Frictional Investigation of Magnetorheological Finishing (MRF) for Optical Glasses and Hard Ceramics

C. Miao,S. N. Shafrir, H. J. Romanofsky, J. Mici, J. C. Lambropoulos, and S. D. JacobsUniversity of RochesterLaboratory for Laser Energetics

Optical Fabrication andTesting Topical Meeting

Rochester, NY19–23 October 2008

Elapsed time (s)

Systemat rest

Part depressedinto ribbon (10 s)

Systemat rest

Normal force

Drag force

In-situfrictional force sensors

In-situfrictional force sensors

A frictional investigation of MRF for optical glasses and hard ceramics is carried out

G8620

• Adual-loadcellisusedforsimultaneous,in-situ measurement of frictional forces on optical glasses and hard ceramics

• Dragforcedecreaseslinearlywithincreasingmaterialhardness, whilenormalforcesaturatesathighhardness

• Coefficientoffrictioniscloselycorrelatedwithmechanicalpropertiesof materials and the particles in the MR fluid

Summary

We demonstrate that both drag and normal forces play an important role in material removal in MRF for both glasses and ceramics

The drag and normal forces are measured in MRF

G8621

• Employadual-loadcellforrealtime,simultaneousmeasurement of both drag force and normal force on materials during MRF

• Determinehowdragforce(FD) and normal force (FN) respond to material mechanical properties

• Studytherelationshipbetweencoefficientoffriction(COF) and material removal rate

Objectives

MaterialremovalinMRFisknowntobebased on drag force and shear stress

G8622

Preston’s equation (1929)

Shorey’s modification (2000)

ClP = modified Preston’s coefficientn = friction coefficient = FD/FNFD = drag forceAS = spot areax = shear stress = FD/AS

MRR = material removal rateCP = Preston’s coefficientP = normal pressureV = relative velocityFN = normal forceAC = nominal contact area

C PV CAF

V CAF

V CAF

V C VMRR P PC

NP

S

NP

S

DP

nx= = = = =l l l

Materials of interest include optical glasses and hard ceramics

G8623

Materials in each group are listed by the order of increasing Vickers hardness

Materials Mat. ID Grain Size (nm)

Young’s Modulus E

(GPa)

Vickers Hardness Hv (GPa)

Fracture Toughness

Kc (MPa•m1/2)

Source

Optical glasses (100-gload,literaturevalues)Phosphate LHG8 – 62 3.7 0.5 HoyaBorosilicate N-BK7 – 81 6.0 0.8 SchottFused silica FS – 69 7.5 0.8 Corning

Hard ceramics (500-gload,Shafrir2007)Magnesium aluminum oxide

Spinel 100–200 273 14 2.2 TA&T

Aluminum oxynitride ALON 150–250 334 15 2.7 Surmet

Polycrystallinealumina PCA ~0.3 400 22 3.3 CeraNova

Silicon carbide CVC SiC 5–20 460 29 4.5 Trex

MRF spots are taken on a research platform: aspot-takingmachine(STM)

G8624

• Partmountedonanonrotating z-axisslide

• MRFspotcreatedbyloweringnonrotating part into the rotating MR fluid ribbon

• Volumetricremovalrate(VRR) calculated from spot volume and spot time

• STMmachineparametersheldconstantforthiswork

• Allpartspre-polishedflats

Interferometric map of polishing spot on part surface

Flowdirection

STM

Nozzle

Z axis

Z axis

Part

Ribbon

The drag force (FD) and normal force (FN) are measured in situ on the STM using piezoelectric sensors

G8625

• Sensors† measure dynamic, normal and shear forces

• Sensorsrespondtochangesin – substrate type – substrate surface condition – STM machine settings – MR fluid composition

• Allspotstakenonmachine z axis

†Single-axisslimlineshear(K9143B21) and compressive (K9133B21) load-cellmeasuringsystem(KistlerInstrumentCorp.,Amherst,NY).

