Borosilicide Coatings for HighTemperature MoSiB Alloys

1
Borosilicide Coatings for High-Temperature
Mo-Si-B Alloys

• R. Sakidja, J. Werner and J. H. Perepezko

University of Wisconsin - Madison Dept. of
Materials Science Engineering Madison, WI 53706
19th International Conference on Surface
Modification Technology August 1-3, 2005 St.
Paul, Minnesota
Acknowledgement Office of Naval Research (ONR)
(N00014-02-1-0004 )
2
Outline

• Introduction / Background
• High Temperature Requirements
• Mo-Si-B System
• Oxidation Behavior
• - Alloy (Uncoated) Behavior
• - Coating Strategies
• gtSi Pack Cementation Benefit Deficiency
• gt SiB Co-Pack Cementation
• Application to Commercial Alloys (TZM)
• Graded Smart Coating
• Summary

3
The Need for Higher Temperatures
D. Dimiduk and J. Perepezko, MRS Bulletin, 28,
No. 9, 639 (2003)
4
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Mo-Si-B Ternary Equilibrium (at)
T2?Mo-12.5Si-25B
6
Mo-Rich Mo-Si-B Alloy Oxidation
MoO3
SiO2
SiO2 B2O3
Mo3Si
T2
Mo3Si
T2
T2
O2
Mo
Mo
T2
High T
T2
Mo3Si
As-Produced
Time
MoO3 (g)
MoO3 (g)
Time
7
High-temperature oxidation behavior of Mo-Si-B
Alloys
TGA results from cyclic oxidation tests at
1200oC
TGA results between 700oC and 1400 oC
The B/Si ratio is critical to control the
transient stage of oxidation kinetics
Yoshimi et al, Intermetallics, 2002
Mendiratta, Intermetallics, 2002
8
DIFFUSION PATHWAYS AND OXIDE STRUCTURES IN
Mo-Si-B ALLOYS
O
(a)
(b)
B2O3
MoO2
3
2
SiO2
B
MoB
1
Mo2B
T2
Si
Mo3Si
Mo5Si3
BCC

• Cross section BSE image of the Mo-14.2Si-9.6B
  (at) alloy oxidized at 1200 ?C for 100 hr in
  air.
• Schematic illustration of the diffusion pathway
  indicating the phase
• evolution upon oxidation of a Mo-Si-B alloy
  located in the Mo-Mo3Si-T2 three phase field.
  (The numbers indicate
• 1. Mo(ss) BCC phase with internal oxide
  precipitates,
• 2. MoO2
• 3. Borosilicate layer.

J. S. Park, R. Sakidja, and J. H. Perepezko,
"Coating Designs for Oxidation Control of Mo-Si-B
Alloys", Scripta Materialia, 46, p. 765 (2002).
9
(a) TEM image and diffraction pattern for the
MoO2 precipitate formed in the in-situ
borosilicate layer (Outer layer), (b) is HR-TEM
image of the rectangle area in (a).
J. S. Park, R. Sakidja, and J. H. Perepezko,
"Coating Designs for Oxidation Control of Mo-Si-B
Alloys", Scripta Materialia, 46, p. 765 (2002).
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(1) Mo(s)O2MoO2 (2)MoO2(s)(1/2)O2MoO3(l)

Mo-O System, volatile species at 1250K (977 ?C)
Gulbransen et al, 1979. Vertical dotted lines
indicate the Mo(s)/MoO2(s)/MoO3(l) transition
boundaries of Oxygen pressure at 1200 ?C
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Challenges for Mo-Si-B alloys at intermediate
temperatures (650- 750oC)

• Oxidation mechanism map in
  temperature-composition space for selected
  Mo-Si-B alloys
• LOW TEMPERATURE (lt650oC) Solid MoO3 scales
  form gt a slower O diffusion
• HIGH TEMPEARTURE (gt 750oC) Borosilica gt
  Silica form gt a slower O diffusion
• INTERMEDIATE TEMPERATURES MoO3 is a gaseous
  phase and evaporates, but the temp. range is
  still too low for a continuous borosilica glass
  formation gt a faster O diffusion

D. Dimiduk and J. Perepezko, MRS Bulletin, 28,
No. 9, 639 (2003)
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Uncoated Mo-3Si-1B (700oC/30 hrs)
Severe oxidation at 700oC due the absence of
protective borosilica glass The oxide scale is
made of non-protective/evaporating MoO3 consuming
the sample
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1300C 1372C Oxidation Behavior
B2O3 (g)
B2O3 (g)
O2
B2O3 Depleted Borosilicate
O2
SiO2 B2O3
Molybdenum metal borosilicate
Substrate
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Temperature dependence of Oxygen self diffusion
coefficients for SiO2-B2O3 glass
B/Si ratio within Scale is the key
800 ?C
700 ?C
(a) 0 (b) 1 (c) 5 (d) 20 (e) 30 (f)100 mole
of B2O3
Ref. Handbook of Glass Properties
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Si Pack Cementationonto Mo-Si-B Alloys
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Si Pack Cementation on Mo-Si-B alloy
NaF Si(s) Al2O3(s) ? SixFy (g)NaF (s)
Al2O3(s)
Initial period Rate Controlling Step Solid
State Diffusion After Initial period Rate
Controlling Step Gas Diffusion

