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Effect of Sintering Parameters on Room-Temperature Injection Molded, Pressurelessly Sintered Boron Carbide
Components
Erich WeaverNDSEG Research Fellow
Prof. Rodney Trice and Prof. Jeffrey Youngblood
Purdue University, School of Materials Engineering
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Boron carbide is highly valued for its extreme hardness and low density
2Sand Blasting. Dual-teq.com. Web.Boron Carbide Abrasive Powder. 3m.com. WebCeramic Ballistic Plates & Modular Body Armor. steeldefender.com. Web.
• Favorable properties• Density = 2.52 g/cm3
• Hardness = >30 Gpa Vicker’s• Young’s Modulus = 460 GPa• Melting Point = ~2450 °C
• Applications• Sand-blasting nozzles• Grinding and polishing media• Lightweight armor
• Excellent properties require full densities; not trivial for B4C
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Ceramic suspensions with controlled rheology can be used to improve upon colloidal processing techniques
• High ceramic (>50%) and low binder (~5%) content (balance = water)
• Faster burn-out with fewer cracks and bubbles
• High density after pressureless sintering
• Flowable at room temperature
• Ability to use low pressure tooling
• Rheology amenable to variety of processing methods including injection molding and additive manufacturing
3Diaz-Cano, A., et al., “Stabilization of Highly-Loaded Aqueous Suspensions”, Ceramics International (2017)
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Colloidal ceramic suspensions solve many of the problems associated with traditional ceramic injection molding
4
• No need for elevated temperatures during green body forming
• Pressures required to injection mold are very low
Polymer Binder
Ceramic Powder
Binder Burnout and Sintering
Demolding and Drying
Water
High Speed Mixing
Optional Machining
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Can achieve high densities and hardness values using Y2O3 and WC as sintering aids
5
B4C B4C + WC + Y2O3
• Found several sintering aid combinations that work well, notably 5 wt. % WC and 10 wt. % Y2O3 + 5 wt. % WC
20 μm 20 μm
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Can achieve high densities and hardness values using Y2O3 and WC as sintering aids
• Found several sintering aid combinations that work well, notably 5 wt. % WC and 10 wt. % Y2O3 + 5 wt. % WC
• Achieved relative densities of >95 % and hardness values >3200 Vickers
6
0 5 10 15
89
90
91
92
93
94
95
96
Rela
tive D
ensity (
%)
Sintering Aid (wt. %)
B4C
Al
Y2O3
Al2O3
0 5 10 15
2400
2600
2800
3000
3200
3400
3600
3800
Hard
ness (
HV
)
Sintering Aid (wt. %)
B4C
Al
Y2O3
Al2O3
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Success means new equipment! And a good chance to explore sintering parameters in depth
7
• Purchased a new furnace in order to be able to produce larger parts
• During initial testing, actually managed to exceed results from old furnace
• Decided to thoroughly investigate sintering parameters
Upgrade!
New FurnaceOld Furnace
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Polymer Binder
B4C Powder
Binder Burnout and Sintering
Demolding and Drying
Water
High Speed Mixing
Optional Machining
Sintering Aid
Experimental approach
• Sintering aids• 5 wt. % WC + 10 wt. % Y2O3
• Room-temperature injection molding
• Pressureless sintering• Temperature (2300, 2350, 2400 °C)• Hold Time (1, 2, 4 hours)• Ramp Rate (10, 25, 50 °C/min)• Atmosphere (Ar, Ar w/ burn-off, Ar w/ vacuum burn-off)• Baseline: 2350 °C, 2 hr. hold, 25 °C/min, Ar
• Mechanical testing and microstructure analysis• Optical microscopy• SEM• Archimedes density• Vicker’s hardness
8
Backfill with Ar here
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Grain pullout is an issue across all sintering parameters
9
• Weak interfaces between Y phase and B4C make it extremely easy to pull out grains during polishing
• Grain pullout is present in all samples containing Y2O3
50 μm10 μm
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Both WC and Y2O3 react with B4C during sintering
10
• WC reacts with B4C to form W2B5 , occasionally forming platelet structures1-3
• Y2O3 reduces during sintering to form liquid metal, then forms YB6 when cooled4
