Plh S 1359-8368(96)00023-6
ELSEVIER
Composites Part B 28B (1997) 57-69
© 1997 Elsevier Science Limited
Printed in Great Britain. All rights reserved
1359-8368/97/$17.00
Functionally graded materials synthesis via
low vacuum directed vapor deposition
J. F. Groves and H. N. G. W a d l e y
Intelligent Processing of Materials Laboratory, School of Engineering and Applied Science,
University of Virginia, Charlottesville, VA 22903, USA
(Received 20 February 1996; revised 18 April 1996)
The spatially distributed microstructures needed to implement many functionally graded material (FGM)
designs are difficult to realize affordably with today's materials synthesis/processing technologies. To
address this need, a new directed vapor deposition (DVD) technique has been developed and explored as a
potential FGM synthesis tool. The technique exploits supersonic helium jets in combination with electron
beam/resistive evaporation under low vacuum (10-3-10 Torr) conditions to atomistically spray deposit a
wide variety of monolithic and composite materials. Two of the most important processing parameters (the
carrier gas velocity and the deposition chamber pressure) that control deposition are identified, and their
effect upon deposition efficiency for flat and fiber substrates is explored systematically. Under certain
conditions, the DVD approach is found to deposit vapor onto fibers with a significantly higher efficiency
than traditional high vacuum line-of-sight vapor deposition techniques. It can even deposit material onto
surfaces that are not in the line-of-sight of the source. A computational fluid dynamics model has been used
to interpret the experimental observations and to identify the role of carrier gas dynamics in controlling
deposition efficiency and spatial distribution. © 1997 Elsevier Science Limited. All rights reserved
(Keywords: vapor deposition; electron beam evaporation; functionally graded materials; supersonic; gas jet; materials
processing; computational fluid dynamics)
1 INTRODUCTION
Microlaminated and continuous fiber reinforced metal
matrix composites (MMCs) are being investigated in conjunction with functionally graded materials' (FGM) concepts for many high temperature light weight load
bearing aerospace structures I 10. Functional grading in
these applications can occur at the microscale to control
laminate spacings ~1'~2 and to create fiber-matrix interface architectures that chemically protect the fibers, provide low sliding resistance, and reduce thermal expansion
mismatch stresses 9. F G M concepts can also be employed
at the mesoscale. For example, the fiber spacing within a
composite can be varied so that stiff, strong, low thermal
expansion coefficient fibers are concentrated in regions of
highest stress or temperature, Figure 1. These F G M
concepts promise new generations of structural materials
that can withstand the severe thermal and mechanical
stresses encountered during the service of aircraft engines,
marine turbines, power plants, rocket motors, and space
structures 13.
Before F G M s can enter service, tightly controlled
material synthesis techniques must be developed for their
affordable fabrication. Many of the methods under
evaluation for continuous fiber M M C production can be
considered as possible F G M process pathways. A variety
of approaches are being explored for the fabrication
of traditional (i.e. uniformly spaced) fiber reinforced
MMCs including molten droplet spray deposition 14'15,
tape casting with powder slurries 16, the foil-fiber-foil
technique 13, a powder-foil method 13 and vapor deposition
approaches 17. The vapor deposition approach is of particular F G M interest as it seeks to coat fibers uniformly
with an alloy of interest and then use hot isostatic or
vacuum hot pressing to synthesize a composite component. Because vapor deposition makes possible control
of the metal coating thickness, it could facilitate precise
fiber spacing control and could represent a means for
achieving mesoscale functional grading as schematically depicted in Figures l(a) and (b) 13'17. The vapor
deposition route can also be used to create laminated
FGMs, Figure 1 (c).
A range of vapor deposition techniques including
sputtering, chemical vapor deposition (CVD) and thermal vaporization (e.g. electron-beam and resistive heating) have been explored for the metal coating of fibers
and the creation of microlaminates (see Bunshah 18 for a
review of these processes). Sputter deposition is conducted in a relatively high vacuum (~ 10-4 Torr) and has
attracted significant interest since it can vaporize all
metals and alloys and deposit them atomistically with
significantly higher adatom kinetic energy than many
57
FGM synthesis via low vacuum DVD: J. F. Groves and H. N. G. Wadley
conventional thermal evaporation processes is. This
higher adatom energy can be crucial for synthesizing
high quality microstructures, especially at low substrate
temperatures and relatively high deposition rates 19 21.
Despite this advantage, sputter deposition is often not
the deposition method of choice for large volume production of continuous fiber reinforced composites because
of the method's low deposition rate, < 1 #m/min and high
energy input to material production ratio22. Another
deposition method, CVD, can deposit uniform films on
surfaces with complex topologies including the entire
circumference of a fiber. For example, it is used to synthesize the SiC reinforcements used in many MMCs and
to functionally grade their interfaces. However, employing the CVD approach for most of the metals/alloys
envisioned for use in FGM concepts requires the use of
expensive, often highly toxic precursor gases. Furthermore, it utilizes those gases slowly and creates additional
hazardous gases as by-products. It can thus become an
expensive route to FGM synthesisTM.
Thermal evaporation under high vacuum conditions
(10-4-10 8Torr) is widely used for the deposition of
many metals and even some ceramics. For example,
electron beam (e-beam) evaporation from skull melts
allows almost any metal (and some ceramics) to be
evaporated and deposited atomistically, even at very
high rates 17'22 24. The most significant drawbacks of
the high vacuum e-beam coating are the relatively high
cost of a high vacuum system, its relatively low materials
utilization efficiency when coating continuous fibers
(a consequence of line-of-sight deposition), and its nonuniform coating characteristic resulting from a vapor
flux which spreads out with a distribution described
by a cos"0 function where n = 2,3,4 or more (see
Figure 2).