• Reaction Products Mainly
• MoSi2 ? Gas diffusion governs during the process

Solid State Diffusion J Si in solid
Gas Diffusion J Si in gas
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Si-pack cementation on Mo under reducing
environment
CHEMICAL REACTION STEP TO FORM GAS SPECIES
CHEMICAL REACTION STEP TO DEPOSIT Si
SOLID-STATE DIFFUSION STEP
Jin-Kook Yoon, J. Electrochemical Society, 151(6)
B309-318
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Si Pack Cementation on Mo-14.2Si 9.6B
T1 T2 layers develop after heat treatment at
1300oC in air T1 layer serves as the oxidation
resistant layer T2 (MoB) layer functions as the
diffusion barrier layer
R. Sakidja, J. S. Park, J. Hamann and J. H.
Perepezko, "Synthesis of Oxidation Resistant
Silicide Coatings on Mo-Si-B Alloys", Scripta
Materialia 526 (2005) pp. 723-28 .
19
The 001 textured T1 phase is effective in
reducing the thermal stresses due to the thermal
expansion compatibility
K. Ito, T. Hayashi, M. Yokobayashi and H.
Nakumura., Intermetallics 12 (2004) 407-415
EBSP images for (b) T1 phase (c) MoSi2 of the
framed coating area in (a). Grain orientation
normal to the coating surface determined by EBSP
are presented by colors as indicated in the inset
001-100-110 stereographic triangle. The
001 texture results in a thermal expansion in
the a direction (aa)of the T1 phase to be fairly
similar to those of T2 and Mo-rich substrate (
6-7.10-6 oC-1).
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Si-Pack Cementation onto Mo-Si-B Alloys
MoB
Scale in at
T2
Mo-14.2Si-9.6B
Mo-3Si-1B(wt. )
Si
MoSi2
DEFICIENCY The MoB/MoSi2 layer ratio is
dependent on the substrate composition
21
Deficiency in Si-only Pack Cementation
Sensitivity to Substrate Composition
Substrate Mo-3Si-1B (wt )
Due to the low content of Si and B in Mo-rich
Mo-3Si-1B alloys, a continuous layer of
boride/borosilicide can not be established
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Simultaneous Si B Pack Cementation into
Mo-Si-B and TZM Alloys
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Si-Pack Cementation onto Mo-Si-B Alloys
MoB
Scale in at
T2
Mo-14.2Si-9.6B
Mo-3Si-1B(wt. )
Si
MoSi2
DEFICIENCY The MoB/MoSi2 layer ratio is
dependent on the substrate composition
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CO-DEPOSITION with Silicon Boron powder Source
MoB
0.4
0.6
MoB
Scale in at
0.5
0.5
Mo2B
0.6
0.4
X
T2
X
Mo
B
B
0.7
0.3
T
2
0.8
0.2
Mo-14.2Si-9.6B
0.9
0.1
Mo-3Si-1B(wt. )
1.0
0.0
Si
0.0
0.1
0.2
0.3
0.4
0.5
0.6
MoSi2
T1
Mo(ss)
Mo
Si
X
Si
3
MAJOR BENEFIT The MoSi2MoB layer ratio are
independent of the substrate composition
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201 Si/B Pack Cementated Mo-3Si-1B wt
Pack-cementated Mo-3Si-1B (wt ) (I) without and
(II) with a partial substitution of Si with B
(120 B to Si weight ratio) A full development of
the boride phase underneath the silicide phase is
observed with the partial substitution.
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151 Si/B Pack Cementation 1000oC Oxidation (25
hrs) (Mo-3Si-1B wt )
Element Wt. Atom O -K 55.00 69.45
Al-K 1.01 0.76 Si-K 40.36 29.03
Mo-L 3.63 0.76
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251 Si/B Pack Cementation 1400oC Oxidation (30
hrs)
Thin Borosilicate Glass 10 13 mm
Silicide (MoSi2)
COATING STRUCTURE
Silicide (B-doped Mo5Si3)
B-enriched Region
MoB Mo2B
T2
Substrate
Affected Ox. Region lt 15 mm
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Uncoated , Oxidized in Air at 1400oC for 30 hours
Thick Borosilicate Glass
Intermediate Region (Glass Substrate, depleted
of Silicon and Boron)
SUBSTRATE
Region Evaporated/Loss 400 mm Remaining
Oxidized Region 150 mm
Affected Ox. Region 550 mm
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The Si-pack coated Mo-Si-B alloys posses
excellent high temperature oxidation resistance
due to the formation of boride and/or
borosilicide layers

