R
Net-Shaped Fabrication of
henium Components by EB-PVD
Dr. Douglas E. Wolfe
814-865-0316
[email protected]
Fabrication Methods: CVD vs. EB-PVD
Rhenium is a rare, silverywhite heavy transition metal
with several beneficial
properties for high
temperature and oxidation
resistant applications.
Net-shaped Rhenium components such as rhenium coated graphite cores, rhenium tubes,
plates and thrusters have typically
p
yp
y been fabricated through
g chemical vapor
p deposition
p
((CVD).
)
Electron Beam Physical Vapor Deposition (EB-PVD), however, has shown promise as both a
higher quality coating fabrication method with improved microstructure and properties.
Goals for Application
CVD Disadvantages
These properties include:
CVD is generally a slow manufacturing process involving
several steps and allowing little application flexibility. Chlorine
and hydrogen impurities occur and reduce the optimal physical
and mechanical properties of rhenium. An undesirable large
grain structure of rhenium is also produced, which can only be
fixed by several removals and mechanical grindings of the
component during application to create multilayers.
• High melting point of 3186ºC (highest amongst all
metals except for tungsten)
• High temperature strength
• Excellent erosion resistance in high-temperatures
(ie. Rocket engines, hot gas-valves)
• Great tensile and creep-rupture
creep rupture strength at
elevated temperatures (best amongst refractory
metals)
• Resistant to thermal cycling deterioration
• Remains ductile at extreme cold temperatures
• Develop tailored
microstructure techniques
• Improve mechanical
properties
• Achieve rhenium density
greater than 99%
EB--PVD Advantages
EB
EB-PVD is a reliable and highly reproducible process that
eliminates the intermediate fabrication steps. Along with high
deposition rates (5-100 µm/min), EB-PVD offers flexibility in
forming components with tailored microstructure and
properties.
Graphite Cores
Rhenium satellite thruster
(http://ipp.nasa.gov/innovation/Innovation_85/R-16937Web-V8N5/sbircomp.html)
Rhenium Tubes
Rhenium tubes are formed around a molybdenum tube mandrel.
After the rhenium coating process, chemical dissolution is used
to remove the Mo leaving only the Re.
Rhenium coated graphite cores are an
important
p
mechanical component
p
of
diverter valves.
Current CVD Process:
With CVD, volume manufacturing of rhenium coated-graphite cores
is limited to 2-3 balls per run. Rejection rate is high (60-70%) due
to non-concentricity, and non-uniform thickness.
EB-PVD Process:
A higher volume of rhenium coated-graphite cores can be produced (12-18
balls per run) , all with a uniform thickness and 100% concentricity. The
structure is fine grained with more than 250 micro and sub-micron grains
through coating the thickness. There is also no contamination of H2, O2 or Cl2.
Current CVD Process:
EB-PVD Process:
CVD Re tubes contain only 1-3 grains
through the wall thickness, which is
inadequate for welding and bending, and
cannot accommodate pressure under high
temperature bust tests. It is also limited
to manufacturing 2-3 tubes at a time.
EB-PVD Re tubes contain 40-50 grains
through the wall thickness with the
production of sub-micron and nano sized
grains. 11 Mo mandrels can also be
mounted and coated simultaneously while
allowing for rotation as shown to the right.
CVD
100X
CVD
400X
400X
EB-PVD
400X
Coating density >99%, Identical Concentric Spheres
(a) Optical micrograph (low magnification) and (b)
SEM (high magnification) showing
the dense, nanoand sub-microngrained crosssection of the
rhenium coating.
Cross-section
Polished surface
Graphite cores simultaneously charged
into a cylindrical cage of molybdenum
wires and plates. During EB-PVD, the
cage is rotated in the Re vapor.
Re
Graphite
Low magnification optical micrograph of a CVD produced Re tube
with 1-3 grain thickness.
a
Rhenium Coefficient of Thermal Expansion
U n it T h erm al Exp an sio n , d elta l / l x 0.0001
1150oC
High magnification optical micrograph of an EB-PVD produced Re
tube with 40-50 grain thickness.
