measurement and control of ice adhesion to aluminum 6061 alloy

J[ Mech[ Phys[ Solids\ Vol[ 35\ No[ 09\ pp[ 0634Ð0660\ 0887Þ 0887 Elsevier Science Ltd[ All rights reserved\ Pergamon Printed in Great Britain

9911Ð4985:87 ,*see front matterPII ] S9911Ð4985"87#99903Ð2

MEASUREMENT AND CONTROL OF ICE ADHESION TOALUMINUM 5950 ALLOY

PAUL ARCHER0 AND VIJAY GUPTA�Department of Mechanical and Aerospace Engineering\ University of California\

Los Angeles\ CA 89984\ U[S[A[

"Received 19 December 0886 ^ in revised form 04 January 0887#

ABSTRACT

A new experimental strategy for measuring the tensile strength of ice coatings to structural surfaces ispresented[ In this experiment\ a laser!induced compressive stress pulse travels through a 0 mm!thicksubstrate disc that has a layer of ice grown on its front surface[ The compressive stress pulse re~ects into atensile wave from the free surface of the ice and pulls the ice:interface apart\ given a su.cient amplitude[The interface strength was calculated by recording the free surface velocity of an Al substrate using aDoppler interferometer and calculating the stress at the interface using a _nite!di}erence elastic wavemechanics simulation with the free surface velocity as an input[ The test procedure was used to study iceadhesion on 5950 aluminum alloy sheets[ It was found that the adhesion strength of ice to unpolishedaluminum substrates was 163 MPa at −09>C[ This value decreased with temperature\ down to 068 MPaat −39>C[ Interestingly\ this decrement in the tensile strength could be directly related to the existence ofa liquid!like layer that is known to exist on the surface of solid ice till −29>C[ The interface strength wasalso shown to decrease by polishing the Al substrate surface or by adding thin polymer coatings on theunpolished Al substrate[ The sensitivity of the technique to such microstructural changes in the interfacialregion is indicative of the experiments ability to provide basic adhesion data\ which in turn\ can be used tosolve the deicing problem from a fundamental standpoint[ Þ 0887 Elsevier Science Ltd[ All rights reserved[

Keywords ] A[ adhesion and adhesives\ A[ fracture\ B[ ice and snow\ B[ stress waves\ C[ opticalinterferometry[

0[ INTRODUCTION

In cold climates all over the world\ icing on structural surfaces is a common phenom!enon[ Examples include\ ice build!up on power and high!tension lines\ radio andtelevision transmitting and receiving towers\ o}shore oil platforms in cold environ!ments\ roads and foot paths\ automobile windshields and headlights\ and airplanefuselages and wings\ among many others[ Removal of ice coatings from these struc!tures is usually necessary for their safe and reliable operation[

Most research related to deicing has focused on mechanical ice removal strategiesbecause of practical considerations\ and very little has been directed towards gaining

� Author to whom all correspondences should be addressed[0 Current address ] Raytheon Electronic Systems\ 49 Apple Hill Drive\ P[O[ Box 0190\ Tewksbury\ MA

90765!9890\ M:S ANMN12\ U[S[A[

0634

P[ ARCHER and V[ GUPTA0635

a fundamental understanding of the ice adhesion process[ In this regard\ this paper isdi}erent from the rest and provides a quantitative measurement of basic adhesionand its in~uence on such variables as the ice microstructure\ temperature\ and thetype "metal\ polymer\ etc[# and quality "polishing\ roughness# of the structural surface[Before presenting the results\ some terms related to adhesion are introduced and theavailable literature is reviewed[

Basic adhesion is an intrinsic property of the interface and is solely determined bythe atomic structure and chemistry of the interfacial region[ It can be characterizedby either the intrinsic tensile strength "si# or the intrinsic toughness "Gi# of theinterface\ both of which are related to each other via the fundamental interface stress!separation curve "Gupta et al[\ 0883 ^ Mittal\ 0867#[ In contrast\ the total toughnessof the interface or the energy consumed in propagating a unit area of a crack alongan ice:structure interface "or in its vicinity# as may occur during any ice removalprocess\ depends upon many extrinsic parameters such as the specimen geometry\loading rate\ temperature\ substrate roughness\ adsorbed impurities\ interface ~awdensity\ and the ratio of the tensile to shear stress ratio separating the interface[ Eachof these extrinsic parameters in~uence the inelastic "creep# dissipative mechanisms inthe vicinity of the crack tip ^ and consequently\ the fracture resistance or the totaltoughness of the interface becomes larger than the basic adhesion by two orders ofmagnitude and higher[

Ice literature is replete with studies that have measured directly or indirectly thetotal interface toughness[ Jellinek "0846a\ b\ 0851\ 0859\ 0856#\ examined the adhesiveproperties of snow!ice sandwiched between stainless steel\ optically ~at quartz plates\and various polymers and block copolymers applied to aluminum substrates underboth tensile and shear loading[ His tensile experiments were essentially a direct pull!type test and only cohesive breaks through the bulk ice were observed at a maximumstress of about 6 MPa\ implying the interface strength of the ice!solid bond to begreater than the cohesive strength of the ice[ Shear experiments were conducted in asetup resembling the classical charpy impact test apparatus\ where the impactingpendulum head was made to strike the ice plug almost horizontally[ Such tests resultedin adhesive breaks at the interface[ The overall interface shear strength was found todepend upon the loading rate\ degree of surface roughness\ temperature and the typeof the substrate material being tested[ In shear experiments with ice on steel\ adhesivebreaks were predominantly found down to −02>C[ Below −02>C\only cohesivebreaks through the ice were found at a peak stress of only 1 MPa[ The di}erencebetween the shear and tensile strengths was related to the existence of a liquid!likelayer that is known to exist between ice and the structural solid till −29>C[ Theexistence of this layer which was proposed by Faraday "0748# are given in more detaillater while discussing the basic adhesion studies[

