Metal-ceramic contact angles determined by thin film techniques

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44 (1974) 585-597 0 North-Holland

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BY THIN FILM TECHNIQUES

K. PRABRIPUTALOONG

and M. R. PIGGOTT

Materials Research Centre. Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Canada M5S IA4

Received 28 August 1973; revised manuscript

received 7 May 1974

A method of estimating contact angles is described in which 1000 A thick layers of metals are condensed from the vapour. If condensation takes place at temperatures greater than about 0.8 Tm the film is observed to break up into drops which are approximately segments of spheres, and the contact angle may be calculated from measurements on the drop size when the average film thickness is known. The experimental method is straightforward and relatively very fast, yet gives results in good agreement with earlier measurements. Results for the following metals on sapphire are presented: Al, Ag, Au. Cu, In, Sn. In addition results for Al on quartz are discussed, and it is suggested that the method may give meaningful results even when, as in this case, chemical reaction takes place.

1. Introduction It is often useful to know the contact angle in metalceramic systems because of the desirability of achieving good wetting in the manufacture of cutting tools (metal-metal carbide, and metal-diamond combinations) and in the manufacture of fibre and whisker reinforced metals. The sessile drop method has been extensively used, but is not entirely satisfactoryl). The main difficulties are the long time taken to reach equilibrium, and the risk that chemical reactions may interfere with the measurements). It was noticed by one of the authorss) that thin metal films, condensed on heated substrates, often broke up into tiny droplets of approximately circular section. The droplets were fairly uniform in size, and the average size appeared to be a function of film thickness. The effect thus appeared to be due to the contact angle between the metal and substrate. Singh and Murr4) have observed this same phenomenon even at temperatures as low as 0.8 T,,, (T,,, is the melting point of the metal in “K). This paper investigates the possibility of determining contact angles in such films from measurements of droplet size. Since a small drop may reach equilibrium much faster than a large one the use of small drops could speed up the measurement, and make possible some estimation of contact angle 585

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K. PRABRIPUTALOONG

AFiD M. R. PIGGOTT

in reactive systems. Murrs) has recently used the scanning electron microscope to measure the contact angles of somewhat larger drops directly. 2. Evaporation technique Metals, in the form of thin wires of 99.999% purity, or better, were evaporated from a tungsten filament inside a special filament holder designed so that the filament could be clearly seen by reflection, but metal vapour could only escape through a small hole. The substrate was in the form of 1-2 mm thick discs, also of about 99.999% purity and was placed inside a I cm cubic heater box made from tantalum sheet, heated by alternating current. A small hole in the top of the box admitted the metal vapour, so it condensed only at the centre of the disc. The experimental method is described in more detail by Prabriputaloong and Piggott 6). The temperature of the disc was measured using a thermocouple welded to an insulated tantalum sheet on which the disc sat. The accuracy of temperature measurement was about & 10°C. The average film thickness was determined from the weight of wire evaporated and the distance between filament and disc, assuming that the tungsten filament emitted the metal vapour in all directions equally. The accuracy of the filament-specimen distance was better than 1%. The experiments were carried out at a pressure of about 5 x 10m6 tort-, and as soon as the metal had all evaporated, both filament and specimen heater currents were interrupted. A palladium shadowed carbon replica of the specimen was examined in a Phillips EM 300 electron microscope. 3. Type of deposit obtained Fig. 1 shows micrographs of aluminum films ranging in thickness from 300 to 3000 A condensed on sapphire (0001) at 800°C and subsequently cooled to room temperature. It is clear that the droplets have a range of sizes, and in the case of the 3000 A films, some of them are not even approximately sections of spheres. Fig. 2 shows some typical histograms of drop sizes (secondary droplets, having sizes less than 0.45 times the modal value of drop size have been neglected because the volume they contribute is not very significant, volume being proportional to the cube of the apparent diameter) In the case of the 2000 A and 3000 A films the distribution is bimodal, while 300 A and 1000 A films have approximately normal distributions. A plot of the modal values obtained is shown in fig. 3. The bimodal distributions are represented by dumb-bells in the graph. The plot is linear, and goes through the origin. Thus the ratio of average film thickness to

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Fig. 1. Al films of various thicknesses condensed at 800°C on sapphire. Bars indicate 1 pm. (a) Thickness 300 A, (b) 1000 A, (c) 2000 A, (d) 3000 A.

