Scripta METALLURGICA
et MATERIALIA
Vol. 29, pp. 293-298, 1993
Printed in the U.S.A.
Pergamon Press Ltd.
All rights reserved
S T R U C T U R E A N D P R O P E R T I E S OF JET VAPOR D E P O S I T E D
ALUMINUM-ALUMINUM OXIDE NANOSCALE LAMINATES
L.M. Hsiung l, J.Z. Zhang2, D.C. Mclntyre 3, J.W. Golz 2, B.L. Halpern 2, J.J. Schmitt 2 and H.N.G. Wadley 1
I Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903-2442;
2jet Process Corporation, 25 Science Park, New Haven, CT 06511;
Division 1834, Sandm National Laboratories, Albuquerque, N M 87185.
3
.
.
(Received February 8, 1993)
(Revised May 18, 1993)
Introduction
Recently, we have fabricated aluminum-aluminumoxide (presumably A1203) nanoscale laminates using the novel Jet Vapor Deposition
(JVD) process[l-3]. The JVD process is of interest because it uses high speed (He) gas jets in "low vacuum" to inexpensively deposit
multilayered thin films (laminates) at potentially very high rates (0.1 - 1 cm3/min). A preliminary study of the structure and hardness
(strength) of the first of these JVD fabricated laminates is reported below. One reason materials with thin layered structure are of interest
is because they may offer novel mechanical and physical properties (see a recent review by Froes and Suryanarayana[4]). Koehler first
proposed using alternate thin layers of a high and a low shear modulus materials to produce high strength composite materials[5]. There
are many potential approaches for fabricating materialslike this. Conventional eutectic/eutectoid reactions to produce alloys with lamellar
microstructures, the yield strength (Oy) has been correlated with the spacing (~) according to oy = o* + k 4. "n, where o* is a frictional
stress, k is a constant, and n for example equals to 0.5 in Fe-FesC eutectoid pearlite[6] and NisAl-NisNb lamellar eutectics[7] (although
~ = 1 can also fit the data). The yield strength of a NisAI-NisNb lamellar eutecties, produced by directional eutectic solidification,
increases from -750 MPa at a 5 I,tm lamellar spacing to 1.5 GPa at a 1 I~m spacing. However, attempts to further elevate strength by
decreasing the spacing below I I.tm proved unsuccessful because of the onset of cellular solidification and loss oflamellar structure. Other
techniques for synthesizing lamellar or multilayer structures include electron vapor deposition[8-11], r.f. magnetron sputtering[12], and
elcctrodeposition[l 3]. Vapor deposition techniques promise a particularly interesting alternative pathway to the synthesis of useful high
st rcngth layered materials if their rates ofdeposition can be increased. Alpas et al.[l 1] recently measured the tensile properties of AI-A1203
microlaminates grown by an electron-beam vapor deposition (EBVD) process (deposition rate: -3.5 nm/sec). The thickness of the
aluminum oxide layer was kept constant at 5 rim, and the thickness of the metal Al layer was varied from 50 to 500 rim. They found that
the maximum value for Oy (475 MPa) was obtained for ~, = 50 nm and decreased with increases in metal layer thicknesses. Although
lacking the perfection of more slowly grown films, we show that the JVD nanoscale laminates exhibit comparable strength.
Fa,w,fimr,m a l
A schematic illustration of the JVD process is shown in Fig. 1. In principle, it can be used to inexpensively deposit thin or thick
multilaycr films at potentially high rate and in almost unlimited material combinations[l-3]. The JVD vapor source is a nozzle,
incorporated in a "low vacuum" (several hundred Pa), mechanically pumped, fast flow system. Pure helium cartier gas emerges from the
nozzle as a highly collimated sonic or near sonic jet. Atomic or molecular vapor, generated in the nozzle by evaporation, is entrained in
the jet and forms a localized deposit on a downstream substrate. One or more jets can be aimed at a rotating/oscillating substrate carousel.