Recently, a novel Jet Vapor Deposition (JVD TM)
process has been reported, which uses an inert gas jet in
comination with a resistive evaporation source to concentrate and deposit various materials under low vacuum
(~ 1-10 Torr) conditions with high local efficiencies1'2'12,18.
By using multiple jets and reactive codeposition concepts,
the JVD TM process appears to offer considerable potential
for creating laminated or fiber reinforced FGMs 1'2'12'25.
However, a significant drawback to the JVD TM process
is the difficulty it has rapidly evaporating and depositing
the large, uncontaminated volumes of high melting point
or reactive materials (e.g. Ni and Ti) needed for many
FGM applications, principally due to its reliance on
resistive evaporation.
Here, a directed vapor deposition (DVD) process is
described which combines the high rate refractory/reactive
metal evaporation capabilities of electron-beams with
the flexibility/high materials utilization efficiency of a
low vacuum JVDTM-like process 26'27. The design of a
synthesis system that utilizes a modified axial e-beam gun
in a nontraditional (low vacuum) e-beam environment is
addressed, and the fundamental issues that govern its
materials utilization efficiency for both laminated and
fiber reinforced FGM synthesis are explored.
58
a) Coated fibers with differentcoatingthicknesses ~
Ti alloycoatingwith -~
differentcoating
thicknesses~
-
SiC Fibers
b) Post-consolidatedcomposite
with different fiber spacings
Ti alloylayersof different
thickness& spacinc
C) Laminated composite
Ceramic(e.g.,TiN,TiC)layersof different
thickness& spacing
Figure 1 Functional grading concepts employed in MMCs. (a) Fiber
coating thicknesses can be varied. (b) After hot isostatic or vacuum hot
pressing, a composite with a depth dependent fiber fraction is obtained.
(c) Functional grading using laminate layers can also create materials
with depth dependent properties
Flux(l(x, y, z))
l(x, y, Zo) = locosno
|
(n = 2, 3, or 4) L ] ~ ~ "
Uncnat~_d fih~.r
~
---.y
Most thi
coated fi
or
Bent el
per crucible
Continuous
target feed
Figure 2 The high vacuum processing environment of traditional ebeam systems leads to line-of-sight deposition onto fibers, and the
peaked vapor flux distribution results in significant variation in coating
thickness on fibers and flat substrates
2 DVD SYSTEM OVERVIEW
The directed vapor deposition technique uses a combination of electron-beam (or resistive) evaporation
coupled with a supersonic carrier gas jet to deposit a
potentially wide variety of materials.
2.1 Electron beam evaporation
A schematic illustration of the DVD system is shown
FGM synthesis via low vacuum DVD. J. F. Groves and H. N. G. Wadley
in Figure 3. It uses a continuously operating 60 kV/10 kW
e-beam gun to evaporate rapidly a variety of materials
from a continuously fed skull melt contained in a water
cooled crucible. The small beam diameter (0.4 ram) and
the high accelerating voltage (60 kV) of the DVD system's axial e-beam make possible high rate evaporation
in a relatively unexplored low vacuum (10 - 3 - 1 0 Torr)
e-beam processing environment. This low vacuum regime
contrasts with the high vacuum (10 -4 10-8 Tort) environment which has long been used for conventional
electron beam vapor processing systems 22.
To function in this low vacuum environment, the axial
e-beam gun employs differential pumping of the gun
column and a design that reduces gas flow from the low
vacuum processing chamber to the high vacuum e-beam
generation and focusing region of the column while permitting free, relatively undiminished propagation of the
e-beam to the target for evaporation. During processing,
the pressure in the beam generating region within the
e-beam gun is maintained at 10 5-10 -7 Torr by a standard turbomolecular high vacuum pump. Once the ebeam is created in this space, it is transmitted with
minimal energy loss down the gun column into a l0 -3 Torr
pressure region evacuated by a mechanical differential
pumping package and is electromagnetically focused to
a diameter of 0.4mm. Finally, the focused electronbeam emerges into a deposition chamber through a small
(~2.5mm) hole in a replaceable tungsten plug which
separates the gun column from the process chamber.
In the DVD system, the electron-beam impinges upon
a small diameter source rod contained within a watercooled copper crucible, Figure 3. The impinging electrons
heat the rod stock and form a molten evaporant pool
along its top surface. The edges of the rod stock, in
contact with the cooled crucible wall, remain solid. Thus,
a 'skull' encased melt is formed which ensures that the
molten portion of the rod stock comes only into contact
with solid portions of the rod stock. By containing the
melt inside of a solid skull of its own composition,
undesirable melt/crucible reactions leading to contamination of the vapor stream are prevented. This skull
melting evaporation source is similar to that already
widely used in conventional e-beam systems because it
results in contamination-free deposits 22'23. Skull melting
and evaporation is essential for evaporating many of
the reactive, refractory materials (e.g. Ti) envisioned for
use in F G M applications, The design of the crucible
(Figure 3) makes possible continuous replenishment of the
evaporant source as material is vaporized from the top.
2.2 Flux environment/propagation
In the DVD system, the evaporated source material
is entrained in a supersonic inert gas flow where it is
Electron gun
vacuum
==l•High
pump
beam
Purification
system
Differential~
Pressure
gauge
pump
Mass
flow
controller
Heater
v
Mixing chamber
source
material
Contifuous
source
feed e
Pressure
Compressed
helium cylinaer
gauge ' ~
~......
