ELSEVIER Plh S 1359-8368(96)00023-6

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. Wadley 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 I N T R O D U C T I O N

Microlaminated and continuous fiber reinforced metal matrix composites (MMCs) are being investigated in con- junction with functionally graded materials' (FGM) con- cepts 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 inter- face architectures that chemically protect the fibers, pro- vide 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 FGMs can enter service, tightly controlled material synthesis techniques must be developed for their affordable fabrication. Many of the methods under evaluation for continuous fiber MMC 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 par- ticular 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 compo- nent. 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 schemati- cally 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 ther- mal vaporization (e.g. electron-beam and resistive heat- ing) 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 con- ducted 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 pro- duction of continuous fiber reinforced composites because of the method's low deposition rate, < 1 #m/min and high energy input to material production ratio 22. 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 syn- thesize the SiC reinforcements used in many MMCs and to functionally grade their interfaces. However, employ- ing 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. Further- more, it utilizes those gases slowly and creates additional hazardous gases as by-products. It can thus become an expensive route to FGM synthesis TM.

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 non- uniform 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 con- centrate and deposit various materials under low vacuum (~ 1-10 Torr) conditions with high local efficiencies 1'2' 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.

a) Coated fibers with different coating thicknesses ~

Ti alloy coating with -~ - different coating

thicknesses ~

SiC Fibers

b) Post-consolidated composite with different fiber spacings

Ti alloy layers of different thickness & spacinc

C) Laminated composite

Ceramic (e.g., TiN, TiC) layers of 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 e- beam 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 combi- nation 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

58

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 sys- tem's axial e-beam make possible high rate evaporation in a relatively unexplored low vacuum (10 -3-10 Torr) e-beam processing environment. This low vacuum regime contrasts with the high vacuum (10 -4 10 -8 Tort) envi- ronment 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 per- mitting 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 stand- ard turbomolecular high vacuum pump. Once the e- beam 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 electron- beam 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 water- cooled 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 contami- nation 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 FGM 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

==l•High vacuum pump

Purification system

Pressure gauge

beam

Mass flow

controller

Differential~ pump

Mixing chamber

Heater

v source Fibers

material or flat substrate

Contifuous e ~ Pressure source feed Compressed gauge ' ~

helium cylinaer ~ ...... Throttl

plate ~

Mechanical chamber

pump Figure 3 In the low vacuum DVD system, electron beam evaporated source material is transferred to a substrate by a directed gas flow entering 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 operat- ing 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. Experi- ments indicate that electron-beam penetration through an inert gas can be described with reasonable accuracy using a modified Bethe stopping power formula 28. 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 energy 29. 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.

2.3 Deposition

Either flat or fibrous, stationary or rotating/translat- ing substrates can be used in the DVD system. Stationary substrates facilitate study of evaporated source material distribution in the carrier gas stream and basic investiga- tion 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 thick- ness 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 sub- strate during deposition is vital for creating the micro- structures that possess th e properties required for FGM applications ~9.

100

E 10

o E

.~_

c- O 0.1

.1.-,

(~ 0.01 Q.

0.001 0

. . . . . I . . . . . I . . . . I I I I

_ ~ H e

P = 5 Torr

I I I I I I

10 20 30 40 50 60 70

Accelerating voltage (kV) Figure 4 The distinctly different electron propagation distances (Bethe ranges) in helium and argon are illustrated for ambient temperature and 5 Torr pressure conditions

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 depos- ited 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 stoichiometri- cally from a single evaporation source, and use of multiple crucible sources to accomplish the task can be quite inefficient in a traditional e-beam system 22. 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 com- pounds 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.

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.

60

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:

p (1)

To 1 + ~ M 2 T = (2)

and

u = M Tv sT (3)

where Po

P

= Upstream (e.g. the mixing chamber of Figure 3) pressure (Pa),

= 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).

