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April 29, 2003
Landing Gear
Replacement of Chrome Plating with HVOF (High Velocity Oxy-Fuel) applied WC-Co-Cr
coatings on Airbus A380 Landing Gear applications
Roque Panza-Giosa, Chief Material and Process TechnologyGoodrich Landing Gear - Oakville
Canadian Aeronautics and Space Institute 50th AGM and Conference16th Aerospace Structures and Materials Symposium
Montréal 28-30 April 2003
AbstractHard Chrome plating has been used for decades in Landing Gear applications as the primary protection against wear and corrosion. Alternatives to hard Chrome plating are being considered by the aerospace industry, primarily for performance enhancements and environmental acceptability reasons. High Velocity Oxy-fuel (HVOF) applied coatings have matured to the point of offering enhanced performance over Chrome plating, at a comparable price. Among the coatings that can successfully be applied by the HVOF process, WC-16%Co and WC-10%Co-4%Cr boast higher hardness, corrosion and wear resistance than Chrome plating deposits, without the environmental concerns associated with hexavelent Chromium.
Extensive testing of WC-Co-Cr coatings in hydraulic sealing applications has demonstrated that HVOF applied coatings perform better than Chrome plating, but require a finer finish. The corrosion resistance of WC-Co-Cr appears to be somewhat better than that of WC-Co coatings, and either of these HVOF coatings displays significantly better corrosion behaviour than Chrome plating in salt fog environments. Field assessments of landing gear components incorporating HVOF coatings revealed considerably longer wear life in fretting, sliding and rotation wear regimes.
Qualification and validation tests of hydraulic actuators and complete landing gears are underway at Goodrich as part of the Canadian Hard Chrome Alternative Team (CHCAT). A direct comparison between Chrome plating and HVOF coating in bending fatigue is also in progress. This comparison involves 5” and 10” diameter simulated pistons fatigue tested to failure. For each piston diameter, 3 specimens are coated with WC-Co-Cr and 3 specimens are Chrome plated. These full size tests have demonstrated good correlation with laboratory coupon tests, while at the same time they have highlighted a number of major differences in behaviour which are of importance in Landing gear design philosophy.
This paper presents a summary of available test data, provides an update of ongoing tests, and an overview of how the “lessons learned” are being implemented by Goodrich in the Body and Wing landing gears design of the Airbus A380.
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Landing Gear
♦ HVOF PROCESS - PROPERTIES - LIMITATIONS
♦ COATING CHOICES - PROCESS CONTROL
♦ CHROME VS WC-Co/WC-Co-Cr
♦ EFFECT ON FATIGUE
♦ HCAT (Hard Chrome Alternatives Team) INVOLVEMENT� Dash-8 S400 MLG Fatigue� High Cycle Endurance Test of Actuator (seal wear)� Bending Fatigue of Simulated Pistons
♦ HVOF INTEGRITY TETSING
♦ PIN TESTS – aimed at A380
OUTLINE
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Landing Gear
Thermal Spray Coatings
• Plasma Process
• High Velocity Oxyfuel Process(HVOF)
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Landing Gear
Mechanism of HVOF Coating Formation
• Very High Temperatures• Very High Velocities
• High Kinetic Energy (Molten/Semimolten Particles)• Limited Oxidation
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Landing Gear
Properties of HVOF Applied Coatings
• High adhesive strength• Low porosity• Low oxide formation• Excellent for Carbides, Tungsten Carbide coatings• High degree of coating process control• High repeatability (Robotics Control) • Minimum excess required for grinding
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Landing Gear
• Line of sight process• Miniature Plasma and HVOF guns• Extensive development for coating Internal Diameters
Limitations Of HVOF Process
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Landing Gear
Coating Choices
• Tungsten Carbide 17% Cobalt (WC-17Co)• Tungsten Carbide 10% Cobalt 4%Chrome
(WC-10Co-4Cr)• Nickel Aluminides• Chrome Carbides• Triballoys
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Landing Gear
POWDER CONTROL
Method of manufacture (aglomerated,sintered, crashed)ChemistrySizeMorphology
Impurities
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Landing Gear
Process Control• Adhesive Strength 10,000psi• Metallography
