wettability of aluminum on alumina

Wettability of Aluminum on Alumina

SARINA BAO, KAI TANG, ANNE KVITHYLD, MERETE TANGSTAD,and THORVALD ABEL ENGH

The wettability of molten aluminum on solid alumina substrate has been investigated by thesessile drop technique in a 10�8 bar vacuum or under argon atmosphere in the temperaturerange from 1273 K to 1673 K (1000 �C to 1400 �C). It is shown that the reduction of oxide skinon molten aluminum is slow under normal pressures even with ultralow oxygen potential, but itis enhanced in high vacuum. To describe the wetting behavior of the Al-Al2O3 system at lowertemperatures, a semiempirical calculation was employed. The calculated contact angle at 973 K(700 �C) is approximately 97 deg, which indicates that aluminum does not wet alumina ataluminum casting temperatures. Thus, a priming height is required for aluminum to infiltrate afilter. Wetting in the Al-Al2O3 system increases with temperature.

DOI: 10.1007/s11663-011-9544-z� The Minerals, Metals & Materials Society and ASM International 2011

I. INTRODUCTION

UNDERSTANDING the wetting behavior of alu-minum on ceramics is crucial for metal-ceramic produc-tion of composites. The wettability of aluminum onceramics also plays an important role in aluminumfiltration. Alumina is the most common filter materialand is one of the most typical inclusions. Contact anglesbetween alumina and molten aluminum are used tocharacterize wetting. The wettability between aluminumand alumina should describe on how the metal andalumina inclusions or metal and alumina filter materialinteract. In the past, researchers[1–11] measured thewettability between molten aluminum and solid alu-mina. Table I summarizes the previous measurementsavailable in the literature. There is considerable spreadin the experimental results. Therefore, we attempted toexamine the contact angle under high vacuum condi-tions where the oxide film on aluminum is eliminated.

All the experiments in Table I were performed undervacuum (10�9 to 10�7 bar), except those of John andHausner,[1] who measured the contact angle under argonatmosphere with 10�49 bar oxygen partial pressure.Naidich et al.[4] reported their experimental results butdid not mention the vacuum conditions. Even thoughthe contact angles tend to decrease with the increase oftemperature, the results are scattered (Figure 1). Forexample, contact angles at 973 K (700 �C) measured bydifferent authors were in the range of 88 deg to 167 deg.The scatter may be caused mainly by variations in thespecimens (different purity, pretreatment, crystallization,

surface orientation, etc.) used in measurements anddissimilar experimental conditions (vacuum, oxygenpotential, and the material in furnace tube), as listed inTable I. The high vacuum work tends to give low contactangles as indicated by the solid dots for below 10�9 barvacuum in Figure 1. In our experience, an aluminumsessile drop is always covered by an oxide layer when1 bar argon with 10�22 to 10�21 bar oxygen partialpressure is employed. This leads to the anomalous highcontact angle. For instance, a contact angle of 150 degwas measured at 1273 K (1000 �C) for the Al-Al2O3

system in our early experimental work.[12]

The knowledge of the contact angle at castingtemperatures and higher is of industrial importance tounderstand the mechanisms of filtration and of primingthe filters. The aluminum is oxidized (reaction [1]) evenat oxygen partial pressure of 10�49 bar at 973 K(700 �C) (Figure 2).[13] At this pressure, a 3 liter furnacechamber contains less than 1 oxygen molecule. How-ever, this pressure is impossible to achieve in the currentexperimental apparatus. Nevertheless, the oxide layer onthe surface of a molten aluminum drop can be removedif the outgoing flow of gaseous Al2O according toreaction [2] is greater than the incoming flow of oxygen.The equilibrium partial pressure of Al2O (Figure 2),according to reaction [2], is 4.3 9 10�5 bar at 1273 K(1000 �C). Because we held the total pressure in thefurnace under 10�8 bar, the oxide skin of the aluminumdrop was removed. This process allows us to measurethe contact angles between molten aluminum and solidalumina.

