Papers

ICMCTF2004 Session B5: Laser Assisted Coatings and Technologies

Thursday, April 22, 2004 1:30 PM in Room Sunset

Thursday Afternoon

Start	Invited?	Item
1:30 PM		B5-1 Laser Synthesis and Processing of Carbon Nanotubes and Multifunctional Nanocomposites D.B. Geohegan (Oak Ridge National Laboratory); A.A. Puretzky, I.N. Ivanov (University of Tennessee); H.M. Christen, I. Ohkubo (Oak Ridge National Laboratory); S. Jesse (University of Tennessee); C.M. Rouleau, G. Eres, D.H. Lowndes, H. Cui, J.Y. Howe (Oak Ridge National Laboratory) Lasers offer unique capabilities for nanomaterial synthesis, processing, and characterization. In this talk, several uses of lasers for the controllable processing of functional nanomaterials will be presented. Single-wall carbon nanotubes (SWNT) are just one example of a multitude of one-dimensional nanocrystals which are efficiently grown by catalytically-assisted laser-synthesis. Unlike growth by chemical vapor deposition (CVD), exclusively SWNT are easily grown by laser vaporization (LV) of carbon/catalyst targets at high temperatures despite the wide variety of metal catalyst nanoparticle diameters produced when metal vapors condense in a background gas. Time-resolved laser spectroscopy and imaging measurements of the reactants and dynamics inherent in the LV process inside a hot tube furnace will be reviewed which have provided some of the only direct measurements of SWNT growth rates, indications of the available carbonaceous feedstock, and likely mechanisms of growth. Variations in the laser pulse width and repetition rate greatly affect the SWNT yield and chemistry required for purification. Free-running Nd:YAG laser irradiation of composite targets at 20 Hz repetition rates produce equivalent ablation rates and higher yields (up to 70 wt. % SWNT) than nanosecond pulse-irradiation, despite a factor of 1000 decrease in peak laser intensity. Time-resolved diagnostic measurements will be presented which investigate this ablation phenomena. Methods of alignment and processing of loose SWNT into nanocomposites will be presented. Alternatively, nanotubes can be grown aligned on substrates by thermal CVD. We have developed in situ laser reflectivity to measure the growth kinetics of aligned multwall nanotube forests from evaporated metal multilayer films, conrol their length, and optimize their growth to long lengths at high rates. Coordinated combinatorial deposition of metal multilayers by PLD and microRaman laser spectroscopy mapping of the nanotubes resulting from CVD growth will be presented to optimize the catalyst composition and survey the fraction of SWNT grown. Novel experiments combining LV and CVD will also be described. Laser machining and lithographic patterning of the aligned MWNT forests coupled with PVD and liquid infiltration techniques provide opportunities for a variety of sensor and multifunctional composite applications, which will be reviewed. This research was sponsored by DARPA, NASA-Langley Research Center, NASA-Marshall Space Flight Center, the Laboratory-Directed Research and Development Program at ORNL, and the U.S. Department of Energy under contract DE-AC05-00OR22725 with the Oak Ridge National Laboratory, managed by UT-Battelle, LLC.
2:10 PM		B5-3 Pulsed Laser Deposition of C and BN Based Hard Coatings with Low Internal Stress S. Weissmantel, G. Reisse, D. Rost (University of Applied Sciences, Germany) A review will be given on the preparation of carbon and boron nitride based hard coatings by PLD. The conditions under which hard phases of the materials are formed will be presented. Special attention will be given to the preparation of DLC, t-aC and c-BN films. The growth rates, microstructure and properties of those films will be discussed and the special advantages of the PLD method emphasized. The possibility of t-aC and c-BN deposition with high rates, high hardness and low internal stress will be shown. Especially for the preparation of micrometer thick films of those materials a low internal stress and good adhesion are required. Various methods to reduce stress and improve adhesion will be presented. For improving the adhesion interfacial engineering is necessary. In the case of c-BN the use of pulsed laser deposited h-BN intermediate layers was found to increase significantly the adherence. Additional measures for the reduction of internal stress can be taken during, alternating to or after the deposition process including thermal annealing, pulsed laser annealing and ion bombardment as well as implantation. Especially pulsed laser annealing applied alternating to the deposition process proved to be an efficient method for the preparation of stress-free t-aC films with 80 to 85 % sp³ bonds and thicknesses in the µm-range. Its great advantage to conventional thermal annealing is the short time it requires to completely remove stress (the process itself requires only a few µs) and that it is possible to go over directly from deposition to annealing and vice versa. The latter is of particular significance as, due to the high stress of as-deposited t-aC films, the annealing of sublayers of a few 100 nm is necessary for the preparation of thick films and t-aC must be deposited at low substrate temperature below 100°C. In our set-up we have prepared 2 µm thick homogeneous stress-free films on 20 cm² Si- and WC-hard-metal substrate area in about one hour.