• Flexibility/Adhesion• Residual Stress (Almen N)
Porosity <1%
Oxide Content <1%
Hardness >1000 Knoop/Vickers
Thickness ( 0.1 mm/0.004”)
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Landing Gear
Chrome Plating Vs. WC-Co/WC-Co-CrEnvironmental drive
• Hexavalent Cr. is a carcinogen• Chrome baths emit Hex Cr. mist into the air• Level of toxicity higher than Cadmium and Arsenic• Current Permissible Exposure Limit (PEL) 0.1 mg/m3
• Levels are reduced to 0.005 or 0.0005 mg/m3
• Many Chrome plating facilities will close, price will increase, disposal cost will rise
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Landing Gear
Chrome Plating Vs. WC-Co/WC-Co-CrPerformance Drive
Environmentally compliantNon embrittlingHigher hardnessBetter corrosion resistance Better wear resistance (surface critical)Shorter production cyclesMore repeatable process (robotic control)
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Landing Gear
Effect on FatigueLow Cycle Fatigue
HourglassSmooth Gage
Rectangular Kb
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Landing Gear
Effect on Fatigue
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Landing Gear
Effect on Fatigue
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Landing Gear
Effect on Fatigue
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Landing Gear
• Goodrich is using HVOF technology on A380 program, NO Chrome plating
• Developed Process Specifications: Application, Grinding, (Powders)
• Developed Relationship with Suppliers
• Experienced with WC-Co/WC-Co-Cr systems
• Active participant in the HCAT/CHCAT Program(HARD CHROME ALTERNATIVE TEAM)
RISK MANAGEMENT
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Landing GearHCAT
Military CustomersC-HCATOEMs
AirlinesCommercial Customers
Basic process and material development
Limited military flight testing
Landing gear specific HVOF - WC-CoCr
Full scale testing
Landing gear coating performance data
HVOF for overhaul -landing gear
Commercial flight testing
Airline overhaul specs - Commercial
Manufacturers’ specs
Depot maintenance specs - Military
AMCOM
Air Force NAVAIR
Heroux
Goodrich
Messier-Dowty
Lufthansa
Delta United
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Landing Gear
CHCAT Status - Feb. 2003
CONDUCTING THREE TESTS:
1. Fatigue of entire MLG - Dash-8 Series 400.
2. High Cycle (Dithering) wear test on hydraulic actuator.
3. Fatigue of MLG pistons 5” and 10” OD (Cr vs. WC-Co-Cr).
HVOF coating is WC-Co-Cr.
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Landing Gear
• Full Scale Fatigue Qualification of Complete Main Landing Gear Assembly.
CHCAT ACTIVITIES
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Landing Gear
TEST 1: MLG FATIGUE TEST
CURRENT STATUS
14 parts have been HVOF coated (piston, axle, and pins).Components sprayed by Vac Aero and Southwest Aeroservice.Gear has been fatigue test for 2 lives; currently being inspected.
April 29, 2003
Landing Gear
TEST 1: MLG FATIGUE TEST
• CURRENT STATUS• Testing - 10 layers of testing = 5 lives. • Each layer includes 32 blocks of test conditions (load location,
magnitude, direction and number of cycles).• 1 layer of fatigue consists of 30 blocks of ground load conditions
(= 30,000 flight cycles), 1 block of retraction lug test conditions (66,000 gear extend and 33,000 gear retract flight cycles) and 1block of stabilizer brace lock link test conditions (99,000 lockand 33,000 unlock flight cycles).
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Landing Gear
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Landing Gear
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Landing Gear
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Landing Gear
CURRENT STATUS
• Testing considered successful when the gear withstands 5 lives of fatigue loads without initiation of detectable fatigue cracks. 1st life inspection revealed no issues.
• Test has accrued 2.0 lives (40% complete). Some delays with the test rig and changing seals.
• Testing began with elastomeric T-type seals but the seals failed after 330,000 cycles.
• Second type of elastomeric seal failed after 480,000 cycles. New Seal Design has recently been installed.
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Landing Gear
• High Cycle Endurance Qualification Test on Landing Gear Actuator-failure of seals at 50%life-surface finish was 20Ra-surface finish improvedto 3Ra
• Similar experience reported by G-Tweed Shamban, Delta , etc.