Landy and Freiberger "0856# investigated the adhesion strength of ice to variousplastics using a shear apparatus similar to that used by Jellinek[ They attempted tocorrelate the adhesion strengths to physical and chemical properties such as criticalsurface tension of wetting\ contact angle\ coe.cient of thermal conductivity andthermal expansion\ porosity\ dielectric constant\ and ~exural modulus[ They wereunsuccessful in this attempt probably because such tests are unable to discriminatebetween the e}ects of various variables separately[ Rather\ they measure a combinede}ect which can best be considered an average failure stress\ far away from the localinterface stress of interest[

Measurement and control of ice adhesion to aluminum 5950 alloy 0636

More recently\ solid!ice adhesion was investigated using a Raman microprobe shearapparatus "Sonwalkar et al[\ 0882#[ Ice samples of thickness 099Ð199 mm were vapor!deposited on polished substrates of titanium\ copper\ aluminum\ stainless steel\ andTe~on[ The results indicated that the adhesive bonds between the ice and the solidsubstrates are formed primarily by the interaction of oxygen atoms in the ice latticewith the atoms of the solid surface[ When the solid lattice closely matched the icelattice\ high values of adhesive strength were obtained[ The adhesive strength wasalso found to be proportional to the extent of mechanical interlocking and inverselyproportional to the contact angle of a water droplet on the substrate[ Values for theadhesive strengths obtained in the shear apparatus were in relative agreement withthe values obtained by Jellinek "0846#\ in his experiments on steel and polymericsubstrates[

All tests discussed above\ provide only a relative measure of the adhesion of ice tothe substrate of interest[ This is due to the in~uence in all cases of the large extrinsiccomponents[ for example\ Jellinek|s direct tension and pure shear tests "Jellinek\0846a\ b# contained the e}ect of residual stresses at the metal!ice interface trappedduring sample preparation[ These tests also contained the e}ects of dust particle!induced interfacial ~aws as indicated by the low measured bond strength value ofabout 1 MPa\ which is twoÐthree orders of magnitude lower than the expected strengthfrom quantum mechanical calculations[ Also included\ are the e}ects of local creeprelaxations as indicated by the measured non!linear stress!strain response\ e}ects ofstrain rate as indicated by di}erent stress!strain curves at di}erent loading rates\e}ects of substrate surface roughness\ and _nally\ the e}ects of the di}erent localtensile to shear stress ratios at the interface*even though the far!_eld loading waseither pure tension or pure shear[ In fact\ shortly after Jellinek carried out many ofhis studies on ice adhesion\ breakthroughs in interfacial fracture mechanics were madeby Rice and Sih "0854# ^ they found that when a crack resides at an interface betweendissimilar materials\ the local stress state is a combination of shear and tension\ evenwhen the interfacial crack is loaded under far!_eld uniaxial tension "Mode I# or shear"Mode II# loads[ The exact ratio of the tensile to shear stresses depends upon themismatch in the elastic properties of the two materials[ For the steel!ice interface\ ashear stress of about 04) of the far!_eld tensile stress will develop under pure ModeI loading[ So\ without knowing the exact local stress state which is responsible foreither the cohesive\ adhesive\ or mixed failure\ the conclusions drawn by Jellinekassuming the locus of failure caused by either pure applied tension or shear\ remainquestionable[ It seems unlikely that the bond strength of the stainless steel!ice interfaceis as low as 1 MPa\ so there must be concentration sites at the interface which lead tohigher local stresses[

Attempts to understand basic adhesion dates back to the 0749s\ when Faraday"0748# studied the adhesion between two spheres of ice brought into contact[ Hecorrectly explained this adhesion by postulating that there is a thin {{liquid!like|| layerat the surface of ice which seemed to persist until temperatures as low as −29>C[Weyl "0840# later proposed a plausible structure to this liquid layer\ claiming that itis oriented in such a way that the negative oxygen atoms of the water molecules areuppermost in the layer[ In this orientation\ a liquid!like electrical double layer isformed and the surface free energy is minimized[ The thickness of the layer "several


hundred molecular layers thick and decreasing with temperature# was required togradually pass from the outer electrical double layer to the proper structure of solidice[ Since this initial attempt to describe the liquid!like layer on the surface of ice\many other models have been proposed to describe the ice surface[ A good summaryof these is given by Petrenko "0883#[

Most fundamental experimental studies of the ice adhesion process to date havebeen accomplished using the contact angle measurement technique\ where the contactangle u\ made by a drop of water on a solid surface is measured\ and related to thework of adhesion Wad and free energy of the liquid!vapor interface glv using therelationship "Jellinek\ 0846a#

Wad � glv "0¦cosu#¦pe\ "0#

where pe represents the increment in the solid!vapor free energy on account ofadsorbed gases and liquids on the solid surface[ The values of Wad obtained from eqn"0# for the water:solid interface cannot be directly taken as a measure of the work ofadhesion for the ice:solid interface\ especially when a liquid!like layer of varyingthickness is present at the ice:solid interface at di}erent temperatures[ Nevertheless\such contact angle studies can be used for qualitatively determining the degree ofhydrophobicity of various engineering metallic and polymeric substrates as dem!onstrated by Jellinek "0846a#[

Scanning force microscopy "SFM# has recently been used by Nicolayev andPetrenko "0884# to study the surface of ice and the adhesive forces between the icesurface and cantilever tip of the apparatus[ This method allows the study of the icesurface and ice adhesion to solids "the tip of the cantilever# at the molecular level[ Inthis experiment\ a very sharp tip\ mounted to a ~exible cantilever of the SFM\ isdragged over the surface of an ice sample[ De~ections in the ~exible cantilever aredetected by a laser beam re~ected from the cantilever\ providing a vertical resolutionof about 9[0 nm[ Measurements of the force interaction between the tip and thesurface\ as a function of relative tip!sample position\ yield a force curve[ From thisforce curve\ the adhesion force between the tip and the ice surface can be deduced[Then\ the adhesion strength is de_ned as the adhesion force per unit area of the tip[For silicon nitride cantilever tips\ Nickolayev and Petrenko "0884# found an adhesionstrength of 1[6 MPa at −4>C\ with the strength decreasing with decreasing tempera!ture[ Although this technique is promising\ there are a number of questions thatpertain to converting the measured force to the local stress[ First\ the mechanisticconsiderations should include the sharp tip geometry!related singularity e}ects whileconverting the force to the stress[ This should increase the calculated interface stresssigni_cantly[ When operated in the contact mode\ the surface tension e}ects betweenthe tip and the liquid layer will signi_cantly depend upon the tip geometry\ and hence\so would the calculated adhesion stress[ Finally\ the technique is rather impractical\since one will have to change the tip material to aluminum or other materials ofinterest[ Unfortunately\ putting a coating on nitride cantilever tips will not yield basicadhesion data between the coating material and ice\ since it is well known that thein~uence of tip material could persist through 69Ð099 monolayers of the interveninglayers of di}erent materials "Yuan and Gupta\ 0884#[ Additionally\ the grain orien!


