Papers

ICMCTF1999 Session H2: Quasicrystalline Coatings

Tuesday, April 13, 1999 8:30 AM in Sunrise Room

Tuesday Morning

Start	Invited?	Item
8:30 AM		H2-1 Deposition, Microstructure and Abrasive Wear Behavior of Al-Cu-Fe Quasicrystalline Coatings D. Sordelet (Iowa State University) Si crystals are a new class of materials which exhibit previously forbidden rotational symmetries (e.g., 5-fold) and long range aperiodic translational order. Since their discovery in 1982, quasicrystals have been studied not only because of their fascinating atomic structures, but as a result of their useful physical and mechanical properties, which include low surface energy, low coefficients of friction, high hardness, and low thermal and electrical conductivities. Ongoing efforts have been focused on developing surface coatings to utilize unique combinations of these properties.
9:10 AM		H2-3 Surface Energy of Al-Cu-Fe Intermetallics E. Belin-Ferré (LCPMR & GDR CINQ, France); J.M. Dubois (LSG2M - EMN, France); D. Sordelet (Iowa State University, Ames Laboratory) The surface energy of a solid may be obtained from fairly simple wetting experiments with various liquids, thus leading to the total surface energy of the solid gS = gDS + gNS due to dispersive terms (D) araising from London forces and non-dispersive terms (N), mostly of ionic character. We have used this method to study a serie of sintered Al-Cu-Fe intermetallic samples. For the sake of comparison, we have also examined PTFE, Al2O3 single-crystal and metallic specimens. Special attention was paid to icosahedral quasicrystalline materials. The surface energy of the icosahedral alloys was found to have a very small gNS component, with gS close to that of PTFE and significantly below that of the fcc Al sample. This is fairly weak for an alloy, in strong contrast with the surface energy of a non-oxidized metal which is two orders of magnitude larger. As prepared by careful polishing, the quasicrystal surface therefore shows no detectable contribution from the oxide layer. In the quasicrystalline intermetallics of the Al-Cu-Fe system, a slight dependence of the surface energy upon lattice structural quality and purity of the icosahedral phase was pointed out. Furthermore, the surface energy happens to correlate with the Al 3p density of states measured at the Fermi level by X-ray spectroscopy techniques, thus emphasizing the depressed metallic character of the quasicrystal as compared to the other intermetallics. These measurements are in strong support to the potential applications of quasicrystals in the fields of low-adhesion surfaces and of tribology.
9:30 AM		H2-4 Microstructure and Tribological Behaviour of (Al-Cu-Fe) Based Quasicrystalline Films Deposited by Means of Magnetron Sputtering J.M. Soro, C. Comte, T. Huu Loi, A. Billard, J. von Stebut (Ecole des Mines, France) Quasicrystalline Al-based alloys are well known for their specific properties like low thermal conductivity, specific electrical and magnetic properties and low coefficient of friction. In the present study 10 µm (Al-Cu-Fe) films have been deposited by magnetron sputtering on construction steel substrates. The films were characterized by X-Ray diffraction (X.R.D.), Energy Dispersive Spectroscopy (E.D.S.) and scanning electron microscopy. Depending on the deposition conditions (i.e. substrate temperature), we obtain thin films of the same distinct Al₆₂Cu_25.5Fe_12.5 composition from amorphous to quasicrystalline state. Depth sensing microindentation reveals both intrinsic hardness, elasticity and brittleness of the coatings. Multipass, unidirectional friction experiments were run. In the triboscopic mode they yield friction and surface topology modification maps during 500 consecutive dry friction cycles under subcritical contact pressures with respect to cracking in single pass scratching. Brittle through thickness failures and adhesion loss are assessed by using optical and acoustical microscopy
10:30 AM		H2-7 Potential Applications of Quasicrystalline Coatings and Thin Films J.M. Dubois (LSG2M - EMN, France) The task of resolving the structure of quasicrystals is now entering a new age in the sense that the main part of the atomic skeleton is known. In parallel, the study of the properties of quasicrystals is still rather new though it has already opened surprising views. This study is made easier thanks to the availability of stable quasicrystals and of single grained samples the quality of which competes with that of the best crystalline specimens. All such measurements demonstrate that quasicrystals, basically made of seventy atomic percent of aluminium and of transition metals, no longer behave like metals. They are heat insulators and show a very weak electronic conductivity which furthermore increases with temperature. Their optical properties dramatically depart from those of a free electron metal. They exhibit amazingly low friction as compared to usual metallic alloys, catalytic properties and good corrosion resistance. They apparently offer a great potential for hydrogen storage. These properties may find an application in various technological fields, especially thin films and coatings. The talk will review these domains and supply information on the current status of development of the research. Examples along this line are the preparation of low adhesion surfaces, of thermal barriers and of reduced friction and wear-resistant parts produced by plasma spray coating. Thin films are developped for temperature and infra-red sensors or solar light conversion.
11:10 AM		H2-9 Interesting Properties of Films Composed of Very Small Grains Formed From a High-Rate Nanoparticle Beam F.K. Urban, III, A. Housseini-Tehrani, P. Griffiths, G. Fernandez (Florida International University) Only a few films have been deposited from nanoparticle beams in spite of interest dating back to the early 1970s. Early attempts did not condense, could only condense a few "high vapor pressure" solids, or were so low rate as to make growth too slow to be of use. A new type of high-rate deposition system overcomes these impediments. Deposits have been made using a 1 Torr Argon + Helium sputtering and condensation atmosphere followed by nozzle aperture extraction. The beam is highly non-uniform with maximum intensity on-axis, which drops rapidly to zero within less than 1 degree off-axis. Work using copper has begun here and at one other lab at which highly accelerated (10 eV per atom) films were formed. The work to be reported here explores soft-landed nanoparticle films using no acceleration up to as much as 10 eV per atom. The soft landed films are porous with nanocrystalline structure from about 5 to 10 nm crystallites. Copper films are optically transparent and contain a significant (tens of percent) carbon contamination following exposure to air. The change in film structure with acceleration will be reported. SEM, TEM, AES, RBS, and AFM results of Cu and other films will be presented.