Throttl
plate
Fibers
or flat
substrate
~
~
Mechanical
chamber
pump
Figure 3 In the low vacuum DVD system,electronbeam evaporatedsource material is transferredto a substrate by a directed gas flowentering the
chamber through a nozzle
59
FGM synthesis via low vacuum DVD: J. F. Groves and H. N. G. Wadley
accelerated and transported towards a substrate. The
entire gas flow system (Figure 3) has been designed to
enable repeatable, high purity synthesis conditions to be
achieved. High purity carrier gases from compressed gas
cylinders are conducted through a continuously operating purification (gettering) system to reduce oxygen and
moisture levels below one part per billion. From the
purification system, the gas flows into a parallel array of
helium calibrated mass flow controllers capable of
controlling flows of 0.5-200 standard liters per minute
(slm). From the mass flow controllers, the gas passes into
a mixing chamber (where other process gases could be
added) before being routed into a gas flow tube that ends
with a nozzle located inside the deposition chamber. The
deposition chamber is pumped by a large (30000 lpm @
1 Torr) mechanical rotary piston pumping package
capable of maintaining deposition chamber pressures in
the 0.01-10Torr range for the stated gas fluxes. The
deposition chamber pressures are monitored using (gas
type independent) capacitance manometer gauges. The
ratio of gas pressures up and downstream of the nozzle
exit controls the gas jet velocity directly. This pressure
ratio can be increased either by reducing the nozzle exit
diameter or by opening a variable position throttle plate
located between the deposition chamber exit and the
chamber pump (i.e. increasing the gas flow rate out of the
processing chamber).
Helium has been chosen as the primary carrier gas
because of its small electron scattering cross-section and
thus long electron-beam propagation distance. Experiments indicate that electron-beam penetration through
an inert gas can be described with reasonable accuracy
using a modified Bethe stopping power formula28. The
Bethe range is considered to be the total distance that the
beam's electrons can travel through the gas before losing
all of their energy29. Figure 4 shows the predictions of
this model for an electron-beam propagating through
both helium and argon at 5 Torr. While the propagation
of electrons through helium is clearly better, the results
of Figure 4 obviously do not preclude the use of low
partial pressures of other inert (e.g. Ar), or perhaps even
reactive carrier gases.
100
. . . . .
I. . . . .
_
E
I . . . .
~
I
I
H
I
I
e
10
o
E
.~_
cO
.1.-,
0.1
(~
Q.
0.01
P = 5 Torr
0.001
0
I
I
I
I
I
I
10
20
30
40
50
60
70
Accelerating voltage (kV)
Thedistinctlydifferentelectronpropagationdistances(Bethe
ranges)in heliumandargonareillustratedforambienttemperatureand
5Torrpressureconditions
Figure 4
2.4 Multi-element deposition
The DVD system is a versatile synthesis approach
because several carrier gas streams can be used either
simultaneously or sequentially to deposit a variety of
materials. Figure 5 shows a setup using three jets where
two use an inexpensive resistance heater to evaporate less
reactive/low melting point metals. As a result of this
multi-source capability, the materials which can be deposited considerably exceed those achievable with many
traditional approaches. For instance, alloys containing
elements with widely varying vapor pressures (e.g. T i Mg or Ti-Nb alloys) cannot be deposited stoichiometrically from a single evaporation source, and use of
multiple crucible sources to accomplish the task can be
quite inefficient in a traditional e-beam system22. In a
DVD system, separate sources could be used to direct all
of the vapor from each source towards the substrate,
increasing materials utilization efficiency. Additionally,
the DVD system is ideally suited for reactive compound
deposition. Reactive elements can easily be fed into the
processing chamber through the gas system or another
reactive element injection system to reconstitute compounds decomposed during evaporation or to create new
compounds with pure elements evaporated from the
crucible. Such system flexibility considerably expands a
DVD system's FGM synthesis options.
2.3 Deposition
Either flat or fibrous, stationary or rotating/translating substrates can be used in the DVD system. Stationary
substrates facilitate study of evaporated source material
distribution in the carrier gas stream and basic investigation of the vapor interaction with a flat or fiber substrate.
The rotating, translating apparatuses would be used as
part of a deposition strategy 'that controls coating thickness uniformity. They can also simplify heating of the
substrates during deposition (Figure 3). To date, infrared
heater lamps have been used to heat samples to at least
600°C during processing. This ability to heat the substrate during deposition is vital for creating the microstructures that possess th e properties required for FGM
applications ~9.
60
3 PROCESSING FUNDAMENTALS
The capture of e-beam evaporant (at 10-3-10 Torr) in a
supersonic carrier gas jet represents an unexplored
materials synthesis environment with new processing
variables that have unknown effects upon the deposition
efficiency, uniformity, and microstructural quality of
fabricated films. In particular, the velocity and density of
the carrier gas flow both merit study as changes in either
are likely to affect significantly the concentration and
velocity distribution of the vapor entrained in the carrier
gas stream. Such changes are likely to affect the material
utilization efficiency, deposition uniformity, and film
microstructure.
FGM synthesis via low vacuum DVD: J. F. Groves and H. N. G. Wadley
Figure 6 schematically shows a typical synthesis setup
for deposition on a flat substrate. The velocity of the jet
can most simply be estimated by modeling the carrier gas
stream as an isentropic flow of a compressible fluid 3°.
The important governing relationships between pressure,
temperature, Mach number, and jet velocity are given by:
(1)
p
To
T =
where Po = Upstream (e.g. the mixing chamber of Figure
3) pressure (Pa),
P = Downstream (e.g. the deposition chamber
of Figure 3) chamber pressure (Pa),
To = Upstream temperature (K),
T = Downstream temperature (K),
M = Flow's Mach number,
7 = Ratio of the specific heats (5/3 for helium
and argon),
U = Jet velocity (m/s),
T = Absolute temperature (K), and
Rs =Specific gas constant (2077 J/(kg K) for
helium, 208.1 J/(kg K) for argon).