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K. PRABRIPUTALOONG

AND M. R. PIGGOTT

IOOIO0 g

A

SO-

0 “a Y

@-

0 I -

40-

z z 20-

0

DROP

Fig. 2.

Distributions

DIAMETER,

of droplet sizes from Al films condensed

FILM

Fig. 3.

MICRONS

Modal values obtained

at 800°C.

THICKNESS, A

from droplet size distributions. sured for each value plotted.

About 350 drops mea-

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apparent drop diameter is approximately constant. If the drop size were governed by contact angle, we might expect this type of behaviour, as shown in the next section. 4. Effect of constant contact angle Suppose s&icient material is evaporated to form a continuous film having an average thickness ft, and suppose that the film breaks up into uniform co-equal droplets, radius P. For contact angles less than fn, each droplet wifi have an apparent diameter of 2r sina (fig. 4a) where cxis the angle of contact.

b

a

Fig 4.

Spherical dropIets.

The volume of the droplet is Y= 7W* (3r-

W)/3 = rr3r3(1 - cosi%)” (2 +cosa)/3.

0)

The droplets do not appear to be very effciently packed (fig. 1). The volume of film present on unit area of substrate can therefore only be calculated from the volume of individual droplets if the number of droplets per unit area, it, is also known. However, if the packing efficiency is constant and can be evaluated, II can be calculated from drop size. A hexagonal pattern of almost touching drops is the most efficient arrangement, giving a number of droplets per unit area of rr“MX = a/ (J3 d2).

(2)

Let us suppose that the number of drops per unit area observed experimentally is n = e&la*, (3) where e is an e%ciency factor governed by the drop arrangement and lack of proximity.

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K. PRABRIPUTALOONG

We can now equate

the volume

from its average thickness,

AND

M. R. PIGGOTT

of film present

and the total volume

h=nV=2xer3

(I-cos~)~

area calculated

of drops, i.e.,

(2+cosc()/(3J5dZ)

using eqs. (l), (2), and (3). Replacing r by d/(2 since) and rearranging

Similarly,

on unit

(4)

we find that

6.61 h

(1 - coscz)z (2 + coscc)

ed

sin3 c(

(5)

for a > +r, 6.61 h ed

=4-(1

+cosCr)2(2-cosa),

where d is now the true drop diameter (fig. 4b). Thus for constant contact angle and packing efficiency, h/d is constant. Furthermore, using the known value of contact angle for aluminum, the value of e can be determined from the slope of the line in fig. 3. It comes to 0.87, if we take c(= 100” - the approximate mean of Brennan and Pa&s values of 90” and 113” (according to whether the sapphire was heat treated or not) at this temperature. More accuracy at this stage is hardly warranted,

Fig. 5.

1000 8, Al film condensed on sapphire at 530°C.

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since some of Brennan and Pask’s results varied by as much as +5x, even in one set of tests. It should be noted that most of their work was carried out on a different crystallographic face than used in this work (60” to the axis, compared with the basal plane). However, they found the contact angle to be essentially independent of the orientation of the sapphire. This value of e is about that which could be obtained with square packing. Examination of the droplets in the thinner films (e.g. 300 A, fig. 1) shows that the number of droplets (roughly 300 in the picture) corresponds to the measured median drop size (about 0.077 urn) if square packing is assumed. Perfect agreement cannot be expected, since re-evaporation of the metal takes place at 8OO”C, and drop coalescence occurs, leaving substantial bare areas. (Three pairs of coalescing drops are visible on the 300 A film, and bare areas are evident.) At lower temperatures, these processes occur more slowly and a film condensed at 500 “C shows less apparently bare area, fig. 5. Another factor which may prevent perfect agreement is the volume change of aluminum on freezing (6.5%). This could result in shrinkage of the drop by about 2%, and could also account for some of the bare area. However, it is clear that the films do show contact angle effects. They are easy to prepare and examine in the microscope, and are therefore worth considering for use in contact angle measurements. Measuring the sizes of hundreds of drops is clearly a troublesome way of