2:50 PM		B5-5 Laser Crystallization of Sputter-deposited Amorphous Quasicrsytalline Coatings F. Kustas (Engineered Coatings, Inc.); P. Molian, A Sadhu Kumar (Iowa State University); M. Besser, D. Sordelet (DOE Ames Laboratory) Quasicrystalline (QC) (e.g., Al₇₀Fe₁₀Cu₁₀Cr₁₀) materials offer a unique combination of relatively high hardness, low surface energy / low friction, and low thermal conductivity. This desirable suite of properties is reportedly due to their nonconventional translational symmetry and aperiodic atomic ordering. These materials can be formed only after a high temperature (>700°C) anneal and they exhibit brittle behavior in bulk form. Coating deposition alleviates the low toughness issue, but the requirement for a high-temperature anneal degrades the mechanical properties of conventional engineering substrates. To address this latter issue, controlled laser surface treatment of sputter-deposited QC coatings (on Al, Ti, and bearing steel alloys) was performed to convert the amorphous (a) structure into crystalline (c) phases. Characterization of both a-QC and c-QC films included Energy Dispersive Spectroscopy and x-ray diffraction for composition/structure, C-brale indentation for relative toughness, and ball-on-disk (BOD) friction / wear tests. Laser treatment was successful in converting the amorphous structure to the crystalline phase, without significant reduction (<10% for Ti-6Al-4V) in substrate hardness, and it was shown that laser pulse energy influences the final surface finish of the c-QC surface. Laser crystallization was observed to increase the indentation resistance / adhesion of c-QC films on coated Ti-6Al-4V and 52100 steel. Friction / wear tests of c-QC films showed reductions in coefficients of friction, compared to non-coated substrates, of ~40%, 20%, and 25%, respectively, for coated 2024-T3Al, Ti-6Al-4V, and 52100 steel substrates. Reductions in wear damage on c-QC coated surfaces were also observed, compared to non-coated surfaces.
3:10 PM		B5-6 Large Area PLD of nm-multilayers for X-ray Optical Applications R. Dietsch, T. Holz, D. Weissbach (AXO DRESDEN GmbH, Germany); S. Braun (Fraunhofer Institute Material and Beam Technology (IWS) Dresden, Germany) The deposition of nanometer-multilayers on technical relevant substrates, used as X-ray optics, makes extreme demands on the deposition process concerning precision, reproducibility and long term stability. Across a stack of more than 150 layers with single layer thicknesses in the range between 1 to 10 nm a variation of single layer thickness considerably lower than sigma-D = 0.1 nm and an interface roughness below sigma-R = 0.25 nm have to be realized. Thickness homogeneity delta-d/d < 1% and lateral thickness gradients delta-d/delta-x approx. 10-8 have to be guaranteed across macroscopic substrate dimensions. Magnetron sputtering and e-beam evaporation are well established deposition techniques to fabricate X-ray optical multilayers. For particular material combinations and for tailored thickness profiles PLD has become an interesting alternative to these predominant technologies. Within the last years the established 4 - (inch) large area PLD technology of X-ray optical metal/carbon and carbon/carbon multilayers has been up-scaled to the deposition of substrates up to 6- (inch) diameter. As a result of the realized precision and long term stability of the up-scaled PLD process multilayer stacks showing X-ray optical quality can be deposited both on flat and on pre-curved substrates. These multilayers with homogeneous or lateral graded thickness distributions are applied as monochromators, collimators or focusing X-ray optical elements in X-ray analysis.
3:30 PM		B5-7 Laser Gas Alloying of Titanium - Process Technology and Wear Test Results S. Bonss, B. Brenner (Fraunhofer Institute of Materials and Beam Technology, Germany); H.-J. Scheibe (Fraunhofer Institute of Material and Beam Technology, Germany); E. Beyer (Fraunhofer Institute of Materials and Beam Technology, Germany) Laser gas alloying is a very smart process without a need of powder or wire. The alloying element is a gas like nitrogen. The results are remolten layers which contain hard phases like titaniumnitride (TiN). To eliminate present difficulties at laser gas alloying of titanium as not sufficient control of nitrogen content, inadequate oxygen exclusion and pronounced crack formation tendency, a new technology was developed. It is based on a special copper made inert gas bell that ensures on-line controlled constant treatment parameters and reduced cooling velocity. So it is possible to adjust the properties of the laser gas alloyed layers. The thickness can be varied between 0.1 to 1.5 mm and the hardness can be adapted to the application in the range of 500 HV to 1200 HV. Such gas alloyed layers have very high resistance against abrasive and sliding wear. An additional hard amorphous carbon layer deposited by Laser-Arc technology on the gas alloyed layer can be used for achieving minimal friction coefficients in case of sliding wear. The gas alloyed layer can also be used for supporting other films like TiN to increase their bearing load. Results of fatigue tests of gas alloyed layers, including additional amorphous carbon top layer, are presented as well as results of different wear tests.
3:50 PM		B5-8 Effect of TiO₂-doping on the Microstructure and Properties of Laser-clad Ferrous-based Coatings E. Liang, J.M. Yu, M.J. Chao, B. Yuan (Zhengzhou University, PR China) The effects of TiO₂ doping on the properties of laser-clad ferrous-based coatings on carbon steel substrates have been studied. It is shown that good finish and crack-free ferrous-based-composite coatings can be fabricated by laser cladding under a proper amount of TiO₂-doping and suitable laser processing parameters. Proper amount of TiO₂ refines the microstructure and improves the cracking sensitivity considerably without deteriorating or even improving the coatings' are resistance. The microstructural and metallographic analyses by X-ray diffraction (XRD) and scanning electron microscopy (SEM) suggest that proper amount of TiO₂ doping favors the formation of inter-metallic compounds of Cr₁₃Fe₃₅Ni₁₃Ti₇, (FeNi) solid solution and fine titanium boride (TiB₂) particulates and of pseudo eutectic textures. The refinements in microstructure and cracking sensitivity are attributed to both an increase in number of nucleation sites by the formation of TiB₂ in the melt pool and the formation of the pseudo eutectics during the solidification process. However, an over doping with TiO₂ leads to the formation of Ni-Ti solid solution and increases the contents of oxides, which degrades the hardness and wear properties of the laser-clad ferrous-based coatings.