1+ ~ M
2
u =M
Tv sT
(2)
and
(3)
Electron beamevaporated
refractive metals (Ti, Ni, Nb) and
light element(C, Si) source
Vacuum gauge
t 0-3- 10 Torr)
• Chamber
~ pumping
l~l unit
He + metal vapor[I
Vacuum
gauge
~,~
' ,
,.~
L' '
:~y"
~
~ Metal
/o0os,
Infrared
heater
,ao,
(RT'600°C)
H4
Mixing chamber
Resistivelyevaporatedmetals
(Cu, AI, Au, Ag) sources
Figure 5 A multisource DVD system can produce alloys, compounds, and multilayered coatings
E-beam gun
column
1.27cm
nozzle exit
Carrier gas
flow tube
~lO.lcm
/
l
Icm
3.5cm
flat subst~ate
Figure 6 A close-upview of a stationary flat substrate coating experimentalconfiguration shows the geometricrelationships betweencarrier gas flow
nozzle, e-beam gun, crucible, evaporant source, and substrate
61
FGM synthesis via low vacuum DVD: J. F. Groves and H. N. G. Wadley
While equation (3) predicts the carrier gas velocity just
after it exits the nozzle (i.e. as it enters the deposition
chamber), it does not provide insight into the change in
carrier gas velocity or density with position in the
chamber or into the carrier gas stream's interaction with
the vapor atoms. To obtain a first order estimate of
carrier gas/atomic vapor/substrate interactions, the experimental configuration of Figure 6 was modeled using a
computational fluid dynamics (CFD) code (Flow-3D
from AEA-CFDS, Inc.).
To model carrier gas stream variations through the
system, the initial velocity, pressure, and temperature of
the carrier gas at the nozzle exit are required as inputs.
Fluid dynamics research has shown that for transonic
(M = 1) or supersonic flow (M > 1) the velocity, pressure and temperature conditions at the nozzle throat (i.e.
the smallest cross-section through which carrier gas
flows) correspond to 'choked' or Mach 1.0 conditions 31.
Thus, if the upstream (mixing chamber) temperature and
pressure are 293K and 5Torr (667Pa) respectively,
equations (1), (2), and (3) indicate that the model inputs
for the nozzle exit should be 200 K, 2.44 Torr (325 Pa),
and 873 m/s if a helium carrier gas is used.
The initial speed and direction of the evaporated
atoms are also needed as model inputs. The initial kinetic
energy (and thus initial velocity) of the evaporant source
atoms can be calculated from the Boltzmann temperature equation and a kinetic energy equationZ2:
3
1
2
E=~kT~ =~mU
(4)
where E
= The kinetic energy of the evaporated atoms
(J)
k = Boltzmann's constant (1.381 × 10-23 J/K),
Tv = The vaporization temperature of source (K),
m = The mass of an individual evaporated atom
(kg), and
U = Velocity of vapor atom (m/s).
For a typical metal being processed in an e-beam
system, the kinetic energy per evaporated atom is in the
1.6 to 3.20 × 10-2° J (i.e. 0.1-0.2eV) range, corresponding to an atomic speed of 200-2000 m/s. The angular
distribution of vapor atoms leaving the evaporant surface has a cos n 0 distribution where n -- 2,3,4, or more
depending on the local surface shape and pressure 22'23.
After selecting a specific atom type (copper), initial atom
trajectories, an average thermal kinetic energy (N0.1 eV),
and a total mass flow rate (10g/h), the paths from
source-to-substrate for a selection of vapor atoms can be
calculated given an appropriate model for the carrier gas
stream/vapor atom interaction.
The carrier gas/vapor atom interaction arises from the
difference in the velocity vectors of the carrier gas flow
and the vapor atoms being entrained. Flow-3D provides
a particle tracking model, which calculates the change in
velocity of vapor atoms with time as they interact with
the carrier gas. To determine this interaction, the drag
force, FD, exerted on the entrained species by a continuous phase (in this case the carrier gas stream), is
62
calculated using:
1
2
FD = -~Trd pCDIUR]UR
(5)
where FD = Drag force exerted by the carrier gas upon a
vapor particle (N),
d = Entrained vapor particle diameter (m),
p = Vapor particle density (kg/m3),
CD = Drag coefficient, and
uR = Relative velocity of the two components.
Flow-3D's drag coefficient equation has been optimized
to track particles larger than 1 #m. Since the majority of
the particles evaporated are either monatomic atoms or
ions, a modification to the basic code was made. Flow3D's default drag coefficient, CD, in equation (5) was
replaced by an expression developed by Abuzeid et al. 32
for particles less than 1 #m in diameter.
24
CD -- Re . ~
(6)
where Re = Reynolds number of the carrier gas flow,
and
Cc = Cunningham slip correction.
In equation (6), the Reynolds number, Re, is calculated
from:
/
Re -
d" uR
(7)
v
where u is the kinematic viscosity of helium. The Cunningham slip correction is dependent upon the dimensionless Knudson number (Kn):
Cc = 1 + 1.257Kn + 0.40Kn • e - -
.
(8)
The Knudson number, in turn, is dependent upon a
vapor particle's mean free path (A) and its diameter:
2A
Xn = -d-"
(9)
Finally, the mean free path of each vapor particle can be
calculated from:
A
A - NApQ
(10)
where A = Atomic weight (kg/kmol),
N A = A v o g a d r o ' s number (6.02 × 1026 atoms/
kmol),
p = Density (kg/m3), and
Q =Cross-section for an individual a t o m
(10 -20 m2).
The process simulations in Figure 7 show the effect of
the carrier gas flow upon the vapor atom trajectories. In
Figure 7(a) vapor atoms initially traveling towards the
nozzle are in the fast flow portion of the cartier gas jet
longer than vapor atoms traveling initially perpendicular
to the nozzle and the magnitude of their initial velocity
relative to the carrier gas stream (UR) is greater. As a
result they are more sharply turned into the cartier gas
stream and then directed straight onto the substrate.
