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Keith Legg 847-680-9420
Overview of planned additional work
HCAT Program ReviewLong Beach
April 2001
Keith Legg 847-680-9420
What seems to work best? General coating conditions
Too hard (>1300HV) is bad excessive alloying into Co?
Overheating is bad alloy tempering particle overheating and
carbide dissolution Substrate heating improves
splat flow Full gage vs patch coatings
little or no difference (if anything recent data show more tendency for patch delamination)
Recently optimized coatings at Hill AFB doing very well
Survive 240 ksi at R=-0.5 Almen = 10 compressive
Strongest determinant of compressive stress is particle size
Ideal phase structure not yet known
Prop hub data show excellent results
High compressive residual stress (Almen = -20)
Keith Legg 847-680-9420
Issues to be resolved
Demonstration of coating integrity to 240 ksi, R=-1
Test on realistic sample size
Stress corrosion cracking If we need higher coating
compressive stress, does that worsen SCC?
WC-Co vs WC-CoCr In past WC-Co generally
showed less spalling than WC-CoCr. Recent work by Jerry Schell shows little difference.
Can we get equivalent optimized performance?
Optimization and testing Tests of full-scale pins
and cylinders Tests of high residual
stress and newly optimized coatings
Coating optimization for spalling resistance
Note - HVOF coatings can and should be optimized for desired property
Limited SCC and fatigue tests
to ensure spall-optimized coating does not cause problems elsewhere
Keith Legg 847-680-9420
Compressively stressed coatings
Keith Legg 847-680-9420
Effect of residual compressive stress
-400
-300
-200
-100
0
100
200
300
0 10 20 30 40
Stre
ss (
ksi)
Almen=0Almen=-10Almen=-20
Compressive residual stress decreases crack initiation and propagation
Reduces damaging tensile stress, increases compressive stress
might cause delamination if interface cracks build up
Compressive residual stress decreases crack initiation and propagation
Reduces damaging tensile stress, increases compressive stress
might cause delamination if interface cracks build up
240 max tensile
240 max compressive
180 max tensile
300 max compressive
Keith Legg 847-680-9420
Effect of compressive residual stress
40
-100
0 0.010 0.100
Str
ess (
ksi)
0
Depth (inch)
Substrate tensile
Coating compressive
Almen=-10Note - equal area
Almen=-20
Based on Wigren, Volvo Aerospace - actual numbers very rough
Based on Wigren, Volvo Aerospace - actual numbers very rough
Conclusion:• tensile<<compressive• substrate fatigue and SCC will be a little worse, but effects will be low for reasonably attainable compressive coating stresses
Shot peen:Large compressive stress near surfaceMuch smaller tensile stress deeper
Compressive coating:Large compressive stress in coatingMuch smaller tensile stress subsurface
Compressive coating + shot peen:Large compressive stress in coatingSome reduction of compressive near-surface stressSome increase in tensile stress subsurface
More compressive coating:Larger compressive stress in coatingLarger increase in tensile stress subsurface
More compressive coating + shot peen:Larger compressive stress in coatingMore reduction of compressive near-surface stressLarger increase in tensile stress subsurface
Note:Stresses and depths are rough
Keith Legg 847-680-9420
BIG samples
Keith Legg 847-680-9420
Small samples vs large
1/4” dia samples have non-optimal HVOF, but high current density gives better EHC
Most parts are >1” OD LG inner cylinders usually>2.5”
On small parts coating is larger percentage of total area
coating may carry significant load
especially thick coatings
Grazing incidenceGrazing incidence
Normal incidenceNormal incidence
1/4”
Always near normal incidence
Always near normal incidence
1 - 6”dia
Keith Legg 847-680-9420
Large sample testing
NAVAIR (Eui Lee) Evaluate spalling for full-
size samples Axial stress Stop-frame testing Evaluation of spalling
and delamination thresholds
Metcut (Phil Bretz) Same sample OD/ID,
design somewhat different
Stop-frame testing Evaluation of different
coating parameters based on results of small-sample tests
Evaluation of spalling thresholds
Time scale - ASAPTime scale - ASAP
Keith Legg 847-680-9420
Testing of landing gear cylinders
A-10 cylinder testing - Hill AFB
WC-Co Testing of actual
cylinders with newly-optimized coating done at OO-ALC
Flexure, rather than axial loads (simulates use)
Primary concern is performance of thick repair coatings
Simulated LG piston tests - BF Goodrich
WC-CoCr 5” and 10” dia simulated
LG pistons (not actual parts)
Flexural fatigue tests (not specifically spalling tests)
Time scale - in-progressTime scale - in-progress Time scale - 2 or 3 monthsTime scale - 2 or 3 months
Keith Legg 847-680-9420
Rig testing
Messier-Dowty F-18 E/F NLG Drag Brace
fatigue/endurance test spectrum loading including launch loads
F-18 E/F NLG full scale fatigue test
BF Goodrich Bombardier Dash8-400
MLG full scale fatigue qualification test
spectrum loading
Time scale - in-progressTime scale - in-progress
Time scale - begin shortlyTime scale - begin shortly
Keith Legg 847-680-9420
Process validation for high load regime
Testing of high residual stress coatings Small and large samples
Determination of optimum coating structure/phases Evaluation of “good” and “bad” coatings Definition of optimum coating properties and
deposition methods Process optimization for best coating performance at
high load Process window determination Evaluation of fatigue (a few points to define curve) and
stress corrosion crackingTime scale - begin shortlyTime scale - begin shortly
Keith Legg 847-680-9420
Final outcome expected
Determine whether HVOF can reach performance requirements of worst case - launch and landing for carrier-based aircraft
Based on full-scale testing and rig tests
If HVOF coatings cannot be used for worst case
define spalling limits, if less than fatigue life (designer need)
define where HVOF WC can and cannot be used (depot need)
Establish validity, or otherwise, of small-sample tests
Optimal material and process definition for high-load components
Must have adequate window and be doable by OEMs, vendors, depots
Determine whether we will need different process or material definitions for high and low-load parts
Not possible with EHC