tation and geometry that is known to in~uence the adhesion strongly will become asigni_cant factor in interpreting the data\ given the rather small diameter of thecantilever tip[ In light of the above discussion\ the values obtained from this procedurecan only be considered qualitative at the present time[

In summary\ a quantitative measure of basic adhesion between ice and structuralsolids has not been obtained so far\ and yet this appears to be a basic quantity that isneeded to solve the deicing problem\ from a fundamental standpoint[ The aim of theresearch leading to this paper was to develop an experimental procedure for measuringthe tensile strength of ice:structural aluminum interfaces by adapting the recentlydeveloped technology of laser!generated stress pulses[ Previously\ the tensile strengthsof interfaces formed by a variety of metal\ ceramic\ and dielectric _lms and a numberof engineering substrates were measured under ambient conditions by using laser!generated stress pulses "Gupta et al[\ 0889 and 0881 ^ Yuan et al[\ 0882 ^ Gupta andYuan\ 0882 ^ Yuan and Gupta\ 0882 ^ Gupta et al[\ 0883#[ As discussed below\ thisso!called laser spallation experiment is capable of measuring the intrinsic tensilestrength of ~aw!free interfaces[ The present investigation was focused on developingthis procedure further for carrying out ice adhesion measurements at cryogenic tem!peratures\ and to study the e}ects of the type and quality of the substrate surface\including the e}ects of intervening hydrophobic coatings and the interface strengthdependence on temperature[ The basic laser spallation test methodology is discussed_rst\ and this is followed by the sample preparation procedures and the new testapparatus for examining the ice:5950 Al bond strength[

1[ THE LASER SPALLATION TECHNIQUE

In this experiment "Fig[ 0#\ a 1[4 nanosecond "ns# long Nd ]YAG laser pulse ismade to impinge over a 2 mm!dia area on a 9[2 mm!thick aluminum _lm which issandwiched between the back surface of a substrate disc and a 4Ð09 mm thick layerof solid water glass[ This layer is provided as a liquid solution of sodium silicate\which dehydrates within a few minutes when exposed to air\ and leaves behind acontinuous layer of solid SiO1\ covering the aluminum _lm[ The melting!inducedexpansion of aluminum under con_nement generates a compressive stress pulse with0 ns "�09−8 s# rise time directed towards the test coating which is deposited on thesubstrate|s front surface[ The compressive stress wave re~ects into a tensile pulse fromthe coating|s free surface and leads to its spallation "complete removal# at a su.cientlyhigh amplitude[ The critical stress at the interface is calculated by measuring thetransient displacement history of the coating|s free surface "induced during pulsere~ection# by using an optical interferometer "shown towards the right and top inFig[ 0# which is capable of recording fringes from even optically rough surfaces andwith a resolution of only 9[1 ns in the single shot mode[ For a coating of density r\thickness h\ and longitudinal wave velocity\ c\ the interface stress s is calculable fromthe measured transient velocity v "t# using


Fig[ 0[ A schematic diagram of the laser spallation setup along with the laser Doppler di}erential dis!placement!velocity interferometer[

s"h\ t# � 01rc$v0t¦

hc1−v0t−

hc1%[ "1#

The major advantage of this technique over its counterparts is in its ability to providea fundamental measure of the bond strength\ since interface decohesion is achievedat a relatively high strain rate "096 s−09 during which all inelastic dislocation!relatedprocesses are essentially suppressed\ and the measured values relate directly to theatomic structure and chemistry of the interfacial region[ Additionally\ the short risetime of the stress pulse is able to invoke a rather local response of the interface suchthat minute changes in the atomic structure and chemistry are re~ected directly in themeasured strength values[ This was recently demonstrated "Yuan and Gupta\ 0884#on interfaces between sputter!deposited polycrystalline Nb coatings and sapphiresubstrates where structural changes brought about by di}erent heat treatment cyclesand di}erent thicknesses of the intervening Cr and Sb layers were shown to directlya}ect the interface|s tensile strength[


2[ MODIFICATIONS TO THE LASER SPALLATION TECHNIQUE FORICE ADHESION STUDIES

2[0[ Sample preparation

The current investigation involved testing the adhesion strength of ice frozen tosheets of Al!5950 alloy with di}erent surface treatments[ The _rst set of sampleswere tested in the as!manufactured condition[ The second set were polished on ametallographic wheel using a!alumina powder of sizes 0[9 mm\ 9[2 mm and 9[94 mm[The third set were prepared with the aim of investigating the in~uence of hydrophobiccoatings[ For this purpose\ unpolished Al substrates were coated with 0 mm!thickcoatings of polymethylmethacrylate "PMMA# and a polyimide "PI# and tested at−09>C[ The polymer coatings were spin!cast onto the Al substrates\ and then bakedfor 0 h in an oven at 099>C[ The coatings were transparent[ Finally\ the last set ofsamples were used for studying the e}ects of temperature on bond strength[ For this\only the as!manufactured Al surfaces were used and bond strength determined at−19>C\ −29>C\ and −39>C[