CHCAT ACTIVITIES
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Landing Gear
• Purpose: Testing performance of HVOF coating on an aircraft actuator and demonstrate it can withstand constant cycles under endurance stress level for a civil aircraft actuator.
• 3 actuators being tested, 2 chrome plated, 1 WC-Co-Cr coated.
• Chrome finished 8-16 Ra.
• HVOF finished 3 Ra.
TEST 2: HIGH CYCLE WEAR TEST
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Landing Gear
CURRENT STATUS
• Test program is approximately 95% complete.
• To date, scheduled inspections have found the HVOF coating acceptable with no evidence of scratches, cracks or other unacceptable indications.
• No evidence of leakage or wear on the seals.
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Landing Gear
• Full Scale Cycle-to-Failure Fatigue Test on Simulated Landing Gear Pistons, 5” and 10” OD
• Flexural Fatigue, full load reversal to failure
• 3 - 5” and 3 - 10” Cr • 3 - 5” and 3 - 10” HVOF
CHCAT ACTIVITIES
2 x R .060
DETAIL B
(4.00)
4.00
Φ 4.200
2 x R 3.50
R 1.50(30.05)
60.2056.70
41.65
R 4.000SECTION A-A DETAIL B
(NOTCH)
9.1889.183Φ
(.400)
30o
9.359.0159.005Φ
A9.9979.994
Φ
.125
15o
71.7002X 60.70
59.50
A
R 3.265
Lower BearingUpper Bearing Test Piston
Support Beam
Pillow Block
Adapter
Load Cell
Actuator
b b
PinNotch1
22 22
33
SK6093 / SK6094
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Landing Gear
• CURRENT STATUS
• First test: 5” OD Piston (3 Cr and 3 HVOF) - simulate a commercial regional aircraft.
• Second test: 10” OD Piston ( 3 Cr and 3 HVOF) - simulate large commercial jet.
TEST 3: PISTON FATIGUE TEST
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Landing Gear
SCOPE OF TEST
Purpose: “…to compare the fatigue performance of simulated LG piston coated with HVOF applied WC-Co-Cr and electrolytic chrome plating.”
“This test is aimed at deriving fatigue life design factors…”
“The test will be performed until failure of the simulated LG pistons occurs.”
SCOPE OF TEST WAS ALTERED FOR 3rd HVOF 5” PISTON
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Landing Gear
Test Conditions
Bending loads based on fatigue analysis and landing gear fatigue requirements.
Bending stresses approx. 100 ksi at R= -1.
50,000 cycles = 1 block of tests. Inspection after every block.
Expected lives: 285,000 for 5” pistons, 330,000 for 10” pistons.
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Landing Gear
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Landing Gear
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Landing Gear
CURRENT STATUS
• 5” OD pistons have been HVOF sprayed (Vac Aero), ground and superfinished to 4 Ra by Goodrich (with assistance from Jon Devereaux, US Navy).
• Chrome plated pistons were processed in-house by Goodrich. Finished to better than 16 Ra.
• 10” OD pistons are complete, but not HVOF coated.
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Landing Gear
TEST RESULTS
1st chrome plated piston:285,000 + 122,000 cycles. ~100ksi at notch
No spalling of coating prior to failure. Some surface wear noted on chrome plating.
Piston failed at notch.Crack initiated near the piston inner diameter at an inclusion - extended circumferentially outward.
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Landing Gear
CHROME PLATED 5 INCH PISTON
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Landing Gear
CHROME PLATED 5 INCH PISTON
April 29, 2003
Landing GearTEST RESULTS
1st HVOF coated piston:570,000 cycles, i.e. 2 lives, ~ 100ksi at notch 184,500 cycles, ~ 110ksi
Small area of failed coating (approximately .150” x .350”), between 250,000 - 285,000 cycles at bottom.
Visual, MPI, LPI and Barkhausen Noise inspection after 570,000 cycles did not find evidence of substrate cracking.
