27 ksi
(211) reflection
-219 ksi
-125 ksi
-32 ksi
(103) reflection
-100 ksi
-33 ksi
-25 ksi
Fatigue Tested at 110 ksi
(211) reflection
Fatigue Tested at 150 ksi
Fatigue Tested at 220 ksi
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DATA INTERPRETATION FOR THE
110KSI SPECIMEN
 Sample did not fracture, was a run-out at 107 cycles, R=0.1.
 Data for the 103 peak reflection of the 110ksi specimen flawed
due to low intensity of reflecting planes.
 Intensity for the 211 WC peak was sufficient to collect accurate
FWHM data and the d vs. Psi2 was nearly linear with a good fit
indication accurate strain values.
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DATA INTERPRETATION FOR THE
150KSI AND 220KSI SPECIMEN
 d vs. Psi2 plots showed near linear
behavior (accurate stain data).
 Significantly greater intensity for the
211 WC peak vs. the 103 peak
indication more accuracy of the
results.
 Shallow penetration depths of the xray beam at acute angles skew the
near surface values, especially for the
radial stress calculations since the
free surface is defined as ZERO
stress.
 Greater mathematical regression
analysis needed to remove
statistically erroneous data and further
refine the accuracy of the
measurements.
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FRACTOGRAPHY
• JEOL 5900LV was used to examine the coated
specimens before and after fatigue testing.
• Surface morphology, coating /substrate before and
after fatigue testing.
• Fracture surface characterization for both the coating
and substrate.
• Energy Dispersive Spectroscopy was used to evaluate
delaminated coating regions as well as defect at the
fatigue crack origins.
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SURFACE MORPHOLOGY BEFORE FATIGUE TESTING
SHOWING MACHINE LINES AND DEFECTS ON FINISH GROUND
COATING
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SUBSTRATE IMBEDDED GRIT ON THE 110ksi SPECIMEN (UNDER A
DELAMINATED COATING SECTION
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Crack propagation follows intersplat boundaries through the softer
cobalt matrix causing carbide particle decohesion
(110ksi Maximum Applied Stress – Run Out Specimen)
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AT LOWER APPLIED STRESS
LEVELS
 For samples that fractured during the test, below 175ksi maximum
applied stress, the predominant mode of initiation was at a
subsurface defect (4340 tested at 125ksi, R=-1).
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AT HIGHER APPLIED STRESS LEVELS
 For samples that fractured during the test, above 175ksi maximum
applied stress, the predominant mode of initiation was at a surface
defect (4340 tested at 190ksi, R=-1).
 At 165ksi – 175ksi, mixed mode was observed with both surface and
subsurface defect origins.
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Imbedded aluminum oxide grit sites confirmed by XRD, and EDS.
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•
•
•
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SUMMARY
There were reductions in the substrate compressive residual stress
state after abrasive blasting the shot-peened surfaces; however the
overall stress state is still compressive..
Failure mode and crack initiation at lower applied stresses relied on
inherent substrate defects and mechanical properties of the
substrate. Only the reduction in compressive residual stress was
noted as a possible root cause in relative reduction of fatigue life at
these applied stress levels. Substrate defect near the max depth of
the shot-peened layer.
Failure mode and crack initiation at higher applied stresses relied on
interfacial defects from the blasting operation. The reduction in
fatigue life at these applied stress levels is clearly related to the
concentration of defects available for crack initiation.
There were changes in the coating residual stress profile after finish
grinding the coatings as well. Both axial and hoop stress increased
dramatically, whereas radial stress was reduce by half.
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SUMMARY CONTINUED
• Coating fracture behavior was relatively consistent throughout the
applied stress range for the fatigue specimens. Cracks propagated
radially towards the substrate in the cobalt matrix along splat and
WC particle boundaries.
• Substrate fatigue cracks initiated at either subsurface origins, or
interfacial defects (imbedded aluminum oxide grit particles). No
coating crack showed any correlation to base metal fatigue site
initiation.
• At higher applied stresses, coatings delaminated circumferentially
due to the inability to plastically deform with the substrate.
Phenomena was more pronounced for Aermet 100 specimens,
which makes sense due to the increased reduction in area
properties inherent in this alloy.
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CONCLUSIONS AND RECOMMENDATIONS
• The method of substrate surface preparation prior to coating should be
investigated further to reduce the effect on the shot-peened residual
stress.
• Reduction in imbedded grit could significantly decrease the fatigue life
reduction at higher applied stresses.
• Alternative methods.
• Water jet blasting.
• Low pressure, or glancing angle blast process.
• Grit size change.
• Ultrasonic surface cleaning after blasting.
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CONCLUSIONS AND RECOMMENDATIONS
• Low stress grinding and polishing process provide a
beneficial change in residual stress state of applied
coatings. Could affect cracking tendency.
• Increased residual stress on the coating surface should
behave like the substrate shot-peening effects and assist
in hindering crack initiation.
• However, surface defects can mitigate the effects of
residual compressive stress and provide surface crack
initiation sites.
• Ra is a “Gross” measurement of surface profile and other
measurements needed to accurately characterize the
surface morphology.