All aluminum sheets\ regardless of the surface treatment\ were 9[7 mm thick androughly 14Ð49 mm in length and width[ The back side "i[e[\ other than the one treatedabove# of each sample was cleaned in an ultrasonic cleaner in a bath of acetone\rinsed in de!ionized water\ then again cleaned in a bath of de!ionized water in theultrasonic cleaner[ The samples were then air dried and a thin layer "29Ð49 mm# ofliquid water glass "H1SiO2 ="H1O#x# was applied to one surface of the aluminum[ Afterexposure to the air for several minutes\ the water evaporated and a layer of solidwater glass "SiO1# was left on the aluminum[ This layer of water glass serves as thecon_ning layer\ which is critical for the generation of the high!amplitude and quicklydecaying stress pulse discussed by Yuan et al[ "0882#[ Previously\ it was found thatthe generated stress pulse amplitudes and pro_les are quite insensitive to the solidwater glass in the above thickness range\ which in turn\ could be applied reproducibly[

Once the water glass hardened\ samples were placed in a freezer held at −09>C[The samples were allowed to equilibrate to this temperature for about 0 h and thenthe ice was grown on the treated surface of the aluminum\ opposite to the one coveredwith the water glass coating[ To grow ice\ an O!ring with a 14 mm inner diameter wasplaced on the surface[ Then\ room temperature de!ionized water was placed insidethe O!ring\ with the meniscus of the water rising above the thickness of the O!ring"¼1Ð2 mm#[ Normally\ ice formed several minutes after the water was _lled insidethe O!ring[ If ice did not form within this time\ a slight movement of the O!ring wouldcause the water to crystallize[ Figure 1 shows a schematic of such a sample[

After ice formation\ the sample was allowed to sit for an hour in the freezer at−09>C[ The O!ring was then carefully removed from the sample and the ice puckwas shaved ~at\ using a razor blade[ This was done to remove bubbles from the upperpart of the ice which would inhibit examination of the interface by optical microscopy[The ice samples were polycrystalline in nature with columns perpendicular to theinterface[ In all experiments the thickness of the ice was about 9[63 mm and thevariation of the thickness over the entire surface area was normally less than 29[94mm[ Due to the internal re~ection of the stress pulse that occurs at each boundary\


Fig[ 1[ A schematic showing the geometry of the ice plug:aluminum sample[

variation of the ice thickness could lead to a di}erent stress state at the interface dueto di}erent arrival times of the compressive and tensile pulses[ Therefore\ only sampleswith ice thicknesses in the range of 9[6329[94 mm were used in actual experiments[

Figure 2"a# and "b# show high magni_cation view of the unpolished and polishedAl substrates\ respectively[ These surfaces were typical of the surfaces which wereused in the present study[ It should be noted that no quantization of the surfaceroughness was performed "such as surface pro_lometry\ etc[#\ so that any conclusionsbased on surface roughness are qualitative in nature[

As expected\ the ice microstructure was strongly in~uenced by that of the substratesurface below[ Figure 3"a# and "b# show ice grains on unpolished and polished Alsubstrates\ respectively\ using re~ected light in an optical microscope[ The grains arevisible as shadows on the surface of the aluminum substrates[ The grains on theunpolished Al "Fig[ 3"a## are generally hexagonal in nature\ having grain sizes ofabout 0[4 mm and under\ and oriented perpendicular to the interface "so the grainsare visible in the plane of the aluminum#[ On the polished Al substrates "Fig[ 3"b##\the grains are smaller in size "averaging less than 0 mm in width# and\ in general\more irregular in shape than on the unpolished substrates[

Figure 4"a# and "b# show the grain structure of the ice grown on the substratescoated with 0 mm of PMMA and PI\ respectively[ The grains in these micrographsare viewed under double!polarized light[ In order to view the samples in this way\ thelight illuminating the samples had to be polarized in one direction before hitting thesample[ After re~ection from the sample\ the light was polarized along the otherdirection and gathered by the microscope objective[ Figure 4"a# and "b# do not showthe hexagonal structure for the ice grains that is seen on the uncoated aluminumsamples[

Before performing experiments on new samples\ all samples were examined usingan optical microscope[ Occasionally\ some samples would show bubbles or othercontaminants either in the bulk ice or close to the ice!aluminum interface[ Thesesamples were discarded and not used in any laser spallation experiments since thesecontaminants could have caused premature failure and skew the data[


Fig[ 2[ Micrographs showing high magni_cation views of "a# unpolished Al and "b# polished Al[


Fig[ 3[ Micrographs showing the grain structure of ice grown on "a# unpolished Al and "b# polished Alsubstrates under re~ected light in an optical microscope[ The grains show up as shadows on the Al

substrate[

2[1[ Test apparatus

A sample holder similar to the one used by Gupta and Tian "0883#\ was constructedto incorporate the new geometry of the current investigation "Fig[ 5#[ In this holder\stainless steel clips held the aluminum!ice sample in the inner chamber " _g[ 5"a##which was held in the freezer at −09>C until the outer chamber "Fig[ 5"b##\ alreadyin place in front of the YAG laser# was cooled to the desired test temperature[ Once


Fig[ 4[ Micrographs showing the grain structure of ice grown on Al substrates coated with 0 mm of "a# PIand "b# PMMA viewed under double!polarized re~ected light in an optical microscope[ Grain boundaries

are indicated by arrows[

the outer chamber was at the desired temperature\ the inner chamber was placedinside such that the water glass!covered surface of the sample was facing towards theYAG laser source[ Both chambers were then allowed to equilibrate to the desired testtemperature for several minutes[ As before\ a copper coil was immersed in a liquidnitrogen bath and gas ~owed through the coil and into the chambers to control thetemperature[ Due to its availability in the laboratory\ compressed air was used as the


Fig[ 5[ A schematic diagram of the ice sample holder used in the aluminum!ice adhesion experiments ] "a#inner chamber ^ "b# outer chamber[

cooling gas\ as opposed to nitrogen used in the cleavage strength measurements ofice crystals of Gupta and Tian "0883#[ Occasionally\ water vapor and oxygen wouldfreeze inside the coils and the ~ow of air was restricted\ but this never prevented thesetup from cooling the chamber to the desired temperature[ The temperature of thechamber was controllable to within 20>C by adjusting the ~ow rate of the air[The lowest test temperature investigated\ −39>C\ was easily achieved using thisexperimental assembly\

The Nd ]YAG laser pulse communicated with the constrained aluminum surfacevia one of the quartz windows that was provided on the outer chamber[ Similarly\ arear quartz window provided the view of the ice surface as needed to inspect the spallphenomenon[ The quartz used for constructing the window facing the YAG lasersource was transparent to the 0[95 mm wavelength of the YAG laser[ A continuous~ow of room temperature air was blown on the windows to prevent condensation ofvapor[