Vacuum gauge

Electron beam evaporated refractive metals (Ti, Ni, Nb) and

light element (C, Si) source

t 0-3- 10 Torr) • Chamber

~ pumping l~l unit

He + metal vapor [I ~ , ~ , . ~ :~y" ~ Infrared ' , L' ' ~ Metal heater /o0os, ,a o,

Vacuum (RT'600°C) gauge H 4

Figure 5

Mixing chamber

Resistively evaporated metals (Cu, AI, Au, Ag) sources

A multisource DVD system can produce alloys, compounds, and multilayered coatings

1.27cm nozzle exit

E-beam gun column

~lO.lcm

Carrier gas flow tube / l Icm

3.5cm

flat subst~ate

Figure 6 A close-up view of a stationary flat substrate coating experimental configuration shows the geometric relationships between carrier 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 experi- mental 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, pres- sure 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 tempera- ture equation and a kinetic energy equationZ2:

3 1 2 E = ~ k T ~ = ~ m U (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, correspond- ing to an atomic speed of 200-2000 m/s. The angular distribution of vapor atoms leaving the evaporant sur- face 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 con- tinuous phase (in this case the carrier gas stream), is

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 u R = 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. Flow- 3D'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: /

d " u R Re - (7)

v

where u is the kinematic viscosity of helium. The Cun- ningham slip correction is dependent upon the dimen- sionless 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), NA=Avogadro ' 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.

6 2

FGM synthesis via low vacuum DVD: d. F. Groves and H. N. G. Wadley

Other vapor atoms, with velocity vectors generally per- pendicular 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 tra- jectories 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 equa- tions (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.

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 approxi- mately 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 veloc- ity (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

E - b e a m G u n ...... V a p o r Part ic le Trajec tory .... Wal l 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 ioni- zation of both the evaporant and its carrier gas jet during electron-beam evaporation results in luminescence-in the optical spectrum. This provides a convenient oppor- tunity 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 quali- tative 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 indi- cate that the transport of evaporated material to a sub- strate is sensitive to the carrier gas stream density and velocity and to the deposition chamber pressure, This

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 deposi- tion 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 cen- terline of the evaporant source (Figure 6). Gettered helium carrier gas flows were conducted into the process- ing 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 hemi- spherical molten pool. Experimental observation indi- cated 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 cham- ber pressure (above where the peak efficiency is found), the deposition efficiency increases with decreasing Mach number.

The experimental efficiency variations can be recon- ciled 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

64

1.27 diamq

llOZ2

FGM synthesis via low vacuum DVD: J. F. Groves and H. N. G. Wadley

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 rela- tive 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 Increase of the carrier gas Mach number while holding the carrier gas density constant shows that a higher Mach number flow more 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

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 dimen- sion of 5.08 cm and allowed 25 colinear fibers to be mounted vertically with a nominal center-to-center fiber spacing of 2 mm. Fo 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

66

FGM synthesis via low vacuum DVD: J. F. Groves and H. N. G. Wadley

Table 1 Deposition efficiency of Cu onto flat substrate

Chamber Mass Deposition Gas Mach pressure evaporated efficiency flow number (Torr) (g) (%) (slm)

1.45 0.20 2.063 16.3 2.00 1.45 0.35 2.610 34.0 3.95 1.45 0.50 2.501 50.5 7.17 1.45 0.8(/ 2.452 53.4 10.0 1.45 1.14 2.400 52.2 15.0 1.45 1.25 2.421 51.2 17.8 1.45 1.50 2.351 48.7 21.6 1.45 2.00 2.237 40.1 30.0 1.45 2.96 2.213 31.1 47.0

1.65 0.20 2.455 23.3 2.90 1.65 0.35 2.268 46.0 6.05 1.65 0.62 2.386 49.6 10.0 1.65 0.85 2.522 46.6 15.0 1.65 1.25 2,353 39.9 23.8 1.65 1.48 2,292 34.2 30,0 1.65 2.00 2,365 28.2 41,6 1.65 2.79 2,098 20.0 60.0 1.65 3.50 2.159 21.0 77.0 1.65 4.07 1.852 18.5 90.0