4AlðlÞ þ 3O2ðgÞ ¼ 2Al2O3ðsÞ ½1�

4AlðlÞ þAl2O3ðsÞ ¼ 3Al2OðgÞ ½2�

If the aluminum is covered by oxide, then theexperiments give incorrect and high values for thecontact angles. Most of the published results with lowtotal pressure, 10�9 bar, gave low contact angles. Onemust assume that the oxide on the aluminum has been

SARINA BAO, Ph.D., MERETE TANGSTAD, Professor, andTHORVALD ABEL ENGH, Emeritus Professor, are with theDepartment of Materials Science and Engineering, NorwegianUniversity of Science and Technology, N-7491 Trondheim, Norway.Contact e-mail: [email protected] KAI TANG andANNE KVITHYLD, Researchers, are with the Department ofMetallurgy, SINTEF Materials and Chemistry, N-7465 Trondheim,Norway.

Manuscript submitted January 3, 2011.

METALLURGICAL AND MATERIALS TRANSACTIONS B

Table

I.Equilibrium

Contact

Angle

ofMolten

AlonAl 2O

3in

theLiterature

[1–11]

Reference

Atm

osphere

AlandSubstrate

Tem

perature

[K(�C)]

Contact

Angle,deg

Rem

arks

JohnandHausner,1986[1]

Arwith10�49barO

2partial

pressure

99.99pct

pure

Alandsingle

crystal

Al 2O

3

973(700)

90

Thesessiledropmethod.

Aluminafurnace

tubewas

used.

Klinteret

al.,2008[2]

Highpurity

Ar.

Vacuum

betterthan10�9bar

99.99pct

pure

Al

and99.6

pct

polycrystalline

alumina

943(670)

115

Injectiontechnique*

Graphitesyringeandmullite

furnace

tubewereused.

973(700)

93

1003(730)

94

1023(750)

87

1073(800)

88

99.99pct

pure

AlandA-plane(1120)

sapphire

943(670)

122

973(700)

91

1003(730)

84

1023(750)

86

1073(800)

86

1123(850)

~100

1273(1000)

~87

WangandWu,1994[3]

1.2

910�9barvacuum.

10�49barO

2partialpressure

at973K

(700

�C).

Alandsapphire(001)

973(700)

90

±2

Injectiontechnique.

Zroxygen

getterwaslocatedin

thealuminatube.

Naidichet

al.,1983[4]

Vacuum,butnotspecified

inthepaper

99.995pct

pure

Aland

technicalleuco

sapphirewith

99.97pct

Al 2O

3

973(700)

~90


1073(800)

~85

1173(900)

~82

1273(1000)

~80

1373(1100)

~78

1473(1200)

~65

Ksiazeket

al.,2002[5]

Dynamic

vacuum

of2

910�8bar

99.9999pct

pure

Aland

a-Al 2O

3

(>99.9

pct)

953(680)

126


1023(750)

121

1123(850)

96

1223(950)

79

1323(1050)

74

Shen

etal.,2003[6]

Vacuum

approxim

atley

59

10�9bar.

Highpure

Ar+

3pct

H2

Aland(0001)a-Al 2O

3single

crystal

(99.99pct)

973(700)

127


Taheaterwasused.

Authorssaid

that

thewettingis

anonequilibrium

phenome-

nonbecau

seofcontinuous

oxidation.