FN

Z axis

Fluiddirection

FD

Drag forcesensor

Ribbon Wheelrotating clockwise

Part

Normal forcesensor

Drag forcesensor

Normal forcesensor

The sensor output signals are stable throughout the measurement

G8626

• LabViewinterfacerecordsthedragandnormalforcessimultaneously• Datacollectingrate:10datapointspersecond• Forceisaveragedoverthewholespottingtime

15

Forc

e in

New

ton

s

Elapsed time (s)

15 s

FS

Spinel

FN: ~11 NFN: ~9 N

FD: ~3.5 NFD: ~4 N

10

5

0 2 s

Both drag force (FD) and normal force (FN) strongly correlate to material hardness

G8627

Drag force (FD)

• Decreaseslinearlywithincreasinghardness(STM parameters fixed)

Dual-load-cellresults

• PriorworkdoneontheSTMunderdifferentmachineconditions• SignificantexpansionofpriorworkontheSTM

00

1

2

3

F D (

N)

4

5

6

5 10 15

Hv (GPa)

20 25 30

LHG8 FS

FSSapphire

Spinel

ALON

PCASiC

BK7

LHG8

][DeGroote2007

Shorey2001 ][

BK7

y = –0.09x + 4.9R2 = 0.79

• Pressureshowsthesametrendfrom0.15to0.3MPa

Normal force (FN)• Increaseslinearlywithincreasinghardness• Saturatesat~11 N for hard ceramics

Both drag force (FD) and normal force (FN) strongly correlate to material hardness

G8637

Dual-load-cellresults

LHG8

FS

Spinel ALON(Line to guide eye)

PCA SiC

BK7

0

5

F N (

N) 10

15

0 5 10 15

Hv (GPa)

Pre

ssu

re(M

Pa)

20 25 3530

0.30

0.15

The coefficient of friction (FD/FN)shows aninversecorrelationwithmaterialhardness

G8628

InteractionbetweenMRfluidparticlesandthepartismechanicallydominant

LHG8

FS

Spinel ALON

Co

effic

ien

t o

f fr

icti

on

(C

OF

), n

PCA SiC

BK7

1.0

0.8

0.6

0.4

0.2

0.00 10

Hv (GPa)

20 30

Thematerialremovalrateshowsastrongdependence on the process coefficient of friction (FD/FN)

G8629

Both drag and normal forces play an important role in material removal in MRF for glasses and ceramics

LHG8

FS

Spinel

ALON

Coefficient of friction (COF), n

Volu

met

ric

rem

oval

rat

e (m

m3 /

min

)

PCA

SiCBK7

0.8

0.6

0.4

0.2

0.00.0 0.2 0.4 0.6 0.8 1.0

y = 0.9x – 0.1R2 = 0.70

Acknowledgments

• AlexMaltsevandMikeKaplun(LLE) for polishing parts

• ScottRussell(UR)forLabViewsoftwareinterface

• SpineldiskswereprovidedbyTA&T

• CVCSiCmaterialwasprovidedbyTrex

• PolycrystallinealuminadiskswereprovidedbyCeraNovaCorporation.Development of this material by CeraNova is funded by NAVAIR through the U.S. Government SBIR program; SBIR data rights apply

• Continuousfinancialsupport

– Laboratory for Laser Energetics

– HortonFellowship

– U.S. Army Armament, Research, Development, and Engineering Center

– U.S. Department of Energy Office of Inertial Confinement Fusion

A frictional investigation of MRF for optical glasses and hard ceramics is carried out

G8620

• Adual-loadcellisusedforsimultaneous,in-situ measurement of frictional forces on optical glasses and hard ceramics

• Dragforcedecreaseslinearlywithincreasingmaterialhardness, whilenormalforcesaturatesathighhardness

• Coefficientoffrictioniscloselycorrelatedwithmechanicalpropertiesof materials and the particles in the MR fluid

Summary/Conclusions

We demonstrate that both drag and normal forces play an important role in material removal in MRF for both glasses and ceramics