2[2[ Test procedure

2[2[0[ Procedure summary[ The basic structure of the laser spallation experimentdescribed in Section II was used without change except that a di}erent quanti_cationstrategy had to be adopted because of the di.culties encountered in measuring thefree surface velocity of the ice surface[ In the new procedure\ the threshold laser~uence causing the onset of damage at the ice:aluminum interface was determined_rst[ Next\ a bare aluminum surface having identical geometry and stress pulsegeneration conditions\ was subjected to the same laser ~uence that led to the initiationof interface damage in the _rst step\ and the transient free surface velocity resultingfrom the re~ecting stress pulse was recorded by using an interferometer[ This freesurface velocity was used as an input to a _nite!di}erence program which solved theone!dimensional elastic wave equation in an aluminum:ice plug assembly and resultedin the desired peak interface tensile stress[ Although this procedure is not as direct asthe one based on the measurement of the coating|s free surface velocity "as discussedin Section II above#\ it is still less complicated than the original procedure of Gupta\et al[ "0881# which used a simulation to convert the threshold laser energy into thedesired interface stress\ without the use of any interferometer[ In this latter approach"Gupta et al[\ 0881#\ the entire process of stress pulse generation resulting from themelting and associated volumetric expansion of the aluminum _lm\ and its subsequentpropagation to build the tensile stress at the interface\ was modeled[ The variousaspects of the quanti_cation procedure outlined above are preliminary and wellestablished\ and hence only a brief description of each is provided below[

2[2[1[ Determination of the threshold laser ener`y[ At the desired test temperature\a Nd ]YAG laser pulse was _red through the quartz window onto a 2 mm dia areaof the aluminum surface constrained by the water glass layer"Fig[ 5#[ After each shotof the YAG laser\ the ice samples were returned to the cold room and examinedunder an optical microscope[ For the purpose of interface strength measurement\quanti_cation of the initiation of interface damage and not propagation of suchdamage was desired[ Three types of damage were typically seen in most samples\ asis shown in Fig[ 6[ First\ damage occurs at the outer reaches of the incident area orthe stress pulse "labeled A in Fig[ 6#[ This damage is caused by the momentum of thevolume of ice under the in~uence of the stress pulse and the constraint of this volumeof ice by the surrounding ice[ So\ the axial stress sx "normal to the ice surface\ seeFig[ 1# and the con_nement by the surrounding ice create a shear stress which causesthe cracking around the periphery of the stressed spot[ A second type of failure"labeled B in Fig[ 6# is damage within the ice\ that is\ the cracking of columns alongthe grain boundaries[ This damage is caused by the transverse tensile stress\ st\ thatdevelops in the yz plane due to the plane strain constraints of the sample[ The _naltype of failure "labeled C in Fig[ 6# is the interface damage consisting of a voidformation at the interface due to the axial stress sx[ The _rst two types of damagewill not be discussed any further here[ It is the interface damage which is of interestin this investigation[

Interface damage and void formation are more easily seen in Fig[ 7"a# where thelarge decohered regions appear dark compared with the bonded regions immediatelyto the outside and are labeled A[ Normally\ the formation of voids "or one large void


Fig[ 6[ A micrograph showing the three types of failure observed during ice adhesion experiments viewedunder re~ected light in an optical microscope[ "A# pulling out of ice plug ^ "B# cracking of ice columns ^ "C#

interface voids[

in some cases# occurred within the central region of the incident area of the stresspulse\ sometimes extending to the edges of the stress pulse area[ This damage occurredas a result of the axial tensile stress acting to pull the ice away from the aluminum atthe interface[ These voids were easily discernible under the optical microscope\ evenwhen they were small in size\ Fig[ 7"b#[

It is noteworthy that the compressive pulse should not predamage ice since grainboundaries are normal to the interface and hence experience no shear[ Furthermore\the deformation occurs at a strain rate of 09−6 s−0\ which is about _ve orders ofmagnitude higher than the rate at which Picu and Gupta "0884b# observed the grainboundary sliding phenomenon in freshwater columnar ice[

Once damage at the interface was discovered using the optical microscope\ theexperiments were repeated several times to verify that interface damage actuallyoccurred and was repeatable[ Then\ the laser output energy was measured using anenergy meter\ and the laser ~uence "or energy per incident area# was calculated[ Thelaser ~uence at which damage at the interface _rst occurred was then found byrepeated experimentation\ and this was termed the threshold laser ~uence for theparticular aluminum surface treatment at the particular test temperature[ The stresscausing this damage initiation at the interface is a direct measure of the bond strengthbetween the ice and the aluminum[ The method to calculate this stress from therecorded threshold laser ~uence is discussed next[

2[2[2[ Determination of the aluminum free surface velocity usin` interferometry[The wide angle interferometer shown in Fig[ 0 was used for recording the free surfacevelocities from a highly re~ective aluminum surface whose backside was subjected to


Fig[ 7[ "a# A micrograph showing interface voids "labeled A# on a polished Al substrate viewed underre~ected light in an optical microscope ^ "b# Interfacial microbubbles viewed using an optical microscope

at the ice interface with a polished aluminum surface[

the same laser ~uence that led to crack nucleation at the ice:aluminum interface[Except for the ice layer\ every other aspect of the previous sample geometry includingthe water glass layer was identically reproduced[ The interferometer design is some!what similar to that proposed by Amery "0865# and its details have been given earlier"Pronin and Gupta\ 0882#[ A typical fringe record obtained from an Al surface isshown by the solid line in Fig[ 8[ Since the useful duration of the signal is well within


Fig[ 8[ The fringe pattern recorded by using the di}erential displacement!velocity interferometer of Fig[ 0\is shown by the bold line[

the delay time td of the interferometer "�02[2 ns#\ the fringes obtained during thistime are directly related to the surface displacements[ Due to the rather sharp risetime and duration of the signal in the laser spallation setup\ the signal to noise ratioof the recorded fringes is not ideal[ This requires information on the expected velocitypro_le before the fringe data can be unambiguously reduced to the transient velocitypro_le[ To get such information\ a linear velocity interferometer was constructed[ Inthis type of interferometer\ the delay length between the two optical paths is mademuch smaller than the rise time of the velocity pulse so that the photodetector outputis directly proportional to the free surface velocity "Barker\ 0861#[