1073(800)

130

1173(900)

120

1273(1000)

110

1373(1100)

97

Wolfet

al.,1966[7]

1.3

910�7barvacuum

Commerciallypure

Al(99.1

pct)

andsingle

crystalsapphire

973(700)

167


1088(815)

142

1199(926)

115

1310(1037)

90

1423(1150)

70


removed according to reaction [2] and that the contactangle measured is for the Al(l)-Al2O3 system.In the current study, the contact angle of molten

aluminum on alumina substrate in the temperaturerange of 1273 K to 1673 K (1000 �C to 1400 �C) wasmeasured using the sessile drop technique. The contactangle of Al-Al2O3 system has then been extrapolateddown to 973 K (700 �C).Aluminum oxide covers the molten aluminum in the

launder in the cast house. When aluminum enters analumina filter, the oxide layers on the aluminum is brokenup into particles (inclusions) or it could cover parts of thefilter. The strong metal flow, for example, 15 tons perhour, breaks and removes the oxide film on the filter.Once the metal has entered the filter, oxidation is not aproblem. In any case, molten aluminum is in contact withalumina. We wish to study wetting between aluminumand alumina. In the laboratory, this wetting can bestudied only under high vacuum conditions that providethe contact angle between aluminum and alumina.

II. EXPERIMENTAL PROCEDURE

The experimental apparatus is shown schematically inFigure 3. The apparatus essentially consists of a hori-zontal graphite heater surrounded by graphite radiationshields, located in a water-cooled vacuum chamber. Thechamber was fitted with windows to allow a digital videocamera (Sony XCD-SX910CR, Sony Corporation,Millersville, MD) to record the shape of the droplet.The contact angles and linear dimensions of the imageswere measured directly from the image of the drop usingVideo Drop Shape Analysis software (First Ten Ang-stroms, Inc., Portsmouth, VA).[12] We assume symmetryof the drop. After the experiments, no asymmetry wasobserved.The experiments were carried out with the substrate

99.7 pct pure alumina and with 99.999 pct pure alumi-num. The aluminum rod with a diameter of 2 mm wascut into small pieces around 2 mm in length, thenpolished by 500-mesh sandpaper and cleaned withethanol to prevent subsequent oxidation. The averageroughness of the alumina substrate was 393.13 nm.Argon gas was cleaned with an all-pure gas purifier fromALLTECH,* followed by magnesium turnings at 723 K

(450 �C). Inside the vacuum chamber there was also asmall oxygen getter furnace which contained Ti spongeat 923 K (650 �C). The oxygen potential in thegas outlet from the furnace was measured with aRAPIDOX 2100.** When the potential was sufficiently

Table

I.Continued

Reference

Atm

osphere

AlandSubstrate

Tem

perature

[K(�C)]

Contact

Angle,deg

Rem

arks

Carnahanet

al.,1958[8]

1.3

910�7barvacuum

99.99pct

pusreAlandrecrystallized

Al 2O

3oftriangle

RR

quality

1516

±4(1243

±4)

60


Moresistance

heatingelem

ents

wasused.

Authors

did

notattain

asteady

valueatagiven

temperature.

BrennanandPask,1968[9]

(2.6–11.7)

910�8barvacuum

99.999pct

pure

Alandsapphire

(single

crystalAl 2O

3),‘‘water

soaked’’

�

1073(800)

90


Zroxygen

getterwaslocatedin

thegraphitetube.

1173(900)

45

99.999pct

pure

Alandsapphire

(single

crystalAl 2O

3),‘‘heat

treated’’

�

1073(800)

76

1173(900)

55

Nicholas,1968[10]

Vacuum

betterthan

3.9

910�8bar

AlandAL23gradealumina

(>99.5

pct)

943(670)

158


1291(1018)

90

Laurentet

al.,1988[11]

Totalpressure

of4

910�9bar

andO

2partialpressure

approxim

ately

10�20bar

Alandsingle

crystalAl 2O

3933(660)

103

±6


Moresistance

furnace

was

used.

1273(1000)

86

±6

*Injectiontechnique:

Themetaldropisforced

onto

asubstrate

mechanicallyfrom

asyringeonce

theexperim

entalconditionsare

reached.

�Watersoaked:Thesubstratesweresoaked

inwateratroom

temperature

foraboutaweek.

�Heattreated:Thesubstrateswerepreheatedin

vacuum

at1573K

(1300

�C)just

before

theexperim

ent.

