Normal Force and Drag Force in Magnetorheological Finishing

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

Optical Manufacturing and Testing VIIISPIE Optics and Photonics

San Diego, CA4–5 August 2009

Summary

G8620a

• AfrictionalinvestigationofMRFforopticalglassesandhardceramicsis carried out

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

• Dragforcedecreaseslinearlywithincreasingmaterialhardness, whilenormalforcesaturatesathighhardness

• ThemeasuredratioFD/FN, corresponding to the coefficient of friction, iscloselycorrelatedinMRFwiththehardnessvaluesofmaterials

We demonstrate that the measured ratio FD/FN, taking into account both drag and normal forces, is a useful measure of material removal in MRF for both glasses and ceramics.

Objectives

G8621a

• 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–FD/FN) and material removal rate

MaterialremovalinMRFisknowntobebased on drag force and shear stress

G8622a

n = friction coefficient = FD/FNP = normal pressureFD = drag forceAs = spot areax = shear stress = FD/AsKc = fracture toughness

MRR = material removal rateCp = Preston’s coefficientFN = normal forceAc = nominal contact areaV = relative velocityE = Young’s modulusHV = Vickers hardness

Preston (1927):

Shorey et al. (2000):

Miao et al. (2009):

C PV CAF

VMRR p pc

N= =

CAF

V CAF

V C VMRR , , ,p Fs

Np F

s

DpMRF MRF MRF MRFN D

: :n

x= = = xl l l_ _ ^i i h

CK H

E VMRR , ,pc V

2MRF MRF FOM : :x= xl ^ h

Clp,MRF(FN) = modified Preston’s coefficient in terms of FNClp,MRF(FD) = modified Preston’s coefficient in terms of FDClp,MRF(x) = modified Preston’s coefficient in terms of shear stressClp,MRF(x,FOM) = modified Preston’s coefficient in terms of shear stress and FOMFOM = figure of merit, defined as E K Hc v

2

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

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

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

G8637a

Dual-load-cellresults

LHG8

FS

Spinel ALON

Lines to guide eye

PCA SiC

BK7

0

5

No

rmal

forc

e, F

N (

N)

10

15

0 5 10 15

Vickers hardness, Hv (GPa)

20 25 30

Volu

met

ric

rem

oval

rat

e,

VR

R (

mm

3 /m

in)Vo

lum

etri

c re

mov

al r

ate,

V

RR

(m

m3 /

min

)

LHG8

FS

SpinelALONPCASiC

BK7Lines to guide eye

0.6

0.4

0.2

FD (N)0 2 4 6

0.0LHG8

FSSpinel

ALON

PCA

SiC

BK7

0.6

0.4

0.2

FN (N)0 5 10 15

0.0

Line to guide eye

It is possible that neither normal force nor drag force can, by itself, adequately describe removal in MRF

G8818

• Volumetricremovalrateshowsatendencytoincreasewithdragforce

• Thereisnoclearlinearcorrelation

• Volumetricremovalrateexhibits anegativecorrelationwithnormal force across all materials

• Thisinverserelationshipisoppositetotheplottedrelationshipbetweendrag force and the removal rate

The measured ratio FD/FNshowsaninversecorrelationwithmaterialhardness

G8628a

InteractionsbetweenMRfluidparticlesandthepartaredominatedbymechanics.

LHG8

FSSpinel

ALON

F D/F

N

PCASiC

BK7

1.0

0.8

0.6

0.4

0.2

0.00 10

Vickers hardness, Hv (GPa)

20 30

y = –0.016x + 0.6R2 = 0.80

Confidence level: >99%

• ThemeasuredratioFD/FN, taking into account both normal force and drag force, is equivalent to the coefficient of friction

Thematerialremovalrateshowsastrongdependenceon the measured ratio FD/FN

G8629a

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

LHG8

FS

FD/FN

PCA

SiC

BK7

0.6

0.4

0.2

0.00.0 0.2 0.60.4 0.8 1.0

Volu

met

ric

rem

oval

rat

e,V

RR

(m

m3 /

min

)

Line to guide eye

ALONSpinel

The measured ratio FD/FN exhibits a strong correlation withshearstress

G8819

• ThemeasuredratioFD/FN, taking into account the contribution of both drag force and normal force, is an important alternative to shear stress for evaluating material removal for MRF

LHG8

FS

FD/FN

PCA

SiC

BK7

0.15

0.10

0.05

0.000.0 0.2

MRRMRF = Clp, MRF(x) • x • V

0.60.4

Sh

ear

stre

ss, x

(M

Pa)

ALONSpinel

y = 0.09x + 0.05R2 = 0.83

Confidence level: >99%

Acknowledgments

• AlexMaltsevandMikeKaplan(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

G8620a

We demonstrate that the measured ratio FD/FN, taking into account both drag and normal forces, is a useful measure of material removal in MRF for both glasses and ceramics.

Summary/Conclusions

• AfrictionalinvestigationofMRFforopticalglassesandhardceramicsis carried out

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

• Dragforcedecreaseslinearlywithincreasingmaterialhardness, whilenormalforcesaturatesathighhardness

• ThemeasuredratioFD/FN, corresponding to the coefficient of friction, iscloselycorrelatedinMRFwiththehardnessvaluesofmaterials