In this modi_ed interferometer "shown in Fig[09#\ the delay leg of two lenses andone mirror of the interferometer shown in Fig[ 0 is replaced by a de!ionized water!_lled etalon with a mirror on one end[ The etalon was constructed of a plexiglas tubeof 1 in inner diameter and about 6 in length\ capped at the ends by fused silicawindows of 9[4 in thickness[ The fused silica window on the left end had a 4 mm thicklayer of aluminum deposited on the outside to act as mirror M1 as shown in Fig[ 09[Once again\ the apparent position of M1 "M1�# and M0 are equidistant from the BS[Due to the higher index of refraction of water compared to air "¼0[2 compared to0#\ the actual optical position of M1 is at M1��\ yet this is much less than the actualoptical position of M1 in Fig[ 2[ This reduction in actual optical distance traveled


Fig[ 09[ A schematic diagram of the laser spallation technique along with the velocity interferometer withetalon to verify the assumed shape of the velocity pulse[

between the two interferometers reduces the time lag between the delayed beam andthe signal beam to approximately 9[56 ns[ Since the expected rise time of the velocitypulse recorded in this experiment is appreciably larger than 9[56 ns\ the photodetectoroutput is directly proportional to the transient surface velocity[ Figure 00 shows thefringe record "or proportional to velocity pro_le# obtained from the linear inter!ferometer at the same laser ~uence as the one that resulted in the displacementfringe record of Fig[ 8[ It is noteworthy that the linear interferometer only providesinformation proportional to the actual velocity\ with the amplitude of the actualvelocity unde_ned[ This was of no concern here as the aim was just to obtain thepro_le of the surface velocity so that it could be used to judiciously reduce thedisplacement fringe data of Fig[ 8 obtained using the interferometer of Fig[ 0[ It wasfound that the velocity pro_le remained almost identical\ except for the amplitudeover the range of laser ~uences that were needed to initiate damage at the ice:aluminuminterface[ The expected displacement fringe data corresponding to the recorded vel!ocity pro_le with an assumed amplitude\ was determined via integration and matchedwith the displacement fringe pattern recorded using the interferometer of Fig[ 0[ Theamplitude of the recorded velocity pulse was varied till the match between the cal!culated and recorded displacement fringes was the closest[ The dashed line in Fig[8corresponds to the calculated fringe record obtained via the above procedure[ Thecomplete velocity information was next used as an input to the wave propagationsimulation to arrive at the desired interface stress history[


Fig[ 00[ The velocity fringe pro_le as obtained from the etalon interferometer[ The ordinate of the fringerecord was adjusted till a close _t to the recorded fringes in Fig[ 8 was obtained[ Thus\ the as!recorded

fringe record ordinate in units\ of mV was converted into m:s[

2[2[3[ Interface stress calculation usin` a _nite!difference simulation[ The geometryof the ice:aluminum assembly used for the computation is shown in Fig[ 1[ In thecentral region of the stressed area\ the normal strains in the plane of the interface canbe considered fully constrained and hence\ the stress wave propagation through theassembly can be idealized as one!dimensional[ Further\ for ice\ a strain rate of 09−0

s−0 is plenty to invoke an elastic dislocation motion!free response from a polycrystal"Picu and Gupta\ 0884a and b#[ The stress!velocity data in aluminum at strain ratesin the laser spallation experiment\ near 096 sec−0\ is currently unavailable[ Linearextrapolation of data from lower rates\ however\ suggests an elastic response[ There!fore\ the stress state within the aluminum and ice was taken to be elastic throughoutwith the displacements and the resulting stresses in the assembly calculated by solvingthe classical one!dimensional elastic wave equation\ albeit modi_ed by the conditionof zero lateral strains[ Due to this latter condition\ the transverse stresses st developand their magnitude equals the product of the axial stress sx with the term n:"0−n#\where n is the Poisson|s ratio[ The wave equation was solved subject to ] "0# thetraction free condition at the ice|s free surface ^ "1# continuity of traction and velocity


at the ice:aluminum interface ^ and "2# the initial stress condition at the back aluminumsurface given by

s� 9[4rAlcAlV"t# "2#

where rAl is the Al density\ cAl is the speed of a longitudinal wave traveling throughaluminum\ and V"t# is the Al free surface velocity determined from the experimentsdiscussed above[

The simulation modeled the propagation of the stress wave through the aluminum!ice sample "Fig[ 1# for 0 ms by using the _nite di}erence approach[ The Al thicknessused in the program was 9[73 mm while the ice thickness was 9[63 mm[ Furthermore\because of the rather small thickness of the polymeric "0 mm# and liquid!like layer" few hundred _ thick# compared with the stress wave\ they were not identi_ed in thesimulation[ The model incorporated 0999 space nodes and 01\999 time steps\ givingDx�0[47 mm and Dt�9[972 ns[ The density\ one dimensional Young|s modulus"l¦1m#\ Poisson|s ratio\ and longitudinal speed of sound for both Al and ice thatwere used in the simulation are summarized in Table 0[ Speci_c ice adhesion resultson di}erently treated aluminum surfaces are provided next[

3[ RESULTS

For all experiments performed\ regardless of the surface _nish\ applications ofcoatings on the Al\ or test temperature\ it was found that damage initiation at theinterface with ice always occurred between YAG laser ~uences of 0[2×093 J:m1 and1[04×093 J:m1[ The interface stress was calculated at three values of laser ~uencewithin this range "as discussed in Section III#\ and _tted to a curve\ as shown in Fig[01[ Each data point on the curve is the average of seven runs of the _nite!di}erenceprogram with each run using a di}erent velocity record obtained from a fringe recordoutput of the interferometer shown in Fig[ 0[ In all cases\ the velocity pulses input

Table 0[ Material properties for aluminum and ice as used in the _nite differencesimulation