FGM synthesis via low vacuum DVD: d. F. Groves and H. N. G. Wadley
Other vapor atoms, with velocity vectors generally perpendicular to the carrier gas flow, traverse the carrier gas
stream more quickly, entrain in the edge of the main
carrier gas stream, and, upon reaching the substrate and
interacting with the wall jet, deflect up without making
contact with the substrate. Figure 7(b) shows that, as the
pressure in the system increases, the vapor atom trajectories change so that an increased number of atoms
contact the substrate. Figure 7(c) shows that eventually
the pressure in the system can be increased to the point
where many of the vapor atoms are deflected down into
the wall jet and then only make contact with the
substrate by diffusional jumps. Examination of equations (5)-(10) reveals that the extent to which vapor
atoms entrain in the carrier gas flow depends upon the
mean free path of the vapor atoms in the flow and upon
the relative velocity of the vapor atoms and the carrier
gas.
E-beam Gun
......
In the DVD system, the mean free path of the vapor
atoms (A) will change with the carrier gas density (i.e.
chamber pressure) for a fixed evaporation rate. Higher
chamber pressures lead to shorter particle mean free
paths and thus to a reduced time between carrier gas/
vapor atom collisions. If each collision transfers approximately the same momentum from carrier gas to vapor
atom, an increase in the collision rate will lead to faster
vapor atom redirection parallel to the carrier gas flow.
The model embodies this by reducing Kn and Cc as A
decreases, leading to a larger drag force upon the particle
which then redirects the particles more quickly.
Changes in the carrier gas/vapor atom relative velocity (uR) are most easily effected by varying the carrier
gas velocity. Increasing the relative velocity of the carrier
gas (higher Mach number flow) increases the frequency
of carrier gas/vapor atom collisions above the crucible
and thus to more rapid redirection. In addition, the
Vapor Particle Trajectory
~
.... Wall Jet
~-~ . . . . . . . . . . . . . .
b) M a c h !.0, 4.00 T o r r
c) M a c h 1.0, 10.00 T o r r
Figure 7 Simulation of the carrier gas jet interaction with vapor atoms shows how deposition onto the substrate is dependent upon initial vapor atom
trajectory and subsequent interaction with the wall jet
63
FGM synthesis via low vacuum DVD: J. F. Groves and H. N. G. Wadley
momentum transfer from a carrier gas atom to a vapor
atom increases with Mach number, further accelerating
redirection of the vapor atoms above the crucible. These
effects are accounted for in the model through equation
(5). A more subtle effect of a carrier gas velocity increase
for a given chamber pressure is the corresponding
increase in upstream pressure (see equation (1)). This
results in higher carrier gas densities (shorter mean free
paths) above the crucible and at the substrate, which will
contribute to more rapid vapor atom redirection in both
locations.
4 EXPERIMENTAL STUDIES
4.1 Flat substrate coating
4.1.1 Visualization experiments. Excitation and ionization of both the evaporant and its carrier gas jet during
electron-beam evaporation results in luminescence-in
the optical spectrum. This provides a convenient opportunity to visualize the flow during the DVD synthesis
process. By equipping the deposition chamber with
observation ports, it is possible to photograph the carrier
gas/vapor stream/substrate interactions for a variety of
chamber pressures and carrier gas Mach numbers and
to relate these observations to the CFD predictions.
Figures 8 and 9 show the evaporation of Cu (its
luminescence appears green or green/blue), its transport
towards a flat substrate in a He carrier stream (its
luminescence is violet), and the resulting interactions of
the entire flow with the substrate. Many of the qualitative features predicted by the CFD calculations are
evident in the figures. For instance, Figure 8 clearly
shows that an increase in the chamber pressure leads to a
more rapid entrainment of the vapor and a tendency for
it to be transported along, rather than across, the flow's
streamlines. For a given deposition chamber pressure,
Figure 9 shows that an increase in the carrier gas stream's
Mach number results in a similar, more rapid redirection
of the vapor towards the substrate. The experiments also
reveal that a portion of the vapor stream is deflected
parallel to the substrate (i.e. it is entrained in the wall jet)
and can be deposited some distance from the carrier gas
impact point with the substrate (or perhaps not at all).
This phenomenon has important practical consequences
for the deposition efficiency and deposit microstructure.
As the vapor particles are turned into the wall jet, the
component of their velocity vector carrying them towards
the substrate decreases towards zero. Even if the particles
contact and stick to the substrate they do so with a lower
energy (equation (4)), a fact that is likely to affect the
microstructure 19-21'
The above results reveal important insights into
carrier gas/vapor atom/substrate interaction in the
DVD processing environment. The CFD calculations
combined with the experimental flow visualizations indicate that the transport of evaporated material to a substrate is sensitive to the carrier gas stream density and
velocity and to the deposition chamber pressure, This
64
suggests that there could be a range of conditions which
will result in efficient deposition.
4.1.2 Deposition efficiency. To gain further insight
into deposition efficiency, a set of flat substrate deposition efficiency experiments were undertaken. An average
efficiency of deposition was determined by weighing an
evaporant source and deposition substrate before and
after each run. The evaporant source for all experiments
was a 1.27 cm diameter copper rod of five 9's purity. The
deposition substrate consisted of a 10.1 x 10.1 cm glass
square placed normal to the flow at 9.3 cm from the centerline of the evaporant source (Figure 6). Gettered
helium carrier gas flows were conducted into the processing chamber through a 1.27 cm diameter nozzle.
