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www.utsi.edu
Center for Laser Applications
University of Tennessee Space Institute
411 B. H. Goethert Parkway Tullahoma, TN 37388
Surface & Coatings Technology (2009), 203 (9), pp 1281-1287
Molybdenum-on-Chromium Dual Coating on SteelImplementation of fast scanning diode
pumped fiber laser to surface modification
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
2
Molybdenum coating on steelObjective
• Good wear resistance• High hardness • Excellent adhesion
Process
• LISITM (Laser Induced Surface Improvement)• LISITM concept: Laser alloying of pre-placed powder
3
Direct alloying of Mo on Steel: Problems
• Melting point of Mo (~2623oC) is much higherthan that of steel (~1530oC). High dilution !!
• Mo and Fe form high and low temperatureintermetallics.
4
Dilution
5
Precursormixture
Substrate
Laser beam Pores
Capillary action fills up the pores with the molten metal from the substrate
Substrate
Dilution
6
Substrate
1) Composite coating: unmelted precursor particles embedded in the substrate
1
2
Tp >> Ts
Tp > Ts or not enoughLaser power
2) Partial melting of the precursor: coating contains alloy and unmelted particles
Fe-Mo phase diagram
7
Problems & Solution• Intermediate layers of materials that don’t form
intermetallics with Fe and Mo
Mo Fe
Mo FeIML
intermetallics
No intermetallics No intermetallics
Phase diagrams show that Cr, V and Nb are the best intermediate layers (IML)
8
Fe-Cr phase diagram
9
Cr-Mo phase diagram
10
Cr-B phase diagram
11
1630oC
Mo-B phase diagram
12
2180oC
Chemistry & Stoichiometry
• Cr* = Cr + CrB2 eutectic mixture (9:1) - IML• Mo* = Mo + MoB eutectic mixture (7:3) - ML• B gives additional hardness
tem
pera
ture
CrCrB2
CrB2Cr % CrB2
Cr + 10.5% CrB2
13
Process: LISITM
• 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.% binderML Precursor = Mo + 30 wt.% MoB + 85 wt.% binder
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Precursor Deposition
Precursor mixture = Metal Powders + Binder
Precursor mixture
steel
air
Spray gun
15
Laser Deposition
SteelCr IML
Mo Layer
Cr* = 165W, 25mm/s, Hatch 0.1mm @ 355mmMo* = 180W, 25mm/s, Hatch 0.1mm @355mm
16
Fiber laser processing
Scan headFiber
Water-cooledchamber
View port
Hopper
17
Fiber laser results
Substrate: AISI 4130 steel18
Fiber laser results
Cr* layer at high magnification19
Fiber laser – two coatings
20
Fiber laser – two coatings
21 Mo* layer at high magnification
Cr* coating
Microhardness test of chromium layer22
X-ray Cr* layer
X-ray diffractogram of chromium layer23
Mo* on Cr* coating
Microhardness test of molybdenum coating24
Mo* on Cr* x-rays
X-ray diffractogram of molybdenum coating25
Block-on-ring wear tester
ASTM G77 (4 lb load)26
Block-on-ring sliding wear
Sliding wear performance27
PLINT TE68 Gas Jet Erosion Rig
ASTM G76 (silica 2 gm/min at 52 m/s) 28
ASTM G76 dry sand erosion wear
Solid particle erosion performance29
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
30
Thank You