Transition Metal Coatings on Graphite via Laser Processing





   


Note:  Presented at the International Congress on Applications of Lasers and Electro-Optics (ICALEO 2009) on 3rd November 2009. This is the corrected version of Laser Processing on Graphite.

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Keywords:

Laser processing, Graphite, Coatings, Surface alloying, Oxidation resistance, Wear, Erosion, ICALEO
 




Presentation Transcript:


Transition Metal Coatings on Graphite
Via
Laser Processing

D. Rajput*, L. Costa, K. Lansford, A. Terekhov, G. Murray, W. Hofmeister


Center for Laser Applications
University of Tennessee Space Institute
Tullahoma, Tennessee 37388-9700
* Email: drajput@utsi.edu
Web: http://cla.utsi.edu


Outline
> Objective & Motivation
> Introduction to Graphite
> Problems and Possible Solutions
> Laser Processing
> Results & Discussion
> Summary
> Future work


* Objective:

* Thick metallic coatings on graphite, carbon fiber materials, carbon-carbon composites.

* Motivation:

* Protection of carbon from oxidation/erosion
* Integration of carbon and metallic structures
Graphite: Introduction

* Low specific gravity
* High resistance to thermal shock
* High thermal conductivity
* Low modulus of elasticity
* High strength (doubles at 2500oC*)

?High temperature structural material?


Graphite: Problems & Solution
* Low resistance to oxidation at high temperatures
* Erosion by particle and gas streams

* Solution: Well-adhered surface protective coatings !!
* Adherence:
(1) Metal/carbide and carbide/graphite interfaces are compatible since formed by chemical reaction.
(2) Interfacial stresses can be created by the difference in thermal expansion.

Graphite: Surface Protection
? Mismatch in the thermal expansion develops interfacial stresses.
? Large interfacial stresses lead to coating delamination/failure.

Graphite: Surface Protection
* The ideal coating material for a carbon material:

* One that can form carbides, and
* Whose 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 are carbide formers.

* UTSI: Semiconductor grade graphite (7.9 x 10-6 m/m oC)
Graphite: Surface Protection
* Non-transition metal coatings like silicon carbide, silicon oxy-carbide, boron nitride, lanthanum hexaboride, glazing coatings, and alumina have also been deposited.

* Methods used: chemical vapor deposition, physical vapor deposition, photochemical vapor deposition, thermal spraying, PIRAC, and metal infiltration.
Graphite: Laser Processing
* CLA (UTSI): the first to demonstrate laser deposition on graphite.
* Early attempts were to make bulk coatings to avoid dilution 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 of Tennessee Research Corporation.

LISITM on Graphite
* Prepare a precursor mixture by mixing metal particles and a binder.
* Spray the precursor mixture with an air spray gun on polished graphite substrates (6 mm thick).
* Dry for a couple of hours under a heat lamp before laser processing.
* Carbide forming ability among transition metals: Fe * Titanium (<44 ?m), zirconium (2-5 ?m), niobium (<10 ?m), titanium-40 wt% aluminum (-325 mesh), tantalum, W-TiC, chromium, vanadium, silicon, iron, etc.
* Precursor thickness: Ti (75 ?m), Zr (150 ?m), Nb (125 ?m). Contains binder and moisture in pores.


LISITM on Graphite
LISITM on Graphite
LISITM on Graphite
LISITM on Graphite

Focal spot size (Intensity):
LISITM on Graphite
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 spectrometry of the niobium coating
Results: Titanium
Results: Zirconium
Results: Niobium
Proposed Mechanism
* Self-propagating high temperature synthesis (SHS) aided by laser heating. It is also called as combustion synthesis.

* Once triggered by the laser heating, the highly exothermic reaction advances as a reaction front that propagates through the powder mixture.

* This mechanism strongly depends on the starting particle size. In the present study, the average particle size is <25 ?m.
Coating delamination
The coefficient of thermal expansion of titanium carbide is close to that of the graphite substrate than those of zirconium carbide and niobium carbide. Hence, titanium coating did not delaminate.


SIMS of the Niobium Coating
Summary
* Successfully deposited fully dense and crack-free transition metal coatings on graphite substrates.

* All the coating interfaces contain carbide phases.

* Laser assisted self-propagating high temperature synthesis (SHS) has been proposed to be the possible reason for the formation of all the coatings.

* SIMS analysis proved that LISITM binder forms a thin slag layer at the top of the coating surface post laser processing.

Future Work
* Heat treatment

* Advanced characterization (oxidation analysis, adhesion test)

* Calculation of various thermodynamic quantities

* Try different materials !!


Acknowledgements

* Tennessee Higher Education Commission (THEC)

* The Vanderbilt Institute of Nanoscale Science and Engineering (VINSE), Vanderbilt University, Nashville

* National Science Foundation student grant to attend ICALEO 2009.


QUesTions ??

(or may be suggestions)


Thanks !!!