FILM

THICKNESS,

%,

Estimated droplet size of Al films condensed at 8OO”Cr.

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K. PRABRIPUTALOONG

AND

M. R. PIGGOT-I

determining contact angle. If we can assume that droplets of other materials pack in the same way as the aluminum droplets, we should be able to estimate d from a drop count over a known area. This was done for the aluminum films at 8OO”C, and fig. 6 shows that a linear plot is obtained up to 1000 A, and deviations occur above this thickness. The slope of the linear part gives a contact angle of 105 &-5”. Agreement with the literature values is still good, indicating that drop counting works for weil-shaved films (i.e., ones without bimodal distributions). The maximum film thickness suitable for this method seems to be 1000 A. The technique was then applied to other metals, and to aluminum at a different temperature (900X!), and the results compared with literature values. In table 1, these values are presented. It can be seen that in all cases except copper the results agree within the limits of accuracy of the sessile drop method. In every case apart from aluminum the thin film result is smaller than the sessile drop result. This could be because surface impurities, present on the pieces of metal used for sessile drop measurement, have interfered with the melting process. In the case of copper, some oxide is almost certain to be present in the thin film tests, and Chaklader et a1.s) have shown that the contact angle of copper decreases sharply with small additions of oxide. [In the case of aluminum the oxide film is evaporated off in the form of Al,Os).] Alternatively, the discrepancies could arise from the approximations and assumptions made in the thin film method. It should be noted that accurate film thickness measurements using the recently developed quartz crystal weighing method, e.g., the Kronos Film Thickness Monitor, would provide useful supporting data, and reduce the dependence of the method on the assumptions. It even seems that the concept of contact angle has some validity below the melting point. Fig. 5 shows that aluminum films condensed at 130°C below the melting point form droplets, and fig. 7 shows that continuous aluminum TABLE

Contact

1

angles compared with literature values (in all cases the substrate was values from the literature were obtained by the sessile drop method)

A1203;

Contact angle (deg) Metal

Al AU Ag CU

Temperature (“C) 900 1100 960 1230

Small drop

90 13s 132 145

Literature value 90 140 140 163

Reference

Brennan and Pask7) Brennan and Pask ?) Rhee16) Chakiader et al.*)

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Fig. 7.

fniti&y

centi~uous

Fig. 8.

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AI fiim heated for I.5 min at about 500°C.

IO00 A AI film condensed at 420°C.

593

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K. PRABRIPUTALOONG

0

1

AND M. R. PIGGOTI

2

3

4

f(d) Fig. 9.

Right hand side of eqs. (5) and (6) plotted as a function of contact angle.

films, formed by condensation at 25°C show a tendency to break up and form islands on heating to 500°C for 15 min. At lower temperatures droplet formation becomes rather slow, and at 420°C they are no longer circular, and are surrounded by what appear to be fringes of material that has not been fully incorporated, fig. 8. Liquid-like behaviour below the melting point (or lowering of the melting point) of small particles of metals has been investigated in considerable detaillo~ls). A graphical method was used for the evaluation of c( from eqs. (5) and (6). The right hand side of these equations is plotted versus c1in fig. 9. Themethod thus involves the deposition of films 1000 A thick, the estimation of d from a drop count over a known area, the calculation of 7.34 h/d, and the use of fig. 9 (or a table of values) to determine the contact angle.