10 μm
1. Z. Zakhariev, D. Radev, J Mater Sci Lett, 7 (1988), pp. 695-696
2. J. Yin, Z. Huang, X. Liu, Z. Zhang, D. Jiang, J. Eur. Ceram.
Soc., 33 (2013), pp. 1647-1654
3. A. Rahimi, H. Baharvandi, Int. J. Refract. Met. Hard
Mater. (2017)
4. Goldstein, A. et al.,. J. Eur. Ceram. Soc. 27, 695–700 (2007).
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Both WC and Y2O3 react with B4C during sintering
11
20 μm
• WC reacts with B4C to form W2B5 , occasionally forming platelet structures1-3
• Y2O3 reduces during sintering to form liquid metal, then forms YB6 when cooled4
1. Z. Zakhariev, D. Radev, J Mater Sci Lett, 7 (1988), pp. 695-696
2. J. Yin, Z. Huang, X. Liu, Z. Zhang, D. Jiang, J. Eur. Ceram.
Soc., 33 (2013), pp. 1647-1654
3. A. Rahimi, H. Baharvandi, Int. J. Refract. Met. Hard
Mater. (2017)
4. Goldstein, A. et al.,. J. Eur. Ceram. Soc. 27, 695–700 (2007).
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Using WC and Y2O3 together significantly increases the size of W2B5 platelets
12
50 μm
• W is soluble in Y metal at sinteringtemperatures5
• Platelets form almost exclusively at the boundary between Y liquid and B4C
• High aspect ratio due to interface controlled growth
5. E. Lasner & W.D. Schubert. Tungsten: Properties, Chemistry,
Technology of the Element, Alloys, and Chemical Compounds.
1999.
10 μm
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The amount of intragranular carbon inclusion increases at higher temperatures
13
B4C + WC2350 °C
B4C + WC2375 °C
• Carbon inclusions have been previously observed in B4C, authors found that they had no significant effect on mechanical performance6
• Reduced grain boundary area could explain higher number of intragranular inclusion at 2375 °C
50 μm 50 μm
6. T. Sano, E. S.C. Chen, B. Paliwal, M.W. Chen. “Comparison of Slip Cast to Hot Pressed Boron Carbide. MS&T 2008 Ceramic Transactions Proceedings,
Pittsburgh, PA, 5–9 October 2008
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Y2O3 significantly reduces the amount of carbon inclusions, which could explain hardness increases
14
• Significantly fewer carbon inclusions when Y2O3 is added as a sintering aid
• Excess C reacts with O during the reduction of Y2O3
B4C + WC +Y2O3
2350 °C
50 μm
B4C + WC2350 °C
50 μm
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Y2O3 significantly reduces the amount of carbon inclusions, which could explain hardness increases
15
• Despite having lower densities, Y2O3 consistently has higher hardness values than WC
• Previous literature6
performed bulk measurement (Split-Hopkinson Bar), rather than localized measurement like microhardness
6. T. Sano, E. S.C. Chen, B. Paliwal, M.W. Chen. “Comparison of Slip Cast to Hot Pressed Boron Carbide. MS&T 2008 Ceramic Transactions Proceedings,
Pittsburgh, PA, 5–9 October 2008
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Polymer Binder
B4C Powder
Binder Burnout and Sintering
Demolding and Drying
Water
High Speed Mixing
Optional Machining
Sintering Aid
Quick Reminder of experimental approach
• Sintering aids• 5 wt. % WC + 10 wt. % Y2O3
• Room-temperature injection molding
• Pressureless sintering• Temperature (2300, 2350, 2400 °C)• Hold Time (1, 2, 4 hours)• Ramp Rate (10, 25, 50 °C/min)• Atmosphere (Ar, Ar w/ burn-off, Ar w/ vacuum burn-off)• Baseline: 2350 °C, 2 hr. hold, 25 °C/min, Ar
• Mechanical testing and microstructure analysis• Optical microscopy• SEM• Archimedes density• Vicker’s hardness
16
Backfill with Ar here
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Unlike in pellets, the benefits of increasing ramp rate level off at 25 °C
17
10 °C/min• No significant
differences in microstructure between different ramp rates
500 μm
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Unlike in pellets, the benefits of increasing ramp rate level off at 25 °C
18
Baseline (25 °C/min)• No significant
differences in microstructure between different ramp rates
500 μm
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Unlike in pellets, the benefits of increasing ramp rate level off at 25 °C
19
50 °C/min• No significant
differences in microstructure between different ramp rates
500 μm
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The benefits of increasing ramp rate level off at 25 °C
20
• No significant differences in microstructure between different ramp rates
• Increase in density and hardness as ramp rate increases (up to a point)
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The sweet spot for sintering temperature is around 2350 °C
21
• No significant differences in overall microstructure
2300 °C
500 μm
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The sweet spot for sintering temperature is around 2350 °C
22
• No significant differences in overall microstructure
Baseline (2350 °C)
500 μm
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The sweet spot for sintering temperature is around 2350 °C
23
• No significant differences in overall microstructure
2400 °C
500 μm