*ALLTECH is the trademark of Alltech Associated, Inc., 2051Waukegan Road, Deerfield, IL 60015-1899.

**RAPIDOX 2100 is a zirconia oxygen gas analyzer from Cam-bridge Sensotec Limited, 31 Elizabeth Court, St Ives, CAMBS PE275BQ, United Kingdom.


low, the sample was quickly heated to 1223 K (950 �C)in approximately 80 seconds, then heated to 1273 K,1373 K, 1473 K, 1573 K, and 1673 K (1000 �C,1100 �C, 1200 �C, 1300 �C, and 1400 �C) with a heatingrate of 50 K/min (50 �C/min). In all the experiments, thecontact angle and linear dimensions of the droplet wererecorded simultaneously during the isothermal period at1273 K, 1373 K, 1473 K, 1573 K, and 1673 K (1000 �C,1100 �C, 1200 �C, 1300 �C, and 1400 �C).

III. RESULT AND DISCUSSION

A. Vacuum or 1 Bar Ar Gas?

The wettability of aluminum on solid alumina sub-strates depends strongly on the experimental conditions,particularly the atmosphere. A high vacuum environ-ment will promote aluminum evaporation and results in

anomalous low contact angles, as illustrated in Figure 4.The contact angle after evaporation is reduced from hato hr because of the reduced drop size. It seems that thebase diameter does not change. The height of the sessiledrop is reduced also. To evaluate the influence of theatmosphere on the contact angle of the Al-Al2O3 system,preliminary experiments were carried out. The sampleswere held 60 minutes at 1473 K and 1673 K (1200 �Cand 1400 �C) under the 10�8 bar vacuum and 1 barargon with 10�22 to 10�21 bar oxygen partial pressure,respectively. The results are shown in Figure 5.The contact angles obtained in an argon atmosphere

were higher than those determined under vacuumcondition. As shown in Figure 5, at 1473 K (1200 �C)the apparent contact angle was approximately 160 degunder the argon atmosphere, whereas the contact angledecreases from 138 deg to 80 deg in vacuum at the sametemperature. Similar phenomena occurred for the con-tact angles at temperature 1673 K (1400 �C). However,the aluminum droplet disappeared fast in vacuum at1673 K (1400 �C) because of evaporation. The aluminafilm, with the melting point 2345 K (2072 �C), acts as asolid shell, encloses the molten aluminum, and leads tothe overestimate of the contact angle. The thinner theoxide layer, the less the measured contact angle is untilthe surface of molten aluminum is uncovered. Obvi-ously, it takes much less time to remove the oxide skin invacuum than in argon as the argon atmosphere impedesthe gasification as shown by Coudurier et al.[15] Underan argon atmosphere, contact angles at 1673 K(1400 �C) are almost the same as in vacuum at 1473 K(1200 �C). This indicates that high temperature and along time are necessary to remove the oxide skin in anargon atmosphere.Figure 6 shows the preliminary experimental results

obtained from 3 step-heating experiments. The sampleswere isothermally held at 1473 K, 1673 K, and 1873 K(1200 �C, 1400 �C, and 1600 �C) for 1 hour at each

900 1000 1100 1200 1300 1400 150050

75

100

125

150

175 John.1986,Ar with10-49 bar Po

2

Klinter.2008, Vac.<10-9 bar

Wang.1994,1.2*10-9 bar vac. with 10-49 bar Po2

Naidich.1983, vac. but not specified in paper

Ksiazek.2002,2*10-8 bar vac.

Shen.2003,~5*10-9 bar vac. or Ar+3% H2

Wolf.1966,1.3*10-7 bar vac.

Carnahan.1958,1.3*10-7 bar vac.

Brennan.1968,~10-7 bar vac.