5. New contact angle measurements (1) Aluminum on sapphire. Contact angles have been measured in the range 510” to 1 lOO”C, and fig. 10 compares the results obtained in this work with literature values. (2) In and Sn on sapphire. Fig. 11 shows the results obtained for these metals in the range 220°C to 620°C. As with aluminum, contact angle increases as temperature is decreased. (3) Aluminum on quartz. This system is a reactive one14) but reaction rates are low below the melting point’s). Normal approximately spherical droplets, do form at temperatures below the melting point, but at 750°C

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595

NO-

t Champion et al. o Brennan & Pask

160 -

l

Present work

80 -

60 -

I 500

I 600

I 700

I 800

TEMPERATURE,

I 905

I 1000

I 1100

I 1200

“C

Fig. 10. Contact angles of Al on basal plane of sapphire compared with literature values for the basal plane [Champion et aLz)] and for a plane at 60” to the c axis [Brennan and Pask?)].

f 200

300

I 400

I 500

I 600

1 700

TEMPERATURE ,OC

Fig. 11.

Contact angles for In and Sn on sapphire.

596

Fig. 12.

K. PRABRIPUTALOONG

AND M. R. PIGGOTT

Effect of chemical reaction on appearance (b) 750°C.

of droplets Al on SiOz: (a) 67O”C,

180- c. ‘0..

%.

160 -

ALUMINUM 0%.

0%.

l*

a*..

Y s 140g

ON QUARTZ

%* . %

n

h

“*..

’

z 2

120-

\

5 5 0 loov

80-

I

I

500

I

600

700

TEMPERATURE,?

Fig. 13.

Contact angles for Al on quartz.

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the appearance

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of the film changes (fig. 12). Presumably

597

at 750°C the reaction

rate is sufficiently great for a significant amount of reaction product to have formed before the droplets have had time to grow. It appears to be possible, however, to obtain meaningful data in the range 500-700°C (fig. 13). This shows the normal decrease of contact angle with increasing temperature.

6. Conclusion Simple droplet counting over known areas appears to be an adequate method of estimating contact angle from micrographs of condensed films. The method is quick, and gives results in fair agreement with previous work. It is possible to measure contact angles below the melting point of the metal, and to do measurements on reactive systems, so long as the reaction rate is not too great. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16)

J. J. Bikerman, Physical Surfaces (Academic Press, New York, 1970) p. 239. J. A. Champion, B. J. Keene and J. M. Sillwood, J. Mater. Sci. 4 (1969) 39. M. R. Piggott, J. Appl. Phys. 39 (1968) 4438. H. L. Singh and L. E. Murr, Phil. Mag. 16 (1972) 649. L. E. Murr, Mater Sci. Eng. 12 (1973) 277. K. Prabriputaloong and M. R. Piggott, J. Am. Ceram. Sot. 56 (1973) 177. J. J. Brennan and J. A. Pask, J. Am. Ceram. Sot. 51 (1968) 569. A. C. Chaklader, A. M. Armstrong and S. K. Misra, J. Am. Ceram. Sot. 49 (1968) 630. C. R. Alcock, Trans. Brit. Ceram. Sot. 60 (1961) 147. D. J. Turnbull and R. E. Cech, J. Appl. Phys. 21 (1950) 804. M. Takagi, J. Phys. Sot. Japan 9 (1954) 359. M. Blackman and A. E. Curzon, Structure and Properties of Thin Films (Wiley, New York, 1959) p. 217. N. J. Gladkich, R. Niedermeyer and K. Spiegel, Phys. Status Solidi 15 (1966) 181. K. Prabriputaloong and M. R. Piggott, J. Am. Ceram. Sot. 56 (1973) 184. K. Prabriputaloong and M. R. Piggott, J. Electrochem. Sot. 121 (1974) 430. S. K. Rhee. J. Am. Ceram. Sot. 55 (1972) 300.