Piston failed at bearing contact point
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Landing Gear
Seal area
Bearing area
Notch area
HVOF COATING AFTER 285,000 CYCLES
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Landing Gear
HVOF COATING AFTER 285,000 CYCLES
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HVOF COATING AFTER 570,00 CYCLES
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Landing Gear
BUSHING CONFIGURATION
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Landing GearTEST RESULTS
2nd Chrome plated piston:1,000,000 cycles. ~110ksi at notch20,000 cycles. ~135ksi at notch
Piston failed at notch
3rd Chrome plated piston:62,000 cycles. ~125ksi at notch
Piston failed at notch
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Landing Gear
TEST RESULTS2nd HVOF coated piston:33,000 cycles. ~125ksi at notch
Piston failed at bearing contact point, large area of coating failure 2”x0.25”- top and bottom
SCOPE OF TEST ALTERED
Coating Application? (different coating supplier)Understanding of coating failure mechanismComparison of LG spectrum to test conditions
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Landing Gear
TEST RESULTS
InternalNotch
Origin
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Landing Gear
TEST RESULTS
HVOF coating remains
Initiation region
2nd HVOF coated specimen
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Landing Gear
TEST RESULTS
Piston
Bearing Seal
Chrome Piston FailureHVOF Piston
Failure
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Landing Gear
TEST RESULTS3rd HVOF piston, coated by SouthWest Aero :
2,000 cycles ~125 ksi5,000 cycles - some cracking top and bottom10,000 cycles - significant circumferential cracking15,000 cycles - coating separation evident20,000 cycles - coating blistering 2” x 0.15”25,300 cycles - failure at the bearing contact point
initiating at bottom
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Landing Gear
Bearing areaAppearance of coating after 10,000 cycles
X~10
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Landing Gear
Appearance of coating after 25,000 cycles
Blistering
TEST RESULTS
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April 29, 2003
Landing GearSUMMARY
Piston Stress (bending) Cycles Failure Mode
1st Cr 100 ksi 407K ID, @ notch (inclusion)
2nd Cr 110 ksi 1M135 ksi 20K ID, @ notch
3rd Cr 125 ksi 62K ID, @ notch
1st HVOF 100 ksi 570K coating failed (250K)110ksi 185K OD, @ bearing
2nd HVOF 125 ksi 33K OD, @ bearing3rd HVOF 125 ksi 25K OD, @ bearing
April 29, 2003
Landing Gear
SUMMARYHVOF pistons failed at contact point with bearing - Failure originated at OD; Chrome plated pistons failed at notch - Failure originated at ID.Cracking of coating is circumferential, localized and appears tooccur relatively early ~5 K cycles @125ksiCoating failure is cohesive (substrate still coated).Coating application and finish - not a factor.Commercial Landing Gear Stresses are comparable to test - but # of cycles much smaller.
LG spectrum 125 ksi -- 575 cycles/5 lives (120 one life) Test regime 125 ksi -- 5,000 - 7,000 cycles
Test conditions more severe, not representative of LG.
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Landing Gear
Test conditions too severe, not representative of LG.
1) Rigid bearing reaction 2) Single stroke position3) Monotonic cycling
Scope of test changed to simulate LG conditions - 10” test
10” Rig will be re-designed to represent LG lower bearing
Simple spectrum loading will be implemented
Representative bearing will be implemented
FUTURE ACTIONS
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Landing Gear
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Cylinder added
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Landing Gear
TEST DESCRIPTION
•TEST SPECIMENS A total of three test pins will be tested for each of the joint configurations. All pins (specimens) shall be manufactured from a single heat of material, processed, heat treated and coated as a single batch.
•LOAD SPECTRUM A simplified fatigue spectrum shall be used to cyclically load the specimens. This spectrum will be subdivided into load blocks, which will be repeated until five fatigue lives are accumulated. The fatigue spectrum will be representative of a commercial landing gear.
•INSPECTIONS Visual inspections for coating wear or damage will be conducted after each block, with detailed joint inspections after each life. Detailed joint inspections will consist of complete joint disassembly and NDT inspection of pins and relevant hardware.
•EVALUATION The results of the tests will be evaluated by Stress, Design and M&PT and used to validate the use of HVOF on the A380 pins.
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Landing Gear
SUMMARYHVOF applied coatings are considered the primary replacement for hard chrome plating in landing gear applications to combat wear and corrosion.
Performance evaluation of HVOF coatings appear superior to Chrome plating
Spalling and cracking characteristics are very different for Chrome and HVOF coatings, hence a number of tests are underway to understand this behaviour and reduce the risk on the A380 landing gear.
