Synthesis of Nanostructured WC Films by Supersonic Impaction of Nanoparticle Aerosols
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' Department of Physics 2 Texas
Materials Institute-Materials Science and Engineering
3 Department of Chemical Engineering
"4Department
of Electrical and Computer Engineering, University of Texas at Austin, Austin, TX 78712 ABSTRACT Tungsten carbide (WC) coatings were prepared using supersonic impaction of nanoparticles produced by pulsed laser ablation of microparticle aerosols. The influence of experimental parameters such as, carrier gas type and impaction velocity on the structure, composition and physical properties of the resultant particles and films were studied. It was found that stoichiometric, crystalline films could be grown. These films pass both the adhesive lift off and scratch tests. TEM investigations indicate that the laser ablation forms individual particles with mean size of 7 ± 3 nm. At the highest aersol pressures small aggregates were also observed, and adjustment of the gas pressure in the laser interaction cell was found to control the degree of aggregation. Upon impaction, the separate particles form dense, self sintering nanocrystalline films, with helium forming the most dense as determined from SEM images. INTRODUCTION Presently there is a great deal of interest in nanostructured films and coatings. Materials consolidated from nanoparticles have shown improved physical and mechanical properties such as, sinterability and hardness. A natural application of such materials would be in protective and wear resistant coatings to enhance performance and lifetime of moving parts. Important considerations are the creation of hard and adherent coatings that are chemically compatible with substrates while maintaining a low substrate temperature throughout the coating process. Tungsten carbide is well suited for these hard coatings due to its low coefficient of friction, high corrosion resistance in acidic media and its ability to maintain a high degree of hardness at elevated temperatures [I]. The process of supersonic impaction of aerosol particles has been studied in detail [2]. It was shown that particles down to molecular scale can be deposited onto plates with high efficiency. The minimum particle size impacted depends primarily on the extent to which the gas jet expands before encountering the substrate. This expansion can be controlled by the pressure ratio, gas type and nozzle-substrate distance. Aerosol particle impaction has been used to deposit a number of technologically useful films. Kashu et al. used a subsonic gas jet with 20-70 nm particles to produce nanostructured films of TiN and a number of metals [3]. More recently, Rao et al. has demonstrated the growth of SiC films at high rates by supersonically impacting nanoparticles produced in a DC arc source [4-6]. In that process precursors react chemically, in a 193 Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
hot plasma producing SiC of the proper stoichiometry. In this work we describe a complementary physical technique that has many of the same advantages as in the work of Rao and co-workers. The Las
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