liquid phase sintering of boron carbide...16 backfill with ar here. ... l.s. sigl. “processing and...

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

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

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)

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

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

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

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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

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

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

Grain pullout is an issue across all sintering parameters

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• 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

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).

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).

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

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

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

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

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

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

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

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

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)

The sweet spot for sintering temperature is around 2350 °C

21

• No significant differences in overall microstructure

2300 °C

500 μm

The sweet spot for sintering temperature is around 2350 °C

22

• No significant differences in overall microstructure

Baseline (2350 °C)

500 μm

The sweet spot for sintering temperature is around 2350 °C

23

• No significant differences in overall microstructure

2400 °C

500 μm

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

Increasing hold time increased density

25

• No significant differences in overall microstructure

1 hr. hold

500 μm

Increasing hold time increased density

26

• No significant differences in overall microstructure

Baseline (2 hr. hold)

500 μm

Increasing hold time increased density

27

• No significant differences in overall microstructure

4 hr. hold

500 μm

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

Adding a hold at 1350 °C increased density and hardness

29

Baseline (Ar)• No significant

differences in overall microstructure

500 μm

30

Ar w/ Burn-off• No significant

differences in overall microstructure

Adding a hold at 1350 °C increased density and hardness

500 μm

31

Ar w/ Vacuum Burn-off• No significant

differences in overall microstructure

Adding a hold at 1350 °C increased density and hardness

500 μm

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.

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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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

Extra Slides

35

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

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

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

What if we used all the methods for small improvements again now that we’re at the correct temperature?

39

Heating Rate

Atmosphere

???