characterization of boron carbide for inertial confinement ... · pdf filecharacterization of...

Characterization of Boron Carbide for Inertial Confinement Fusion

Carolina Mattsson Lehigh University

Aug 14th, 2013

Funding provided by the NUF program & General Atomics IRD

Greg Randall [email protected]

Inertial Confinement Fusion requires lightweight ablator materials

•  Low atomic numbers •  Robust for fabrication •  Uniformity •  ~100um thick

Boron carbide also offers •  Exceptional Strength •  High Neutron Absorption •  Non-reactivity

Ablation

Boron carbide

Radiation

Fuel


There are various ways to fabricate boron carbide

Hot-Pressing

Chemical Vapor Deposition

Sputtering

~1800˚C, ~50 MPa

B

C

B4C target

V Ar plasma

B4C coating on substrate B4C coating on substrate

Plasma with Gas phase reactants

~200-700˚C ~500˚C

Can buy from Goodfellow This study Coming soon


Sputter coating is one method of depositing films

B and C Atoms Adhere to Substrate

Argon Gas

Electric Field

Ar+ ions in Plasma

Atoms Knocked Off Surface With High KE

1-2 μm/hr


My project has been to fabricate and characterize boron carbide films

Goal is to obtain dense and uniformly textured ~100μm films

Analyze films made with various parameters

Refine coating parameters

Fabricate boron carbide films

-  Thickness -  Size -  Density -  Roughness -  Crystallinity (XRD) -  Stoichiometry -  Microstructure -  Toughness -  Stress

Develop characterization techniques


Microstructure and stress are interrelated and the most important properties to control

Stress Microstructure

Lower Ar Pressure St

ress

Zone 1 Zone T

Windischmann, 1991

Density

ten

sile

c

om

pre

ssiv

e


Summary of sputtering runs to date RF: ~200˚C or 0.16Tm DC: ~700˚C or 0.32Tm

20mTorr

10mTorr

6mTorr

3.3mTorr

10mTorr

6mTorr

Coating Rate: 1-2 um/hr


We see lessening of columnar structure with lower pressure, except with DC and extreme angles

RF DC

Crystalline?

Lower pressures and angles are generally more uniform, which is preferred

20mTorr

10mTorr

6mTorr

3.3mTorr

10mTorr

10mTorr zoomed out

6mTorr


Properties

Surface roughness -- Wyko Density - Microbalance, ImageJ, Dual Confocal

15.2 mm2

Ideal ** 2.52 g/cm3

Hot Pressed 2.45

Average RF sputter

2.38 ±0.05

Average DC sputter Slightly higher? Ra: 7.70nm

Crystalline Structure -- XRD

1000

800

600

400

200

080604020

1000

800

600

400

200

080604020

1000

800

600

400

200

080604020

1000

800

600

400

200

080604020

Xra

y D

iffra

ctio

n In

tens

ity

2θ (deg)

Crystalline B4C with graphite inclusions

RF Sputtered on Al2O3

Cgraphite (002)

Stoichiometry-- Auger

100806040200

200150100500

Ato

mic

%

Cycles

Boron

Carbon

* Absolute % not accurate, needs standard

Hot-pressed Sputtered

Sputtered samples are 35% more boron-rich than hot-pressed

Greatest uncertainty in thickness measurements

Amorphous

5mm

Insaco Hot-Pressed


Stress properties

Residual stress – from coating process

Surface stress – from coefficient of thermal mismatch

Modified Stoney’s Formula

Bent coating off Cu substrate

100

150

200

250

300

350

0 1000 2000 3000 4000 Thic

kne

ss fr

om

leve

l (um

)

Position along sample (um)

Dual Confocal Scan

R=-16.1mm

Theoretical CTE mismatch stress gives :

High compressive stress

All DC Runs

High tensile stress

All RF Runs

Ta up B4C up •  Thin Tantalum pieces bend in the

direction of the stress

•  Lower pressure increases the absolute value of the stress

•  Coating stresses do not cause the film to bend

CTE matched substrate is important

R = -13.4mm For B4C coated on Copper


0

100

200

0 5 10 15

Thic

kne

ss (

um

)

Ar Pressure (mTorr)

Thickness at pop-off

Spontaneous delaminaiton at a critical thickness

Incoming Particle Energy

Stre

ss σ

c

Zone 1 Zone T

Windischmann, 1991

DC coatings do not pop off

Controlled pop-off during RF runs

~170um

~42um

hcr

hcr

5mm

5mm

σc


20 mTorr KIC 1.3 MPa m1/2

3.3 mTorr KIC 2.2 MPa m1/2

Goodfellow KIC 2.2 MPa m1/2

One can estimate the fracture toughness from the pressure σdown under which cracking first occurs  72%

50% Spontaneous cracking, even through laser cuts

Less, not through laser cuts

Power

20mTorr 6mTorr 3.3mTorr

Evaluating fracture toughness

KIC is a material property denoting the resistance of a material to fracture propagation

Boron Carbide KIC ~ 2.5 MPa m1/2

Qualitative Quantitative

Some interesting crack patters warrant further study.

Apply pressure σdown

Shorter cracks


Conclusions from my work this summer

•  Achieved more control over the sputtering process –  Controllable pop-offs and coatings that remain on

Al2O3 substrates –  Predictable trend found in coating stress

•  Obtained coatings with better properties –  Approaching Zone T microstructure –  Near-theoretical fracture toughness –  Consistent density


Recommendations for future work on the subject

•  Plasma-enhanced sputtering •  Determining interfacial energy involved •  More precise strength and toughness testing

–  Microhardness on the way

•  Auger standard for more reliable B/C ratio •  XRD testing on DC runs to check for

crystallinity •  Stress calculations from laser cutting •  Polishing studies •  Coating on curved surfaces


Acknowledgements

•  Greg Randall

•  Hongwei Xu

•  Jack Knipping •  Don Wall •  Chris Hill

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