The moving substrates are thereby "scanned" and coated uniformly. Multiple jets, when operated simultaneously, yield alloys, or
sequentially, multilayers (laminates)[3]. Here, aluminum-aluminum oxide layer laminates were grown by means of an A1 "wire feed" jet
source. Alvaporwascreatedin the nozzle by feedingan aluminumwire, undercomputercontrol, toahotBN-sheathedWfilament. High
purity (99.99%) molecular O 2 was periodically admitted through a pulsed valve just downstream of the nozzle. When the 02 flux was off,
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AI layers were deposited; when 02 was on, aluminum oxide (presumably A1203) was formed by reactive deposition at the surface of the
growing film. The pulsed valve, duty cycle, AI wire feed velocity, and carousel rotation frequency were chosen to allow alternating
deposition of nanometer dimension AI and aluminum oxide layers at nominal rates of -5.6 nrn/sec over an area of 50 can2. The laminate
was deposited at room temperature on a carbon coated glass slide with a layer of salt between the glass and carbon to permit detachment.
The resulting laminates had a nominal aluminum layer thickness of between 15 and 30 nm. The nominal thickness of each oxide layer
was approximately 2.5 nm.
A thin layer of epoxy was used to attach free standing (100 pm thick) laminates to fused quartz slides to facilitate nano-hardness
measurements. Hardnesswasmeaanredusingnanoindentationtechniques[14]onaNanoinstrumentslnc.
nano-indenter equipped with
a triangular pyramid diamond tip. The sides of the pyramid made an angle of 65.3 ° with the normal to the base of the indenter. The
triangular indentations were approximately 7.4 times larger than the indentation depth, and were made at a maximum load of 0.049 N
in a load control mode. Loads were recorded for every 25 nm ofindenter displacement. Hardness values were corrected for thermal drift
and elasticity. Both plan-view and cross-sectional TEM samples were prepared from a laminate (10 I~m thick) by standard dimpling and
ion-milling procedures. To reduce irradiation damage, ion-milling was performed at 4 V, 0.5 m.A under liquid nitrogen temperature. The
total milling times were less than 4 h. Samples were examined using either a Phillips-400T or a Joel-400OEX high resolution transmission
electron microscope.
Results and D i s c u s s i o n
Microstructuw
Figure 2(a) is a plan-view TEM micrograph showing the microstructure of a typical laminate. Only the reflections corresponding to
the metallic AI could be found in the selected-area diffraction (SAD) pattern shown in Fig 2(b). This suggests that the aluminum oxide
is amorphous, in agreement with the findings of Alpas et al.[ I 1] and Chou et al.[12]. Figure 2(a) shows that the microstructure of the A1
layers. This consisted of AI clusters or islands 50 - 120 nm in diameter. Within these islands were numerous crystalline AI grains (ranging
in size from 5 to 50 nm); they are more readily seen in the dark-field image shown in Fig. 2(c). Note that the gaps between AI clusters with
a bright contrast shown in Fig. 2(a) were presumably voids formed when the AI layer was first deposited. Those voids were subsequently
filled with aluminum oxide as the oxide layer was deposited. Further examinations of the nanograins nucleated within the AI clusters (by
tilting the TEM sample) indicates that these grains were rarely in contact with one another (i.e. few grain boundaries were observed). We
therefore postulate that the metal between the nanograins was amorphous. To confirm this postulation, the microstructure was also
examined by high resolution transmission electron microscopy (HRTEM) under many-beam conditions. The result is shown in Fig. 3.
While amorphous aluminum oxide was observed between two AI clusters, two crystalline AI grains ( -8 nm in diameter) were found within
an AI cluster. The growth direction of the two crystalline AI grains was determined to be [I 11]. One can see that these two crystalline AI
grains partially overlap and are rotated -25 ° with respect to each other, resulting in the formation of Moir* fringes appearing at the upper
right of Fig. 3. The width (d) of the Moir~ fringes was -0.32 nm, in agreement with the relation: d = d / 0 , where d (= 0.14 nm) is the
(220)AI interplanar spacing, and 0 is the rotation angle in radians. The image surrounded the nanocrystals in Fig. 3 appears to be
featureless, i.e. the A1 layer was partially amorphous - a surprising result in comparison with the findings of Alpas et al.[l 1] where only
crystalline AI layers were observed following ambient temperature deposition. To assure that the amorphization was not an artifact
induced by ion milling during sample preparation, an aged sample (500°C, 2h) prepared by the same procedures also was examined. No
amorphous AI was found within the aged sample.