• Mass change in isothermal and cycle oxidation
  tests for Si-pack coated Mo-9Si-18B
• SEM cross-section image of the alloy after
  oxidation at 1500oC for 50 hours showing
• the T1 phase layer and MoB/Mo2B T2 layer
  underneath
• Mass loss in the 1500oC cycle ox. test 8x10-3
  mg/cm2

REF Yamaguchi et. al., MRS Symp. Proc. 753 (2003)
30
Oxidation Test at 700oC for 30 hrs(Mo-3Si-1B wt
)
UNCOATED
COATED
2.2 mm
The coated sample remains intact virtually no
consumption of thickness
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Application to TZM Alloys
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TZM Composition 0.5 Ti, 0.08 Zr, balance Mo H.
C. Starck Inc., Cleveland, OH, USA
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DEVELOPMENT OF HIGH-TEMPERATURE GRADED SMART
COATING
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HIGH-TEMPERATURE Mo-TM-Si-B SMART COATING

• Key Issues
• Thermodynamic Stability of High-Melting TM-Oxide
• BOROSILICA ? SILICA interchange during thermal
  cycling
• Low diffusivity of metalloids in Mo-TM-Si-B
  coating
• Low density of Mo-TM-Si-B coating

36
Weight density of the Mo-Ti-Si-B alloys
R. Sakidja and J. H. Perepezko, in Materials for
Space Applications, MRS. Symp. Proc. No. 851,
edited by Chipara, M., Edwards, D. L., Benson,
P. and Phillips, S. (MRS, Pittsburgh, PA, 2005),
p. NN11.11.
37
Mo-5Ti-20Si-10B Oxidized in air at 1100oC for 10
hours
8 mm
titania
Cross-sectioned
Plan-view - near surface
Formation of titania near the surface and
borosilica underneath (matrix)
R. Sakidja and J. H. Perepezko, in Materials for
Space Applications, MRS. Symp. Proc. No. 851,
edited by Chipara, M., Edwards, D. L., Benson,
P. and Phillips, S. (MRS, Pittsburgh, PA, 2005),
p. NN11.11.
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Mo-20Ti-20Si-10B Oxidized in air at 1100oC for 10
hours
Titania oxide outer layer develops with an
increasing Ti substitution
R. Sakidja and J. H. Perepezko, in Materials for
Space Applications, MRS. Symp. Proc. No. 851,
edited by Chipara, M., Edwards, D. L., Benson,
P. and Phillips, S. (MRS, Pittsburgh, PA, 2005),
p. NN11.11.
39
ALLOYING STRATEGY ON Mo-Si-B SYSTEM
BCC T2 A15 (Mo-Si-B Cr/V)
Mo-Si-B
BCC T2 T1 (Mo-Si-B W/Nb/Ta)
BCC T2 D88 (Mo-Si-B Hf/Ti/Zr)
BCC T2 A15
A15 T2 T1
BCC Stabilizer Ti, Zr, Hf, V, Nb, Ta, Cr,W T2
Stabilizer Ti, Zr, Hf, V, Nb, Ta, Cr,W T1
Stabilizer W, Nb, Ta A15 Stabizer
Cr, V D88 Stabilizer Ti, Zr, Hf
D. Dimiduk and J. Perepezko, MRS Bulletin, 28,
No. 9, 639 (2003)
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Only a minimum W alloying is required to
modify the phase equilibrium from A15 T2 T1
to BCC T2 T1
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Pack Cementation in Mo-W-Si-B Alloys
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J. H. Perepezko, J. S. Park, R. Sakidja and S.
Kim, "Elevated Temperature Coating Layer Designs
for Mo-Si-B Alloys", in High Temperature
Corrosion and Materials Chemistry IV, Symposium
Proc. Vol. No. 2003-06, edited by Opila, P. Hou,
T. Maruyama, B. Pieraggi, M. McNallan, D.
Shifler, and E. Wuchina, p. 310 (2003).
45
J. H. Perepezko, J. S. Park, R. Sakidja and S.
Kim, "Elevated Temperature Coating Layer Designs
for Mo-Si-B Alloys", in High Temperature
Corrosion and Materials Chemistry IV, Symposium
Proc. Vol. No. 2003-06, edited by Opila, P. Hou,
T. Maruyama, B. Pieraggi, M. McNallan, D.
Shifler, and E. Wuchina, p. 310 (2003).
46
Graded Structures in Mo-Cr-Si-B alloys after
oxidation in air
GRADED METAL OXIDES
BOROSILICATES
OXIDE STRUCTURES IN Mo-35Cr-10Si-20B (700oC/6h)
47

• SUMMARY
• OXIDATION OF Mo-Si-B ALLOYS
• - High temperature protective layers/
  oxidation rate too high
• - Low Temperature non-protective
• - Alloy composition (Viscosity and Oxygen
  diffusivity) - Control through B/Si
• COATING DESIGN
• - Si-Pack Cementation
• Effective, but dependent on substrate
  composition--variability
• - B Si Co-Deposition pack cementation
• - Effective even for TZM 
• - Future Development in Graded Structures of
  Smart Coating