Producing Dense B4C Components Using Room-Temperature Injection Molding and Pressureless

Sintering

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

Several techniques can sinter B4C to full density, but they come with limitations

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Method Advantages Disadvantages

Spark Plasma Sintering (SPS)• High densities• Short cycle time• Small particle size

• High cost• Simple geometries

Hot-Pressing (HP)• Lower cost than HIP or SPS• High densities• Small particle size

• Higher cost than PS• Simple geometries

Hot-Isostatic Pressing (HIP)• Complex geometries• High densities• Small particle size

• High cost• Must be pre-fired to closed porosity

Pressureless Sintering (PS)• Low cost• Complex geometries

• Extensive grain growth due to high temp and time required

• Difficult to get high densities

Traditional ceramic injection molding has a lot of appeal for making complex shapes, but also several critical limitations

• Appealing for highly complex shapes

• Near-net shape parts

• High production rates

• Limitations

• High pressure/elevated temperatures• Expensive tooling that takes a long time to

machines

• Large amounts of polymer binder • Long/complicated binder burnout cycles (i.e.

slow)

• Problems densifying during sintering (i.e. cracks and pores)

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Polymer Binder

Ceramic Powder

Shear Mixing

Ceramic/Polymer Mixture

Binder Burnout and Sintering

Demolding

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

5Diaz-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

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• 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

Polymer binder Injection Pressure Injection Temperature

Traditional IM > 35 vol% > 150 MPa > 220 ⁰C

Purdue IM < 5 vol% < 0.15 MPa 22 ⁰C

Major benefit is ability to use 3-D printed molds for lower cost and rapid turnaround time

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12 mm 10 mm 7 mm

Combined experimental approach

• Attrition Milling• WC milling media

• Ethanol

• Room-temperature injection molding

• Pressureless sintering• Ar, 25 °C/min, 2075 °C, 4 hrs.

• Mechanical testing and microstructure analysis• Vicker’s hardness

• 4-point flexure (B bars)8

Polymer Binder

Attrition Milled Powder

Binder Burnout and Sintering

Demolding and Drying

Water

High Speed Mixing

Optional Machining

Attrition Milling

Ceramic Powder

Injection molding without sintering aids doesn’t reach our desired mechanical properties

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• No sintering aids

• Attrition milling results in small amount of WC in the microstructure

• Lower hardness and flexure strength compared to hot-pressed

• Low density

Relative Density (%)

Hardness (HV) Flexure Strength (MPa)

90.05 ± 0.49 2420 ± 47 295 ± 15

We selected sintering aids that have been used in small amounts to densify B4C

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Sintering Aid Melting Point (°C) Sintering Temperatures (°C)

Sintering Aid Amount (wt. %)

Al 660 2050-2150 1-5

Al2O3 2072 2075-2150 1-5

Y2O3 2425 2100-2250 7.5-36

Goldstein, A. et al., B4C/metal boride composites derived from B4C/metal oxide mixtures. J. Eur. Ceram. Soc. 27, 695–700 (2007).

Lee, C. H. & Kim, C. H. Pressureless sintering and related reaction phenomena of Al2O3-doped B4C. J. Mater. Sci. 27, 6335–6340 (1992).Mashhadi, M., Taheri-Nassaj, E. & Sglavo, V. M. Pressureless sintering of boron carbide. Ceram. Int. 36, 151–159 (2010).

15 wt. % Y2O3

Updated experimental approach

• Attrition Milling• WC milling media• Ethanol

• Sintering aids• Al (1, 3, 5 wt. %)• Al2O3 (1.87, 3.71, 5.52 wt. %)• Y2O3 (5, 10, 15 wt. %)

• Room-temperature injection molding

• Pressureless sintering• Ar, 25 °C/min, 2075 °C, 4 hrs.

• Mechanical testing and microstructure analysis• Vicker’s hardness• 4-point flexure (B bars)

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Polymer Binder

Attrition Milled Powder

Binder Burnout and Sintering

Demolding and Drying

Water

High Speed Mixing

Optional Machining

Sintering Aid

Attrition Milling

Ceramic Powder

Sintering aids effect on microstructure

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B4C +

3.71 wt. % Al2O3

B4C +

5 wt. % Y2O3

B4C +

2 wt. % AlB4C

Oxides reduce to base metals during sintering, and form new phases during cooling

• Carbothermic reduction at sintering temperatures

• Forms borides or Me-B-C phases during cooling

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20 30 40 50 60 70 80

0

2000

4000

6000

8000

10000

12000

Co

un

ts2θ (°)

B4C + Al2O3

B4C + Al

B4C + 5.52 wt. % Al2O3

B4CAl3BCAlB2

WC

B4C + Al

B4C + Al2O3

Al3BC

AlB2

WC

0 1 2 3 4

92

93

94

95

Re

lative

De

nsity (

%)

Sintering Aid (wt. %)

Al

Al2O3

Al, Al2O3, and Y2O3 all significantly increased density

• Previously shown for Ti and TiO2 that oxides are better when normalized

• Small window of benefit for Al2O3 over Al

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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

Levin, L., et al., The Effect of Ti and TiO 2 Additions on the Pressureless Sintering of B 4 C. Metall. Mater. Trans. a 30A, 3201–3210 (1999).

0 5 10 15

150

200

250

300

350

400

Fo

ur

Po

int

Fle

xu

ral S

tre

ng

th (

MP

a)

Sintering Aid (wt. %)

B4C

Al

Y2O3

Al2O3

Flexural strength did not increase in line with increasing densities

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Typical B4C flexural strength values

Previous B4C-Al work

Previous B4C-Y2O3 work

Goldstein, A. et al. J. Eur. Ceram. Soc. 27, 695–700 (2007).Mashhadi, M. et al., Ceram. Int. 36, 151–159 (2010).

There are several possible explanations for why sintering aids aren’t beneficial to flexural strength

• Agglomeration during injection molding?

• Grain coarsening during sintering?

• Grain pullout from sample preparation?

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B4C + 5 wt. % Y2O3

B4C + 2 wt. % AlMashhadi, M., Taheri-Nassaj, E. & Sglavo, V. M. Pressureless sintering of boron carbide. Ceram. Int. 36, 151–159 (2010).

0 5 10 15

2400

2600

2800

3000

3200

3400

3600

3800

Ha

rdn

ess (

HV

)

Sintering Aid (wt. %)

B4C

Al

Y2O3

Al2O3

Hardness improved significantly with all three sintering aids

Typical B4C hardness values

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Previous B4C-Al workPrevious B4C-Y2O3 work

Goldstein, A. et al. J. Eur. Ceram. Soc. 27, 695–700 (2007).Mashhadi, M. et al., Ceram. Int. 36, 151–159 (2010).

Summary

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• Room-temperature injection molding can be used to produce near-net, complex shapes

• Pressureless sintering with sintering aids can be used to densify these shapes to >95% relative density and with >3200 HV

• Ongoing work to explain the reduction in flexural strength when sintering aids are used

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Acknowledgements

Faculty Members Prof. Rodney Trice

Prof. Jeffrey Youngblood

Group MembersWilly Costakis

Andrew SchlupAnnie Brenner

Angel PeñaJorge RamírezTess Marconie

UndergraduatesBenjamin Stegman

Lillian Koczwara

ACerS Engineering Ceramics DivisionONR

Rod PetersonTroy Hendricks

NDSEG Fellowship Program