—––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––Property Aluminum Ice

Density "kg:m1# 1699 807One!dimensional 000 8[3Modulus "l¦1m# "GPa#n 9[234 9[22c "m:s# 5398 2199Reference Brandes and Brook "0881# Hobbs "0863#—––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––


Fig[ 01[ A plot of the calculated interface stress as a function of laser ~uence[

into the _nite!di}erence program\ were of similar shape "that is\ similar rise and decaytimes# and varied only in amplitude for di}erent threshold laser ~uence levels[ Thiswas con_rmed experimentally[ This method of determining the interface stressexpedited the data collection process[

The results for the ice adhesion experiments performed in this investigation aresummarized in Table 1[ Those results which show a 2 value were calculated from themethods described in Section III[ The others were determined by reading the interfacestress from Fig[ 01 according to the experimentally determined threshold laser ~uence[The number of experiments in which interface damage was observed at the indicatedinterface stress is shown after the stress level in Table 1[ The number after 2 representsthe error range resulting from the laser pulse!to!pulse variation\ recording and reduc!ing the interferometer data\ and sample preparation procedures[ Details of this esti!mation can be found in Archer "0885#[

The maximum strain rate calculated in the _nite!di}erence program was found tobe about 1×096 s−0[ This\ of course\ depends upon the exact shape of the stresspulse and the actual stress amplitudes\ but since the velocity pulses found with theinterferometer were of the same shape and varied only slightly in amplitude\ the stress


Table 1[ Results for the laser spallation experiments on the Al!ice interface system withvarious aluminum surface treatments

—––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––Interface strength

Substrate Surface Temp ">C# "MPa# No[ of experiments

Unpolished Al −09 163206[0 5Unpolished Al −19 081 3Unpolished Al −29 072 5Unpolished Al −39 068 4Polished Al −09 079[725[3 7Al¦0 mm PMMA −09 089[227[9 7Al¦0 mm PI −09 070 4—––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––All temperatures shown for the experiments are 20>C[ The number of experiments in whichinterface damage was observed at the indicated stress is shown after the stress[

pulses in all experiments were very similar in shape[ Therefore\ the strain rate in allexperiments is in the order of 096 s!0\high enough to justify the elastic wave propagationsimulation[

4[ DISCUSSION

As discussed in Section I\ many studies have been performed attempting to measurethe ice adhesion strength to various substrates[ yet none\ to date\ have been able tocharacterize the intrinsic interface tensile strength[ The current investigation providesdata closer to the true value of intrinsic interface stress than any other report to date[Thus\ data obtained here cannot be directly compared with the results of previousstudies\ but the results are very promising in that the trends of the data follow intuitionand current theories on the existence of the liquid!like layer at the interface[ Qualitativecomparisons are made with other investigations wherever appropriate[

4[0[ Temperature dependence

Table 1 shows the adhesion strength of ice to the unpolished Al substrates to betemperature!dependent[ This temperature dependence is plotted in Fig[ 02 along witha best _t of an exponential function of the form

sx �s9¦a exp14T:b "3#

where s9 is 068 MPa and a and b are _tting parameters whose values are shown inthe _gure[ with falling temperatures\ the interface strength falls\ leveling o} to a valueof about 068 MPa at −39>C[ This temperature dependence is contrary to that foundby Jellinek in tensile tests of ice on polystyrene substrates "Jellinek\ 0846a#\ but in


Fig[ 02[ A plot of the temperature dependence of the interface strength of ice on unpolished aluminumsubstrates and the best _t of an exponential function[

agreement with Nickolayev and Petrenko "0884# in SFM tests of ice adhesion tometals[

This temperature!dependence of adhesion data on unpolished Al substrates can beexplained using the existence of a liquid!like layer at the Al!ice interface[ At relativelyhigh temperatures "−09>C#\ the value of adhesion is the largest[ The liquid!like layer islargest at temperatures close to the freezing point of water and decays with decreasingtemperature "Weyl\ 08409[ Since the layer is liquid!like\ it should be treated asincompressible with Poisson|s ration equal to 9[4[ In this case\ the transverse stress"st# developed within the liquid!like layer equals the axial stress "sx#\ and now moreenergy is dissipated in the transverse direction[ Because of this\ a much larger stressis needed to accomplish interface separation[ At −19>C\ the interface strength hasdropped dramatically\ which also correlates with a corresponding decrement in theliquid layer thickness[ below −19>C\ the value of the interface strength levels o} toabout 079 MPa[ It is a temperature between −14>C and −29>C where the liquid!likelayer disappears "Weyl\ 0840 ^ Jellinek\ 0856 ^ Hosler et al[\ 0846#[ In this case\ the


damping e}ects of the liquid!like layer would be removed completely\ and the interfacestress would correspond directly to the bond strength between the aluminum and ice[The _nal two data points " for −29>C and −39>C# support this view\ as they are thesame[ In order to verify this hypothesis\ however\ more experiments would have tobe performed at intermediate temperatures "between !09>C and −29>C#\ as well astemperatures between 9>C and −09>C[ Interestingly\ the decrement of the liquid!likelayer with temperature follows an exponential curve\ too\ "Weyl\ 0840#\ and thisprovides further credence to the above explanation tying the strength decrement tothe features of the liquid!like layer[

4[1[ Surface rou`hness

As stated earlier\ the discussion of the relationship of substrate surface rough!ness to adhesion strength is purely qualitative\ meaning no data on surface rough!ness was collected and can be compared to actual values of adhesion strength[This should not preclude a discussion of such a relationship[\ however\ as repeatableexperiments reported herein attest to the fact that such a relationship does exist[

As shown in Table 1\ the adhesion strength of ice on aluminum can be greatlyreduced by decreasing the surface roughness of the aluminum[ That is\ by re_ning thesurface quality from that shown in Fig[ 2"a# to that shown in Fig[ 2"b#\ the stresscausing interface damage was reduced from 163 MPa to 070 MPa[ This decrease inthe adhesion strength can be explained as follows ] the smooth Al surface shows manysmall\ thin scratches\ while the rougher surface appears to have only larger hills andvalleys "as evident in Fig[ 2#[ These scratches on the polished Al surface act asadditional nucleation sites for the ice[ This is witnessed in the smaller and moreirregular grain structure seen on the polished Al substrates in Fig[ 3"b#\ as opposedto the larger grains more closely resembling the classic hexagonal structure that iceassumes on the unpolished Al "Fig[ 3"a##[ The larger fraction of intersecting grainboundaries must lead to larger defect density at the interface\ and in addition\ providesites for mechanical stress concentration[ A more detailed discussion is outside thescope of the present investigation[