Figure 4(a) shows a typical cross-sectional micrograph of a laminate. The layered structure of the laminate is evident from this
micrograph. The mean thickness of the AI layers is - 16.5 nm. For these rapidly deposited films, the A1 layers were not totally continuous,
and the aluminum/aluminum oxide interfaces were not planar. The coexistence of amorphous (diffuse rings) and crystalline AI (labeled
refraction rings) within the laminate is revealed in the selected-area diffraction (SAD) pattern shown in Fig. 4(b). In addition, stacking
of the At clusters occurs within the laminate [see Fig. 4(c)]. It is unclear at present whether the amorphous layer results from limited
surface/bulk diffusion due to high deposition rate/low temperature or the incorporation of oxygen in the metal layer.
Nanohsrd~:~
Four hardness measurements were made on a 100 lxm thick laminate. The average maximum displacement for four indentations was
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825 ± 40 nm. Scanning electron microscopy inspection of the indents showed no indentation cracks. There was also no evidence of
cracking in the load-displacement data. The average measured hardness (H) from four indents was 2.3 ± 0.2 GPa. This corresponds to
an estimated yield strength (H/3) of approximately 770 ± 60 MPa[ 15]. This compares favorably with the hardness of as-deposited A1/Fe
laminates synthesized by the EBVD method[10]: H -- 2.25 GPa (estimated yield strength -750 MPa) for layer thickness of A128.8 nm and
of Fe 0.7 nm. Although we do not, at the present time, have enough data available to correlate the strength of the laminates with the AI
layer thickness at nanometer range, a comparison between the estimated ay of the JVD laminates and that of the EBVD laminates
reported by Alpas et al.[l 1] is shown in Fig. 6. Even though these first JVD laminates lack the perfection of the EBVD laminates, their
strength (770 ± 60 MPa with a 16.5 nm thick AI layer) was close to that extrapolated from the result of Alpas et al.: ay (MPa) = 30 +
3.2.U~'~(mm-"~). It is unclear if the above relation (equation) is still valid at Al layer thicknesses smaller than 50 nm, since the theoretical
maximum achievable strength of the AI-Al20.-~(amorphous) laminates predicted by the Koehler's model was -560 MPa[5]. If Koehler's
model is valid, the extra strength (-210 MPa) measured for the JVD laminates might be due to other strengthening mechanisms, for
instance: interstitial atom strengthening. It is worth noting that the Koehler's model predicts a strength for the AI/Fe laminates of -730
MPa, in agreement with the experimental value (-750 MPa) mentioned above. More experimental data are required to resolve this.
Sugary
The novel Jet Vapor Deposition (JVD) method has been used to produce Al-aluminum oxide laminate samples with nanoscalc
microstructure. The microstructure and properties of Al-aluminum oxide nanoscale laminates deposited at room temperature have been
investigated using electron diffraction, electron microscopy and nanoindentation techniques. Preliminary results show that an imperfect,
but still layered, structure was formed within the laminates. The aluminum oxide layers were entirely amorphous, whereas the A1 layers
were mixed amorphous/crystalline. The Al layers were not totally continuous; they contained many Al clusters (20 to 120 nm in diameter)
within which many crystalline AI grains (5 - 50 nm in diameter) had nucleated. A yield strength of 770 ± 60 MPa was achieved for an A1
layer thickness of 16.5 nm.