A precisely controlled e-beam power was used for
each test. The beam power was initially set at 60 W and
the power increased in 60 W increments every 30 s until a
beam power of 1200 W was achieved. This beam power
was maintained for 10 rain before being reduced to 60 W
for 30 s and then shut off. At the start of each run the flat
top of the copper rod stock was positioned about 0.5 mm
above the top edge of the crucible. Upon melting, the flat
top of the rod stock formed a hemispherical molten pool.
During a deposition run, the copper rod was periodically
raised to maintain the entire top of the rod as a hemispherical molten pool. Experimental observation indicated that this e-beam power cycle and evaporant source
position led to consistent evaporant flux from the molten
pool into the helium carrier gas stream with no evidence
of molten copper spitting during any of the runs.
Figure 10 shows the combination of Mach number/
chamber pressures that are experimentally accessible for a
variety of nozzle exit diameters and throttle plate positions
when using helium as the carrier gas. The Mach number
and chamber pressure variations were made by changing
the number of standard liters per rain (slm) of carrier gas
entering the.system and by varying the relative open/closed
position of the throttle plate located in front of the
chamber pump (Figure 3). From the measured upstream/
downstream pressures, equation (1) was used to compute a
Mach number for each flow condition. For the 1.27 cm
diameter nozzle, the experimental Mach numbers ranged
from 1.45 to 1.95 as the deposition of chamber pressures
ranged from 0.20 to 4.07 Torr.
Table 1 shows the dependence of the deposition
efficiency and evaporated mass upon the flow conditions.
Figure 11 shows graphically how the deposition efficiency
changes with Mach number and deposition chamber
t.
pressure. The deposition efficiency initially rises, goes
through a maximum and then decreases as the deposition
chamber pressure increases. At a fixed deposition chamber pressure (above where the peak efficiency is found),
the deposition efficiency increases with decreasing Mach
number.
The experimental efficiency variations can be reconciled by comparison with the flow visualizations of
Figures 7, 8 and 9. Figures 7(a), 8(a), and 9(a) show
that when the carrier gas density or Mach number is
FGM synthesis via low vacuum DVD: J. F. Groves and H. N. G. Wadley
1.27
diamq
llOZ2
a) Mach 1.70, 0.20 Torr
m
b) Mach !.70, 0.80 Torr
c) Mach 1.70, 3.20 Torr
Figure 8 Similar carrier gas density
v e r s u s vapor position trends predicted using Flow-3D are evident experimentally. For a constant Mach number,
increases in carrier gas density lead to increased vapor transport along, as opposed to across, carrier gas flow lines. Deflection of gas and vapor at the
substrate is especially evident in (c)
sufficiently low, a portion of the vapor atoms can diffuse
completely across the stream lines of the carrier gas flow
and never reach the substrate. Indeed, for a Mach
number/chamber pressure combination close to zero, the
vapor distribution must return t o a traditional cos"0
distribution of trajectories. In such a case (0 ~ 90°), a
very small fraction of the flux would be directed towards
the substrate.
As the carrier gas velocity and density are increased,
the vapor diffuses into the center of the carrier gas jet,
is transported to the substrate, contacts the substrate,
and forms a deposit near the impact point of the jet on
the substrate. Figure 7 shows that when the vapor is
entrained in the jet center, it is more likely to impact the
substrate rather than be redirected into the wall jet and
travel parallel to the substrate with the wall jet. Once the
carrier gas Mach number or density becomes sufficiently
great, the vapor is confined to the bottom edge of the jet
(Figures 7(c), 8(c), and 9(c)) and is deflected into the
wall jet that propagates parallel to the substrate. These
effects are embodied in the model as an increase in relative velocity (UR) and a decrease in mean free path (A).
Although some vapor is scattered to the surface from
the wall jet, the majority does not contact the surface,
resulting in a significant decrease in average deposition
efficiency near the jet impact point on the substrate.
4.2 Fiber coating
From the above results, the presence of the substrate
65
FGM synthesis via low vacuum DVD." J. F. Groves and H. N. G. Wadley
a) Mach 1.40, 0.80 Torr
b) Mach 1.70, 0.80 Torr
c) Math !.95, 0.80 Torr
Figure 9 Increaseof the carrier gas Mach number while holding the carrier gas density constant shows that a higher Mach number flowmore readily
entrains vapor particles in carrier gas flow lines rather than allowing diffusion across the streamlines
3.0
2.5
= 2.0
g
1.5
1'00
1
2
3
4
"5
Chamber pressure (Tort)
Figure 10 Changes in nozzle size, throttle plate position, and carrier
gas flux can significantly vary the Mach number (velocity) of the carrier
gas jet used to transport the vapor flow to the substrate. Shaded regions
represent available processing conditions for each nozzle
66
is seen to have a significant effect upon the carrier gas
stream flow and a varying effect on vapor deposition
efficiency, depending u p o n process conditions. To learn
more about this effect and to assess the potential for fiber
coating, a fiber deposition efficiency study was conducted