Nicholas.1968,Vac.<3.9*10-8 bar

Laurent.1988,4*10-10 bar vac. with10-20 bar Po2

Con

tact

ang

le/o

Temperature/K

John,1986Wang,1994Naidich,1983

Fig. 1—Contact angle of Al on Al2O3 as function of temperature reported in.[1–11]

900 1000 1100 1200 1300 1400 1500 1600 1700

-50

-40

-30

-20

-10

0

PO2

by Reaction (1)

PAl2O

by Reaction (2)

Log

( P/b

ar)

Temperature/K

Fig. 2—Equilibrium partial pressure of gas species in reactions [1]and [2] calculated from FactSage v6.2.


temperature. As expected, contact angles in argon areapproximately 40 deg higher than in vacuum. Again, itindicates that the oxide skin is more stable under argonatmosphere. Every 200 K (200 �C) increase of temper-ature results in reduction of the contact angle by 20 to25 deg in an argon atmosphere. The contact angles at1673 K and 1873 K (1400 �C and 1600 �C) could not bemeasured under vacuum mainly because of the strongaluminum evaporation.

Based on the preceding preliminary experimentalresults, we conclude that the atmosphere has a signif-icant impact on the wetting behavior of molten alumi-num on alumina. Because of the low diffusion rate ofAl2O(g) at high pressure, it is difficult to reduce the

oxide layer according to reaction [2] at 1 bar atmo-sphere even with low oxygen potential. Hence, it isrecommended to use high vacuum; 10�8 bar vacuum isemployed in this study. In addition to the atmosphere,the roughness of substrate, mass of aluminum droplet,temperature, holding time, oxide film thickness, alumi-num evaporation, different substrate pretreatments, andsurface orientation of substrates influence the wettingbehavior, as shown in Table I. The preceding parame-ters in each trial are reasonably the same in the currentstudy employing the same materials with the samepretreatment.

B. Wetting Properties

Figures 7 and 8 show the time-dependent wettingproperties of aluminum on alumina at 1273 K (1000 �C)under vacuum conditions. The first experiment (240409)failed because of the relatively small size aluminumdrop. No stable contact angles were determined. How-ever, stable contact angles were detected for the secondmeasurement (061210) with a relatively large aluminumdrop. In the later experiments, the contact anglesdecrease rapidly from 145 deg to 124 deg during thefirst 50 minutes, followed by a relatively slow reductionto approximately 70 deg extending to 750 minutes in thestage 1. Removal of the oxide layer and evaporation ofmolten aluminum are reasons for the reduction of the

Fig. 3—Schematic of the sessile furnace.

Fig. 4—Schematic illustration of the effects of evaporation on theprofile and parameters of a sessile drop[14] (R: the sessile drop baseradius; h: contact angle for aluminum on the substrate; H: the sessiledrop height).

0 10 20 30 40 50 60

40

60

80

100

120

140

160

Con

tact

ang

le/o

Time/min

1473K,10-8 bar vacuum 1673K,10-8 bar vacuum 1473K,1 bar Ar,Po2=10-21 bar

1673K,1 bar Ar,Po2=10-22 bar

Fig. 5—Contact angle vs time for Al on Al2O3 at 1473 K and1673 K (1200 �C and 1400 �C) in Ar and vacuum.

0 20 40 60 80 100 120 140 160 18020

40

60

80

100

120

140

160

1873 K1673 K

Con

tact

ang

le/o

Time/min

1 bar Ar, PO2

=10-23-10-21bar

10-8 bar vacuum

1473 K

Fig. 6—Contact angle vs time at 1473 K and 1873 K (1200 �C and1600 �C) in Ar and vacuum.


contact angle. In the second stage, the base diameter isnearly stable and the equilibrium contact angle (67–70 deg) was obtained. The time needed for removal ofthe oxide layer at 1273 K (1000 �C) is estimated to beapproximately 750 minutes. In stage 1, reduction of theoxide layer is the main reason for the decrease of contactangle.

