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Laser Processing on Graphite
MSE 503 Seminar - Fall 2009
08-27-2009
CLA Conference Room, UT Space Institute, Tullahoma, TN - 37388, USA
Deepak RajputGraduate Research AssistantCenter for Laser Applications
University of Tennessee Space InstituteTullahoma, Tennessee 37388-9700
Email: [email protected] Web: http://drajput.com
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Outline
Introduction to Graphite
Problems and Possible Solutions
Laser Processing
Results & Discussion
Summary
Future work2
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Carbon (Atomic number: 6 / 1s22s22p2)
Carbon: Introduction
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Graphite (sp2)
Diamond (sp3)
Fullerenes (molecular form / cage-like structure)
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Allotropes of Carbon
Eight allotropes
a) Diamondb) Graphitec) Lonsdaleited) C60
e) C540
f) C70
g) Amorphous Carbonh) Carbon Nanotube
Image source: http://en.wikipedia.org/wiki/Carbon4
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Graphite: Introduction
Low specific gravityHigh resistance to thermal shockHigh thermal conductivityLow modulus of elasticityHigh strength (doubles at 2500oC*)
“High temperature structural material”
*Malmstrom C., et al (1951) Journal of Applied Physics 22(5) 593-6005
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Graphite: Introduction
Low resistance to oxidation at high temperaturesErosion by particle and gas streams
Solution: Well-adhered surface protective coatings !!Adherence:
(1) the ability of the coating elements to wet thesurface of the carbon material (wettability).
(2) the difference in the values of coefficient ofthermal expansion of the coating and that of the carbonmaterial.
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Graphite: Surface Protection
The ability of a material to wet the surface of carbon depends onthe contact angle between the melt and the carbon material.It can be determined from the Young-Dupre equation:
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)cos1( θσ +=aW
Wa = work of adhesionσ= surface tension of the meltθ= contact angle
“The smaller the contact angle, the betterthe wettability of the metal.”
1) Wettability
Image source: David Quéré (2002) Nature Materials 1, 14 – 15.
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Graphite: Surface Protection
a) When the melt solidifies on the carbon substrate,significant internal stresses develop at the coating-substrate interface.
b) If the interfacial stress are large enough, the interfacefails and the coating delaminates.
c) The reason for this failure is the weak cohesive strengthof the coating surface.
d) The cohesive strength of the coating depends on thedifference in the values of coefficient of thermalexpansion of the coating and that of the substrate. Thelarger the difference, the weaker the cohesive strengthof the coating.
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2) Thermal Expansion
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Graphite: Surface Protection
The ideal coating material for a carbon material:One that can wet the carbon material andWhose coefficient of thermal expansion is close to that of the carbon substrate.
The coefficient of thermal expansion of a carbon material depends on the its method of preparation.
Transition metals wet the carbon materials efficiently.
UTSI: Semiconductor grade graphite (7.9 x 10-6 m/m oC)
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Graphite: Surface Protection
Transition metals have partly-filled d orbitals. They cancombine strongly with carbon.They form strong covalent bonds with carbon.Transition metals and their carbides, nitrides, oxides, andborides have been deposited on carbon materials.Non-transition metal coatings like silicon carbide, siliconoxy-carbide, boron nitride, lanthanum hexaboride,glazing coatings, and alumina have also been deposited.Methods used: chemical vapor deposition, physical vapordeposition, photochemical vapor deposition, thermalspraying, PIRAC, and metal infiltration.
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Graphite: Laser Processing
CLA (UTSI): the first to demonstrate laser deposition ongraphite.Early attempts were to make bulk coatings to avoiddilution in the coating due to melting of the substrate.Graphite does not melt, but sublimates at room pressure.Laser fusion coatings on carbon-carbon composites.Problems with cracking.CLA process: LISITM !!LISITM is a registered trademark of the University ofTennessee Research Corporation.
