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 1of xx
Outline Introduction to Graphite Problems and Possible Solutions Laser Processing Results & Discussion Summary 2 Future work 2of xx
Carbon: Introduction Carbon (Atomic number: 6 / 1s 2 2s 2 2p 2 ) Graphite (sp 2 ) Diamond (sp 3 ) Fullerenes (molecular form / cage-like structure) 3 3of xx
Allotropes of Carbon Eight allotropes a) Diamond b) Graphite c) Lonsdaleite d) C 60 e) C 540 f) C 70 g) Amorphous Carbon h) Carbon Nanotube 4 Image source: http://en.wikipedia.org/wiki/carbon 4of xx
Graphite: Introduction Low specific gravity High resistance to thermal shock High thermal conductivity Low modulus of elasticity High strength (doubles at 2500 o C*) High temperature structural material 5 *Malmstrom C., et al (1951) Journal of Applied Physics 22(5) 593-600 5of xx
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. 6 6of xx
Graphite: Surface Protection 1) Wettability 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: W a = σ ( 1+ cosθ ) W a = work of adhesion σ= surface tension of the melt θ= contact angle The smaller the contact angle, the better the wettability of the metal. 7 Image source: David Quéré (2002) Nature Materials 1, 14 15. 7of xx
Graphite: Surface Protection 8 2) Thermal Expansion a) When the melt solidifies on the carbon substrate, significant internal stresses develop at the coatingsubstrate interface. b) If the interfacial stress are large enough, the interface fails and the coating delaminates. c) The reason for this failure is the weak cohesive strength of the coating surface. d) 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. 8of xx
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 o C) 9 9of xx
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. 10 10 of xx
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: LISI TM!! LISI TM is a registered trademark of the University of Tennessee Research Corporation. 11 LISI: Laser Induced Surface Improvement 11 of xx
LISI TM on Graphite 12 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<Mn<Cr<Mo<W<V<Nb<Ta<Ti<Zr<Hf Titanium (<44 μm), zirconium (2-5 μm), niobium (<10 μm), 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 of xx
LISI TM on Graphite 13 Two-step Processing Chamber 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. 13 of xx
LISI TM on Graphite 14 of xx C L A 14
LISI TM on Graphite T = 800 o C Graphite track 15 Copper induction heating element Process variables: laser power (W), scanning speed (mm/s) focal spot size (mm), laser pass overlap (%), 15 of xx
LISI TM on Graphite Focal spot size (Intensity): Focal plane (Max intensity) I = P/spot area 16 Laser beam: near-gaussian, 1075±5 nm Image source: Rajput D., et al (2009) Surface & Coatings Technology, 203, 1281-1287 16 of xx
LISI TM on Graphite D X Laser pass overlap (%) X % = x100 D 17 Overlap important to get complete melting because the beam is near-gaussian 17 of xx
LISI TM 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 18 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 Å Microhardness 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 + ions 18 of xx
Results: Titanium XRD of the titanium coating surface (A) and its interface with the graphite substrate (B) Oxygen: LISI TM binder or traces in the chamber 19 SEM micrographs of the titanium coating. 900-1100 HK 19 of xx
Results: Zirconium XRD of the zirconium coating surface (A) and its interface with the graphite substrate (B) SEM micrographs of the zirconium coating 20 Delamination and crack appear in some locations ~ 775 HK 20 of xx
Results: Niobium XRD of the niobium coating surface (A) and its interface with the graphite substrate (B) 21 SEM micrographs of the niobium coating 620-1220 HK 21 of xx
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 μm. 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. 22 22 of xx
SIMS of the Niobium Coating 23 Chemical Image of as received Nb coating A: Potassium, B: Magnesium C: Oxygen, D: Carbon Mass Spectrum A: as received B: slightly ground 23 of xx
LISI TM on Graphite Mandrels Laser Powder Deposition Laser Powder Deposition of W-TiC Cermelt Rocket Nozzles on Graphite Mandrels ICALEO 2008 Temecula, CA 24 24 of xx
LISI TM on Graphite Mandrels 0.6 mm pre-placed layer: 1500 W, 0.2 mm/s linear, 0.2 rotation/s 100 um 10 um 25 25 of xx
Summary 26 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 LISI TM binder forms a thin slag layer at the top of the coating surface post laser processing. 26 of xx
Future Work Heat treatment Advanced characterization (oxidation analysis) Calculation of various thermodynamic quantities Publish the results in a good journal 27 27 of xx
Questions?? (or may be suggestions) 28 of xx 28
Thanks!!! photos published without permission 29 of xx