Figures 9 and 10 are the experimental results at1373 K (1100 �C). The diagrams can be divided into twostages. Similar to the results shown in Figures 7 and 8,the contact angles decrease rapidly from 158 deg to87 deg in the first 58 minutes. Then, a slow reduction incontact angle occurs until approximately 63 deg(approximately 259 minutes). A decreasing sessile dropvolume indicates that the reduction of the oxide layerand evaporation of aluminum take place simulta-neously. In the second stage, the base diameter becomesnearly stable and the equilibrium contact angle (60–63 deg) has been obtained. The time needed for removalof the oxide layer at 1373 K (1100 �C) is estimated to beapproximately 259 minutes. The contact angles seem todecay exponentially with time at 1273 K and 1373 K(1000 �C and 1100 �C).

Figure 11 and Figure 12 show similar results at1473 K (1200 �C). It clearly shows two stages for thevariation of contact angles. A sharp decrease of contact

angles from 118 deg to 53 deg (approximately80 minutes) has been observed in the stages I becauseof the high aluminum evaporation rate. In the secondstage, the sessile drop base diameter becomes stable,which allows the equilibrium contact angle (47–57 deg)to be measured. It is found that the reduction of theoxide layer at 1473 K (1200 �C) takes approximately80 minutes.

0 200 400 600 80060

80

100

120

140

160 061210 240409

III

Con

tact

ang

le/o

Time/min

Fig. 7—Contact angle vs time for Al on Al2O3 at 1273 K (1000 �C)in 10�8 bar vacuum.

0 200 400 600 8001

2

3

4

5

6

7

8

III

Sess

ile v

o. &

Bas

e di

a.

Time/min

061210,Sessile volume/ul 061210,Base diameter/mm 240409,Sessile volume/ul 240409,Base diameter/mm

Fig. 8—Sessile volume and base diameter vs time for Al on Al2O3 at1273 K (1000 �C) in 10�8 bar vacuum.

0 50 100 150 200 250 30060

80

100

120

140

160

III

Con

tact

ang

le/o

Time/min


0 50 100 150 200 250 3001

2

3

4

5

6

7

IIISe

ssile

vo.

& B

ase

dia.

Time/min

Sessile Volume/ul Base diameter/mm

Fig. 10—Sessile volume and base diameter vs time for Al on Al2O3

at 1373 K (1100 �C) in 10�8 bar vacuum.

0 20 40 60 80 10012014016018020040

50

60

70

80

90

100

110

120

III

Con

tact

ang

le/o

Time/min



The measured contact angles for the same system at1573 K (1300 �C) are shown in Figures 13 and 14.Again, two stages are shown clearly in the diagrams. Adecrease of both sessile drop volume and base diameterin stage 2 indicates that very high evaporation takesplace. However, it is clear that the relatively stablecontact angle of 50–55 deg occurs (from around 40 min-utes) in the second stage. The total oxide layer removaltime at 1573 K (1300 �C) is approximately 40 minutes,which is much shorter than at 1273 K (1000 �C) and1373 K (1100 �C).

Great efforts were made to obtain the Al-Al2O3

contact angle at even higher temperatures, for example,1673 K (1400 �C) under vacuum condition. Unfortu-nately, the rapid evaporation at temperatures higherthan 1573 K (1300 �C) prevents us from obtainingreliable results.

C. Calculation of Contact Angle

Consider a solid surface in contact with a liquid in thepresence of a vapor phase. If the liquid does not coverthe solid completely, then the liquid surface will intersectthe solid surface at a contact angle h. The equilibrium

value of h, used to define the wetting behavior of theliquid, obeys the classical equation of Young:

cos h ¼ rSV � rSL

rLV½3�

We have not taken into account the effect of thecurvature of the aluminum droplet.[16,17] Most of thereported surface tension measurements pertain to‘‘oxygen-saturated’’ samples.[18] According to Millsand Su,[18] the surface tension of oxygen-saturated[18]

molten aluminum can be evaluated:

rLV ¼ 875� 0:18 T� TMð Þ ¼ 1043� 0:18T ðmNm�1Þ½4�

Eustathopoulos, Nicholas and Drevet[14] reported theexperimental surface energies of alumina by the multi-phase equilibrium technique. The extrapolation of theexperimental data to low temperatures gives a value of1400 ± 500 mN m�1, which agrees roughly with thefirst principle simulation results[19] (Figure 15).