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The sweet spot for sintering temperature is around 2350 °C
24
• No significant differences in overall microstructure
• Maximum density was achieved at 2350 °C
• Hardness values could not be measured in 2300 or 2400 C samples due to issues with grain pullout
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Increasing hold time increased density
25
• No significant differences in overall microstructure
1 hr. hold
500 μm
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Increasing hold time increased density
26
• No significant differences in overall microstructure
Baseline (2 hr. hold)
500 μm
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Increasing hold time increased density
27
• No significant differences in overall microstructure
4 hr. hold
500 μm
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Increasing hold time increased density
28
• No significant differences in overall microstructure
• Increasing hold time increased density, but with diminishing returns
• Hardness values could not be measured in 1 hr. or 4 hr. samples due to issues with grain pullout
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Adding a hold at 1350 °C increased density and hardness
29
Baseline (Ar)• No significant
differences in overall microstructure
500 μm
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30
Ar w/ Burn-off• No significant
differences in overall microstructure
Adding a hold at 1350 °C increased density and hardness
500 μm
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31
Ar w/ Vacuum Burn-off• No significant
differences in overall microstructure
Adding a hold at 1350 °C increased density and hardness
500 μm
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32
• No significant differences in overall microstructure
• Well established that a hold between 1300-1500 °C boils off boria4,7
• Adding a burn-off step significantly increases density, and the vacuum burn-off also resulted in higher hardness values
Adding a hold at 1350 °C increased density and hardness
7. L.S. Sigl. “Processing and mechanical properties of boron carbide sintered with TiC”. Journal of the European Ceramic Society. 1998.
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Summary
33
• Pressureless sintering can be used to sinter B4C + WC, B4C + Y2O3, and B4C + WC + Y2O3 to >95% relative density.
• Best practices for pressureless sintering are to burn off in vacuum at 1350 °C, high ramp rate (>25 °C/min), sinter at 2350 °C for 4 hrs.
• Despite having lower densities compared to WC, adding Y2O3
consistently demonstrates higher hardness values
• Ongoing work to determine effects on grain size and to combine best parameters
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34
Acknowledgements
Faculty Members Prof. Rodney Trice
Prof. Jeffrey Youngblood
Group MembersWilly Costakis
Andrew SchlupTess Marconie
Rodrigo Orta GuerraOlivia Brandt
Averyonna Kimery
UndergraduatesYew Wei (Mike) See
ACerS Basic Science DivisionCTTSO
Brady RussellEric Walzer
NDSEG Fellowship ProgramJHU APL
Adam MaisanoBruce TrethewayMorgan Trexler
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Extra Slides
35
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New stuff to add• Seemingly no reduction of grain growth due to precipitate drag
from carbon inclusions – soluble at high temperatures?
• New story order– General microstructure observations as a whole
• Grain pullout
• Y2o3 reduction and liquid phase sintering
• W2b5 platelets
• Platelets showing up almost exclusively at borders between y liquid and b4c
• Carbon inclusion– Paper claimed no performance on mechanical properties
• Y2o3 reducing carbon inclusions– Could explain hardness benefits
– Kolsky bar guys claimed no difference, but they were testing sample as a whole rather than a localized area like in hardness testing
– Benefits from different sintering parameters36
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1
2
3
4
5
6
2”
9.5”
7.5”
5.5”
4”
11”
Sintering parameters made many small improvements to results, but none large enough to explain the difference
37
Hot Zone Location Heating Rate
Atmosphere (Vacuum)
• Both increasing the heating rate and sintering in vacuum slightly increased density, but not enough to match the old results
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The solution turned out to be increasing the sintering temperature significantly – the old furnace was miscalibrated
38
Carbolite 2350 °C Carbolite 2375 °CCentorr 2100 °C
R.E. Taylor et al., Spectral Emissivity at High Temperatures, AFOSR Annual Technical Report (1980).
• The old Centorr furnace was underreporting temperature due to an error in the e-slope value of the pyrometer
• Increasing the temperature used in the Carbolitefurnace increased density even beyond what had been achieved in the Centorr
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What if we used all the methods for small improvements again now that we’re at the correct temperature?
39
Heating Rate
Atmosphere
???