11 LISI: Laser Induced Surface Improvement
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LISITM on Graphite
Prepare a precursor mixture by mixing metal particlesand a binder.Spray the precursor mixture with an air spray gun onpolished graphite substrates (6 mm thick).Dry for a couple of hours under a heat lamp before laserprocessing.Carbide forming ability among transition metals:Fe<Mn<Cr<Mo<W<V<Nb<Ta<Ti<Zr<HfTitanium (<44 μm), zirconium (2-5 μm), niobium (<10μm), titanium-40 wt% aluminum (-325 mesh), tantalum,W-TiC, chromium, vanadium, silicon, iron, etc.Precursor thickness: Ti (75 μm), Zr (150 μm), Nb (125 μm).Contains binder and moisture in pores.12
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LISITM on Graphite
1,2,12,13 – Overhead laser assembly; 4 – Argon; 16,17 – mechanical & turbo pumps 7 – sample, 8 – alumina rods, 9 – induction heating element, 18 – RF supply.
Two-step Processing Chamber
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LISITM on GraphiteC
L
A
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Graphite
LISITM on Graphite
Process variables: laser power (W), scanning speed (mm/s)focal spot size (mm), laser pass overlap (%), 15
T = 800 oC
Copper induction heating element
track
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Focal spot size (Intensity):
LISITM on Graphite
Focal plane(Max intensity)I = P/spot area
Laser beam: near-Gaussian, 1075±5 nmImage source: Rajput D., et al (2009) Surface & Coatings Technology, 203, 1281-128716
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LISITM on Graphite
Laser pass overlap(%)
X
D
100% xDX
=
Overlap important to get complete melting because the beam is near-Gaussian17
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LISITM on Graphite: Results
Scanning electron microscopy
X-ray diffraction of the coating surface
X-ray diffraction of the coating-graphite interface
Microhardness of the coating
Secondary ion mass spectrometery of the niobium coating
SEM was done at the VINSE, Vanderbilt University (field emission SEM)X-ray diffraction was done on a Philips X’pert system with Cu Kαat 1.5406 ÅMicrohardness was done on a LECO LM 300AT under a load of 25 gf for 15 seconds (HK)SIMS was done on a Millbrook MiniSIMS: 6 keV Ga+ ions18
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Results: Titanium
SEM micrographs of the titanium coating.
XRD of the titanium coating surface (A) and its interfacewith the graphite substrate (B)
Oxygen: LISITM binder ortraces in the chamber
19 900-1100 HK
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Results: Zirconium
SEM micrographs of the zirconium coatingDelamination and crack appear in some locations
XRD of the zirconium coating surface (A) and its interfacewith the graphite substrate (B)
20 ~ 775 HK
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Results: Niobium
SEM micrographs of the niobium coating
XRD of the niobium coating surface (A) and its interfacewith the graphite substrate (B)
21 620-1220 HK
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Proposed Mechanism
Self-propagating high temperature synthesis (SHS) aidedby laser heating. It is also called as combustion synthesis.Once ignited by the laser heating, the highly exothermicreaction advances as a reaction front that propagatesthrough the powder mixture.This mechanism strongly depends on the starting particlesize. In the present study, the average particle size is <25μm.The coefficient of thermal expansion of titanium carbideis close to that of the graphite substrate than those ofzirconium carbide and niobium carbide. Hence, titaniumcoating did not delaminate.
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SIMS of the Niobium Coating
A: Potassium, B: MagnesiumC: Oxygen, D: Carbon Mass Spectrum
A: as received B: slightly ground23
Chemical Image of as received Nb coating
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LISITM on Graphite Mandrels
Laser Powder Deposition
Laser Powder Deposition of W-TiC Cermelt Rocket Nozzles
on Graphite Mandrels
ICALEO 2008Temecula, CA
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0.6 mm pre-placed layer: 1500 W, 0.2 mm/s linear, 0.2 rotation/s
100 um 10 um
LISITM on Graphite Mandrels
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Summary
Successfully deposited fully dense and crack-freetransition metal coatings on graphite substrates.
All the coating interfaces contain carbide phases.
Laser assisted self-propagating high temperaturesynthesis (SHS) has been proposed to be the possiblereason for the formation of all the coatings.
SIMS analysis proved that LISITM binder forms a thinslag layer at the top of the coating surface post laserprocessing.
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Future Work
Heat treatment
Advanced characterization (oxidation analysis)
Calculation of various thermodynamic quantities
Publish the results in a good journal
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Questions ??
(or may be suggestions)
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Thanks !!!
photos publishedwithout permission