0 20 40 60 80 100 120 140 160 180 2000

1

2

3

4

5

6

III

Sess

ile v

o. &

Bas

e di

a.

Time/min

Sessile volume/ul Base diameter/mm



0 20 40 60 80 100 120

50

60

70

80

90

100

110

III

Con

tact

ang

le/o

Time/min


0 20 40 60 80 100 1200

1

2

3

4

5

IIISess

ile v

o. &

Bas

e di

a.

Time/min

Sessile volume/ul Base diameter/mm



0

200

400

600

800

1000

1200

1400

1600

1800

2000

900 1100 1300 1500 1700 1900Temperature/ K

Sol

id A

l 2O

3 su

rfac

e en

ergy

/ m

N/m Eustathopoulos

error limit

average

σSV = 2077-0.7083T

Fig. 15—The surface energy of solid Al2O3 vs temperatures.


The work of adhesion EA is equal to the sum of thesurface tension of molten aluminum, rLV, and thesurface energy of solid alumina, rSV, minus interfacialenergy, rSL.

EA ¼ rLV þ rSV � rSL ½5�

When the two surfaces are brought into contact andreplaced by the interface, the work of adhesion holdsthe two surfaces together. The work of adhesion be-tween molten metal and the solid oxide is a measureof the bond strength between the two media. Girifalcoand Good[20] concluded that the work of adhesion isproportional to the geometric average of the surfaceenergies of two media:

EA ¼ 2uffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

rLVrSVp ½6�

where u is a function of the molar volumes of the liquidand the solid. In the current work, the deviation betweenthe calculated and measured interfacial energies isminimized regarding the coefficient u as temperaturedependent. Figure 16 shows the fitted line for thecoefficient u. The proportionality factor increasesmonotonically with temperature. The experimentalresults at low temperatures reported by John andHausner,[1] Klinter et al.,[2] and Wang and Wu[3] havebeen included. They all achieved a total pressure lowerthan 10�9 bar, except John and Hausner.[1] The otherresults reported in the literature[4–11] seem to haveemployed either a high total pressure or less purematerials.

Based on the preceding discussion, the followingequation is obtained for the interfacial energy ofaluminum and alumina:

rSL¼rLVþrSV� 1:55�14:38exp �0:0029Tð Þð Þ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffirLVrSVp

½7�

where rLV and rSV are given by Eq. [4] and Figure 15.The equation is applicable for the temperature rangefrom 973 K (700 �C) up to 1773 K (1500 �C). FromEq. [7], the contact angles of the aluminum and alu-mina at various temperatures under vacuum can becalculated using Eq. [3]. At the melting point of

aluminum, rSL = 1.63 J/m2 is obtained from Eq. [7].Nikolopoulos et al.[21] proposed a similar semiempiri-cal relation (Eq. [8]) for the metal and alumina or zir-conia interface at the melting point of the metal

rSL ¼RO

RAl� xy� VAl

VAl2O3

þ 1

� �2=3ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

rLVrSVp ½8�

where R is the radii of the ions; V is the molar volume;and x = 2, y = 3 for AlxOy. RO/RAl = 2.80,VAl = 11.36 cm3/mol, and VAl2O3

= 25.62 cm3/mol. Itgives rSL = 1.66 J/m2 at 933 K (660 �C), which agreeswell with our calculation results in even thoughNikolopoulos et al.[21] has not included aluminum intheir correlations.Figure 17 shows the calculated contact angle with the

measured values at various temperatures. The contactangle between aluminum and alumina at 973 K (700 �C)is approximately 97 deg, which indicates that aluminais not wetted by molten aluminum at the castingtemperature.It is observed that wetting of the Al-Al2O3 system

improves with increasing temperature. To initiate filtra-tion of aluminum, it may be necessary to use a primingheight up to 400 mm[22,23] or to heat the filter to thetemperature above the melting point of aluminum. Inour filtration pilot trials, the metal height above the filterwas approximately 200 mm. Once the metal has infil-trated the filter, metal height can be reduced. To preventthe oxidation, it is advantageous to employ a cover gasof argon or nitrogen.[24,25]