The lowering of adhesion strength by polishing the substrate surface was also foundby Jellinek "0851#\ although his tests were performed under shear and on stainlesssteel substrates[ In addition\ Sonwalkar et al[ "0882\ found in shear experiments thatthe adhesion strength was proportional to the amount of the mechanical interlockingof ice on the substrate[ Instead of varying the substrate surface roughness\ theyvaried the actual substrates from stainless steel\ copper\ aluminum\ titanium\ topolytetra~uoroethylene "PTFE#[ The stainless steel\ copper and aluminum showedthe largest pit size\ which allowed the ice to interlock on these substrates more thanon titanium and PTFE[ Similarly\ the hills and valleys of the rough Al substrate"in the present investigation# provide a greater likelihood of interlocking and thuscontributed to the increased adhesion strength over polished Al substrates\ whichhave fewer anchoring sites[


4[2[ Addition of polymer coatin`s

The addition of polymer coatings a}ected both the growth of the ice on the Alsubstrates and the measured adhesion strength[ The surface of the polymers is rela!tively ~at\ with few pits\ and therefore few nucleation sites for ice growth[ This impliesthat ice growth will be quite random\ as is evident from Fig[ 4"a# and "b#\ which showthe ice grain structure for substrates coated with PMMA and PI\ respectively[ Thematerials chosen for this investigation were done so because of their availability andtheir durability to repeated testing[ For the present investigation\ the two polymersexhibit slight polarity\ yet not to the extent of Al1O2\ which is inevitably found on theAl surface[ This provided few bonding sites for the ice on the coated substrates"compared with Al alone# which contributes to the rather low " for this study# adhesionstrength values of 070 MPa and 089 MPa for ice on PMMA and PI\ respectively[However\ as discussed below\ there is a potential to lower this strength further[

The structure of PI shows that there are many oxygen atoms available on bothsides of its chain to which hydrogen atoms form the liquid!like layer can attach[ Incomparison\ PMMA lacks the exposed oxygen that PI displays\ but it does have apolar side group\ COOCH2[ Thus\ even though the PMMA coatings on Al substratesprovide fewer bonding sites to the liquid layer in comparison to the PI\ the PMMAlayer is by no means completely hydrophobic[ Attempts to manufacture such coatingsusing the concepts of self assembled monolayers of polymeric material is currently inprogress[

4[3[ Transverse crackin` of ice columns

Finally\ using the simulation\ the transverse tensile stress that led to the crackingof the grain boundaries was estimated to be 36[3 MPa from data obtained at −09>C[These cracks were observed to begin "Fig[ 6# within the bulk of the ice away from theinterface and the free surface[ This value of the grain boundary strength is muchsmaller than the cleavage strengths across the prismatic planes of 0[2 GPa "Guptaand Tian\ 0883#[

5[ CONCLUSIONS

A basic understanding of the nature and strength of this bond is crucial to solvethe deicing problem from a fundamental standpoint[ Motivated by such goals\ a newstrategy to measure the tensile strength of interfaces between ice and structuralsurfaces was developed and presented in this paper[ Because the interface separationis accomplished at an ultra high strain rate "096 s−09#\ all inelastic deformation duringthe interface separation process are essentially suppressed and the measured strengthis the actual strength of the interface[ The tensile strength of ice with the as!manu!factured 5950 Al surfaces was measured as a function of temperature "−09Ð−39>C#\and the results indicated an exponential decrease in the bond strength with decreasein the temperature\ 163 MPa at −09>C to 068 MPa at −39>C[ The e}ect of polishing


the aluminum surface was to reduce the adhesion dramatically\ from 163 MPa forthe as!manufactured surfaces to 070 MPa for surfaces polished on a metallographicwheel using a!alumina powder of sizes 0[9 mm\ 9[2 mm and 9[94 mm[ This was alsoconsistent with results of other investigators "Jellinek\ 0851 ^ Sonwalkar et al[\ 0882#[The same holds true with the experiments on Al substrates coated with polymer _lms[As in other investigations "Jellinek\ 0846a\ jellinek et al[\ 0867#\ polymer coatedsubstrates exhibited lower adhesion strengths when compared with untreated metalsubstrates[ The interface between polymer and ice was the weakest\ and for polyimideand PMMA\ tensile strength values of 089 and 070 MPa were recorded[ This resultshows the promise of developing better hydrophobic coatings with no outstandingpolar groups[

An understanding of the fundamental adhesion is only one step in resolving theoverall ice:adhesion engineering problem[ In any deicing or mechanical ice removalprocess\ the energy consumed in propagating a crack along an ice:structure interfaceor in its vicinity is much larger than what is required to overcome the intrinsic tensilestrength of the interfacial bonds\ and often termed as the total rou`hness G of theinterface[ This depends upon many extrinsic parameters including the specimengeometry\ loading rate\ creep e}ects\ temperature\ roughness and adsorbed impuritieson structural surfaces\ interface ~aw density\ and the ratio of the tensile to shear stressratio at the interface[ Experiments that can sort out the role played by each of theseprocess variables separately are vital for a complete understanding of the ice adhesionprocess[ The results of such a study should be also useful in designing e.cient iceremoval strategies[

ACKNOWLEDGEMENTS

This work was funded via the Army Research O.ce Contract Nos[ DAAL92!81!G!9149and DAAH93!85!0!9909\ and the O.ce of Naval Research Grant No[ N99903!82!0!0095 forwhich we are grateful to Drs Russell Harmon\ Tom Swean\ Y[ D[ S[ Rajapakse\ Tom Curtin\Robert Reeber and Wilbur Simmons of these agencies[

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measurement and control of ice adhesion to aluminum 6061 alloy

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