1.75 0.20 2.685 26.9 3.50 1.75 0.35 2.324 45.6 7.20 1.75 0.54 2.222 49.6 10.0 1.75 0.75 2.624 46.6 15.0 1,75 1,25 2,430 31.8 30.0 1.75 1.85 2.418 20.6 45.0 1.75 2.42 1.947 16.0 60.0 1.75 3.50 1.946 11.5 90.0

1.82 (1.20 2.419 34.3 4.00 1.82 0.35 2.605 45.8 8.15 1.82 0.49 2.591 49.0 10.0 1.82 0.66 2.568 46.6 15.0 1.82 I. 17 2.304 31.1 30.0 1.82 1.50 2.133 23.0 41.0 1.82 2.15 2.076 14.2 60.0 1.82 3.15 1.746 11.6 90.0 1.82 3.60 1.898 9.1 105.0

1.95 0.20 3.029 33.1 5.10 1.95 0,35 2.732 45.0 10.15 1.95 0,41 2.564 50.0 10.0 1.95 0,55 2.563 45.1 15.0 1.95 0.97 2.281 30.7 30.0 1.95 1,25 2.051 22.9 40.2 1.95 1.62 1.987 16.2 55.0

0.6 Mach 1.45

~' 0.5 . ~ - 7 Math 1.75 G) ~ !.w]~:~ Mach1.82 "~o \ " , \ L - - ~ Machl.95

0.4 ;j , , ' ,

~) ~i 1 Vx

\~ . , o 0.3 , , . ".A

¢ , . . . . . . . ~ . . . . . • , ~ 0.1 . . . . . •

0.0 ---- ~ 1 2 3

C h a m b e r pressure (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 depos- ited using similar gettered helium carrier gas stream conditions. During each run, the frame itself was tem- porarily 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 dis- tance 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 propa- gation 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 line- of-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 essen- tially 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 cruc- ible. 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 sub- strate 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

Chamber Mass Deposition Gas Mach pressure evaporated efficiency flow number (Torr) (g) (%) (slm)

1.50 0.25 2.224 8.81 2.85 1.50 0.50 1.837 15.0 7.05 1.50 1.00 1.921 13.4 13.2 1.50 2.00 1.500 11.3 31.0 1.50 ,3.00 1.100 10.3 51.0 1.50 4.00 0.831 8.06 70.0

1.65 0.10 2.411 1.28 1.12 1.65 0.25 2.054 10.5 3.83 1.65 0.50 2.440 13.2 9.50 1.65 1.00 1.866 11.4 19.0 1.65 2.00 1.856 10.3 41.0 1.65 3.00 1.372 8.89 65.0 1.65 4.00 0.747 6.29 89.0

1.80 0.10 2.199 2.09 1.50 1.80 0.25 2.811 10.2 5.30 1.80 0.50 2.638 12.2 9.80 1.80 1.00 2.058 10.2 24.0 1.80 2.00 1.799 8.45 53.8 1.80 3.00 0.652 6.13 83.0 1.80 4.00 0.372 5.11 112.0

1.95 0.10 2.195 3.32 2.00 1,95 0.25 2.435 11.3 6.90 1,95 0.50 2.064 10.6 12.0 1,95 1.00 1.398 8.23 31.0 1.95 2.00 0.703 6.97 67.0

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 effi- ciency 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 pro- cess uses a, combination of e-beam evaporation under low vacuum conditions and carrier gas vapor entrainment

Incident Vapor Direction

2.50

2.00 ~E

1.50

I1) t~ 1.00

> o.so

n-

0.00

Figure 12

' r

//~'~'--.-..,~ ~ 5 0 Non line-0f-

~] Area Fraction Occupied by

r Fibers (7.1"/o)

1 2 3 4

Chamber pressure (Torr)

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)

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)

68

FGM 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 depos- ited material, and their effect upon deposition efficiency has been determined. The study reveals that, under selected carrier gas velocity and chamber pressure con- ditions, the technique can deposit vapor onto fibers at twice the efficiency of traditional, high vacuum, line-of- sight vapor deposition systems.

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.

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