using test conditions similar to those employed for the
planar samples. An array of 142#m diameter SCS-6
fibers (supplied by Textron Specialty Materials, Lowell,
MA) was set up in an aluminum frame for these coating
studies. Each square frame had an inside edge dimension of 5.08 cm and allowed 25 colinear fibers to be
mounted vertically with a nominal center-to-center fiber
spacing of 2 mm. F o r this fiber spacing, 7.1% of the
cross-sectional area of the frame was occupied by fibers
(thus, if a uniform flux were incident upon the fibers and
F G M synthesis via low vacuum DVD: J. F. Groves and H. N. G. Wadley
Table 1 Deposition efficiency of Cu onto flat substrate
Mach
number
Chamber
pressure
(Torr)
Mass
evaporated
(g)
Deposition
efficiency
(%)
Gas
flow
(slm)
1.45
1.45
1.45
1.45
1.45
1.45
1.45
1.45
1.45
0.20
0.35
0.50
0.8(/
1.14
1.25
1.50
2.00
2.96
2.063
2.610
2.501
2.452
2.400
2.421
2.351
2.237
2.213
16.3
34.0
50.5
53.4
52.2
51.2
48.7
40.1
31.1
2.00
3.95
7.17
10.0
15.0
17.8
21.6
30.0
47.0
1.65
1.65
1.65
1.65
1.65
1.65
1.65
1.65
1.65
1.65
0.20
0.35
0.62
0.85
1.25
1.48
2.00
2.79
3.50
4.07
2.455
2.268
2.386
2.522
2,353
2,292
2,365
2,098
2.159
1.852
23.3
46.0
49.6
46.6
39.9
34.2
28.2
20.0
21.0
18.5
2.90
6.05
10.0
15.0
23.8
30,0
41,6
60.0
77.0
90.0
1.75
1.75
1.75
1.75
1,75
1.75
1.75
1.75
0.20
0.35
0.54
0.75
1,25
1.85
2.42
3.50
2.685
2.324
2.222
2.624
2,430
2.418
1.947
1.946
26.9
45.6
49.6
46.6
31.8
20.6
16.0
11.5
3.50
7.20
10.0
15.0
30.0
45.0
60.0
90.0
1.82
1.82
1.82
1.82
1.82
1.82
1.82
1.82
1.82
(1.20
0.35
0.49
0.66
I. 17
1.50
2.15
3.15
3.60
2.419
2.605
2.591
2.568
2.304
2.133
2.076
1.746
1.898
34.3
45.8
49.0
46.6
31.1
23.0
14.2
11.6
9.1
4.00
8.15
10.0
15.0
30.0
41.0
60.0
90.0
105.0
1.95
1.95
1.95
1.95
1.95
1.95
1.95
0.20
0,35
0,41
0,55
0.97
1,25
1.62
3.029
2.732
2.564
2.563
2.281
2.051
1.987
33.1
45.0
50.0
45.1
30.7
22.9
16.2
5.10
10.15
10.0
15.0
30.0
40.2
55.0
0.6
Mach 1.45
~' 0.5
G)
"~o
0.4
.
\
;j
",
,
\
,',
0.3
~i1
Vx
,,.
".A
.......
¢,
,~
Math 1.75
Mach1.82
Machl.95
\~ . ,
o
~)
~-7
!.w]~:~
L--~
~
~. . . . .
.....
0.1
0.0 ----
1
~
2
•
•
3
C h a m b e r p r e s s u r e (Tort)
F i g u r e 11 At low chamber pressures (low carrier gas fluxes), the
carrier gas momentum is insufficient to redirect the vapor stream
towards the substrate, resulting in low efficiency readings. Once the
chamber pressure is increased beyond about 0.50Torr, a significant
decrease in material transfer to the substrate is seen as Mach number
and carrier gas density increase
deposition occurred only on surfaces that were 'insight'
of the flux~ the average deposition efficiency would be
7.1%).
For this study, (99.999% purity) copper was again
eval~orated from a 1.27 crn diameter rod stock and deposited using similar gettered helium carrier gas stream
conditions. During each run, the frame itself was temporarily covered with aluminum foil to ensure that copper
deposition on the frame was not included in the weight
change measurements. A single source-to-substrate distance of 9.3 cm was employed with the frame centered on
the carrier gas stream. For this study the effect of Mach
number (between Mach 1.50 and 1.95) and chamber
pressure (from 0.10 to 4.00Torr) upon the efficiency
and distribution of material deposition onto fibers was
investigated. While the position of the crucible was
unchanged from the fiat substrate runs, the gun to
crucible distance was increased to 7.1cm to eliminate
possible vapor stream interaction with the bottom of the
gun column at low gas flows. The increased beam propagation distance (7.1cm versus 2.0cm) in the chamber
resulted in greater e-beam energy losses. Thus, to obtain
a fiber experiment vapor flux rate comparable to that
of the fiat substrate coating work, a modified e-beam
heating cycle was used. Once the fixed-position e-beam
was initially turned on at 60 W, the e-beam power was
increased in increments of 60 W every 20 s until a beam
power of 1500 W was achieved (after 8 rain). This beam
power was maintained for 11.5 rain before being reduced
to 60 W for 30 s and then shut off'.
Table 2 lists deposition efficiency results for the fiber
coating study. Figure 12 plots the relationship between
Mach number, chamber pressure, and relative fiber
coating efficiency (compared to that achieved by a lineof-sight deposition process). Interestingly, carrier gas
stream density/Mach number/deposition efficiency trends
similar to those observed for fiat substrate coating were
apparent. The deposition efficiency measured in this
study is an average value over the surface area of the
array. The non-uniform flux resulted in fibers at the
center of the array having significantly thicker coatings
than those at the sides.
At low chamber pressures, the average deposition
efficiency was less than that of a line-of-sight process,
Figure 12. This arose because much of the vapor was
not entrained in the flow and therefore not directed
towards the fibers. For runs with a chamber pressure
P > 0.5Torr, visual observation indicated that essentially all o f the vapor was passing through the frame. The
average efficiency went through a maximum (of more
than twice that of a line-of-sight process) at P ~ 0.5 Torr,
and, in the higher pressure regime (P > 0.5Torr), the
deposition efficiency decreased with increasing chamber
pressure and with increasing Mach number.