This research was supported by the Defense Advanced Research Projects Agency (W. Barker, Program manager) and the Office of
Naval Research (S. Fishman, Program manager) under Contract No. N00014-91-J-4089. Microhardness measurements were per formed
by T.M. Rice of Sandia National Laboratories-New Mexico and the work at Sandia was supported by the Department of Energy under
Contract No. DEACIM76DP00789.
Rr,f.ex.c,a m n
1. B.L. Halpern, J. Colloid & Interface Sci., 86 (1982), p. 337.
2. J.J. Schmitt, U.S. Patent No. 4,788,082, November 29 (1988).
3. B.L. Halpem, J.J. Schmitt, Y. Di, J.W. Golz, D.L. Johnson, D.T. McAvoy, D. Wang, and J.-Z. Zhang, Metal Finishing, p. 37,
December 1992.
4. F H . Froes and C. Suryanarayana, JOM, 41, (June 1989), p. 12.
5. J.S. Koehler, Phys. Rev., B2 (1970), p. 547.
6. W. Heller, in Rail Steels, $ T P 644, ASTM, Philadelphia, 1979.
7. E.R. Thompson et al., Proc. Conf. In Situ Composites II, eds. M.R. Jackson et al., p. 539, Xerox Individualized Publishing, Lexington,
Mass., 1976.
8. S.L. Lehoczky, J. Appl. Phys. 49, (1978), p. 1814.
9. R.W. Springer and D.S. Catlett, Thin Solid Films, 54 (1978), p. 197.
10. R.L. Bickerdike et al., Internat. J. Rapid Solidification, 1 (1984-85), p. 305.
I I. A.T. Alpas, J.D. Embury, D.A. Hardwick and R.W. Springer, J. Mater. Sci., 25 (1990), p. 1603.
12. T.C. Chou, T.G. Nieh, S.D. McAdams, G.M. Pharr and W.C. Oliver, J. Mater. Res., 7 (1992), p. 2774.
13. T. Foecke and D.S. Lashrnore, Scripta Metall. Mater., 27 (1992), p. 651.
14. W.C. Oliver et al., in ASTM STP 889, eels. P.J. Blau and B.R. Lawn (ASTM, Philadelphia, PA, 1986), p. 90.
15. R.F. Bishop, R. Hill and N.F. Mott, Proc. Phys. Sci., 42 (1945), p. 147.
,n
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NANOSCALE
LAMINATES
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~ a s Inerl
~1torr ~
x
Vaporsource
t~it I V"lOSm/s
Substrate
~"-~ted
gasjet
Fig. 1 Jet vapor source. The deposit vapor is generated near the nozzle exit by mechanisms such as
thermal vaporization and sputtering. The dotted arrows indicate substrate motion in two dimensions
for large area coverage and uniformity.
C
Fig. 2 (a) Plan-view, bright-field (BF) TEM micrograph showing a typical microstructure
of an aluminum-aluminumoxide laminate deposited at room temperature; (b) a selected-area
diffraction (SAD) pattern generated from the area in (a), and (c) plan-view, dark-field (DF)
TEM image obtained from the same area in (a).
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Fig. 3 Plan-view HRTEM micrograph showing the formation of crystalline AI grains within an AI cluster.
i
Fig. 4 (a) Bright-field (BF) cross-sectional TEM mierograph showing a typical morphology of layer structure
fomaed in a laminate; (b) selected-area diffraction (SAD) pattern reveals the coexistence of amorphous layers
and crystalline AI in the laminate; (c) bright-field cross-sectional TEM micrograph showing stacking of AI
clusters within the laminate.
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Interlamellar Spacing (nm)
50O
100
50
25
I
I
I
I
10
/
m
AI- A I 2 0 a
700
297°K
600"¢:~ 500
•
-
•
O0
.~D
>-
JVD
Aipaset al.
30oI
209
100
0
I
100
I
200
I
300
Reciprocal Root of
Interlamellar Spacing, ~"/' (mm"v')
Fig. 5 A plot of the yield strength versus AI layer thickness of both EBVD synthesized[11] and JVD synthesized
aluminum-aluminum oxide laminatesi The mean deviation is indicated by the vertical bars.