IV. CONCLUSIONS

In the filtration of aluminum using an alumina filter,aluminum is in contact with alumina. Therefore, it isimportant to study the wetting behavior of aluminum onalumina. The wetting behavior between molten alumi-num and solid alumina in the temperature range 1273 Kto 1673 K (1000 �C to 1400 �C) under argon atmosphere

900 1000 1100 1200 1300 1400 1500 16000,0

0,5

1,0

1,5

John.1986,Ar with10-49 bar Po2

Klinter.2008,vac.<10-9 bar Wang.1994,1.2*10-9 bar vac. with 10-49 bar Po

2

This work, 10-8 bar vac. Fitted line

2ϕ

Temperature/K

John.1986Wang. 1994

Fig. 16—The coefficient u fitted to literature data and this work.

1000 1200 1400 160040

50

60

70

80

90

100

110

120

John.1986,Ar with10-49 bar Po2

Klinter.2008,vac.<10-9 bar Wang.1994,1.2*10-9 bar vac. with 10-49 bar Po

2

Naidich.1983,vac. but not specified in paper This work, 10-8 bar vac. Calculated

Con

tact

ang

le/o

Temperature/K

John.1986Wang. 1994

Fig. 17—The calculated and measured contact angle vs temperaturefor Al on Al2O3.


and vacuum has been determined using the sessile droptechnique. The equilibrium contact angles are 67–70 degat 1273 K (1000 �C), 60–63 deg at 1373 K (1100 �C), 47–57 deg at 1473 K (1200 �C), and 50–55 deg at 1573 K(1300 �C). The equilibrium contact angle cannot bemeasured at 1673 K (1400 �C) because of the rapidaluminum evaporation rate. The times for removal of theoxide layer on the aluminum drop can be estimated fromthe experimental curves, approximately 750 minutes at1273 K (1000 �C), 259 minutes at 1373 K (1100 �C),80 minutes at 1473 K (1200 �C), and 40 minutes at1573 K (1300 �C).

To predict the wetting behavior of the Al-Al2O3

system at lower temperatures, a semiempirical calcula-tion for the temperature dependence of contact anglewas used in the current paper. The contact anglesdecrease exponentially with increasing of temperatures.The calculated contact angle of Al-Al2O3 system at973 K (700 �C) is 97 deg, which is in good agreementwith the experimental values in the literature.

The effect of atmosphere on the contact angle ofAl-Al2O3 system was also examined in this article. Underan argon atmosphere, even with ultralow oxygen partialpressure, the oxide skin on the aluminum drop was notremoved efficiently. However, under high vacuum con-ditions, the oxide skin is removed, and equilibriumcontact angles between aluminum and alumina can, thus,be determined from the sessile drop measurement.

Improved wetting of aluminum on alumina withtemperature is an advantage in getting molten metal toinfiltrate alumina. In priming alumina filters, it is neces-sary to have a metal height above the filter or to increasethe temperature. However, filtration may proceed atlower temperatures once metal has entered the filter.

ACKNOWLEDGMENT

This research was carried out as part of the NorwegianResearch Council (NRC) funded BIP Project (No.179947/I40) Remelting and Inclusion Refining of Alumi-num (RIRA). It includes the following partners: HydroAluminum AS, SAPA Heat Transfer AB, Alcoa NorwayANS, NTNU, and Sintef. Funding by the industrial part-ners and NRC is acknowledged gratefully. Thanks arealso given to Tone Anzjøn for assisting in the sessile droptests.

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wettability of aluminum on alumina

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