For experiments conducted at chamber pressures less
than 0.5 Torr, the explanation of the deposition efficiency
experiments is much the same as in the flat substrate case;
there are not enough carrier gas/vapor atom collisions to
redirect the vapor to the substrate. Instead, much of the
67
FGM synthesis via low vacuum DVD." J. F. Groves and H. N. G. Wadley
vapor diffuses across the carrier gas streamlines. For
higher carrier gas Mach numbers and chamber pressures,
vapor redirection occurs more rapidly above the crucible. Once the carrier gas velocity or chamber pressure is
high enough (>0.5 Torr) to direct the vapor through the
frame, the vapor deposition trends are the result of gas
stream/substrate interaction. The results of the flat substrate Flow-3D modeling work appear to also explain the
fiber coating results. As the chamber pressure of carrier
gas Mach number increases, the pressure around the
fibers increases. The pressure increase will lead to a
decrease in the mean free path of the vapor atoms and
thus more rapid vapor redirection around the fibers.
However, use of the continuum model to explain the
fiber coating results is not valid since the continuum
Table 2 Deposition efficiency of Cu onto stationary fibers
Mach
number
Chamber
pressure
(Torr)
Mass
evaporated
(g)
Deposition
efficiency
(%)
1.50
1.50
1.50
1.50
1.50
1.50
0.25
0.50
1.00
2.00
,3.00
4.00
2.224
1.837
1.921
1.500
1.100
0.831
8.81
15.0
13.4
11.3
10.3
8.06
2.85
7.05
13.2
31.0
51.0
70.0
1.65
1.65
1.65
1.65
1.65
1.65
1.65
0.10
0.25
0.50
1.00
2.00
3.00
4.00
2.411
2.054
2.440
1.866
1.856
1.372
0.747
1.28
10.5
13.2
11.4
10.3
8.89
6.29
1.12
3.83
9.50
19.0
41.0
65.0
89.0
1.80
1.80
1.80
1.80
1.80
1.80
1.80
0.10
0.25
0.50
1.00
2.00
3.00
4.00
2.199
2.811
2.638
2.058
1.799
0.652
0.372
2.09
10.2
12.2
10.2
8.45
6.13
5.11
1.50
5.30
9.80
24.0
53.8
83.0
112.0
1.95
1,95
1,95
1,95
1.95
0.10
0.25
0.50
1.00
2.00
2.195
2.435
2.064
1.398
0.703
3.32
11.3
10.6
8.23
6.97
2.00
6.90
12.0
31.0
67.0
2.50
'
2.00
//~'~'--.-..,~ ~ 5 0
~E
Gas
flow
(slm)
model is based on the assumption that the critical
dimension of the system (e.g. the fiber diameter, 142 #m)
is much greater than the mean free path of the vapor
particles (25-100/~m), clearly not the case. This rule is
embodied in the Knudson number (Kn = A/L) which
states that continuum equations apply only if Kn << 131.
In the present case, the Knudson number indicates that
explanation of the deposition phenomenon will require
the use of free molecular flow and kinetic theory of gases
concepts. Still, visual observations of these fiber coating
runs revealed the presence of a small shock structure
at the fiber surface facing the gas flow. The presence of
the shock suggests that the fibers form a microwall
jet around their diameter and that free molecular flow
modeling could well yield a similar relationship between
pressure/velocity processing conditions and deposition
efficiency as currently defined by the continuum model.
Finally, the fiber coating experiments revealed a peak
deposition efficiency over twice the 7.1% expected for
line-of-sight deposition. Scanning electron microscopy
(Figure 13) of these samples shows that, for conditions of
maximum efficiency, vapor deposits not only on the front
of the fiber facing the incoming vapor but also on the
fiber's sides and its back. The backside fiber coating
phenomenon appears to be a manifestation of atomic
scattering from the flow as it passes the fiber 25. This
results in a significantly increased vapor deposition efficiency over that observed for (line-of-sight) traditional
e-beam deposition.
5 CONCLUSIONS
The design of a directed vapor deposition system for
FGM synthesis has been described and analyzed. The process uses a, combination of e-beam evaporation under
low vacuum conditions and carrier gas vapor entrainment
Incident Vapor Direction
r
Nonline-0f-
1.50
I1)
t~ 1.00
>
o.so
~]
r
n-
Area Fraction
Occupied by
Fibers (7.1"/o)
0.00
1
2
3
4
Chamber pressure (Torr)
Figure 12 As with fiat substrate coating, low chamber pressures (low
carrier gas fluxes) do not entrain the vapor and transport it towards the
fiber array, resulting in low deposition efficiencies. Once most of the
vapor passes through the frame, carrier gas Mach number/density
trends similar to those observed with fiat substrate coating are evident.
(Note that some efficiencies are well above line-of-sight efficiencies)
68
Figure 13 Vapor deposition is evident upon the sides (~15 #m thick)
and back (~5 #m thick) of this fiber which was held stationary during
the deposition process (M = 1.50, P = 0.50Torr)
F G M synthesis via low vacuum DVD: J. F. Groves and H. N. G. Wadley
to enable high rate directed vapor deposition of a wide
variety of metals, alloys and some ceramics. The carrier
gas velocity and deposition chamber pressure have been
identified as two important flow parameters that affect
the efficiency, distribution, and microstructure of deposited material, and their effect upon deposition efficiency
has been determined. The study reveals that, under
selected carrier gas velocity and chamber pressure conditions, the technique can deposit vapor onto fibers at
twice the efficiency of traditional, high vacuum, line-ofsight vapor deposition systems.
10
I1
12
13
14
ACKNOWLEDGEMENTS
The authors are grateful to the Advanced Research
Projects Agency (W. Barker, Program Manager) and
NASA (D. Brewer, Technical Program Monitor) for
support of the design of this synthesis route through
NASA grant NAGW 1692 and to the Air Force Office of
Scientific Research (W. Jones, Program Manager) for
support of its use to synthesize high temperature FGMs.
We would also like to thank H. G. Wood for his advice
regarding the computational fluid dynamics modeling.
15
16
17
18
19
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