Laser Processing on Graphite





   
Note:  There were some mistakes in this presentation that have not been corrected. The corrected version was presented at the International Congress on Applications of Lasers and Electro-Optics (ICALEO) on 3rd November 2009, and is available here (ICALEO 2009 presentation).

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Keywords:

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




Presentation Transcript:


Laser Processing on Graphite?

MSE 503 Seminar - Fall 2009

08-27-2009

CLA Conference Room, UT Space Institute, Tullahoma, TN - 37388, USA
Deepak Rajput
Graduate Research Assistant
Center for Laser Applications
University of Tennessee Space Institute
Tullahoma, Tennessee 37388-9700
Email: drajput@utsi.edu Web: http://drajput.com

Outline
* Introduction to Graphite

* Problems and Possible Solutions

* Laser Processing

* Results & Discussion

* Summary

* Future work

Carbon: Introduction

Carbon (Atomic number: 6 / 1s22s22p2)


Allotropes of Carbon
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: Introduction
* Low resistance to oxidation at high temperatures
* Erosion by particle and gas streams

* Solution: Well-adhered surface protective coatings !!
* Adherence:
(1) the ability of the coating elements to wet the surface of the carbon material (wettability).
(2) the difference in the values of coefficient of thermal expansion of the coating and that of the carbon material.

Graphite: Surface Protection
* The ability of a material to wet the surface of carbon depends on the contact angle between the melt and the carbon material.
* It can be determined from the Young-Dupre equation:


Graphite: Surface Protection
? When the melt solidifies on the carbon substrate, significant internal stresses develop at the coating-substrate interface.
? If the interfacial stress are large enough, the interface fails and the coating delaminates.
? The reason for this failure is the weak cohesive strength of the coating surface.
? The cohesive strength of the coating depends on the difference in the values of coefficient of thermal expansion of the coating and that of the substrate. The larger the difference, the weaker the cohesive strength of the coating.


Graphite: Surface Protection
* The ideal coating material for a carbon material:
* One that can wet the carbon material 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 wet the carbon materials efficiently.

* UTSI: Semiconductor grade graphite (7.9 x 10-6 m/m oC)
Graphite: Surface Protection
* Transition metals have partly-filled d orbitals. They can combine strongly with carbon.
* They form strong covalent bonds with carbon.
* Transition metals and their carbides, nitrides, oxides, and borides have been deposited on carbon materials.
* 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 spectrometery 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 ignited 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.
* 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
LISITM on Graphite Mandrels

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)

* Calculation of various thermodynamic quantities

* Publish the results in a good journal

Questions ??

(or may be suggestions)


Thanks !!!