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 drajput@utsi.edu / 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

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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.

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Dilution

5

Precursormixture

Substrate

Laser beam Pores

Capillary action fills up the pores with the molten metal from the substrate

Substrate

Dilution

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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

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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)

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Fe-Cr phase diagram

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Cr-Mo phase diagram

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Cr-B phase diagram

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1630oC

Mo-B phase diagram

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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

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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

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Laser Deposition

SteelCr IML

Mo Layer

Cr* = 165W, 25mm/s, Hatch 0.1mm @ 355mmMo* = 180W, 25mm/s, Hatch 0.1mm @355mm

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Fiber laser processing

Scan headFiber

Water-cooledchamber

View port

Hopper

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Fiber laser results

Substrate: AISI 4130 steel18

Fiber laser results

Cr* layer at high magnification19

Fiber laser – two coatings

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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

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Thank You