Transcript Molybdenum-on-Chromium Dual Coating on Steel Implementation of fast scanning diode pumped fiber laser to surface modification Surface & Coatings Technology (2009), 203 (9),

Molybdenum-on-Chromium Dual Coating on Steel
Implementation of fast scanning diode
pumped fiber laser to surface modification
Surface & Coatings Technology (2009), 203 (9), pp 1281-1287
www.utsi.edu
Center for Laser Applications
University of Tennessee
Space Institute
411 B. H. Goethert Parkway
Tullahoma, TN 37388
Deepak Rajput
[email protected] / http://drajput.com
Molybdenum coating on steel
• Improves wear resistance (self lubricant)
• Low coefficient of thermal expansion
• High resistance to scuffing under sliding contact
• Problem: low hardness (approx. 160 VHN)
• Carbon addition improves the hardness of Mo
• Processes widely used: flame and plasma spraying
• Problems with thermal spraying: porosity & adhesion
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Molybdenum coating on steel
Objective
• Good wear resistance
• High hardness
• Excellent adhesion
Process
• LISITM (Laser Induced Surface Improvement)
• LISITM concept: Laser alloying of pre-placed powder
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Direct alloying of Mo on Steel: Problems
• Melting point of Mo (~2623oC) is much higher
than that of steel (~1530oC). High dilution !!
• Mo and Fe form high and low temperature
intermetallics.
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Dilution
Laser beam
Pores
Precursor
mixture
Substrate
Capillary action fills up the pores with the molten metal from the substrate
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Dilution
1) Composite coating: unmelted precursor particles embedded in the substrate
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Substrate
Tp >> Ts
2) Partial melting of the precursor: coating contains alloy and unmelted particles
Substrate
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Tp > Ts or not enough
Laser power
Fe-Mo phase diagram
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Problems & Solution
• Intermediate layers of materials that don’t form
intermetallics with Fe and Mo
Mo
Mo
intermetallics
No intermetallics
IML
No intermetallics
Fe
Fe
Phase diagrams show that Cr, V and Nb are the best intermediate layers (IML)
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Fe-Cr phase diagram
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Cr-Mo phase diagram
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Cr-B phase diagram
1630oC
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Mo-B phase diagram
2180oC
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Chemistry & Stoichiometry
• Cr* = Cr + CrB2 eutectic mixture (9:1) - IML
• Mo* = Mo + MoB eutectic mixture (7:3) - ML
• B gives additional hardness
CrB2
temperature
Cr
Cr
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Cr + 10.5% CrB2
% CrB2
CrB2
Process: LISITM
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Laser Induced Surface Improvement
Uses pre-placed powder (precursor)
Precursor = Metal powders + Binder
Dry for few hours
Laser process
IML Precursor = Cr + 10.5 wt.%CrB2 + 50 wt.% binder
ML Precursor = Mo + 30 wt.% MoB + 85 wt.% binder
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Precursor Deposition
Precursor mixture = Metal Powders + Binder
Precursor mixture
air
Spray gun
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steel
Laser Deposition
Cr* = 165W, 25mm/s, Hatch 0.1mm @ 355mm
Mo* = 180W, 25mm/s, Hatch 0.1mm @355mm
Mo Layer
Cr IML
Steel
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Fiber laser processing
Fiber
Scan head
Hopper
View port
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Water-cooled
chamber
Fiber laser results
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Substrate: AISI 4130 steel
Fiber laser results
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Cr* layer at high magnification
Fiber laser – two coatings
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Fiber laser – two coatings
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Mo* layer at high magnification
Cr* coating
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Microhardness test of chromium layer
X-ray Cr* layer
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X-ray diffractogram of chromium layer
Mo* on Cr* coating
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Microhardness test of molybdenum coating
Mo* on Cr* x-rays
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X-ray diffractogram of molybdenum coating
Block-on-ring wear tester
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ASTM G77 (4 lb load)
Block-on-ring sliding wear
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Sliding wear performance
PLINT TE68 Gas Jet Erosion Rig
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ASTM G76 (silica 2 gm/min at 52 m/s)
ASTM G76 dry sand erosion wear
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Solid particle erosion performance
Summary
• Direct diode fiber laser can effectively perform
surface alloying.
• Cr is an efficient intermediate layer for Mo
coatings on steels.
• Hardness of Mo coating is approx. 1100 VHN.
• Dry sliding wear resistance of Mo is 10 times
that of steel and 4 times that of chromium
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Thank You