Current CVD Process:
Production of rhenium thrusters by CVD is limited to one at a time due
to little flexibility inside the CVD chamber.
25
1050oC
High magnification optical micrograph of a CVD produced Re tube
with 1-3 grain thickness.
20
EB-PVD
EB
PVD Process:
P
N t h
Net-shape
th
thruster
t
f b i ti
fabrication
with
ith EB-PVD
EB PVD was
demonstrated with a titanium coating over a graphite mandrel in place of
rhenium.
High flexibility of the EB-PVD process allowed for the
simultaneous creation of two thrusters while depositing the desired
uniform thickness. Future efforts to duplicate this process with a rhenium
coated Mo mandrel by EB-PVD are underway due to the promising results
for the titanium coating.
CVD Re Tube IV-A-1 #337 Run 1
15
CVD Re Tube IV-A-1 #337, Run 2
Penn State EB-PVD Re CTE-1
10
Penn State EB-PVD Re CTE-2
Penn State EB-PVD Re CTE-3
Part VI ISO-63 Specimen 1 Run 1
5
Part VI ISO-63 Specimen 2 Run 1
Carbonne JP1091 Specimen 1
Run 1
0
0
1000
2000
3000
4000
5000
6000
Temperature, °F
Optical micrograph showing layered deposit of 40 mil thick rhenium
plate by EB-PVD with tensile strength 72 KSI and hardness 283VHN.
Typical tensile strength of CVD Re is 50 KSI and hardness 245VHN.
Graph comparing the coefficient of thermal expansion
of CVD and EB-PVD deposited Rhenium on graphite
plates. CTF values were comparable in both methods.
(a) Simulated titaniumcoated graphite mandrel
thruster and (b) titaniumcoated graphite thruster
produced by EB-PVD having
advantages such as high
production rates with
tailored microstructure.
Rhenium on graphite plates produced by EB-PVD were estimated to have equivalent or better physical and mechanical
properties than conventional CVD. The EB-PVD plates were found to have higher hardness (283 VHN) than CVD plates
(245 VHN), and exhibited textured grain growth with micron and sub-micron sized microstructure. The deposited rhenium
1450 C
was also found to be free from impurities such as copper and other potential contaminants from the vacuum chamber.
The finer grained and harder microstructure of the EB-PVD Re plate is anticipated to exhibit superior mechanical strength
with 30% improvement over the CVD method of application.
o
b
Net-shape Thrusters
Rhenium Plates
Microstructure Tailoring Methods
Microstructure and hardness can be
altered through temperature annealing.
Annealing the temperature of rhenium
deposited plates to 1100-1450ºC
causes
grain
growth
and
a
corresponding
softening
of
the
rhenium plate (275 VHN at 1000ºC to
224 VHN at 1450ºC). SEM of rhenium
plates as a function of annealing
temperature are shown to the right.
The direction
of coating
grain growth occurs parallel
to the incoming vapor flux
from EB-PVD. Altering the
vapor incidence angel (VIA)
by moving the substrate to
different positions during
EB-PVD can be used as a
method of tailoring a “zigzag” microstructure.
As deposited
at 1000
1000ºC
ºC
1250ºC
1250
ºC
1050ºC
1050
ºC
1350ºC
1350
ºC
1150ºC
1150
ºC
1450ºC
1450
ºC
290
40µm
Average Vicker's Hardness Number (VHN)
Tailoring the
microstructure
of the rhenium
coating is
important for
enhancing
desired
mechanical
properties and
can be done
with ease in
the EB-PVD
process.
280
270
260
250
240
230
220
210
40Optical
μm micrograph showing the
coating growth direction.
0
200
400
600
800
1000
1200
1400
1600
Inert gas heat treatment temperature (oC)
Average Vicker's Hardness Number (VHN0.050) as a function of heat treatment
temperature for rhenium-coated graphite in argon atmosphere for 4 hrs.
SEM showing grain size of rhenium plates as
a function of annealing temperature.
Electron Beam Physical
Vapor Deposition has
been proven for fabrication of net
net-shaped
shaped
rhenium components.
Future applications of
EB-PVD will provide a
flexible
alternative
manufacturing process
with improved material
properties.